Retrogressive Evolution Of Pelitic Granulite In The Larsemann Hills,East Antarctica

The Larsemann Hills are located in the middle of the Prydz Bay in the East Antarctica.The study of pelitic granulite in this area is helpful to understand the metamorphic evolution of the Pan-African metamorphic event and to further study the structuaral attribution and significance of the Pryds Bay.This paper mainly studies the pelitic granulite in the Larsemann Hills.Through detailed petrographic observations,mineral chemical analysis,U-Pb dating and phase equilibrium modeling,the metamorphic age of pelitic granulite in the region are discussed.Metamorphic trajectories and the establishment of new PT conditions provide new constraints on the regional geological structure and metamorphic evolution.Detailed petrographic observations of four samples?LSM178-2,LSM337-1,LSM319-2,LSM302-10?in this area show that the metamorphic mineral assemblages are mainly garnet+orthopyroxene+biotite+plagioclase+K-feldspar+quartz+ilmenite,garnet+cordierite+biotite+plagioclase+quartz+ilmenite,garnet+cordierite+biotite+plagioclase+quartz+ilmenite and garnet+biotite+sillimanite+spinel+K-feldspar+quartz+magnetite+ilmenite.Affected severely by metamorphism,no peak mineral composition can be seen.The results of mineral chemical analysis indicate that the garnet component in some sample is pyrope-rich garnet belongs to the almandine and has a weak ring zone.he metamorphic age corresponding to the development of the major mineral assemblage of sample LSM178-2 should be⁵43±5.7Ma.The age for the dark gray rim of zircon in sample LSM302-10 is 531±5.7Ma and the age for the light gray rim is 509±5.9Ma.The metamorphic ages may reflect Pan-African tectonic thermal event that lasted for a relatively long period of time?⁵40-510Ma?.Combined with the analysis of mineral assemblages and petrographic observations and using the THERMOCALC3.45 program in Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-O?NCKFMASHTO?and MnO-containing system to perform thermodynamic simulation on four samples of pelitic granite in this area.The estimated peak P-T conditions of the sample garnet-orthopyroxene pelitic granulite?LSM178-2?are T=830-870?,P=6.1-8.9Kbar,which retrogressed to T<705?,P=3.6-5.4Kbar.The garnet-cordierite pelitic granulite?LSM337-1?peak temperature and pressure conditions are T=780-840?,P=4.8-5.5Kbar,and then undergo degeneration.The garnet-cordierite pelitic granulite?LSM319-2?has a temperature and pressure range of T=600-635?and P=4.1-5.1Kbar after undergoing degeneration.The peak P-T ranges of the garnet-sillimanite-spinel gneiss?LSM302-10?are T=860-1050?,P=4.8-10.2Kbar,followed by retrograde conditions of T=820-840?,P=4.7-5Kbar.Comprehensive analysis shows that its retrogressive evolution path is the P-T path of near-isobaric cooling.t Combined with chronological studies and existing data in related areas,the analysis of the metamorphism of medium-low pressure high-temperature compacted granulite facies in the Larsemann Hills may reflect the uplift and extension during the late stage of orogenesis.