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Pegmatitic dyke-like carbonate rocks mainly composed of very coarse-grained calcite, are a rare type of carbonate rocks found in some of orogenic belts in the world. These specific carbonate rocks generally occur intimately with high-temperature granulites and marbles. In the Proterozoic Highland Complex of Sri Lanka which is a segment of the Mozambique suture, they are associated with marbles and granitic pegmatites, and intercalated with high-grade calc-silicate gneisses and highly folded ortho- and para-gneisses. These pegmatitic carbonate rocks do not show any signs of metamorphic or deformed overprint, but instead well preserve igneous textures and contain various silicate crustal xenoliths. The calcite crystals occur as euhedral to subhedral grains and are large in size from 1 to 15 cm. The diverse colors of calcite from white to yellow and blue derive from mineral inclusions and their own compositions. Non-carbonate minerals, commonly present in typical carbonatites such as phlogopite, apatite, clinopyroxene, olivine, plagioclase, iron oxides and spinel, are all found in the rocks. Meanwhile, a skarn-type assemblage of wollastonite, garnet, clinopyroxene and sulfide occurs in contact between the carbonate rocks and gneiss xenoliths, which probably resulted from antiskarn reactions. Chemical compositions of major constituent minerals (calcite, dolomite and apatite) of the carbonate rocks are intermediate between typical marbles and mantle-derived carbonatites and akin to crustal-origin carbonatites worldwide. We thus classify the studied rocks as 'anatectic carbonatite pegmatite', and suggest that they originated from the melting of a mixture of marbles and surrounding silicate rocks at crustal levels during high-temperature metamorphism. [ABSTRACT FROM AUTHOR]

Abstract The Early Permian in the present-day Europe area was a time when a major tectonic shift occurred, leading from the tectonic collapse of the Variscan orogeny to the crustal extension and thinning that characterized the Early Permian times. Crustal extension was associated with extensive magmatism at different crustal levels: from gabbro in the lower crust or at the mantle/crust transition to subaerial high-SiO2 volcanic activity. In the whole Southalpine Domain, the Early Permian intrusive bodies occur from the west (e.g. Ivrea-Verbano Complex and “Graniti dei Laghi”) to the east (Ifinger, Brixen and Cima d’Asta intrusive complexes). Among these, in the central Southern Alps (comprised between the Giudicarie Belt and the Lake Como), minor intrusive complexes also occur. The Val Biandino Intrusive Suite consists of two magmatic units: the Val Biandino Quartz-Diorite (VBQD) and the Valle di San Biagio Granite (VSBG). The first of them consists of gabbro-diorite to granodiorite bodies associated with leucocratic cordierite-bearing granitic dikes that intruded the pre-Permian basement. To the west, a W-dipping normal fault of Permian age represents the boundary between this unit and the Valle di San Biagio porphyric granite. All rock varieties of the Val Biandino Intrusive Suite display a high-K calc-alkaline affinity with metaluminous to peraluminous character. Field crosscutting relationships point to a late generation of the cordierite granites of the Val Biandino Quartz-Diorite unit with respect to the more mafic types. SHRIMP U–Pb zircon dating provided an age of 285.2 ± 1.9 Ma for a cordierite granite of the Val Biandino Quartz-Diorite unit and 283.2 ± 1.9 Ma for the porphyric Valle di San Biagio Granite. Geochemical data suggest that gabbro-diorite, quartz-diorite, granodiorite and leucogranite are not co-magmatic. The existing gaps in term of SiO2 wt% and the higher HREE contents in mafic and intermediate rocks with respect to granite coupled with similar LREE in all rocks support this hypothesis. The high Rb/Sr ratios (> 1) in leucogranite, together with the occurrence of white mica and cordierite point to a significant contribution of crustal partial melting in their genesis. The Val Biandino Intrusive Suite was likely formed through the interaction of magma genesis at the mantle/crust transition and partial melting of the heterogeneous pre-Permian basement of the Southalpine Domain. This scenario explains the strong heterogeneity displayed by the relatively small intrusive complex and the strong crustal signature exhibited by all the magmatic types of the Val Biandino Intrusive Suite.

Although high-silica granites can provide unique insights into the maturity of the continental crust and rare metal enrichment, the origin of high-silica granitic magmatism remains uncertain. In this paper, we present an integrated study of zircon U-Pb geochronology and trace elements, whole-rock geochemistry, and Sr-Nd isotopes for two typical high-silica granites (namely, the Longbao granitic porphyry and the Yushu granite) found in the North Qiangtang terrane, central Tibetan Plateau. Zircon geochronological data indicate that these high-silica granites crystallized at 217-214 Ma. All the samples from the Longbao granitic porphyry and the Yushu granite exhibited high SiO2, low MgO, depletion of Ba, Nb, Sr, P, and Ti, and enrichment of Th and U. They exhibited relatively high (87Sr/86Sr)i ratios of 0.7120-0.7136 and low eNd(t) values of -8.58 ~ -7.58; together with their old ages according to the two-stage Nd model (1.6-1.7 Ga), these features indicate the involvement of crustal materials. Geochemical and isotopic variation indicated that the high-silica granites studied were mainly produced by the dehydration melting of a muscovite (Ms)-bearing source, and that the Triassic turbidites might be a good candidate for the magma source. Combining this evidence with new regional studies, it can be concluded that partial melting of Triassic turbidites induced by slab roll-back might be the key factor controlling the origin of Late Triassic magmatism in the North Qiangtang terrane. [ABSTRACT FROM AUTHOR]

Determining crustal melting in parallel with geodynamic evolution provides critical information on plateau crustal thickening and uplift. Here we investigate the timing and duration of crustal melting through in situ analysis of zircon U–Pb ages, trace elements and Hf–O isotopes, and whole-rock elements and Sr–Nd–Hf isotopes for the granites and high-grade metamorphic rocks from the Pamir Plateau. Zircon dates record protracted crustal melting for both Central Pamir (43–33 and 22–12 Ma) and South Pamir (28–10 Ma). The Pamir Cenozoic granites are characterized by significant elemental and isotopic heterogeneity. The elemental variability within the Pamir Cenozoic granites is dominantly attributed to fractional crystallization of K-feldspar and plagioclase with subordinate biotite from a variably fractionated melt, and the accumulation of early crystallized feldspar during magma ascent. Peritectic mineral entrainment and accessory mineral crystallization had some influence on the geochemical characteristics of the garnet-bearing leucogranite dikes. Zircon Hf isotopes and whole-rock Sr–Nd–Hf isotopes show secular variations for both Central and South Pamir granites. The Central Pamir granites show a mild decrease in whole-rock ƐNd(t) values from Eocene (−4.3 to −4.9) to Miocene (−6.2 to −7.7), and the zircon ƐHf(t) values decrease from c. 40 Ma (+2 to −5) to c. 10 Ma (−4 to −8). In contrast, the South Pamir granites have highly variable whole-rock Sr–Nd–Hf (87Sr/86Sr(i) = 0.7053 to 0.7830; ƐNd(t) = −31.5 to +0.2; ƐHf(t) = −40.0 to +8.2) and zircon Hf isotopes (ƐHf(t) = −31.7 to +7.5) and display a strong decrease in ƐNd(t) and ƐHf(t) values from c. 25 Ma to c. 13 Ma. Geochemical and isotopic data indicate that both the Central and South Pamir experienced crustal melting from juvenile lower crust to ancient lower-middle crustal materials, and Indian crustal materials were incorporated into the melt region of the South Pamir leucogranites from c. 20 Ma. Our study highlights a causal link between a chain of events that includes magma underplating induced by lithosphere thinning and slab breakoff, lithosphere delamination and underthrusting of Indian lithosphere, and formation of the Cenozoic granites in Pamir. This series of processes are incorporated here into a comprehensive model for the geodynamic evolution of the Pamir during the India–Asia collision. [ABSTRACT FROM AUTHOR]

The Macusani Volcanics and related rocks (SE Peru) are well known examples of erupted strongly peraluminous rare-metal rich silicic magmas. We used the phenocryst/matrix glass method to determine relevant mineral/melt partition coefficients to facilitate modeling of such systems. Concentrations of trace (Li, Be, B, Al, P, Ti, Mn, Zn, Ge, Rb, Sr, Zr, Nb, Sn, Cs, Ba, La, Sm, Eu, Gd, Yb, Ta, W, Pb and U) and major elements were measured in obsidians, matrix glasses, glass inclusions, phenocrysts and phases from biotite reaction zones by Laser-Ablation ICP-MS and Electron Microprobe Analysis. The derived phenocryst/melt partition coefficients for quartz, plagioclase, sanidine, biotite, muscovite, andalusite and ilmenite are compared with values from reference silicic magmas and literature. Mineral/biotite partition coefficients are calculated for tourmaline and hercynite. The database is then applied to model chemical fractionation in Macusani magmas. Fractional crystallization and partial melting trends (with the B concentration used as a proxy for the fraction of residual liquid) generate progressively more evolved (higher Rb, Nb, Sn, Cs and Ta and lower Sr, Ba and Pb) residual liquids. The trace element concentrations in obsidians are reproduced for 50–70% crystallization of matrix glasses from ash-flow tuffs. This demonstrates that differentiation of magmas parental to the ash-flow tuffs can generate the highly evolved obsidian-forming liquids. Based on this, we present an updated model for crustal melting and the generation of the Macusani magmas. A metapelitic component dominates the source region but calcic plagioclase cores enriched in Sr, Ba and La suggest an affiliation with mafic magmas, which were likely potassic to ultrapotassic. The presence of a mafic component is consistent with the high heat fluxes in the source region and accounts for specific magmatic variables (i.e., middle crustal anatectic zone, very reducing fO 2 , high F). Petrogenetic processes differ significantly between Macusani Volcanics suites and two-mica leucogranites despite both having nearly identical source rocks, mineral assemblages and compositions. [ABSTRACT FROM AUTHOR]

The Cenozoic Alpine, and Paleozoic Variscan and eo-Variscan collisional belts are compared in the framework of the Wilson cycle considering differences between cold and hot orogens. The W. Alps result of the opening and closure of the Liguro-Piemonte ocean, whereas the Paleozoic Eovariscan and Variscan orogenies document multiple ocean openings and collisions in space and a polyorogenic history in time. Jurassic or Early Ordovician break-up of Pangea or Pannotia megacontinents led to the formation of passive continental margins, and the opening of Liguro-Piemonte, or Rheic, Tepla-Le Conquet, andMedio-European oceans, respectively. In Paleozoic or Mesozoic, microcontinents such as Apulia and Sesia or Armorica and Saxo-Thuringiawere individualized. The oceanic convergence stage was associated with the development of arcs and back-arc basins in the Variscan belt but magmatic arcs are missing in the W. Alps, and inferred in the Eo-variscan one. Though the nappe stack is mainly developed in the subducted European or Gondwana crust in the western Alps and Eo-variscan cases, the Moldanubian nappes formed in the upper plate in the Variscan case. The Alpine and Variscan metamorphic evolutions occurred under ca. 8 °C/km and 30 °C/km gradients, respectively. During the late- to post-orogenic stages, all belts experienced "unthickening" accommodated by extensional tectonics, metamorphic retrogression, and intramontane basin opening. The importance of crustal melting, represented by migmatites, granites, and hydrothermal circulations in the Variscan and Eo-Variscan belts is the major difference with the W. Alpine one. The presence, or absence, of a previous Variscan or Cadomian continental basement might have also influenced the rheological behavior of the crust. [ABSTRACT FROM AUTHOR]

The Velay anatectic dome in the Variscan French Massif Central exposes a low-pressure–high-temperature metamorphic sequence, which represents an ideal natural laboratory for documenting the behavior of rare-metals and fluxing elements during crustal melting. We investigated the silicate and bulk-rock geochemistry of sub- to suprasolidus metapelites and orthogneisses, as well as related granites, and performed forward thermodynamically constrained geochemical modeling to quantify the respective effects of melting pressure, temperature, H2O activity, and protolith composition on the Li and F contents of granitic melts. We find that biotite compositions are good proxies of melt compositional evolutions during prograde melting. The crystallization of peritectic cordierite at low pressure (< 5 kbar) and "water-fluxed" melting both inhibit the Li enrichment of anatectic melts. Metapelite-derived melts consistently show modest Li–F contents, and a decoupling is observed as melts with the highest Li concentrations (~ 200–400 ppm) are produced below 750 °C, whereas F-richest melts (~ 0.2–0.4 wt%) are produced above 750 °C near the biotite-out isograd. Peraluminous orthogneiss anatexis can generate a melt that is concomitantly enriched in both F (~ 0.3–1 wt%) and Li (~ 600–1350 ppm) at relatively low temperature (< 750 °C), which can evolve toward rare-metal granite compositions (~ 10,000 ppm Li; ~ 2 wt% F) after 80–90 wt% of fractional crystallization. Melting of felsic meta-igneous rocks followed by magmatic differentiation is thus a viable mechanism to form Li-F-rich rare-metal granites and pegmatites, providing a direct link between protracted crust recycling and rare-metal magmatism in late-orogenic settings. [ABSTRACT FROM AUTHOR]

The Cenozoic Alpine, and Paleozoic Variscan and eo-Variscan collisional belts are compared in the framework of the Wilson cycle considering differences between cold and hot orogens. The W. Alps result of the opening and closure of the Liguro-Piemonte ocean, whereas the Paleozoic Eo-variscan and Variscan orogenies document multiple ocean openings and collisions in space and a polyorogenic history in time. Jurassic or Early Ordovician break-up of Pangea or Pannotia megacontinents led to the formation of passive continental margins, and the opening of Liguro-Piemonte, or Rheic, Tepla-Le Conquet, and Medio-European oceans, respectively. In Paleozoic or Mesozoic, microcontinents such as Apulia and Sesia or Armorica and Saxo-Thuringia were individualized. The oceanic convergence stage was associated with the development of arcs and back-arc basins in the Variscan belt but magmatic arcs are missing in the W. Alps, and inferred in the Eo-variscan one. Though the nappe stack is mainly developed in the subducted European or Gondwana crust in the western Alps and Eo-variscan cases, the Moldanubian nappes formed in the upper plate in the Variscan case. The Alpine and Variscan metamorphic evolutions occurred under ca. 8 °C/km and 30 °C/km gradients, respectively. During the late- to post-orogenic stages, all belts experienced “unthickening” accommodated by extensional tectonics, metamorphic retrogression, and intramontane basin opening. The importance of crustal melting, represented by migmatites, granites, and hydrothermal circulations in the Variscan and Eo-Variscan belts is the major difference with the W. Alpine one. The presence, or absence, of a previous Variscan or Cadomian continental basement might have also influenced the rheological behavior of the crust.

The southern US and northern Mexican Cordillera experienced crustal melting during the Laramide orogeny (c. 80–40 Ma). The metamorphic sources of melt are not exposed at the surface; however, anatectic granites are present throughout the region, providing an opportunity to investigate the metamorphic processes associated with this orogeny. A detailed geochemical and petrochronological analysis of the Pan Tak Granite from the Coyote Mountains core complex in southern Arizona suggests that prograde metamorphism, melting, and melt crystallization occurred here from 62 to 42 Ma. Ti-in-zircon temperatures (T Ti-zr) correlate with changes in zircon rare earth elements (REE) concentrations, and indicate prograde heating, mineral breakdown, and melt generation took place from 62 to 53 Ma. T Ti-zr increases from ~650 to 850 °C during this interval. A prominent gap in zircon ages is observed from 53 to 51 Ma and is interpreted to reflect the timing of peak metamorphism and melting, which caused zircon dissolution. The age gap is an inflection point in several geochemical-temporal trends that suggest crystallization and cooling dominated afterward, from 51 to 42 Ma. Supporting this interpretation is an increase in zircon U/Th and Hf, a decrease in T Ti-zr, increasing zircon (Dy/Yb)n, and textural evidence for coupled dissolution–reprecipitation processes that resulted in zircon (re)crystallization. In addition, whole rock REE, large ion lithophile elements, and major elements suggest that the Pan Tak Granite experienced advanced fractional crystallization during this time. High-silica, muscovite± garnet leucogranite dikes that crosscut two-mica granite represent more evolved residual melt compositions. The Pan Tak Granite was formed by fluid-deficient melting and biotite dehydration melting of meta-igneous protoliths, including Jurassic arc rocks and the Proterozoic Oracle Granite. The most likely causes of melting are interpreted to be a combination of (1) radiogenic heating and relaxation of isotherms associated with crustal thickening under a plateau environment, (2) heat and fluid transfer related to the Laramide continental arc, and (3) shear and viscous heating related to the deformation of the deep lithosphere. The characteristics and petrologic processes that created the Pan Tak Granite are strikingly similar to intrusive suites in the Himalayan leucogranite belt and further support the association between the North American Cordilleran anatectic belt and a major orogenic and thermal event during the Laramide orogeny. [ABSTRACT FROM AUTHOR]

Ignimbrite flare-ups are rare periods of intense silicic volcanism during which the pyroclastic volume and eruptive frequency is more than an order of magnitude higher than background activity. Investigating the compositional differences between flare-up and steady-state magmas provides critical constraints on the petrogenetic causes for the event and can offer unique opportunities to investigate the role of large-scale tectonic or geodynamic processes in arc magmatism. In this study, we focus on the bimodal Deschutes Formation ignimbrite flare-up of Central Oregon, which erupted unusually high volumes of pyroclastic material 6.25–5.45 Ma from a new axis of volcanism in the Cascades arc. This episode is marked by increased eruption rates and eruption of more silicic compositions relative to the Quaternary Cascade arc, which rarely erupts rhyolites. Ignimbrites are crystal-poor (<10%) dacite to rhyolites (mostly 65–77 wt.% SiO2) with anhydrous mineral assemblages and higher FeO/MgO, Y, Eu/Eu*, MREE and Zr/Sr, indicating drier magmatic evolution compared to the Quaternary arc, and are more similar to those from the rear-arc High Lava Plains (HLP) province that lies to the east. Magnetite-ilmenite oxybarometry indicates that Deschutes Formation felsic magmas tend to be hotter and more reduced (NNO-1 to NNO) than the Quaternary arc (NNO to NNO + 1.5). Rhyolite-MELTS geobarometry suggests complex storage of diverse Deschutes Formation magmas within the shallow crust (50–250 MPa), and the common co-eruption of multiple plagioclase populations, pumice compositions, and compositionally banded pumice suggest variable degrees of mixing and mingling of distinct magmas. Deschutes magmas also have low δ18Oplagioclase values that indicate partial melting and assimilation of hydrothermally altered shallow crust. Trace element systematics and rhyolite-MELTS modeling suggests that felsic pumice cannot be produced by simple fractionation of co-erupted mafic pumice or basaltic lavas, and requires a crustal melting origin, and trace elements and Pb isotopes suggest that young mafic crust may have been the primary protolith. We suggest that partial melting produced low-Si rhyolite melt (~72 wt.%) that acted as both a parent for the most evolved rhyolites, and as a mixing endmember to create the dacite to rhyodacite magmas with heterogenous plagioclase populations. Unlike the predominantly calc-alkaline basalts erupted in the Quaternary Cascade arc, Deschutes Formation primary basalts are mostly low-K tholeiites, indicative of decompression melting. These are similar to the compositions erupted during a contemporaneous pulse of low-K tholeiite volcanism across the whole HLP that reached into the Cascades rear-arc. We suggest that intra-arc extension focused decompression melts from the back-arc into the arc and that tensional stresses allowed this high flux of hot-dry-reduced basalt throughout the crustal column, causing partial melting of mafic protoliths and the production of hot-dry-reduced rhyolite melts. Depletion of incompatible elements in successive rhyolites implies progressive depletion in fertility of the protolith. Extension also allowed for the establishment of a robust hydrothermal system, and assimilation of hydrothermally-altered rocks by magmas residing in a shallow, complex storage network lead to low δ18O melts. Our findings suggest the integral role that extensional tectonics played in producing an unusual ignimbrite flare-up of hot-dry-reduced rhyolite magmas that are atypical of the Cascades arc and may be an important contributor to flare-ups at arcs worldwide. [ABSTRACT FROM AUTHOR]

The magmas that formed each Devonian, S-type, volcanic complex in central Victoria were generated through partial melting of a distinct assemblage of rock types in the crustal source region. The more silicic rhyolites were formed independently of the rhyodacitic to dacitic magmas, rather than as differentiates. Within every complex, each ignimbritic unit is also heterogeneous in its chemical and isotopic characteristics. Thus, in this part of Paleozoic Australia, magma heterogeneity occurred on at least three scales—the individual rock unit, the complex and across the region. This means that the Selwyn Block, the logical source of the ignimbrite magmas, is highly heterogeneous, probably in three dimensions. The statistically significant differences in the ages of the various complexes suggest that the heat sources for partial melting of the deep crust were themselves rather local in extent, and probably reflected numerous asynchronous intrusions of mantle-derived mafic magmas, most likely in relatively small batches. Thus, there should be no thick underplated or intraplated layer of gabbroic rock in the deep crust here. Thinner bodies of hot mafic magmas were probably emplaced in different places and at different levels, inducing local high-grade metamorphic conditions. Published radiometric dates for various volcanic units suggest that the magmatic heating of the crust occurred over a period of at least 17 Myr, although maximum intensity occurred between ca 375 and ca 371 Ma. Thus, a model of the deep crust here should involve numerous, perhaps thin metasedimentary units with highly variable ages and original clay contents, as well as sills of Late Devonian mafic rocks scattered both vertically and horizontally and intruded episodically. The dominantly meta-igneous sources of crustally derived I-type magmas generally lie at higher structural levels, where they did not experience metamorphic temperatures sufficient to cause the higher-temperature melting reactions needed to form I-type volcanic magmas. Each Late Devonian, S-type, volcanic magma in central Victoria formed through melting of a distinct assemblage of crustal source rocks, most likely within the Selwyn Block. The rhyolites formed independently of the rhyodacitic to dacitic magmas, not as differentiates. The mafic magma heat sources for partial melting of the deep crust were emplaced as numerous, relatively small batches, in sheets distributed horizontally and vertically. The dominantly meta-igneous and shallower crustal sources of I-type magmas did not experience the higher temperatures required to form I-type volcanic magmas. [ABSTRACT FROM AUTHOR]

Both continental crust and depleted mantle are characterized by subchondritic Nb/Ta, leading to a mass imbalance when compared to the bulk Earth. Even though several potential high-Nb/Ta reservoirs in Earth's core and undepleted mantle have been proposed, little attention has been given to those in the crust. Here we present bulk-rock and rutile geochemical data for samples from a lower crustal pelitic granulite, North China Craton, which exhibit systematic variation in their Nb and Ta contents. High-temperature (HT) and ultrahigh-temperature (UHT) granulite residues exhibit Nb/Ta ratios that are close to chondritic and subchondritic, respectively, whereas leucosomes from UHT granulite mostly have suprachondritic Nb/Ta. These variations are best explained via competition for Nb and Ta between biotite and rutile during metamorphism, although initial bulk-rock Nb/Ta values also have an effect. As biotite preferentially incorporates Nb over Ta, the early stages of biotite dehydration melting produce a high-Nb/Ta residue and a low-Nb/Ta melt; however, geochemical modeling suggests that once biotite is depleted, the Nb/Ta ratio of the system is instead controlled by rutile growth, which promotes the formation of a lower Nb/Ta residue and a higher Nb/ Ta melt, even though the volume of melt produced at this stage may be small. We propose that in situ and in-source leucosomes and leucocractic veins in UHT terranes may retain a high-Nb/Ta geochemical signature. However, residual crustal-derived A2-type granites that experience significant fractionation of Nb- or Ta-bearing minerals during crystallization or contamination from other low-Nb/Ta sources cannot retain this high-Nb/Ta ratio, even though these ratios are generally higher than that of S-type granites. Anhydrous partial melting of metapelite can generate Nb-rich melts, such that high-temperature leucosomes, in addition to related A2-type granites, may represent significant Nb deposits. [ABSTRACT FROM AUTHOR]

Almandine-rich garnets from a Neogene andesite of Slovakia can be divided into two main types. Garnet megacrysts are magmatic and form a chemically homogeneous group that contains, on average, about 5 wt% CaO and 4.5 wt% MgO as petrogenetically significant components. Garnets occurring in lithic fragments and garnets aggregated in garnetite lenses are characterised by Ca-poor cores (CaO

Summary: The Martian crustal dichotomy (MCD) between the southern highlands and the northern lowlands is the planet’s most ancient crustal structure, but its origins and evolution remain enigmatic. Understanding of the MCD comes largely from present-day and shallow crustal constraints. Lacking ancient and deeper constraints, hypotheses for the origin of the MCD range from an early giant impact, partial melting from sustained mantle convection, or some combination. We investigate with seismological modeling the best-preserved case of the “antipodal effect”—energy from an impact that concentrates and induces uplift and fracturing promoting volcanism at its antipode—the Hellas crater and the Alba Patera volcano on Mars. The volcano is latitudinally offset ∼2° (∼119 km) from the expected antipode, and we explore whether the MCD can explain this deflection. Variations across the MCD in topography, thickness, and composition have only minor effects. Simulations capable of sufficiently decelerating southern surface waves require the presence of 2%–5% more partial melt in the southern highlands. As the age of impact ca. 4 billion years ago post-dates the formation of the MCD, our partial melting results thus imply that, with or without an early giant impact, the MCD was modified by mantle convection in order to supply enough heat for crustal melts for several hundreds of millions of years after Mars formation.

Silicic large igneous provinces (SLIPs) are periods of particularly voluminous felsic volcanism in the geologic record. Previous work has suggested an overall long lifespan for SLIPs of 20 to 40 Myr, commonly punctuated by shorter-lived 'flare-ups' of higher volcanic productivity (1–5 Myr), but detailed studies of individual flare-up events are lacking. The Jurassic Chon Aike SLIP (CASP) is the product of an exceptional geological event wherein voluminous felsic volcanism (ca. 219,000 km3) was generated predominately via crustal anatexis over 45 Myr. We focus on the Late Jurassic El Quemado Complex (EQC), which marked the final stages of felsic volcanism for the CASP. In-situ U-Pb ages for the EQC previously suggested a duration of ∼5 Myr; however, new high-precision CA-ID-TIMS ages indicate the total durations of ignimbrite successions were shorter than 350 kyr. A compilation of zircon U-Pb ages for the eight CASP formations in Patagonia and the Antarctic Peninsula reveals changes in volcanic duration between formations deposited in the intraplate relative to the continental margin, suggesting a spatial control over the magmatic lifespans of geographically restricted systems. We suggest that the rate of magmatism in the CASP was primarily controlled by heterogeneities in the crust, largely between Proterozoic igneous crust and younger, metasedimentary crust that was recently accreted relative to the timing of Jurassic volcanism. The observed duration of volcanism was modulated by these differences in crustal properties from the injection of mafic magmas into the lower crust during extension and mantle upwelling, following rollback of the subducting oceanic slab towards the Paleo-Pacific margin of Gondwana. The short duration in the EQC is the product of unique overlapping conditions that favored crustal melting and resulted in the voluminous ignimbrite flareup (104 km3) of some of the Earth's highest δ18O magmas measured. • New U-Pb ages reveal CASP ignimbrites erupted faster than previously suggested. • Volcanic rates of the CASP were modulated by crustal heterogeneities. • Crustal anatexis of readily fusible metasediments formed voluminous high δ18O magmas. [ABSTRACT FROM AUTHOR]

To investigate the mechanism of magma emplacement of the intruded zoned pluton, we report high-precision Ca isotopic data of 23 granitoids (including granodiorite and quartz diorite), 5 mafic enclaves, and 7 hornblende and plagioclase mineral pairs from the Early Cretaceous Fangshan pluton, Beijing, China. Δ44/40Ca Hbl-Pl of rocks span from 0.02 ± 0.15‰ to 0.23 ± 0.12‰, which likely reflects equilibrium inter-mineral isotope fractionation. The granodiorites exhibit ∼0.30‰ variation in Ca isotopic compositions, with δ44/40Ca ranging from 0.47 ± 0.08‰ to 0.77 ± 0.09‰. The diorites and the enclaves yield δ44/40Ca of 0.51 ± 0.03‰ to 0.64 ± 0.05‰, and 0.54 ± 0.06‰ to 0.66 ± 0.11‰, respectively, which are lower than that of the upper continental crust (0.72 ± 0.10‰). Ca isotopic variation cannot be ascribed to K decay due to their low ε Ca values (< ±1.0). Theoretical study showed that Hbl fractional crystallization could counteract Ca isotope fractionation effect of their co-existing Pl crystallization. Source heterogeneity cannot account for Ca isotopic variation because δ44/40Ca does not correlate to (87Sr/86Sr) i. Instead, the Ca isotopic variation is likely controlled by partial melting and residual mineralogy. Specifically, δ44/40Ca of the granodiorites are negatively correlated with (Dy/Yb) N , reflecting various magma pools derived from different crustal depths with clinopyroxene and garnet residua. Modeling calculations suggest that the low Ca isotopic signature can be caused by ∼10% partial melting of sources with various proportions of garnet and clinopyroxene (from 5% garnet + 95% clinopyroxene to 40% garnet + 60% clinopyroxene). The diorites intruded earlier than the granodiorites, which can be slightly contaminated by surrounding basement rock. Ca isotopic compositions of enclaves are similar to those of their host rocks. Considering the spatial structure of the Fangshan pluton, it should be assembled by at least four magma pulses derived from various crustal depths through ballooning. Our results show that Ca isotopes can be used as a potential tool to trace magma source depth and decipher the process of magma emplacement of intrusive pluton, which is important to understand the continental crustal growth. [ABSTRACT FROM AUTHOR]

The generation of felsic melts (through open or closed system processes) within ocean island volcanoes has been a key area of study since their identification. At Ascension Island in the south Atlantic, explosively erupted felsic melts have, to date, demonstrated a marked absence of signs of magma mixing and crustal assimilation. Here we present the first observations of a fall deposit from Ascension Island recording both macro- and micro-scale evidence for magma mingling. Geochemical analyses of mineral and glass phases, coupled with volatile concentrations of melt inclusions highlight the role of lower-crustal partial melting to produce rhyolitic magmas. Glass textures and the lack of zoning in major mineral phases indicate that mingling with a mafic melt occurred shortly prior to eruption. These inferences of a deep rhyolite production zone, coupled with rapid ascent rates highlight the challenges in forecasting a similar style of eruption at Ascension Island in the future.

The Porteirinha Block (PB) provides an important record of the origin and evolution of South American crust. The PB records the generation of Paleoarchean TTG, Mesoarchean anatexis, and Paleoproterozoic crustal reworking. It is exposed as a basement inlier in the Araçuaí Orogen as a result of a tectonic involvement of the continental margins of São Francisco Congo Paleocontinent (PSFC) during the collisional processes of Gondwana amalgamation at the Neoproterozoic. The PB basement unit is the Porteirinha Complex (PC), which consists of TTG orthogneisses, migmatites, amphibolites, metaultramafics rocks and Neoarchean granitoids. In this paper, we discuss the evolution of the TTG orthogneisses and migmatites from PC based on field, geochronological and whole-rock geochemistry data. Zircon U–Pb ages (LA-ICP-MS) show a complex and polycyclic evolution of these rocks. Concordant ages were obtained for crystallization of TTG magma at ca. 3.4 Ga and the geochemical signature supports the idea of TTG generation by partial melting of hydrated mafic crust under garnet-amphibolite facies conditions. Zircon U–Pb ages also suggest crustal reworking events at 3.1 Ga. Metatexite and diatexite domains related to partial melting of TTG orthogneisses are individualized. The migmatites record processes of crustal anatexis and melt generation that occurred between 2.8 and 2.9 Ga (LA-ICP-MS, U–Pb in zircon) and are linked to crustal reworking and heat generation events recognized regionally. The new data presented in this work allow us to establish correlations between Archean and Paleoproterozoic units of the PB and those that occur regionally, particularly those related to Gavião block, where a series of stages and events of early crustal evolution are shared. • Archean crustal evolution of TTG orthogneisses from the São Francisco Congo Paleocontinent. • Zircon U–Pb geochronology shows igneous ages of 3.4 Ga for the precursor TTG magma of the Porteirinha Complex orthogneisses. • The orthogneisses show a typical TTG signature and were generated by partial melt of thickened lower crust. • Meso-Neoarchean crustal anatexis are recorded in metatexites and diatexites domains. • Successive stages of magmatism and crustal reworking have been identified since the Paleoarchean. [ABSTRACT FROM AUTHOR]

Intrusion‐related gold deposits (IRGDs) occur in the Eastern Desert (ED) of Egypt within magmatic districts that are exploited for tungsten and tin mineralization. IRGDs and intrusion‐related rare metal deposits (IRRMDs) are almost invariably linked with the late to post collisional Younger Granites (YGs) that have three successive phases (I, II and III). At ∼635–630 Ma, the ED underwent a transition in deformation style from compressional to extensional and a switch from subduction with crustal thickening to delamination with crustal thinning. This transition was concurrent with the emplacement of a short magmatic pulse (∼635–630 Ma) that represents a transition between orogenic gold deposits and IRGDs. K‐rich calc alkaline granites (phase I and II of the YGs) hosting IRGDs like the Hangalia deposit were emplaced during the time span 630–610 Ma. Alkaline magmatism began at 610 Ma, coexisting with the K‐rich calc‐alkaline magmatism over the 610–590 Ma time span, where the Fawakhir (598 ± 3 Ma) and Um Had (596 ± 2 Ma) granites that host the IRGDs were emplaced. In time, the alkaline magmatism became more alkaline giving rise to phase III of the YGs that hosts IRRMDs. A distinct metallogenic epoch comprising both IRGDs and IRRMDs, was undergoing extreme growth at ∼600 Ma. [ABSTRACT FROM AUTHOR]

The crust of the eastern Tibetan Plateau is cut through by several ancient fracture zones, which existed prior to the Cenozoic uplift of eastern Tibet. However, the role of these pre‐existing crustal weak zones (CWZs) in the Cenozoic evolution of eastern Tibet is still poorly understood. Here, we conduct a series of 2D thermo‐mechanical simulations to investigate the role of CWZs in the uplift of eastern Tibet. Our results show that CWZs embedded in the continental lithosphere facilitate crustal thickening via folding and thrusting deformation and uplift of a rugged surface at the early stage of convergence. As crustal thickening continues, the lowermost crust begins to partially melt due to strain and radioactive heating. With continued thickening and heating, melt volume gradually increases, leading to the lower crust melt preferentially accumulating where the CWZs are situated. At a later stage, the partially molten layer becomes thick enough and starts to flow in a channel with respect to its upper and lower solid layers. CWZs behave as windows to steer the ascension of the partial melts, leading to melt doming through the CWZs. The melt doming contributes to a second local surface uplift. Parameter tests demonstrate that increasing the number of CWZs can accelerate such geodynamical processes, and that the CWZ orientation controls the polarity of lithospheric underthrusting. We suggest that the Cenozoic episodic uplift of the eastern Tibetan Plateau, revealed by thermochronological studies, was likely caused by a combination of pure shear crustal thickening and melt doming along the pre‐existing CWZs. Key Points: Crustal weak zones facilitate the formation of fold‐thrust structures under compression, they also cause upwelling of the partially molten lower crustThe partially molten lower crust forms channel flow when weak materials are extracted by the convection due to crust upwellingThe Cenozoic episodic uplift of the eastern Tibet perhaps caused by the combination of pure shear crustal thickening and melt doming [ABSTRACT FROM AUTHOR]

Zircon inheritance is a common phenomenon in igneous rocks, although more frequent in granitoids. Zircons inherited from granite magmas mostly come from the source, not from wall rocks or xenoliths. Consequently, they can provide invaluable information about the source materials, melting temperature, and melt segregation conditions. Miller et al. (Geology 31:529–532, 2003) divided granite rocks according to their zircon saturation temperature (TZr) into "hot" (TZr > 800 °C, with little or no inherited zircon) and "cold" (TZr < 800 °C; with abundant inherited zircon). Nevertheless, we have found that coeval and neighboring two-mica granites with TZr < 750 °C, presumably derived from similar sources, may have a radically different inheritance, from about 95 to near 0%. This paper aims to understand the reasons for these differences, in particular, and the survival of source zircons in granitoids, in general. To this end, we modeled the relationships between source composition, temperature, pressure, water content, zircon solubility, and melt fraction, on one hand; and melt production and zircon solution rates, on the other hand. Our results foresee that zircon survival during crustal melting is more probable if the source is a fertile peraluminous metasedimentary rock than if it is a metaluminous source with similar SiO2. Elevated zircon inheritance is characteristic of mid-crustal S-type, water-rich granite magmas generated within 4.5 and 6 kbar. Moderate or no inheritance is characteristic of water-poor granite magmas, because their sources require higher temperatures to produce the same melt fraction. Fast melt extraction does not cause perceptible effects on our models, because melt generation is slower than zircon dissolution, except in the case of crustal underplating by hot mafic magmas. We propose to refine the "hot" and "cold" classification by splitting the "cold" granites (TZr < 800 °C) into two categories, "dry" with little inheritance and "wet" with a very high zircon inheritance. Wet granites require a source water-fluxed from outside. They are characteristic of mid-crustal anatectic complexes with highly fertile gneisses alternating with unfertile mica-rich metapelites. We suggest that the extra water should come in most cases from dehydration reactions in the unfertile metasedimentary rocks beneath the crustal section undergoing anatexis. [ABSTRACT FROM AUTHOR]

The ~183 Ma Karoo Continental Flood Basalt (CFB), southern Africa, formed during a period of major crustal extension prior to Gondwana breakup. To explain the 'lithospheric contamination' observed in its whole‐rock chemical and isotopic data various models have been proposed, such as different subcontinental lithospheric mantle (SCLM) sources or interaction between a mantle plume and SCLM or subducted material. A re‐assessment of data in the literature from the Northern Karoo Province, combined with new petrographic, whole‐rock chemical and Sr‐Nd isotopic data of basaltic and dacite/trachydacite samples from the southern Lebombo suggest that chemical diffusion between low‐Ti basaltic/picritic magmas and high‐SiO2 rhyolite parent melt may have caused this 'lithospheric contamination'. Such slow chemical diffusion, which is posited to have developed during formation of this CFB, can explain the bimodal Karoo basalt‐rhyolite association, the wide variation in SiO2 content (~62–78 wt%) in the dacite/rhyolites, and variable 87Sr/86Sr (183 Ma) and ɛNd (183 Ma) isotope ratios of the Karoo picrites and basalts (0.7037 to 0.7102; 3.58 to −33.52, respectively) and dacites/rhyolites (0.7032 to 0.7152; 0.44 to −17.19). The intermediate incompatible trace element and isotopic compositions of the Karoo high‐Ti basalt/picrite suggest that these rocks could have evolved by magma mixing through chemical diffusion. Our geochemical model for the northern Karoo CFB province involves existence of a voluminous hot (>1,000°C) low‐Ti basaltic magma chamber with remnant picritic melts at shallow crustal depth (~4 km) during major crustal extension. Magma storage was relatively protracted, resulting in significant volumes of partial melt of the surrounding granitoids of the Kaapvaal Craton under relatively anhydrous condition, leading to generation of A‐type high‐SiO2 rhyolite parent magma at pressure of ~1.5 kbar and temperature of ~930°C. This was followed by chemical diffusion of selected elements [LREE, HFSE, and some LILE (K, Rb, Ba, Pb)] and self‐diffusion of Sr and Nd isotopes in these two melts, which generated the observed litho‐type variations in Karoo CFB. [ABSTRACT FROM AUTHOR]

The widespread Linzizong volcanic successions (LVS) in the Lhasa Terrane was a magmatic response to the tectonic transition from Neo‐Tethyan oceanic subduction to the India–Asia collision, which has been the subject of many studies; however, studies on the LVS in the western Lhasa Terrane are scarce compared to those in the east, limiting our understanding of the tectonic transition. Zircon U–Pb ages of two rhyolitic samples from the Shiquanhe region in the west of the Lhasa Terrane indicate that at least part of the Zenong Group, previously thought to have formed during the Early Cretaceous, was actually erupted during the Late Palaeocene (ca. 54 Ma) and belongs to the LVS. The rhyolites are characterized by high SiO2 and Al2O3, moderate K2O and Na2O, and low MgO, CaO, and FeO contents, enrichment in Rb and Th, depletion in Sr, Ba, Eu, Nb, and Ta, and have relatively uniform whole‐rock εNd(t) (−1.90 to −0.89) and zircon εHf(t) (+0.95 to +5.03) values. These geochemical characteristics indicate that they were formed by the partial melting of juvenile continental crust with subsequent plagioclase, biotite, amphibole, and Fe–Ti oxide fractionation. Combined with the results of previous studies, our data show that the western Lhasa Terrane underwent intensive Early Cenozoic crustal growth, similar to what occurred in the east, due to underplating of mantle‐derived magma resulting from the roll‐back and eventual break‐off of the Neo‐Tethys slab after the initial continental collision. [ABSTRACT FROM AUTHOR]

Garnets in metapelitic paragneisses from the southern Drosendorf unit in the Austrian part of the Bohemian Massif exhibit two episodes of growth during the Variscan orogeny, which can be distinguished on textural and chemical grounds. The first garnet (grt1) records evidence of high-grade metamorphism in the Late Devonian (Frasnian–Famennian), while the second garnet (grt2) formed by a second high-grade event in the Early Carboniferous (Visean). Both garnet generations contain abundant inclusions, of which monazite, rutile and crystallised melt droplets are particularly useful for reconstructing P–T–t conditions. The Late Devonian age (373 ± 9 Ma) for the first episode of garnet growth was obtained from chemical dating of monazite inclusions in grt1. Metamorphic conditions during the first episode of garnet growth are estimated to have been between 0.7 and 0.8 GPa at 680–700 °C and 0.95–1.10 GPa at 745–785 °C. There followed a phase of cooling and exhumation, after which the second garnet (grt2) were formed beginning under amphibolite facies conditions and continuing prograde to peak conditions of 0.95–1.10 GPa and 745–785 °C, which are similar to those of the first garnet forming event. Subsequently, the rocks experienced near isothermal decompression to 0.5–0.8 GPa. Chemical dating of both monazite inclusions in grt2 and the matrix provide a Visean age (343 ± 3 Ma). A study of detrital zircons in these paragneisses revealed zircon forming events at around 1.2, 1.5 and 1.8 Ga, suggesting an Avalonian provenance. The lack of zircons younger than 1 Ga and the presence of Cadomian metamorphic monazite relics (652 ± 15 Ma) indicates an Early Neoproterozoic deposition age for the sedimentary protolith likely. Our documentation of a Late Devonian high-grade metamorphic event in rocks derived from Avalonian corroborates tectonic models which assume that frontal parts of the Armorican terrane had already docked with Avalonia by this time. Unlabelled Image • Two garnet generations give evidence of a polymetamorphic evolution. • Monazite dating revealed Devonian and Carboniferous metamorphism. • Detrital zircons confirm an Avalonian origin of precursor sediments. [ABSTRACT FROM AUTHOR]

The presence of nanogranitoids (crystallized melt inclusions showing a granitic s.l. composition) in regionally metamorphosed migmatitic and granulitic terranes was documented for the first time in 2009. This important finding represented a hallmark event for crustal petrologists, demonstrating that the pristine composition of anatectic melts may be preserved and recovered in small objects hosted in peritectic minerals of high-grade metamorphic rocks. Many further occurrences were documented in rocks worldwide, from diverse geodynamic settings, from the Archean to Miocene, and these inclusions turned out to have diverse composition (from silicatic to carbonatitic). Therefore, we propose "nanorocks" as a comprehensive name for these crystallized melt inclusions. In the last decade, many studies demonstrated their utility in characterizing mechanisms of melting and in tracking crustal magma evolution. Although scholars are now able to easily recognize nanorocks in high-grade metamorphic rocks, only a few research groups remelt them by means of experimental devices because of a time-consuming preparation. However, when cutting-edge techniques are applied in addition to more routine ones, nanorocks do open new perspectives in crustal petrology. [ABSTRACT FROM AUTHOR]

It is well-acknowledged that the northern margin of the Gondwana supercontinent was affected by a major magmatic event at late Cambrian (Furongian) to early Ordovician (Tremadocian–Floian) times. However, an accurate assessment of its extent, origin, and significance is partly hampered by the incomplete characterization of the numerous gneiss massifs exposed in the inner part of the Variscan belt, as some of them possibly represent dismembered and deformed Furongian–Lower Ordovician igneous bodies. In this study, we document the case of the “Cézarenque–Joyeuse” gneiss complex in the Cévennes parautochthon domain of the French Massif Central. The gneisses form decametre- to kilometre-thick concordant massifs interlayered within a pluri-kilometric sequence of mica- and quartz schists. They encompass two main petrological types: augen gneisses and albite gneisses, both typified by their blue and engulfed quartz grains with the augen facies differing by the presence of centimetre-sized pseudomorphs after K-feldspar and the local preservation of igneous textures. Whole-rock geochemistry highlights that many gneisses have magmatic ferrosilicic (acidic with anomalously high FeOt and low CaO) compositions while others are akin to greywackes. Collectively, it is inferred that the bulk of the Cézarenque–Joyeuse gneisses represents former rhyodacite lava flows or ignimbrites and associated epiclastic tuffs. Volumetrically subordinate, finer grained, and strongly silicic leucogneisses are interpreted as microgranite dykes originally intrusive within the volcanic edifices. LA–ICP–MS U–Pb dating of magmatic zircon grains extracted from an augen gneiss and a leucogneiss brackets the crystallization age of the silicic magmas between 486.1 ± 5.5 Ma and 483.0 ± 5.5 Ma which unambiguously ties the Cézarenque–Joyeuse gneisses to the Furongian–Lower Ordovician volcanic belt of SW Europe. Inherited zircon date distributions, Ti-in-zircon and zircon saturation thermometry demonstrate that they formed by melting at 750–820 °C of Ediacaran sediments. Zircon Eu/Eu* and Ce/Ce* systematics indicate that the melts were strongly reduced (fO2 probably close to the values expected for the iron–wüstite buffer), possibly because they interacted during ascent with Lower Cambrian black shales. This would have enhanced Fe solubility in the melt phase and may explain the peculiar ferrosilicic signature displayed by many Furongian–Lower Ordovician igneous rocks in the northern Gondwana realm. We infer that crustal melting resulted from a combination of mantle-derived magma underplating in an intracontinental rift setting and anomalously elevated radiogenic heat production within the Ediacaran sedimentary sequences.

Inherited zircon ages and geochemical features of late Cadomian and late Variscan granitoids from the north-eastern Peloritani Mountains (NE Sicily) and the western Aspromonte Massif (SW Calabria) shed new light on the sources and processes involved in poly-orogenic granitoid magmatism. The two groups of strongly peraluminous granitoids have similarities in trace element abundance patterns, and Sr and Nd isotopic compositions consistent with both being derived from crustal sources, possibly with a minor contribution from mantle-related components. Comparison of the granite compositions with those of experimental melts derived from various metaigneous and metasedimentary sources indicates that both groups of granitoids originated exclusively from different degrees of melting of similar greywacke-dominated turbidite. Abundant inherited zircon cores from representative samples of metamorphosed late Cadomian (545 ± 5 Ma) granite and late Variscan (300 ± 4 Ma) leucogranodiorite have the same range of U–Pb ages, from Early Paleoproterozoic to latest Neoproterozoic, with main age clusters at ∼0.55 and ∼0.63 Ga, and minor age clusters at ∼0.95 and ∼2.5 Ga. The pattern of detrital zircon ages from a paragneiss, host rock to the late Cadomian granite, is the same, indicating, in conjunction with the geochemistry, that both granites originated by partial melting of deeper crustal equivalents of those paragneisses. The same crustal sequence melted during successive orogenies under different thermal regimes and in different post-collisional tectonic settings, giving rise to granitoid associations with different ages and geochemical features largely reflecting the melting conditions. On the other hand, the zircon inheritance patterns and specific chemical features of S-type granitoids reflect the nature of their crustal magma sources, independently from the effects of the thermal regime or tectonic setting at the time of magmatism.

The solubility of apatite in anatectic melt plays an important role in controlling the trace-element compositions and isotopic signatures of granites. The compositions of glassy melt inclusions and nanogranitoids in migmatites and granulites are compared with the results of experimental studies of apatite solubility to evaluate the factors that influence apatite behavior during prograde suprasolidus metamorphism and investigate the mechanisms of anatexis in the continental crust. The concentration of phosphorus in glassy melt inclusions and rehomogenized nanogranitoids suggests a strong control of melt aluminosity on apatite solubility in peraluminous granites, which is consistent with existing experimental studies. However, measured concentrations of phosphorus in melt inclusions and nanogranitoids are generally inconsistent with the concentrations expected from apatite solubility expressions based on experimental studies. Using currently available nanogranitoids and glassy melt inclusion compositions, we identify two main groups of inclusions: those trapped at lower temperature and showing the highest measured phosphorus concentrations, and melt inclusions trapped at the highest temperatures having the lowest phosphorus concentrations. The strong inconsistency between measured and experimentally predicted P concentrations in higher temperature samples may relate to apatite exhaustion during the production of large amounts of peraluminous melt at high temperatures. The inconsistency between measured and predicted phosphorus concentrations for the lower-temperature inclusions, however, cannot be explained by problems with the electron microprobe analyses of rehomogenized nanogranitoids and glassy melt inclusions, sequestration of phosphorus in major minerals and/or monazite, shielding or exhaustion of apatite during high-temperature metamorphism, and apatite–melt disequilibrium. The unsuitability of the currently available solubility equations is probably the main cause for the discrepancy between the measured concentrations of phosphorus in nanogranites and those predicted from current apatite solubility expressions. Syn-entrapment processes such as the generation of diffusive boundary layers at the mineral-melt interface may also be responsible for concentrations of P in nanogranitoids and glassy melt inclusions that are higher than those predicted in apatite-saturated melt. [ABSTRACT FROM AUTHOR]

Mid-crustal melting is widely documented under orogenic plateaus. However, the mechanism for its generation and its role in the evolution of orogenic plateaus remain poorly understood. Here we investigate the physical controls and consequences of crustal melting in plateau evolution using 3-D thermo-mechanical modeling method. Our simulations demonstrate that the convergence causes orogenic crust to have more upper crustal compositions and more radiogenic elements. The persistent radioactive and shear heating provides heat source required for in situ partial melting of felsic rocks at mid-crustal depths. Slower convergence rate favors earlier emergence and larger concentration of mid-crustal melt. Radioactive heating accumulated during crustal thickening plays the primary role in generating mid-crustal partial melting, which is consistent with previous 2-D model results. Shear heating promotes crustal melting, but its role is secondary to radioactive heating. Our simulations also show that basal heating produces a broad-area increase of the crust temperature and surface elevation and, to a lesser extent, promotes mid-crustal melting, because convergence-driven heat advection dominates the transport of the newly-added heat. The significant mid-crustal melting dramatically reduces the viscosity of the middle crust and eventually leads to channelized outward flow of the melt-weakened layer. This has the potential to cause mechanical decoupling of the layers above and below the melt channel and the marginal dominance of mid-crustal melting. We suggest that the widespread low velocity or resistivity zones observed in Tibet are caused by interconnected mid-crustal melting, which is an inherent feature of large hot orogenic plateaus. • Physical controls on mid-crustal melting are investigated using numerical modeling. • Radioactive heating accumulated as crustal thickening plays the primary role. • Melt-weakening effect leads to decoupling between the upper and lower crust. [ABSTRACT FROM AUTHOR]

• Zircon and monazite inclusions in the pre-kinematic corundum from mylonitic gneisses were dated to be ∼28 Ma. • These data give an upper age limit on the left lateral shearing in the Ailaoshan massif. • The siliceous melt inclusions in the pre-kinematic corundum indicate the presence of pre-shearing crustal anatexis event. The Ailaoshan-Red River shear zone (ASRR) in SE Tibet accommodated about 20% convergence of the Cenozoic India-Asia continental collision which resulted in complicated thermal history during Paleocene to Miocene. The Ailaoshan massif, in the middle part of ASRR, is characterized by extensive mylonitization and migmatization, but the exact timing of the large scale left lateral shearing and whether the crustal partial melting occurred prior to or coeval with the left lateral shearing remain controversial. The detailed structural observation suggests the pre-kinematic nature of the Ailaoshan corundum with respect to the left lateral shearing. LA-ICP-MS U-Pb dating of metamorphic zircon and monazite inclusions in the pre-kinematic Ailaoshan corundum obtained a weighted 206Pb-238U ages of 28.0 ± 0.5 Ma (MSWD = 3.3; N = 27) and 28.3 ± 0.3 Ma (MSWD = 1.8; N = 18), respectively, which gave an upper limit on the initiation of the left lateral shearing in the Ailaoshan massif. Moreover, the presence of protogenetic siliceous melt inclusions in the pre-kinematic corundum indicates that the Cenozoic crustal anatexis of the Ailaoshan massif has occurred before the left lateral shearing. Along with available geochronological data, the crustal partial melting of the Ailaoshan massif was attributed to joint results, which at least include the Neoproterozoic convergence, the Triassic Indosinian Orogeny, and the Cenozoic evolution as a response to the India-Asia continental collision. [ABSTRACT FROM AUTHOR]

Mylonitic orthogneisses in the Kathmandu area, central Nepal have been investigated using whole-rock and mineral chemistry, Rb-Sr isotopes, and zircon U-Pb age dating. Zircon REE patterns determined from orthogneisses are characterized by enriched HREE patterns and the prominent Eu anomalies, consistent with a magmatic origin. The U-Pb zircon age dating and Ti-in-zircon thermometry revealed crystallization took place ca. 1.74 Ga at temperatures of 705–765℃; typical of felsic magmatism in the crust. Whole-rock data from most orthogneisses in this study and from similar rocks in previous studies span the 'syn-collisional' and 'post-collisional' fields on various tectonic discrimination diagrams, while some data also plot in rift-related magmatism fields. The peraluminous compositions, very high Sr isotopic ratios (0.865–3.585) and high Th and U concentrations for all orthogneisses in this study indicate that mylonitic orthogneisses are largely of S-type crustal origins. The new data presented herein, combined with that of previous studies, outline at least two Palaeoproterozoic magmatic episodes: 1) ca. 1.92–1.90 Ga rift-related magmatism derived from mantle melting and 2) 1.84–1.74 Ga crustal melting, resulting from burial of the Indian basement during thermal subsidence after rifting. This two-stage Palaeoproterozoic magmatism in Nepal occurred along the northern passive margin of the Indian basement during and/or after the breakup of the Columbia supercontinent. [ABSTRACT FROM AUTHOR]

Abstract Devonian intermediate-felsic igneous rocks are widespread in the Gorny Altai terrane and they were suggested to form in a continental arc setting that was built on the late Neoproterozoic to early Ordovician Kuznetsk-Altaiintra-oceanicsubduction-accretion system (KAIOSS). 1 1 KAIOSS, Kuznetsk-Altaiintra-oceanicsubduction-accretion system Petrogenesis of these rocks will not only provide information on geodynamic mechanisms for continental arc magmatism but also shed light on how accretionary orogens can be differentiated and matured, both of which are pivotal in understanding the continental generation and evolution. This study is focused on igneous suites from the early Devonian Ongudai and middle Devonian Kuratin complexes. The former complex is dominated by andesites and basaltic andesites. Zoning patterns of clinopyroxene phenocrysts and whole-rock geochemical modeling suggest that these volcanic rocks formed mainly by magma mixing with substantial input of mantle-derived mafic melts, signifying significant crustal growth. The latter complex comprises a volcanic suite (ca. 383–382 Ma) ranging from andesites to rhyolites and minor granitic intrusions (ca. 385–382 Ma). The rhyolitic endmembers of the volcanic suite were probably sourced from the late Neoproterozoic to early Ordovician meta-sedimentary sequences of the KAIOSS. In contrast, the andesitic endmembers possibly formed by partial melting of mixed crustal components comprising the late Neoproterozoic magnesian andesites (or their geochemical equivalents) and more depleted basaltic rocks of the KAIOSS. Geochemical data demonstrate that the granitic intrusions were most likely products of melts from the late Neoproterozoic to Cambrian arc-type basaltic rocks of the KAIOSS. Taken together, the whole igneous suite of the Kuratin complex documents strong crustal differentiation. Of particular importance is that the highly heterogenous crustal components involved likely mirror the pre-Devonian crustal architecture that was induced by prolonged subduction-accretion of the KAIOSS. Our study suggests that heterogeneous crustal melting by underplating of hydrous basaltic melts in continental arcs is a plausible way to transform previous immature oceanic materials in accretionary orogens into differentiated and mature continental crust. This process contributes significantly to the magmatic diversity in subduction zones and crustal growth and differentiation in accretionary orogens. Highlights • Heterogenous crustal sources were involved in the studied continental arc magmatism. • Source heterogeneity and input of mantle melt contributed to the magmatic diversity. • Substantial crustal differentiation and maturation were achieved. [ABSTRACT FROM AUTHOR]

Abstract The Gongga Shan batholith is a complex granitoid batholith on the eastern margin of the Tibetan Plateau with a long history of magmatism spanning from the Triassic to the Pliocene. Late Miocene–Pliocene units are the youngest exposed crustal melts within the entire Asian plate of the Tibetan Plateau. Here, we present in-situ zircon Hf isotope constraints on their magmatic source, to aid the understanding of how these young melts were formed and how they were exhumed to the surface. Hf isotope signatures of Eocene to Pliocene zircon rims (ɛ Hf (t) = −4 to +4), interpreted to have grown during localised crustal melting, are indicative of melting of a Neoproterozoic source region, equivalent to the nearby exposed Kangding Complex. Therefore, we suggest that Neoproterozoic crust underlies this region of the Songpan–Ganze terrane, and sourced the intrusive granites that form the Gongga Shan batholith. Localised young melting of Neoproterozoic lower or middle crust requires localised melt-fertile lithologies. We suggest that such melts may be equivalent to seismic and magnetotelluric low-velocity and high-conductivity zones or "bright spots" imaged across much of the Tibetan Plateau. The lack of widespread exposed melts this age is due either to the lack of melt-fertile rocks in the middle crust, the very low erosion level of the Tibetan plateau, or to a lack of mechanism for exhuming such melts. For Gongga Shan, where some melting is younger than nearby thermochronological ages of low temperature cooling, the exact process and timing of exhumation remains enigmatic, but their location away from the Xianshuihe fault precludes the fault acting as a conduit for the young melts. We suggest that underthrusting of dry granulites of the lower Indian crust (Archaean shield) this far northeast is a plausible mechanism to explain the uplift and exhumation of the eastern Tibetan Plateau. Graphical abstract Highlights • Neoproterozoic crust underlies Gongga Shan. • Bright spots beneath Tibetan Plateau relates to localised melts. • Dry granulites of the lower Indian crust were underthrust. [ABSTRACT FROM AUTHOR]

Abstract Jurassic-Cretaceous granitoids are widespread in eastern continental China and have been considered by many as resulting from paleo-Pacific subduction. However, the actual mechanism of their petrogenesis remains speculative. In order to address this important problem and on the basis of our regional study, we hypothesized that the coastal granitoids may result directly from the paleo-Pacific plate subduction, whereas the widespread granitoids in the continental interiors ultimately result from dehydration of the paleo-Pacific slab stagnated in the mantle transition zone (Niu et al., 2015). Here, we present the very first study testing this hypothesis. We sampled 18 Jurassic-Cretaceous granitoid plutons along a ~ 1300 km long traverse parallel to the inferred paleo-Pacific subduction from the southeast coastline to the Xiaoqinling in the continental interiors and carried out a detailed study on these plutonic samples, including zircon U Pb geochronology, bulk-rock major and trace element compositions and Sr-Nd-Pb-Hf isotopic characteristics. These plutons give varying zircon crystallization ages of ~146 to 100 Ma. They are mostly granitic and minor granodioritic, quartz monzonitic and syenitic in composition, enriched in large ion lithophile elements (LILEs), depleted in high field-strength elements (HFSEs) and have varying negative Sr and Eu anomalies. The plutons in the continental interiors show significant positive correlations of Nd (ε Nd (t) = −25.5 to −10.9) and Hf (ε Hf (t) = −31.5 to −11.3) isotopes with Pb isotopes (206Pb/204Pb (t) = 15.827 to 17.622), with the enriched endmember characterized by low ε Nd (t), ε Hf (t) and 206Pb/204Pb (t). The plutons towards the coastal region have relatively high ε Nd (t) (−9.0 to −5.2), ε Hf (t) (−11.2 to −4.1) and 206Pb/204Pb (t) (18.051 to 18.349). The coastal granitoids are best explained as resulting directly from subduction slab dehydration induced mantle wedge melting and resultant crustal anatexis, whereas the interior granitoids are best interpreted as resulting from mature crustal anataxis caused by basaltic magmatism associated with mantle lithosphere thinning, ultimately triggered by dehydration of paleo-Pacific slab stagnant in the mantle transition zone. Highlights • The interior granitoids have different sources and petrogenesis than coastal granitoids. • The interior granitoids result from basal hydration weakening induced crustal melting. • The coastal granitoids result directly from the paleo-Pacific plate subduction. [ABSTRACT FROM AUTHOR]

We report on the oxygen isotope composition of Jurassic rhyolites from a silicic large igneous province, the Chon Aike Province (Patagonia, Argentina). Quartz is shown to have refractory behavior with respect to diffusional oxygen isotope exchange, making it a robust tracer of magmatic processes. Detailed secondary ion mass spectroscopy (SIMS) transects across 24 quartz crystals reveal homogeneous, but elevated, oxygen isotope values (10.9–12.5‰). None of the analyzed grains display distinct discontinuities in 18O values. Late hydrothermal exchange is limited to a few tens of micrometers next to cracks, some grain boundaries, and inclusions. No correlation with igneous zoning as revealed by cathodoluminescence (CL) was found. Finally, quartz crystals display little to no inter-grain variability at a sample or outcrop scale. Zircons (7.5–10.1‰), in contrast, display significant inter–crystalline oxygen isotopic heterogeneity (>2.0‰) at a sample scale, but core-rim analyses reveal no systematic variations. This is interpreted to confirm the antecrystic nature of zircons, while quartz crystals mostly are phenocrysts. The studied quartz and zircon provide, hence, complementary information on the evolution of the magmatic system of the Chon Aike Province. Zircon likely captures information about the deeper source region, in contrast to quartz that will record the last stages of the magmatic system and thus might provide important information on the buildup and duration of magma chamber processes in the upper crust. The data illustrate that quartz—in the absence of recrystallization—can retain its magmatic signature and is thus a useful tracer of pre-eruptive magmatic processes. The high δ18O values of both zircon and quartz require a significant (>50%) crustal—most likely sedimentary— contribution in the melt formation process, either via assimilation or anatexis. This conclusion yields new constraints on petrological models for the Chon Aike Province. [ABSTRACT FROM AUTHOR]

Anatectic magmas form plutons or accumulate in the core of anatectic domes. Both scenarios have distinct implications on the behaviour of the continental crust during orogenic evolution from collision to collapse. Considering a stepwise extraction of melt, we simulate the evolution of anatectic melt and of solid residues produced in the crust from collision to collapse using thermodynamic modelling. We also simulate the effect of entrainment of source material (restite-unmixing and peritectic assemblage entrainment) on the compositional range of the resulting magmas. The results are then compared to the compositions of lower crustal xenoliths and of peraluminous granites in both plutons and anatectic dome in the southeastern French Massif Central (SE-FMC). From our calculations, we identify two type of anatectic melts (1) cool-and-wet produced at low-temperature (< 800 °C) which release fluids during crystallisation and (2) hot-and-dry produced at high-temperature (> 750 °C) which only release fluids at the end of crystallisation. When emplaced around 0.4 GPa, cold-and-wet melts are produced by muscovite-dehydration melting reactions; hot-and-dry are produced by biotite-dehydration melting. In the SE-FMC, the Velay dome is cored by the Velay granite, intruded by small bodies of Velay leucogranite and surrounded by plutons made of either two mica leucogranite (MPG) or cordierite-bearing granite (CPG). MPG and Velay leucogranite compositions are best reproduced by cool-and-wet magmas. CPG and Velay granite compositions are best reproduced by hot-and-dry magmas. Melt extraction after biotite dehydration melting leaves residues that are similar in composition to lower-crustal xenoliths. Magmas forming plutons migrate freely toward the upper crust forming plutons with distinct compositions. On the contrary, to form a dome, magmas are retained on the way up. The emplacement and accumulation of magma at deeper level enhances (or trigger) melting due to the addition of heat (from hot-and-dry) and fluids (from cool-and-wet). The accumulation of magma and the in situ melting increases melt fraction and has consequence to weaken the middle crust and leads to the formation of an anatectic dome. We suggest that magmas are retained due to lithological heterogeneities in the crust. In the case of the Velay dome, a large orthogneiss formation similar to the Velay orthogneiss formation may have played that role. [ABSTRACT FROM AUTHOR]

The garnet-biotite-sillimanite enclaves from El Hoyazo are quenched anatectic metapelites found within peraluminous dacites (Betic Cordillera, SE Spain), representing a residual lower crust in the area after 40–60% of melt extraction. Anatexis occurred concomitantly with deformation in a regional metamorphic setting during the Upper Miocene at the base of the continental crust. Previous studies have provided detailed information on the pressure-temperature evolution, the sequence of melting reactions, and associated melt proportions and compositions. They show that enclaves mostly record peak metamorphic assemblages, mineral compositions, and, likely, microstructures, with minor changes upon entrapment within the dacite magma and rapid ascent and extrusion. The enclaves still preserve a proportion of the primary melt, that solidified to glass in abundant melt inclusions (MI) and matrix melt, permitting the study of the microstructural relationships between melt and residue. This study focuses on the geometry of the glass network at the microscale that, combined with the previously reported anatectic history, helps shed light on the mechanisms and history of melt drainage from these rocks. A representative sample of the enclaves was investigated by synchrotron μ-XRF and scanning electron microscopy to map the distribution of glass and minerals on three thin sections cut perpendicularly to the foliation. The combination of major and trace element μ-XRF distribution maps and detailed backscattered electron images evidence the presence of a pervasive and mostly interconnected glass network through the studied centimeter-scale sections. Interconnection is due to the crosscutting of films and glass-rich domains oriented parallel and at high angle with foliation. Although enclaves lost ~40–60% of melt, they still contain ~10–15% of glass, with a considerable proportion of it stored within the Mix, which is an aggregate of micrometer-sized fibrolitic sillimanite and glass. The distribution of glass (former melt) is not in textural equilibrium with the solid residue and resembles the interconnected network of deformation bands observed in migmatites of anatectic terranes at the mesoscale. Microstructural studies of melt pseudomorphs in migmatites and granulites of anatectic terranes are scarce, but the following remarkable interpretations can be made combining our observations of these enclaves: melt formed an interconnected network during anatexis that permitted melt segregation and extraction, though melt-residue textural disequilibrium is the rule rather than the exception. The proportion of melt present in residual migmatites can be much higher than the permeability threshold for crustal protoliths; in this particular study, two reasons for this might be that (1) melt was still being produced and flowing through the residual migmatite right before disaggregation and inclusion within the host dacite, where additional melt drainage was impeded by the hydrostatic stress field, and (2) a particular microstructure produced at the onset of anatexis, such as the Mix, acted as a trap for melt impeding or delaying melt segregation. [ABSTRACT FROM AUTHOR]

Abstract The Maures-Tanneron Massif (MTM), together with Corsica and Sardinia, represent the Southeastern branch of the huge European Variscan belt. A continuous evolution from continental collision to exhumation is described from ca. 350 Ma to ca. 320 Ma, based on an extended compilation of available geological and geochronological data. This geological compilation is complemented with thermo-mechanical modelling that tests the conditions under which deep-seated HP units can melt and exhume massively during continued convergence. We simulate in two-dimensions the development of gravitational instabilities in a partially melting crust, depending on rheological layering and heat sources, and accounting for temperature dependent elasto-visco-plastic rheologies. In order to reproduce PT exhumation patterns in the MTM, over the appropriate time-scale (> 20 My) and spatial extent (> 100 km), a best fit was obtained with a convergence rate of 0.5 cm/yr and moderate diffusive surface processes. The contribution of radiogenic heating in the pre-stacked felsic crustal units is crucial. A lateral alternation with mafic units is also required in order to prevent lateral spreading of the orogen. A low viscosity partially molten crust (1020 Pa·s, thus rather felsic) accompanies crustal exhumation in a decoupled mode from the deeper mantle. A shallow asthenosphere below the orogen (LAB at ~ 70 km depth, e.g. shallow slab break-off) produces too warm and sudden exhumation as opposed to a competent mantle lithosphere (120 km depth LAB) that restrains it. The MTM witnesses the typical competition between far-field plate convergence and internal body forces, and our study pleads for a progressive evolution of transpression towards perpendicular extension from ca. 320 Ma. Highlights • A geological synthesis is combined with thermo-mechanical models to interpret exhumation of the South European Variscan Belt, • The orogenic root exhumed diapirically over > 100 km² during convergence at ~ 0.5 cm/yr for > 20 My, • A bulk viscosity of 10^20 Pa.s appropriately simulates the formation and rise of partially molten mush at these scales. • Alternation of felsic crustal units against a mafic buttress is necessary to model the observations, • Structures and dynamics show a continuous transition from continental collision to perpendicular extension. [ABSTRACT FROM AUTHOR]

In the Emeishan Large Igneous Province (ELIP), western Yangtze Block, SW China, minor amounts of silicic volcanic rocks (∼5%) are spatially and temporally associated with flood basalts. The distribution and origin of these silicic rocks are keys to understanding the processes associated with mantle plume activities. In the Binchuan and Panzhihua areas, southwestern ELIP, there are Nb-Ta-poor and Nb-Ta-rich rhyolites. Nb-Ta-poor rhyolites interlayered with basalts in the Binchuan area occur in the lower part of the Emeishan volcanic sequence and have a crustal origin. They are characterized by negative Nb-Ta anomalies, low ε Nd (t) and high 87 Sr/ 86 Sr(i) values, and zircon saturation temperatures (836–852 °C). Nb-Ta-rich rhyolites are associated with the main phase of the Emeishan volcanic sequence in the Binchuan and Panzhihua areas and are characterized by positive Nb-Ta anomalies, oceanic island basalt (OIB)-like trace element ratios, low 87 Sr/ 86 Sr(i) ratios, positive ε Nd (t) and ε Hf (t) values, and high A/CNK ratios and Fe∗s. In addition, they have high Ga/Al ratios, high field strength elements (HFSEs), and high zircon saturation temperatures (>920 °C), typical of aluminous A-type silicic rocks, similar to the spatially distributed syenitic intrusions of the ELIP. These Nb-Ta-rich rhyolites are best interpreted as hybrid products of crystal fractionation of high-Ti basaltic magmas, coupled with minor crustal assimilation (6–14%), on the basis of Rhyolite-MELTS and EC-AFC simulations. The temporal and spatial distributions of these two types of rhyolite reflect various extents of thermal and mass exchange between mantle-derived basaltic magma and crustal material above a mantle plume. When the plume head reached the base of the western Yangtze Block, enriched components in the lithospheric mantle were melted and the melts partially ponded near the crust-mantle boundary or in the lower crust where they induced partial melting of crustal materials to form Nb-Ta-poor rhyolites. During the main phase of volcanism, large magma chambers and extensive plumbing systems developed due to decompressional melting of the mantle plume. In this stage, abundant mafic-ultramafic and felsic intrusions were formed, and Nb-Ta-rich rhyolites and A-type granites were emplaced. [ABSTRACT FROM AUTHOR]

Strongly peraluminous fractionated S-type granites are important indicators of compositional maturity of the continental crust. Here we report the finding of strongly peraluminous fractionated S-type granites from the western Baoshan Block in the southeastern Tibetan–Himalayan orogenic belt. We present LA ICP-MS zircon U–Pb ages from biotite- and two-mica- granites which reveal two distinct periods of magmatism at 121.8 ± 1.1 Ma and 81.9–79.7 Ma. These Cretaceous granites are strongly peraluminous (A/CNK = 1.18–1.31), high-K calc-alkaline, enriched in Rb, Th, K, and Pb, and depleted in Ba, Sr and Ti. They also show high normative- corundum contents of 2.84–4.42 wt%, moderately high P 2 O 5 content (0.22–0.28 wt%) and high differentiation index (DI) of 93.1–96.8, similar to the features of fractionated S-type granites. The low total Fe 2 O 3 T + MgO + TiO 2 contents (<3.0 wt%) are also consistent with the results from experimental studies on S-type granite melts. The S-type granites from Baoshan show high Al 2 O 3 /TiO 2 (93.8–182.9), Rb/Ba (6.19–23.5), and Rb/Sr (17.0–30.5) values, and low CaO/Na 2 O (0.09–1.19). They also display negative εNd( t ) values (−11.7 to −13.6) and εHf( t ) ranges (−5.47 to −0.77 for biotite granites and −7.67 to −0.81 for two-mica granites) suggesting evolved and heterogeneous continental crustal sources for the magma. The initial 206 Pb/ 204 Pb (17.527 to 19.277), 207 Pb/ 204 Pb (15.772 to 15.888), and 208 Pb/ 204 Pb (39.237 to 39.498) isotopic ratios are also consistent with lower to upper crustal affinity. Our data, in conjunction with those from experimental studies suggest that the strongly peraluminous S-type granites were derived from partial melting of metasedimentary, pelite-dominated source within continental crustal basement, similar to typical fractionated S-type granites from the Jiangnan Orogen (SE China), Erzgebirge (Central Europe), Eastern Australia and the low-temperature SP granites in the Himalayas. We correlate the Early- to Late- Cretaceous magmatism with two-stage melting of fertile continental materials following the final closure and termination of Bangong-Nujiang Tethys ocean. [ABSTRACT FROM AUTHOR]

The Qinling Orogenic Belt in central China, which resulted from continent–continent collision, is an excellent area for the study of collision-related magmatism. An integrated study including detailed field investigations, petrography, mineral and whole-rock geochemistry, zircon U–Pb–Hf–O isotopes, and geochemical modeling was carried out on the Huayang intrusive complex—a key magmatic intrusion in the South Qinling Belt—in order to understand the nature and melt source regions of magmatism associated with continental collisional orogenesis. The Huayang intrusive complex is composed of ∼207–202 Ma medium to fine-grained granite, coarse to medium-grained granite of the same age, ∼214–207 Ma tonalite and granodiorite, and rare ∼218–213 Ma mafic xenoliths. The mafic xenoliths are characterized by enriched large ion lithophile elements, with zircon εHf(t) values of -6·8 to +4·1 and average zircon δ18O of 6·1‰, which suggests that the xenoliths may represent melts derived from phlogopite-bearing lithospheric mantle. The tonalites and granodiorites exhibit high Sr/Y and La/Yb, but low Rb/Sr, with variable zircon εHf(t) values of -6·7 to +1·.9 and zircon δ18O values of 5·3‰ to 9·0‰. We suggest that they were derived from partial melting of Neoproterozoic, low δ18O basaltic rocks with a minor input of mafic magma. These melts underwent fractional crystallization and assimilated high δ18O crustal materials during magma ascent and emplacement. The coarse to medium-grained granitic rocks have zircon εHf(t) values of -7·3 to +1·5, with low zircon δ18O values (average 5·7‰). The medium to fine-grained granitic rocks have zircon εHf(t) values of -14·7 to +1·1, with high zircon δ18O values (average 8·4‰). Both of these granitic rock types show similar whole-rock geochemistry, with metaluminous to strongly peraluminous compositions, and are characterized by intermediate to low Sr/Y values. We propose that the coarse to medium-grained granites originated from partial melting of low δ18O Neoproterozoic metabasaltic to metatonalitic rocks, and that the medium to fine-grained granites were derived from high δ18O Neoproterozoic metagreywackes. Both granitic magma types experienced plagioclase-dominated fractional crystallization during magma ascent and emplacement. The data suggest that three different source materials were involved in magmatism in the South Qinling Belt: 1) the lithospheric mantle; 2) low δ18O Neoproterozoic metabasaltic to tonalitic rocks, and 3) high δ18O Neoproterozoic metagreywackes. Slab break-off and/or dehydration of the subducted slab may have induced the melting of the sub-continental lithospheric mantle and caused subsequent crustal melting by heating the base of the crust. The results of this study suggest that magmatism in continental collisional orogens is not only generated by heating from radioactive element decay during crustal thickening. [ABSTRACT FROM AUTHOR]

Peralkaline syenite and granite dykes cut the Straumsvola nepheline syenite pluton in Western Dronning Maud Land, Antarctica. The average peralkalinity index (PI = molecular Al/[Na + K]) of the dykes is 1.20 ( n = 29) and manifests itself in the presence of the Zr silicates eudialyte, dalyite and vlasovite, and the Na-Ti silicate, narsarsukite. The dykes appear to have intruded during slow cooling of the nepheline syenite pluton, and the petrogenetic relationship of the dykes and the pluton cannot be related to closed-system processes at low pressure, given the thermal divide that exists between silica-undersaturated and oversaturated magmas. Major and trace element variations in the dykes are consistent with a combination of fractional crystallization of parental peralkaline magma of quartz trachyte composition, and internal mineral segregation prior to final solidification. The distribution of accessory minerals is consistent with late-stage crystallization of isolated melt pockets. The dykes give an Rb-Sr isochron age of 171 ± 4.4 Ma, with variable initial Sr/Sr ratio (0.7075 ± 0.0032), and have an average ε of − 12.0. Quartz phenocrysts have δO values of 8.4-9.2‰, which are generally in O-isotope equilibrium with bulk rock. Differences in the δO values of quartz and aegirine (average Δ = 3.5‰) suggest aegirine formation temperatures around 500 °C, lower than expected for a felsic magma, but consistent with poikilitic aegirine that indicates subsolidus growth. The negative ε (< − 10) and magma δO values averaging 8.6‰ (assuming Δ = 0.6‰) are inconsistent with a magma produced by closed-system fractional crystallization of a mantle-derived magma. By contrast, the nepheline syenite magma had mantle-like δO values and much less negative ε (average − 3.1, n = 3). The country rock has similar δO values to the granite dykes (average 8.0‰, n = 108); this means that models for the petrogenesis of the granites by assimilation are unfeasible, unless an unexposed high-δO contaminant is invoked. Instead, it is proposed that the peralkaline syenite and granite dykes formed by partial melting of alkali-metasomatised gneiss that surrounds the nepheline syenite, followed by fractional crystallization. [ABSTRACT FROM AUTHOR]

We have identified a new crustally derived granite pluton that is related to the Emeishan Large Igneous Province (ELIP). This pluton (the Wase pluton, near Dali) shows two distinct SHRIMP zircon U-Pb age groups (~768 and ~253 Ma). As it has an intrusive relationship with Devonian limestone, the younger age is interpreted as its formation, which is related to the ELIP event, whereas the ~768 Ma Neoproterozoic-aged zircons were inherited from Precambrian crustal component of the Yangtze Block, implying the pluton has a crustally derived origin. This is consistent with its peraluminous nature, negative Nb-Ta anomaly, enrichment in light rare earth elements, high Sr/Sr ratio (0.7159-0.7183) and extremely negative ε(Nd) values (−12.15 to −13.70), indicative of melts derived from upper crust materials. The Wase pluton-intruded Devonian strata lie stratigraphically below the Shangcang ELIP sequence, which is the thickest volcanic sequence (~5400 m) in the whole ELIP. The uppermost level of the Shangcang sequence contains laterally restricted rhyolite. Although the rhyolite has the same age as the Wase pluton, its geochemical features demonstrate a different magma origin. The rhyolite displays moderate Sr/Sr (0.7053), slightly negative ε(Nd) (−0.18) and depletions in Ba, Cs, Eu and Sr, implying derivation from differentiation of a mantle-derived mafic magma source. The coexistence of crustally and mantle-derived felsic systems, along with the robust development of dike swarms, vent proximal volcanics and thickest flood basalts piles in Dali, shows that the Dali area was probably where the most active Emeishan magmatism had once existed. [ABSTRACT FROM AUTHOR]