藏北大横山—弯岛湖一带的中—晚三叠世地层——对若拉岗日构造带地质演化的地层学制约

Stratigraphic constraints of the Mid-Late Triassic strata on the geological evolution of the Rola Kangri tectonic belt in the Daheng Mountain-Wandaohu area, northern Tibet, China

在藏北大横山地区二岔沟一带的硅质岩、硅质泥岩中首次发现了拉丁晚期—卡尼早期的放射虫动物群,建立了二岔沟组。二岔沟组在区域上出露比较广泛,并往往与蛇绿岩相伴产出,系蛇绿岩的上覆沉积岩系,根据其中的放射虫化石的时代及获得的蛇绿岩Sm-Nd全岩等时线年龄值232Ma±11Ma,推断蛇绿岩形成于中三叠世晚期—晚三叠世早期。若拉岗日群的解体和二岔沟组、弯岛湖蛇绿岩的基本特征,表明大横山地区中三叠世晚期—晚三叠世早期为多岛洋盆扩张的鼎盛时期,晚三叠世—早侏罗世早期的消减闭合、碰撞造山运动形成了大横山构造蛇绿混杂岩带。

The late Latinian-early Camian radiolarian fauna was for the first time found in siliceous rocks and siliceous mudstone in the vicinity of Erchagou of the Dahengshan area, northern Tibet, and the Erchagou Formation has been established. The Erchagou Formation is widely exposed at a regional scale and usually occurs in association with ophiolites. It is a sedimentary sequence overlying the ophiolites. According to the age of the radiolarian fossils therein and available Sm-Nd isochron isotope age （232±11 Ma） of ophiolites, it is inferred that the ophiolites originated in the late Middle Triassic to early Late Triassic. The disintegration of the Rola Kangri Group and basic characteristics of the Erchagou Formation and Wandaohu ophiolite suggest that the late Middle Triassic to early Late Triassic in the Dahengshan area witnessed the culmination of the archipehgoic ocean basin spreading. The Late Triassic-early Ea/ly Jurassic comumption and closing of the ocean basin and collisional orogeny resulted in the formation of the Dahengshan tectonic ophiolitic mélange zone.