藏南康马奥陶系及其底砾岩的发现并初论喜马拉雅沉积盖层与统一变质基底的关系

Discovery of the Ordovician and its basal conglomerate in the Kangmar area, southern Tibet-with a discussion of the relationof the sedimentary cover and unifying basement in the Himalayas

藏南北喜马拉雅拉轨岗日带康马岩体西南侧,奥陶系及其底砾岩覆盖于前奥陶系拉轨岗日群(POL)之上,后者被515～485Ma拉轨岗日构造穹隆带花岗岩侵入,沿不整合面又被泛非运动最晚期的基性脉岩侵入。奥陶系与前奥陶系的接触关系为伸展不整合,与喜马拉雅甚至冈底斯带有关剖面完全可以对比。这一不整合面即是冈瓦纳大陆北缘统一变质基底和沉积盖层的分界。冈瓦纳大陆统一变质基底的形成始于震旦纪末的“泛非运动”,其终止时间在喜马拉雅及以北地区可以延续至寒武纪—奥陶纪之交,表现为区域上不断的伸展—拉张—裂解的构造环境。由此可以认为伸展构造亦是控制统一变质基底与沉积盖层的形成机制之一。

In southern Tibet, Ordovician and its basal conglomerate cover the pre-Ordovician Lhagoi Kangri Group on the southwest side of the Kangmar rock body in the Lhagoi Kangri belt of the North Himalaya, the latter is intruded by the 515-485 Ma old granite of the Lhagoi Kangri structural dome and again by the latest-stage basic dike of the Pan-African movement along an unconformity between the Ordovician and pre-Ordovician. This extensional unconformity may well correlate with the related section in the Himalayas and even the Gangdise belt and is just the boundary between the unifying basement on the northern margin of the supercontinent Gondwana and sedimentary cover. The unifying basement of Gondwana began to form in the 'Pan-African movement' at the end of the Sinian, and the time of formation may have continued to the turning between the Cambrian and Ordovician in the Himalaya and to its north, which is manifested by the tectonic environment of regional continuous extension and breakup. Therefore, the extensional movement is also one of the mechanisms for controlling formation of the unifying metamorphic basement and sedimentary cover.