漠河盆地西缘漠河组形成时代及物源区构造环境判别

大兴安岭北部漠河盆地广泛发育漠河组,其形成时代、物源区特征及大地构造背景等对研究盆地的形成与演化具有重要意义。本文在漠河盆地西缘恩和哈达地区漠河组内发现了2层凝灰岩夹层。采用LA-ICP-MS U-Pb测年方法进行了锆石U-Pb年代学研究,分别获得(158±1)Ma、(157±2)Ma的年龄,表明漠河组形成于晚侏罗世早期。砂岩岩石地球化学分析结果显示,漠河组砂岩SiO_2含量高(70.20%～76.13%),SiO_2/Al_2O_3值为4.99～5.87,K_2O/Na_2O值为0.59～1.62;在稀土元素球粒陨石标准化图解上,曲线与活动大陆边缘的稀土元素曲线吻合较好;样品富集高场强元素(Th、Hf)和大离子亲石元素(Rb、K),亏损高场强元素(Nb、Ta、Ti、P)和大离子亲石元素(Ba、Sr),微量元素蛛网图曲线与活动大陆边缘相似。地球化学物源分析图解和大地构造背景判别表明:漠河组砂岩的母岩主要来源于石英质沉积岩和酸性火山岩的混合区。砂岩碎屑锆石LA-ICP-MS U-Pb测年结果显示,碎屑锆石年龄与盆地南缘前寒武系基底和各期花岗岩类岩石的时代一致。综合研究认为,漠河组沉积物主要来自盆地南缘额尔古纳、兴安等地块,物源区既有古老陆壳物质,也有年轻的火成岩,大地构造背景以活动大陆边缘为主。

Stratigraphic age of the Mohe Formation in the western margin of Mohe Basin and tectonic environment discrimination of provenance

The Mohe Formation is widely developed in Mohe Basin which is located in the northern part of the Da Hinggan Mountains. Its formation age, provenance characteristics and tectonic background are of great geological significance for the study of the formation and evolution of the basin. Two interbeds of tuff were discovered in the Mohe Formation in the western margin of Mohe basin, Enhe-Hada area. Zircon U-Pb dating by LA-ICP-MS U-Pb method yields the weighted average ages of (158±1) Ma and (157±2) Ma respectively, indicating that the Mohe Formation rocks were formed in early Late Jurassic. The geochemical analysis of Mohe Formation sandstone shows that the content of SiO₂ is high (70.20%-76.13%), the value of SiO₂/Al₂O₃ is 4.99-5.87, and the value of K₂O/Na₂O is 0.59-1.62. In the chondrite-standardized diagram of rare earth elements, the curve is consistent with that of the active continental margin. In the high field strong elements and large-ion lithophile elements, the samples are enriched in Th, Hf, Rb and K, and deficit in Nb,Ta, Ti, P, Ba and Sr. The cobweb curve of trace elements is similar to that of the active continental margin. The geochemical source analysis and tectonic background discrimination show that the parent rocks of the Mohe Formation sandstone were mainly originated from the mixing of the quartzite sedimentary rocks and the acidic volcanic rocks. The ages of the detrital zircons in the sandstone coincide with the ages of the Precambrian basement and granitic rocks in the southern margin of the basin. The formation age, provenance characteristics and tectonic setting of the Mohe Formation, which is widely developed in the Mohe Basin in the north of the Great Hinggan Mountains, are of great significance to the study of the formation and evolution of the basin.