全吉地块花岗片麻岩中镁铁质岩包体的矿物演化和锆石定年与古元古代区域变质作用

摘要

A wall-rock-derived meta-mafic enclave was discovered in the Eady-Paleoproterozoic Delingha monzogranitic gneiss in the middle part of the Quanji Block. The enclave includes three roclss, from the margin to the center, being hornblende plagioclase pyroxenite, amphibolite, and garnet-hornblende schist. Three generations of mineral assembla ges have been identified from petrological evidence and microtextural relationship in the hornblende plagioclase pyroxe nite. The generation I is of clinopyroxene＋ brownish green homblende＋ plagioclase＋ quartz＋ ilmenite, the genera tion Ⅱ is of bluish green homblende＋ plagioclase＋garnet and the generation Ⅲ is of actinolite＋albite＋zoisite. The generation I assemblage indicates the peak metamorphic condition of upper amphibolite-facies. Zircons from the hornblende plagioclase-pyroxenite all display multi-faced crystals; their CL images are characterized by high-temperature recrystallization. LA-ICPMS dating of the zircons yielded an upper intercept age of 1 913±38 Ma (MSWD=0.15),suggesting peak metanorphism in the late Paleoproterozoic. It is suggested that all regionally developed monzogranit ic gneisses in the Quanji Block underwent regional metamorphism with the Dakendaban Group, in response to the global assembly event of the Columbia supercontinent.%全吉地块中段古元古代早期的德令哈二长花岗片麻岩中发育一个来自围岩的变质镁铁质岩包体.该包体的岩性从核部到边缘依次为角闪斜长辉石岩、斜长角闪岩和石榴石角闪片岩.角闪斜长辉石岩的显微结构指示三个世代的矿物演化,世代Ⅰ为单斜辉石+棕绿色普通角闪石+斜长石+石英+钛铁矿,世代Ⅱ为蓝绿色普通角闪石+斜长石+石榴子石,世代Ⅱ为阳起石+钠长石+黝帘石.世代Ⅰ指示其峰期变质达高角闪岩相条件.角闪斜长辉石岩的锆石全部为浑圆状多晶面晶体,其CL图像具高温变质结晶特征.LA-ICPMS 技术测定获得上交点U-Pb年龄为(1 913±38)Ma(MSWD=0.15),指示峰期变质发生在古元古代未.研究表明,全吉地块中区域性分布的古元古代早期的二长花岗片麻岩与达肯大坂岩群在响应全球Columbia超大陆汇聚事件中一起发生了区域变质作用.