阿拉善东部波罗斯坦庙杂岩含石榴子石黑云斜长片麻岩的p-t演化及独居石U-Pb年龄OA北大核心

The Boluositanmiao Complex in the eastern part of the Alxa Block,is one of the high-grade metamorphic complexes,which has important geological significance for the late Paleozoic tectonic evolution of the Alxa Block.In this paper,a combined study of detailed geological investigation,petrography,LA-ICP-MS monazite U-Pb dat-ing and phase equilibrium modeling,were carried out for felsic gneisses of the Boluositanmiao Complex,which is used to reveal their metamorphic evolution and tectonic implications.Felsic gneisses are mainly composed of garnet-bearing biotite-plagioclase gneiss and biotite-plagioclase gneiss.The garnet-bearing biotite-plagioclase gneiss sam-ples ALS2202 and ALS2204 record peak mineral assemblage of garnet+biotite+muscovite+ilmenite+plagioclase+K-feldspar+quartz+melt.Combined with phase equilibrium modeling,the peak p-t conditions of two samples were constrained at 727～751 ℃/820～1 030 MPa and 730～751 ℃/930～1 070 MPa,respectively.In addition,the gar-net-biotite-plagioclase-quartz(GBPQ)geothermobarometry yields relatively low metamorphic p-t conditions of 629～672℃/410～490 MPa and 620～657℃/330～400 MPa,which correspond to the retrograde cooling.Based on the above results,a clockwise p-t evolution was obtained for the garnet-bearing biotite-plagioclase gneiss samples ALS2202 and ALS2204.LA-ICP-MS monazite U-Pb analyses obtained the weighted mean 206Pb/238U ages of 268±4 Ma and 255±2 Ma for the garnet-bearing biotite-plagioclase gneiss sample ALS2207 and the biotite-plagioclase gneiss sample ALS2210,respectively,which may represent the timing of peak metamorphism or anatexis.Based on the mineral assemblage,it suggests that the protolith of these felsic gneisses should be sedimentary rocks.Com-bined with the clockwise p-t path and previously published data in the region,this indicates a significant crustal thickening process in the middle and late Permian,which may be related to the closure of the Paleo-Asian Ocean.