西藏冈底斯带西段狮泉河地区林子宗群火山岩地球化学特征、锆石U-Pb年龄及地质意义

林子宗群火山岩广泛发育在冈底斯带的南部,长期以来被认为是印度与亚洲大陆碰撞过程中的火山响应,对冈底斯带南缘的地质演化具有重要意义。迄今对林子宗群火山岩的研究主要集中在冈底斯带的中东部,未见有冈底斯带西部地区林子宗群火山岩的研究报道。对冈底斯带西部狮泉河地区的林子宗群火山岩进行了年代学与元素地球化学研究。结果表明,研究区年波组主要包括玄武安山岩、安山岩与流纹岩,属钙碱性—高钾钙碱性系列;帕那组火山岩主要为流纹岩,属于高钾钙碱性和钾玄岩系列。岩石富集大离子亲石元素Rb、Th、U等,亏损高场强元素Nb、Ta、Ti、Sr、Ba等,与冈底斯带中东部林子宗群火山岩一致,具有岛弧火山岩的特征。此外,帕那组流纹岩LA-ICP-MS锆石U-Pb年龄为53.9±0.5Ma,比冈底斯带中东部帕那组形成时代更早。结合前人的研究成果表明,印度与亚洲大陆的碰撞具有西早东晚的特点。

Geochemistry, zircon U-Pb age and implications of the Linzizong Group volcanic rocks in Shiquan River area, western Gangdise belt, Tibet

Located in the southern part of Gangdise belt, the Linzizong Group volcanic rocks are regarded as the magmatic response to the continental collision between India and Asia, which is essentially important in understanding the evolutionary history of the Gangdise belt. The present research is only focused on central-western part of Gangdese belt, while the research on the western part is very insufficient. The authors conducted for the first time a systematic study of geochronology and major and trace element geochemistry of the Linzizong Group volcanic rocks in Shiquan River area in western Gangdise belt. The results indicate that the Nianbo Formation is of the calc-alkalic and high-potassic calc-alkalic series and is composed mostly of basaltic andesite, andesite and rhyolite, whereas the Pana Formation is of the high-potassic calc-alkalic to Shoshonite series with rhyolite. The Linzizong Group volcanic rocks in Shiquan River area are enriched in Rb, Th and U and depleted in Nb, Ta, Ti, Sr and Ba, which is similar to characteristics of central-eastern Gangdise belt, indicating the arc-related volcanics. The LA-ICP-MS zircon U-Pb age of Pana Formation in Shiquan River area is 53.9±0.5Ma, which is older than that of central-eastern Gangdise belt. The results combined with previous researches show that the initial collision between India and Asia continent took place earlier in western Gangdise belt but late in central-eastern Gangdise belt.