Progressive Metamorphism Of Metabasites In The Baoyintu Group,Central-western Inner Mongolia

The Baoyintu Group,which is distributed in the areas of Langshan,Tugurige and Damaoqi in the central-western Inner Mongolia,mainly composed of mica-schist,quartz schist,quartzite,metabasite and marble.The metabasites in Geraobao,Baoyintu and Tugurige are amphibolite occurring as bedded or lenticular bodies within mica-schist and they are characterized by different mineral assemblage.The garnet amphibolite in Geraobao mainly contains garnet,hornblende,plagioclase,rutile and ilmenite,the amphibolite in Baoyintu contains hornblende,plagioclase and sphene and the garnet epidote amphibolite in Tugurige mainly contains garnet,hornblende,epidote,rutile and ilmenite.Geochemical study shows that the amphibolites are characterized by tholeiitic basalt series.The amphibolites show right-dipping REE patterns with∑REE=79.29×10^-6～150.3×10^-6,（La/Yb）_N=2.17～8.70 andδEu=0.66～1.12,and they have no obvious negative anomaly in Ta,Nb and Ti,showing an affinity to enriched mid-ocean ridge basalts（E-MORB）.Discrimination diagram indicates that the protolith of amphibolites may have formed in an intra-continental rift setting.Theε_Nd（T）ratios（-0.6～3.5）indicate that the protolith of amphibolites may have suffered weak crustal contamination during the emplacement.Phase equilibria modeling shows that sample LS01 from Geraobao,LS19 from Baoyintu and LS35 from Tugurige experienced clockwise P-T paths including prograde,peak and near isothermal decompression or decompression with cooling stages.The peak P-T condition of sample LS01 is estimated to be～11 kbar/～735℃with the peak mineral assemblage predicted to be garnet+hornblende+plagioclase+biotite+diopside+rutile+melt.However,the P-M（H₂O）pseudosection shows that the diopside would not be retained due to the saturated fluid penetration during the post-peak evolution.The peak P-T condition of sample LS19 is estimated to be～8kbar/650℃with the peak mineral assemblage predicted to be hornblende+plagioclase+sphene.The peak P-T condition of sample LS35 is estimated to be～8kbar/675℃,and the peak mineral assemblage is predicted to be garnet+hornblende+epidote+rutile.The different peak P-T conditions of amphibolites are indicative of the progressive metamorphism in the Baoyintu Group.Zircon LA-ICP-MS U-Pb dating indicates that the metamorphic zircons in garnet amphibolite sample LS01 are aged at394±8Ma.Consequently,it is inferred that the protolith of metabasite in the Baoyintu Group may have formed in the Neoproterozoic rifting basins in the northern margin of North China Craton.In the middle Devonian,the Baoyintu group as a part of the North China Craton,was involved in the orogenic process associated with the closure of the Paleo-Asian Ocean and experience progressive metamorphism of medium P/T series.