冈底斯岩浆弧东段沉积岩的早新生代变质作用及构造意义

Early Cenozoic metamorphism of the sedimentary rocks from the eastern Gangdese magmatic arc and its tectonic implications

青藏高原南部冈底斯岩浆弧形成于中生代以来新特提斯洋俯冲和新生代印度与欧亚大陆碰撞过程中。岩浆弧东部出露的中-新生代变质岩是研究大陆地壳组成、生长和再造的理想窗口。本文选择早新生代的变质沉积岩--石榴子石矽线石云母片岩和石榴子石黑云母片麻岩进行了岩石学和锆石U-Pb年代学研究。研究结果表明,石榴子石矽线石云母片岩由石榴子石、黑云母、白云母、斜长石、钾长石、矽线石、石英和金红石组成,峰期变质条件为730~750℃和0.78~0.81GPa。片岩和片麻岩中的锆石由继承的碎屑核和变质边组成,变质边给出的206Pb/238 U年龄为51~72Ma,继承碎屑核给出的206 Pb/238 U(或207 Pb/206 Pb)年龄在314~3286 Ma之间,并具有340Ma、550Ma、1100~1200Ma、1500~1600Ma和1800Ma年龄峰值,表明这些变质岩的原岩可能形成于晚石炭世。本文研究表明,冈底斯弧东段的古生代沉积岩在早新生代的碰撞造山过程中被埋藏到约25km深的中-下地壳,经历了角闪岩相变质作用和部分熔融,由此导致了岩浆弧中-下地壳组成和同碰撞岩浆岩化学成分的变化。

The Gangdese magmatic arc,south Tibet,was formed during the Mesozoic Andean orogeny resulted from the northward subduction of the Neo-Tethyan oceanic plate beneath the Eurasian continent and the Cenozoic collisional orogeny resulted from the collision between Indian and Asian continents.The Mesozoic to Cenozoic metamorphic rocks in the eastern Gangdese arc provide a window into investigating the growth and reworking of continental crust.The present paper conducts a petrological and geochronological study of the metasedimentary rock(garnet-sillimanite-mica schist and garnet-biotite gneiss)from the eastern Gangdese arc.The petrographical study shows that the garnet-sillimanite mica schists have a mineral assemblage of Grt+Bt+Ms+Pl+Kfs+Sil+Q+Rt.Phase equilibria modelling shows that the peak metamorphic conditions of the garnet-sillimanite mica schists are 730~750℃and0.78~0.81 GPa.This indicates that the schists and associated gneisses experienced the upper amphibolitefacies metamorphism and partial melting.The cathodoluminescence images show that the zircons from the schists and gneisses have a core-rim structure with an inherited detrital core and a metamorphic overgrowth rim.The U-Pb dating data shows that the zircon metamorphic rims have an age range of 51~72 Ma,while the detrital cores have a wide ages ranging from 314 Ma to 3286 Ma,with five dominant age populations at340 Ma,550 Ma,1100~1200 Ma,1500~1600 Ma and 1800 Ma,respectively.These results indicate that the sedimentary rocks were probably deposited at the Late Carboniferous,and underwent the Early Cenozoic metamorphism and partial melting.We suggest that the Late Paleozoic sedimentary rocks were buried into the middle to lower crust during the Early Cenozoic collisional orogeny,and in trun resulted in significant changes of the component of lower crust,and the chemical composition of syn-collisional magmatic rocks of the Gangdese magmatic arc.