为了探讨小兴安岭南部晚三叠—早侏罗世岩浆活动及构造演化,查明其岩石成因、形成年代及构造背景,笔者对小兴安岭南部晚三叠—早侏罗世二长花岗岩和正长花岗岩的岩石学、地球化学及同位素年代学等进行研究.结果表明,二长花岗岩和正长花岗岩锆石具明显岩浆震荡生长环带,为岩浆锆石,LA-MC-ICP MS 锆石 U-Pb 测年分别为(202.5±1.9)Ma、(191.5±1.4)Ma,时代为晚三叠—早侏罗世.岩石地球化学特征显示其具有高硅、富钾、富碱、低钠的特征,为铝质高钾钙碱性系列的I型花岗岩;富集Rb、K、Th、Hf等元素,亏损Ba、Sr、Ti、Nb、P等元素;轻稀土元素较重稀土富集,δEu=0.37～0.58,铕具明显负异常.结合区域对比及综合研究,初步认为小兴安岭南部花岗岩形成于古太平洋板块向欧亚大陆下俯冲形成的活动大陆边缘环境.
Geochemical characteristics,formation time and geological significance of Late Triassic-Early Jurassic granites in southern Xiao Hinggan Ling(Mts.)
To investigate the magmatic activity and tectonic evolution of Late Triassic-Early Jurassic in south-ern Xiao Hinggan Ling(Mts.),and to identify its petrogenesis,formation time and tectonic setting,petrology,geochemistry and isotopic chronology of Late Triassic-Early Jurassic monzogranites and syenogranites in Xiao Hing-gan Ling(Mts.)have been studied.The results show that the zircons of monzogranites and syenogranites are mag-matic zircons with obvious magmatic oscillations and growth belts.The LA-MC-ICP MS zircons have U-Pb dates of(202.5±1.9)Ma and(191.5±1.4)Ma,respectively,being Late Triassic to Early Jurassic.The geochemical characteristics show that the rocks are high silicon,rich potassium,rich alkali and low sodium,which is a Ⅰ-type granite of aluminous high potassium calc-alkaline series.Enrichment of Rb,K,Th,Hf,and deficit of Ba,Sr,Ti,Nb,P.Light rare earth elements are more abundant than heavy rare earth elements with an obvious negative anomaly(δEu=0.37～0.58).Based on regional correlation and comprehensive study,it is preliminarily concluded that the tectonic setting of the granites in southern Xiao Hinggan Ling(Mts.)is an active continental margin environ-ment formed by the subduction of the paleo-Pacific plate to Eurasia.
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