新疆博格达南缘后碰撞期陆内裂谷和水下滑塌构造

The post-collision intracontinental rifting and olistostrome on the southern slope of Bogda Mountains, Xinjiang

本文较为系统地研究了哈密市七角井-车轱辘泉和乌鲁木齐市白杨沟地区晚古生代后碰撞期陆内裂谷和滑塌构造的几何学、岩石学和地球化学特征。研究表明,在博格达山南缘的七角井-车轱辘泉一带,玄武岩和流纹岩彼此互层产出,构成厚度较大的后碰撞期双峰式火山岩系,呈近东西方向平行造山带展布,堆积在早二叠世红色磨拉石之上;在白杨沟一带,后碰撞水下滑塌堆积岩与火山岩共存,构成一个厚1300m的滑塌构造剖面,由水下滑塌堆积岩夹双峰式火山岩带、枕状熔岩-气孔状玄武岩带和硅质、泥砂质岩带所组成。其底部以北东走向的拆离断层和晚石炭世灰岩、砂岩岩层接触。在岩石地球化学特征上,火山岩的SiO2含量在53%-67%之间出现间断,呈现双峰式火山岩的特点。其中,流纹岩SiO2平均71.65%,CaO2.15%,Na2O与K2O含量相近(3.2%-3.7%),含碱量>含钙量,ANKC值>1.1;轻稀土富集、稀土总量高,铕亏损,具明显Eu负异常。与之共生的玄武岩则以富硅贫碱为特征,SiO2平均51.05%,TiO2含量变化较大,轻稀土轻度富集,铕异常不明显,属拉斑系列玄武岩类。其特征和东非阿法尔裂谷的双峰式火山岩相近,反映陆内裂谷环境。对滑塌堆积岩带中的辉绿玢岩作锆石U-Pb测年,获289±5Ma年龄值,相当于二叠纪初期。从晚二迭叠世开始,全区处于相对稳定的剥蚀-堆积环境中,形成了厚度较大的磨拉石,并逐渐演化为三叠纪的准平原化环境。

A regional-scale tectonic event of closure of ocean and collision-orogeny look place in the interval from Late Devonian to Late Carboniferous in the Xinjiang area. At the late stage of collision, some tectonic changes occurred in the piedmont area. The preliminary framework of mountains-basins of northern Xinijiang was composed of both Late Carboniferous-Early Permian orogenic belt and piedmont basins. In the direction toward continent, post-collision intracontinental rifting and olistostrome developed well. The bedded diabase and bimodal volcanic rocks were distributed in the Early Permian strata in the piedmont area. The most typical area is located on the southern slope of Bogda Mountains, namely the Qijiaojing-Cheguluquan segment of Hami city and the Baiyanggou section of Urümqi. This paper studied the geometrical, petrologieal and geochemical features of intracontinent bimodal volcanic rocks and olistostrome. Of them, post-collision olistostrome is the first time discovery by us. In the Qijiaojing-Cheguluquan segment, basalt and rhyolite are ranged alternately that overlie the Early Permian red molasses. This post-collision bimodal volcanic series with a big thickness is distributed in parallel with the sub-E-W mountain extension. In the Baiyanggou section, post-collision underwater oiistostrome co-exists with undenvater eruption and is composed of olistostrome zone, pillow lava and pore basalt zone, and siliceous shale and siltstone zone; the bottom plane is contacted with Late Carboniferous limestone and sandstone series by a NEE-trending detachment fault. Geochemically, rhyolitic rocks are characterized by average contends of SiO2 71.65% , CaO 2. 15% , Na2O≈K2O 3.2% -3.7%, (Na2O+K2O) >CaO, ACNK value >1. 1; high Σ REE and rich in LREEs, depleted in Eu and marked negative Eu anomalies. In contrast with it, basaltic rocks are marked by rich in SiO2 (51. 05% on average) and poor in ( Na2O + K2O) , weakly rich in LREEs, weak depleted in Eu, belonging to the tholeiite series. Theses features are similar with the bimodal volcanic series of Alfa rift (East Africa) , reflecting a within-continent rift environment. The bedded diabasic porphyrite in oiistostrome yields a U-Pb age ol 289±5Ma on zircon grains, equal to the Early Permian time. Since the Late Permian, studied region was in a stable sedimentary setting, forming molasses with large thickness that overlie the olistostrome, and then evolved gradually into a Triassic pene-planed environment that is characterized by uplifting-erosion of ranges and accumulation of basins.