藏南东部麻玛沟地区早古生代与中新世岩浆作用及其意义

喜马拉雅造山带东部错那县麻玛沟地区发育多种类型的花岗片麻岩和淡色花岗岩。锆石SHRIMP U-Pb地质年代学研究结果表明：花岗片麻岩(MM15)原岩结晶年龄为500.7±4.5Ma，含石榴子石淡色花岗岩中携带的继承性核部锆石年龄为498.6±3.4Ma，表明该地区经历了早古生代的岩浆作用事件。淡色花岗岩的结晶年龄区间为15.7~25.1Ma之间，为白云母脱水熔融的产物，可能是晚元古代-早古生代花岗质岩石发生低程度部分熔融的结果。锆石形态学表明该区的花岗片麻岩和淡色花岗岩均为过铝质花岗岩，并相对富集Cs、Rb、U、Pb，亏损Zr、Hf和低Nb/Ta比值，属于造山型花岗岩，支持该区域古生代岩浆作用事件与俯冲-碰撞造山作用相关，不是被动大陆边缘构造背景。结合前人数据推断：(1)从晚元古代末期开始，原特提斯洋向印度大陆的初始俯冲为自东向西的俯冲扩展模式；和(2)喜马拉雅造山带中新世淡色花岗岩为白云母脱水熔融和水致白云母熔融共同作用的结果，岩浆活动至少存在五个相。

Early-Paleozoic and Miocene magmatism of the Mama Valley and their significance, eastern South Tibet

The Mama Valley area of the Greater Himalayan Crystalline Sequence (GHCS) contains various types of granitic gneisses and leucogranites. New zircon SHRIMP U-Pb dating results indicate that the protolith of the granitic gneiss (Sample MM15) crystallized at 500.7±4.5Ma, and the inherited core of zircon grains from the garnet-bearing leucogranite contain a substantial population of zircon grains with an age of 498.6±3.4Ma. The new data together with literature data suggest that the Mama Valley also experienced a major episode of magmatism at the Early Paleozoic time. In addition, the magmatic zircon rims of the leucogranite yield ages from 15.7Ma to 25.1Ma, suggesting a protract anatectic process in this area. Geochemical characteristics suggest that the leucogranites were derived from muscovite dehydration melting, which may be achieved by low-degree melting of Neoproterozoic to Early-Paleozoic granites. The granitic gneiss and leucogranite have high Cs, Rb, U, and Pb, but low Zr, Hf, and Nb/Ta ratios. Zircon morphologies indicate that they are of peraluminous and orogenic granite, consistent with their bulk trace element compositions. Thus, the Early Paleozoic magmatic event occurred in a subduction to collision setting rather than in a passive margin setting. Combined with previous data, we infer that: (1) the subduction of the Proto-Tethys oceanic lithosphere beneath the Indian continent proceeded from the east to the west with time; and (2) both muscovite dehydration melting and fluxed-melting of muscovite are dominant melting mechanisms for the formation of the Miocene leucogranites in Himalayan belt, and the Miocene leucogranites represent the products from at least 5 episodic anatectic processes.