西藏西北部浅变质石英砂岩岩石学特征及其构造意义

羌塘盆地中央隆起带主要由浅变质石英砂岩组成,由于化石匮乏、变形强烈,长期以来对这套浅变质石英砂岩成因的争议颇大,导致了对藏西北地区前中生代构造演化的认识长期模糊不清,其中,何处才是冈瓦纳大陆北界就是一个长期争论的议题。在西藏西北部,近东西走向的布尔嘎错断裂带将北部的查多岗日地块与南部的南羌塘地块分隔开,浅变质石英砂岩广泛出露于这两地块之内。沿近东西走向布尔嘎错断裂带断续产出的冈玛错蓝片岩、蛇绿岩等岩片大体呈南北向逆冲于浅变质石英砂岩之上,因此早期曾认为布尔嘎错断裂带是冈瓦纳大陆之北界。本文的调查与研究发现查多岗日和南羌塘地块内浅变质石英砂岩岩相学特征完全相同,均主要由石英组成,遭受了绿片岩相的变质作用,形成了钠长石+绿泥石+白云母组合,充填于早期石英颗粒之间,钠长石交代钾长石。这两地的浅变质石英砂岩均被未变质钙质胶结的钾长石石英砂岩平行不整合覆盖。碎屑锆石的阴极发光分析与U-Pb定年结果进一步证实了查多岗日与南羌塘地块内浅变质石英砂岩内的锆石来源完全相同。这些证据充分反映了查多岗日与南羌塘地块早期构造演化过程相似,源自同一大陆。碎屑锆石定年结果进一步表明浅变质石英砂岩的最大沉积年龄为520±8Ma,该岩石再被约480Ma的花岗岩脉侵入,因此其很可能形成于晚寒武世,而不整合面之上沉积岩的最大沉积年龄为460±8Ma,表明该不整合面上、下沉积岩之间存在明显的沉积间断,证实了该平行不整合面形成于奥陶纪。不整合面之下的浅变质石英砂岩因此与杨耀等(2014)报道的荣玛组相同,不整合面之上未变质长石石英砂岩则属于中上奥陶统塔石山组。查多岗日地块因此是西藏境内最北端的冈瓦纳大陆的碎块。在西藏西北部,冈瓦纳大陆北界为龙木错-帮达错-(83°40'E、35°N)-红脊山-荣玛乡。布尔嘎错断裂带不是冈瓦纳大陆之北界。

Petrology of low-grade metamorphic quartz sandstones within northwestern Tibetan regions: Implications to the tectonic evolution of the northwestern Tibet

The central upheaval of Qiangtang basin is mainly composed of low-grade metamorphic quartz sandstones. Due to fossil scarcity and strong deformation, the genesis of the low-grade metamorphic quartz sandstones has remained a controversy for a long time, leading to the misunderstanding of pre-Mesozoic tectonic history of the northwestern Tibetan regions. For example, it is still debating where the northern border of the Gondwanan supercontinent within the Tibet is. At the northwestern Tibetan regions, the Chaduogangri terrane in the north and South Qiangtang terrane in the south were separated by the EW-trending Buerga fault zone. Low-grade metamorphic quartz sandstones are widely exposed in the two terranes. The Gangmacuo blueschists and ophiolitic rocks discontinuously exposing within the Buerga fault zone structurally overlay the low-grade metamorphic quartz sandstones. Therefore, the Buerga fault zone was regarded as the northern border of the Gondwanan supercontinent in the early times. The investigations in this study have revealed that the low-grade metamorphic quartz sandstones occurring in the Chaduogangri and South Qiangtang terranes, respectively, are exactly similar. Both are mainly composed of quartz, and minor K-feldspar, experiencing greenschist facies metamorphism to yield a new assemblage of albite + muscovite + chlorite among early quartz grains. Early K-feldspar was normally as relics in the quartz and albite grains. The low-grade metamorphic quartz sandstones in the Chaduogangri and South Qiangtang terranes are parallel-unconformably covered by the K-feldspar sandstones. Moreover, the U-Pb dating and CL images have further revealed that the detrital zircons from the low-grade metamorphic quartz sandstones in the Chaduogangri and South Qiangtang terranes, respectively, are derived from same sources. These lines of evidence thus suggest that the tectonic history of the Chaduogangri terrane is similar to that of the South Qiangtang terrane. The maximum sedimentary ages of the low-grade metamorphic quartz sandstones beneath the parallel unconformity in the South Qiangtang terrane are 520±8Ma. The low-grade metamorphic quartz sandstones were intruded by the granitoids with ages of 480Ma, suggesting that the rocks were formed during Late Cambrian times probably. The maximum sedimentary ages of the K-feldspar sandstones above the parallel unconformity are 460±8Ma, suggesting a significant depositional hiatus between the two sandstones. This further confirms that the parallel unconformity was formed during the Ordovician times. Therefore, the low-grade metamorphic sandstones beneath the parallel unconformity belong to Rongma Formation reported by Yang et al. (2014), and the K-feldspar quartz sandstones above the unconformity belong to Tashishan Formation. The Chaduogangri terrane is, therefore, a fragment of Gondwanan supercontinent in the northernmost of the Tibetan regions. In the northwestern Tibet, the northern border of Gondwanaland is Longmu Cuo-Bangda Cuo-83°40''E, 35°N-Hongji Mountains-Rongma village instead of the Buerga fault zone.