东南极拉斯曼丘陵泥质麻粒岩的变质作用演化

Metamorphism Evolution of Pelitic Granulites from the Larsemann Hills,East Antarctica

普里兹湾拉斯曼丘陵代表了东南极一条重要的早古生代的～530Ma泛非期(Pan-African)高级构造活动带。然而,该区早期的晚元古代的～1000Ma格林维尔期(Grenvellian)高级变质作用的演化历史至今仍有争论。该区呈透镜状产出的泥质麻粒岩峰期矿物组合(M1)为石榴石+堇青石+斜方辉石+钾长石+石英,峰期石榴石变斑晶发育堇青石或堇青石+斜方辉石反应边(M2)。利用Thermocalc程序在KFMASH模式体系对该泥质麻粒岩进行的定量模拟表明,其峰期矿物组合是由反应石榴石+黑云母+石英=堇青石+斜方辉石+钾长石+熔体形成的。利用Themocalc平均P-T计算方法获得峰期M1变质P-T条件为～0.9GPa和～900℃,而叠加的M2组合反映了一个减压冷却的过程,其变质P-T条件为～0.7GPa和800～850℃。结合已有的年代学数据,认为该区泥质麻粒岩的峰期M1矿物组合反映晚元古代(～1000Ma)格林维尔期挤压D1构造事件,而叠加的M2矿物组合与M3蠕虫状结构则形成于早古生代泛非期(～530Ma)D2～D3高级扭压剪切构造期间。该扭压事件导致了面状高低应变带的发育以及进步花岗岩和伟晶岩的侵入。

The Larsemann Hills in Prydz Bay represent an important high--grade tectonic active belt in East Antarctica in early Paleozoic at -530 Ma （Pan-African） . However, the early Late Proterozoic -1000 Ma （Grenvellian） high-grade metamorphic history of the region is still controversial. A pelitic granulite lens in the region preserves peak assemblage grt-crd-opx-ksp-qtz （M1）, and peak garnet grains develop cordierite or crd-opx corona （M2）. Quantitative modelling for the Al-rich pelitic granulite via Thermocalc program has been undertaken in the KFMASH model system, suggesting that the peak assemblage was formed by the crossing of the reaction grt＋bt＋qtz = crd＋opx＋ksp＋melt. The average P-T calculations indicate that M1 peak metamorphic P-T conditions reached -0.9 GPa and - 900 ℃, while the overprinting M2 assemblage reflects a decompression-cooling path with peak P-T conditions of -0.7 GPa and 800-850 ℃. Combined with available geochronological data and field relationships, the peak M1 metamorphic assemblage preserved in the Al-rich pelitic granulite is considered to be associated with the Late Proterozoic （-1000 Ma） Grenvellian compressional tectonic D1 event, whereas the overprinted M2 assemblages and M3 myrmeckites may have formed during the Early Paleozoic （-530 Ma） Pan-African high-grade transpressional tectonic D2-D3 events, which led to the development of planar high- and low-strain zones and the intrusion of the Progress Granite and pegmatites in the region.