The Early Cretaceous Magmatism And Tectonic Setting Of The Southern Segment Of The Great Xing'an Range

The Great Xing'an Range is located in the northeast of China.Since Paleozoic,this area has experienced the superimposition and transformation of the Paleo-Asian,the Mongol-Okhotsk Ocean and Paleo-Pacific tectonic systems,and developed large-scale magmatism,the Early Cretaceous magmatic rocks are the most intense,but the origin and tectonic setting of the magmatic rocks are still controversial.In view of the problems existing in the study of Early Cretaceous magmatic rocks in the Southern Great Xing'an Range,based on the systematic petrology,zircon chronology and geochemistry analysis of Lanjiayingzi gabbro-diorite pluton in the north of Linxi Country,genesis of the pluton and the Early Cretaceous tectonic setting are discussed.Zircon U-Pb dating reveals that the pluton was formed in Early Cretaceous(142Ma-147Ma).Geochemistry of the pluton is characterized by 52.79%-54.86%of SiO2 contents,1.15%-1.71%of K2O contents and 3.49%-4.05%of Na2O contents,with 0.75-0.85 of A/CNK,which indicates that the pluton belongs to a calc-alkaline metaluminous magmatic rock.It's MgO and TFe2O3 contents vary from 5.11%-7.10%and 7.53%-9.35%respectively,with 34.16 to 40.41 of Mg#values.Combined with the girdle of plagioclase and the poikilitic texture of olivine in clinopyroxene,it is suggested that the pluton was the product of the fractional crystallization of mantle-derived primary magma.In addition,the pluton is relatively rich in LILEs such as Rb and Ba,and depletion in HFSEs such as Nb and Ta,with+4.7 to+10.0 of zircon ?Hf(t)values.which indicates that the pluton was derived from a depleted lithospheric mantle that experienced metasomatism from the subduction fluid.Based on the regional geological data,the Early Cretaceous magmatic rocks in southern Great Xing'an can be divided into two stages:the early stage of Early Cretaceous(150Ma-135Ma)and the late stage of Early Cretaceous(135Ma-115Ma).The early stage of Early Cretaceous magmatic rocks have bimodal characteristics,indicating that they are the product of extensional processes,Spatially,the Early Cretaceous magmatic rocks distribute in the Southern Great Xing'an Range parallel to the Mongol-Okhotsk tectonic belt,it is suggested that the formation of the Early Cretaceous magmatic rocks in the Southern Great Xing' an Range has relation with the post-collision extensional setting resulting from closure of the Mongol-Okhotsk Ocean.The late stage of Early Cretaceous magmatic rocks indicates continuous series from basic to acidic.In spatially,the magmatic rocks in the Southern Great Xing'an Range is consistent with the subduction zone of the Paleo-Pacific Ocean.Thus,it is believed that the formation of the magmatic rocks of the late stage is related to the subduction of the Paleo-Pacific.