东准噶尔晚石炭世双峰式火山岩年代学、地球化学及其构造意义

Geochronology,Geochemistry and Geological Significance of the Late Carboniferous Bimodal Volcanic Rocks in the Eastern Junggar

新疆东准噶尔卡拉麦里造山带晚石炭世双峰式火山岩很好地记录了中亚造山带晚古生代时期洋陆转换阶段复杂的岩浆作用过程,对该过程的详细剖析能更好地理解中亚造山带的地质历史.通过该区域晚石炭世巴塔玛依内山组火山岩详细的岩石学、地球化学、锆石U-Pb年代学和Sr-Nd-Pb同位素组成的研究,并结合区域上已有的研究成果,获得了如下认识:(1)该套火山岩组合形成于晚石炭世早期320.2±4.2Ma,为晚石炭世早期陆相喷发的产物.火山岩具明显的双峰式组合的特征,基性端元由碱性系列和拉斑系列的玄武岩、玄武质粗面安山岩组成;酸性端元由粗面岩和流纹岩组成,成分上相当于A型花岗岩;(2)岩石地球化学和同位素特征显示,该套双峰式火山岩来源于不同的岩浆源区,基性岩来自于亏损的地幔源区,在岩浆上升过程中发生橄榄石及单斜辉石的分离结晶作用并遭受了地壳混染,而酸性岩来自于下地壳的部分熔融;(3)该套双峰式火山岩产出于后碰撞末期的构造环境,由于洋壳的拆沉作用而引发软流圈上涌,使得上覆的地幔发生部分熔融产生岩浆,同时由于底侵作用导致地壳下部发生部分熔融,喷发出地表形成该双峰式火山岩套,这套双峰式火山岩的出现,标志着东准噶尔卡拉麦里地区造山作用进入尾声.

The study region locates in the southwest margin of Central Asian Orogenic belt, which recorded several volcanic events of different tectonic environment. The volcanic rocks from Late Carboniferous Batamayineishan Formation includes the alkaline series and sub-alkaline series, which formed in 320.2±4.2 Ma. They have a SiO2interrupt ranging from 56% to 67.5%. The basaltic rocks and the felsic rocks are closely associated. They show a typical characteristic of bimodal volcanic rocks. Their geochemistry and isotope characteristics suggesting that the basaltic rocks come from a depleted mantle source whereas the felsic rocks come from a source due to anatectic melting of the lower crust and show characteristic of A-type granites. The assemblages of volcanic rocks and the geochemical information of volcanic rocks from Batamayineishan Formation indicate they formed in apost-collision environment. Combining the research results ofthis region, the authors consider the Batamayineishan Formation were generated mainly in the end stage of post-collision of Kalamaili orogenic, which began after cessation of collision, and involved extensional detachment of the thickened lithosphere,leading to extension and thinning of the crust. © 2016, Editorial Department of Earth Science. All right reserved.