Provenance and ages of the Altyn Complex in Altyn Tagh: Implications for the early Neoproterozoic evolution of northwestern China

摘要

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 900-940. Ma, containing inherited zircon cores ranging in age from ~1100. Ma to 1800. Ma. Zircons in the orthogneisses have metamorphic rims that yielded ages clustered from 454 to 508. Ma. The paragneiss from Yaganbuyang yielded an estimated maximum depositional age of 835 ± 10. Ma and a metamorphic age of 458 ± 6. Ma, and most detrital zircons were dated at 835-1864. Ma. The paragneiss from Bashiwake yielded an estimated maximum depositional age of 1362 ± 18. Ma and a metamorphic age of 907 ± 14. Ma, and most detrital zircons were dated at 1150-1630. Ma. A leucosome from migmatite in Jianggalesayi yielded a metamorphic age of 417 ± 2. Ma, with detrital cores dated at ~1073-1789. Ma. 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. 485-510. Ma, ca. 454-458. Ma and ca. 417. Ma) in the Altyn Complex, overlap the main phases of the South Altyn granitic rocks (ca. 500-410. Ma), 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.