Metamorphism of gneisses in the Stornes Peninsula and adjacent region of the Larsemann Hills: Phase equilibrium modelling and zircon geochronology

Abstract

东南极拉斯曼丘陵斯图尔内斯半岛及邻区发育不同类型的片麻岩。本文结合岩相学、相平衡模拟以及年代学分析等方法对该地区石榴石斜方辉石片麻岩（样品LSM178-2）和石榴石夕线尖晶石片麻岩（样品LSM302-10）进行综合研究，结果显示片麻岩经历了中低压高温麻粒岩相变质作用。样品LSM178-2峰期温压条件为T=830~870℃，P=6.1~8.9kbar，后期退变至T<705℃，P=3.6~5.4kbar。石榴石夕线尖晶石片麻岩（LSM302-10）保存的峰期温压范围为T=860~1050℃，P=4.8~10.2kbar，之后退变至T=820~840℃，P=4.7~5kbar。样品LSM178-2主要矿物组合对应的变质年龄为~543±5.7Ma。样品LSM302-10中锆石暗灰色边年龄为531±5.7Ma，浅灰色边年龄为509±5.9Ma，这些变质年龄可能反映了持续较长时间的泛非期构造热事件（ca.540~510Ma）。结合相关区域已有资料分析，拉斯曼丘陵西部斯图尔内斯半岛及邻区片麻岩记录的中低压高温麻粒岩相变质作用可能反映了造山作用后期的抬升与伸展。;Different types of gneiss are exposed in the Stornes Peninsula and its adjacent region of the Larsemann Hills, East Antarctica. In this paper, a comprehensive study of garnet-orthopyroxene gneiss (Sample LSM178-2) and garnet-sillimanite-spinel gneiss (Sample LSM302-10) is carried out by combining petrography, phase equilibrium modelling and geochronological analysis. The results indicate that the gneisses experienced medium-low pressure high-temperature granulite facies metamorphism. The estimated peak P-T conditions of the Sample LSM178-2 are T=830~870℃, P=6.1~8.9kbar, which retrogressed to T<705℃, P=3.6~5.4kbar. The peak P-T ranges of the garnet-sillimanite-spinel gneiss (Sample LSM302-10) are T=860~1050℃, P=4.8~10.2kbar, followed by retrograde conditions of T=820~840℃, P=4.7~5kbar. The metamorphic age corresponding to the development of the major mineral assemblage of Sample LSM178-2 should be ~543±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 (ca. 540~510Ma). Combined with the existing data in the relevant regions, the medium-low pressure high-temperature granulite facies metamorphism recorded by the gneisses in the Stones Peninsula and its adjacent region in the west of the Larsemann Hills may reflect the uplift and extension during the late stage of orogenesis.