青海省乌兰县早古生代低压高温变质岩:柴北缘存在双变质带?

LP/HT metamorphic rocks in Wulan County,Qinghai Province:An Early Paleozoic paired metamorphic belt on the northern Qaidam Basin?

乌兰县二郎洞南侧一带的矽线石榴黑云斜长片麻岩矿物组合为石榴子石＋黑云母＋长石＋石英＋矽线石,是典型的低压高温高角闪岩相岩石.岩石学和矿物化学研究表明其峰期变质P-T条件为：T=677~696℃,P=0.349~0.419 GPa.LA-ICP-MS锆石U-Pb测年获得二郎洞南侧低压高温矽线石榴黑云斜长片麻岩的峰期变质年龄为450~483 Ma.研究区内同时期岛弧岩浆作用、混合岩化作用的存在,以及低压高温峰期变质的P-T条件,共同指示二郎洞一带低压高温高角闪岩相岩石形成于柴达木地块与祁连地块之间的原特提斯洋向北俯冲过程中,俯冲形成的岛弧岩浆侵入并加热大陆地壳,在该热异常强烈区内形成了低压高温高角闪岩相岩石.通过分析该套低压高温变质岩与柴北缘高压-超高压变质带内岩石的变质属性、峰期变质时代和大地构造位置关系,二郎洞地区低压高温高角闪岩相岩石可能与其南部柴北缘大洋俯冲阶段形成的高压变质岩构成了双变质带.

Low-pressure/high-temperature（LP/HT） sillimanite-garnet-biotite-plagioclase gneisses occur south of Erlangdong area in Wulan County on the northern margin of the Qaidam Basin. They consist of garnet,biotite,plagioclase and sillimanite,which have experienced high amphibolite-facies metamorphism with peak metamorphic conditions of T=677–696°C and P=0.349–0.419 GPa. Twenty-nine spot analyses on zircons using LA-ICP-MS method yield a concordant 206Pb/238U age of 483–450 Ma representing the time of the peak LP/HT metamorphism. We suggest that the LP/HT high amphibolite-facies rocks were formed by the northward subduction of the Proto-Tethyan Ocean beneath the Qilian Block but before final continental collision with the Qaidam Block. A strong thermal anomaly associated with arc magmatism and migmatization affected the base of the Qilian Block resulting in formation of the LP/HT high amphibolite-facies rocks in Erlongdong area. The age of these LP/HT rocks are the same as nearby high-pressure assemblages and the spatial relationships between the two metamorphic facies indicate this to be a paired metamorphic belt. We interpret this to have developed along the northern margin of the Qaidam Basin during the Ordovician.