Study On Metamorphic Rock Series Of ‘Jiageda Formation'in The Moerdaoga Area In Northern Great Xing'an Range

The metamorphic rock series of the “Jiageda Formation” are mainly distributed on the Erguna-Xing'an massif,with the Mashan Group,Xinghuadukou Complex,Zhalantun Group,Dongfengshan Group,etc.They have been considered as the Precambrian basements of the micro continental blocks in the northeast of China.Meanwhile,it is also an important "window" for people to study the evolution history of early crust in Northeast China.However,there are still many controversies about the formation timing,tectonic setting and metamorphic process of “Jiageda Formation”.This paper makes a detailed study on the metamorphic rocks of “Jiageda Formation”,which are exposed in the Moerdaoga area of the northern part of Great Xing'an Range,in order to explore the new evidence for the evolution of the early continental crust evolution.The “Jiageda Formation” metamorphic rocks of study area are exposed to phyllite and mica schist,with a small amount of metamorphic sandstone.In this paper,the garnet and staurolite mica schist,which records the multi-stage metamorphic process,is the main research object.According to the mineral assemblage and chemical characteristics,the temperature and pressure conditions of three stages are obtained: the prograde metamorphic stage: T=560~575?,P=6.5~6.8kbar;the near peak stage: T=580~600?,P=7.0~7.2kbar;retrograde cooling stage: T=550~570?,P=6.0~5.0kbar.Theses three stages constitute a typical clockwise P-T path,which suggesting that the metamorphism of the “Jiageda Formation” could be related to the subduction-collision and exhumation processes.The results show that phyllite and mica schists are enriched in Al and K and metamorphic sandstones are enriched in Si,based on the major and trace elements tests.These three groups exhibit similarity in trace elements geochemistry,with strong enrichment in light REEs and negative Eu anomalies.The primitive mantle-normalized trace element pattern shows strong depletion in Sr.Furthermore,Nb,Ta,P,Ti are relatively depleted,and most high field-strength elements are enriched.The protoliths of the studied samples are mainly graywake with a small amount of clay rocks.The uppercrust felsic sources with possible old sediment component make a contribution to their provenance.The tectonic discrimination diagrams show that the protoliths were deposited in an active continental margin setting and had a large number of clastic rocks deposited in continental island arc setting.The sediments show low maturity with moderate chemical weathering.Zircon U-Pb geochronology study results show that the detrital zircons in the metamorphic rocks of the “Jiageda Formation” are mostly magmatic origin.The distribution of the detrital zircons is wide,which concentrated in Neoproterozoic(1.0-0.8 Ga)and Paleoproterozoic(2.0-1.8 Ga).It implied that the material sources of “Jiageda Formation” were mainly derived from Neoproterozoic and Paleoproterozoic,and the strata began to receive sedimentation in the late Neoproterozoic.The igneous rocks originally defined as “Jiageda Formation” were mostly formed in the Paleozoic-Mesozoic and did not belong to the Precambrian basement,which should be disintegrated.