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Olivine-rich lavas in the Margi area, Cyprus, are part of the boninitic Upper Pillow Lava series of the Troodos ophiolite. Re-Os isotope compositions of both whole rock and olivine phenocrysts of these lavas have been measured to determine the initial 187Os/188Os isotope compositions of the primary boninitic magma. The Margi lavas have high and variable whole rock Re-Os isotope compositions (187Re/188Os: 1.060–8.613; 187Os/188Os: 0.1307–0.1599), with (87Sr/88Sr) 90Ma (0.705936 ± 0.001021), ε Nd (90 Ma) (5.6 ± 2.5), and δ18O (8.59 ± 0.38 ‰). In contrast, olivine phenocrysts, which host spinel and melt inclusions, have low and constant 187Os/188Os from 0.1301 to 0.1336 and low 187Re/188Os from 0.1350 to 1.557. Initial 187Os/188Os of these olivine phenocrysts (0.1300 ± 0.0041) is lower than that of whole rock (0.1373 ± 0.0135). These facts indicate that the olivine phenocrysts faithfully record initial 187Os/188Os of the primary boninitic magmas, which are comparable to the 187Os/188Os of primitive mantle, whereas whole rock Re-Os isotopes have been affected by secondary processes after olivine crystallization. Further geochemical modeling indicates that, although Os can be mobilized during slab dehydration, the contribution of crustal Os is insufficient to significantly raise the 187Os/188Os of the mantle wedge in a nascent subduction zone. Overall, our findings provide new insights about Os recycling in the early stages of subduction. [ABSTRACT FROM AUTHOR]

Concentrically zoned pyrite aggregates (CZPA) are common in sediment‐hosted massive sulfide (SHMS) deposits and have been widely used to interpret the ore‐forming processes. There is considerable uncertainty, however, over the formation of aggregates that are oscillatorily zoned and contain randomly‐orientated pyrite microcrystals. Guided by the results of examination of the micro‐textures of CZPA and in‐situ chemical analyses, we conducted a quantitative diffusion‐reaction simulation to assess the mechanism of CZPA formation. Our study shows that oscillatory zoning results from the feedback between the diffusion of reactants and the nucleation‐growth of Fe‐sulfides. Externally derived Fe2+ diffuses into the early diagenetic sediments containing decomposing organic matter (2CH2O + SO42− = 2HCO3− + H2S) and reacts with H2S to form a pyrite layer via an intermediate pathway (Fe2+ + H2S → FeS + 2H+, FeS + H2S → FeS2 + H2). This growth of pyrite layers depletes the local concentration of reactants and suppresses nucleation until the diffusive reaction front advances and another layer is formed. Intermediate phases, for example, mackinawite, nucleate instead of pyrite, because of their greater ease of nucleation due to the low surface tension, and lead to the domination of nucleation over growth. The nucleation of mackinawite and occurrence of siderite in the CZPA are consistent with a low temperature, high pH, anoxic early diagenetic environment. Our study demonstrates that CZPA in SHMS deposits are formed by intrinsic self‐organizational processes rather than by extrinsic changes of ore‐forming fluids. The CZPA in SHMS deposits are thus indicative of their diagenetic origin with Fe2+ infiltrated and diffused from hydrothermal fluids into the sediments. Plain Language Summary: Oscillatory zoning patterns are a phenomenon commonly encountered in scientific disciplines that include biology, geology, chemistry and atmospheric physics. To decipher these patterns, we employ a unifying principle that leverages a reaction‐diffusion approach and successfully replicate the textures observed in a massive sulfide deposit, via numerical simulations. In addition, we show that factors such as the concentration, distribution, and mobility of reactant species, such as the iron ion, sulfide, and organic matter in sediments, influence the texture of patterns at both the millimeter and nanometer scales. Our proposed numerical models yield results that closely align with the oscillatory zoning patterns observed in pyrite‐siderite from the Xinqiao massive sulfide deposit in Central China. The broader impact of this work lies in the development of a computer simulation‐based protocol, which sheds light on the processes forming massive sulfide deposits. This, in turn, has the potential to significantly aid the exploration for new metal and sulfide resources beneath the surface of the Earth. Key Points: Reactive‐diffusion simulations of the formation of concentrically zoned pyrite aggregates in sediment‐hosted massive sulfide depositsThe ore textures were the result of an intrinsic self‐organized process rather than extrinsic changes of chemical compositionsProtocols for deciphering diagenetic environments affected by the infiltration of Fe2+‐rich hydrothermal fluids [ABSTRACT FROM AUTHOR]

The genesis and timing of formation of the giant Bayan Obo deposit, the world's largest rare earth element (REE) deposit in the western part of the late Paleozoic northern North China continental arc (NCA), are highly controversial due to complex mineral assemblages and reported ages of mineralization. We conducted new zircon U‐Pb and 40Ar/39Ar dating of metamorphic and igneous Neoarchean to Permian mid‐to upper‐crustal rocks exhumed along a north−south corridor across the western NCA and its retroarc foreland. The results show that the mid‐ to upper‐crust of the western part of the NCA has been strongly affected by thermal modifications during arc construction in the late Paleozoic, while the retroarc foreland remained thermally stable. Our results first reveal an unusually warm upper‐crust around Bayan Obo during the late Paleozoic with high geothermal gradients of 50.0 ± 8.3 to 88.3 ± 8.3°C/km and strong thermal modification of the upper‐crust during arc construction. This unusually warm upper‐crust and high geothermal gradients resulted in intensive thermal perturbations and recrystallization of REE‐bearing minerals in the Bayan Obo deposit, as well as formation of high‐grade REE ores and complete or partial resetting (U‐)Th‐Pb isotopic systems of REE minerals. Our identification of the unusually warm upper‐crust and high geothermal gradients in the western part of the NCA provides important constraints on genesis, timing and thermal modification of the giant Bayan Obo deposit, as well as other REE deposits with complex isotopic ages. Plain Language Summary: The Bayan Obo deposit is the largest rare earth element (REE) deposit in the world. Located in North China, its formation and timing of mineralization have been debated for over 60 years. We conducted thermochronometric investigations on metamorphic and igneous minerals (hornblende, muscovite and biotite) of the mid‐to upper‐crusal rocks around Bayan Obo and concluded that anomalously high geothermal gradients led to extensive crustal mobilization and enrichment of the REE minerals to form the ore deposits. We suggest that these thermal perturbation events were also responsible for resetting isotopic clocks (e.g., (U‐)Th‐Pb systems) of REE minerals, ultimately causing uncertainties in the mineralization ages of the Bayan Obo deposit and geological reconstructions of the region. Key Points: Archean‐Paleoproterozoic basement rocks in the northern North China continental arc have young 40Ar/39Ar ages of 290−220 MaAn unusually warm upper‐crust around Bayan Obo had high geothermal gradients of 50.0 ± 8.3°C/km to 88.3 ± 8.3°C/km during the late PaleozoicThe world's first largest Bayan Obo rare earth element deposit was strongly modified during arc construction in the late Paleozoic [ABSTRACT FROM AUTHOR]

To understand the oxygen fugacity (fO2) during subduction initiation, we examined proto‐arc boninites and associated mantle peridotites from the Troodos ophiolite, Cyprus. The Troodos ophiolite represents an exhumed piece of oceanic lithosphere that formed during subduction initiation in Neo‐Tethys in Late Cretaceous. Dunites and surrounding harzburgites in the Kokkinorotsos podiform chromite deposit of the ophiolite were formed by reactions between peridotites and ascending boninitic magmas. The olivine‐chromite oxybarometers show that the fO2 values (∆QFM −0.41 ± 0.37) of the dunites are slightly higher than those (∆QFM −0.85 ± 0.52) of the harzburgites under mantle conditions, indicating that the mantle fO2 was raised limitedly by percolation of the boninitic magmas. Meanwhile, the boninitic upper pillow lavas in the Margi area also recorded relatively reduced fO2 values (∆QFM −0.52 ± 0.20) during olivine‐chromite crystallization, comparable to the fO2 of mid‐ocean ridge basalts. Shallow‐level processes (i.e., magma ascent, crystallization, crustal assimilation, and degassing) appear to have limited influence on the estimated fO2 values in Troodos. The similarly low fO2 values of the mantle peridotites and boninitic magmas suggest that the Troodos primary boninitic magmas were perhaps not as oxidized as the Izu‐Bonin‐Mariana (IBM) proto‐arc boninitic magmas, though both mantle sources have captured the slab fluids released during subduction inception. This difference might be attributed to the different nature of the slab components (i.e., a more reduced subduction input for Troodos). Our study thus suggests that subducted slabs might not always release oxidized fluids, and the oxidation processes might be different for each nascent subduction zones. Plain Language Summary: Subduction zones have played a key role in mass exchange between the Earth's surface and its interior, and are one of the main factories for the generation of Earth's magmas. The redox state of arc lavas produced at subduction zones controls the formation of important mineral deposits. In fact, subduction zone lavas are believed to be generally more oxidized than mid‐ocean ridge lavas, but the origin of their oxidized nature and how their redox state change during the birth of a subduction zone remains poorly understood. Boninites are special Mg‐ and Si‐rich but Ti‐poor lavas (SiO2 > 52 wt.%, MgO > 8 wt.%, and TiO2 < 0.5 wt.%), and are typical products of magmatism that formed during subduction initiation. Here, we show that the primary boninitic magmas of the Troodos ophiolite of Cyprus are in fact no more oxidized than mid‐ocean ridge basalts, even though their mantle source interacted with the slab fluids released from the incipient subducted plate. Therefore, we propose that proto‐arc magmas, that formed as the down‐going slab began foundering, might not always be oxidized during the inception of a subduction zone. Key Points: Percolation of boninitic magmas raised the oxygen fugacity of the Troodos mantle peridotites to a limited extentThe primary boninitic magmas of the Troodos ophiolite may not be more oxidized than mid‐ocean ridge basaltsThe oxidation processes of the Troodos proto‐arc mantle may be different from those of the Izu‐Bonin‐Mariana proto‐arc mantle [ABSTRACT FROM AUTHOR]

The compositions of chromitites and dunites from Moho transition zone (MTZ) of the Coto block of the Zambales ophiolite, Philippines, are used to investigate the geodynamic transition from anhydrous to hydrous magmatism during subduction initiation (SI). Chromite grains in the chromitites have Cr# values [100 × Cr/(Cr + Al)] and TiO2 contents ∼35–50 and 0.05–0.30 wt.%, respectively, intermediate between those of chromite in typical MORB‐like lavas (Cr#, ∼20–60; TiO2, ∼0.6–1.7 wt.%) and boninites (Cr#, ∼70–85; TiO2, <0.4 wt.%). Olivine grains in the dunites have δ7Li values varying from ∼−2‰ to +21‰ with most between +10‰ and +15‰, beyond that of normal mantle (+4 ± 2‰) but comparable to those of some arc lavas (up to +12‰). The data set indicates that parental magmas of the high‐Al chromitites originated from hydrated harzburgitic mantle sources and formed temporally between MORB‐like and boninitic magmatism during SI, resulting from the early stage of flux melting in the Zambales proto‐forearc mantle. Modeling of Li diffusion reveals that the MTZ cooled down at a minimum rate of 0.1°C/yr in order to preserve the large δ7Li variation of olivine in the dunites, comparable to the thermal conditions below ultra‐slow to slow spreading ridges. Such a stage of transitional magmatism, although displaying notable slab contributions, took place at a sluggish period of slab rollback and asthenospheric upwelling, leading to a trough level of heat flow and magma production during the entire course of SI. Plain Language Summary: Subduction initiation (SI) is a prerequisite for starting plate movement. It is featured by rollback of subducted oceanic slabs in the mantle and upward flow of deep mantle materials into the shallow mantle wedges, resulting in mid‐ocean ridge‐like spreading settings above the retreating slabs and mid‐ocean ridge basalt‐like (MORB‐like) lavas. Heated by surrounding hot mantle, slabs gradually release fluids into the overlying mantle, inducing H2O‐rich magmatism such as boninitic ones. Although MORB‐like and boninitic lavas are accepted to originate from H2O‐poor and H2O‐rich mantle sources, respectively, few details were revealed on how transition between the two contrasting types of magmatism is achieved. This work did chemical analyses and modeling on some SI‐related rocks that show affinity to transitional MORB‐boninitic magmatism. Our results show the transitional magmas originated from mantle sources chemically similar to those of boninites, but formed under cooler conditions than both MORB‐like and boninitic lavas. Development of such transitional magmatism reveals that the sources of SI‐related magmas became increasingly rich in fluids, and there was a cooling period between the MORB and boninitic magmatism, possibly due to slow slab rollback and sluggish upwelling of deep hot mantle below the SI‐induced spreading centers during the transitional period. Key Points: Parental magmas of high‐Al chromitites were derived from hydrous harzburgitic mantleHigh‐Al chromitites formed at the early stage of flux melting during subduction initiationThe δ7Li variation of olivine in dunites reveals a period of sluggish slab rollback [ABSTRACT FROM AUTHOR]

Scandium is important in modern technology and is regarded as a strategic metal in many countries. It is highly dispersed in Earth's crust and rarely forms independent minerals. Clinopyroxene is the most important Sc-bearing mineral in some world-class deposits hosted in mafic–ultramafic intrusions, which are also the major source of laterite-hosted Sc deposits. However, the factors controlling Sc distribution in minerals have been little explored, impeding the understanding of the geochemical behavior of Sc and why it is common in some clinopyroxene grains. The newly discovered Mouding Sc deposit in SW China is hosted in a zoned intrusion composed, from core to rim, of monzogabbro, syenogabbro, gabbro, magnetite clinopyroxenite, and clinopyroxenite. Clinopyroxene in the intrusion is diopsidic in composition with high Sc contents (80–105 ppm). In-situ trace element mappings of diopside crystals reveal homogeneous, zoned, swallow-tailed, and hourglass internal Sc distribution patterns. These patterns can be produced through kinetically controlled incorporation of Sc on different crystal faces. The preferential substitution of Sc can take place on the {1 0 0}, {1 1 0} and {0 1 0} prism faces because of the high flexibility of the octahedral M1 protosites. The fast growth of diopside, which facilitates kinetically controlled crystallization, is dominated by textural coarsening and promoted by the hydrous parental magmas with low viscosities and active convection. The active flow and efficient interstitial communication of the magma can direct compatible elements from the magma into clinopyroxene, thus favoring formation of Sc-rich grains. Our study provides a feasible way to study intra-grain variations of Sc in minerals and emphasizes that kinetic effects may play a critical role in Sc distribution and enrichment in hydrous magmatic Sc deposits. We also show that disequilibrium crystallization may be more pervasive than previously thought, and the hourglass zoning of clinopyroxene can provide valuable information on this process. [ABSTRACT FROM AUTHOR]

• The main mineralization age of the Xianquan deposit is the Early Ordovician (478 ± 33 Ma). • Metals (e.g., Hg and Tl) in the Xiangquan deposit were mainly sourced from venting hydrothermal fluids. • The ore-forming fluids of the Xiangquan deposit probably originated from circulated, evolved seawater which picked up metals from country rocks. Sediment-hosted stratiform sulfide (SHSS) deposits commonly contain large amounts of strategic metals, such as Tl, Hg, Cd, Ge and Ga, however, sources of these metals remain debate. Here we present new elemental and Hg isotopic data to constrain the metal sources for the Xiangquan Tl deposit (South China), a unique SHSS deposit with 250 t Tl at an average grade of 928 ppm. Stratiform pyrite orebodies are hosted in the Ordovician marine sedimentary succession of mostly limestone with minor mudstone. Pyrite, the dominant ore mineral, is rich in Hg (up to 1360 ppm). Bulk ores and pyrite separates from this deposit have significant variations of δ202Hg (−5.48 to −0.65 ‰) and Δ199Hg (−0.01 to 0.36 ‰). The Δ199Hg values are mostly positive, similar to those of seawater and marine sediments. Ore petrography, trace element enrichment coefficients, rare earth element patterns, Re-Os ages (478 ± 33 Ma) and S isotope compositions of pyrite indicate that Hg and other metals were derived from hydrothermal fluids venting at the seafloor. We suggest that the ore-forming fluids of the Xiangquan deposit were predominantly originated from large-scale circulation of evolved seawater, which leached Hg and Tl from the marine country rocks. These metal-rich fluids migrated upward, vented to the seafloor, and mixed with anoxic H 2 S-rich seawater to precipitate Hg- and Tl-rich pyrite. Our study highlights that the combined application of Hg isotopes and trace elements could be used to trace the sources of metals in hydrothermal ore deposits such as SHSS deposits. [ABSTRACT FROM AUTHOR]

Microbial and hydrothermal venting activities on the seafloor are important for the formation of sediment-hosted stratiform sulfide (SHSS) deposits. Fe isotopic compositions are sensitive to both microbial and hydrothermal activities and may be used to investigate the formation of these deposits. However, to the best of our knowledge, no Fe isotopic studies have been conducted on SHSS deposits. In the Devonian Dajiangping SHSS-type pyrite deposit (389 Ma), South China, laminated pyrite ores were precipitated from exhalative hydrothermal fluids, whereas black shales were deposited during intervals with no exhalation. Pyrite grains from black shales mostly display positive δ56Fe -py (0.01–0.73‰), higher than marine sediments (ca. 0‰), due to pyrite deriving Fe from basinal shuttled Fe(III) (hydr-)oxides and slowly crystallizing in pores of sediments with equilibrium fractionation, except for negative δ56Fe -py (−0.17‰ to −0.24‰) of two samples caused by mixing of Fe from underlain laminated ores. The positive δ34S -py (3.50–24.5‰) of black shales reflect that sulfur of pyrite originated from quantitative reduction of sulfate in closed pores of sediments. In contrast, pyrite grains of laminated ores have negative δ56Fe -py (−0.60‰ to −0.21‰), which were not only inherited from the negative δ56Fe of hydrothermal fluids but also caused by kinetic fractionation during rapid precipitation of a pyrite precursor (FeS) in hydrothermal plumes. These ores have negative δ34S -py (−28.7‰ to −1.82‰), because H 2 S for pyrite mineralization was produced by bacterial sulfate reduction (BSR) in a sulfate-rich seawater column or shallow sediments. The δ56Fe -py values of laminated ores co-vary positively with δ34S -py and δ13C -carbonate along the ore stratigraphy, with δ13C -carbonate values ranging from −12.0‰ to −2.50‰. However, they correlate negatively with aluminum-normalized total organic carbon (TOC/Al 2 O 3). Organic carbon is thus considered to enhance the production of H 2 S by BSR activities, increase pyrite precipitation rates and promote the expression of kinetic fractionation of Fe isotopes. Intriguingly, in the ore units with vigorous hydrothermal venting activities, δ56Fe -py , δ34S -py and δ13C -carbonate values display a consistently increasing trend. Such results suggest that venting hydrothermal fluids significantly inhibited the H 2 S production of BSR, which then reduced the pyrite crystallization rate and decreased the kinetic fractionation of Fe isotopes. Our study reveals that the formation of SHSS deposits relies on H 2 S from microbial activities and metals from hydrothermal exhalation on the seafloor, but that vigorous exhalation can inhibit microbial activities and thus sulfide precipitation rates. The integrated use of Fe, S, and C isotopes can effectively elucidate these dynamic interactions between hydrothermal venting and microbial activities during the formation of SHSS deposits. [ABSTRACT FROM AUTHOR]

• Ediacaran-Cambrian marine sediments deposited at the northern margin of Gondwana have abnormally high Hg contents. • Hg anomalies across the Ediacaran-Cambrian transition were dominantly led by an enhanced terrestrial input rather than volcanic eruptions. • Shallow ocean O 2 -deficiency led by oxidative weathering and ocean upwelling and was causally linked to the disappearance of Ediacara biota. Significant perturbations in ocean redox states have been thought to be one of the major causes for the biological evolution from the Ediacara biota to the Cambrian fauna. However, the driving force of these changes in the ecosystem across the Ediacaran–Cambrian (E–C) boundary is still a matter of debate. In this study, we reported stratigraphic variations of Hg content and Hg isotopes in E–C sediments from the Mussoorie section, Lesser Himalaya, India. Dolomites from the late Ediacaran Krol D and E members have increasing Hg/Al ratios and decreasing Δ199Hg values from the base upwards, which were most likely originated from a prolonged and enormous terrestrial-Hg input into the E–C ocean. The overlying cherty phosphorites in the lowest Tal Group are highly rich in Re relative to Mo, indicative of a suboxic-anoxic depositional environment on the earliest Cambrian shelf that was coincident with the disappearance of the Ediacara biota. Such an O 2 -deficient condition was possibly led by a consumption of atmospheric O 2 during the oxidative weathering of sedimentary organic matter and shallow-water eutrophication, with evidence from the coupled δ202Hg and δ13C values in late-Ediacaran dolomites and high Cd/TOC and Zn/TOC ratios in early Cambrian phosphorites. The overlying black shales have decreasing contents of Hg and micronutrients (Cd and Zn) and increasing values of Δ199Hg, representing notable declines of terrestrial flux and seawater nutrient at the Cambrian Stage 2 before the major phase of Cambrian diversification. Therefore, we conclude that enhanced continental erosion and pervasive upwelling had led to major changes in redox state and nutrient content in shallow seawaters on shelves, which in turn had promoted the biological evolution across the E–C boundary. [ABSTRACT FROM AUTHOR]

• The Pobei mafic–ultramafic complex is composed of multiple intrusive phases emplaced at ca. 283 Ma. • Multiple injections of magma from a deeper chamber to the shallow magma chamber to form the current Pobei complex. • The ultramafic bodies were formed from the injection of olivine- and sulfide-laden crystal slurry and have the potential of Ni-Cu sulfide mineralization. • The gabbroic body with strong crustal contamination and high differentiation has the reef type Cr-V-PGE ore potential. A number of mafic–ultramafic intrusions with variable degrees of Ni-Cu sulfide mineralization occur in the northeastern Tarim Block and East Tianshan Orogenic Belt (NW China). Among them, the Pobei complex is composed of a gabbro body intruded by several ultramafic bodies including Poyi and Poshi. These bodies have similar mineralogy and chemical compositions and were derived from a common magma. Olivine crystals from the Poyi and Poshi bodies generally have high Fo contents with the highest Fo value of 89.5 and Ni up to 3300 ppm Ni, indicative of crystallizing from Ni-undepleted, high-Mg basaltic magmas. Sr-Nd-Pb isotopic compositions show that more mafic rocks show relatively low degrees of crustal contamination, whereas more evolved rocks experienced higher degrees of contamination, especially in the upper part of the complex. Olivine crystals from both the Poyi and Poshi bodies show sharp decrease of Ni with decreasing Fo values, indicating sulfide segregation in these two bodies. Gabbros may have potential of reef-type chromite mineralization at the deep level, and V-bearing magnetite mineralization at higher stratigraphic level corresponding to the current surface level, but the potential of PGE mineralization is low. We propose that the ultramafic bodies of the Pobei complex may have been formed from the injection of olivine- and sulfide-laden crystal slurry from the bottom of a large magma chamber. The Ni-Cu mineralization was triggered by the crustal contamination. The Pobei complex was formed by the multiple intrusions of magmas to form the current complex. [ABSTRACT FROM AUTHOR]

• Devonian ironstone deposits in South China are typical of stratiform deposits formed near costal shore lines. • Hydrothermal fluids were involved with variable contributions from continental weathering. • These deposits are spatially associated with Devonian SEDEX pyrite and Mn deposits, but they formed in different rifting basins. • Hydrothermal activities in rifting basins were essential in the formation of various stratiform deposits in a passive continental margin. Devonian ironstone deposits in South China are widely distributed and are major Fe resources of China. They are spatially associated with coeval SEDEX sulfide and Mn deposits. Stratiform orebodies of these ironstone deposits are part of the middle and late Devonian shallow marine sedimentary sequences. The main ironstone layers are interlayered with silty or muddy shales and limestones. The majority of these deposits contain hematite ores and some deposits contain siderite ores. Hematite ores display ooidal textures with Fe-rich ooids and they have Hg isotopic compositions indicative of multiple sources of metals, but trace elemental concentrations are supportive of sources of Fe from seafloor hydrothermal fluids. The Devonian ironstone deposits in South China were formed in a passive continental margin under shallow water conditions. These deposits and coeval SEDEX sulfide and Mn deposits were formed in the same continental shelf but in different sub-basins when the South China Block drifted northward after it was separated from Gondwana. [ABSTRACT FROM AUTHOR]

Rare earth elements (REEs) have become increasingly important to our modern society due to their strategical significance and numerous high technological applications. Regolith-hosted heavy rare earth element (HREE) deposits in South China are currently the main source of the HREEs, but the ore-forming processes are poorly understood. In these deposits, the REEs are postulated to accumulate in regolith through adsorption on clay minerals. In the Zudong deposit, the world's largest regolith-hosted HREE deposit, clay minerals are dominated by short, stubby, nanometer-scale halloysite tubes (either 10 or 7 Å) and microcrystalline kaolinite in the saprolite and lower pedolith and micrometer-sized vermicular kaolinite in the humic layer and upper pedolith. A critical transformation of the clay minerals in the upper pedolith is coalescence and unrolling of halloysite to form vermicular kaolinite. Microcrystalline kaolinite also transformed to large, well-crystalline vermicular kaolinite. This transformation could result in significant changes in different physicochemical properties of the clay assemblages. Halloysite-abundant clay assemblages in the deep regolith have specific surface area and porosity significantly higher than the kaolinite-dominant clay assemblages in the shallow soils. The crystallinity of clay minerals also increased, exemplified by decrease in Fe contents of the kaolinite group minerals (from ~1.2 wt% in the lower saprolite to ~0.35 wt% in the upper pedolith), thereby indicative of less availability of various types of adsorption sites. Hence, halloysite-abundant clay minerals of high adsorption capacity in deep regolith could efficiently retain the REEs released from weathering of the parent granite. Reduction in adsorption capacity during the clay transformation in shallow depth partially leads to REE desorption, and the released REEs would be subsequently transported to and adsorbed at deeper part of the soil profile. Hence, the clay-adsorbed REE concentration in the lower pedolith and saprolite (~2500 ppm on average) is much higher than the uppermost soils (~400 ppm on average). Therefore, weathering environments that favor the release of the REEs in the shallow soils but preservation of halloysite in the deep regolith can continuously adsorb REEs in the clay minerals to form economically valuable deposits. [ABSTRACT FROM AUTHOR]

The role of recycled slab-mantle interaction in the formation of mantle-derived rocks during the Paleo-Pacific subduction remains to be tested. In this study Nb-enriched basalts (NEBs) are identified in Shanggao, western part of the Gan-Hang belt, South China. These volcanic rocks have 40Ar/39Ar ages of ∼90 Ma. They are tholeiitic to calc-alkaline in composition and have low SiO 2 (50.1–53.4 wt%), high MgO (5.86–7.10 wt%), and Nb (10.7–14.3 ppm) contents. These NEBs have high Nb/La (0.91–0.99) and Nb/U (14.8–21.0) ratios and chondrite-normalized patterns with slightly enriched light rare earth elements (LREEs) ((La/Yb) N = 5.32–6.52). Their primitive mantle-normalized patterns are similar to E -MORB-like characteristics. These features indicate that these NEBs were derived from a mixed source between the asthenospheric mantle and slab-derived adakitic components in the garnet stability field. Their Sr-Nd-Pb-Hf isotopic compositions also reflect the contribution of the subducted oceanic crust including minor sediments in the source. These NEBs are spatially associated with adakitic rocks, A-type granites, and related mineral deposits in the Gan-Hang belt, clearly supporting a tectonic model in which the Cretaceous ridge subduction and slab window opening occurred during the enhanced slab rollback of the Paleo-Pacific plate. The melting of residual eclogites caused by episodic upwelling of the asthenospheric mantle can account for the formation of the NEBs in a back-arc extensional setting. • We report new findings of Nb-enriched basalts (NEBs) in the Gan-Hang Belt. • The basalts yield 40Ar/39Ar ages of ∼90 Ma. • The NEBs were derived from the depleted mantle enriched by slab-derived melts. • The NEBs correspond to the slab window opening during ridge subduction. [ABSTRACT FROM AUTHOR]

This Special Issue brings together a variety of recent studies on mostly Tethyan ophiolites in China, Pakistan, Turkey, Albania, Cyprus, and Iran, as well as two papers dealing with ophiolites in Cuba and one reporting on chromitites in the Philippines. These papers cover a range of topics ranging from magmatic processes in mantle peridotites, to hydrothermal alteration and associated mineralization in ophiolites as well as continuing attempts to understand the origin and evolution of the enigmatic Xigaze ophiolite of Tibet, which has puzzled multiple authors for decades. With the recognition that most ophiolites were either formed in, or modified by suprasubduction zones, the focus of ophiolite studies has shifted from discoveries and descriptions of new bodies to specific, detailed investigations of magmatic processes to better understand the roles played by mantle mineralogy, geochemistry, partial melting, magma mixing, and SSZ fluids in the mantle and crustal sequences. Thus, nearly half of the papers in this Issue are based on these topics. And a key question addressed by several of the studies is the extent to which the records of these processes have been overprinted or even obliterated by mantle processes. [ABSTRACT FROM AUTHOR]

Combined Fe isotopic and olivine fabric analyses have been conducted on the nodular chromitites from the Luobusa ophiolite, Tibet, to study the physico-chemical effects of plastic flow in modifying chromitites and their associated mantle peridotites below spreading centers. The Luobusa nodular chromitites are generally composed of fresh dunitic matrices and well-aligned chromite nodules. Olivine grains in both the dunitic matrices and chromite nodules underwent pressure solution, and those in the dunitic matrices have prevalent Fe-rich stripes parallel to their kink bands and sometimes show lattice dislocation-induced preferred orientations of (1 0 0). Chromite grains in the chromite nodules have Cr# ∼ 75–80 and δ56Fe values from −0.122 to 0.067. The Fo and δ56Fe values of olivine in the dunitic matrices vary from 96 to 99 and 0.165 to 0.294, respectively, distinguishably higher than those of olivine in ordinary ophiolitic dunites (mostly 90–95 and 0.050–0.150). The calculated Mg-Fe exchange temperatures between olivine in the dunitic matrices and chromite in the chromite nodules range from 700 to 900 °C, overall higher than those for ordinary dunites and chromitites (∼600–800 °C). Combined with numerical modeling of diffusion of Fe in olivine, our study reveals that the abnormally high Fo and δ56Fe values of olivine in the dunitic matrices of nodular chromitites were not controlled by their parental magmas but caused by significant loss of Fe from olivine under subsolidus conditions. Intensive plastic deformation in nodular chromitites largely promoted the migration of Fe from olivine and facilitated the Mg-Fe exchange between the chromite nodules and dunitic matrices, maximizing the Fo and δ56Fe values of olivine. However, such an effect is expected to be negligible in chromite-barren dunites and harzburgites because such rocks do not have enough chromite to generate effective chemical-potential gradients for Fe migration. The high Fo values of olivine in nodular and massive chromitites require fast cooling rates (∼0.05–0.1 °C/yr) in the surrounding mantle, consistent with settings below intermediate-fast spreading ridges. Boudinization of small chromitite bodies in less competent dunites under 900–1200 °C is a potential mechanism for generating the nodular texture, also accounting for the en-echelon or bead-like occurrences of chromitite bodies in ophiolites. Effective mantle flow below spreading centers plays an important role in shaping the textures and compositions of mantle rocks, explaining the absence of deformation-related nodular chromitites in layered mafic-ultramafic intrusions. [ABSTRACT FROM AUTHOR]

Abstract Titanium (Ti) is typically hosted in detrital minerals in marine sediments and has long been considered to be immobile during diagenesis. In this study, the authigenic titania minerals, brookite and anatase, are observed in early Cambrian carbonaceous shales from the Meishucun and Zhajin sections of South China, respectively. Black shales in the Meishucun section have total organic carbon (TOC) contents from 1.6 to 3.9 wt% and HI (hydrogen index) values from 3.8 to 20 mg HC/g TOC, whereas black shales in the Zhajin section have much higher TOC (7.1–15.6 wt%) but lower HI (<2.0 mg HC/g TOC) and contain abundant bitumen (~3 vol%). Brookite in black shales from the Meishucun section crystallized invariably along cleavages of detrital biotite. This intimate association suggests that the Ti required to form titania minerals was derived from detrital biotite and that Ti was mobilized only on a nano- to micro-meter scale. In contrast, anatase aggregates in black shales from the Zhajin section are intergrown with bitumen. It is proposed that Ti in these shales was mobilized in low pH organic-rich fluids and, subsequently, preferentially precipitated as anatase with increasing pH. The mobilization of inert Ti in black shales is indicative of a possible diagenetic or post-diagenetic elemental redistribution in carbonaceous rocks. Our study further demonstrates that proxies based on geochemical component of black shales can be employed to estimate redox state of ancient oceans, but must be applied with caution. [ABSTRACT FROM AUTHOR]

Abstract The Li-O isotopes of olivine and pyroxene from the mantle sequence of the Luobusa ophiolite, a fragment of Neo-Tethyan forearc lithosphere in Tibet, reveal a series of magmatic processes and geochemical evolution in a nascent mantle wedge during subduction initiation. Olivine grains from the ophiolitic mantle sequence display δ18O values similar to those of normal mantle (+5.18 ± 0.2‰), but have large δ7Li variations, ∼+4‰ to +13‰ in harzburgites and ∼+1‰ to +11‰ in high-Cr chromitites. Clinopyroxene grains in cpx-bearing harzburgites have finger-like protrusions and LREE-depleted patterns comparable to those of clinopyroxene in abyssal peridotites, and also show high Li concentrations (∼2–8 μg/g) and negative δ7Li values (∼−15‰ to −8‰), which are beyond the ranges of normal upper mantle (<1.3 μg/g and +4 ± 2‰, respectively). Our dataset suggests that the nascent Luobusa harzburgitic mantle wedge was penetrated by limited amounts of slab-derived fluids at an early stage, leading to decoupling of high δ7Li but normal mantle-like δ18O values of the mantle rocks. The hydrated harzburgites were later refertilized by depleted mantle-derived melts. This process generated the cpx-bearing harzburgites with light-δ7Li clinopyroxene, and also imposed normal mantle-like Li isotopic signatures on some previously hydrated high-δ7Li mantle areas at deeper levels. Subsequent melting in both the high-δ7Li and normal mantle-like Li isotopic mantle regimes generated the parental magmas of the high-Cr chromitites with high-Ca boninitic affinities, accounting for the large δ7Li variation of olivine in the chromitites. Slab rollback below the nascent mantle wedge induced asthenospheric upwelling and a high geothermal gradient, which played a more direct role than slab dehydration in triggering the magmatic events of refertilization and formation of the high-Cr chromitites. Numerical modeling of inter-mineral Li diffusion in cpx-bearing harzburgites suggests an interval of <1000 years between the two magmatic events and rapid slab rollback before the generation of the high-Cr chromitites. Intra-oceanic emplacement of the Luobusa proto-forearc lithosphere was possibly finished within 106 years after the chromitite formation. [ABSTRACT FROM AUTHOR]

• The weathering crusts have thicknesses typically of more than 10 m regolith over alkaline complexes in the ELIP. • Regolith over the Jijie and Shuikoujing complexes has Sc concentrations two to five times richer than parent rocks. • ∼1,100 to ∼3,500 tonnes of Sc metal are estimated as Exploration Target in the Luoci region, ELIP. • Regolith-hosted Sc deposits over alkaline complexes are potentially important Sc resources. Scandium (Sc) is one of several important critical metals with increasing applications in modern industry. However, Sc is highly dispersed in nature and is rarely sufficiently concentrated to form Sc-dominated deposits. Therefore, finding new Sc resources is now urgently needed to ensure future sustainable supplies. We report, for the first time, the potential Sc resources in regolith developed over pyroxenites of the Jijie and Shuikoujing alkaline complexes of the Permian Emeishan Large Igneous Province (ELIP), SW China. The weathering profiles are variable, but typically consist of soft pedolith in the upper part and solid saprolith in the lower part. Overall, the pedolith and saprolith in Jijie and Shuikoujing have average Sc grades of 83.2 ppm to 91.5 ppm and 57.4 ppm to 58.3 ppm, respectively, which are two- to five-fold those of fresh bedrocks. There are abundant pyroxene relicts in both the pedolith and saprolith. Scandium is mainly hosted in montmorillonite and goethite, which may be much easier and more suitable for mining than the bedrock. At the early stage of weathering, Sc concentrations correlate positively with the abundances of secondary montmorillonite. The highest Sc concentration coincides with the appearance of goethite which is the major host of Sc during the late stage of weathering. However, Sc is depleted in the transition zone between the saprolith and pedolith, which might be ascribed to the absence of both montmorillonite and/or goethite. Our preliminary study estimates ∼1,100 to ∼3,500 tonnes of Sc metal in the regolith developed above the two alkaline complexes. With the increasing market interest and improvements in recovery methods, these Sc resources in Jijie and Shuikoujing can be considered potentially important Sc deposits. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The world's W and Sn reserves are about 3.1 and 12 million tons, among which South China accounts for around 45% and 11%, respectively. • W-Sn deposits are concentrated in the Nanling region of central South China and the Yangtze River region of northeastern South China. • Recent studies of W-Sn mineral deposits in South China have advanced our understanding of the distribution and origin of these deposits. • This special issue brings together the latest information in 23 papers that deal with many aspects of the Mesozoic W-Sn mineral deposits. [ABSTRACT FROM AUTHOR]

The Neoproterozoic (840 Ma) Sin Quyen deposit in northwestern Vietnam contains replacement Cu-LREE-Au orebodies in Proterozoic metasedimentary rocks. In this deposit, LREE-bearing minerals include allanite-(Ce), monazite-(Ce), chevkinite-(Ce), and fluorapatite. Fluorapatite from orebodies has undergone variable degrees of metasomatic alteration. Samarium-neodymium isotopic analyses were conducted on altered fluorapatite, and also on allanite-(Ce) and monazite-(Ce), to investigate whether such metasomatism can affect the Sm-Nd isotope system. Allanite-(Ce) and monazite-(Ce) have 147Sm/144Nd ratios ranging from 0.0359 to 0.0549, and 143Nd/144Nd ratios from 0.51147 to 0.51172. Their initial 143Nd/144Nd values at the time of mineralization range from 0.51126 to 0.51148, but mostly cluster between 0.51135 and 0.51145. Thus, the primary ore-forming fluids were relatively homogeneous in their Sm-Nd isotopic compositions. In the 147Sm/144Nd vs. 143Nd/144Nd diagram, the compositions of allanite-(Ce) and monazite-(Ce) generally plot along a Sm-Nd isochron of 840 Ma, implying that the Sm-Nd isotopic systems of these minerals were either closed or only slightly modified. In contrast, altered fluorapatite crystals have 147Sm/144Nd ratios varying from 0.0667 to 0.1348, and 143Nd/144Nd ratios from 0.51160 to 0.51199. The calculated initial 143Nd/144Nd ratios range widely from 0.51114 to 0.51141, with most values lower than those of the allanite-(Ce) and monazite-(Ce). In the 147Sm/144Nd vs. 143Nd/144Nd diagram, their compositions mostly plot below the 840-Ma Sm-Nd isochron. Petrographic observations and trace elemental analyses show that metasomatic modification of fluorapatite grains led to increases of their Sm/Nd ratios. The unaltered domains in the grains have Sm/Nd ratios varying from 0.114 to 0.200, with an average value of 0.161; whereas the altered domains have Sm/Nd ratios varying from 0.111 to 0.254, with an average value of 0.183. The increased Sm/Nd ratios can cause the calculated initial 143Nd/144Nd ratios to be lower than actual initial isotopic ratios, and can also result in compositional deviations from the reference Sm-Nd isochron. This study demonstrates that the traditionally assumed inert Sm-Nd isotopic system can be metasomatically disturbed due to changes in the Sm/Nd ratio. Therefore, care must be taken when interpreting the Sm-Nd isotopic data from apatite/apatite-rich rocks that have undergone metasomatic alteration. [ABSTRACT FROM AUTHOR]

SEDEX sulfide deposits hosted in black shale and carbonate are common in the South China Block. The Dajiangping pyrite deposit is the largest of these deposits and is made up of stratiform orebodies hosted in black shales. Sandstone interlayered with stratiform orebodies contains detrital zircon grains with the youngest ages of 429 Ma. Pyrite from the orebodies has a Re-Os isochron age of 389 ± 62 Ma, indicative of formation of the hosting strata and syngenetic pyrite ores in the mid-late Devonian. The hosting strata is a transgression sequence in a passive margin and composed of carbonaceous limestone in the lower part and black shales in the upper part. The ore-hosting black shales have high TOC (total organic carbon), Mo, As, Pb, Zn and Cd, indicating an anoxic-euxinic deep basin origin. The high redox proxies, V/(V + Ni) > 0.6 and V/Cr > 1, and the positive correlations of TOC with Mo and V in black shales are also consistent with an anoxic depositional environment. The Dajiangping deposit is located close to the NE-trending Wuchuan-Sihui fault, which was active during the Devonian. The mid-late Devonian mineralization age and the anoxic-euxinic deep basinal condition of this deposit thus imply that the formation of this deposit was causally linked to hydrothermal fluid exhalation in an anoxic fault-bounded basin that developed in a carbonate platform of the South China Block. The regional distribution of many Devonian, stratiform, carbonaceous sediment-hosted sulfide deposits along the NE-trending fault-bounded basins in South China, similar to the Dajiangping deposit, indicates that these deposits formed at a basin developed in the passive margin setting of the South China Block during the Devonian. This environment was caused by the break-up and northward migration of the South China Block from Gandwana. [ABSTRACT FROM AUTHOR]

The Baoshan scheelite deposit, located in the Nanling region, South China, is a skarn-type tungsten ore deposit. Orebodies occur in the contact zone between Carboniferous limestone and Baoshan granitic pluton. The pluton consists mostly of coarse-grained, porphyritic, and fine-grained biotite granites, which have zircon U-Pb ages of 172.4 ± 0.3 Ma, 166.6 ± 0.3 Ma, and 156.6 ± 1.4 Ma, respectively. These rocks have similar geochemical characteristics with high SiO 2 , total alkalis, high differentiation indexes and high Rb/Sr ratios of S-type granites formed by partial melting of upper continental crust. The porphyritic granites display the highest degree of fractionation, and are most closely associated with the formation of skarn ores. The skarns are zoned mineralogically outward from garnet, through clinopyroxene, to wollastonite, and overprinted by retrograde minerals. Three stages of skarnization are recognized: Early prograde, late prograde and retrograde. The early prograde stage is mainly composed of Al-rich garnet (Grs 60–76 ) and Mg-rich clinopyroxene (Di 41–62 ) with subordinate Mo-rich scheelite (Sch 59–86 ), all of which are typically overgrown by late prograde assemblages of Fe-rich garnet (And 67–95 ), Fe-rich clinopyroxene (Hd 81–87 ), and Mo-depleted scheelite (Sch 97–98 ). The retrograde stage is characterized by Al-rich garnet (Grs 40–66 ) and pure scheelite (Sch 100 ), accompanied by hydrous and sulfide minerals. Minor subcalcic garnet with magnetite sometimes occurs in the limited endoskarns. Fluid inclusions of the early prograde minerals homogenize at 390.5 to >550 °C, and one inclusion in wollastonite has a salinity of 13.0 equiv. wt% NaCl. Inclusions in minerals of the late prograde and retrograde stages have homogenization temperatures of 196.9–377.4 °C and 160.4–291.4 °C, respectively; their salinities 2.4–19.0 equiv. wt% NaCl and 1.4–11.6 equiv. wt% NaCl, respectively. Accordingly, the high-temperature and moderate- to high-salinity fluids in the early skarn stages are typical of magmatic fluids originating granites, whereas the lower temperature and variable salinity in the later stages suggest mixing with meteoric water. [ABSTRACT FROM AUTHOR]

Scheelite is the main ore mineral in skarn-type tungsten deposits, and a common accessory mineral in a variety of rock-types. The Baoshan deposit in South China is one of the most important polymetallic scheelite skarn deposits in China, hosting 40,000 t of WO 3 with economic concentrations of Zn, Cu, and Ag. It is hosted by a calcic skarn that is zoned outwards mineralogically from garnet-clinopyroxene, through clinopyroxene-garnet, to wollastonite, and overprinted by retrograde minerals. Scheelite occurs in both the prograde and retrograde skarns, and is complexly zoned. On the basis of its textures, the scheelite was classified into three types. Scheelite I and II belong to the early and late prograde stages, respectively, and Scheelite III precipitated during the retrograde stage. The molybdenum (Mo) content of these scheelite types ranges from 54 ppm to 24 wt%, and the total rare earth element content ranges from 12 to 321 ppm. Rare earth element (REE) concentrations and chondrite-normalized REE profiles vary with the distribution of major elements. The profiles indicate variable degrees of REE enrichment, which correlates negatively with the Mo content. Molybdenum-rich scheelite displays a negative Eu anomaly, and Mo-poor scheelite a positive Eu anomaly. Crystal structure provided the first-order control on the minor and trace element composition of the scheelite. Incorporation of REE 3 + into scheelite was controlled partly by a coupled substitution involving Mo. The lattice strain model was used to estimate scheelite-fluid partition coefficients for the REE from the contents of these elements in the scheelite and to predict the relative distributions of the REE in the ore-forming fluids. It is proposed that conditions were initially oxidizing, leading to strong incorporation of Mo in Scheelite I, that they became more reducing with the crystallization of Scheelite II containing lesser Mo, and that during retrograde skarn formation there was a return to oxidizing conditions due to an influx of meteoric waters, which altered Scheelite II giving rise to the formation of Scheelite III. The study shows that the composition of scheelite recorded the history of the Baoshan hydrothermal system, and that the behaviour of the REE could be used to quantitatively reconstruct the changing physicochemical conditions during ore formation. [ABSTRACT FROM AUTHOR]

[Display omitted] • LA-ICP-MS in-situ U-Pb age of garnet in pegmatite is 216.7 ± 5.9 Ma. • Potentially two pulses of pegmatitic melts were emplaced in Jiajika. • Garnet grains show decoupled major and trace element zonings. • Incipient pegmatitic melts had high Li contents as recorded by garnet cores. Granitic pegmatites can be extremely rich in Li, an important metal for modern industries. Being the Asia's largest hard-rock Li polymetallic deposit, the Jiajika Li deposit is associated with numerous pegmatite dikes in the eastern Tibetan Plateau. The major Li resources in Jiajika are mainly hosted in fine-grained phases of pegmatite dikes. In this study, samples were collected from the typically zoned No. 9 dike, which inwardly consists of fine-grained albite phase, medium-grained albite phase, coarse-grained albite-microline phase, massive microline phase and massive quartz phase. Garnet is one of the common accessory minerals in the fine-grained albite phase. We report the first in-situ LA-ICP-MS U-Pb age of garnet from pegmatite dikes, yielding an age of 216.7 ± 5.9 Ma, which is consistent with the age of the associated granitic pluton. Combined with published isotopic dating results in Jiajika, two age groups can be identified: ∼210 to ∼ 220 Ma and ∼ 190 to ∼ 200 Ma, which are suspected to be resulted from two stages of thermal events, probably separate pulses of pegmatitic melts. The late pulse of melts might be responsible for the weak deformation of garnet grains and Fe-enriched rims along the deformed boundaries. The early pulse of melts derived from the Majingzi-Jiajijiami granitic pluton shows an indistinct time path from granite, to fine-grained phases, to medium- to coarse-grained phases, which resulted in the trace elemental core-rim textures within individual garnet grains. The garnet cores have typically higher trace element concentrations than the rims, and the boundaries between cores and rims are usually sharp and sinuous, which are proposed to record the amalgamation, dissolution and reprecipitation processes. The growth of cores was not in equilibrium with other phases by strong undercooling, and thus inherited the initial Li signature of the incipient pegmatitic melts, which can be up to 373 ppm. Accordingly, it implies that Li and other incompatible element contents of the incipient pegmatitic melts in Jiajiaka could be enriched in a significant amount at the early stage, which may, to some extent, explain why major Li resources are hosted in very-fine to fine-grained phases in Jiajika. [ABSTRACT FROM AUTHOR]

It has been known that the lithospheric mantle underneath eastern China is geochemically heterogeneous, with different isotopic compositions across the South China and North China blocks. The distribution of Cenozoic basalts (<23 Ma) with abundant mantle xenoliths in eastern China is mainly controlled by lithospheric-scale faults. In this paper, we report new Li and O isotopic data for these mantle xenoliths and demonstrate that the mantle was variably modified via the Pacific subduction. Most olivine and orthopyroxene grains from the peridotite xenoliths in the South China block display lower δ18O values (4.43 to 5.77‰ and 4.77 to 5.55‰; respectively) than those of normal mantle, possibly reflecting impregnation of the slab melt of altered gabbroic precursors in the mantle transition zone (MTZ). The positive correlation between Li (1.40–6.54 ppm) and δ7Li (−28.41 to 20.83‰) of olivine grains suggests that the SCLM was metasomatized by hydrated slab-derived, isotopically heavier Li melts penetrating the frontal mantle wedge. The decoupling of Li and O isotopic systems implies that both hydrated mantle wedge and MTZ control the mass transportation and melt/fluid flow circulation beneath South China via the Pacific subduction. Comparatively, the higher δ18O values (4.90–6.41‰) and negative correlations between Li (1.39–6.02 ppm) and δ7Li (−23.55 to 22.54‰) in olivine grains from the North China block suggest a major contribution from the eclogitic slab that was metasomatized by the carbonatitic agent. Our findings corroborate the wide storage of carbonated eclogites in the MTZ beneath North China. We highlight that the stagnant slab in the MTZ might have controlled the refertilization of the lithospheric mantle beneath North China. [ABSTRACT FROM AUTHOR]

The Mianhuadi mafic complex in the Ailao Shan-Red River (ASRR) shear zone, SW China, consists of granulite and mylonite, formed by granulite-facies peak metamorphism and retrograde amphibolite-facies metamorphism, with later local mylonitic deformation. Textures and mineral relics indicate that the protolith, represented by the mineral assemblage (M 0 ) of olivine, clinopyroxene, plagioclase and Fe-Ti oxides, was emplaced as a mafic intrusion in the Late Permian as part of the Emeishan large igneous province. Two kinds of metamorphic mineral assemblages were identified: the peak assemblage (M 1 ) of garnet + orthopyroxene + clinopyroxene + plagioclase + magnetite + ilmenite, and the retrograde assemblage (M 2 ) composed of hornblende surrounding orthopyroxene and garnet, and hornblende + spinel symplectite around garnet. Pressure-temperature estimates yield 657–722 °C and 7–9 kbar, and 649–676 °C and 5–6 kbar for the M 1 and M 2 stages, respectively, roughly defining a clockwise P-T path with near-isothermal decompression following the peak metamorphism. These metamorphic conditions are quite unlike those (pressure up to 14 kbar) of the Late Permian-Middle Triassic metamorphic event in the region. In addition, foliated biotite from mylonites has an 40 Ar- 39 Ar plateau age of 34.3 ± 0.4 Ma, indicating that the ASRR shear zone was active at or before the Late Eocene. Combined with previous data, our new results indicate that the granulite-facies metamorphism of the Mianhuadi mafic complex occurred most likely during the Eocene, followed by rapid exhumation and ductile sinistral shearing in the Late Eocene. The formation of the granulite was likely related to the upwelling of the asthenosphere due to the delamination of overthickened lithospheric mantle after the India-Asia collision. [ABSTRACT FROM AUTHOR]

The Sin Quyen deposit in northwestern Vietnam is composed of Fe-Cu-LREE-Au ore bodies hosted in Proterozoic metapelite. There are massive and banded replacement ores with variable amounts of monazite-(Ce) and chevkinite-(Ce) crystals, which have undergone fluid-induced alteration. Monazite-(Ce) and chevkinite-(Ce) were deposited from high-temperature fluids in the early ore-forming stage, but became thermodynamically unstable, and thus were altered to other phases in later ore-forming stages. The alteration of monazite-(Ce) formed a three-layered corona texture, which commonly has relict monazite-(Ce) in the core, newly formed fluorapatite in the mantle, and newly formed allanite-(Ce) in the rim. In some cases, the original monazite-(Ce) was completely consumed, forming a core of polygonal fluorapatite crystals rimmed by allanite-(Ce) crystals. The formation of allanite-(Ce) and fluorapatite at the expense of monazite-(Ce) indicates that the later-stage fluids had high Ca/Na ratios and relatively low temperatures. Chevkinite-(Ce) was variably replaced by an assemblage of allanite-(Ce) + aeschynite-(Ce) ± bastnäsite-(Ce) ± columbite-(Fe) ± ilmenite. The replacement of chevkinite-(Ce) by mainly allanite-(Ce) and aeschynite-(Ce) required low-temperature, Ca-, LREE-, and Nb-rich metasomatic fluids, probably with relatively low fO2. Mass-balance calculations were made to investigate the hydrothermal element mobility. It is assumed that Th was immobile during the alteration process of monazite-(Ce). Light (and middle) REE from La to Tb, U, As, and Ge were variably lost relative to Th, while heavy REE from Dy to Lu, HFSE (e.g., Nb, Ta, Zr, and Hf) and Sr were variably gained relative to Th. Regarding the alteration of chevkinite-(Ce), some major elements in chevkinite-(Ce), such as Ti, La, and Ce, were obviously removed from the system during alteration, whereas Ca, Al, Nb, U, and HREE were needed to be variably supplied by the metasomatic fluids. Concerning the hydrothermal mobility of trace elements, previous studies demonstrated that REE and HFSE can be commonly reserved in the system during alteration, consistent with the traditionally assumed immobile nature of these elements. In contrast, this study shows that REE and HFSE can be mobilized on at least the hundreds of micrometers scale. This may be related to the high flux and strong chemical reactivity of the metasomatic fluids. [ABSTRACT FROM AUTHOR]

South China hosts the most abundant and largest tungsten (W) deposits in the world, being a famous W metallogenic region. Located at the eastern part of the South China Block, which was formed by amalgamation of the Yangtze and Cathaysia Blocks during the Neoproterozoic, these W deposits were mainly formed during the Mesozoic. The W mineralization is dominanted by greisen, quartz-vein, skarn, and porphyry types, all of which are genetically related to the evolution of highly fractionated granitoids. Four episodes of W mineralization are recognized: (1) Late Triassic (230–210 Ma) in the central and western parts of South China; (2) Middle Jurassic (ca. 170 Ma) to Early Cretaceous (ca. 140 Ma) in the interior of South China, with the mineralization being concentrated in southern Jiangxi Province between 165 and 150 Ma; (3) Early Cretaceous (136–120 Ma) with deposits across South China; and (4) Late Cretaceous (100–80 Ma) mainly in the southwestern parts of South China. These four periods of mineralization are closely related to the closure of paleo-Tethys and subduction of the paleo-Pacific plate. In the Late Triassic, these two events caused local extensional environments, facilitating emplacement of the peraluminous granitoids, and formation of the W deposits. In the Middle Jurassic, break-off of the subducting oceanic plate resulted in emplacement of highly fractionated granites in the Nanling region. Later anticlockwise rotation of the paleo-Pacific plate created widespread S-type granitoids and associated Middle Jurassic to Early Cretaceous W mineralization in the interior of South China. Since 136 Ma, rollback of the subducting Pacific plate resulted in weak W mineralization across South China. Finally, a change of direction in the retreating plate from SE to ESE resulted in intensive mineralization of the southwestern part of South China. [ABSTRACT FROM AUTHOR]

The North Kostobe gold deposit in the Kalba gold province, eastern Kazakhstan, is tectonically located in the Chara shear zone of the western Altaids. The Chara shear zone separates the Kazakhstan microcontinent and Siberia craton which collided in the late Carboniferous. In the North Kostobe deposit, Au mineralization is distributed along an E-W striking shear zone linked to the NW-SE trending regional faults, and is mainly concentrated in quartz-sulfide-carbonate veins or in disseminated sulfides hosted in carbonaceous metasedimentary rocks. Sulfide minerals are mainly pyrite and arsenopyrite, and were formed in three generations. The first generation is dominated by pyrite (py1) occurring as microcrystal aggregates, followed by the second generation including euhedral arsenopyrite and compact pyrite (py2) locally overgrowing early py1 grains as rims; third generation of pyrite (py3) is present in barren micro-fractures crosscutting early mineralized rocks. Native gold is present in cracks of brecciated arsenopyrite grains and as inclusions in py2. Investigations on fluid inclusions in auriferous quartz veins indicate that the ore-related fluids are CO 2 -bearing, with homogenization temperatures of 288 °C and low salinity (1.42 to 8.03 wt% NaCl equiv). The fluids have calculated δ 18 O and δD values ranging from 9.96 to 11.86‰ and from − 75 to − 97.1‰, respectively, suggesting that they were most likely metamorphic in origin. The first and second generations of ore sulfides have similar δ 34 S values (− 6 to + 2.6‰) which possibly indicating a common sulfur source as the hosting sedimentary rocks. In contrast, py3 grains have a huge range of δ 34 S values (− 40 to + 54.5‰), indicating a biogenic source for the sulfur. On the other hand, in-situ Pb isotopic compositions of different generations of sulfides show a similarly mixed lead source, possibly involved orogen, mantle and lower crust Pb reservoirs. We propose that the gold-bearing fluids of the North Kostobe gold deposit were most likely derived from gold-rich, pyritic carbonaceous sedimentary rocks through dehydration during the metamorphism that was related to the collision between the Kazakhstan microcontinent and Siberia craton in the late Carboniferous. It is thus concluded that the North Kostobe gold deposit shares many characteristics with typical orogenic gold deposits in terms of tectonic settings, mineralization styles, fluid compositions and source of fluids and gold. Our new findings thus have important implications for regional exploration in the Kalba gold province. [ABSTRACT FROM AUTHOR]

The paleo-Moho transition zone (MTZ) of the Kop ophiolite in NE Turkey is mainly composed of dunites, which are locally interlayered with chromitites and contain minor relics of harzburgites. Large Fe isotopic variations were observed for magnesiochromite (− 0.14 to 0.06‰) and olivine (− 0.12 to 0.14‰) from the MTZ rocks. In individual samples, magnesiochromite has lighter Fe isotopic compositions than olivine, which was probably caused by subsolidus Mg–Fe exchange between them. Both magnesiochromite and olivine display an increasing trend of δ 56 Fe along a profile from chromitite to dunite. This trend reflects continuous fractional crystallization in a magma chamber, which resulted in heavier Fe isotopes concentrated in the evolved magmas. In each cumulate cycle of chromitite and dunite, dunite was formed from relatively evolved melts after massive precipitation of magnesiochromite. Mixing of more primitive and evolved melts in the magma chamber was a potential mechanism for triggering the crystallization of magnesiochromite, generating chromitite layers in the cumulate pile. Before mixing happened, the primitive melts had reacted with mantle harzburgites during their ascendance; whereas the evolved melts may lie on the olivine–chromite cotectic near the liquidus field of pyroxene. Variable degrees of magma mixing and differentiation are expected to generate melts with different δ 56 Fe values, accounting for the Fe isotopic variations of the Kop MTZ. [ABSTRACT FROM AUTHOR]

Both prograde and retrograde skarns from the Tengtie iron deposit, South China, contain rounded, euhedral, and anhedral zircon grains. Rounded grains were originally derived from detritus in carbonate rocks and were incorporated into the skarns. Euhedral and anhedral crystals are intergrown with various skarn minerals and are clearly hydrothermal in origin. These hydrothermal grains have low (Sm/La)N ratios and high La contents relative to typical magmatic ones and display fat LREE and subdued fattening of HREE chondrite-normalized patterns, similar to those of zircon crystallized from Zr-saturated fluids. Prograde skarns also contain baddeleyite rimed by zircon, which record a period of low Si activity during prograde skarnization relative to original magmatic-hydrothermal fluids. Hydrothermal zircon grains from Tengtie have variable Eu anomalies and slightly positive Ce anomalies, indicating that they may have crystallized from highly heterogeneous, but generally reducing fluids. They have low δ18O values (-5.1 to -2.7 ‰), suggesting the involvement of meteoric fluids. Fluorine-rich fluids played an important role in remobilizing and transporting some high field strength elements (HFSE), including Zr, from the host granites into the skarn system. Reaction between HFSE-bearing fluids and carbonate rocks at the prograde stage decomposed F complexes to deposit HFSE-rich skarn minerals and baddeleyite. At the retrograde stage, alteration of the HFSE-rich skarn minerals released HFSE, including Zr and Sn, consequently producing a mineral assemblage of zircon, cassiterite, and retrograde skarn minerals. Dating results of zircons from the Tengtie skarn system by SIMS indicates roughly less than several million years duration for skarnization. Our study indicates that Zr was not only mobile locally under favorable conditions, but was also readily transported and deposited in different stages of skarnization. [ABSTRACT FROM AUTHOR]

The Emeishan large igneous province (ELIP) in SW China is characterized by voluminous high-Ti and low-Ti basalts and spatially associated Fe-Ti oxide-bearing mafic-ultramafic and syenitic/granitic intrusions. The Baima layered mafic intrusion in the central part of the ELIP is surrounded by syenitic and granitic rocks and contains a Lower Zone of interlayered Fe-Ti oxide ores, troctolites and clinopyroxenites and an Upper Zone of isotropic olivine gabbros and gabbros (UZa) and apatite gabbros and Fe-Ti-P oxide ores (UZb). Polycrystalline mineral inclusions, for the first time, were observed in primocryst plagioclase from the basal part of the UZa through to the top of the UZb and consist mostly of clinopyroxene, plagioclase, magnetite, ilmenite and apatite with minor orthopyroxene, sulfide and hornblende. These minerals are commonly anhedral and form irregular shapes. Daughter plagioclase usually crystallizes on the walls of host primocryst plagioclase and has An contents typically 3–6 An% lower than the host plagioclase. Daughter clinopyroxene has similar Mg# but lower TiO 2 and Al 2 O 3 contents than primocryst clinopyroxene. These polycrystalline mineral inclusions are considered to crystallize from melts contemporaneous with host plagioclase. The compositional differences between daughter and primocryst minerals can be attributed to equilibrium crystallization in a closed system of the trapped melt inclusions in contrast to fractional crystallization and possible magma replenishment in an open system typical for primo-cumulates of large layered intrusions. Heated and homogenized melt inclusions have variable SiO 2 (33–52 wt%), CaO (7–20 wt%), TiO 2 (0.1–12 wt%), FeO t (5–20 wt%), P 2 O 5 (0.2–10 wt%) and K 2 O (0–2.2 wt%). The large ranges of melt compositions are interpreted to result from heterogeneous trapping of different proportions of immiscible Si-rich and Fe-Ti-rich silicate liquids, together with entrapment of various microphenocrysts. The separation of micrometer-scale Si-rich melts at least started from the lower part of the Upper Zone. We thus infer that the Baima high-Ti basaltic magmas evolved into the field of immiscible Fe-rich and Si-rich melts. Prolonged fractional crystallization of olivine, plagioclase and clinopyroxene from the Fe-rich melts formed gabbro and olivine gabbro in the UZa, before the melts became saturated in Fe-Ti oxides and apatite and formed apatite-rich Fe-Ti oxide ores in the UZb. Immiscible Si-rich melts migrated upwards and coalesced to form fayalite syenite around the Baima intrusion. Silicate liquid immiscibility may have played an important role in the petrogenesis of Fe-Ti oxide-rich layered mafic intrusions and syenitic rocks in the ELIP and elsewhere. This mechanism may be common and may explain the traditionally thought bi-modal assemblages of large igneous provinces in intra-continental extensional settings. [ABSTRACT FROM AUTHOR]

The Jiangnan Fold Belt in the South China Block has been traditionally assumed to be Mesoproterozoic in age and related to the global Grenville orogeny. Sedimentary successions in the Jiangnan Fold Belt archive direct record of tectonic evolution; however, they have not yet been evaluated properly. The Lushan massif, comprising Kangwanggu and Xingzi groups, is the major basement complex in the Jiangnan Belt. Regional correlation of these two groups is poorly constrained, such as with the Shuangqiaoshan group, and thus their role in the regional tectonic evolution is not clear. Detrital zircon U–Pb ages suggest that the Xingzi and Kangwanggu groups were deposited at 820–810 and ca. 830 Ma, respectively. They are composed of dominantly felsic to intermediate volcanic detritus, as indicated by the relatively high Th/Cr (0.24–0.06) ratios and radiogenic Nd isotopes (ε Nd(t) values = + 1.5 to − 2.9) of the sedimentary rocks. An overwhelming abundance of Neoproterozoic (ca. 860–810 Ma) angular, detrital zircon grains in both the groups indicates derivation chiefly from locally distributed syn-sedimentary igneous rocks. A predominance of zircons with ages close to the time of deposition implies a convergent plate margin setting for Kangwanggu and Xingzi groups. Geochemical signatures, such as La-Th-Co and Th-Sc-Zr/10 plots for Xingzi and Kangwanggu sedimentary rocks also underline tectonically active settings, consistent with the arc affinity of the associated mafic and felsic volcanic rocks. In contrast to the dominant Neoproterozoic detritus in the Kangwanggu sandstone, argillaceous rocks of the Xingzi group received additional input of pre-Neoproterozoic detritus. Moreover, the Xingzi argillaceous rocks have ε Nd(t) values (+ 0.9 to − 2.9) slightly lower than those of the Kangwanggu sandstones (+ 1.5 to 0.0), indicating contribution from mature crustal materials exposed during progressive uplift of continental basement during orogenesis. These features suggest the Kangwanggu and Xingzi groups' record in a convergent continental margin environment during the formation of the Jiangnan Fold Belt. Persistence of the back-arc basin until ca. 811 Ma suggests that the South China Block occupied a peripheral position in the configuration of the Rodinia supercontinent. [ABSTRACT FROM AUTHOR]

• The ELIP in SW China has a component of carbonatite-alkaline complexes that were not previously well known. • Pyroxenites of these complexes are composed of clinopyroxene with high Sc contents and can form low grade but high tonnage of Sc ores. • Exploration Targets for Sc in pyroxenites in three representative complexes have ∼12,000 to ∼36,000 tonnes of Sc metal at grades of ∼30 to ∼70 ppm. • Carbonatite-alkaline complexes of the ELIP may become a potentially important Sc metallogenic province and may be targets for exploration of REEs. Economically viable Sc deposits in the world are scarce and new and alternative resources are now urgently needed to ensure future sustainable supplies. We report here an essentially unknown, yet potentially important, type of Sc deposit that is associated with carbonatite and alkaline igneous complexes of the Permian Emeishan Large Igneous Province (ELIP), SW China. The ELIP is well known for its variably mineralized intrusions and flood basalts. Associated alkaline igneous complexes are minor in volume but important as possible sources of Sc and other REEs. Several such complexes occur in the N-S-trending Kangdian belt, where they are typically composed of pyroxenite, gabbro, and nepheline syenite. Some complexes also contain jacupirangite, melteigite, ijolite, and urtite. We show that some of these complexes contain carbonatites. The alkaline mafic-ultramafic rocks typically belong to either sodic or potassic magmatic series with high alkali compositions (up to 15.4 wt% Na 2 O + K 2 O). Pyroxenites in these complexes are composed of clinopyroxene with variable amounts of apatite and magnetite and have 39 to 71 ppm Sc. The Sc concentrations of these rocks may be high enough to justify mining under the current prices of Sc, which is nearly 5 times that of gold. Our preliminary study indicates a minimum of ∼12,000 to ∼36,000 tonnes of Sc metal in three representative complexes. This type of mineralization may ultimately contribute to a reliable and sustainable Sc supply chain in the future. In addition to Sc in the pyroxenites, carbonatites in these complexes will also be potentially important targets for REE exploration. [ABSTRACT FROM AUTHOR]

Samples of an early olivine gabbro in the lower plutonic sequence of the Troodos ophiolite were studied to constrain their thermal history and the compositions of their parental magma. These rocks exhibit typical orthocumulate textures and petrographic evidence of reaction between a migrating interstitial magma and a crystal mush, including embayed margins of olivine crystals and irregular ragged grain boundaries of clinopyroxenes, indicating dissolution of olivine and crystallization of clinopyroxene, followed by crystallization of plagioclase within the framework of olivine and clinopyroxene. All the minerals exhibit relatively homogeneous compositions, without any clear zoning in either their major or trace elements, indicating that the minerals and migrating magma reached equilibrium. The rare earth element (REE) distribution between clinopyroxene and plagioclase records an equilibrium temperature of about 1300 °C, but the Mg-exchange geothermometer yielded a lower temperature of about 1000 °C. These different closure temperatures indicate a rapid cooling history with a cooling rate of −2 °C/year log units. The low TiO 2 contents and additional trace element compositions of clinopyroxene show that the parental magma of the gabbro was related to boninitic magmas of the ophiolite. Consequently, we show that some boninitic lavas in the upper oceanic crust have been affected by melt-rock reaction process during their ascent through the lower crust, and therefore their composition cannot be directly used to constrain the composition of the primary boninites. • Gabbros of the Troodos ophiolite display petrographic evidence of reaction between a migrating magma and a crystal mush. • Trace elements of clinopyroxene indicate that the migrating magma is related to boninites in the UPL. • Some boninitic lavas affected by melt-rock reaction process cannot represent composition of the primary boninite. [ABSTRACT FROM AUTHOR]

The Yinachang deposit in the Kangdian region, Southwest China, contains large amounts of Fe, Cu and REE, and formed at ∼1700 Ma. In this deposit, there are three stages of alteration and mineralization, including pre-ore Na–(Fe) alteration, Fe–(REE) mineralization, and Cu–(REE) mineralization. In the Fe–(REE) mineralization stage, REE-rich fluorapatite, with total REE concentrations ranging from 10,700 to 34,000 ppm, formed together with magnetite. In the following Cu–(REE) mineralization stage, large amounts of REE, especially LREE, were leached out of the REE-rich fluorapatite due to the interaction between fluorapatite and Cl-, F-, CO 2 -, and Ca-rich, but REE-unsaturated fluids. The leaching of REE was associated with the removal of Si, Na, Th, U, Pb, and Ba, and modification of the oxygen isotope signature in the fluorapatite. During a ∼840 Ma tectonothermal event, REE-rich fluorapatite underwent the second interaction with oxidized, F-, CO 2 -, and possibly Cl-rich, but Na- and Ca-deficient fluids. Due to fluid–fluorapatite interaction, REE were removed from the fluorapatite, but were immediately reincorporated into new phases within the fluorapatite. Thus, the altered fluorapatite contains abundant REE mineral inclusions, including bastnäsite-(Ce), monazite-(Ce), and minor xenotime-(Y). A very small portion of the LREE were transported out of the fluorapatite, and formed bastnäsite-(Ce) and monazite-(Ce) grains in the vicinity of the altered fluorapatite. In addition to the metasomatism of fluorapatite, allanite and “primary” synchysite-(Ce) from the Cu–(REE) mineralization stage were also altered with variable replacement of allanite by an assemblage of synchysite-(Ce) + chlorite ± bastnäsite-(Ce) and “primary” synchysite-(Ce) by bastnäsite-(Ce). These styles of alteration were possibly synchronous with the second alteration phase of the fluorapatite. This study demonstrates that REE can be mobilized during multiple stages of hydrothermal activities and highlights the significant controls fluid compositions have on REE transportation and deposition in hydrothermal system. [ABSTRACT FROM AUTHOR]

The Early Cretaceous Shangzhuang Fe-Ti oxide-bearing layered mafic intrusion in the Yanshan Belt northeast of Beijing is coeval with the giant Late Mesozoic igneous province in the eastern part of the North China Craton (NCC). This magmatic event was associated with lithospheric thinning and thus the igneous rocks have been used to characterize the nature of the Mesozoic mantle beneath the NCC. The Shangzhuang mafic pluton intruded a large granodioritic complex and crystallized at ~ 850–872°C at a depth of 13–14 km. It is composed, from the base upward, of troctolite, Fe-Ti oxide-bearing gabbronorite and gabbro. Rocks from this intrusion have low initial 87 Sr/ 86 Sr ratios (0.7053–0.7058), negative initial ε Nd values (− 9.4 to − 10.7), highly differentiated LREE and nearly flat HREE patterns indicative of an EMI-like mantle source unaffected by upper crustal contamination. The occurrence of Fe-Ti oxide ore layers, magnetite-ilmenite exsolution lamellae in hornblende and high TiO 2 contents of the silicate rocks indicate that they formed from Fe- and Ti-rich ferrobasaltic magmas, which may have been generated by addition of magmas from a deeper mantle source. The presence of orthopyroxene, high-Mg ilmenite (up to 8.5 wt %), hornblende, biotite and high oxygen fugacities calculated from coexisting titanomagnetite-ilmenite pairs can be explained by derivation from an enriched EMI-type mantle modified by fluids from a subducted slab and mixed with asthenospheric or deeper-mantle materials in an extensional setting. Exposure of the complex occurred during large-scale uplift (at least 13 km) and exhumation of the Yanshan orogenic belt in the Early Cretaceous. Chemical metasomatism triggered by water and enhanced by heat from a deep magma source may have played an important role in removing the ancient cratonic root, generating partial melting of the lithospheric mantle and producing coeval magmatic activity in the Mesozoic eastern NCC. [ABSTRACT FROM AUTHOR]

The Tuwu porphyry Cu deposit in the eastern Tianshan Orogenic Belt of southern Central Oceanic Orogen Belt contains 557 Mt ores at an average grade of 0.58 wt.% Cu and 0.2 g/t Au, being the largest porphyry Cu deposit in NW China. The deposit is genetically related to dioritic and plagiogranitic porphyries that intruded the Carboniferous Qieshan Group. Ore minerals are dominantly chalcopyrite, pyrite and enargite. Porphyric diorites have Sr/Y and La/Yb N ratios lower but Y and Yb contents higher than plagiogranites. Diorites have highly variable Cu but nearly constant PGE contents (most Pd = 0.50–1.98 ppb) with Cu/Pd ratios ranging from 10,900 to 8,900,000. Plagiogranites have PGEs that are positively correlated with Cu and have nearly uniform Cu/Pd ratios (5,100,000 to 7,800,000). Diorites have concentrations of Re (0.73–15.18 ppb), and 187 Re/ 188 Os and 187 Os/ 188 Os ratios lower but common Os contents (0.006–0.097 ppb) higher than plagiogranites. However, both the diorites and plagiogranites have similar normalized patterns of rare earth elements (REE), trace element and platinum-group elements (PGEs). All the samples are characterized by the enrichments of LREE relative to HREE and display positive anomalies of Pb and Sr but negative anomalies of Nb and Ta in primitive-mantle normalized patterns. In the primitive mantle-normalized siderophile element diagrams, they are similarly depleted in all PGEs but slightly enriched in Au relative to Cu. Our new dataset suggests that both the diorite and plagiogranite porphyries were likely evolved from magmas derived from partial melting of a wet mantle wedge. Their parental magmas may have had different water contents and redox states, possibly due to different retaining time in staging magma chambers at the depth, and thus different histories of magma differentiation. Parental magmas of the diorite porphyries are relatively reduced with less water contents so that they have experienced sulfide saturation before fractional crystallization of silicate minerals, whereas the relatively more oxidized parental magmas with higher water contents of the plagiogranite porphyries did not reach sulfide saturation until the magmatic-hydrothermal stage. Our PGE data also indicates that the Cu mineralization in the Tuwu deposit involved an early stage with the enrichments of Au, Mo and Re and a late stage with the enrichment of As but depletion of Au–Mo. After the formation of the Cu mineralization, meteoric water heated by magmas penetrated into and interacted with porphyritic rocks at Tuwu, which was responsible for leaching Re from hosting rocks. [ABSTRACT FROM AUTHOR]

Magnetite is common in many ore deposits and their host rocks, and is useful for petrogenetic studies. In the Khetri copper belt in Rajasthan Province, NW India, there are several Cu-(Au, Fe) deposits associated with extensive Cu ± Fe ± Au ± Ag ± Co ± REE ± U mineralization hosted in phyllites, schists and quartzites of the Paleoproterozoic Delhi Supergroup. Ore bodies of these deposits comprise dominantly disseminated and vein-type Cu-sulfide ores composed of chalcopyrite, pyrite, and pyrrhotite intergrown with minor magnetite. There are also Fe-oxide ores with minor or no Cu-sulfides, which are locally overprinted by the mineral assemblage of the Cu-sulfide ores. In addition to the Fe-oxide and Cu-sulfide ores, the protolith of the Delhi Supergroup includes banded iron formations (BIFs) with original magnetite preserved (i.e. magnetite-quartzites) and their sheared counterparts. In the sheared magnetite-quartzites, their magnetite and quartz are mobilized and redistributed to magnetite and quartz bands. Trace elemental compositions of magnetite from these types of ores/rocks were obtained by LA-ICP-MS. The dataset indicates that different types of magnetite have distinct concentrations of Ti, Al, Mg, Mn, V, Cr, Co, Ni, Zn, Cu, P, Ge and Ga, which are correlated to their forming environments. Magnetite grains in magnetite-quartzites have relatively high Al (800–8000 ppm), Ti (150–900 ppm) and V (300–600 ppm) contents compared to those of BIFs in other regions such as the Yilgarn Craton, Western Australia and Labrador, Canada. The high Al, Ti and V contents can be explained by precipitation of the magnetite from relatively reduced, Al–Ti-rich water possibly involving hotter, seafloor hydrothermal fluids derived from submarine mafic volcanic rocks. Magnetite in sheared magnetite-quartzites is generally irregular and re-crystallized, and has Ni, Mn, Al, Cu and P contents lower than the magnetite from the unsheared counterparts, suggesting that the shearing-related mobilization is able to extract these elements from original magnetite. However, elevated contents of Ti, V, Co, Cr, Ge and Mg of the magnetite in the sheared magnetite-quartzites can be ascribed to involvement of external hydrothermal fluids during the shearing, consistent with occurrence of some hydrothermal minerals in the samples. Compositions of magnetite from the Fe-oxide and Cu-sulfide ores are interpreted to be controlled mainly by fluid compositions and/or oxygen fugacity ( f O 2 ). Other potential controlling factors such as temperature, fluid–rock interaction and co-precipitating minerals have very limited impacts. Magnetite in the Cu-sulfide ores has higher V but lower Ni contents than that of the Fe-oxide ores, likely indicating its precipitation from relatively reduced, evolved fluids. However, it is also indicated that the two types of magnetite do not show large distinctions in terms of concentrations of most elements, suggesting that they may have precipitated from a common, evolving fluid. We propose a combination of Ge versus Ti/Al and Cr versus Co/Ni co-variation plots to discriminate different types of magnetite from the Khetri copper belt. Our work agrees well with previous studies that compositions of magnetite can be potentially useful for provenance studies, but also highlights that discrimination schemes would be more meaningful for deposits in a certain region if fluid/water chemistry and specific formation conditions reflected in compositions of magnetite are clearly understood. [ABSTRACT FROM AUTHOR]

The Sokoman Iron Formation in the Labrador Trough, Canada, a typical granular iron formation (GIF), is coeval with the ~ 1.88 Ga Nimish volcanic suites in the same region. It is composed of the Lower, Middle and Upper Iron Formations. In addition to primary and altered magnetite in iron formations of the Hayot Lake, Rainy Lake and Wishart Lake areas, magnetite in volcanic breccia associated with the iron formation is identified for the first time in the stratigraphy. Trace elemental compositions of the most primary, altered and volcanic brecciated magnetite of the Sokoman Iron Formation were obtained by LA–ICP-MS. Commonly detected trace elements of magnetite include Ti, Al, Mg, Mn, V, Cr, Co and Zn. These three types of magnetite have different trace elemental compositions. Primary magnetite in the iron formation has a relatively narrow range of compositions with the depletion of Ti, Pb, Mg and Al. Magnetite from volcanic breccia is rich in Ti, Al, V, Mn, Mg, Zn, Cu and Pb, indicative of crystallization from mantle-derived magmas. Altered magnetite in the iron formation shows a relatively wide range of trace elemental compositions. Mineralizing fluids associated with magmas that generated the ~ 1.88 Ga Nimish volcanic suites circulated through the sedimentary piles to further enrich the iron formations and to form magnetite with variable compositions. The comparisons of different types of primary and altered magnetite in the iron formation in the region show distinct provenance discrimination. Our findings also support the origin of iron formations in association with multiple stages of exhalative volcanic and hydrothermal processes. [ABSTRACT FROM AUTHOR]

The Taihe, Baima, Hongge, Panzhihua and Anyi intrusions of the Emeishan Large Igneous Province (ELIP), SW China, contain large magmatic Fe–Ti–(V) oxide ore deposits. Magnetites from these intrusions have extensive trellis or sandwich exsolution lamellae of ilmenite and spinel. Regular electron microprobe analyses are insufficient to obtain the primary compositions of such magnetites. Instead, laser ablation ICP-MS uses large spot sizes (~ 40 μm) and can produce reliable data for magnetites with exsolution lamellae. Although magnetites from these deposits have variable trace element contents, they have similar multi-element variation patterns. Primary controls of trace element variations of magnetite in these deposits include crystallography in terms of the affinity of the ionic radius and the overall charge balance, oxygen fugacity, magma composition and coexisting minerals. Early deposition of chromite or Cr-magnetite can greatly deplete magmas in Cr and thus Cr-poor magnetite crystallized from such magmas. Co-crystallizing minerals, olivine, pyroxenes, plagioclase and apatite, have little influence on trace element contents of magnetite because elements compatible in magnetite are incompatible in these silicate and phosphate minerals. Low contents and bi-modal distribution of the highly compatible trace elements such as V and Cr in magnetite from Fe–Ti oxide ores of the ELIP suggest that magnetite may not form from fractional crystallization, but from relatively homogeneous Fe-rich melts. QUILF equilibrium modeling further indicates that the parental magmas of the Panzhihua and Baima intrusions had high oxygen fugacities and thus crystallized massive and/or net-textured Fe–Ti oxide ores at the bottom of the intrusive bodies. Magnetite of the Taihe, Hongge and Anyi intrusions, on the other hand, crystallized under relatively low oxygen fugacities and, therefore, formed net-textured and/or disseminated Fe–Ti oxides after a lengthy period of silicate fractionation. Plots of Ge vs. Ga + Co can be used as a discrimination diagram to differentiate magnetite of Fe–Ti–(V) oxide-bearing layered intrusions in the ELIP from that of massif anorthosites and magmatic Cu–Ni sulfide deposits. Variable amounts of trace elements of magmatic magnetites from Fe–Ti–(P) oxide ores of the Damiao anorthosite massif (North China) and from Cu–Ni sulfide deposits of Sudbury (Canada) and Huangshandong (northwest China) demonstrate the primary control of magma compositions on major and trace element contents of magnetite. [ABSTRACT FROM AUTHOR]

The Nanling Range in South China hosts numerous world-class W–Sn deposits and some Fe deposits. The Mesozoic Tengtie Fe skarn deposit in the southern Nanling Range is contemporaneous with the regional Sn mineralization. The deposit is composed of numerous ore bodies along the contacts between the late Paleozoic or Mesozoic carbonate rocks and the Yanshanian Lianyang granitic complex. Interaction of the magma with hosting dolomitic limestone and limestone formed calcic (Ca-rich) and magnesian (Mg-rich) skarns, respectively. The Tengtie deposit has a paragenetic sequence of the prograde stage of anhydrous skarn minerals, followed by the retrograde stage of hydrous skarn minerals, and the final sulfide stage. Magnetite in the prograde and retrograde skarn stages is associated with diopside, garnet, chlorite, epidote, and phlogopite, whereas magnetite of the final stage is associated with chalcopyrite and pyrite. Massive magnetite ores crosscut by quartz and calcite veins are present mainly in the retrograde skarn stage. Laser ablation ICP-MS was used to determine trace elements of magnetite from different stages. Some magnetite grains have unusually high Ca, Na, K, and Si, possibly due to the presence of silicate mineral inclusions. Magnetite of the prograde stage has the highest Co contents, but that of the sulfide stage is extremely poor in Co which partitions in sulfides. Magnetite of magnesian skarns contains more Mg, Mn, and Al than that of calcic skarns, attributed to the interaction of the magma with compositionally different host rocks. Magnetite from calcic and magnesian skarns contains 6–185 ppm Sn and 61–1246 ppm Sn, respectively. The high Sn contents are not due to the presence of cassiterite inclusions which are not identified in magnetite. Instead, we believe that Sn resides in the magnetite structure. Regionally, intensive Mesozoic Sn mineralization in South China indicates that concurrent magmatic–hydrothermal fluids may be rich in Sn and contribute to the formation of high-Sn magnetite. Our study demonstrates that trace elements of magnetite can be a sensitive indicator for the skarn stages and wall-rock compositions, and as such, trace elemental chemistry of magnetite can be a potentially powerful fingerprint for sediment provenance and regional mineralization. [ABSTRACT FROM AUTHOR]

The Bayan Obo Fe-REE-Nb deposit in northern China is the world's largest light REE deposit, and also contains considerable amounts of iron and niobium metals. Although there are numerous studies on the REE mineralization, the origin of the Fe mineralization is not well known. Laser ablation (LA) ICP-MS is used to obtain trace elements of Fe oxides in order to better understand the process involved in the formation of magnetite and hematite associated with the formation of the giant REE deposit. There are banded, disseminated and massive Fe ores with variable amounts of magnetite and hematite at Bayan Obo. Magnetite and hematite from the same ores show similar REE patterns and have similar Mg, Ti, V, Mn, Co, Ni, Zn, Ga, Sn, and Ba contents, indicating a similar origin. Magnetite grains from the banded ores have Al + Mn and Ti + V contents similar to those of banded iron formations (BIF), whereas those from the disseminated and massive ores have Al + Mn and Ti + V contents similar to those of skarn deposits and other types of magmatic-hydrothermal deposits. Magnetite grains from the banded ores with a major gangue mineral of barite have the highest REE contents and show slight moderate REE enrichment, whereas those from other types of ores show light REE enrichment, indicating two stages of REE mineralization associated with Fe mineralization. The Bayan Obo deposit had multiple sources for Fe and REEs. It is likely that sedimentary carbonates provided original REEs and were metasomatized by REE-rich hydrothermal fluids to form the giant REE deposit. [ABSTRACT FROM AUTHOR]

The Tengchong volcanic field east of the Burma arc in SW China comprises numerous Quaternary volcanoes. The volcanic rocks can be grouped into four units, numbered 1–4 from oldest to youngest. Units 1, 3 and 4 are composed of trachybasalt, basaltic trachyandesite and trachyandesite, respectively, whereas Unit 2 consists of hornblende dacite. Primary minerals in the dacite include plagioclase, clinopyroxene, amphibole and magnetite, all set in a glassy groundmass. Some of the dacites are moderately to highly weathered and show effects of low temperature alteration, with loss on ignition (LOI) up to 9.6 wt% and positive correlations of LOI with Al 2 O 3 , but negative correlations with SiO 2 , CaO and Na 2 O. Fresh dacites (LOI <3 wt%) belong to the calk-alkaline series and have pronounced negative Ti, Nb and Ta anomalies. They have whole-rock 87 Sr/ 86 Sr ratios ranging from 0.7090 to 0.7101, εNd values from −10.8 to −11.8, and εHf values from −5.1 to −6.8. Many of the large plagioclase crystals in the fresh dacites have reverse zoning, ranging from An 55 in the cores to An 72 in the rims. The smaller phenocrysts have relatively uniform An values between 56 and 50. One plagioclase crystal has an An value of 33.5. Plagioclase in the fresh dacite has relatively high Sr contents and 87 Sr/ 86 Sr ratios ranging from 0.7050 to 07125 except for one value of 0.7516, whereas plagioclase in the altered dacite has low Sr contents and highly variable Sr isotopic compositions (0.7039–0.7138), indistinguishable from those of the fresh rocks. Both An values and Sr isotopic compositions of the plagioclase indicate mixing of mantle- and crustally-derived magmas. We therefore propose that mantle-derived basaltic magma caused partial melting of the lower–middle crust and that mixing of the mafic and felsic magmas produced the dacite in staging magma chambers. The dacites contain minerals crystallized from both the mafic and felsic magmas, as well as a few xenocrysts plucked from the country rocks. [ABSTRACT FROM AUTHOR]

Formation of Proterozoic massif-type anorthosites is known to be related to polybaric process involving early high-pressure crystallization of plagioclase and high-Al orthopyroxene megacrysts at mantle–crust boundary, followed by emplacement of plagioclase-dominated mushes to shallow-level crust. Therefore, the plagioclase megacrysts record important information about the magmatic sources and crystallization process of anorthosites. The ~ 1.74-Ga Damiao complex, North China, comprises > 90 vol.% anorthosites and leuconorites containing abundant plagioclase megacrysts associated with minor high-Al orthopyroxene megacrysts (HAOMs). The plagioclase megacrysts are generally euhedral to subhedral (mostly 1 to 30 cm in diameter), and some of them contain very fine lamellae of orthoclase and undefined Fe–Ti-rich minerals (1–5 μm). The HAOMs occur as subhedral grains or as angular grains with an intercumulus relationship to the plagioclase megacrysts. They contain abundant thin, regular lamellae of exsolved plagioclase (15–20 vol.%), indicative of originally high-Al features (originally 6.0–7.3 wt.% Al 2 O 3 ). Based on the Al-in-orthopyroxene geobarometry, the HAOMs and associated plagioclase megacrysts are constrained to be crystallized together at pressures of 9.4–11.2 kbar (33–36 km), indicative of their crystallization at lower crust depths or crust–mantle boundary. In contrast, orthopyroxene grains from late differentiation phases such as oxide-apatite gabbronorites do not contain plagioclase lamellae, and have much lower Al 2 O 3 contents (1.5 to 1.7 wt.%), indicating final crystallization at < 5 kbar (< 15 km). Four plagioclase megacrysts analyzed in this study have slightly different compositions, but collectively they all display comparable, oscillatory variations of An values (43–55), Sr (1100–1800 ppm), Ba (800–1400 ppm), La (2 to < 7 ppm) and 87 Sr/ 86 Sr ratios (0.70283–0.70466) from center to rim. Although Sr content is also likely related to pressures, our work suggests that the oscillatory chemical and Sr isotopic signature of the plagioclase megacrysts was mainly controlled by the compositions of the magmas, which temporally changed in the deep magma chamber. A mixing-assimilation modeling, based on Sr contents and initial 87 Sr/ 86 Sr ratios of synchronous mantle-derived mafic dykes and ancient lower crustal xenoliths in North China, suggests that the parental magma was initially a depleted mantle-derived basaltic magma that assimilated with ~ 30% lower crustal materials (Al 2 O 3 = 15–24 wt.%; Sr = 800–2000 ppm) or partial melts of the lower crust when ponding at the base of lower crust. We consider that an increasing of Al 2 O 3 in the basaltic magma due to assimilation may have triggered saturation of plagioclase, because the oscillatory chemical and isotopic patterns are consistent from center to rim in all plagioclase megacrysts, indicative of initiation of the assimilation processes at the very beginning of plagioclase crystallization. Our study supports the model that the parental magmas were derived from partial melting of the depleted mantle combined with all-important crustal contamination in deep magma chambers or during rising of crystal mushes, and this may also account for variable isotopic signature for different phases in many anorthosite suites. [ABSTRACT FROM AUTHOR]

The Yangtze Block has a Paleoproterozoic and Archean basement and was traditionally thought to have been cratonized after the Neoproterozoic. Recent studies reveal that the southwestern part is remarkably different from other parts of the block. Major Fe–Cu deposits are present in the southwestern part where they form the Kangdian IOCG metallogenic province in southern China and northern Vietnam. Large Fe–Cu deposits include Lala and Xikuangshan to the north, Yinachang in the center, Dahongshan and Sin Quyen to the south and possibly Shilu to the south-east. These deposits are characterized by an association of Fe-oxides and Cu-sulfides with REE, Mo, Co, Ag, and Au as common by-products. The ore deposits are hosted in variably metamorphosed, late Paleoproterozoic rocks (~ 1.65–1.7 Ga) of the Dahongshan, Dongchuan, Hekou, Sin Quyen, and Shilu Groups, although the age of the Shilu Group in Hainan is poorly constrained. Molybdenite separates from the Yinachang and the Lala deposits have Re–Os model ages of 1.66 and ~ 1.0 Ga. These ages suggest that the Fe–Cu deposits formed in multiple mineralization events over an exceptionally broad time range. There was a minor hydrothermal overprint at ~ 1.45 Ga. Biotite, amphibole, and quartz from several deposits have 40 Ar/ 39 Ar plateau ages ranging from 850 to 780 Ma, but cluster at 830–820 Ma. These 40 Ar/ 39 Ar ages are considerably younger than the Fe–Cu deposits and are interpreted as the result of thermal resetting of the K–Ar system during the Neoproterozoic, consistent with the extensive Neoproterozoic igneous activity in the Yangtze Block. The mineralization styles of the 1.66 Ga deposits in the southwestern Yangtze Block are similar to those of contemporaneous IOCG deposits in other blocks within the Columbia supercontinent, including those in the Leichhardt and Calvert Superbasins, the Mount Isa Inlier, Australia, and those in the Khetri copper belt, NW India. Together with similar hosting strata and associated igneous rocks, the occurrence of these deposits leads us to propose that these blocks were adjacent to each other in the reconstructed Columbia supercontinent. Both South China and India remained together during the assembly of the supercontinent Rodinia and have undergone extensive Neoproterozoic magmatism that overprinted the Fe–Cu deposits. [ABSTRACT FROM AUTHOR]

Abstract: Numerous Fe-Cu deposits with mineralization styles similar to iron oxide-copper gold (IOCG) deposits form the Kangdian Fe-Cu metallogenic province, southwestern (SW) China. As one of the largest deposits in the region, the ~1.0Ga Lala Fe-Cu deposit is hosted in a Paleoproterozoic volcanic-sedimentary succession named the Hekou Group which is alternately intruded by ~1.0Ga doleritic plutons. This deposit has a paragenetic sequence evolving from Stage I of Na-alteration to Stage II of Fe mineralization, and finally to Stage III of Cu-(Mo, REE) mineralization, coeval with mafic-felsic intra-plate magmatism in the region. This study conducted in-situ Sr isotopic analyses on apatite and carbonate, aiming to resolve the long controversial issue regarding the origin of the Fe and Cu mineralizing fluids in the deposit. Apatite of Stage II has 87Sr/86Sr ratios varying from 0.71380 to 0.72733, much higher than those of synchronous igneous rocks in the region (0.7074 to 0.7091), but similar to the Paleoproterozoic host rocks (0.71368 to 0.71837 at ~1.0Ga). This similarity indicates that radiogenic Sr of the Fe mineralizing fluid was dominantly sourced from the host rocks. Apatite and calcites of Stage III have 87Sr/86Sr ratios (0.75758–0.79293) much higher than apatite of Stage II and the host rocks but similar to the Archean basement rocks (as high as 0.80 at ~1.0Ga) beneath the cover of the Yangtze Block, suggesting that the highly radiogenic Sr isotopic composition of the Cu mineralizing fluid was mainly inherited from the old basement rocks. In combination with previous C-O-S isotopic data indicating a magma-hydrothermal origin, it was suggested that the Fe mineralizing fluid was exsolved from a mafic magma that generated the ~1.0Ga doleritic plutons, and inherited radiogenic Sr from the host rocks during fluid-rock interaction. By contrast, the Cu mineralizing fluid might have been sourced from another pulse of magmatic, Cu-Mo-REE- and CO2-rich fluid which have once interacted with Archean basement rocks prior to mineralization. The source of such a Cu-Mo-REE-rich fluid was not well constrained in current study but was inferred to be exsolved from a hidden felsic magma. We propose that intrusions of the bimodal magmas in Kangdian are responsible for regional hydrothermal circulation which led to Fe-Cu-(Mo, REE) mineralization in the Kangdian province. [Copyright &y& Elsevier]

Abstract: Iron isotopic compositions of olivine, clinopyroxene and titanomagnetite of the Baima Fe–Ti oxide-bearing layered mafic intrusion, SW China, are used to investigate Fe isotopic fractionation during the formation of the Fe–Ti oxide ore bodies and to constrain the origin of silicate minerals and Fe–Ti oxides. The Baima intrusion comprises the Lower Zone of interlayered troctolite, clinopyroxenite and oxide ores and the Upper Zone of gabbros. Significant differences of Fe isotope values between olivine ( ), clinopyroxene ( ), and titanomagnetite ( ) were observed in rocks and ores of the Lower Zone. Iron isotopic fractionation between olivine and clinopyroxene, olivine and titanomagnetite and clinopyroxene and titanomagnetite exhibits large variations of 0.06‰ to 0.22‰, 0.12‰ to 0.27‰, and 0.00‰ to 0.20‰, respectively, suggesting disequilibrium fractionation. This disequilibrium cannot be explained by thermal or chemical gradient-induced kinetic fractionation or by possible subsolidus exsolution of granular ilmenite from titanomagnetite. Instead, it could be attributed to crystallization of silicates and titanomagnetite from two immiscible Si-rich and Fe-rich liquids. Continuous segregation of the Fe-rich liquid changed the Fe isotopic composition of the Si-rich liquid and thus the Fe isotopic compositions of olivine and clinopyroxene crystallized from it, resulting in disequilibrium Fe isotopic fractionation between them. The effect of oxygen fugacity on the crystallization order of titanomagnetite and ilmenite from the Fe-rich melts, on the other hand, gave rise to the disequilibrium Fe isotope fractionation between titanomagnetite and olivine/clinopyroxene. [Copyright &y& Elsevier]

Abstract: The ~260Ma Abulangdang ultramafic intrusion in the Panxi region, SW China, is part of the Emeishan large igneous province (ELIP). The intrusion is composed of dunite and lherzolite with minor olivine gabbro. Olivine crystals in the dunite have Fo values from 89.0 to 86.5mol%. Chromite grains in both the dunite and lherzolite contain more than 0.5wt.% TiO2 and show a trend of increasing Fe3+ with increasing Fe2+/(Mg+Fe2+). Rocks of the intrusion have γOs(t) values from +0.1 to +1.2 and εNd(t) values from −1.9 to +2.9, similar to the high-Ti picrites, high-Ti flood basalts and Fe–Ti oxide-bearing, gabbroic intrusions of the ELIP. Modeling indicates that the Abulangdang intrusion formed by accumulation of olivine and chromite from a high-Ti picritic magma, whereas the high-Ti flood basalts and Fe–Ti oxide-bearing, gabbroic intrusions formed from derivative, evolved magmas after the early fractionation of minerals from the high-Ti picritic magma. Both the dunite and lherzolite have relatively high PGE concentrations and primitive mantle-normalized chalcophile element patterns with positive Os and Ru anomalies, in contrast to PGE-poor high-Ti flood basalts with negative Os and Ru anomalies. This is consistent with the retention of Ru–Os–Ir trace phases with olivine and chromite in the formation of the Abulangdang intrusion. Using a clinopyroxene geobarometer, the crystallization pressure of the Abulangdang intrusion is calculated to be 7.1 to 8.3kbar, equivalent to a depth of ~21 to 24km, which is deeper than that estimated for the Fe–Ti oxide-bearing, gabbroic layered intrusions of the ELIP (~5kbar). We propose that large volumes of mantle plume-derived high-Ti picritic magma underplating along the Moho boundary underwent early fractionation before eruption, and some high-Ti picritic magma ascended through a fossil conduit where the Abulangdang intrusion formed. Residual, evolved magmas either from the conduits or underplating magma chamber formed Fe–Ti oxide-bearing, gabbroic layered intrusions or high-Ti flood basalts. In this fashion, the Abulangdang intrusion resembles ultramafic portions that are missing in the Fe–Ti oxide-bearing, gabbroic intrusions of the ELIP. [Copyright &y& Elsevier]