满洲里—额尔古纳地区中生代花岗岩的锆石U-Pb年代学与岩石组合:对区域构造演化的制约

Zircon U-Pb Chronology and Assemblages of Mesozoic Granitoids in the Manzhouli-Erguna Area,NE China:Constraints on the Regional Tectonic Evolution

本文对满洲里地区灵泉盆地、包格德乌拉盆地及额尔古纳地区上护林盆地和恩和盆地及周边的原确定为古生代和中生代的花岗质岩石进行了岩石学和锆石LA-ICP-MSU-Pb年代学研究,以便揭示研究区中生代的构造演化历史。研究区内12个代表性花岗岩中的锆石均呈自形—半自形晶,显示出典型的岩浆生长环带,结合其较高的Th/U比值(0.31～3.63),暗示其为岩浆成因。测年结果表明,该区中生代花岗质岩浆活动可划分成以下三期:(1)中三叠世岩浆活动,可进一步划分成241Ma和229Ma两期岩浆事件,241Ma黑云母正长花岗岩和229Ma正长花岗岩的存在可能与古亚洲洋闭合后的伸展环境有关;(2)早-中侏罗世岩浆事件,可进一步划分成(180±5)Ma和(171±2)Ma两期岩浆事件,黑云母二长花岗岩-正长花岗岩组合,结合其斑岩型Mo矿的存在,反映研究区处于活动陆缘的构造背景,可能与蒙古-鄂霍茨克洋的俯冲作用有关;(3)早白垩世早期岩浆活动,可进一步划分成(140～150)Ma和(134±2)Ma两期岩浆事件,前者与区域内发育的吉祥峰组火山岩形成时代相近,后者的火口充填型产状表明它们应是该期岩浆事件演化晚期的产物,该期岩浆事件在松辽盆地以东地区的缺乏暗示它们形成于伸展环境,并与蒙古—鄂霍茨克缝合带的演化有关。

LA-ICP-MS zircon U-Pb dating data of the Mesozoic granitoids from Lingquan,Baogedewula and Shanghulin basins in Manzhouli and Erguna area are obtained to constrain the regional tectonic evolution.Zircons from 12 representative granitoids are euhedral-subhedral in shape and display typical oscillatory zoning in CL images as well as high Th/U ratios of 0.31~3.63,implying their magmatic origin.The dating results indicate that the Mesozoic granitic magmatism in the region can be divided into three stages,i.e.,the Middle Triassic,Early-Middle Jurassic,and Early Cretaceous magmatism.The Middle Triassic granitoids can be further subdivided into two stages,241 Ma biotite syenogranite and 229 Ma syenogranite,which could be formed under an extensional environment after the final closure of the Paleo-Asian ocean in the late Paleozoic or early Mesozoic.The Early-Middle Jurassic granitoids can be further subdivided into two stages,180±5 Ma biotite monzogranite and 171±2 Ma syenogranite.Combined with the existence of the coeval porphyry-type Mo deposit,our data suggest that the Early-Middle Jurassic magmatism could be formed under an active continental margin which was related to the subduction of the Mongol-Okhotsk oceanic plate beneath the Erguna Massif.The Early Cretaceous magmatic rocks consist of 140-150 Ma granite porphyry and 134±2 Ma quartz-monzonite porphyry.The former is similar to the Jixiangfeng Formation in time,the latter occurs as crater fills and could belong to the late evolutionary products of this magmatism.Combined with the lack of coeval volcanic rocks in the eastern Jilin-Heilongjiang provinces,the data suggest that these volcanic rocks could be formed under an extensional setting related to the evolution of the Mongol-Okhotsk orogen.