羌塘北缘上新世恒梁湖火山岩内含石榴子石长英质捕虏体岩石学与矿物化学特征  

Petrographical and mineral chemical characteristics ofgarnet-bearing quartzofeldspathic xenolith in theHenglianghu Pilocene volcanic rock,northern Qiangtang terrane

作　　者：冀磊 任玉峰 贺日政[1] JI Lei;REN Yufeng;HE Rizheng(Chinese Academy of Geological Sciences,Beijing 100037,China;Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China)

机构地区：[1]中国地质科学院,北京100037 [2]中国地质科学院地质研究所,北京100037

出　　处：《地质学报》2022年第7期2441-2449,共9页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号41761134094,42074112,41574086,41274095);中国地质科学院基本科研业务费项目(编号JKY202014);中国地质调查项目(编号DD20190015)联合资助成果。

摘　　要：本文以羌塘地体北缘出露面积最大的上新世恒梁湖火山岩为研究对象,对火山岩内含石榴子石长英质捕掳体的岩相学、显微结构及矿物化学综合研究,揭示出此类捕掳体系深部地壳超高温部分熔融残余,其中石榴子石以高Mg~#值(41~46)为特征。传统地质压力计估算其发生部分熔融的压力为1.18~1.53 GPa,对应深度约39~50 km,火山岩与捕掳体相互作用的温压条件为950~1050℃,1.30~1.66 GPa,对应深度约43~55 km。火山岩熔体快速上升冷却过程中,熔体沿捕掳体裂隙或边缘与石榴子石发生反应形成斜方辉石和斜长石,而火山岩基质内辉石及长石表现出强烈的成分环带。此研究结果首次揭示了北羌塘北缘中-深地壳岩石部分熔融所产生的熔体及其岩相学和矿物化学特征,亦为此区域地壳深部面波低速异常及强各向异性提供了深部岩石学方面的限定。This study is focussed on the largest Pliocene(3.2~2.5 Ma) volcanic rocks from the Henglianghu area in the north margin of the Qiangtang terrane.Detailed petrographical and mineral chemical work on the garnet-bearing quartzofeldspathic xenolith and phenocrysts revealed that melt in the deep crust(40~50 km) was trapped when the volcano erupted.The garnets in xenolith are characterized by extremely high Mg^(#)(Mg^(2+)×100/(Mg^(2+)+Fe^(2+))) values(41~46) which indicated they were formed under ultrahigh temperature metamorphism.Garnet-plagioclase geobarometer calculation for the xenolith yields 1.18~1.53 GPa corresponding to 39~50 km depth.Clinopyroxene-orthopyroxene geothermometers applied to the volcanic melt give temperature ranges from 950℃ to 1050℃.During ascending stage,volcanic melt interacted with the garnet along the xenolith fractures or mineral boundaries and transformed to orthopyroxene and plagioclase.Pressure estimated by garnet-orthopyroxne-plagiocalse geobarometers is 1.30~1.66 GPa(43~55 km).Pyroxene and plagioclase in the matrix exhibit distinct compositional zoning as the volcanic rock composition evolved.Our results not only first exhibit the texture and mineral chemical features of melt in the middle-deep crust of the northern Qiangtang terrane but also provide petrological constraints on the low S-wave and high anisotropy anomaly in this region.

关 键 词：羌塘北缘 上新世火山岩 含石榴子石捕掳体 矿物化学 温压计算 

分 类 号：P588.14[天文地球—岩石学] P574.2[天文地球—地质学]