西藏东部昌都地区三叠纪岩相古地理

TRIASSIC LITHOFACIES AND PALAEOGEOGRAPHY IN QAMDO, EAST TIBET

昌都地区三叠纪岩相古地理演化可划分为5个阶段,各阶段的古构造与古地理格架既有继承性,又有差异性.早三叠世承袭了晚二叠世末期的古地理,古陆面积扩大,海域沉积只占据狭窄部分,并且从这个时期起,已展现出弧—盆体系中的岛、海格架雏形;中三叠世沿袭了早三叠世弧—盆体系,弧内盆地发生强烈拗陷,堆积了巨厚的多物源浊积岩与幕式弧火山岩;晚三叠世卡尼早期因东侧金沙江洋的消亡,发育磨拉石沉积,仅在局部地区表现为陆棚→斜坡→盆地沉积;晚三叠世诺利早期,全区范围内发生一次规模空前的海侵,碳酸盐沉积向东西两侧古陆超覆,弧—盆区岛海格局从此消失;晚三叠世诺利晚期以后,海水自东向西退出,海陆过渡相与滨岸陆屑沉积广泛覆盖全区.研究表明,三叠纪时昌都地区曾经存在着一个类似于现今印度尼西亚岛、海的古地理格局.

Development of Triassic lithofacies and palaeogeography can be divided into 5 stages in Qamdo area. There were not only inheritances but also differentiations in palaeotectonic and palaeogeographic frameworks of different stages. Palaeogeography of the Early Triassic inherited that of the end of the Late Permian , palaeocontinental area enlarged, and marine sediments only occupied narrow area; and from then, embryonic grillwork of island-sea displayed in arc-basin systems. In the Middle Triassic, arc-basin systems of Early Triassic were inherited, and intra-arc basin largely depressed and accumulated thick multi-source turbidity rocks and episodic arc volcanic rocks. In Early Carnian of the Late Triassic, molasse sediments developed, and shelf, slope and basin deposits only occurred in local areas. In Early Norian of the Late Triassic, the largest scale transgression had taken place the whole area, carbonate sediments on-lapped on palaeocontinent eastwards and westwards, and framework of island-sea from arc-basin systems was extinct from then. After Late Norian of the Late Triassic, water retreated from north and west, and sea-continental transitional and littoral terrigenous deposits covered the whole area. It turns out that there was an island-sea framework in the Triassic in Qamdo area similar to that in Indonesia and Haiti today.