东南极拉斯曼丘陵早古生代麻粒岩相变质事件——Sm-Nd同位素年代学证据

EARLY PALEOZOIC EVENT OF LOW-PRESSURE GRANULITE FACIES METAMORPHISM IN THE LARSEMANN HILLS, EAST ANTARCTICA: EVIDENCE FROM Sm-Nd ISOTOPIC CHRONOLOGY

对东南极拉斯曼丘陵地区典型的暗色麻粒岩及其主要矿物组合、区域最常见的变质矿物石榴石的Sm-Nd同位素年代测定表明,本区主期低压麻粒岩相变质作用发生在早古生代早期(泛非事件)。暗色麻粒岩—紫苏辉石—角闪石—斜长石Sm-Nd等时年龄为540±75(2σ)Ma。片麻状浅色花岗岩—石榴石—钾长石Sm-Nd等时年龄为497±7(2σ)Ma。野外地质证据还表明,本区的构造面貌和变质记录主要是早古生代早期热事件形成的,并与这一时期部分熔融事件关系密切。

Sm-Nd isotopic dates of both a representative mafic granulite sample (G212-3, M_1) and a gneissic leucogranite sample(W20401,M_1) indicate that the low-pressure granulite facies metamorphism in the Larsemann Hills, East Antarctica occurred in earlier Early Paleozoic ('Pan-African Event'). The former (G212-3), which consists of orthopyroxene-hornblende -plagioclase-whole rock yields an internal isochron of 540±75 (2σ) Ma; the latter (W20401), which is made up of garnet-K-feldspar-whole rock gives an internal isochron of 497±7 (2σ)Ma. Evidence from field geology shows that the low-pressure granulite facies metamorphism in the area was accompanied by extensive patial melting as well.