西藏羌塘地体南部多龙增生杂岩早三叠世辉长岩的发现及其地质意义
The discovery of the Early Triassic gabbro rocks of the Duolong accretionary complexes in southern Qiangtang terrane of Tibet and its geological significance
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摘要: 西藏多龙矿集区是班公湖-怒江缝合带北缘在增生楔基础上发育的超大型岛弧斑岩铜金矿床。对多龙增生杂岩中的辉长岩进行了LA-ICP-MS锆石U-Pb定年及主量、微量元素分析和Sr-Nd同位素组成研究,获得了多龙增生杂岩中辉长岩的成岩时代。由LA-ICP-MS测得的辉长岩锆石206Pb/238U年龄为246.0±1.4Ma,代表了岩浆的结晶年龄。辉长岩以低钛、富钠、富镁、贫钾、富集轻稀土元素和大离子亲石元素(Ba、U、La、Sr)、亏损高场强元素Nb和Ta为特征,具有与岛弧玄武岩相似的主量和微量元素组成,相对于原始地幔具高Sr、低Nd的同位素组成和古老的Nd同位素二阶段模式年龄(TDM2=0.54~0.99Ga),属于增生楔基础上发育的板内岛弧辉长岩。由此认为,多龙矿集区早三叠世末岩浆活动很可能是班公湖-怒江洋向北俯冲诱发洋壳物质与岩石圈地幔物质相互作用的结果。该发现和认识为班公湖-怒江洋早三叠世向北俯冲,提供了重要的岩浆作用证据。Abstract: The outcropped Late Triassic-Jurassic accretionary complexes are located along northern Bangong Co-Nujiang suture zone where a large subduction-type Duolong porphyry copper-gold ore concentration area is developed. Bulk-rock major and trace elements, Sr-Nd isotope and LA-ICP-MS zircon U-Pb data are reported for gabbro rocks sampled from the Duolong accretionary complexes. The 206Pb/238U age of zircon in the gabbro analyzed by LA-ICP-MS is 246.0±1.4Ma, which represents the crystallization age of the magma. Na, Mg, LREE and large ion lithophile elements (Ba, U, La and Sr) are concentrated whereas Ti, K and highfield strength elements (Nb and Ta) are deficient. These characteristics suggest that the geochemistry of the gabbro is similar to that of the island arc type basalt. The isotopic compositions of the Sr and Nd ((87Sr/86Sr)i=0.707605~0.709778, (143Nd/144Nd)i=0.512515~0.512678, and TDM2=0.54~0.99Ga) suggest that gabbro is developed on the arc in an intraplate environment. It is considered that the end of early Triassic igneous activity probably resulted from the northward subduction of the Bangong Co-Nujiang Ocean crust which interacted with the lithospheric mantle material. The discovery and understanding provide the northward subduction of the Bangong Co-Nujiang ocean crust in early Triassic with important evidence of magmatism.
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