漠河盆地二十二站组砂岩形成时代及物源区构造环境判别

大兴安岭北部的漠河盆地广泛发育中生界二十二站组砂岩，其形成时代一直备受争议.首次利用碎屑锆石LA-ICP-MS U-Pb法测年对其形成时代作出较为精确的限定，并结合地球化学特征对其物源区及大地构造背景进行了探讨.研究结果表明，二十二站组砂岩碎屑物磨圆度较低、分选差，表现出源区相对不稳定，快速剥蚀、搬运及沉积的特征.锆石颗粒粗大，具有清晰的震荡环带，Th/U值为0.35～1.07，为典型的岩浆锆石.测年结果显示，90个测点年龄主要分布在3个群落:152～170Ma，峰值年龄约为158Ma，这一年龄区间揭示了二十二站组沉积成岩的下限为晚侏罗世；179～193Ma，峰值年龄约为190Ma；205～214Ma，峰值年龄约为210Ma.3个年龄峰值为蒙古-鄂霍茨克洋演化过程中一系列岩浆活动的地质记录，其中210Ma与190Ma峰值年龄与盆地南缘额尔古纳地块晚三叠世、早侏罗世的岩浆事件相吻合，而158Ma峰值年龄则对应于盆地北缘晚侏罗世的岩浆事件.主、微量元素构造判别图解揭示其物源区具有活动大陆边缘的特征，同时锆石定年数据显示其没有任何前中生代的碎屑物，暗示了该3期岩浆岩均形成于额尔古纳地块北缘的中生代活动大陆边缘环境，并为二十二站组的形成提供了主要碎屑物质.由此说明，二十二站组砂岩形成于晚侏罗世蒙古-鄂霍茨克洋闭合后的造山过程中，漠河盆地南北两侧物源区快速剥蚀、快速搬运与快速沉积的环境. 

Geochronology of Ershi'erzhan Formation Sandstone in Mohe Basin and Tectonic Environment of Its Provenance

o address the controversy of the Ershi'erzhan Formation sandstone formation ages in the Mohe basin of the northern Great Khingan Range, this study presents an accurate limit to its formation ages using the LA-ICP-MS detrital zircon U-Pb dating for the first time in this paper, and discusses its provenance and tectonic setting combined with the geochemistry of major and trace elements. The results show that the fragmental material in sandstone has poor psephicity and sorting, indicating that source region is relatively unstable with characteristics of rapid denudation, transportation and sedimentation. The grains of detrital zircons are big with clear oscillatory zoning, and Th/U values range from 0.35-1.07, which is indicative of a magmatic origin. Zircon U-Pb dating results show that 90 dating data are distributed in the following three periods, namely, 152-170Ma with the peak of 158Ma, standing for the lower limit of Ershi'erzhan Formation sedimentary rocks at Late Jurassic; 179-193Ma with the peak of 190Ma and 205-214Ma with the peak of 210Ma. The three peak ages are the evolution records of the Mongolia-Okhotsk ocean. The peak ages of 210Ma and 190Ma are consistent with the Late Triassic and Early Jurassic magmatic events of south margin of the basin in the Erguna massif, the peak age of 158Ma fits the Late Jurassic magmatic event of north margin of the basin. The geochemical diagrams of major and trace elements reveal that the provenance has the background of active continental margin. Meanwhile, the zircon dating shows that there is no Pre-Mesozoic clastics in the Ershi'erzhan Formation. It suggest that these were generated within an active continental margin setting at the north margin of the Erguna massif in Mesozoic, and provided the most clastics for the Ershi'erzhan Formation. Combining with the previous studies, it is concluded that the Ershi'erzhan Formation sandstone was formed in Late Jurassic under the environment of rapid denudation, transportation and sedimentation, controlled by the uplift of the Mongolia-Okhotsk orogenic belt, indicating that the eastern Mongolia-Okhotsk ocean has already closed in Late Jurassic.