The Late Triassic basic magmatism and tectonic implication in Leiwuqi area, eastern Tibet

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羌塘地体东部类乌齐地区是龙木措-双湖古特提斯洋和班公湖-怒江特提斯洋的叠加控制区域，该地区三叠纪的岩浆活动同时受两个大洋演化的影响，导致该地区岩浆岩的构造属性尚不明确。本文对类乌齐地区吉塘岩群内出露的变辉长岩的岩石学、锆石U-Pb年代学和Lu-Hf同位素以及岩石地球化学进行了研究。LA-ICP-MS锆石U-Pb测年结果显示变辉长岩结晶年龄为212~206Ma（晚三叠世）。岩石富集LREEs、Th，亏损Nb、Ta、Zr、Hf和Ti，表现出典型的岛弧火山岩的地球化学特征，较低的ε_Hf（t）值（-10.6~-3.2）表明岩浆来自中元古代岩石圈地幔部分熔融或富集地幔，较高的Th和LREEs含量以及Th/Yb和Th/Nb比值表明岩浆源区有不同程度俯冲沉积物的加入。结合区域上前人的研究成果，本文认为类乌齐地区变辉长岩是形成在班公湖-怒江特提斯洋向北俯冲在羌塘地体下的大陆弧背景下。;The Leiwuqi area in the eastern Qiangtang terrane is a tectonic superposition region, of which the tectonic evolution was controlled by both the Longmu Tso-Shuanghu Paleo-Tethyan Ocean and the Bangong-Nujiang Tethyan Ocean. The Triassic magmatism occurred in this area has been suggested to be related to the evolution of these two ancient oceans, which leads to controversy about the tectonic settings of the Triassic magmatic rocks. In this paper, we investigated the metagabbro of Jitang Group in the Leiwuqi area with petrology, geochemistry, zircon U-Pb geochronology and Lu-Hf isotope. The zircon U-Pb dating result indicates that the metagabbro crystallized at the Late Triassic (c. 212~206Ma). The metagabbro displays enrichment in LREEs and Th, and depletion of Nb, Ta, Zr, Hf and Ti, consistent with those of arc-related magmatic rocks. The negative zircon ε_Hf(t) values (-10.6~-3.2) of the metagabbro indicates it was probably derived from the partial melting of the Meso-proterozoic continental lithospheric mantle or enriched mantle. The high LREEs and Th contents and Th/Yb and Th/Nb ratios suggest the involvement of subducted sediments to the magma source. Combined with previous published data, it is concluded that the metagabbro formed in an arc-related setting related to the northward subduction of the Bangong-Nujiang Tethyan Ocean beneath the Qiangtang terrane.