西藏特提斯喜马拉雅石英砂岩研究

Quartz arenites are the most typical andstable sedimentary rock and are the main research objects in sedimentology. Quartz arenites occupy the longest studying history among all the sedimentary rocks. The purposes for quartz arenite researches were aimed on the petrology at the first periods. At present the purposes are on the temporal and spatial dynamics of tectonic evolution. However, the disputes about controlling factors and the formation mechanism of provenance and depositional environment of quartz arenites still exist. In Tethyan-Himalaya, where the quartz arenites developed better than elsewhere of the world, a continuous and thick Devonian to Paleogene marine succession wide spreads. In total, eight quartz arenite units of different ages were well developed. This proposed project will study these eight units developing from Devonian Boqu Formation to Paleogene Gydula Formation. The provenances and depositional environments will be studied and improved by geohronology, thermochronology and geochemistry, also by traditional detrital framework modes, grain size andheavy mineral assemblages analysis. We hope that this study will explore the important controlling factors of Tethyan-Himalayan quartz arenites, and also establish their formation mechanism. In addition, this study will provide the sedimetological constraints for several important events during the evolution of New-Tethys Ocean.

石英砂岩是沉积学研究中最主要、最典型和最稳定的岩石类型之一，且研究历史最长，目前已经从岩类学研究发展到了与构造演化相关的时空动力学研究。然而其物源和沉积环境等方面的控制因素和成因机制却存在着广泛的争议。特提斯喜马拉雅发育从泥盆系至古近系全球罕见的、序列完整的、厚度巨大的海相沉积，同时也是我国，乃至全球石英砂岩最为发育的地区。本项目以藏南地区特提斯喜马拉雅泥盆系波曲组至古近系基堵拉组多达8层且时代连续的石英砂岩层为研究对象，通过地质年代学、热年代学和地球化学，以及传统的骨架颗粒记数统计、粒度分析、重矿物组合等方法和手段研究石英砂岩的物源和沉积环境，探究特提斯喜马拉雅石英砂岩的关键控制因素，建立其成因机制，为新特提斯洋演化过程中的重要地质事件提供沉积学约束。

石英砂岩所含颗粒成分单一，信息量相对较小，有关石英砂岩的研究一直没有取得突破性进展。特提斯喜马拉雅最显著的特征是发育着寒武纪以来的规模巨大的、典型的、结构类型多变的、序列完整的多套石英砂岩，为项目的研究提供了良好研究内容。本项目在执行过程中，针对泥盆系波曲组、石炭系纳兴组、二叠系基龙组、三叠系德日荣组、侏罗系普布嘎组和门卡墩组顶部以及古新统基堵拉组进行了岩相学、沉积学、地层学、重矿物分析和碎屑锆石U-Pb年代学等方面的研究。研究结果显示：泥盆纪到三叠纪特提斯喜马拉雅物源稳定，陆源碎屑主要来源其南部的印度次大陆。泥盆纪波曲组沉积时部分来源于西澳大利亚和拉萨地体的陆源碎屑进入特提斯喜马拉雅被动大陆边缘盆地，石炭纪以来由于Pinjarra造山带地貌高地的出现该沉积通路被阻断，特提斯喜马拉雅变为相对局限的被动大陆边缘盆地。早二叠世印度北部被动大陆边缘盆地涵盖了特提斯喜马拉雅到仲巴微地体之间的全部区域，沉积学可以定性地论证“大印度”的存在。考虑到印度板块中生代～55°的逆时针旋转，新特提斯洋打开之前印度北部大陆边缘盆地的走向是南北向，而随着新特提斯洋裂谷作用结束和新特提斯洋打开，走向变为东西向，并持续到三叠世末。广泛发育的基堵拉组石英砂岩指示陆源碎屑显著增多，与德干大火成岩省喷发导致的印度克拉通隆升和再活化有关。项目成果为印度北部被动大陆边缘前裂谷期到后裂谷期的物源裂解、盆地展布样式和演化史提供了新证据；为揭示新特提斯洋演化提供沉积学方面的信息。