Geochemical Characteristics And Metamorphism Of Amphibolite Of Jinshuikou Group In Gouli Area,Qinghai Province

Gouli area is located in the middle of the East Kunlun orogenic belt,which is the junction of the central Kunlun Tectonic Belts and southern Kunlun Tectonic Belts.Its unique geographical location makes it an important research place to study regional metamorphism and evolution.Jinshuikou Group,a Precambrian metamorphic basement rock series,is widely exposed in the region.This set of metamorphosed supracrustal rocks with high amphibolite facies-low granulite facies is an ideal target for studying regional metamorphism and metamorphic evolution.In this paper,Gouli area is selected as the key research area,and metabasite(plagioclase amphibole)in this area is taken as the research object.The petrology,petrography,mineral chemistry,rock geochemistry,zircon U-Pb chronology and Lu-Hf isotopic characteristics of plagioclase amphibole are studied in detail,revealing the mineral composition,metamorphic periods,metamorphic P-T path,formation age and metamorphic age,protolith type and structural background of plagioclase amphibole in this area.This study has achieved the following results and understandings:The metabasite of metamorphic basement in the study area is plagioclase amphibole,and the mineral associations are plagioclase and amphibole.Based on the Petrographical and mineral chemical characteristics of rocks,amphibole is divided into two stages of metamorphic mineral associations,and the temperature and pressure conditions of each stage are estimated by using conventional geothermobarometry: in the peak metamorphic stage(M2),the mineral associations are hornblende and andesine,and the temperature and pressure conditions are642.91～590.91 ℃ and 0.45～0.71 GPa;The metamorphic mineral associations are hornblende(tremolite)and oligoclase,and the temperature and pressure conditions are 557.15～522.37℃ and 0.12～0.34 GPa.The rock has a clockwise P-T trajectory of temperature rising and pressure rising,which indicates that the rock was formed in a subduction-collision environment.The study of petrogeochemical characteristics,protolith types and tectonic environment of amphibolite show that the protolith of plagioclase amphibolite is subalkaline basalt,which was formed in the initial rift environment.The zircon U-Pb geochronology and Lu-Hf isotopic characteristics of rocks were studied by LA-ICP-MS technique,and it was found that the rocks were formed at the age of 1829～1 608 Ma,belonging to Proterozoic-Mesoproterozoic.The metamorphic age is417±55 Ma,belonging to early Paleozoic.Lu-Hf isotopic characteristics of typical zircon in rocks show that the magma source of rocks is depleted mantle,which is contaminated by lower crust materials.The metamorphic age of plagioclase amphibolite reflects that a tectonic thermal event occurred in the region in the early Paleozoic.The age of this thermal event is consistent with that of the subduction-collision of the Proto-Teyhys Ocean(about 500Ma),which indicates that this tectonic thermal event is related to the subduction-collision of the Proto-Teyhys Ocean.