雅鲁藏布大峡谷地区蛇绿混杂岩带初步研究

The preliminary study of the ophiolitic melanges along the Yarlung Zangbo Grand Canyon, Xizang

雅鲁藏布大峡谷地区主要有南迦巴瓦群 (Pt1 nj)、大拐弯群 (Mz)和冈底斯群 (Pt1 gd)三个岩石地层单元。其中大拐弯群主要分布在楔入体的东缘 ,主要由变玄武岩 /辉绿岩、辉石岩、镁质橄榄岩、石英岩和大理岩等组成 ,岩石类型和化学成分特征与蛇绿岩套类似。但其岩石化学性质与典型的大洋中脊蛇绿岩差别较大 ,形成环境为具有陆壳性质的中等—慢速扩张的小型洋盆 ,属陆间海 /弧后盆地。自 4 5Ma以来 ,该地区经历了强烈的褶皱、隆升和剥蚀作用 ,使下部地壳暴露地表并形成无与伦比的世界第一大峡谷。

Three lithostratigraphic units are recognized along the Yarlung Zangbo Grand Canyon, Xizang: Namjagbarwa Group (Pt 1nj), Daguaiwan Group (Mz) and Gangdise Group (Pt 1gd). The Daguaiwan Group intermediate between the Namjagbarwa Group and Gangdise Group is developed dominantly in the northeastern and southeastern parts of the Yarlung Zangbo Grand Canyon, and consists of metabasalts or diabases, pyroxenites, magnesian peridotites, quartzites and marbles. These rocks show many similarities in rock type and chemical composition to ophiolitic suites but differences in lithochemistry from typical mid oceanic ridge ophiolites. They are interpreted to form in a moderately to slowly extended small oceanic basin which has the features of the continental crust and may be ascribed to an intercontinental sea or a back arc basin. The intense folding, uplifting and denudation of the study area since 45 Ma allowed the lower crust to have been subjected to more extensive erosion to form the giant canyon.