大兴安岭南段罕庙地区石英二长闪长岩锆石U-Pb年龄及地球化学特征

Zircon U-Pb ages and geochemical characteristics of the quartz monzonite diorite rocks from Hanmiao area in the southern segment of the Da Hinggan Mountains

为确定罕庙地区石英二长闪长岩的形成时代、成因及地质意义,对罕庙地区石英二长闪长岩体进行了岩相学、LA-MC-ICP-MS锆石U-Pb年龄及地球化学研究。结果表明,石英二长闪长岩中的锆石为岩浆成因,^(206)Pb/^(238)U年龄加权平均值为134±1Ma,侵入时代属于早白垩世晚期。石英二长闪长岩中SiO_2含量为63.44%~68.96%,K_2O含量为2.04%~3.84%,Na_2O/K_2O值介于0.47~1.17之间,铝饱和指数A/CNK介于2.12~2.55之间,属于钙碱性-高钾钙碱性过铝质花岗岩类。微量元素特征为富集大离子亲石元素(如Rb、K)和活泼不相容元素(如Th、U),强烈亏损高场强元素(如Nb、Ta、P、Ti)。结合区域构造演化背景,该岩体受俯冲流体改造,与早白垩世晚期古太平洋板块斜向俯冲于欧亚大陆之下的弧后伸展减薄等综合作用有关。

In order to determine the geochronology,petrogenesis and tectonic background of the quartz monzonite diorite rocks in Hanmiao area, the authors conducted the research on petrology by means of LA-MC-ICP-MS zircon U-Pb geochronology and geochemistry. The results show that the zircons from the quartz monzonite diorite rocks are magmatic zircons,and the zircon 206Pb/ 238U dating yielded 134±1Ma（n=22,MSWD=0.42）,suggesting late Early Cretaceous. The values of SiO2, K2O and Na2O/K2O are 63.44%-68.96%, 2.04%-3.84% and 0.47-1.17 respectively. The aluminum saturation indexes （A/CNK） range from 2.12 to 2.55.The rocks thus belong to calc-alkaline- high-K calc-alkaline series. The rocks are enriched in LILE （Rb, K） and incompatible elements（Th, U）, strongly depleted in HFSE such as Nb, Ta, P and Ti. Combined with the study of regional tectonics the authors hold that the quartz monzonite diorite rocks might have been affected by subducted slab fluids. It was closely related to back-arc extension tectonics and lithospheric thinning after the subduction of Paleo-Pacific plate towards Eurasia plate.