Geochronological, Geochemical Characteristics And Tectonic Setting Of Granitic Rocks In The Hulin Area, Heilongjiang Province

This paper elucidates the distribution range,characteristics,forming age,petrogenesis,magma source,and tectonic environment of the Hulin granitic rocks of the northern Khanka massif,based on the studies of field characteristics,petrology,petrography,LA-ICP-MS zircon U-Pb ages,and whole-rock major and trace element geochemistry.Then,the paper confirms the regional granitic magma activity which the Hulin granitic rocks are subordinative,and discusses the tectonic-coupling relationship between the rocks and the Tanlu Fault,Hulin Basin,Hulin Schist and Early Cretaceous NE-trending magmatic-volcanic-metamorphic rocks in eastern NE China.Finally,the paper defines the geodynamic setting of the Hulin granitic rocks.The analysis of isotope chronology of LA-ICP-MS zircon U-Pb shows that the Hulin granitic rocks produced at 127.8 ± 2.8 Ma?granodiorite porphyry dike from northern Taihe village?,114.1 ± 1.4 Ma?granodiorite pluton from the Tiansheng area?,115.2 ± 1.4 Ma?biotite adamellite pluton from the Xishan area?,and 114.9 ± 1.4 Ma?granodiorite porphyry dikes from northern Qingfeng village?respectively.All of them formed in the early stage?131¹18 Ma?and middle stage?116¹11 Ma?of the Early Cretaceous NE-trending granitic magmatism in eastern NE China,which belongs to part of the regional magma activity.Whole-rock major and trace element geochemistry indicates that: The samples have low TiO₂,Fe₂O₃ and P₂O₅ contents,and relatively high Na₂O content,belong to the metaluminous to weakly peraluminous,medium-K and calc-alkaline series,are enriched in LREE and large ion lithophile elements?e.g.,Rb,Ba,and K?,and are depleted in HREE and high field strength elements?e.g.,Nb,P,and Ti?.Moreover,the integrated studies on field characteristics,mineral composition,major and trace element geochemistry,discriminant diagrams of petrogenesis,magma source,and tectonic environment of Hulin granitic rocks show that: The samples are classified to I-type granite,the primary magma originated from the partial melting of mafic rocks from thickened lower crust and was effected by the partial melting of enriched mantle caused by fluid metasomatism in a subduction zone,and the samples formed in an extensional environment of continental arc.Combined the above results with regional research data,I conclude that the Hulin granitic rocks,Tanlu Fault,Hulin Basin,Hulin Schist,and Early Cretaceous NE-trending magmatic-volcanic-metamorphic rocks in eastern NE China share a tectonic-coupling relationship.All of them produced in the Early Cretaceous NW-SE extensional environment,and the extensional intensity shows intensive at the early stage and weak at the late stage.The formation of Hulin granitic rocks and the Early Cretaceous extensional activity are related to a geodynamic setting characterized by lithospheric gravitational instability of pre-Cretaceous thickened lithosphere,and lithospheric delamination and thinning triggered by the Early Cretaceous subduction of the Pacific Plate.