Granitoids and Crustal Growth in the East-Kunlun Orogenic Belt 论文

摘要

The East-Kunlun orogenic belt (abbrev. EKOB) is one of major tectono-magmatic belts in the Tibetan plateau. Four stages of granitoid plutonism took place in Precambrian (the Proterozoic), early Paleozoic (∈-D3), late Paleozoic-early Mesozoic (D3-T3), and late Mesozoic-Cenozoic (after early Jurassic), respectively. Among them, the late Paleozoic-early Mesozoic, especially Triassic granitoids are predominant. The basement of EKOB formed in late Paleo-Proterozoic. The early Paleozoic tectono-magmatic event sequence is comparable to that in the North- Qilian orogenic belt, and became a part of the Qilian-East-Kunlun Caledonian orogenic system. However, EKOB was involved in the Paleo-Tethyan tectonic regime during late Paleozoic-early Mesozoic period, and yielded an integrated Variscian-Indosinian orogenic cycle, similar to the Sanjiang Paleo-Tethyan orogeny. The south-Kunlun suture zone during that time was the main boundary between the north and the south China continents. Subsequently, EKOB was involved in the Tibetan collisional orogenic system in the Cenozoic since Indo-Eurasia collision. While the continental crust of EKOB mainly formed in late Paleo-Proterozoic, juvenile crust was also generated in the Phanerozoic time, similar to the Xing,anling-Mongolia, the Gangdese and the Andes orogenic belts. There are abundant conclusive evidences of underplating and magma mixing in the East-Kunlun granitoid belt. Initial values of Sr/86Sr of the East-Kunlun granitoids are mostly less than 0.710 and eNd(t) values of them range from-9.2 to 87 +3.6. These features imply that inputting of mantle materials and mixing between mantle-and crust-derived materials played an important role in crustal generation and evolution in EKOB during the Phanerozoic time. According to SHRIMP U-Pb dating of zircons from granitoid host rocks, mafic microgranular enclaves (MMEs) and associated gabbros, two major events of underplating and magma mixing took place in EKOB in the early-middle Devonian (394-403 Ma) and the middle Triassic (239-242 Ma), corresponding to the stage from the ending of subduction to the initiation of collision during the Caledonian and Variscian-Indosinian orogeny, respectively.