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The development of trench-arc-backarc (TABA) systems is uniquely associated with modern-style plate tectonics on Earth. The Qilian orogenic belt in NW China records the evolution history of the Proto-Tethys Ocean at the transition time from the Proterozoic to Phanerozoic. This paper presents systematic studies of petrography, U–Pb chronology and geochemistry on various rocks from a middle-ocean ridge (MOR)-type ophiolite belt, active continental margin and back-arc basin in the Qilian orogenic belt to address the development of a modern-style TABA system. Arc magmas include felsic intrusions with ages of 531 to 477 Ma and felsic-mafic arc volcanic rocks with ages of 506 to 439 Ma, showing distinctive features of typical magmatic rocks formed at an Andean-type continental margin. The back-arc basin is recorded by a 490- to 448-Ma suprasubduction zone (SSZ)-type ophiolite with boninite, and Silurian turbidite flysch formation. We establish a three-stage tectonic history from the initiation of subduction to the formation of a mature Japan-Sea-type back-arc basin at the active continental margin in the Early Paleozoic era. (1) Northward subduction of Proto-Tethys Ocean initiated and the Andean-type continental arc developed at ~530 to 500 Ma with continual crustal thickening; (2) a tectonic transition occurred from an Andean-type active continental margin to a West Pacific-type active continental margin at ~500 to 490 Ma with rapid thinning of crust to ~35 km; and (3) mature ocean basins and back-arc-basin (BAB) ophiolites were formed in the back-arc extensional environment at ~490 to 450 Ma. [ABSTRACT FROM AUTHOR]

The Qilian orogen marks the northern boundary of the Proto-Tethys Ocean and consists of the North Qilian belt, the Central Qilian block and the South Qilian belt. The metamorphic P–T–t path of the Central Qilian block and the tectonothermal processes that affected it are crucial to understanding the closure of the ocean. This paper presents a systematic investigation of the metamorphic P–T–t path of amphibolites from the Precambrian Huangyuan Group and Maxianshan Group in the Central Qilian block. An amphibole 40Ar/39Ar age of 456.7 ± 5.2 Ma from a garnet amphibolite and a 427.8 ± 3.8 Ma muscovite 40Ar/39Ar age from an orthogneiss in the Huangyuan Group, and amphibole and biotite 40Ar/39Ar ages of 408.7 ± 2.2 Ma and 349.9 ± 2.0 Ma from an amphibolite and a 361.4 ± 2.0 Ma biotite 40Ar/39Ar age from an orthogneiss in the Maxianshan Group record cooling stages after peak metamorphism. Phase equilibrium modeling of all amphibolite samples indicates clockwise P–T paths. A garnet amphibolite from the sillimanite zone in the Huangyuan Group recorded peak metamorphic conditions of 5.6–6.6 kbar and 730–770°C, reflecting granulite-facies metamorphism. Peak P–T conditions for a garnet amphibolite from the garnet zone are constrained to 580–600°C and 6–7 kbar. The amphibolite sample from the Maxianshan Group indicates peak P–T conditions of ~5.2 kbar and ~630°C. The Central Qilian block was affected by both southward subduction of the North Qilian Ocean and northward subduction of the South Qilian Ocean, which resulted in paired metamorphic belts along the northern margin of the Central Qilian block and accretion of the Hualong block to its southern margin. The subducted North Qilian oceanic slab detached and the remainder was exhumed at 453–445 Ma, accompanied by the extension and retrograde metamorphism of the overlying crust of Huangyuan Group at 457–428 Ma. Subsequent cooling of the Maxianshan Group indicates that the thermal regime relaxed during post-collisional extension at c. 409–350 Ma between the Hualong block and the Central Qilian block. [ABSTRACT FROM AUTHOR]

The mechanism behind the destabilization of the North China Craton (NCC) remains a contentious topic among researchers. Large-scale Cretaceous adakitic magmatism in the NCC offers insights into the decratonization process. This study focuses on the Huashan and Laoniushan plutons located in the Lesser Qinling on the southern margin of the NCC and compiles published data for coeval adakitic rocks to investigate the role of water in adakitic rock petrogenesis during the peak destruction of NCC. Both the Huashan and Laoniushan plutons exhibit adakitic signatures, including high Sr (193–1080 ppm), low Yb (<14.8 ppm) and Y (<1.24 ppm) concentrations, as well as high Sr/Y (18–100) and La/Yb (24–58) ratios. The zircon Hf–O isotope compositions suggest that the primary source for the Huashan and Laoniushan plutons is the mafic lower crust of NCC. Nevertheless, there are significant differences in trace element characteristics between the two plutons. Specifically, the Huashan pluton displays higher Na2O/K2O ratios, lower levels of Rb, Rb/Sr, Nb, Ta content, and a weak Eu anomaly in comparison to the Laoniushan pluton. These variations in geochemical attributes cannot be accounted for by mechanisms like mantle-derived magma mixing, crustal contamination, or fractional crystallization processes. Instead, these disparities are attributed to distinct modes of crustal anatexis, involving both water-fluxed and dehydration melting. Subsequently, we conducted thermodynamic simulations of the melting process of mafic lower crust under different pressure (0.5–1.5 GPa) and water content conditions (1–3 wt.%). The simulation results suggest that the Huashan pluton is most likely formed through water-fluxed melting in a scenario with normal crustal thickness (1 GPa). On the other hand, the Laoniushan pluton might have originated from dehydration melting under normal crustal thickness and pressure conditions. Notably, high pressure (>1.5 GPa) is not necessary for the formation of intracontinental adakitic rocks. The release of water from metasomatized lithospheric mantle and subsequent hydration of the lower continental crust triggers extensive adakitic magmatism in the NCC. These findings emphasize the significance of deep water cycling in understanding large-scale magmatic events and illuminate the decratonization mechanism. [ABSTRACT FROM AUTHOR]

Apatite can incorporate significant amounts of H2O and CO2, potentially recording volatile abundance and behavior during magma evolution. Here we conducted in situ measurements of CO2 and H2O concentrations in apatite, along with elemental compositions, from two contemporaneous alkaline volcanic suites (Tianbao and Tudiling) in the South Qinling belt in central China to better understand the CO2 and H2O contents and behavior in evolving alkaline melts. Apatite from alkali basalts in Tianbao contains variable CO2 contents ranging from <100 ppm to 2.7 wt.% and H2O contents ranging from 0.1 to 0.6 wt.%. Apatite from REE-enriched trachytes, which evolved from alkali basalt, shows significantly lower CO2 contents and a continuous decrease in H2O during magma fractionation. The observations suggest that CO2 loss commenced at the early stages of magma evolution, whereas significant H2O loss occurred during subsequent magma fractionation in tandem with REE-enrichment. In comparison, apatite grains from the Tudiling trachyte, which is genetically linked with carbonatite, contain higher CO2 contents (0.6 to 1.5 wt.%) but lower REE concentrations than the Tianbao trachytes. Apatite in the Tudiling trachyte is inferred to have crystallized from a carbonated alkaline magma prior to significant CO2 loss and the separation of Tudiling carbonatitic melts, where subsequent liquid immiscibility led to REE enrichment into the carbonatitic melts. The volatile characteristics of apatite from the two volcanic suites provide valuable insights into two different evolutionary processes of alkaline/carbonatitic magmas, the behavior of CO2 and H2O, and the enrichment of REE in alkaline systems. [ABSTRACT FROM AUTHOR]

Li-Cs-Ta (LCT) rare-element pegmatites occur as late-stage and highly fractionated bodies at the margins of regionally zoned granite pegmatite fields. The evolution of the granitic pegmatite system, including its rare-metal metallogeny, is often difficult to determine due to complex textures involving variable crystal size and a heterogeneous chemical composition. The Renli-Chuanziyuan pegmatite field (South China) displays a well-developed regional zonation sequence, involving a core of biotite-, two-mica- and muscovite monzogranites (MMs) that grades outward into microcline (K-zone), microcline-albite (K-Na-zone), albite (Na-zone) and albite-spodumene (Na-Li-zone) pegmatites. Monazite and the Th, Ca–end-member (i.e. cheralite) provide valuable age, rare earth element (REE) geochemical and Sm-Nd isotopic data for understanding the regional zoning process within the Renli-Chuanziyuan pegmatite. Monazite (from the MM and the K-, K-Na- and Na-zone pegmatites) and cheralite (from the Na-Li-zone pegmatite) have variable compositions and complex internal microtextures. The monazite and cheralite grains contain irregular areas with subtle heterogeneous BSE response along cracks and grain margins, suggesting that they have experienced alkali-bearing fluid-aided modification. However, these features are rarely seen in monazite from the K-zone pegmatite. Common Pb contamination and/or Pb loss during fluid-aided modification may have disturbed the monazite and cheralite U-Th-Pb isotopic system, due to the differential mobility of U, Th and Pb. The unaltered Na-zone monazite and Na-Li-zone cheralite yielded Th-Pb ages of 140.42 ± 2.30 Ma (2 σ, mean standard weighted deviation (MSWD) = 2.4, n = 14) and 139.58 ± 2.15 Ma (2 σ, MSWD = 2.9, n = 21), respectively. The unaltered MM, K-zone and K-Na-zone monazite yielded 206Pb-238U ages of 138.03 ± 2.18 (2 σ, MSWD = 2.5, n = 18), 140.39 ± 2.18 (2 σ, MSWD = 3.0, n = 20) and 140.58 ± 2.14 Ma (2 σ, MSWD = 2.0, n = 52), respectively. These ages for the four pegmatite zones are temporally consistent with a syngenetic origin for the magmatic sequence of biotite-, two-mica- and MM and the pegmatite system and rare-metal (Li-Nb-Ta-Rb-(Cs)-(Be)) mineralization. The Sm-Nd isotopic analyses of the unaltered monazite and cheralite from the MM and four pegmatite zones yield similar initial Nd isotopic composition with εNd(t) = −9.9 to −7.9, indicating an identical single-source region (i.e. the Neoproterozoic South China lower crust). The Sm/Nd ratios display a gradual decrease across the four pegmatite zones from the unaltered K-zone monazite to Na-Li-zone cheralite, i.e. 0.39–0.63 (avg. = 0.43) for K-zone, 0.29–0.35 (avg. = 0.31) for K-Na-zone, 0.26–0.30 (avg. = 0.28) for Na-zone and 0.21–0.27 (avg. = 0.24) for Na-Li-zone. Such progressive variations suggest their derivation from the same parental magma, which experienced varying degrees of fractionation before the extraction of pegmatitic melts. Comprehensive monazite and cheralite geochemistry, as well as in situ U-Th-Pb and Sm-Nd isotopic results indicate that Rayleigh-type fractional crystallization controls the mineralogical and geochemical evolution from a chemically zoned granite source. [ABSTRACT FROM AUTHOR]

Carbonatite intrusions host the world's most important light rare earth element (LREE) deposits, and their formation generally requires extraordinary fertile sources, magmatic evolution, and hydrothermal events. However, carbonatitic magma evolution, particularly the role of fractional crystallization and contamination from silicate rocks in REE enrichment, remains enigmatic. The Maoniuping world-class REE deposit in southwestern China, is an ideal target to decipher magmatic evolution and related REE enrichment as it shows continuous textual evolution from medium- to coarse-grained calcite carbonatite (carbonatite I) at depth, to progressively pegmatoidal calcite carbonatite (carbonatite II) at shallow levels. In both types of calcite carbonatites, four generations of calcite can be classified according to petrographic and geochemical characteristics. Early-crystalizing calcite (Cal-I and Cal-II) are found in carbonatite I and exhibit equigranular and a polygonal mosaic textures, while late calcites (Cal-III and Cal-IV) in carbonatite II are large-size oikocrysts (>0.5 mm in length) with strain-induced undulatory extinction and bent twinning lamellae. All these generations of calcite yield similar, near-chondritic, Y/Ho ratios (26.6–28.1) and are inferred to be of magmatic origin. Remarkably, gradual enrichment of MgO, FeO and MnO from Cal-I to Cal-IV is coupled with a significant increase in REE contents (~800 to 2000 ppm), with LREE-rich and gentle-to-steep chondrite-normalized REE patterns ((La/Yb)N = 3.1–26.8 and (La/Sm)N = 0.9–3.9, respectively). Such significant REE enrichment is ascribed to protracted magma fractional crystallization with initial low degree of fractional crystallization (fraction of melt remining (F) = ~0.95) evolving to late stage (F = 0.5–0.6) by formation of abundant calcite cumulates. Differential LREE and HREE behavior during magma evolution largely depend on separation of phlogopite, amphibole, and clinopyroxene from the carbonatitic melt, which is indicated by progressively elevated (La/Yb)N ratios ranging from 3.1 to 26.8. The four generations of calcite have significantly different C and Sr isotopic compositions with δ13CV-PDB decreasing from −3.28 to −9.97‰ and 87Sr/86Sr increasing from 0.70613 to 0.70670. According to spatial relations and petrographic observations, the relative enrichment of δ13C and depletion in 87Sr/86Sr ratios of Cal-I and Cal-II show primary isotopic characteristics inherited from initial carbonatitic magma. By contrast, the variable Sr and C isotopic compositions of Cal-III and Cal-IV are interpreted as the results of contamination by components derived from silicate wall rocks and loss of CO2 by decarbonation reactions. To model such contamination processes, Raleigh volatilization and Monte Carlo simulation have been invoked and the model results reveal that carbonatitic melt-wall rock interaction requires 40% radiogenic Sr contamination from silicate rocks and 35% CO2 degassing from carbonatitic melt. Moreover, positive correlations between decreasing δ13C values and increasing REE contents, together with bastnäsite-(Ce) precipitation, indicate further REE accumulation during the contamination processes. In summary, alongside REE-rich magma sources, the extent of fractional crystallization and contamination during carbonatitic magma evolution are inferred to be important mechanisms in terms of REE enrichment and mineralization in carbonatite-related REE deposits worldwide. [ABSTRACT FROM AUTHOR]

The bulk rock composition of granitoids reflects the composition of their source and the conditions of partial melting, which are functions of the geodynamic setting in which they formed. Granitoids in active continental margins (continental arcs) are dominated by calc-alkaline rocks with subordinate alkaline compositions, although how these different magma compositions formed is not well understood. Neoproterozoic magmatic rocks are widely distributed along the western margin of the Yangtze Block in South China to form the >1000-km long Panxi continental arc system, which is dominated by granitoids with minor mafic–ultramafic and intermediate plutons. The granitoids are subdivided into sodic and potassic variants that occur as belts along the western and eastern sides of the continental arc, respectively. Sodic granitoids from the western part consist of tonalite, granodiorite, and monzogranite with crystallisation ages ranging from 870 Ma to 740 Ma. They have low K2O/Na2O ratios (0.1–1.0) and high Na2O contents (3.5–6.7 wt%), high but variable SiO2 (61–75 wt%) concentrations, and negative to positive whole-rock εNd(t) values (−1.7 to +2.9). Zircon grains from the sodic granitoids have εHf(t) values ranging from +0.3 to +9.6 and δ18O from 3.90‰ to 7.71‰. The potassic granitoids from the eastern side consist of monzogranite and syenogranite with crystallisation ages from 820 Ma to 790 Ma. They have high K2O/Na2O ratios (0.6–2.2), K2O (2.6–5.9 wt%) and SiO2 contents (69–78 wt%), but whole-rock εNd(t) (−0.9 to +2.9) and zircon εHf(t) (+1.8 to +12.9), and δ18O values (2.98‰ to 6.41‰) similar to those of the sodic granitoids. The isotopic compositions of both the sodic and potassic granitoids are similar to those of spatially- and temporally-related mantle-derived (mafic to ultramafic) rocks, and are considered to have been derived from juvenile mafic continental crust. Phase equilibrium modelling shows that the H2O content of the granitoid source rocks played a key role in their petrogenesis, both in lowering solidus temperatures and in controlling the compositions of the derived partial melts. Our results indicate that calc-alkaline sodic granitoids can be formed by water-fluxed melting of juvenile mafic crust at 750–900°C and 9–12 kbar in which the required H2O was derived from the dewatering of underplating mafic arc magmas. By contrast, the potassic granitoids were generated by fluid-absent (H2O-undersaturated) partial melting of a similar juvenile mafic source at 725–900°C and 6–9 kbar. We conclude that the sodic granitoids were derived from partial melting of the newly-formed mafic lower crust in the continental arc, whereas the potassic granitoids were likely generated in the back-arc setting induced by upwelling of asthenospheric mantle. [ABSTRACT FROM AUTHOR]

Serpentinites of the mantle wedge were identified from a composite oceanic–continental subduction zone that occurs as the Hong'an orogen in east–central China. They were comprehensively investigated by an integrated approach of whole-rock major and trace elements, mineral major and trace elements, whole-rock and olivine O isotopes, and phase equilibrium modeling as well as zircon U–Pb ages, trace elements, and Hf-O isotopes. These serpentinites show high Mg# and MgO/SiO2 ratios but low Al2O3/SiO2 ratios as well as low Ti contents, resembling those from the mantle wedge rather than the subducting oceanic slab. Petrographic observations were combined with CIPW norm calculations to suggest that the protoliths of these serpentinites are refractory peridotites in the mantle wedge. Whole-rock and mineral compositions indicate that these serpentinites experienced multiple episodes of metamorphism and metasomatism during the tectonic development from oceanic subduction in the Carboniferous to continental collision in the Triassic. The enrichments of Th, U, light rare earth elements and high field strength elements in the whole rock and the finding of zircon inside them indicate their formation through metasomatism of the mantle wedge peridotites by subducting crust-derived fluids. The ca. 310 Ma zircon domains exhibit positive ε Hf(t) values of 4.5 to 19.1 and variable δ18O values similar to those of oceanic-type eclogites in the Hong'an orogen but different trace element compositions, indicating their formation through metasomatism by fluids from the subducting Paleotethyan oceanic crust. The ca. 430 Ma zircons exhibit U–Pb ages, trace elements and Hf-O isotopes similar to those of the protolith magmatic zircons in the oceanic-type eclogites. The older zircon domains also exhibit similar element and isotope compositions to inherited zircons from the high- to ultrahigh-pressure metamorphic rocks in the Hong'an orogen. Thus, these relict zircons would be physically transferred into the serpentinites by metasomatic fluids originating from the subducting Paleotethyan oceanic crust. High Th/U ratios for the ca. 310 Ma zircons imply their formation through the metasomatic reaction rather than direct precipitation from the subducting oceanic crust-derived fluids. The metasomatic zircons of Triassic age show negative ε Hf(t) values of −16.0 to −2.7, indicating their formation through metasomatic overprinting by deeply subducting continental crust-derived fluids in the continental subduction channel during the Triassic continental collision. In this context, the mantle wedge peridotites were first metasomatized in the Carboniferous by the subducting oceanic crust-derived fluids. Afterwards, part of the metasomatized peridotites were off-scrapped into the oceanic subduction channel, where they were hydrated by further subducting oceanic crust-derived fluids to form antigorite serpentinite at forearc depths. The antigorite serpentinite was carried to deeper depths during subsequent continental subduction and underwent dehydration there. This gave rise to metamorphic olivine with extremely high Fo values of 96.7 to 97.6 and higher MnO but lower NiO contents than common mantle olivine. The coexistence of lizardite, brucite and magnesite in the serpentinites indicates their retrograde processes of serpentinization and carbonation on the subsurface. Therefore, the present observations and interpretations of orogenic serpentinites provide insights into the multistage fluid metasomatism at different depths during the tectonic transition from oceanic subduction to continental collision. [ABSTRACT FROM AUTHOR]

The origin of lamprophyres associated with large igneous provinces (LIPs) remains controversial, particularly whether they are derived by direct melting of mantle plumes, or from previously metasomatized domains in thermally perturbed subcontinental lithosphere. Here, we report the petrological and geochemical characteristics of a recently identified suite of alkaline lamprophyres (sannaites) that represent the final pulse of magmatism in the Permian Tarim LIP in NW China. The sannaites display porphyritic texture with phenocrysts of olivine, clinopyroxene, hornblende, phlogopite, and titanomagnetite in a groundmass of plagioclase, clinopyroxene, nepheline, hornblende, biotite, and titanomagnetite with minor pyrite and apatite. Carbonate ocelli and almost pure albite in the groundmass are interpreted to have crystallized from immiscible carbonate and hydrous fluids, respectively, produced by late-stage magmatic segregation. The rocks show low to moderate SiO2 (37.7–49.3 wt.%) and MgO (2.74–9.91 wt.%), together with high Fe2O3T (up to 22.7 wt.%) and alkali contents (up to 9.02 wt.% Na2O + K2O). They are characterized by high incompatible element abundances, especially a marked enrichment in large-ion lithophile elements (Rb and Ba) and light rare-earth elements (e.g. La and Ce) relative to P and high-field-strength elements (e.g. Ti). They show a relatively restricted range of δ66Zn values between 0.22‰ and 0.46‰ with an average of 0.37 ± 0.04‰ (2SE, n  = 10), which is marginally heavier than that of MORBs (0.27 ± 0.05‰). Their (87Sr/86Sr)t values range from 0.7035 to 0.7061, εNd(t) from −0.97 to +5.62, and δ26Mg from −0.36‰ to −0.17‰ (n  = 8), the latter being consistent with those of global MORBs. Based on the correlation between Zn isotopes and TiO2–FeO concentrations, we infer that the heavy Zn isotopes in some of the sannaites resulted from fractional crystallization of Fe–Ti oxide minerals. The whole rock geochemical features of these rocks (negative K anomalies and enrichment in large-ion lithophile elements) and rhyolite–MELTS simulations suggest that the primary magmas of the sannaites were derived from an amphibole-bearing enriched lithospheric mantle source. Metasomatism and related formation of amphibole-bearing metasomatized mantle may be linked to sublithospheric melts/fluids derived from the Tarim plume in the earlier stages of plume activity, rather than slab-derived fluids or carbonate melts as suggested in previous studies for other alkaline mantle-derived magmas. Partial melting may have been triggered by the thermal input from the Tarim plume during a later stage. This study suggests that exotic, alkali-rich magmas can be produced during the multi-stage evolution of large mantle plumes, involving complex cycles of lithospheric mantle metasomatism and later melting of previously enriched domains. [ABSTRACT FROM AUTHOR]

Crystallization and solidification pathways in crustal magma chambers control the chemical and textural evolution of igneous rocks. However, deciphering these codes from completely solidified cumulates is challenging and requires a full understanding of growth processes of minerals and textures of rocks. In this study, we collected samples from a ca. 380-m-thick drill core intersecting the ultramafic sequence of the Xinjie layered intrusion in SW China, which is composed of wehrlite with poikilitic clinopyroxene (Cpx), and clinopyroxenite and olivine clinopyroxenite with euhedral to subhedral Cpx grains. Using micro-X-ray fluorescence and electron probe micro-analyzer mapping techniques, we found that the Cpx grains in clinopyroxenite and olivine clinopyroxenite are characterized by sector-zoned, Cr-rich cores, whereas the Cpx oikocrysts in wehrlite only show Cr-rich cores without sector zoning. The sector-zoned cores of the Cpx grains are widespread in clinopyroxenite and olivine clinopyroxenite and consist of Si–Mg-rich hourglass sectors and Cr–Al–Ti-rich prism sectors, indicating rapid growth of the Cpx grains due to moderate degrees of undercooling, which may be induced by fast cooling during initial emplacement of Cpx-saturated magma into cold country rocks. The sector-zoned Cpx grains thus likely constructed the initial crystal framework in the magma chamber. In contrast, the absence of sector zoning in Cr-rich cores of the Cpx oikocrysts indicates that an olivine-saturated magma may have intruded a high-temperature, Cpx-laden crystal mush so that magma undercooling was significantly reduced or eliminated and no sector zoning developed in the Cpx oikocrysts. Such a process may also result in the spatial variation of the medians of Cpx–Cpx–plagioclase dihedral angles (Θccp) in clinopyroxenite, which shows a significant reduction from an original impingement angle of ~60° in the middle and upper parts to ~40° in the basal part of the sequence. The reduction of Θccp could be attributed to melt-present textural equilibration in the Cpx-laden mush that was sustained by additional heat supplied by massive olivine-saturated melt input in the ~40-m-thick basal part. Here, we propose that the ultramafic sequence of the Xinjie layered intrusion may have formed by the intrusion of primitive, olivine-saturated magma into an early-formed Cpx-laden mush in a manner of sill-like sheets. This study provides new perspectives on the early history of a magma chamber and emphasizes the role of sill intrusion of primitive magmas into crystal mush in the formation of ultramafic sequences in layered intrusions, which has been relatively overlooked in previous studies. [ABSTRACT FROM AUTHOR]

The eastern Paleo-Tethyan Jinshajiang–Ailaoshan–Song Ma orogenic belt in the Southeast Tibetan Plateau represents the tectonic boundary between the Indochina and South China blocks. Regional Cenozoic lithospheric strike-slip movements caused by the India–Asia collision have modified many of the pristine geological records associated with the Indosinian orogeny. The lack of reliable petrological evidences, particularly of high-pressure (HP) metamorphism, has hindered the tracing of the evolutionary history of the Paleo-Tethyan orogenic belt. We report the mineralogy, geochemistry, geochronology and thermodynamic modelling of eclogite lenses/blocks and their host garnet–phengite schists from the Song Ma Suture Zone of Northwest Vietnam and reveal their protolith origins and metamorphic evolution. The eclogites are geochemically similar to mid-ocean ridge basalt, showing weak depletions in high field-strength elements, positive to slightly negative whole-rock ɛNd(t) ratios (−1.69 to +5.15) and slightly high 87Sr/86Sr(t) ratios (0.70466–0.70834). The igneous protolith of the eclogites formed in the late Silurian (425.4 ± 3.6 Ma; igneous zircon dating, 638–459 Ma TDM dating), which makes them the oldest known mid-ocean ridge mafic rocks in an ancient Paleo-Tethyan Ocean. Various eclogites preserve similar peak mineral assemblages (garnet + omphacite + phengite + rutile + quartz ± epidote/clinozoisite ± kyanite ± winchite) under indistinguishable peak conditions. Their prograde P–T trajectories show a uniform pattern of progressive heating and weak compression along a geothermal gradient of 8°C–10°C/km, indicating a warm subduction setting at a low angle that contrasts with the cold oceanic subduction recorded in the adjacent Paleo-Tethyan suture zones. Combined with detrital U–Pb ages from previous studies, our geochronological data indicate that the host schists originated from continental crust within the South China Block and underwent a pervasive Triassic HP metamorphism related to the Indosinian orogeny. The eclogites and host schists yielded HP metamorphic ages of 239–234 Ma and retrogressive amphibolite-facies metamorphic ages of 231–229 Ma, suggesting rapid cooling (14.0°C–21.6°C/Myr) and exhumation (6.4 km/Myr) from the mantle to the crust. The Song Ma eclogites and their host schists formed when the final ocean closure transitioned to the initial Indochina–South China collision. The Song Ma eclogites and their host schists provide essential information on the opening and closure of the eastern Paleo-Tethys and the subsequent amalgamation of Southeast Asian continental fragments. [ABSTRACT FROM AUTHOR]

Newly discovered meta-troctolite enclaves in Early Paleozoic Huanggang diorite within dominantly meta-volcaniclastic greenschist to amphibolite facies Erlangping unit in central China extends the known extent of granulite-facies metamorphism in the Tongbai Orogen. Evidence for this granulite facies event is preserved in coronitic and symplectitic textures developed in the meta-troctolites enclaves. The primary assemblage comprises olivine and plagioclase with minor orthopyroxene, clinopyroxene, spinel and amphibole. The late magmatic stage comprising an orthopyroxene (Opx1) corona around olivine and clinopyroxene corona around spinel formed by reaction of cumulus and intercumulus phases with trapped melt at around 1040–1060°C near the solidus. During troctolite cooling, metamorphic solid-state replacement of primary olivine and plagioclase produced orthopyroxene (Opx1)/clinopyroxene (Cpx1) + vermicular spinel (Sp1) symplectite, followed by an inclusion-free amphibole (Amp1) layer surrounded by amphibole + spinel symplectite (Amp2 + Sp2). Microstructural, pseudosection and mineral isopleth analysis coupled with prominent compositional zoning in the Opx1 corona and chrome-rich spinel (Cr-Sp) suggests that the symplectites formed by diffusion-controlled solid-state replacement at the interface of reactant minerals, probably during cooling to granulite-facies conditions of ~820–880°C and 6.7–7.0 kbar. U–Pb dates of rare metamorphic zircon, as indicated by growth zonation and identification of metamorphic minerals enclosed in grains by Raman probe, are ca. 450 Ma. The reaction textures record the transition from late magmatic to granulite-facies metamorphism with the heat supplied by magma upwelling at the base of Erlangping arc triggered by extension of Erlangping back-arc. [ABSTRACT FROM AUTHOR]

The geochemistry of lavas combined with textural constraints can provide important information on magmatic processes occurring at inaccessible depths. Here we carry out a detailed textural and geochemical study on 0.23 Ma contemporaneous and adjacent Qiulin, Longbao, and Dayukou volcanoes in the intraplate Quaternary Datong volcanic field (DVF) in the North China Craton. Lavas from the three tholeiitic volcanoes have similar major and trace element compositions with their trace element patterns resembling enriched mid-ocean ridge basalts (E-MORB). However, distinct textures are observed between the samples from the three volcanoes. Samples from Qiulin are generally poorly-phyric (<10 vol. %) containing orthopyroxene, plagioclase, and olivine phenocrysts and monomineralic orthopyroxene glomerocrysts set in a groundmass of plagioclase, clinopyroxene, orthopyroxene, and Fe–Ti oxides. In contrast, samples from both Longbao and Dayukou are highly phyric (up to ~50 vol. %) and consist mainly of plagioclase, orthopyroxene, clinopyroxene, and olivine phenocrysts with mm- to cm-sized polymineralic glomerocrysts enclosed in a groundmass comprising plagioclase, clinopyroxene, olivine, and Fe–Ti oxides. Crystal types and proportions vary significantly between the glomerocrysts of Longbao and Dayukou even on a thin section scale. The phenocrysts and glomerocrysts at the two volcanoes commonly show resorbed and embayed margins and sieve textures. Moreover, pyroxenes present as phenocrysts and glomerocrysts are always characterised by corona textures containing orthopyroxene cores mantled by clinopyroxene. Notably, orthopyroxene is never observed in contact with groundmass. The relatively low Mg# (75–85) and Ca content in the cores indicate that the orthopyroxenes from all three volcanoes are crystallised early from the magma rather than being mantle xenocrysts. Thermobarometry and kernel density estimation indicate that orthopyroxenes in all three volcanoes initially crystallised at ~7 kbar (23 km depth), whereas clinopyroxenes crystallised at ~4 kbar (13 km) and the final crystallisation pressure is ~3 kbar (10 km). Thermodynamic modelling by COMAGMAT suggests early near-liquidus crystallisation of the orthopyroxene at high pressures, followed by partial dissolution likely associated with decompression. The relatively larger proportions of phenocrysts compared with glomerocrysts in Qiulin samples implies that the magma reservoir was only partially turned into mushes at the periphery. Magma ascending via the melt-rich interior of the magma reservoir formed the crystal-poor Qiulin lava. In contrast, those ascending through the mushes at the peripheries of the deeper reservoir and discrete shallower storage regions formed the highly phyric Longbao and Dayukou lavas. The pervasive occurrence of sieve textures and complex mineral composition profiles in both phenocrysts and glomerocrysts in Longbao and Dayukou samples belie complicated open system crystallisation histories of these magmas. We conclude that open system processes involving mush disaggregation and magma recharge in multi-level plumbing systems played an important role in the petrogenesis of these intracontinental tholeiites. Moreover, the important role of orthopyroxene in the petrogenesis of these tholeiites distinguishes them from many other tholeiite suites like MORB, as well as those erupted through thickened oceanic crust like in Iceland and the Shatsky Rise. This study highlights the importance of combining textural constraints with geochemical analyses. [ABSTRACT FROM AUTHOR]

Compaction is important to the expulsion and compositional modification of interstitial liquid of crystal mush in the solidification of layered intrusions hosting Fe–Ti oxide mineralization. However, the compaction manner and compositional evolution of interstitial liquid after the onset of abundant Fe–Ti oxide crystallization in crystal mush are still enigmatic. In this study, we collected samples from a drill core profile across the major Fe–Ti oxide-rich segment of the Bijigou intrusion, a large and differentiated layered intrusion in Central China, and carried out a detailed study on the late-stage magmatic microstructures of rocks and crystal size distributions (CSDs) and spatial distribution patterns (SDPs) of clinopyroxene and plagioclase to examine the compaction manner of interstitial liquid. The Fe–Ti oxide-rich segment is composed of, from the base upward, a Fe–Ti oxide ore layer (~80 m), a troctolite unit (~105 m), an oxide gabbro unit (~153 m) and an apatite–oxide–gabbronorite unit (~165 m). The CSD curves for both plagioclase and clinopyroxene grains of the samples from the Fe–Ti oxide ore layer display gentle slopes for small crystals and consistent slopes for large crystals. In addition, there is a positive correlation of the SDP R -value (ratio of observed and predicted nearest neighbor distance of plagioclase/clinopyroxene) and mineral mode except plagioclase/clinopyroxene of the samples. The interstitial liquid was likely expelled by intensive pressure solution concomitant with crystallization of abundant Fe–Ti oxides, evidenced by the positive correlation of aspect ratio and alignment factor of plagioclase and the low fraction of trapped liquid (F TL) of 4–6% for the Fe–Ti oxide ore layer. The cumulus plagioclase of the Fe–Ti oxide ore layer was partially dissolved during pressure-solution compaction, leading to ambient Fe-rich interstitial liquid becoming Al-enriched in local places so that abundant amphibole and hercynite are present in the replacive symplectites adjacent to cumulus plagioclase. Above the Fe–Ti oxide ore layer, plagioclase grains in the samples from the troctolite unit show consistent slopes of the CSD curves and negative correlation of SDP R -value and mineral mode in the SDP plot, indicating that the interstitial liquid was mainly expelled by mechanical compaction, with F TL ranging from 7 to 9%. The CSD curves for clinopyroxene grains of the samples from the oxide gabbro unit have been modified by Ostwald ripening, but the expulsion of interstitial liquid in this unit is also attributed to mechanical compaction due to higher compaction rate than accumulation rate. This study demonstrates that the compaction manner of interstitial liquid can change from pressure solution to mechanical compaction with dramatically diminishing crystallization of Fe–Ti oxides. The pressure-solution compaction accompanying with large amounts of Fe–Ti oxides can remarkably modify the composition of interstitial liquid. This study also demonstrates that a combined measurement of CSDs and SDPs for cumulus minerals can be used to quantify the compaction manner of interstitial liquid during solidification of layered intrusions. [ABSTRACT FROM AUTHOR]

Dabie-type porphyry Mo deposits have recently been identified as a new subtype of porphyry Mo deposits, but several questions remain about the role of ore-related magmas in the formation of this type of deposit, as well as distinctions in genetic processes with the well-studied Climax-type porphyry Mo deposits. Here, mineral and melt inclusions from the giant Qian'echong deposit, Dabie orogen, central-eastern China, were studied in order to improve our understanding of the nature and the role of ore-related magmas in the genesis of Dabie-type porphyry Mo deposits. The magmatic and hydrothermal evolution of the system was reconstructed based primarily on the analysis of quartz-hosted silicate melt and mineral inclusions, in concert with field and petrographic relations as well as previously published U–Pb and Re-Os geochronology. Ore-related magmas at Qian'echong include, from early to late, quartz porphyry dikes, rhyolite porphyry dikes, granite porphyries (stock and dikes) and a newly discovered, deep-seated monzogranite porphyry stock. Based on TitaniQ thermobarometry, these lithologies were sourced from a ~16–19-km deep (500–600 MPa) magma chamber and underwent nearly isothermal decompression to ~7 km (200 MPa), at temperatures ranging from 720°C to 690°C. According to the trace element composition of melt inclusions, in combination with published whole-rock Nd isotopic compositions, the early magma evolved from quartz porphyry to rhyolite porphyry through fractionation crystallization and thus became a crystal mush. This viscous crystal mush was subsequently re-melted to produce the granite porphyries and was injected with melts from a different source to generate the monzogranite porphyry. At Qian'echong, all melt inclusions have low concentrations of Mo (2–8 ppm), >5 wt. % H2O, and little to no F (≤0.26 wt. %), arguing against the requirement for ore-related magmas to be enriched in Mo to form large porphyry Mo deposits. Rayleigh fractionation modeling shows that the concentration of Mo in the quartz and granite porphyries increased through fractionation of quartz, feldspars, biotite, magnetite, and ilmenite and that Mo was subsequently depleted in the melt through fluid exsolution, exclusively in the rhyolite and monzogranite porphyries, as a result of magma ascent and decompression. This suggests that, in addition to ore-related granite porphyry, both the rhyolite and the monzogranite porphyries also contributed to the hydrothermal mineralization. This study confirms that the formation of porphyry Mo deposits does not rely on abnormally high concentrations of Mo in ore-related magmas but instead requires efficient extraction of Mo from large volumes of magmas, with normal concentrations of Mo. Unlike the Climax-type deposits where multiple pulses of ore-forming fluids are delivered from convecting shallow magma chambers, Mo mineralization in Dabie-type deposits was achieved by the assembling of ore-forming fluids from successively emplaced, relatively deep intrusions. Although the Dabie- and Climax-type Mo deposits are respectively associated with I-type and A-type granitoids in the Dabie orogen, it is suggested that both types of magmas are derived from the partial melting of subducted Yangtze continental crust and that it is the tectonic transition from compressional to extensional settings that controlled the different styles of porphyry Mo mineralization in this orogen. [ABSTRACT FROM AUTHOR]

This study explores the effects of high-pressure crystallization and immiscible sulfide melt segregation under mantle conditions on the compositional variation of basaltic magmas, using clinopyroxene megacrysts in the Cenozoic basalts of eastern China. These clinopyroxene megacrysts are large (up to >10 cm in size) and homogeneous at the grain scale. They were crystallized from variably evolved parental magmas and then captured by their host basalts. The large and systematic variations of [Sm/Yb]N, Lu/Hf, Fe/Mn, Sc/La, Ni and Cu with Mg# in the clinopyroxene megacrysts suggest their co-precipitation with garnet and with immiscibility between sulfide and silicate melts. This is consistent with the appearance of garnet megacrysts in the host basalts and abundant sulfide globules in the clinopyroxene megacrysts. The covariation between Ni contents of sulfide globules and Mg# of the clinopyroxene megacrysts suggests a genetic relationship between sulfide globules and clinopyroxene megacrysts. High-pressure crystallization of clinopyroxene and garnet results in decrease of Mg# and concentrations of CaO, MnO and heavy rare earth elements (e.g. Yb) and increase of Fe/Mn and [Sm/Yb]N in the residual melts. Therefore, geochemical characteristics of low Mg#, low CaO and MnO contents and high Fe/Mn and [Sm/Yb]N in basalts do not necessarily indicate a pyroxenite mantle source. In addition, caution is needed when applying the olivine addition method to infer the primary compositions of alkali basalts without considering the effects of high-pressure crystallization of clinopyroxene and garnet. The calculated pressure (P) and temperature (T) conditions of the clinopyroxene megacrysts are close to those of the lithosphere–asthenosphere boundary (LAB) beneath eastern China, and the low primitive [Sm/Yb]N (~4.0) of melts parental to the clinopyroxene megacrysts suggests final equilibration at relatively low pressures most likely beneath the LAB. Hence, a melt-rich layer is expected close beneath the LAB. Melt pools in this melt-rich layer provide a stable and closed environment for the growth of compositionally homogeneous clinopyroxene megacrysts. As a result, melts in these melt pools are compositionally evolved with low and variable Mg#. Subsequent pulses of melt aggregation/supply from depths with primitive compositions and high Mg# will disturb these melt pools, cause magma mixing and trigger the eruption of magmas carrying clinopyroxene and garnet megacrysts. [ABSTRACT FROM AUTHOR]

Harzburgite–dunite associations in the Yushigou ophiolitic complex (North Qilian suture, NW China) offer insights into melt migration and melt–rock interaction above a subduction zone. Based on detailed petrographic and in situ analyses, we propose a three-stage model to describe the history of melt channel formation in oceanic arc mantle. In the first stage, high-degree melt extraction was followed by widespread infiltration of small-volume melts, which formed the harzburgite that makes up most of the Yushigou complex. The Yushigou harzburgites thus have highly refractory major-element compositions (olivine Fo = 91·1–93·2, spinel Cr# [atomic 100 × Cr/(Cr + Al)] = 55·2–62·9, whole-rock Mg# = 90·5–92·0, Al2O3 = 0·29–0·74 wt%, CaO = 0·35–0·54 wt%, and low Na2O = 0·02–0·05 wt%) but slight enrichment in the light rare earth elements (LREE). In the second stage, high-volume focused melts infiltrated into the mantle wedge and produced two types of dunite, with high-Cr# (mainly 63·1–73·6) versus low-Cr# (23·6–33·7) spinel. In the low-Cr# dunites, spinels have significantly fewer inclusions and lower Fo values (88·7–90·1) in olivine than in the high-Cr# dunite (Fo = 89·9–90·5), together with higher modal abundances of interstitial sulfides. The two groups of dunite show similar extremely low TiO2 (<0·01 wt%) but different igneous clinopyroxene trace-element patterns (ΣLREE/ΣHREE ≈ 1 in low-Cr# dunite versus ΣLREE/ΣHREE >10 in high-Cr# dunite), indicating distinct differences in the infiltrating melts. The low-Cr# dunite is a cumulate from an anhydrous Al- and S-enriched basaltic melt, whereas the high-Cr# dunite was produced by reaction of harzburgite with a carbon-bearing, alkaline hydrous silicate magma. The confluence of these melt migrations finally formed a channelized dunite network, and later magmatic activity was restricted to these channels. Pulsated melt supply after chromite crystallization formed pervasive sieve-textured rims around spinel in both kinds of dunite, but these are rare in the harzburgite. The third stage was marked by fluid-dominated metasomatism recorded by millimeter-scale veined conduits, which contain carbonate (dolomite and magnesite), amphibole, phlogopite and compound crystal assemblages in both kinds of dunite. CH4–N2–graphite-dominated fluid inclusions are widespread in all lithologies, recording the volatiles transported during the last metasomatic event. The Yushigou complex thus provides a detailed example of interaction between multiple batches of melt or fluid and a lithospheric mantle wedge dunitic channel system. The results of this study further suggest that the formation of podiform chromite requires melt participation, and the nature and origins of the melts can be diverse. [ABSTRACT FROM AUTHOR]

Two suites of amphibole-rich mafic–ultramafic rocks associated with the voluminous intermediate to felsic rocks in the Early Cretaceous Laiyuan intrusive–volcanic complex (North China Craton) are studied here by detailed petrography, mineral and melt inclusion chemistry, and thermobarometry to demonstrate an in situ reaction-replacement origin of the hornblendites. Moreover, a large set of compiled and newly obtained geochronological and whole-rock elemental and Sr–Nd isotopic data are used to constrain the tectono-magmatic evolution of the Laiyuan complex. Early mafic–ultramafic rocks occur mainly as amphibole-rich mafic–ultramafic intrusions situated at the edge of the Laiyuan complex. These intrusions comprise complex lithologies of olivine-, pyroxene- and phlogopite-bearing hornblendites and various types of gabbroic rocks, which largely formed by in situ crystallization of hydrous mafic magmas that experienced gravitational settling of early crystallized olivine and clinopyroxene at low pressures of 0·10–0·20 GPa (∼4–8 km crustal depth); the hornblendites formed in cumulate zones by cooling-driven crystallization of 55–75 vol% hornblende, 10–20 vol% orthopyroxene and 3–10 vol% phlogopite at the expense of olivine and clinopyroxene. A later suite of mafic rocks occurs as mafic lamprophyre dikes throughout the Laiyuan complex. These dikes occasionally contain some pure hornblendite xenoliths, which formed by reaction-replacement of clinopyroxene at high pressures of up to 0·97–1·25 GPa (∼37–47 km crustal depth). Mass-balance calculations suggest that the olivine-, pyroxene- and phlogopite-bearing hornblendites in the early mafic–ultramafic intrusions formed almost without melt extraction, whereas the pure hornblendites brought up by lamprophyre dikes required extraction of ≥20–30 wt% residual andesitic to dacitic melts. The latter suggests that fractionation of amphibole in the middle to lower crust through the formation of reaction-replacement hornblendites is a viable way to produce adakite-like magmas. New age constraints suggest that the early mafic–ultramafic intrusions formed during ∼132–138 Ma, which overlaps with the timespan of ∼126–145 Ma recorded by the much more voluminous intermediate to felsic rocks of the Laiyuan complex. By contrast, the late mafic and intermediate lamprophyre dikes were emplaced during ∼110–125 Ma. Therefore, the voluminous early magmatism in the Laiyuan complex was probably triggered by the retreat of the flat-subducting Paleo-Pacific slab, whereas the minor later, mafic to intermediate magmas may have formed in response to further slab sinking-induced mantle thermal perturbations. Whole-rock geochemical data suggest that the early mafic magmas formed by partial melting of subduction-related metasomatized lithospheric mantle, and that the early intermediate to felsic magmas with adakite-like signatures formed from mafic magmas through strong amphibole fractionation without perceptible plagioclase in the lower crust. The late mafic magmas seem to be derived from a slightly different metasomatized lithospheric mantle by lower degrees of partial melting. [ABSTRACT FROM AUTHOR]

There are more than 80 porphyry (or skarn) Mo deposits in northeastern China with Jurassic or Cretaceous ages. These are thought to have formed mainly in a continental arc setting related to the subduction of the Paleo-Pacific oceanic plate in the Jurassic and subsequent slab rollback in the early Cretaceous. The Jurassic Daheishan porphyry Mo deposit is one of the largest Mo deposits in NE China, which contains 1·09 Mt Mo with an average Mo grade of 0·07 %. To better understand the factors that could have controlled Mo mineralization at Daheishan, and potentially in other similar porphyry Mo deposits in NE China, the geochemical and isotopic compositions of the ore-related granite porphyry and biotite granodiorite, and the magmatic accessory minerals apatite, titanite and zircon from the Daheishan intrusions, were investigated so as to evaluate the potential roles that magma oxidation states, water contents, sulfur and metal concentrations could have played in the formation of the deposit. Magmatic apatite and titanite from the causative intrusions show similar εNd(t) values from –1·1 to 1·4, corresponding to TDM2 ages ranging from 1040 to 840 Ma, which could be accounted for by a mixing model through the interaction of mantle-derived basaltic melts with the Precambrian lower crust. The Ce and Eu anomalies of the magmatic accessory minerals have been used as proxies for magma redox state, and the results suggest that the ore-forming magmas are highly oxidized, with an estimated ΔFMQ range of +1·8 to +4·1 (+2·7 on average). This is also consistent with the high whole-rock Fe2O3/FeO ratios (1·3–26·4). The Daheishan intrusions display negligible Eu anomalies (Eu/Eu* = 0·7–1·1) and have relatively high Sr/Y ratios (40–94) with adakitic signatures; they also have relatively high Sr/Y ratios in apatite and titanite. These suggest that the fractionation of amphibole rather than plagioclase is dominant during the crystallization of the ore-related magmas, which further indicates a high magmatic water content (e.g. >5 wt%). The magmatic sulfur concentrations were calculated using available partitioning models for apatite from granitoids, and the results (9–125 ppm) are indistinguishable from those for other mineralized, subeconomic and barren intrusions. Furthermore, Monte Carlo modelling has been conducted to simulate the magmatic processes associated with the formation of the Daheishan Mo deposit, and the result reveals that a magma volume of ∼280 km3 with ∼10 ppm Mo was required to form the Mo ores containing 1·09 Mt Mo in Daheishan. The present study suggests that a relatively large volume of parental magmas with high oxygen fugacities and high water contents is essential for the generation of a giant porphyry Mo deposit such as Daheishan, whereas a specific magma composition (e.g. with unusually high Mo and/or S concentrations) might be less critical. [ABSTRACT FROM AUTHOR]

The role of subducting oceanic crust-derived fluids in generating mafic arc magmatism has been widely documented. However, the subducting oceanic crust is generally composed of basaltic igneous crust and seafloor sediment, which may give rise to different compositions of liquid phases causing metasomatism of the mantle wedge. Because of the similarity in enrichment of fluid-mobile incompatible elements in the two sources of subduction zone fluids, it has been a challenge to distinguish between them when studying the products of mafic arc magmatism. This difficulty is overcome by a combined study of whole-rock Li isotopes and zircon O isotopes in addition to whole-rock major-trace elements and Sr–Nd–Hf isotopes in Late Paleozoic mafic igneous rocks from southwestern Tianshan in western China. Zircon U–Pb dating yields consistent ages of 313 ± 3 Ma to 305 ± 1 Ma for magma crystallization. The mafic igneous rocks exhibit arc-like trace element distribution patterns and depleted whole-rock Nd–Hf isotopes but slightly high (87Sr/86Sr)i ratios of 0·7039– 0·7057. They also show positive zircon εHf(t) values and slightly higher zircon δ18O values of 5·2–7·6‰. There are covariations of whole-rock Sr isotopes with Th/La and Rb/Nb ratios, indicating a contribution from terrigenous sediment-derived fluids to their mantle source in addition to basaltic igneous crust-derived fluids. Based on the slightly higher zircon δ18O values but variably lower whole-rock δ7Li values of -0·8 to 3·5‰ for the target rocks than those of mantle respectively, both altered oceanic basalt- and terrigenous sediment-derived fluids are identified in the mantle source of these mafic igneous rocks. Model calculations for trace elements and Sr–Nd–Li isotopes further confirm that the geochemical compositions of these mafic igneous rocks can be explained by chemical reaction of depleted MORB mantle peridotite with the mixed fluids to generate ultramafic metasomatites at subarc depths. Therefore, chemical metasomatism of the mantle wedge is a key mechanism for the incorporation of crustal components into the source of arc-like mafic igneous rocks above oceanic subduction zones. [ABSTRACT FROM AUTHOR]

Accretionary orogens are characterized by voluminous juvenile components (recently derived from the mantle) and knowing the origin(s) of such components is vital for understanding crustal generation. Here we present field and petrological observations, along with mineral chemistry, zircon U–Pb age and Hf–O isotope data, and whole rock geochemical and Sr–Nd isotopic data for the c.320 Ma Ulungur intrusive complex from the Central Asian Orogenic Belt. The complex consists of two different magmatic series: one is characterized by medium- to high-K calc-alkaline gabbro to monzogranite; the other is defined by peralkaline aegirine–arfvedsonite granitoids. The calc-alkaline and peralkaline series granitoids have similar depleted mantle-like Sr–Nd–Hf isotopic compositions, but they have different zircon δ18O values: the calc-alkaline series have mantle-like δ18O values with mean compositions ranging from 5·2 ± 0·5‰ to 6·0 ± 0·9‰ (2SD), and the peralkaline granitoids have low δ18O values ranging from 3·3 ± 0·5‰ to 3·9 ± 0·4‰ (2SD). The calc-alkaline series were derived from a hydrous sub-arc mantle wedge, based on the isotope and geochemical compositions, under garnet peridotite facies conditions. This study suggests that the magmas underwent substantial differentiation, ranging from high pressure crystallization of ultramafic cumulates in the lower crust to lower pressure crystallization dominated by amphibole, plagioclase and minor biotite in the upper crust. The peralkaline series rocks are characterized by δ18O values lower than the mantle and enrichment of high field strength elements (HFSEs) and heavy rare earth elements (HREEs). They likely originated from melting of preexisting hydrothermally altered residual oceanic crust in the lower crust of the Junggar intra-oceanic arc. Early crystallization of clinopyroxene and amphibole was inhibited owing to their low melting temperature, leading to HFSEs and HREEs enrichment in residual peralkaline melts during crystallization of a feldspar-dominated mineral assemblage. Thus, the calc-alkaline and peralkaline series represent episodes of crust generation and reworking, respectively, demonstrating that the juvenile isotopic signature in accretionary orogens can be derived from diverse source rocks. Our results show that reworking of residual oceanic crust also plays an important role in continental crust formation for accretionary orogens, which has not previously been widely recognized. [ABSTRACT FROM AUTHOR]

The chilled marginal rocks of the Panzhihua layered intrusion of the Emeishan Large Igneous Province (ELIP), SW China, including microgabbros and olivine–phyric gabbros, are similar to coeval high-Ti basalts in the Panzhihua area in both major and trace element compositions. The olivine–phyric gabbros are characterized by depleted Nd isotopic compositions (εNd(t) = +1·15 to +4·18) and initial 87Sr/86Sr (0·7043–0·7052) similar to those of the Panzhihua layered series and the associated high-Ti basalts. The microgabbros have similar initial 87Sr/86Sr (0·7045 to 0·7054) but more enriched Nd isotopic compositions (εNd(t)=−1·49 to +0·06), suggesting that the magma was contaminated by lower crustal materials. The microgabbros have average compositions of 45·5 wt % SiO2, 8·5 wt % MgO and 13·5 wt % FeOT, which are within the ranges of the associated high-Ti basalts. The average TiO2 content in the microgabbros is 2·7 wt %, which is close the lower end of the high-Ti basalts (TiO2 = 2·5 wt %). Coexisting olivine–phyric gabbros contain abundant coarse-grained olivine phenocrysts (20–60 vol. %) and fine-grained clinopyroxene + plagioclase. Cr-spinel inclusions enclosed in the olivine phenocrysts have highly variable Cr2O3 contents (5·9–33·8 wt %) within a single host crystal. Using the average composition of the chilled microgabbros as a starting composition, MELTS simulation indicates that such a magma can well reproduce the mineral compositions and assemblages in the overlying layered series of the Panzhihua intrusion, implying that the average composition of the chilled microgabbros is similar to the parental magma for these rocks. The chilled marginal rocks (microgabbros and olivine–phyric gabbros) have high S contents (up to 1·48 wt %), but low abundances of platinum group elements (PGE). The total amounts of PGE in these rocks vary from 8·6 to 32·9 ppb. The Cu/Pd ratios of these rocks range between 0·75 × 104 and 3·44 × 104, which are significantly higher than mantle values and indicate previous sulfide removal from the magma at depth. The parental magma with elevated Cu/Pd contains more MgO (>8 wt %) than the coeval PGE-undepleted basalts (MgO = 3·2–4·5 wt %), indicating that sulfide saturation was not triggered by extensive fractionation. Nevertheless, the mantle-like Sr–Nd–O–S isotopic compositions of the mineralized Fe–Ti oxide-bearing intrusions of the ELIP also suggest that S saturation was not related to crustal contamination. We suggest that the contamination and fractionation of an early pulse of mantle-derived S-undersaturated magma in a deep-seated magma chamber most likely caused S saturation and sulfide liquid segregation within the lower crust. The chilled microgabbro formed by influx of the residual PGE-depleted, but Nd isotope enriched, magma. A subsequent pulse of S-undersaturated magma then reached S saturation during the early stages of differentiation as a result of the resorption of the previously formed sulfide liquid. This Nd isotope and PGE-depleted magma was then successively emplaced into a shallow crustal chamber along with captured olivine, generating the olivine–phyric gabbros and associated Fe–Ti oxide deposits. Our results support the interpretation that the Fe–Ti oxides directly crystallized from a basaltic parental magma at an early stage of differentiation and then formed the ore layers by gravitational settling. The composition of the parental magma does not support the hypothesis that the early crystallization of Fe–Ti oxides was controlled by high Fe and Ti concentrations in the parental magma. The moderately high oxygen fugacity (FMQ + 1∼FMQ + 2·5) of the parental magma, which was inherited from an oxidized mantle source, may account for the early saturation of Fe–Ti oxides. [ABSTRACT FROM AUTHOR]

To understand fully the role of amphibole in the fractionation of Nb/Ta during arc magma evolution, we conducted experiments with mid-K and high-K basalts to determine amphibole/melt Nb, Ta and Ti partition coefficients (DNb, DTa and DTi) at variable conditions of bulk TiO2, P, T, H2O and fO2. The experimental results show that, at crustal pressures, amphibole is the most important crystalline phase in hydrous basaltic systems. The amphibole/melt Nb, Ta, and Ti partitioning results are 0.16-0.90 for DNb, 0.13-0.68 for DTa, 1.81-10.63 for DTi and 0.76-2.81 for DNb/DTa. Bulk TiO2 and fO2 show no observable effects. T and H2O, in addition to the compositions of amphibole and melt, are the main affecting factors. DNb, DTa, DTi and DNb/DTa increase with decreasing temperature, amphibole Mg# and melt H2O content and increasing melt polymerization. During cooling and crystallization of arc magmas at crustal pressures, amphibole Mg# decreases and melt polymerization increases, leading to significant increase in amphibole/melt DNb, DTi and DNb/DTa. Nb/Ta fractionation in evolved melts will thus be enhanced with crystallization progress. Meanwhile, melt H2O content will increase with the degree of crystallization, which slows down the increase in these D values. Therefore, the trend and extent of Nb/Ta fractionation in the melt by amphibole critically depends on temperature and melt H2O content. Only low temperatures or low H2O contents at high temperatures lead to high D values. For arc magmas with an average H2O of ~3.9wt %, DNb and DNb/DTa are in general >0.40 and >1.20, respectively, which explains why amphibole fractionation results in lower Nb/Ta ratios in evolved arc magmas. The bulk Nb/Ta fractionation trend during arc magma evolution appears to be generally controlled by fractional crystallization of amphibole. Experimental and modeling results suggest that amphibole is a main fractionating phase during arc magma evolution and continental crust formation. [ABSTRACT FROM AUTHOR]

As the major component, Archean granitoids provide us with an insight into the formation of the early continental crust. We report the study of a series of Neoarchean granitoids, including tonalite-trondhjemite-granodiorite (TTG) and potassic granitoids, in the Xingcheng region of the eastern North China Craton. Zircon U-Pb dating shows that the TTG granitoids were emplaced in the Neoarchean within a 75 Myr period (2595-2520 Ma), with coeval mafic magmatic enclaves, followed by intrusion of potassic granitoids. The geochemistry of the TTG granitoids is consistent with partial melting of Mesoarchean enriched mafic crustal sources at different depths (up to 10-12 kbar equivalent pressure) during a continental collision event. The potassic granitoids are derived from either low-degree melting of Mesoarchean enriched mafic crustal sources or remelting of Mesoarchean TTGs in response to post-collisional extension, and were hybridized with Neoarchean mantle-derived mafic melts to various degrees. The TTG and potassic granitoids in the Xingcheng region record the evolution from collision of micro-continental blocks to post-collisional extension, consistent with other studies, suggesting that the amalgamation of micro-continental blocks is what gave rise to the cratonization of the North China Craton at the end of the Archean. The rock assemblage of these granitoids resembles those of syn- and post-collisional magmatism in Phanerozoic orogenic belts, and the estimated average composition is similar to that of the present-day upper continental crust, suggesting that a prototype upper continental crust might have been developed at the end of the Archean from a mixture of TTG and potassic granitoids. Together with concurrent high-grade metamorphism in the North China Craton, we conclude that collisional orogenesis is responsible for continental cratonization at the end of the Archean in the North China Craton. [ABSTRACT FROM AUTHOR]

A study of whole-rock major and trace element and Sr-Nd-Pb-Hf isotope compositions, combined with zircon U-Pb ages and Hf-O isotopes, for postcollisional intermediate volcanic rocks from the Dabie orogen, China provides constraints on the origin of andesitic magmas and insights into slab-mantle interaction in continental subduction channels. The volcanic rocks exhibit variable contents of SiO2 (50·28-63·86 wt %), MgO (1·18-4·65 wt %), (Fe2O3)T (3·60-8·53 wt %), Al2O3 (12·92-18·95 wt %), Na2O (2·08-6·30 wt %) and K2O (0·73-5·25 wt %). They are mainly trachyandesites, with lesser amounts of basaltic trachyandesite, andesite, dacite and trachyte, characterized by subduction-like trace element distribution patterns showing enrichment of large ion lithophile elements and light rare earth elements but depletion of high field strength elements. The volcanic rocks have relatively enriched Sr-Nd-Pb-Hf isotope compositions, with high initial 87Sr/86Sr ratios of 0·7075-0·7110, highly negative eNd(t) values of -23·1 to -15·0, εHf(t) values of -29·8 to -18·3 and elevated 207Pb/204Pb and 208Pb/204Pb ratios at given 206Pb/204Pb ratios. Zircon Hf-O isotope analyses yield negative εHf(t) values of -31·0 to -17·8 and δ18O values of 4·4-6·8 for syn-magmatic domains. Zircon U-Pb dating yields consistent Early Cretaceous ages of 124 ± 3 to 130 ± 2Ma for magma emplacement. Residual zircon cores yield Triassic, Neoproterozoic and Paleoproterozoic U-Pb ages, consistent with the ages of tectonothermal events involving ultrahigh-pressure metamorphism and protolith formation in the Dabie orogen. The zircon cores also yield a few low to negative δ18O values. An integrated interpretation of these geochemical characteristics is that the andesitic magmas were derived by partial melting of metasomatized zones in the orogenic lithospheric mantle. The metasomatites were generated by reaction of subcontinental lithospheric mantle wedge peridotite with felsic melts that originated from deeply subducted continental crust during continental collision in the Triassic. Melt-peridotite reaction in a subduction channel is therefore a key to the origin of the mantle sources of andesitic magmas in collisional orogens. [ABSTRACT FROM AUTHOR]

The positive correlations between whole-rock concentrations of Cr and Os, Ir and Ru (referred to collectively as iridium-like platinum-group elements, IPGE) in both sulphide-poor plutonic and volcanic rocks suggest that chromite contributes to the collection of these elements during the early stages of sulphide-undersaturated magma differentiation. However, it is not clear whether these correlations are the result of IPGE partitioning into chromite or whether other minerals are involved. Positive correlations between MgO and IPGE have been observed, suggesting that these elements could be incorporated into olivine as well as into chromite. Alternatively, given the siderophile nature of IPGE, they may crystallize as discrete minerals together with chromite, as suggested by the presence of platinum-group minerals in chromite-rich rocks such as mantle podiform and crustal stratiform chromitites. To investigate the effect of chromite crystallization on the distribution of IPGE and Rh in picritic magmas, we have determined the content of these elements in chromites from the Emeishan Large Igneous Province (southwestern China) by in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The time-resolved analysis signals are generally uniform, indicating that the IPGE and Rh are homogeneously distributed within the chromite crystals. The median concentrations are 30 ppb Os, 23 ppb Ir, 248 ppb Ru and 21 ppb Rh. The incorporation of Rh in chromites appears to be influenced by the oxidation state of the magma from which they crystallize because there is a positive correlation between the degree of inversion of the chromite structure [expressed by Fe3+/(Cr + Al + Fe3+)] and the Rh content of the analyzed crystals. In contrast, the enrichments of Os, Ir and Ru in chromites appear to be controlled by other parameters such as temperature or the S content of the magma. Based on empirical calculations, we determined that partition coefficients between chromite and melt are higher for Ru (127) than for Os (23), Ir (27) and Rh (32). Despite these high partition coefficients, mass-balance calculations show that chromite does not account for all the IPGE and Rh concentrations in the rocks, with chromite accounting for maxima of 84, 49, 22 and 20% of the whole-rock Ru, Rh, Ir and Os budgets, respectively. In situ LA-ICP-MS and scanning electron microscopy analyses reveal the presence of micrometric-sized PGE-rich minerals, including laurite (RuS2), Os-Ir 6 Ru alloys, sperrylite (PtAs2) and Pt-Fe (6 IPGE and Rh) alloys, in association with chromite crystals. The presence of these minerals may account for the balance of IPGE and Rh. Alternatively, if IPGE and Rh are compatible with olivine, the balance may be accounted for by the large amount of olivine crystals found in the Emeishan picrites. Based on numerical modelling, we conclude that chromite, olivine and platinum-group minerals all contribute to the collection of IPGE and Rh during the early stages of picritic magma differentiation. Also, we establish that the preferential incorporation of Ru into chromite is responsible for the negative Ru anomalies observed in the PGE patterns of the Emeishan flood basalts. On the other hand, the relative importance of olivine and platinum-group minerals in controlling IPGE and Rh remains uncertain and is a key subject for further investigation. [ABSTRACT FROM AUTHOR]

A combined study of zircon U-Pb ages, whole-rock chemistry, Sr-Nd isotopes and in situ Lu-Hf isotopic ratios in zircons was carried out on Permian monzogranites and mafic microgranular enclaves (MME) and coeval massive gabbros in the northern Alxa block, North China. The data obtained were used to constrain magma sources and petrogenetic processes involved in the generation of this igneous suite. Zircon U-Pb dating yields ages of 271 ± 1 Ma, 270 ± 1 Ma and 276-270 Ma for the monzogranites, MME and massive gabbros, respectively. Two populations of MME, gabbroic (SiO2 <49 wt %) and dioritic (SiO2 >53 wt %) enclaves, are identified. They represent mafic to intermediate magmas quenched in a partially crystallized granitic (sensu lato) host; evidence to support this conclusion includes their fine-grained textures, sinuous margins and diffuse contacts with the host monzogranites. Back-veins and xenocrysts of quartz and plagioclase, as well as various disequilibrium textures and mineral assemblages, indicate mingling or mixing processes. The two magma systems, mafic and felsic, have broadly similar isotopic characteristics with whole-rock initial 87Sr/86Sr(i) ratios ranging from 0·7075 to 0·7077 in the monzogranite host and from 0·7067 to 0·7069 in the MME, with εNd(t) values ranging from -10·2 to -12·4 in monzogranites and from -8·2 to -9·9 in the MME. Zircon εHf(t) values of the monzogranites, gabbroic enclaves and massive gabbros show a wide range and significant overlap from -1·2 to -15·2, -4 to -13·3 and -5·4 to -19·5, respectively; the maximum frequency value of the Hf model age is almost coincident with the whole-rock Nd model age. Mafic, gabbroic rocks similar in composition to some enclaves form layered synplutonic intrusions several metres in thickness and more than 100 m in lateral extent. A mixing test based on mass balance for whole-rock major element compositions reveals that mixing was an efficient process between two coeval magmas. The fraction of felsic magma involved in the hybrid rocks ranged from 0·19-0·29 in the dioritic enclaves to 0·84 in granodiorite and good linear fits (r² > 0·9) are obtained using the average composition of gabbroic enclaves and the most felsic monzogranite as end-members. Although the monzogranites and enclaves may be derived from distinct magma sources, they share similar isotopic signatures, pointing to interaction processes in the source region. The combination of information from geochronology, petrology, whole-rock geochemistry and isotopic compositions leads us to conclude that the processes of hybridization and magma mixing were effective at both the level of emplacement in the shallow crust and at depth in the magma source region. Hybridization in a source region within the lithospheric mantle, involving mantle and crustal source rocks, produced magma bimodality with strong geochemical affinities between end-members and clear calc-alkaline arc signatures, compatible with a subduction setting. A plausible subduction erosion plus relamination model, which differs from classical models based on mafic magma underplating, is proposed. Accordingly, the Yamatu monzogranites are argued to have been generated from granitic melt segregated and ponded in buoyant silicic diapirs, which formed by melting of subducted mélanges in the lithospheric mantle and eventually relaminated to the lower crust. They represent the melts that metasomatized the mantle region during a pre-Permian subduction event. The massive gabbros were formed by decompression melting of the previously metasomatized mantle. The gabbroic enclaves and gabbro layers of the Yamatu pluton are interpreted as magmas formed by decompression melting of this modified hydrated mantle. The dioritic enclaves represent hybrid liquids generated from reaction between granitic melts, which were derived from subducted mélanges, and the hydrated mantle or melts derived from it. [ABSTRACT FROM AUTHOR]

The Fanshan intrusion in the North China Craton (NCC) is concentrically zoned with syenite in the core (Unit 1), surrounded by ultramafic rocks (clinopyroxenite and biotite clinopyroxenite; Unit 2), and an outer rim of garnet-rich clinopyroxenite and orthoclase clinopyroxenite and syenite (Unit 3). The intrusive rocks are composed of variable amounts of Ca-rich augite, biotite, orthoclase, melanite, garnet, magnetite and apatite, with minor primary calcite. Monomineralic apatite rocks, nelsonite and glimmerite exclusively occur in Unit 2. Geochemically, the Fanshan rocks are highly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), moderately depleted in high field strength elements (HFSE), and have a limited range of Sr-Nd-O isotopic compositions. The similar mineralogy, mineral compositions, and trace element characteristics of the three units suggest that all the rocks are co-magmatic. The parental magma is ultrapotassic and is akin to kamafugite. Very low-degree partial melting of metasomatized lithospheric mantle best explains the geochemistry and petrogenesis of the parental magmas of the Fanshan intrusion. We propose that the mantle source may have been metasomatized by a hydrous carbonate-bearing melt, which has imprinted the enriched Sr-Nd isotopic signature and incompatible element enrichment with conspicuous negative Nb-Ta-Zr-Hf-Ti anomalies and LREE enrichments. The mantle source enrichment may be correlated with oceanic sediment recycling during southward subduction of the Paleo-Asian oceanic plate during the Carboniferous and Permian. We propose that crystal settling and mechanical sorting combined with repeated primitive magma replenishment and mixing with previously fractionatedmagma is the predominant process responsible for the formation of the apatite ores. [ABSTRACT FROM AUTHOR]

The early stages of magmatic processes operating at mantle depths beneath continental arcs are poorly known. The chemical compositions of minerals and rocks, mineral Sr–Nd–Hf–O isotopes and zircon U–Pb ages of garnet clinopyroxenite dykes from the Shenglikou peridotite massif (North Qaidam Orogen, NE Tibet, China) were studied to constrain their sources and genesis, and the dynamic processes that controlled pyroxenite formation beneath an early Paleozoic active continental margin. Major-element compositions of bulkrocks suggest that the pyroxenitic protoliths were cumulates segregated from a melt, which was extracted from a peridotite-dominated mantle source. Bulk-rock and mineral trace-element patterns show strong enrichment in fluid-mobile elements (e.g. Cs, Rb, Ba, Th, U, K, Pb and Li) and marked negative anomalies in the high field strength elements relative to rare earth elements, similar to the characteristics of melts derived from a volatile-rich sub-arc mantle. Enriched Sr and Nd initial isotopic compositions at 500 Ma (87Sr/86Sr of 0·70919–0·71774 and εNd of −16·3 to −3·4) are in contrast to the highly radiogenic Hf isotope compositions (similar to those of the depleted-mantle reservoir) and to the uncontaminated upper-mantle δ18OV-SMOW (garnet: 5·6 ± 0·3‰, 2SD, n = 61; zircon: 5·9 ± 0·3‰, 2SD, n = 28). These decoupled isotopic signatures suggest that the melt source was located in a convective mantle wedge (controlling the Hf and O isotopes) that had been pervasively metasomatized by fluids from a subducted Proto-Tethys oceanic slab (controlling the Sr–Nd isotopes and highly incompatible elements). Zircons with two groups of U–Pb ages (430 ± 5 Ma and 401 ± 7 Ma) were generated by recrystallization events, corresponding to UHP metamorphism and a major uplift stage during the North Qaidam orogeny, respectively. The combined evidence reveals a picture of continental arc magmatism at mantle depths and subsequent continental collision. The subduction of the Proto-Tethys oceanic slab beneath the southern Qilian margin triggered flux melting of the metasomatized convective mantle wedge and generated hydrous arc magmas. These primitive magmas intruded into the overlying lithospheric mantle and segregated the cumulates parental to the Shenglikou pyroxenites. Subsequent continental subduction incorporated fragments of the mantle-wedge peridotite (containing pyroxenite dykes) at ∼430 Ma and carried them to shallow depths during exhumation at ∼400 Ma. [ABSTRACT FROM PUBLISHER]

Layered mafic intrusions (LMI) are sporadically distributed in the Early Permian Tarim large igneous province (LIP), NW China, and are crosscut by numerous contemporaneous dykes. The Xiaohaizi wehrlite intrusion is composed mainly of olivine (Fo69–75), clinopyroxene (Mg# = 75–84), intercumulus plagioclase (An53–86) and Fe–Ti oxides. Both petrography and mineral compositions suggest that olivine and clinopyroxene crystallized earlier than plagioclase and Fe–Ti oxides. The dykes are of alkali basalt to trachyandesite with low Mg# (35–39). The least-contaminated dykes display strong rare earth element (REE) fractionation, enrichment of Nb and Ta, and depletion of Pb relative to other similarly incompatible elements, bearing strong similarity to ocean island basalts (OIB). This, together with their positive εNdi values (4·3–4·8), is consistent with derivation from an enriched asthenospheric mantle source. Clinopyroxenes in the wehrlites display convex-upward chondrite-normalized REE patterns. The melts in equilibrium with these clinopyroxenes have very similar trace element compositions to those of the crosscutting dykes, suggesting a similar mantle source shared by the Xiaohaizi wehrlite intrusion and dykes. The Xiaohaizi wehrlite intrusion is characterized by Sr–Nd isotopic disequilibrium between clinopyroxene and plagioclase separates: 87Sr/86Sri (0·7038–0·7041) and εNdi (1·0–1·9) of clinopyroxene are lower and higher than the respective ratios of intercumulus plagioclase (87Sr/86Sri = 0·7042–0·7043, εNdi = 0·4–1·0). The 87Sr/86Sri and εNdi of clinopyroxene separates correlate positively and negatively with Zr/Nb, respectively, implying variable degrees of crustal contamination during the formation of the Xiaohaizi wehrlite intrusion. 87Sr/86Sri increases and εNdi decreases with increasing Ca content of plagioclase, indicating that higher An plagioclases experienced higher degrees of contamination. This can be explained by assimilation of continental crust through a turbulent magma ascent (ATA) process. However, this ATA model fails to account for the positive correlation between the Mg# and εNdi of clinopyroxene separates. The isotopic disequilibrium in the Xiaohaizi LMI is more probably generated during an assimilation and fractional crystallization process involving Archean–Neoproterozoic basement and carbonates as contaminants. [ABSTRACT FROM PUBLISHER]

Multiphase solid inclusions within cumulus silicates, particularly olivine, in Fe–Ti oxide ores from the Lower Zone of the Baima intrusion, Emeishan large igneous province, SW China, have been identified for the first time using 2-D scanning electron microscope and 3-D high-resolution X-ray computed tomography. These inclusions are spherical to subspherical and range from 100 to 300 µm in diameter. They are composed dominantly of titanomagnetite and ilmenite with minor apatite, hornblende, phlogopite and pyrrhotite. The titanomagnetite in the inclusions has a low Cr content (<700 ppm) similar to the interstitial titanomagnetite, suggesting that these inclusions cannot be early crystallized mineral aggregates. In contrast, the spherical shape of these inclusions provides evidence of early trapped liquids from which these minerals crystallized. Based on the composition and modal proportions of the daughter mineral phases within the inclusions, the trapped liquids are estimated to have 82·1–59·6 wt % FeOT, 11·4–18·5 wt % TiO2, 2·69–6·12 wt % Al2O3, 1·40–4·47 wt % MgO, 0·87–4·93 wt % SiO2 and ∼1 wt % volatiles including F, S, Cl, P and H2O. Such a liquid composition deviates substantially from that of the slightly evolved ferrobasaltic magmas inferred to be parental to the Fe–Ti–(V) oxide-bearing mafic–ultramafic intrusions of the Emeishan large igneous province. It is thus speculated that these trapped liquids are immiscible Fe–Ti-rich melts formed upon cooling of ferrobasaltic magma. The net-textured and disseminated oxide ores have titanomagnetite compositions similar to those in the inclusions, suggesting that the oxide ores of the Baima intrusion also formed from Fe–Ti-rich melts immiscibly separated from ferrobasaltic magmas. We propose that immiscible Fe–Ti-rich liquids with high density percolated down through crystal-bearing silicate magma and crystallized an interconnected Fe–Ti oxide network interstitial to olivine, plagioclase and clinopyroxene. This study highlights that immiscible separation of Fe–Ti-rich liquids from ferrobasaltic magmas is an important mechanism in the formation of magmatic Fe–Ti–(V) oxide deposits hosted in mafic–ultramafic layered intrusions. [ABSTRACT FROM AUTHOR]

Garnetite pods containing >60 mol % garnet in association with garnet-bearing quartz-rich lenses are sporadically distributed in the Khondalite Belt, North China Craton. The mineral assemblages and microtextures of garnetite occurring in quartzo-feldspathic sillimanite–garnet gneiss in the Xiaoshizi area allow definition of the metamorphic history of the area. Formation of garnet poikiloblasts coupled with their enclosing and matrix minerals quartz, sillimanite, plagioclase, biotite, rutile and ilmenite defines an M1 assemblage consistent with partial melting and subsequent melt separation. Thermodynamic modeling of bell-shaped Ca zoning in garnet poikiloblasts confirms that this occurred during decompression. A retrograde M2 assemblage replacing M1 garnet or sillimanite is defined by two microdomains: coronas of Grt2 + Crd (M2a) where quartz is present, and Spl + Crd symplectite (M2b) where quartz is absent. The final metamorphic stage led to the coronal assemblage Grt3 + Opx + Crd + Pl + Bt (M3) surrounding M1 garnet. Comparison of the observed mineral proportions and compositions with the predictions of phase equilibria modeling in the NCKFMASHTO system for the effective bulk compositions, in combination with Zr-in-rutile thermometry, results in the following P–T conditions for the metamorphic events: 820–850°C (up to 950°C) and 8·5–9·5 kbar for M1; 850–865°C and 7·4–7·6 kbar for M2; 710–720°C and 6·4–6·6 kbar for M3. The clockwise P–T path thus defined for M1 to M3 evolution is interpreted to reflect the onset of extension and exhumation of the Khondalite Belt at ∼1890 Ma, following the emplacement of S-type granitoids at ∼1920–1890 Ma and ultrahigh-temperature metamorphism at ∼1920 Ma that resulted from collision of the Yinshan and Ordos Blocks at ∼1950 Ma. [ABSTRACT FROM PUBLISHER]

One of the major geophysical discoveries concerning the Tibetan Plateau is the existence of unusually weak layers in the mid- to lower crust, a characteristic widely interpreted as the result of crustal melting. This interpretation, however, remains highly contentious, particularly when applied to northern Tibet where crustally derived magmatic rocks are scarce. Here we report the finding of tourmaline-bearing mica and biotite rhyolites in the Bukadaban–Malanshan area, southern Kunlun Range, near the northern margin of the Tibetan Plateau. Zircon U–Pb and whole-rock or mineral 40Ar–39Ar analyses suggest that these rocks erupted between 9·0 and 1·5 Ma. These rocks are geochemically similar to Himalayan leucogranites (interpreted as crustal melts), with strongly peraluminous compositions, high SiO2 contents (69·0–76·0 wt %), and clear negative Eu, Ba and Sr anomalies. They have low &varepsilon;Nd (–5·8 to –8·6) and high 87Sr/86Sr (0·7125–0·7178), 206Pb/204Pb (18·59–18·70), 207Pb/204Pb (15·49–15·63) and 208Pb/204Pb (38·31–38·74) isotopic compositions as well as magmatic zircon &varepsilon;Hf (–0·7 to –5·0) compositions similar to those of global marine sediments and Proterozoic–Triassic sedimentary rocks in northern Tibet. We suggest that the Bukadaban–Malanshan rhyolites were generated by dehydration melting of metasedimentary rocks at 0·5–1·2 GPa and 740–863°C. Our data not only confirm the occurrence of a partially molten zone in the mid- to lower crust beneath northern Tibet but also constrain the crustal melting to have existed from middle Miocene to Quaternary times. Adopting the crustal flow model, we further argue the importance of outward flowing of the melt-weakened crust in the formation of crustal inflation, surface uplift, and earthquakes along the northern margin of the Tibetan Plateau. [ABSTRACT FROM PUBLISHER]

Two eclogites, one from the southern Dabie Shan, China, and the other from the western Erzgebirge, Germany, were studied because of amphibole and clinozoisite–epidote formation either as typical porphyroblasts or as oriented fine-grained matrix minerals. A nearly isobaric prograde path was derived for the eclogite from the Erzgebirge, mainly on the basis of garnet zonation and the reconstruction of P–T pseudosections in the system SiO2–TiO2–Al2O3–MgO–MnO–FeO–CaO–Na2O–K2O–H2O–O2. Peak P–T conditions were 2·8 GPa and 670°C. The retrograde path shows significant cooling during pressure release. In contrast, the Dabie Shan eclogite contains sparse Mg-rich garnet cores pointing to a granulite stage. Subsequently, peak-pressure conditions of 3·5–4·0 GPa were reached at 620°C, followed by a retrograde path that was initially nearly isothermal and then similar to that of the Erzgebirge eclogite. Most of the amphibole and clinozoisite–epidote grew during early retrogression in the pressure interval 1·8–2·3 GPa (Erzgebirge) or 2·4–2·7 GPa (Dabie Shan) owing to infiltration of hydrous fluids amounting to ∼1 wt % and 0·5 wt % H2O, respectively. As a result of this hydration omphacite was partially decomposed, but the remaining omphacite demonstrates that infiltration of H2O essentially ceased at 1·8 GPa (Erzgebirge) or 2·4 GPa (Dabie Shan). The fluid infiltration and hence the growth of amphibole and clinozoisite–epidote is proposed to have occurred in a deep-seated subduction channel during ascent of eclogitic material. Subsequently, the eclogites were captured between colliding continental plates and further exhumed in an exhumation channel. [ABSTRACT FROM AUTHOR]

Flood basalts are one of the remaining enigmas in global mantle petrology. They come in enormous quantities of up to 106 km3 of mantle-derived melt, and they erupt in rather short time intervals of only a few million years. Throughout geological history, all continents have been periodically flooded by dominantly basaltic and rare picritic magmas that can differ widely within the same province in terms of their major element (e.g. high- and low-Ti series), trace element, and radiogenic isotope compositions, suggesting significant compositional heterogeneity within the mantle source regions tapped. In this study of the Late Permian Emeishan large igneous province (ELIP) in SW China picrite lavas from thick stratigraphic successions at Binchuan and Yongsheng represent the low-Ti and high-Ti ‘classic’ end-members of continental flood basalt magmatism, respectively. This study focuses on the petrochemical variability of the Emeishan magmas, and the genetic links between the suites and their respective mantle sources, based upon estimates of the chemical compositions of the primary melts represented by homogenized melt inclusions hosted by exceptionally primitive olivine (up to 92 mol % Fo in both suites) and Cr-spinel (Cr# 64–72 mol % in Binchuan and 65–80 mol % in Yongsheng) phenocrysts. The average compositions of the melt inclusions and their overall chemical variability, together with the presence of picrites in the province (e.g. Lijiang and Dali localities) with compositions intermediate between the low- and high-Ti end-members, suggest that numerous parental magma batches contributed to a diverse spectrum of more differentiated basaltic magmas within the ELIP. The end-member and intermediate magma compositions are confirmed by the compositions of phenocrysts (Ni and Mn abundances in olivine, Ti abundances in Cr-spinel and clinopyroxene, and trace element abundances in clinopyroxene). The end-member melt and phenocryst compositions (e.g. Gd/Yb in bulk-rocks, melt inclusions and clinopyroxene and Ni–Mn systematics in olivine) suggest a peridotite and garnet pyroxenite mantle source for the low- and high-Ti end-members, respectively. The Sr and Nd isotopic compositions of the two end-member magmas are similar [87Sr/86Sri ∼0·7045; &varepsilon;Nd(t) ∼ +1·7] and are considered to reflect a source in the subcontinental lithospheric mantle rather than the convective asthenosphere or a deep mantle ‘plume’. [ABSTRACT FROM AUTHOR]

Igneous rock textures reflect the cooling history of the parental magma. Combined with chemical data, they can provide physical and chemical information about the evolution of a magma body. The petrographic textures and chemical compositions of 21 coarse- and fine-grained granite samples along an ∼250 m horizontal outcrop of the Shanggusi granite porphyry are presented in this case study. The coarse-grained granite porphyry is an early intrusion, and the fine-grained granite dykes, mostly intruded into the granite porphyry, are later intrusions. The studied samples have nearly homogeneous major element bulk-rock and mineral compositions, but show large variations in their trace element compositions and textural characteristics. The trace element data suggest the influence of hydrous fluids (possibly enriched in CO2, F, and Cl) in the evolution of the plutonic body. Textural analysis of the coarse-grained granite porphyry indicates that the crystal size distribution (CSD) slopes, intercepts and total numbers of groundmass decrease from the center to the margin of the intrusion in contrast to the maximum diameter of the crystals (Lmax) (average length of the four largest quartz crystals for each sample); however, most fine-grained samples and the groundmass of the coarse-grained samples show concave-down CSDs, indicating textural coarsening. Quartz CSDs in the coarse-grained samples are kinked, with a steep-sloped log–linear section representing small crystals (<1 mm) and a shallow-sloped log–linear section representing large crystals (>1 mm). These two crystal populations are interpreted as resulting from a shift in cooling regime. The straight CSDs of two fine-grained samples may be due to a different cooling history. In general, the spatial variation of the CSD patterns can be attributed to various degrees of overgrowth and mechanical compaction. The quartz phenocrysts in several coarse-grained samples exhibit a high degree of alignment, which may be the result of magmatic flow. By integrating the field geology, geochemistry and quantitative textural data from the horizontal profile of the Shanggusi granite porphyry, it is suggested that hydrous fluids at the top of the intrusion not only controlled the fractionation of elements but also affected its cooling history. Fluid migration-controlled undercooling can explain the solidification processes in the Shanggusi intrusion, and may also be prevalent in other fluid-rich shallow intrusions. Quantitative integration of textural and geochemical data for igneous rocks can contribute to our understanding of the relationships between physical and chemical processes in a magma system, and provide relatively comprehensive insights into the petrogenesis of granites. [ABSTRACT FROM AUTHOR]

Serpentinized massif peridotite in the Xugou, Su-Lu ultrahigh-pressure (UHP) metamorphic belt, eastern China, preserves texturally old (porphyroclastic) ortho- and clinopyroxene with up to two generations of lamellae of garnet, clinopyroxene and Mg-chromite. Their crystallographic orientation with respect to the host pyroxene is consistent with an origin by solid-state exsolution. Comparison of integrated mineral chemistry with simplified and natural chemical datasets suggests that both aluminous precursor pyroxenes were in equilibrium at a minimum pressure of ∼4 GPa and within a temperature range of about 1300–1500°C. Steep isopleths of Ca in orthopyroxene imply that exsolution occurred during cooling. Al diffusion modelling suggests growth of widely spaced lamellae in orthopyroxene down to about 900°C. Integrated Al contents between wide lamellae record a minimum of 4 GPa pressure during cooling. Compositionally uniform exsolved minerals were formed at 4·3 ± 0·3 GPa and 730 ± 30°C and reflect a cratonic geotherm with about 33 mW m−2 surface heat flow. The peridotite matrix mineral assemblage of olivine + orthopyroxene ± garnet ± Mg-chromite ± clinopyroxene ± phlogopite records strain-induced recrystallization that partially to completely replaced precursor porphyroclasts. The recrystallized minerals lack lamellar exsolution. Recrystallized orthopyroxene, with Al2O3 at 0·13 wt %, indicates conditions of 5·5 ± 0·3 GPa and 760 ± 30°C, which are higher-grade metamorphic conditions than those preserved in the chemically equilibrated exsolution microstructures. Both estimates overlap with the range reported for the Early Mesozoic UHP metamorphism in the region (4·0–6·7 GPa and 760–970°C). Major element melt models applied to previously published Xugou peridotite data suggest high degrees of melt extraction (30–35 %) in the garnet peridotite stability field (3–4·5 GPa) until garnet and clinopyroxene exhaustion. Coincidence in pressure and in the order of temperature of equilibration of precursor pyroxenes and peridotite melting implies that peridotite formation occurred at ∼135 km depth in the subcontinental lithospheric mantle (SCLM) beneath the Archaean North China Craton. Subsequent refertilization, mineral exsolution and chemical re-equilibration during long-term cooling in the SCLM occurred prior to deformation and incorporation of the mantle fragment into the continental crust during UHP metamorphism at a minimum depth of 170 km. Because the Xugou precursor pyroxenes and peridotite formed at depths greater than the regional SCLM (c. 90 km), we infer that the orogenic peridotite massif formed part of the former hanging wall of the Archaean SCLM, which delaminated after the Late Mesozoic. [ABSTRACT FROM AUTHOR]

The 134–130 Ma Han-Xing diorite–monzodiorite complex intrudes the central part of the North China block (NCB). The high-Mg diorites have intermediate SiO2, high MgO, Cr and Ni contents, elevated Mg-numbers (Mg#), and are characterized by high Sr and Ba, highly fractionated rare earth elements (REE), and strong Nb and Ta depletions. These geochemical signatures are similar to those of high-Mg adakite and sanukitoid magmas that form through complex crust–mantle interactions. We have investigated felsic magma–peridotite interactions in the Han-Xing diorite complex by undertaking a detailed petrological and bulk-rock and mineral trace element study. The high-Mg diorites contain olivine xenocrysts and dunite xenoliths that have been partly resorbed by the quartz-bearing matrix. These textures, coupled with the high Cr and Ni, and low Ca, Al, P, Y, Sc and Zr contents, provide evidence that the olivines were derived from a depleted peridotite. The assimilation of olivine produced diorite with highly variable MgO, Cr, Ni and Mg# values. However, all rocks have similar incompatible element patterns, indicating that they originate from a common pristine magma. The relative timing of olivine assimilation is constrained by reversely zoned clinopyroxene and amphibole crystals. The cores of the reversely zoned minerals have low Mg# values (clinopyroxene, ∼75; amphibole, ∼60) and REE characteristics indicating an origin by crystallization from a felsic magma compositionally similar to the monzodiorites. Inclusions of andesine, K-feldspar and biotite are present in the low-Mg# cores of reversely zoned clinopyroxenes, and are further proof of the felsic character of the pristine magma. The high-Mg (up to 90) midsections of the reversely zoned minerals have high Cr and Ni contents and lower but subparallel REE, indicating assimilation of peridotite by the monzodiorite magma. An eclogite residue for the pristine magma is precluded by the presence of plagioclase inclusions, and low Na2O and Al2O3 and unfractionated heavy REE in the low-Mg# cores of the reversely zoned clinopyroxenes. The maximum depth of the peridotite assimilation is estimated to be ∼20 km, using the Al-in-amphibole geobarometer for the high-Mg# midsection of a reversely zoned amphibole. This provides further evidence that the interaction of felsic magma with peridotite occurred at crustal rather than mantle depths. We suggest that the peridotite was emplaced at middle or upper crustal levels by previous orogenic processes. The original monzodiorite magma was probably produced in an extensional setting by partial melting of lower crust. Our observations indicate that the delamination model cannot be applied to the petrogenesis of the Han-Xing high-Mg diorite. The interaction of felsic melt with ultramafic rocks at crustal pressures has not been considered as a mechanism for the generation of high-Mg adakites and sanukitoids so far. This might have important implications for crustal evolution. Whereas in previous models sanukitoids have been related to either crustal growth by partial melting of enriched mantle peridotite or crust destruction through lower crust delamination, in the case reported here, they document crustal fractionation through anatexis of lower crustal rocks. [ABSTRACT FROM PUBLISHER]

It is now well established that lithospheric thinning of the eastern North China Craton (NCC) occurred during the Phanerozoic. The cause and extent of the thinning, however, remains highly debated. In this study, mantle xenoliths from the Paleozoic Mengyin kimberlites, along with xenoliths from the Cenozoic Penglai and Shanwang basalts in Shandong Province, are investigated via traditional petrographic and elemental, Sr–Nd–Hf–Os isotopic and platinum-group element (PGE) analyses. Late Archean Os model ages of c. 2·5 Ga for the Mengyin peridotites provide confirmation that refractory, Archean lithospheric mantle existed beneath the easternmost portion of the NCC during the Paleozoic. Some Paleoproterozoic lithospheric mantle fragments may also be present in the Mengyin xenolith suite. In contrast, the spatially associated Penglai and Shanwang peridotite xenoliths are more fertile, and have Sr–Nd isotopic compositions similar to the depleted mantle. They differ dramatically from the Archean peridotites sampled by the Mengyin kimberlites. Osmium model ages for single samples range from mid-Proterozoic to modern, similar to variations observed in Phanerozoic convecting upper mantle as sampled by modern abyssal peridotites. This suggests that the present lithospheric mantle beneath the eastern NCC formed in the Phanerozoic, despite the fact that Os model ages extend back to the Proterozoic. Some samples from the Penglai suite yield Proterozoic Lu–Hf clinopyroxene mineral isochron ages that are consistent with the Os model ages, suggesting that Hf isotopes in modern convective upper mantle can preserve evidence for ancient melt depletion, similar to Os isotopes. Our results are consistent with thinning of the eastern NCC as a result of foundering of the deep crust and lithospheric mantle, but are inconsistent with stretching or refertilization models, as remnants of Archean mantle are expected to be present in these scenarios. The match between 187Os/188Os in convective upper mantle and the eastern China lithopspheric mantle also precludes models that seek to explain the present lithosphere as being due to lateral translation of Proterozoic lithosphere. [ABSTRACT FROM PUBLISHER]

The South Tianshan high/ultrahigh-pressure (HP–UHP) metamorphic belt, NW China is characterized by extensive pelitic–felsic schists with numerous blocks of eclogite, ultramafic rock and marble. The pelitic–felsic schists preserve two generations of garnet-bearing mineral assemblages: (1) an early generation that involves garnet, phengite, glaucophane, and quartz/coesite with or without paragonite; (2) a late generation that involves albite, hornblendic amphibole, quartz and grossular-rich, pyrope-poor garnet. Early generation garnet has growth zoning involving core to rim increases in pyrope content, coupled with increased or constant grossular content. Pseudosection modeling of these growth zoning textures in three samples (T311, T314, and T316) reflects prograde paths and peak conditions of c. 32 kbar at 550–570°C, c. 22.5 kbar at 550°C, and c. 22–23 kbar at 540–550°C, respectively. These P–T estimates are consistent with those recovered from eclogite blocks hosted by the schists. Phase equilibrium modeling predicts that the early garnet would have mainly grown in mineral assemblages involving lawsonite, jadeite and chloritoid, with or without coesite or carpholite, distinct from the assemblages now in the matrix. The post-peak decompression of the pelitic–felsic schists is inferred to have involved two stages. The early stage decompression is characterized by dehydration reactions involving lawsonite and carpholite at P > 20 kbar, coupled with mode and compositional changes of garnet, glaucophane and phengite. The late-stage decompression after lawsonite disappearance led to the rocks being fluid-absent. Under such fluid-absent conditions, the solid transition of jadeite to albite occurs at P ≈ 14 kbar, and hornblendic amphibole forms at 12–13 kbar. Most garnet grains are somewhat changed in composition, to produce a pyrope-poor and grossular-rich outer rim. In contrast to above-solidus conditions, decompression of the pelitic–felsic schists at subsolidus conditions does not destabilize phengitic muscovite and tends to preserve their peak mineral assemblages if there is no intensive fluid infiltration. In the South Tianshan belt, buoyancy of subducted metasediments with respect to mantle rocks could be one of the major reasons for fast exhumation of the HP–UHP rocks. [ABSTRACT FROM PUBLISHER]

Cenozoic, post-collisional, potassic and ultrapotassic igneous rocks in the North Qiangtang, Songpan–Ganzi and North Kunlun terranes of the northern Tibetan Plateau are distributed along a semi-continuous, east–west-trending, volcanic belt, which is over 1200 km in length. Spatially, there is a close association with major strike-slip faults, thrust faults and pull-apart basins. The ages of these magmatic rocks range from 45 Ma to the present (the youngest known eruption occurred in 1951); they are shoshonitic, compositionally similar to K-rich subduction-related magmas, and range in SiO2 from 44 to 66 wt %. There is a relative enrichment of large ion lithophile elements (LILE) and light rare earth elements (LREE) in the most primitive magmatic rocks (MgO >6 wt %) in the North Qiangtang terrane compared with those in the Songpan–Ganzi and North Kunlun terranes; correspondingly, the primitive magmas have higher 87Sr/86Sr and 206Pb/204Pb, and lower 143Nd/144Nd ratios in the North Qiangtang terrane than in the Songpan–Ganzi and North Kunlun terranes. The dominant factors that control the geochemical characteristics of the magmas are an enriched asthenospheric mantle source composition, the degree of partial melting of this source, and the combined processes of crustal assimilation and fractional crystallization (AFC). Enrichment of the asthenosphere is considered to have occurred by incorporation of subducted sediments into the mantle wedge above a subducted slab of Indian lithosphere during India–Asia convergence. Continental lithospheric mantle, metasomatically enriched during earlier episodes of subduction, may have also contributed a source component to the magmas. Trace element modelling indicates that the mantle source of the most primitive magmas in the North Qiangtang terrane contained higher amounts of subducted sediment (0·5–10%) compared with those in the Songpan–Ganzi and North Kunlun terranes (<2%). The degrees of partial melting required to generate the primitive potassic and ultrapotassic magmas from the enriched mantle sources range from <0·1% to ∼15% in the three major basement terranes. Energy-constrained AFC model calculations show that the more evolved magmatic rocks (MgO <6 wt %) are the results of AFC processes in the middle crust in the North Qiangtang terrane and the upper crust in the Songpan–Ganzi and North Kunlun terranes. We propose that the ultimate driving force for the generation of the post-collisional potassium-rich magmatism in north Tibet is the continuous northward underthrusting of the Indian continental lithosphere following India–Asia collision. This underthrusting resulted in upwelling of hot asthenosphere beneath north Tibet, squeezed up between the advancing Indian lithosphere and the backstop of the rigid Asian continental lithosphere. Asthenospheric upwelling may have also contributed to uplift of the northern Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Two apparently distinct, sub-parallel, paleo-subduction zones can be recognized along the northern margin of the Tibetan Plateau: the North Qilian Suture Zone (oceanic-type) with ophiolitic mélanges and high-pressure eclogites and blueschists in the north, and the North Qaidam Belt (continental-type) in the south, an ultrahigh-pressure (UHP) metamorphic terrane comprising pelitic and granitic gneisses, eclogites and garnet peridotites. Eclogites from both belts have protoliths broadly similar to mid-ocean ridge basalts (MORB) or oceanic island basalts (OIB) in composition with overlapping metamorphic ages (480–440 Ma, with weighted mean ages of 464 ± 6 Ma for North Qilian and 457 ± 7 Ma for North Qaidam), determined by zircon U–Pb sensitive high-resolution ion microprobe dating. Coesite-bearing zircon grains in pelitic gneisses from the North Qaidam UHP Belt yield a peak metamorphic age of 423 ± 6 Ma, 40 Myr younger than the age of eclogite formation, and a retrograde age of 403 ± 9 Ma. These data, combined with regional relationships, allow us to infer that these two parallel belts may represent an evolutionary sequence from oceanic subduction to continental collision, and continental underthrusting, to final exhumation. The Qilian–Qaidam Craton was probably a fragment of the Rodinia supercontinent with a passive margin and extended oceanic lithosphere in the north, which was subducted beneath the North China Craton to depths >100 km at c. 423 Ma and exhumed at c. 403 Ma (zircon rim ages in pelitic gneiss). [ABSTRACT FROM AUTHOR]

The Dexing adakitic porphyries (quartz diorite–granodiorite porphyries), associated with giant porphyry Cu deposits, are located in the interior of a continent (South China). They exhibit relatively high MgO, Cr, Ni and Sr contents, high La/Yb and Sr/Y ratios, but low Yb and Y contents, similar to adakites produced by slab melting associated with subduction. However, they are characterized by bulk Earth-like Nd–Sr isotope compositions (ɛNd(t) = −1·14 to +1·80 and (87Sr/86Sr)i = 0·7044 – 0·7047), and high Th (12·6–27·2 ppm) contents and Th/Ce (0·19–0·94) ratios, which are different from those of Cenozoic slab-derived adakites. Sensitive High-Resolution Ion Microprobe (SHRIMP) geochronology studies of zircons reveal that the Dexing adakitic porphyries have a crystallization age of 171 ± 3 Ma. This age is contemporaneous with Middle Jurassic extension within the Shi-Han rift zone, and within-plate magmatism elsewhere in South China, indicating that the Dexing adakitic porphyries were probably formed in an extensional tectonic regime in the interior of the continent rather than in an arc setting. Their high Th contents and Th/Ce ratios, and Middle Jurassic age, argue against an origin from a Neoproterozoic (∼1000 Ma) stalled slab in the mantle. Taking into account available data for the regional metamorphic–magmatic rocks, and the present-day crustal thickness (∼31 km) in the area, we suggest that the Dexing adakitic porphyries were most probably generated by partial melting of delaminated lower crust, which was possibly triggered by upwelling of the asthenospheric mantle due to the activity of the Shi-Hang rift zone. Moreover, the Dexing adakitic magmas must have interacted with the surrounding mantle peridotite during their ascent, which elevated not only their MgO, Cr and Ni contents, but also the oxygen fugacity (fO2) of the mantle. The high fO2 could have induced oxidation of metallic sulfides in the mantle and mobilization of chalcophile elements, which are required to produce associated Cu mineralization. Therefore, the Cu metallogenesis associated with the Dexing adakitic porphyries is probably related to partial melting of delaminated lower crust, similar to the metallogenesis accompanying slab melting. [ABSTRACT FROM AUTHOR]

The Panzhihua gabbroic layered intrusion is associated with the 260 Ma Emeishan Large Igneous Province in SW China. This sill-like body hosts a giant Fe–Ti–V oxide deposit with 1333 million ton ore reserves, which makes China a major producer of these metals. The intrusion has a Marginal zone of fine-grained hornblende-bearing gabbro and olivine gabbro, followed upward by Lower, Middle, and Upper zones. The Lower and Middle zones consist of layered melanogabbro and gabbro composed of cumulate clinopyroxene, plagioclase, and olivine. These zones also contain magnetite layers. The Upper zone consists chiefly of leucogabbro composed of plagioclase and clinopyroxene with minor olivine. Most rocks in the body show variable-scale rhythmic modal layering in which dark minerals, primarily clinopyroxene, dominate in the lower parts of each layer, and lighter minerals, primarily plagioclase, dominate in the upper parts. The oxide ores occur as layers and lenses within the gabbros and are concentrated in the lower parts of the intrusion. Ore textures and associated mineral assemblages indicate that the ore bodies formed by very late-stage crystallization of V-rich titanomagnetite from an immiscible oxide liquid in a fluid-rich environment. The rocks of the Panzhihua intrusion become more evolved in chemistry upward and follow a tholeiitic differentiation trend with enrichment in Fe, Ti, and V. They are enriched in light rare earth elements relative to heavy rare earth elements, and exhibit positive Nb, Ta, and Ti anomalies and negative Zr and Hf anomalies. The silicate rocks and oxide ores of the Panzhihua intrusion formed from highly evolved Fe–Ti–V-rich ferrobasaltic or ferropicritic magmas. The textures of the ores and the abundance of minor hydrous phases indicate that addition of fluids from upper crustal wall-rocks induced the separation of the immiscible oxide melts from which the Fe–Ti–V oxide ore bodies in the lower part of the intrusion crystallized. [ABSTRACT FROM AUTHOR]

The Luobusa ophiolite, Southern Tibet, lies in the Indus–Yarlung Zangbo suture zone that separates Eurasia to the north from the Indian continent to the south. The ophiolite contains a well-preserved mantle sequence consisting of harzburgite, clinopyroxene (cpx)-bearing harzburgite and dunite. The harzburgite contains abundant pods of chromitite, most of which have dunite envelopes, and the cpx-bearing harzburgites host numerous dunite dykes. Dunite also exists as a massive unit similar to those of the mantle–crust transition zones in other ophiolites. All of the dunites in the ophiolite have a similar mineralogy, comprising mainly olivine with minor orthopyroxene and chromite and traces of clinopyroxene. They also display similar chemical compositions, including U-shaped chondrite-normalized REE patterns. Mantle-normalized PGE patterns show variable negative Pt anomalies. Detailed analysis of a chromite-bearing dunite dyke, which grades into the host cpx-bearing harzburgite, indicates that LREE and Ir decrease, whereas HREE, Pd and Pt increase away from the dunite. These features are consistent with formation of the dunite dykes by interaction of MORB peridotites with boninitic melts from which the chromitites were formed. Because the transition-zone dunites are mineralogically and chemically identical to those formed by such melt–rock reaction, we infer that they are of similar origin. The Luobusa ultramafic rocks originally formed as MORB-source upper mantle, which was subsequently trapped as part of a mantle wedge above a subduction zone. Hydrous melts generated under the influence of the subducted slab at depth migrated upward and reacted with the cpx-bearing harzburgites to form the dunite dykes. The modified melts ponded in small pockets higher in the section, where they produced podiform chromitites with dunite envelopes. At the top of the mantle section, pervasive reaction between melts and harzburgite produced the transition-zone dunites. [ABSTRACT FROM PUBLISHER]

Chemical variations along with changes in microstructure of the principal constituent minerals make it possible to identify at least four equilibrium stages in the evolution of the Yangkou garnet peridotite in the Su-Lu ultrahigh-pressure metamorphic belt, eastern China: Stage I—a primary garnet lherzolite stage represented by coarse-grained (a few millimeters size) porphyroclastic aluminous pyroxenes + chromian spinel ± garnet; Stage II—an ultrahigh-pressure (UHP) stage defined by fine-grained matrix phases (0·1–0·3 mm size) of garnet + extremely low-Al orthopyroxene + high-Na clinopyroxene + chromite; Stage III—a medium-pressure stage defined by fine-grained mineral aggregates (<0·1–0·2 mm size) mainly composed of aluminous spinel + high-Al orthopyroxene in the matrix; Stage IV—an amphibolite- to greenschist-facies stage defined by poikiloblastic amphibole. Orthopyroxene–clinopyroxene thermometry and an empirical spinel barometer give temperatures of around 800–830°C and pressures of 1·2–2·9 GPa for porphyroclasts of Stage I. Garnet–orthopyroxene, garnet–clinopyroxene and empirical spinel geothermobarometers give relatively uniform P–T conditions for the matrix garnet–orthopyroxene–clinopyroxene–chromite assemblage of Stage II (∼730–760°C and 3·6–4·1 GPa). Aluminous spinel–olivine pairs in the aggregates give ∼650–680°C at 1·0–1·5 GPa for Stage III. The granulation of the studied rocks may have taken place during subduction (Stages I and II) of the host peridotite. The lack of kelyphitic rims around the matrix garnet and the preservation of incompatible assemblages in the matrix of the studied rocks is due to extremely low H2O activity during the early decompression stage. The peak temperature of associated eclogites in the Yangkou UHP unit is 700–800°C at 3·1–4·1 GPa. These observations suggest that the mantle fragment (garnet peridotite) and the crustal fragment (eclogite) in the Yangkou UHP unit both experienced a common UHP metamorphic event and that the exhumation path obtained by these reference points supports nearly isothermal decompression of the unit. The Yangkou UHP unit is considered to have been exhumed as part of a larger crustal mass (c. >10 km in size) along with the surrounding orthogneisses. Such isothermal decompression paths have also been reported from other areas of the Su-Lu belt and the Dabie Mountains. As the orthogneisses volumetrically account for >90% of these areas, buoyancy-controlled uplift probably played an important role in the exhumation of the UHP rocks, at least from the upper mantle to the lower crust. [ABSTRACT FROM AUTHOR]

Distinct equilibration temperatures, deformation and trace element characteristics are observed in amphibole-bearing and amphibole-free peridotite xenoliths from Nushan, Sino-Korean Craton, eastern China. Amphibole-free peridotites are predominantly deformed, fine-grained (∼1 mm) and equilibrated at 990–1110°C. Their cpx are characterized by either light rare earth element (LREE)-depleted or relatively flat REE patterns with only a slight depletion in high field strength elements (HFSE). LREE enrichment is generally associated with Fe-rich samples, consistent with ‘wall-rock’ metasomatism adjacent to basaltic veins. In contrast, amphibole-bearing peridotites are less deformed, coarse-grained (>3 mm) and display chemical zonation in the pyroxenes suggesting cooling from 1050 to 850°C. Their cpx show a large variation in LREE (Cen = 1·7–68) and almost constant heavy rare earth element (HREE) contents (Ybn = 9·8–11·6). The highest LREE contents occur in cpx from amphibole-rich samples, coupled with Fe enrichment, strong enrichment in Th and U, and marked depletion in the HFSE. These characteristics may be accounted for by combined ‘wall-rock’ and ‘diffuse’ metasomatism involving an agent rich in volatiles and incompatible elements. As such the Nushan xenoliths could represent samples from two spatially separate metasomatic aureoles. Conversely, the cryptic and modal metasomatism could be genetically related, because the amphibole-peridotites and Fe-rich amphibole-free samples show similar Sr–Nd isotopic ranges that are indistinguishable from those of the Cenozoic basalts from eastern China. The different metasomatic assemblage and the trace element composition can be accounted for in terms of P–T control on amphibole stability and progressive chemical evolution of asthenosphere-derived melts during upward migration. Trace element signatures and metasomatic assemblages, together with the fertile composition of the Nushan peridotites, suggest an origin as newly accreted lithosphere rather than as relic cratonic mantle. Metasomatism may have occurred after late Mesozoic lithospheric thinning, which marked a dramatic change in lithospheric architecture beneath the Sino-Korean Craton. [ABSTRACT FROM AUTHOR]

Geochronological (K-Ar or 40Ar/39Ar), major and trace element, Sr-Nd-Pb isotopic and mineral chemical data are presented for newly discovered Cenozoic volcanic rocks in the western Qiangtang and central Lhasa terranes of Tibet. Alkali basalts of 65-45 Ma occur in the western Qiangtang terrane and represent primitive mantle melts as indicated by high mg-numbers [100 × Mg/(Mg + Fe)] (54-65), Cr (204-839 ppm) and Ni (94-218 ppm) contents, and relatively low ratios of 87Sr/86Sr (0·7046-0·7061), 206Pb/204Pb (18·21-18·89), 207Pb/204Pb (15·49-15·61) and 208Pb/204Pb (38·42-38·89), and high ratios of 143Nd/144Nd (0·5124-0·5127). In contrast, younger volcanic rocks in the western Qiangtang terrane (∼30 Ma) and the central Lhasa terrane (∼23, ∼13 and ∼8 Ma) are potassic to ultrapotassic and interpreted to have been derived from an enriched mantle source. They are characterized by very high contents of incompatible trace elements, negative Ta, Nb and Ti anomalies, and radiogenic Pb isotopic compositions (206Pb/204Pb = 18·43-19·10; 207Pb/204Pb = 15·64-15·83; 208Pb/204Pb = 39·14-39·67). 87Sr/86Sr (0·7088-0·7092) and 143Nd/144Nd (∼0·5122) ratios of the western Qiangtang terrane potassic lavas are similar to those of 45-29 Ma potassic volcanic rocks in the north-central Qiangtang terrane, whereas 87Sr/86Sr (0·7167-0·7243) and 143Nd/144Nd (∼0·5119) ratios of central Lhasa terrane lavas are similar to those of 25-16 Ma ultrapotassic volcanic rocks in the western Lhasa terrane. The 65-45 Ma alkali basalts in the western Qiangtang terrane, along with widespread calc-alkaline volcanic rocks of this age in the Lhasa terrane, may be related to roll-back of a previously shallow north-dipping slab of Tethyan oceanic lithosphere beneath Tibet. Subduction as opposed to convective thinning of continental lithosphere is favored to explain potassic volcanism in Tibet because of its occurrence in distinct, east-west-trending belts (45-29 Ma in the Qiangtang terrane; 25-17 Ma in the northern Lhasa terrane; 16-8 Ma in the southern Lhasa terrane) and temporal and spatial relationships with major thrust systems. [ABSTRACT FROM AUTHOR]

The Songpan–Garzê Orogenic Belt (northeastern Tibet Plateau) experienced polyphase deformation and metamorphism that is best developed in the Danba Domal Metamorphic Terrane (DDMT), in which three tectonometamorphic events can be identified. The first event (D1–M1) is characterized by prograde sericite- to kyanite-grade Barrovian metamorphism during late Indosinian (∼205–190 Ma) crustal thickening and shortening. A subsequent early Yanshanian (∼165 Ma) sillimanite- to migmatite-grade event (M2) developed during predominantly east–west compression (D2). A final greenschist-facies event (M3) is best developed in shear zones of probable Himalayan age. P–T conditions during M1 varied from ∼3–5 kbar and ∼410–530°C (biotite zone) to 5·3–8 kbar and 570–600°C (staurolite and kyanite zones), and during M2 from 4·8–6·3 kbar and 640–680°C (sillimanite zone) to 5·8–6·2 kbar and 660–725°C (migmatite zone). Clockwise P–T–t segments were inferred for the staurolite, kyanite and sillimanite zones. Muscovite-dehydration melting duringM2 was largely responsible for the generation of migmatites and locally voluminous pegmatites. The polyphase tectonometamorphic evolution of the eastern Tibet Plateau, as documented in the Danba area, resulted from interactions between the Indian, Tibet, and the South and North China Blocks. The eastern Tibet Plateau experienced limited uplift during the Mesozoic, followed by large-scale uplift and rapid cooling during the Tertiary Himalayan Orogeny. [ABSTRACT FROM PUBLISHER]

We report petrological and geochemical data on ultramafic pillow lavas from Late Palaeozoic marine sequences in Yunnan, SW China. These lavas have >26 wt % MgO, euhedral to subhedral olivine phenocrysts and acicular or quench clinopyroxene crystals with or without microlitic plagioclase in a devitrified and altered glassy matrix. These ultramafic lavas are compositionally komatiitic (>18 wt % MgO), but we term them high-Mg picrites because they lack spinifex-textured olivine. Although olivines in these picrites are cumulate crystals, causing the high MgO contents of the bulk rocks, the high forsterite content of these olivines, Fo = 0·902 ± 0·011, suggests that the primitive magmas parental to the picrites would have had ∼17–19% MgO, which is similar to the estimates for primary magmas of the Cretaceous Gorgona komatiites in the literature. The calculated liquidus temperature of the primitive magmas parental to the Yunnan picrites is ∼1400 ± 25°C, which would imply a mantle potential temperature of ∼1540 ± 30°C. This is inconsistent with magma generation beneath ocean ridges or in the mantle wedge above a subduction zone, but is consistent with a mantle plume origin, which is fully supported by the trace element characteristics of the magmas. [ABSTRACT FROM PUBLISHER]