东南极拉斯曼丘陵夕线石榴二长片麻岩的格林威尔期变质作用和构造意义

东南极拉斯曼丘陵出露的中-高级变质岩是普里兹湾基底岩系的主要组成部分,是研究普里兹湾高级区组成与演化的理想对象。本文对拉斯曼丘陵米洛半岛地区的夕线石榴二长片麻岩进行了岩石学、相平衡模拟和地质年代学研究。研究结果表明拉斯曼丘陵夕线石榴二长片麻岩矿物组合为石榴子石+堇青石+斜长石+钛铁矿+夕线石+石英±黑云母±钾长石,经历了格林威尔期(~980Ma)高温变质,其峰期阶段(M1)的矿物组合为石榴子石+斜长石+钾长石+钛铁矿+夕线石+石英,变质温压条件为:~820℃、~9.6kbar。夕线石榴二长片麻岩的岩浆锆石比变质锆石具有更低的LREE和更高的HREE含量,其原岩的沉积年龄不早于~1073Ma,物源主要为中元古代岩浆岩(S-型花岗岩)。拉斯曼丘陵在格林威尔期经历了陆-陆碰撞,进而快速折返、剥蚀作用的过程。

Grenvillian metamorphism of sillimanite-garnet feldspar paragneiss in the Larsemann Hills, East Antarctica and tectonic implications

The medium- to high-grade metamorphic rocks exposed in the Larsemann Hills of East Antarctica are the main components of the Prydz Bay basement rock series. They are ideal for studying the composition and evolution of the Prydz Belt. In this paper,detailed petrological analyses,phase equilibrium modeling,and zircon analyses were performed on the sillimanite-garnet feldspar paragneiss from the Mirror Peninsula in the Larsemann Hills. The study results show that the paragneiss consists of the mineral assemblage garnet+cordierite+plagioclase+ilmenite+sillimanite+quartz±biotite±K-feldspar and underwent HT metamorphism in the Grenvillian Period (~980Ma). The peak metamorphic assemblages (M1) are represented by the matrix minerals garnet+plagioclase+K-feldspar+ilmenite+quartz,the phase equilibrium modeling shows that the metamorphic conditions of the paragneiss are ~820℃,~9.6kbar. The maximum deposition age of the paragneiss is constrained between ~1073Ma and 980Ma. It was formed by denudation and deposition of the Mesoproterozoic magmatic rocks,and the source rock belongs to S-type granite. The magmatic zircons in the paragneiss have lower LREE and higher HREE content than those of the metamorphic zircons in the same rock. During the Grenvillian Oeriod,the Larsemann Hills had a near-isothermal depressurization clockwise metamorphic P-T trajectory,experienced the process of continent-continent collision and then rapid exhumation and sedimentation,the S-type granite was associated with this process.