浙江龙游石榴石角闪岩(退变榴辉岩):华夏加里东期碰撞造山事件的新证据  

Retrograde garnet amphibolite from eclogite of the Zhejiang Longyou area: New evidence of the Caledonian orogenic event in the Cathaysia block

作　　者：陈相艳[1,2] 仝来喜[3] 张传林 朱清波 李亚楠 

机构地区：[1]中国地质科学院研究生部,北京100037 [2]南京地质调查中心,南京210016 [3]中国科学院广州地球化学研究所,广州510640

出　　处：《科学通报》2015年第13期1207-1217,共11页Chinese Science Bulletin

基　　金：中国地质调查局地质调查项目(1212011121098)资助

摘　　要：在华夏地块浙江龙游地区发现了榴辉岩退变形成的石榴石角闪岩,其变质演化可分为3个阶段,分别为峰期榴辉岩相阶段（M1）,矿物组合为绿辉石＋石榴子石变斑晶（核部）＋石英;峰后后成合晶阶段（M2）,矿物组合为石榴子石变斑晶（边部）＋单斜辉石变斑晶＋斜长石,峰期榴辉岩相绿辉石在该阶段形成榴辉岩典型的降压分解结构;退变质角闪岩相阶段（M3）,矿物组合为角闪石＋斜长石,并在石榴子石边部形成角闪石和斜长石的反应边、后成合晶以及斜长石冠状体.由于强烈的退变质过程的影响,传统矿物温压计方法只能得到退变质角闪岩相的温压条件,获得T=664~691℃,P=0.68~0.73 GPa.石榴子石成分环带表明该退变榴辉岩在峰后经历了一近等温降压过程.因此,结合岩相学分析、石榴子石成分剖面和退变质角闪岩相温压数据,可作出退变质过程的顺时针P-T轨迹,与碰撞造山过程一致.故该退变榴辉岩是华夏地块加里东期碰撞造山过程的产物,是华夏地块加里东期碰撞造山事件的新证据.Eclogites are commomly regarded as one of the key indicators of plate convergence and associated orogenesis. In this study we report the discovery of retrograde garnet amphibolite from eclogite of the Longyou area of Zhejiang Province. Detailed petrology reveals a three-stage metamorphic history based on mineral inclusions, reaction textures and pseudomorphs. Peak metamorphism （M1） is defined by high-Ca garnet cores and the presence of omphacite, as indicated by clinopyroxene ＋ plagioclase symplectites. Symplectites of clinopyroxene ＋ plagioclase that pseudomorph omphacite are indicative of the decompression stage （M2）. Symplectites of hornblende ＋ plagioclase and the presence of plagioclase coronas around garnet are indicative of formation during cooling （M3）, under P-T conditions of P=0.68--0.73 GPa and T=664-691~C. As a result of the intense retrograde metamorphism, only precise P-T conditions of the third stage were determined through conventional thermobarometry while garnet compositional profiles reveal decompression process during its growth. Based on the petrographic textures, garnet compositional profiles and thermobarometry, a clockwise P-T path has been defined, consistent with the occurrence of an orogenic event. Therefore, we suggest that the retrograde garnet amphibolite from eclogite of the Longyou area formed during the the Caledonian collisional orogenic event between the South China Block （amalgmation between Yangtze and Cathaysia in early Neoproterozoic） and an unknown block in the process of these blocks assemblage to eastern Gondwana.

关 键 词：石榴石角闪岩 退变榴辉岩 P-T轨迹 华夏地块 碰撞造山 

分 类 号：P588.3[天文地球—岩石学]