西藏林周县扎雪地区林子宗群帕那组火山岩的地球化学特征及其地质意义

The Geochemical Characteristics of the Pana Volcanic Rocks of the Linzizong Group in the Zhaxue Area,Linzhou County,Tibet and Its Geological Implication

分布于西藏林周县扎雪地区的林子宗群帕那组火山岩,主要由流纹质粗安质熔岩和火山碎屑岩组成。岩石总体上反映出高Al高K的特点,Al2O3的质量分数平均为14.55%,K2O+Na2O的质量分数平均为8.78%,K2O质量分数平均为5.77%,K2O/Na2O比值平均为2.07,显示了高钾钙碱性钾玄质火山岩的特征。稀土配分曲线呈轻稀土富集的右倾形式,并具有中等-强烈的负Eu异常。微量元素Rb、Th强烈富集,Ta、Ba、U相对富集,而Y、Yb、Sr等相对亏损。岩石地球化学特征显示了陆壳重熔的特点。研究表明,扎雪地区帕那组火山岩是40 Ma左右陆壳加厚并部分熔融形成的,是印度板块和欧亚板块后碰撞作用的产物。

The Pana volcanic rocks of the Linzizong Group in Zhaxue area, Linzhou County, Tibet are mainly composed of rhyolitic-trachy lavas and volcanoclastic rocks. Its geochemical characteristics （14.55%0 of Al2O3 8.78% of K2O＋Na2O, 5.77% of K2O, K2O/Na2O=2.07） suggest that the Pana Formation volcanic rocks belong to high-Al, high-K calc alkaline-shoshonite series. LREE in the volcanic rocks were slightly enriched with moderate to strong Eu anomalies. In the volcanic rocks, Rb and Th were intensively enriched, Ta, Ba and U moderately enriched while Y, Yb and Sr relatively depleted. The lithogeochemical study suggests that the Pana Formation volcanic rocks were formed about 40 Ma by anatexis of the thickened continental crust in the post-collision process between Indo plate and Eurasia plate.