大兴安岭北段裸河组物源及其构造环境:来自地球化学和碎屑锆石年代学的证据

Provenance and tectonic environment of Luohe Formation in northern Great Xing'an Range:evidence from geochemistry and detrital zircon age

为探讨大兴安岭北段罕达气地区出露的裸河组形成的大地构造背景,对研究区裸河组泥质粉砂岩中的碎屑锆石进行LA—ICP—IMS U—Pb年代学分析和地球化学分析等工作。定年结果显示:本区裸河组<1 000 Ma的年龄主要可分为两组,444～550 Ma:峰值年龄460 Ma; 700～1 000 Ma,峰值年龄为780 Ma,反应物源区岩浆事件的年龄,其中最小的年龄限定裸河组形成时代下限为晚奥陶世。主量元素及稀土元素特征显示碎屑岩特征与长英质岩石一致,轻稀土富集,重稀土亏损,弱Eu负异常,指示裸河组物源区具有活动大陆边缘构造背景。结合样品岩相学、稀土元素特征及前人资料,推断在早古生代时期研究区受嫩江洋俯冲影响,其沉积环境可能与弧后盆地沉积有关。

The study of detrital zircon LA-ICP-MS U-Pb chronology and geochemical analysis are carried out on the argillaceous siltstone of Luohe Formation to explore the geotectonic background of the Luohe Formation that exposed in the Handaqi region in the northern Great Xing’an Range. The results show that the detrital ages < 1 000 Ma of the Luohe Formation can be divided into two groups,i. e.,444 ~ 550 Ma with a 460 Ma peak and 700 ~1 000 Ma with a 780 Ma peak,reflecting the ages of magmatic events in the provenance. The minimum age suggests that the lower limit of Luohe Formation was the Late Ordovician. The characteristics of the major elements and the rare earth elements show that the characteristics of clastic rocks are consistent with felsic rocks,with high LREE,low HREE and weaker Eu negative anomalies,indicating that the provenance of Luohe Formation was in active continental margin tectonic setting. Integrating sample petrography,characteristics of the rare earth elements and previous data,it is concluded that in the Early Paleozoic,the study area was influenced by the subduction of the Nenjiang Ocean and the depositional environment might be related to the back arc basin sedimentation.