Petrogenesis And Geological Significance Of Paleoproterozoic Metabasic Rocks In Xiuyan Area,East Liaoning Province

The metabasic sedimentary rocks of Liaohe Group are widely distributed in xiuyan area of east Liaoning Province.These metabasic sedimentary rocks have metamorphosed from green schist facies to amphibolite facies and even granulite facies,recording a series of tectonic-thermal events from Paleoproterozoic to present.In the process of regional geological mapping in Liaodong,the author found that there are a large number of paleoproterozoic metabasic rocks in Liaohe group.For the further study of the tectonic evolution of the paleoproterozoic of the Liaohe group in the Jiao-Liao-Ji belt,the author studies these metabasic rocks from many aspects,such as field geological characteristics petrography,zircon,chronology and geochemistry.By deeply Exploring the genesis of these metabasic rocks deeply which will provides new evidence and data for the study of tectonic evolution in the paleoproterozoic of Liaohe Group.The paleoproterozoic metabasic rocks in Xiuyan area are mainly divided into plagioclase diorite modified by strong deformation and shallow metabasic,non-deformable,metamorphic diabase/gabbro.These metamorphic rocks mainly intrude into paleoproterozoic monzonites,the lieryu rock formation and some marble containing diopside of dashiqiao rock formations.They are scattered and varied in shape,including vein,lump,stony,lenticular,etc.Microscopic observations show that these metamorphic rocks have metamorphised from the chlorite phase to the amphibole phase.The main minerals of them are plagioclase,amphibole,chlorite,actinolite and diopside.According to the U-Pb chronology of zircon,the metabasic rocks were formed in two different stages,the first stage is ~2.16 Ga,~2.13 Ga and had experienced metamorphism between 1.90 to 1.87Ga;the second stage is 1.87 Ga.Geochemical studies showing that the metabasic rocks formed before orogenesis are mainly calc-alkaline series,loss-making high field strength elements(HFES)U,Nb,Ta,medium enrichment of large ionic lithophile elements(LILE,e.g.Ba?K?Pb),light and heavy rare earth fractionation is obvious.The formation of these metabasic rocks is related to the post-arc extensional environment.The variable gabbro formed after orogenic action have formed in to the early stage of regional uplift(1.87Ga~1.85Ga).It belongs to the series of porphyry basalt.It has the characteristics of loss-rich HFES Th,U,Nb,Ta,medium-enriched LILEs(e.g.Ba,K,Pb).The variable pyrochlore was formed in the late stage of regional uplift and belongs to the calc-alkaline basalt series.loss-rich HFES Nb,Ta and Ti,enriched LILEs(e.g.Rr,Ba,Pb).The metagabbro formed after orogeny have formed in the early stage of regional uplift(1.87 Ga ~1.85Ga).The formation of metamorphic rocks after orogenic period is related to mantle upwelling,which may be formed by partial melting of the "left" mantle wedge before orogenic period.Combined with previous studies,this paper holds that the tectonic evolution of the Liaohe group in paleoproterozoic was recorded by the metabasic rocks formed before and after the orogenic period.The metabasic rocks before the orogenic period are formed in the tectonic environment of the back-arc basin.It is shown that the ancient Proterozoic Liaohe rock group was deposited in the environment of the back-arc basin,and Jiao-Liao-Ji belt experienced the process of early continental crust stretching and cracking to form ocean basin,and the process of deposition.The metabasic rocks after orogenic period are formed in the initial rifting environment caused by subduction plate fragmentation and mantle upwelling,and indicating that the tectonic evolution of Liaohe Group in ancient Proterozoic has a clockwise P-T-t trajectory.