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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]

We report here for the first time long prism/needle-shaped polycrystalline quartz aggregates and square/parallelogram-shaped single-phase quartz inclusions in omphacite and garnet of ultrahigh pressure (UHP) eclogite from the Yinggelisayi area in the South Altyn UHP belt. Based on their morphology and mineral composition, these quartz inclusions in omphacite and garnet are quartz paramorphs after stishovite. The minimum peak pressure of the eclogite is estimated to be >8–9 GPa at 800–1000 °C based on the stability field of stishovite. This new evidence, together with previous discovery of kyanite + spineal exsolutions in former stishovite of the pelitic gneiss from the same region, suggests a ultra-deep subduction and exhumation of the South Altyn continental rocks to/from mantle depths in the stability field of stishovite. Evidence of ultra-deep subduction of continental materials might be more common and diverse than previous thought. [ABSTRACT FROM AUTHOR]

The Altyn Complex in Altyn Tagh, NW China is commonly considered as the Archean–Paleoproterozoic basement of the Tarim craton. It consists predominantly of mafic and felsic orthogneisses and metasedimentary rocks. Seven samples for laser ablation ICP-MS U–Pb zircon dating were collected from felsic orthogneisses, granites, paragneisses and leucosomes within the Altyn Complex that are exposed at Jianggalesayi and Danshuiquan in the west, at Bashiwake in the middle, and at Yaganbuyang in the east of South Altyn. Our results indicate that both granite and orthogneisses were emplaced at 900–940Ma, containing inherited zircon cores ranging in age from ∼1100Ma to 1800Ma. Zircons in the orthogneisses have metamorphic rims that yielded ages clustered from 454 to 508Ma. The paragneiss from Yaganbuyang yielded an estimated maximum depositional age of 835±10Ma and a metamorphic age of 458±6Ma, and most detrital zircons were dated at 835–1864Ma. The paragneiss from Bashiwake yielded an estimated maximum depositional age of 1362±18Ma and a metamorphic age of 907±14Ma, and most detrital zircons were dated at 1150–1630Ma. A leucosome from migmatite in Jianggalesayi yielded a metamorphic age of 417±2Ma, with detrital cores dated at ∼1073–1789Ma. Integrated with the previous U–Pb zircon geochronological data, we propose that the Altyn Complex is not the Archean–Paleoproterozoic crystallized basement of the Tarim craton as previously believed. We suggest that these granitoids magmatism and precursors of metasedimentary units in the Altyn Complex were produced at an active margin outside of the Tarim craton during early Neoproterozoic time. Early Palaeozoic multistage metamorphic ages (ca. 485–510Ma, ca. 454–458Ma and ca. 417Ma) in the Altyn Complex, overlap the main phases of the South Altyn granitic rocks (ca. 500–410Ma), indicating that this complex developed during the early Palaeozoic collisional orogeny in the Altyn Tagh. Based on a combination of Grenville-age detritus and the record of late Neoproterozoic extensional tectono-thermal activity, we place the Altyn Complex on the margin of a Grenville-age orogen which represents part of the juxtaposition of continental masses within the Rodinia Supercontinent. The similarity in age of magmatism and metamorphism in the Altyn Tagh–North Qaidam–Qilian–East Kunlun suggest the existence of an early Neoproterozoic active continental margin system along the northeastern margin of Tibetan Plateau. [ABSTRACT FROM AUTHOR]