Zircon U-Pb Age,Geochemistry And Genesis Of The Early-Middle Ordovician Granites In Baiyinna Area,Heilongjiang

This paper,detailed chronological and geochemical studies of the Early-Middle Ordovician granites have been carried out,and new knowledge has been gained in the formation age,geochemical characteristics,magmatic evolution and tectonic setting of the Early-Middle Ordovician granites.LA-ICPMS zircon U-Pb dating results show that:the magmatic activity time of Early-Middle Ordovician granites in Baiyinna area ranges from 480 to 460 Ma.The Early-Middle Ordovician granite assemblage is composed of granodiorite,monzonite and syenite,belonging to intermediate-acid-acid rocks.Geochemical characteristics show that the Ritman index is less than 3.3,belonging to calc-alkaline series rocks.The K₂O-SiO₂diagrams show that the rocks are calc-alkaline-high potassium calc-alkaline series.The absolute value of A/CNK is less than 1.1,and the injection point of the sample shows weak peraluminous granite.The distribution patterns of rare earth elements are similar,all of which have the trend of left high right low right inclination,light rare earth enrichment,heavy rare earth fractionation and negative Eu anomaly,indicating that the Early-Middle Ordovician granite may be the product of the crystallization differentiation of the homologous magma.The geochemical characteristics of trace elements are high enrichment of large ion lithophile elements?Rb,Ba,Th,U?and loss of high field strength elements?P,Nb,Ta,Ti?,which indicates that their magma may be the product of crustal melting.The characteristics of Early-Middle Ordovician granite rock assemblage are consistent with those of post-orogenic granite assemblage.In the discriminant diagram of tectonic environment,all of them fall into the post-collisional granite area,which indicates that the tectonic background of the formation is post-orogenic or post-collisional environment.