东昆仑造山带牦牛山组火山岩SHRIMP锆石U-Pb年龄及其构造意义

SHRIMP zircon U-Pb ages and tectonic significance of Maoniushan Formation volcanic rocks in East Kunlun orogenic belt,China

东昆仑造山带牦牛山组磨拉石建造不整合覆盖在前泥盆系之上,记录了东昆仑早古生代洋盆关闭的时间。对格尔木南锯齿山一带牦牛山组上部火山岩段的英安岩进行了SHRIMP锆石U-Pb测年,11颗岩浆锆石206Pb/238U年龄加权平均值为(406.1±2.9)Ma,这表明牦牛山组上部火山岩形成于早泥盆世,进一步佐证了根据牦牛山组底部磨拉石中火山岩夹层的岩浆锆石U-Pb年龄所得出的结论:东昆仑早古生代洋盆关闭的时间为晚志留世—早泥盆世,而非传统认为的晚泥盆世。

The Maoniushan Formation of molasses rests unconformably on pre-Devonian strata and records the closure time of the Early Paleozoic ocean.We made zircon U-Pb dating for dacite from upper volcanic rock Member of Maoniushan Formation by SHRIMP.11 grains analyses on zircons indicate weighted mean 206Pb/238U ages of （406.1±2.9）Ma,interpreted to be the time of the formation of volcanic rocks.Therefore,it is suggested that the time of the formation of the Maoniushan Formation should be in the Early Devonian.It also supports the conclusion made from zircon U-Pb dating for rhyolite rocks from bottom volcanic rocks of Maoniushan Formation by LA-MC-ICP-MS,which the closure time of the Early Paleozoic ocean should be in the Late Silurian-Early Devonian.