西藏林芝地区林芝岩群麻粒岩及时代讨论

林芝地区雅鲁藏布江大峡谷西侧的尼洋河、雅鲁藏布江河谷地带,广泛出露有属冈底斯岩浆弧基底岩系的林芝岩群。笔者等在2001—2004年进行1∶25万林芝县幅区域地质调查中,于雅鲁藏布江东岸的里龙及西岸扎西绕登一带,在林芝岩群下部中高级变质基性岩中新发现呈透镜状、饼状、瘤状、长条状、豆荚状等零星分布的麻粒岩。初步研究,化学成分具有富MgO,高CaO,低TiO2、K2O,Al2O3/TiO2比值接近20,与世界上典型玄武质科马提岩特征相似;稀土总量较底,类似于中—基性火山岩;原岩可能为一套富镁的超镁铁—镁铁质火山岩系。时代属古元古代。

The granulites and ages of the Nyingchi Group Complex in the Nyingchi region,Xizang

The Nyingchi Group Complex assigned to the basement series of the Gangdise magmatic arc crops out along the valleys of the Nyang and Yarlung Zangbo Rivers on the western side of the Yarlung Zangbo Grand Canyon in the Nyingchi region, Xizang. During the geological survey in recent years,the granulites are identified as lenses, cakes, nodules and pods in places in the intermediate- to high- grade metamorphic basic rocks in the lower part of the Nyingchi Group Complex in Nailung on the eastern bank and Zhaxiraodeng on the western bank of the Yarlung Zangbo River.The granulites are characterized by higher MgO and CaO contents, lower TiO_2 and K_2O contents,and Al_2O_3/ TiO_2 ratios of nearly 20, similar to those of representative basaltic komatiite in other places of the world.The lower rare earth element amounts show the similarity to the intermediate- basic volcanic rocks. Their prolioths may be represented by Mg- rich ultramafic and mafic volcanic rocks which are traced back to the Palaeoproterozoic.