西藏南木林县罗扎地区早白垩世流纹岩锆石U-Pb年龄及地球化学特征

Zircon U-Pb age and geochemistry of Early Cretaceous rhyolite in Luozha area of Namling County,Tibet

笔者在对冈底斯弧背断隆带罗扎地区北侧大面积近东西向分布的火山岩进行研究时,通过LA-ICP-MS同位素测试技术,获得锆石U-Pb年龄值为(129.8±1.4)Ma(95%置信度),首次在冈底斯弧背断隆带获得早白垩世火山岩同位素测年数据。同时,其化学特征显示具有高的SiO2、K2O、K2O/Na2O,明显富集大离子亲石元素Rb、Ba、Th、U,而亏损Nb、Ta、Ti,并与中冈底斯带则弄群110 Ma地壳重熔形成的酸性火山岩和上地壳具有非常相似的蛛网曲线,表明其应该为地壳物质重熔的产物。在微量元素环境判别图中,均显示为弧火山岩,结合早白垩世所处的大地构造背景,推测其形成与雅鲁藏布江洋壳向北俯冲有关。

The authors studied the nearly EW-trending volcanic rocks in vast areas of northern Luozha zone along the Gangdise arc-ridge fault-uplift zone,obtained the LA-ICP-MS zircon age of 129.8±1.4 Ma,and proved for the first time the existence of Early Cretaceous volcanic rocks in the Gangdise arc-ridge fault-uplift zone.The volcanic rocks are characterized by high SiO2,K2O and K2O/Na2O values,the enrichment of LILEs（Rb,Ba,Th and U） and the depletion of Nb,Ta and Ti.Their cobweb curve is on the whole the same as that of the upper crust and the acidic volcanic rocks owing to crustal remelting of Zenong Group of the Gangdise zone at 110 Ma.These data indicate that the volcanic rocks might be the remelting products resulting from the dehydration of crustal materials.According to the environmental discrimination diagrams of trace elements,the authors consider that they are arc volcanic rocks.In combination with the tectonic background of Early Cretaceous period,it is thought that they were most likely associated with northward subduction of the Yarlung Zangbo River oceanic crust.