柴达木地块寒武纪变质作用——来自相平衡模拟和独居石U-Pb年代学的约束  

Cambrian metamorphism of the Qaidam block: Constraints from phase equilibrium modeling and monazite U-Pb dating

作　　者：滕霞 张建新[1] 毛小红[1] 路增龙 周桂生 武亚威 郭祺 TENG Xia;ZHANG JianXin;MAO XiaoHong;LU ZengLong;ZHOU GuiSheng;WU YaWei;GUO Qi(Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;School of Earth and Space Sciences,Peking University,Beijing 100871,China)

机构地区：[1]自然资源部深地动力学重点实验室,中国地质科学院地质研究所,北京100037 [2]北京大学地球与空间科学学院,北京100871

出　　处：《岩石学报》2022年第11期3247-3258,共12页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(42072237、41630207);地质调查项目(DD20190006、DD20221649)联合资助.

摘　　要：柴达木地块位于青藏高原东北缘的祁连-阿尔金-昆仑早古生代造山系之中,它的西段出露一套(超)高温变质岩组合:变泥质岩、长英质片麻岩、基性麻粒岩、钙硅酸盐岩、含橄榄石大理岩及少量Mg-Al麻粒岩。本文以相平衡模拟和独居石U-Pb年代学为主要手段,限定柴西缘变泥质岩的变质作用P-T-t轨迹。变泥质岩记录了顺时针P-T轨迹,其中,压力峰期条件约为0.89GPa和800℃,温度峰期条件约为0.64GPa和825℃,退变质条件为0.58GPa和800℃至0.37±0.05GPa和702±50℃。变泥质岩的独居石U-Pb年龄集中在517~496Ma之间,三个样品的加权平均年龄分别为508±2Ma(MSWD=2.0)、506±1Ma(MSWD=1.7)、506±1Ma(MSWD=1.3)。本文将其解释为独居石的形成年龄,并认为独居石主要形成于退变质过程中的残留熔体结晶阶段(0.58GPa和800℃左右)。结合已有的工作可以得出,柴达木地块西段麻粒岩相变质作用具有顺时针P-T轨迹和1000~1300℃/GPa的峰期T/P值,并且在高温变质条件(>800℃)持续了超过30Myr。该类高T/P型变质作用最有可能发生在大型碰撞造山带内,可以与冈瓦纳大陆内部的晚泛非期的高T/P型变质作用对比,很可能是与冈瓦纳大陆最终拼合有关的晚泛非期造山事件的体现。The Qaidam block is located in the Qilian-Altun-Kunlun Early Paleozoic orogenic system,northeast of the Tibetan Plateau.Its western part exposes a set of(ultra)high-temperature metamorphic rocks,consisting of metapelites,felsic gneisses,mafic granulites,calc-silicates,olivine-bearing marbles,and minor Mg-Al granulites.This contribution presents a P-T-t path for metapelites in the western Qaidam block,determined mainly using phase equilibrium modeling and monazite U-Pb geochronology.The western Qaidam block metapelite records a clockwise metamorphic P-T path with a P_(max) condition of ca.0.89GPa and 800℃,a T _(max) condition of ca.0.64GPa and 825℃,and retrograde conditions of 0.58GPa and 800℃to 0.37±0.05GPa and 702±50℃.Monazite from three metapelite samples have U-Pb ages of 517~496Ma,with weighted mean ages of 508±2Ma(MSWD=2.0),506±1Ma(MSWD=1.7),506±1Ma(MSWD=1.3).These ages are interpreted as the formation ages of monazites,which mainly crystallized near the solidus condition(ca.800℃and 0.58GPa).Combined with the previous work,we suggest that the western Qaidam block underwent granulite-facies metamorphism along a clockwise P-T path and sustained a high-temperature condition(>800℃)for more than 30Myr,with the peak T/P of 1000~1300℃/GPa.This type of high-T/P metamorphism most likely occurred within a large orogen,and is comparable to regional high-T/P type metamorphism within the Gondwana,indicating that the Late Pan-African orogeny was probably manifested as high-T/P type metamorphism in the Qaidam block.

关 键 词：柴达木地块 变泥质岩 高T/P型变质作用 相平衡模拟 独居石年代学 

分 类 号：P588.34[天文地球—岩石学] P597.3[天文地球—地质学]