西藏羌塘地块西部多玛地区与中部荣玛地区早中二叠世地层对比研究

The Qiangtang Block is bounded by the Longmu Co-Shuanghu suture zone in the north and the Bangong-Nujiang suture zone in the south in the Qinghai-Tibet Plateau. The palaeogeography of the Qiangtang Block during the Late Palaeozoic has been a focus of debate recently. The Permian strata are widely distributed in the Qiangtang Block. However, the Lower and Middle Permian depositional sequences from the Domar area in the western part of the Qiangtang Block differs substantially from those from the Romgma area in the central part of the Qiangtang Block. Whether they are from the same block or from the accretionary complex with different origin is enormously important for the reconstruction of the palaeogeography. This project is designed to establish the depositional sequences, faunal succession and chronostratigraphy of two different types of depositions, have a correlation to see if they originated from the same block, and reconstruct their relationships with adjacent blocks/areas. Knowing these will have potential to shed light on the palaeogeographic evolution of the peri-Gondwanan blocks during the Early and Middle Permian.

西藏羌塘地块位于龙木错－双湖缝合带与班公湖－怒江缝合带之间。其晚古生代的古地理演化是近年来青藏高原研究争论的焦点之一。羌塘地块西部的多玛地区和中部的荣玛地区存在两套不同的早中二叠世沉积层序，它们之间是属于同一地块的不同沉积相还是属于不同来源的增生楔是认识羌塘地块二叠纪古地理演化的重要科学问题。本项目拟对羌塘地块西部和中部两套不同的早中二叠世地层的沉积层序、动物群组合和年代地层展开研究， 判断它们之间的关系以及与相邻地块之间的亲缘性，从而在大范围内恢复早中二叠世时期冈瓦纳北缘不同陆块之间的关系，推测早中二叠世冈瓦纳北缘古地理演化。

西藏南羌塘地块是基墨里陆块的重要组成部分，其二叠纪的地层划分以及与相邻地块的对比与古地理重建密切相关。项目在几年的研究中针对羌塘盆地内部南羌塘和北羌塘的对比、南羌塘地块内部东西向的对比，以及南羌塘地块与相邻其它陆块的对比展开了详细的工作。取得如下几个重要认识。①近年来有关南、北羌塘在二叠纪时是否属于同一块体仍然有较大的争议。项目在羌塘热觉茶卡一带发现了早二叠世特有的Sphaeroschwagerina蜓类动物群，证实它和南羌塘属于完全不同的生物地理区系，确定了羌塘盆地中部的高压变质是二叠纪分隔南北羌塘盆地的最大古特提斯洋盆位置所在地。②南羌塘中部空谷晚期含Vediproductus，Cryptospirifer腕足动物群，表明南羌塘地块在空谷期时是以暖水动物群为主。③南羌塘地块东部吞龙共巴组和中部鲁谷组底部的动物群面貌一致，表明南羌塘地块绝大部分地区不属于增生楔，而是同一个块体的不同沉积相。④南羌塘和拉萨地块的区别明显，一方面南羌塘晚二叠世含有Palaeofusulina暖水动物群，明显暖于拉萨地块；另一方面，拉萨地块中二叠世普遍含有Nankinella-Chusenella蜓类动物群，明显不同于南羌塘的同期动物群组合。这些差异性表明班公湖-怒江洋开启时间早于中二叠世。⑤在多年工作的基础上，把南羌塘的二叠系分为三个类型。由西向东，沉积的变化趋势是浅水—斜坡—海山，这一趋势可能与南羌塘从冈瓦纳北缘裂解有关。这些研究进展对于认识南羌塘在二叠纪的古地理以及同相邻地块的古地理关系提供了关键线索和证据。