东准噶尔东北缘阿舒达斯晚石炭世碱性花岗岩SHRIMP锆石U-Pb定年、岩石地球化学特征及构造意义

LA-ICP-MS Zircon U-Pb Dating,Geochemical Characteristics and Tectonic Significance of Ashudas Late Carboniferous Alkaline Granite in Northeast Margin of Eastern Junggar,Xinjiang

准噶尔盆地周缘出露大量晚古生代花岗岩,与古亚洲洋的闭合及陆陆碰撞造山过程密切相关。对东准噶尔东北缘阿舒达斯一带碱性花岗岩进行年代学和岩石地球化学研究,通过对SHRIMP锆石U-Pb测定,确定该地区碱性花岗岩结晶年龄为(319.3±2.5)Ma。岩石化学组成上碱性花岗岩具高Si O2(70.16%~75.64%)、富碱(Na2O+K2O=8.57%~10.15%)和贫Ca O(0.26%~0.73%)、低Al2O3(11.99%~14.07%)特征。微量元素上富集大离子亲石元素Rb,Th和高场强元素Nb,Zr,Hf,亏损Ba和Sr元素,轻重稀土元素分馏较明显,(La/Yb)N=3.07~6.77,具强Eu负异常,δEu=0.14~0.3。稀土元素分布模式呈右倾海鸥型,具极高的全岩锆石饱和温度,为873℃~918℃,平均894℃,显示典型A型花岗岩特征。据前人研究成果,阿舒达斯一带A型花岗岩为乌伦古碱性花岗岩带的一部分,可能是幔源岩浆底侵,导致富Nb玄武岩部分熔融后与其发生的岩浆混合,经一定程度陆壳混染和结晶分异作用的产物,其形成标志着东准噶尔北缘造山阶段的结束和板内伸展环境的开始。

A large number of late Paleozoic granites are found around junggar basin,it is closely related to the closure of the ancient Asian ocean and the process of land-land collision orogeny.Geochronology and petrochemistry study of alkaline granites in the area of ashudas,northeastern margin of eastern Junggar.The crystal age of the alkaline granites was determined to be(319.3±2.5)Ma by the LA-ICP-MS zircon U-Pb determination in this area.The alkali granite is characterized by high SiO2(70.16%~75.64%),high alkaline(Na2O+K2O=8.57%~10.15%),low CaO(0.26%~0.73%)and low Al2O3(11.99%~14.07%).It is enriched in light rare earth elements(LREE),Rb,Ta,Zr,Hf,Th and depleted in Ba,Sr.The fractionation of the light and heavy rare earth elements is obvious((La/Yb)N=3.07~6.77),showing right dipping lines with strong negative Eu anomaly and obvious right-leaning.It also has extremely high zircon saturation temperature(873~918℃,average 894℃),it shows the characteristics of typical A-type granite.The data presented here in conjunction with previously published data suggest that Ashudasi A-type granite has been formed by partial melting of the predominantly Nb-rich basalt in the lower crust,which was caused by mantle-derived magma underplating in a post-collision or intracontinental setting and influenced by crustal contamination and crystallization differentiation.Its formation marks the collision event in the northern margin of the East Junggar and the beginning of intraplate extensional environment.