东南极拉斯曼丘陵镁铁质麻粒岩的变质作用演化

Metamorphism evolution of mafic granulite from the Larsemann Hills,East Antarctica

拉斯曼丘陵（Larsemann Hills）位于东南极普里兹构造带的中部，研究该区麻粒岩的变质作用演化对于理解普里兹带的构造属性至关重要。通过对该区含石榴石镁铁质麻粒岩转石详细的岩相学观察表明，峰期前进变质阶段矿物组合（M1）由角闪石＋斜方辉石＋单斜辉石＋斜长石＋黑云母＋钛铁矿4-石英±磁铁矿组成，其峰期矿物组合（M2）为石榴石＋斜方辉石＋单斜辉石＋角闪石＋钛铁矿4-磁铁矿±石英，而代表后期与降压有关的叠加变质组合（M3）为斜方辉石＋斜长石＋单斜辉石＋黑云母＋钛铁矿-4磁铁矿。矿物化学分析，结果显示其中石榴子石和斜方辉石具有弱的成分环带特征。利用THERMOCALC软件在NCFMASHTO体系下对该麻粒岩进行了详细的热力学模拟，结合传统温压计和平均温压计算结果，得出不同阶段温压条件分别为650—750℃／5．5—6．5kb（M1），850—950℃／8—8．5kb（M2），800—900℃／5．5—7．5kb（M3）。其变质作用演化为典型的峰期后近等温减压的（ITD）顺时针P—T轨迹。通过区域上镁铁质麻粒岩的对比分析，我们认为该镁铁质麻粒岩可能来源拉斯曼丘陵基岩露头。结合已有的年代学资料，表明该镁铁质麻粒岩的峰期变质事件可能对应于晚元古代格林威尔期构造事件，而后期退变质作用与早古生代的泛非期构造事件有关，意味着泛非期普里兹带可能是陆内造山带。

The Larsemann Hills are located in the central part of the Prydz tectonic belt in East Antarctica. The study of granulite metamorphism of this area is essential to understanding the tectonic nature of the Prydz belt. The petrographic study of the garnet- bearing mafic granulite boulder collected from the region demonstrates that pre-peak prograde mineral assemblage （ M1 ） consists of hb ＋ opx ＋ cpx ＋ pl ＋ bi ＋ ilm ＋ q ＋ mt, and peak metamorphic assemblage （ M2 ） comprises g ＋ opx ＋ cpx ＋ hb ＋ ilm ＋ mt ＋ q, whereas the symplectite opx ＋ pl ＋ bi ＋ ilm ＋ mt represents a superimposed assemblage （ M3 ） associated with a decompression event. Mineral chemistry shows that garnets and orthopymxenes in the mafic granulite have weak compositional zonings. Through the use of THERMOCALC program, the thermodynamic modeling in the NCFMASHTO system has been undertaken for the mafic granulite. Combined with conventional thermobarometers and the average P-T estimates, P-T conditions of the different metamorphic stages are estimated as 650 ~ 750~C/5.5 - 6. 5kb （ M1 ）, 850 - 950~C/8 N 8.5kb （ M2）, and 800 ~ 900~C/5.5 - 6. 5kb （ M3 ）, respectively. Its metamorphism evolution of the mafic granulite is a typical post-peak isothermal decompression （ITD）clockwise P-T path. Compared with the mafic granulites in this region, we argue that this sample may originate from basement in Larsemann Hills. Combined with available chronological data in the region, it is suggested that the peak metamorphism likely corresponds to the Greenville high-grade tectonism, whereas post-peak retrograde metamorphism may be related to the Pan-African high-grade tectonism, implying that the Pan- African Prydz belt could be an intracontinental mobile belt.