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[Display omitted] • Magmatism formed during the initial stage (ca. 480Ma) and advanced stage (ca. 450Ma) of exhumation of subducted continental crust have been recognized in South Altyn western China. • Two stages of magmatism exhibits evidence for the partial melting of subducted continental crust during differential exhumation process of South Altyn in Palaeozoic. The early Palaeozoic continental subduction–collisional belt in South Altyn (SA) western China distributes a large amount of (ultra)-high-pressure ((U)HP) metamorphic and granitic rocks associated with subduction, collision, and exhumation. In this study, four granitic rocks were collected from Bashenwake located in eastern SA. The ages of the garnet-bearing alkali-feldspar granite (GAFG), garnet-bearing granitic gneiss (GGG), alkali-feldspar granite (AFG), and syenogranite (SG) are 458.0 ± 4.1, 483.0 ± 4.1, 450.0 ± 2.4, and 482.0 ± 4.9 Ma, respectively. Whole-rock major and trace elements and Sr–Nd–Pb isotope, and zircon Hf isotope compositions for all samples exhibit characteristics of a continental crustal source. Based on partial melting modeling, the source temperature of the GGG, GAFG, AFG and SG was concluded as 750–780 °C, 730–760 °C, 730–770 °C and ca. 800 °C, respectively. The source pressure was estimated as ca. 0.8 GPa for the GGG, GAFG and AFG, and ca. 1.2 GPa for SG. Combined with studies on (U)HP metamorphic rocks in SA, this study reveals that GGG is formed by metamorphism while SG is formed from partial melting of upper crustal rocks heated by upwelling mantle during the initial stage of exhumation (ca. 480 Ma) of subducted continental crust in SA. During further stage of exhumation of the subducted continental crust at ca. 450 Ma, the GAFG and AFG were formed from partial melting of GGG as a result of decreased pressure and further mantle upwelling. Consequently, the exhumation age of subducted continental crust in eastern SA is ca. 30 Myr earlier than that in western SA. Thus, the geochronology and petrogenesis of four granitic samples in this study exhibit one of very few evidence for partial melting of subducted continental crust during the exhumation and also provide magmatism exhibits evidence for the differential exhumation model of eastern SA in Palaeozoic. [ABSTRACT FROM AUTHOR]

In this paper, we report zircon U–Pb dating, Hf isotopes, geochemical and Sr–Nd isotopic data, with the aim the petrogenesis and regional tectonic evolution of Early Devonian bimodal volcanic rock in the South Altyn, NW China. New LA-ICPMS zircon U–Pb isotopic data constrained them at ca. 406 Ma. The mafic samples are characterized by high Fe, Cr and Ni contents, low Ti and Mg contents, slightly enriched LREE patterns, and low (La/Yb) N , La/Nb and La/Ta ratios, and positive ε Nd ( t ) values (+3.3 to +3.4), indicating that they were likely derived from strong batch-melting of the asthenosphere in the spinel facies field. The felsic rocks show an A-type affinity, with high alkalis, Fe, Ga, Zr, Nb, Ce and Y contents, low Mg, Sr content, high Rb/Sr and Ga/Al ratios, enrichment in LILE (e.g., Rb, K, Th, U and LREE) and depletion in Ba, Sr, Nb, Ta, P and Ti, and fractionated REE patterns with very strong negative Eu anomalies. These features, along with distinct ε Nd ( t ) values (−0.5 to +2.3) and mostly positive ε Hf ( t ) (−0.29 to +5.18), indicate that the felsic rocks were mainly generated by partial melting of the crust in low pressure and high temperature conditions, and simultaneously underwent slight magma mixing of such melts with mantle magma. According to the petrogenetic schemes and geological background of the Early Devonian bimodal volcanic rocks (tholeiite and A-type dacite–rhyolite), they should have formed in a post-collisional extensional setting. Moreover, on the basis of spatial and temporal distribution, and formation mechanism, the tectonic magmatic evolution of the early Paleozoic South Altyn Tagh could be divided into three stages: ① 505–472 Ma (continental collision), the magmatite formed under high-pressure conditions due to the deep subduction and initial tearing of continental slab; ② 467–450 Ma (continental slab break-off), the magmatite formed at high temperature and low pressure in virtue of felsic upper crust uplifting and mantle magma underplating; and ③ 432–385 Ma (post-collisional extension), the magmatite consists of A-type granites and bimodal volcanic rocks, which are the products of the interaction between mantle and crust, and asthenosphere upwelling. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Anataxis of the Grt-bearing biotite gneiss was identified in the Jianggalesayi area. • Protolith, peak & retrograded metamorphic, anatectic ages were obtained for the gneiss. • Protolith ages of 1099–1072 Ma and formation age of 417 Ma were for the felsic veins. • Biotite dehydration accounted for the anatexis of the gneiss between 456 and 417 Ma. • Crystallization of the melt took place at the amphibolite facies stage of the gneiss. Abstract Partial melting of the garnet (Grt)–bearing biotite gneiss from the Jianggalesayi area in the southern Altyn Tagh high pressure (HP) – ultra high pressure (UHP) metamorphic belt has been identified and reported here. Felsic veins were distributed along the foliation plane and deformed together with the Grt–bearing biotite gneiss, which dominated the main outcrop and preserved diagnostic features of anataxis. Highly cuspate and irregular quartz (Qz)-melt and plagioclase (Pl)-melt surrounded or stretched into biotite and/or quartz grains with obvious eroded boundaries further provided the microstructural evidences for partial melting of the gneiss. CL images showed that zircons from the Grt–bearing biotite gneiss were characterized by multizone structures of core – Rim I – Rim II – Rim III. The different zircon zones from the gneiss had integrated recordings of the protolith ages 836–1613 Ma; the peak metamorphic age of 503 ± 5 Ma; the retrograded age of 456 ± 4 Ma; and the anatectic age of 418 ± 3 Ma. Zircons from felsic veins also exhibited core – rim structures on CL images. The felsic veins yielded similar protolith ages (1099–1072 Ma) and anatectic age (417 ± 2 Ma) as the host gneiss. The zircons from the host gneiss and felsic veins had plotted identical crustal evolution curves in the εHf(t) vs. U-Pb age diagram with identical T DM ages. The U-Pb ages and Lu-Hf isotope both suggested that the felsic veins were internally derived by the partial melting of the host gneiss. Moreover, the high content of Al 2 O 3 and Rb in felsic veins indicated that the decomposition of biotite accounted for the partial melting of the gneiss. The temperatures calculated by Ti–in–zircon thermometer for the anatectic zircons from the felsic veins (582–694 °C) were identical to the conditions of the amphibolite facies stage of the Grt–bearing biotite gneiss (T = 619–646 °C), which indicated that the crystallization of the melt had taken place during the amphibolite facies stage of the exhumation. The anatectic ages (418 ± 3 Ma and 417 ± 2 Ma) of the gneiss and felsic veins were determined to be within the range of the fourth episode of the early Paleozoic magmatism (420–385 Ma), which had formed in a post-collisional extensional setting. Therefore, the combination of the magmatic heat events and stress release may have played an important role in the partial melting of the gneiss. [ABSTRACT FROM AUTHOR]

Ultrahigh pressure (UHP) rocks are often overprinted by high pressure (HP) granulite-facies metamorphism during exhumation. Here we investigate retrograde eclogite from the Lvliangshan area in North Qaidam which is associated with abundant felsic veins. The eclogite adjacent to the vein shows conversion to garnet amphibolite. Petrographic, mineralogical, geochemical, geochronological data suggest that the eclogite has been experienced peak eclogite facies metamorphism at c. 440 Ma, followed by granulite facies retrogression at c. 420 Ma, and amphibolite facies overprinting at <420 Ma. The retrograde eclogite has microstructural records for partial melting with elongated plagioclase grains occurring as veinlets along the garnet-ilmenite grain boundaries and interspersed in clinopyroxene. Moreover, highly cuspate, wedge–shaped and triangular plagioclase grains are distributed along the garnet boundaries, and banded felsic veins (Pl + Qz) occur along the boundaries of amphibole in garnet amphibolite. Whole-rock geochemistry suggests that the felsic veins have 438.1–1106 ppm Sr and 2.87–12.18 ppm Y with Sr/Y ratios from 81.98–166.2, exhibit pronounced positive Eu anomaly, and depletion of HREE, resembling those of adakitic rocks. Felsic veins contain zircon grains with an U-Pb age of ∼422 Ma, similar to the age of HP granulite–facies metamorphism (420 Ma) of retrograde eclogite. Combined with the identical Hf isotope compositions between the granulite–facies zircon grains from the host retrograde eclogite and those from the felsic vein, we infer closed system partial melting during HP granulite facies retrogression of the retrograde eclogite. The tonalitic (high Sr/Y ratio and enrichment of LREE) chemical feature, low Rb/Sr and high Ba/Rb ratios of the felsic veins suggest that zoisite and sodic amphibolite dehydration-melting together account for the formation of felsic veins during exhumation from the eclogite–facies to granulite–facies stages. [ABSTRACT FROM AUTHOR]

[Display omitted] • Clockwise P–T evolution is recorded by the South Altyn Tagh felsic gneiss. • The felsic gneiss was exhumed from eclogite-facies to granulite-facies within 20 Myr. • HP–HT eclogite-facies records were preserved in the weakly deformed felsic gneiss. • The HP mineralogical records were further eliminated by the strong deformation. Felsic gneisses are usually the dominant rock types in most high-pressure (HP) and ultrahigh-pressure (UHP) terranes, although their peak metamorphic conditions and complete P–T paths are rarely known. This lack of knowledge is attributed to the high variance of mineral assemblages at HP/UHP conditions of the felsic gneisses, or to the effects of retrograde-related overprinting. Thus, it is difficult to distinguish whether felsic gneisses underwent HP/UHP metamorphism. In this paper, we focus on the felsic gneiss that has been previously documented as having formed under HP granulite-facies metamorphism with peak P–T of > 1.15 GPa and > 860 °C from the Danshuiquan locality in the South Altyn HP – UHP belt. The Danshuiquan felsic gneiss is strongly deformed and is spatially associated with weakly deformed garnet-rich felsic gneiss. On the basis of detailed microstructure observations on mineral assemblages and phase equilibria modelling, three stages of metamorphic evolution for the weakly deformed garnet-rich felsic gneiss are identified. The first stage is represented by garnet cores, revealing a prograde P–T path from 1.5 GPa/750 °C to 2.1 GPa/900 °C. The second stage is characterized by garnet mantles with multiphase inclusions, defining HP eclogite-facies metamorphic conditions of 2.2–2.6 GPa/950–1100 °C. The third stage is represented by the composition of garnet rims, biotite, and plagioclase, suggesting an HP granulite to amphibolite facies decompression cooling process to 1.1–1.3 GPa/710–770 °C. In contrast, the strongly deformed felsic gneiss just records a deformation-related decompression process from 1.1 to 1.2 GPa/750–760 °C to 0.87–0.98 GPa/760–770 °C, and finally to 0.83 GPa/740 °C. Zircon U–Pb dating yields c. 920 Ma protolith ages, c. 500 Ma eclogite-facies ages, and c. 480 Ma granulite to amphibolite facies retrograde ages for the Danshuiquan felsic gneisses. Our results show that the Danshuiquan felsic gneisses underwent a similar metamorphic evolution process to that of Yinggelisayi HP–UHP rocks in the eastern segment of the South Altyn Tagh. Most HP mineralogical records tend to be eliminated by the strong deformation at crustal depths (∼30 km), meaning that the peak metamorphic conditions of the studied felsic rocks in UHP metamorphic terranes are previously underestimated. [ABSTRACT FROM AUTHOR]