【Title】PRELIMINARY STUDY ON METAMORPHIC BASIC ROCKS IN THE GROVE MOUNTAINS, EAST ANTARCTICA
【摘要】东南极格罗夫山存在一套经历了峰期麻粒岩相变质作用的镁铁质麻粒岩和斜长角闪岩。对含石榴石的镁铁质麻粒岩的详细研究则显示了近等温降压 (ITD)的顺时针PT演化轨迹 ,与拉斯曼丘陵有相似的演化历史。岩石的主量元素组成和玄武岩一致 ,并且具有拉斑玄武岩演化趋势。进一步的地球化学研究表明 ,这套玄武岩为洋岛型玄武岩 (OIB)和洋中脊型玄武岩(MORB)的组合。OIB型具有大体类似的地球化学性质 ,它们均富集Ti(TiO2 =2 .68% )、REE( =2 0 2 μg/g)、LREE[(La/Yb) N=4.8]、Ti/Y( =343)、Zr/Y( =3.1 ) ,具洋岛玄武岩的特征 ,推测岩浆来源于富集地幔源区 (EM)。而MORB型以低Ti (TiO2 =1 .1 %— 1 .31 % ) ,明显低于OIB的P的含量 (P2 O5=0 .1 %— 0 .2 % ) ,低REE ( 4 7— 93μg/g)、LREE/HREE( 2 .2 7— 2 .5 4 )、(La/Yb) N( =1 .30— 1 .62 )为特征 ,具洋中脊玄武岩的特征。MORB和OIB组合的出现说明在泛非期该区可能存在过洋盆。
【Abstract】Mafic granulites and amphibolites that underwent granulite facies metamorphism are outcropping in the Grove Mountains (GMs), East Antarctica. The mafic granulites underwent a near isothermal decompression (ITD) P T history from 0.93GPa and 800℃ (M1) to 0.64GPa and 805℃ (M2). This clockwise P T path can be explained in a model involving extensional collapse and exhumation of thickened crust, which was common with that of Larsemann Hill, East Antarctica. The compositions of major elements from the rocks are consistent with those of basalt and have evolutional current of tholeiite. Based on more geochemical signatures, two types of basalt are recognized, namely, ocean island basalt (OIB) and mid ocean ridge basalt (MORB). Type 1 rocks have similar geochemical characteristics, with high in Ti (TiO 2 =2.68%), REE (=202μg/g), LREE \[(La/Yb) N =4.8\], Ti/Y (=343) and Zr/Ti (=3.1), which are similar to those of OIB, and are considerable to be products of magma from enriched mantle sources (EM). Type 2 rocks are characterized by low Ti (TiO 2 = 1.1-1.31% ), REE (47-93μg/g), LREE/HREE (2.27-2.54), (La/Yb) N (=1.30-1.62) and lower P (P 2 O 5 =0.1-0.2%) than those of OIB, which are similar to those of MORB. The presence of such rock assemblages suggests the existence of an ocean basin in this region during Pan Africa ages.