南阿尔金尤努斯萨依退变榴辉岩的发现及地质意义

Discovery of the Retrograded Eclogite in South Altyn and It’s Geological Significance

南阿尔金高压-超高压变质带基性麻粒岩的变质演化历史是地学界长期广泛关注的问题之一.研究发现,南阿尔金尤努斯萨依地区出露的石榴子石辉石岩保留了榴辉岩相变质矿物组合:石榴子石+绿辉石+金红石,曾经历榴辉岩相变质作用,后期叠加了麻粒岩相和角闪岩相两期变质作用改造,依据LA-ICP-MS锆石原位微区U-Pb定年和微量元素分析,获得两期变质年龄,分别为(495±4)Ma和(448±5)Ma,两者分别显示了榴辉岩相和麻粒岩相变质锆石稀土元素组成特征.而且,在~500 Ma锆石微区发现石榴子石、绿辉石包裹体,~450 Ma微区出现单斜辉石,指示该岩石榴辉岩相变质时代为(495±4)Ma,在(448±5)Ma经历麻粒岩相变质叠加.结合已有研究,该岩石是陆壳岩石在~500 Ma发生俯冲-深俯冲后在~450 Ma经历折返和退变质的产物,两期变质年龄代表了同一地质事件的不同阶段.南阿尔金地区呈透镜状产出的基性麻粒岩至少部分经历过榴辉岩相变质.

The metamorphic evolution history of mafic granulites in the South Altyn HP-UHP metamorphic belt has long been widely concerned.In this study,the garnet pyroxenite exposed in the Younusisayi area of South Altyn Tagh retained eclogite-facies metamorphic minerals:Grt+Omp+Ru,and it had experienced eclogitefacies metamorphism.The granulite-facies and amphibolite-facies metamorphic minerals were also found in the rock.Based on LA-ICP-MS zircon U-Pb dating and trace element analysis,two metamorphic ages of(495±4)Ma and(448±5)Ma were obtained.They showed the REE patterns similar to eclogite-facies and granulite-facies metamorphic zircons,respectively.In addition,garnet and pyroxene inclusions were found in~500 Ma zircon microzones,and clinopyroxene appeared in~450 Ma microzones,indicating that the rock underwent eclogite-facies and granulite-facies metamorphis at(495±4)Ma and(448±5)Ma,respectively.Combined with previous research results,our study show that the rock experienced crustal subduction and eclogite-facies metamorphism at~500 Ma,and then exhumation and retrograde metamorphism happened at~450 Ma.The two metamorphic ages represent different stages of the same geological event.At least part of the mafic granulites in South Altyn area had experienced eclogite-facies metamorphism.