东昆仑金水口地区格林威尔期超高温麻粒岩

The Grenvillian-aged UHT granulite in Jinshuikou region,East Kunlun Orogenic Belt

格林威尔期构造事件是了解罗迪尼亚超大陆形成的关键。本文报道了东昆仑造山带东段金水口地区古生代花岗岩中新发现二辉麻粒岩包体,其峰期变质矿物组合为单斜辉石+紫苏辉石+钙长石+石英+磁铁矿。通过锆石U-Pb测年,我们确定二辉麻粒岩样品的峰期变质年龄为995±34Ma,并受到泥盆纪(~417Ma)构造热事件的叠加改造。利用单斜辉石-斜方辉石温压计估算出该区二辉麻粒岩变质峰期温度867~1079℃,压力4.6~8.9kbar,属于低压超高温变质的温压范围,可能形成于高地温梯度的岛弧环境。该二辉麻粒岩是首次在东昆仑地区发现的格林威尔期超高温麻粒岩,代表罗迪尼亚超大陆汇聚过程中低压高温变质的产物。该发现对了解东昆仑造山带前寒武纪基底的构造属性和起源有重要意义。

Grenvillian-aged tectonic events are critical for understanding the formation of Rodinia supercontinent.This paper report newly discovered two-pyroxene granulite enclaves in Jinshuikou district,eastern part of East Kunlun Orogenic Belt.The mineral assemblage consists of clinopyroxene,hypersthene,anorthosite,quartz and magnesite.Zircon grains recovered from the studied sample are spherical in shape and their cathodoluminescence images show weak-zoned or fir-tree structures.Together with their low Th/U ratio(0.01~0.21),these zircons are believed to be of typical metamorphic origin.U-Pb analyses of these zircons reveal that the granulite experienced peak metamorphism at 995±34Ma,and overprinted by a late thermal event at Devonian(~417Ma).The peak metamorphic conditions of the granulite are estimated to be T=867~1079℃and P=4.6~8.9kbar using various geothermobarometers.The orthopyroxene exsolution lamellae in clinopyroxene also imply a ultra-high temperature condition of>1000℃.This ultrahigh-temperature(UHT)metamorphism occurred in a high thermogradient environment,most likely an arc setting.This is the first time to determine the Grenville-age UHT granulite in the East Kunlun Orogenic Belt,which represents the product of assembly of Rodinia supercontinent.Our discovery of the Grenville-age granulite provides new insights for tectonic properties and origin of the Precambrian basement in the East Kunlun Orogenic Belt.