Early Paleozoic Granite in the Talate Mining District, Chinese Altay, and its Geological Significance for the Altay Orogenic Belt

Zircon dating,geochemical and Nd-Sr isotopic analyses have been determined for samples from two granitic intrusions in the Talate mining district,Chinese Altay.Our data suggest that these intrusions were emplaced from 462.5 Ma to 457.8 Ma.These rocks have strong affinity to peralumious S-type granite and are characterized by prominent negative Eu anomalies(δEu=0.20–0.35),strong depletion in Ba,Sr,P,Ti,Nb,Ta and positive anomalies in Rb,Th,U,K,La,Nd,Zr,Hf.Nd-Sr isotopic compositions of the whole rock show negativeεNd(t)values(-1.21 to-0.08)and Mesoproterozoic Nd model ages(T2 DM=1.20–1.30 Ga).Their precursor magmas were likely derived from the partial dehydration melting of Mesoproterozoic mica-rich pelitic sources and mixed with minor mantle-derived components,under relatively low P(≤1 kbar)and high T(746–796℃)conditions.A ridge subduction model may account for the early Paleozoic geodynamic process with mantle-derived magmas caused by Ordovician ridge subduction and the opening of a slab window underplated and/or intraplated in the middle–upper crust,which triggered extensive partial melting of the shallow crust to generate diverse igneous rocks,and provided the heat for the crustal melting and juvenile materials for crustal growth.

New insights into the multiple magmatic fluid generations from scheelite in the Bastielieke W-polymetallic deposit,Chinese Altay(NW,China)

Scheelite is the main ore mineral in skarn-type tungsten(W-Mo,W-Sn,and W-Cu)deposits,and is also a good proxy for ore-fluid evolution and mineralization.The Bastielieke deposit is the first medium-size Wpolymetallic skarn deposit discovered in the Chinese(Xinjiang,NW China)Altay.Scheelite grains at Bastielieke are distributed in biotite granite,skarn and quartz-fluorite veins.They exhibit different textures,and can be divided into four types and six subtypes,including those in granite(Schm),prograde skarn(SchI),retrograde-altered rocks(SchII),and those in late-stage quartz-fluorite veins(SchIII).SchIa and SchIb were formed in the early and late prograde stage,respectively.SchI displays homogeneous texture,enrichments of light rare earth elements(LREEs)relative to heavy REEs(HREEs),and significantly negative Eu anomalies.SchIa has higher Sr-Mo contents and LREE/HREE than SchIb.SchII shows patchy texture by overgrowth and dissolution-reprecipitation,and can be subdivided into dark(SchIIa)and light(SchIIb)zone based in CL imaging.All SchII grains are LREE-enriched with negative Eu anomalies and relatively low LREE/HREE ratios.SchIIb has much higher W-Mo-Nb-Sr contents than SchIIa,which is ascribed to late-stage hydrothermal modifications.Schm and SchIII display homogeneous texture and similar MREE-enriched patterns,as well as very low Mo-W-Sr and different Nb contents.The texture and compositional variations in Bastielieke scheelites reveal that two magmatic fluids derived from different granitic magma reservoirs were involved in the mineralization.The earlier orefluid is relativly oxidized and has low HREE contents,forming the early prograde skarn-stage mineralization.Episodic influxes of later F-rich granitic magmatic fluids may have modified the earlier scheelite compositions,leading to multistage W enrichment and varying intragrain compositions.

Impact of residual zircon on Nd-Hf isotope decoupling during sediment recycling in subduction zone

The subduction factories in convergent plate margins exert crucial control on recycling terrestrial components and returning to the overlying crust. The Nd and Hf isotopic systems provide potential tracers to evaluate these processes. Here we present a case where these isotopic systems are decoupled in a suite of granites from the Chinese Altai, showing a wide range of εHf(t) values(from -4.7 to +10.8) in contrast to a limited range of εNd(t) values(from -5.8 to -1.9). The zircon xenocrysts occurring frequently in these rocks show markedly negative εHf(t) values(from -34.3 to -6.5) and positive d7 Li values(from +12.5 to +18.2). We propose a model to explain the observed relationship between residual zircon and Nde Hf isotope decoupling. We suggest that the Altai granites originated from partial melting of subducted slab components under relatively low temperature conditions which aided the residual zircon from oceanic sediments to inherit and retain a significant amount of177 Hf in the source, thereby elevating the^(176) Hf/^(177) Hf ratio of the melt, and decoupling from the^(143) Nd/^(144) Nd ratio during the subsequent magmatic processes. Our study illustrates a case where sediment recycling in subduction zone contributes to decoupling of Nd and Hf isotopic systems, with former providing a more reliable estimate of the source characteristics of granitic magmas.

Nd-Hf Isotopic Decoupling of the Silurian–Devonian Granitoids in the Chinese Altai: A Consequence of Crustal Recycling of the Ordovician Accretionary Wedge?

Voluminous Silurian–Devonian granitoids intruded a greywacke-dominated Ordovician accretionary wedge in the Chinese Altai. These granitoids are characterized by significant Nd-Hf isotopic decoupling, the underlying mechanism of which, so far, has been poorly understood. This issue is addressed in this study by the integration of our new and regional published geological and geochemical data. Geological studies indicated a close spatial relationship between the regional anatexis of the Ordovician wedge and the formation of the granitoids, which is characterized by a gradual textural evolution from the partial molten Ordovician wedge sedimentary rocks(the Habahe Group) to the granitoid bodies. Compositionally, these granitoids and the Ordovician Habahe Group rocks displayed close geochemical similarities, in the form of arclike trace elemental signatures as well as comparable Nd isotopic characteristics. Combined with regional available data, we suggest that the Silurian–Devonian granitoids originated from the immature and chemically primitive Habahe Group rocks. Since Nd and Hf isotopic data for the Habahe Group rocks show significant Nd-Hf isotopic decoupling, we propose that the Silurian–Devonian granitoids inherited the Nd and Hf isotopic signatures from their sources, i.e., the Habahe Group rocks. In other words, the Nd-Hf decoupling in the Habahe Group rocks is the primary causative factor leading to the prevailing Nd-Hf isotopic decoupling of the Silurian–Devonian granitoids in the Chinese Altai.