内蒙古锡林浩特岩群岩石学特征及变质温压条件

Petrology and metamorphic temperature-pressure conditions of Xilinhot Group,Inner Mongolia,China

锡林浩特岩群出露于内蒙古锡林浩特市东南部,指原锡林郭勒杂岩中表壳岩部分,为一套片麻岩夹层状斜长角闪岩、磁铁石英岩和变粒岩等的变质岩组合。选取岩群中片麻岩及斜长角闪岩进行岩石学及岩相学分析,其中斜长角闪岩主要矿物组合为角闪石+斜长石;片麻岩样品中见夕线石+钾长石矿物组合,石榴子石具明显进变质环带,所以在计算其形成条件时选取了生长于峰期变质阶段的特定部位。运用角闪石-斜长石、石榴子石-黑云母矿物温压计分别估算锡林浩特岩群中准同时形成的变质基性火山岩及变质碎屑岩的变质温压区间。综合二者计算结果,得锡林浩特岩群峰期变质温压条件为660～707℃,0.5～0.6GPa。变质达高角闪岩相。其变质年龄为1 000Ma左右,推测锡林浩特岩群角闪岩相变质为中元古代末期锡林浩特微陆块与其他陆块碰撞的结果。

Xilinhot Group,the supracrustal rock part of Xilin Gol complex,outcropped in the southeastern vicinity of Xilinhot of Inner Mongolia,is composed of gneiss with layers of amphibolite,magnetite quartzite and granofels.In this paper,we research the petrology of gneiss and amphibolite from Xilinhot Group.The amphibolites mainly contain amphibole＋plagioclase assemblage;the sillimanite＋K-feldspar assemblage appears in gneiss.The garnet from gneiss exhibits a pronounced compositional zoning resulting from progressive metamorphism.In such a case,the garnet domains that grow at peak metamorphic stage were chosen to calculate its formation temperatures.The p-T conditions of metabasic volcanic and metaclastic rocks penecontemporaneously formed in Xilinhot Group were appraised by garnet-biotite and hornblende-plagioclase geothermobarometry.The results suggest that the peak metamorphism of the Xilinhot Group had reached high amphibolite facies and its p-T ranges were 660-707 ℃and 0.5-0.6 GPa.Its metamorphic age is about 1000 Ma.The amphibolite facies metamorphism of Xilinhot Group might be result from the collision between Xilinhot microcontinent and other continental blocks.