黑龙江呼玛地区早侏罗世二长花岗岩的岩石成因及地质意义

The petrogenesis and geological significance of early Jurassic monzogranites in the Huma area of Heilongjiang Province

本文通过岩石学、年代学、岩石地球化学等手段,对位于兴安地块东北部的黑龙江呼玛地区早侏罗世二长花岗岩进行岩石成因和构造背景的研究。结果表明,区内早侏罗世二长花岗岩的形成时间集中在(176.3±0.8)Ma~(177.7±1.1)Ma,具有高硅、中低钾、中等镁铁及贫钙特征,属高钾钙碱性系列;富集轻稀土和大离子亲石元素,亏损重稀土和高场强元素,Eu负异常不明显(0.70~1.50);随SiO 2含量的增加P 2 O 5含量递减,有相对高的Sr/Y(9.67~148.07)和La N/Yb N(7.66~54.12)比值,低的Mg#(23.07~39.37)Cr(6.00~11.60)×10^(-6)和Ni(4.69~9.39)×10^(-6)值,具有I型花岗岩特征。其Nb/Ta比值略高于大陆地壳均值,Th含量介于地壳与地幔丰度值区间,表明有幔源成分的加入。通过构造背景判图显示,大部分样品落入火山弧环境中,结合区域上相近年代侵入岩的分布特征及岩石地球化学特征规律,推测岩浆形成于与蒙古-鄂霍茨克洋南向俯冲有关的大陆边缘弧环境。

By means of petrology,chronology and lithogeochemistry,this article studies the early Jurassic monzogranites in the Huma area of Heilongjiang Province,located in the northeast of the Great Xing'an Mountains,revealing their petrogenesis and tectonic background.The diagenesis time of the Early Jurassic monzogranites in the study area ranges from(176.3±0.8)Ma to(177.7±1.1)Ma,characterized by high silicon,medium to low potassium,moderate magnesium iron,and poor calcium in lithogeochemistry.The rocks belongs to the high potassium calcium alkaline series,enriched in light rare earth elements(LREE)and large ion lithophilic elements(Rb,K,Sr,Ba,etc.),but depleted in heavy rare earth elements(HREE)and high field strength elements(Nb,Ta,Ti,P,Zr,etc.),with no significant negative Eu anomaly(0.70-1.50).Compare to the increase of SiO 2,the monzogranites shows a decrease trend in P 2 O 5,with relatively high Sr/Y(9.67-148.07)and La N/Yb N(7.66-54.12)ratios,low Mg#(23.07-39.37),Cr(6.00-11.60)×10^(-6),and Ni(4.69-9.39)×10^(-6) values,indicating the characteristics of I-type granites.Its Nb/Ta ratio is slightly higher than the average of the continental crust,and the Th content is between the abundance values of the crust and mantle,indicating the addition of mantle derived components.The tectonic background map shows that most of the samples fell into a volcanic arc environment.Based on the distribution characteristics of intrusive rocks of similar ages in the region and its geochemical characteristics,it is speculated that the magma formed in a continental margin arc environment related to the southward subduction of the Mongolian-Okhotsk Ocean.