摘要
冈底斯岩浆弧东段的中-新生代变质岩是揭示岩浆弧深部组成与形成演化的窗口。本文对该地区的变质沉积岩——夕线石榴黑云片岩进行了岩石学和锆石年代学研究。岩相学和矿物化学研究表明,夕线石榴黑云片岩具有三期矿物组合:(1)进变质矿物组合为Grt+Pl+Bt+Ms+Qz+Ep+Rt+Ilm,其保存于石榴石和斜长石变斑晶核部;(2)峰期矿物组合为Grt+Pl+Bt+Ms+Qz+Rt+Ilm,为包裹在石榴石变斑晶幔部和基质中的矿物;(3)退变质矿物组合是Grt+Pl+Bt+Sil+Qz+Ilm,为石榴石的冠状体或产在基质中。相平衡模拟表明,岩石的进变质、峰期和退变质条件分别为600~650℃和9~13kbar、~820℃和~16kbar、~680℃和~7kbar,表明岩石具有一个顺时针型的变质作用P-T轨迹,其峰期发生在高压和高温麻粒岩相条件下,并经历了明显的部分熔融。锆石U-Pb定年表明,岩石的变质作用发生在~89Ma。本文和现有研究结果表明,夕线石榴黑云片岩作为晚白垩世岩基中的包体记录了大体积幔源岩浆岩底垫和增生导致的岩浆弧地壳显著加厚过程,以及加厚弧根的高温和高压变质与深熔作用。
The Mesozoic to Cenozoic metamorphic rocks in the eastern Gangdese arc provided a window into investigating the deep components,formation and evolution of the arc. The present paper conducts a petrological and geochronological study of the sillimanitegarnet-biotite schist. The petrography and mineral composition show that the rock experienced three stages of metamorphism. The early prograde metamorphic mineral assemblage is Grt + Pl + Bt + Ms + Qz + Ep + Rt + Ilm. These minerals are included within the core of porphyroblastic garnet and plagioclase. The peak metamorphic mineral assemblage is Grt + Pl + Bt + Ms + Qz + Rt + Ilm,represented by the mantle of porphyroblastic garnet and plagioclase,and also matrix minerals. The late retrograde mineral assemblage is Grt + Pl +Bt + Sil + Qz + Ilm,which is represented by the minerals in the symplectitic corona after the garnet. Phase equilibria modelling shows that the prograde,peak and retrograde metamorphic conditions of the sillimanite-garnet-biotite schist are 600 ~ 650℃ and 9 ~12. 5 kbar, ~ 820℃ and ~ 16 kbar,and ~ 680℃ and ~ 7 kbar,respectively. This indicates that the sillimanite-garnet-biotite schist has experienced a clockwise P-T path with the peak metamorphism of high-pressure and high-temperature granulite-facies and significantly partial melting. The zircon U-Pb dating data shows that the sillimanite-garnet-biotite schist was metamorphosed at the Late Cretaceous of ~ 89 Ma. The present study and the previous results indicate that the underplating and accretion of voluminous mantlederived magma resulted in significant crustal thickening of the eastern Gangdese magmatic arc,as well as the high-pressure and hightemperature metamorphism and associated partial melting of the thickened lower crust,including the meta-gabbros and the hosted metamorphic supracrustal rocks.
引文
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