小兴安岭南部早侏罗世二长花岗岩形成时代、地球化学特征及地质意义

Formation age,geochemical characteristics and geological significance of the Early Jurassic monzonitic granites in southern Xiao Hinggan Mountains

通过对小兴安岭南部二长花岗岩的LA-ICP-MS锆石U-Pb同位素定年和岩石地球化学分析,确定了其形成时代及岩石成因。测得二长花岗岩的同位素年龄为188±2Ma,形成于早侏罗世。地球化学特征显示其富硅、富碱,CaO、Fe203、TiO2、MnO、MgO和P2O5的含量较低,A/CNK=0.95~1.12,A/NK=1.17~1.32,属于准铝-弱过铝质、高钾钙碱性系列岩石;富集大离子亲石元素Rb、K和高场强元素Hf、Zr、Th,相对亏损大离子亲石元素Ba、Sr和高场强元素Nb、Ta、Ti和P;稀土元素总量(∑REE)较高,配分曲线分布型式为轻稀土元素(LREE)相对富集、重稀土元素(HREE)相对亏损的右倾型,表现出轻微的负Eu异常。元素地球化学特征表明,二长花岗岩显示出I型花岗岩特征。结合区域研究资料,小兴安岭南部早侏罗世二长花岗岩的形成应与古大平洋板块向欧亚大陆下的俯冲作用和蒙古-鄂霍茨克洋向额尔古纳地块之下的俯冲作用,即双向俯冲作用的弧后伸展环境相对应,其岩浆起源于下地壳物质的部分熔融。

In this paper,the formation age and petrogenesis of the monzonitic granites were confirmed through LA-ICP-MS zircon U-Pb dating and geochemical analysis in southern Xiao Hinggan Mountains.It is found that the isotopic age of monzogranite is 188±2Ma,suggesting Early Jurassic.The geochemical characteristics show that Si and ALK are abundant,CaO,Fe2O3,TiO2,MnO,MgO and P2O5 are relatively poor,with A/CNK being 0.95~1.12 and A/NK being 1.17~1.32,implying aluminum-weakly peraluminous high potassium calc-alkaline series granites;They are enriched in large ion lithophile elements Rb,K and high field strength elements Hf,Zr,Th,and depleted in large ion lithophile elements Ba,Sr and high field strength elements Nb,Ta,Ti and P.The total rare earth elements(∑REE)are abundant,the distribution curve shows that the light rare earth elements are more abundant than the heavy rare earth elements and show right-inclined pattern,with weak negative Eu anomalies.The geochemical characteristics of elements indicate that the granites belong to I-type granites.Combined with regional research data,it is suggested that the Early Jurassic monzonitic granites in southern Xiao Hinggan Mountains were formed under an extensional environment similar to back-arc basin which might have been related to double-subductions of the Paleo-Pacific plate beneath the Eurasian continent and the Mongol-Okhotsk oceanic plate beneath the Ergima Massif,and the magmas originated from partial melting of lower crustal materials.