西藏冈底斯南缘普夏洋岛的识别及其意义

Identification of the Puxia oceanic island in the southern margin of the Gangdise belt, and its geological significances

关键词:  普夏洋岛  冈底斯  晚三叠世  增生杂岩  新特提斯洋

Key words:  Puxia oceanic island  Gangdise  the Late Triassic  accretionary complex  New Tethys

摘要:

高出海平面的洋岛和低于海平面的海山是成熟大洋最重要的特征。笔者通过野外调查，于西藏自治区贡嘎县昌果乡普夏东侧的“原桑日群”中新识别出普夏洋岛。通对普夏洋岛的岩石学、地球化学研究及锆石U-Pb定年，认为普夏洋岛具有典型的“玄武岩+灰岩”岩石组合，其中的玄武岩为典型的洋岛玄武岩，形成于以成熟洋壳为基底的洋岛海山环境。普夏洋岛玄武岩的锆石U-Pb定年结果为203.9±1.6Ma和219.5±2.0Ma，表明其形成时代为晚三叠世。综合研究认为，普夏洋岛是雅鲁藏布江特提斯洋发现的较早的洋岛海山之一，证明雅鲁藏布江新特提斯洋在三叠纪已具有成熟的洋壳，普夏洋岛是雅鲁藏布江新特提斯洋向北俯冲形成的桑日群增生杂岩的重要组成部分。普夏洋岛的发现，丰富了新特提斯洋的研究内容，为反演研究区地质构造演化提供了新证据。

Abstract:

The Puxia oceanic island, located at the Changguo area, Gongga, Tibet,is a newly identified late Triassic oceanic island from the previous Sangri Group by the authors of this paper. By carrying out detailed geological survey and studies on rock associations, rock fabrics, rock geochemistry and zircon U-Pb dating, the authors believe that the Puxia oceanic island sequences, which are mainly composed of interbed basalt and limestone rocks, occurred in an oceanic island tectonic setting in the late Triassic. Our studies indicate that the studied area already occurred the mature oceanic crust as early as the late the late Triassic, according to our zircon U-Pb dating ages of 203.9 ±1.6Ma and 219.5 ±2.0Ma obtained from the basalts of the Puxiao ceanic island sequences.The Puxiao ceanic island sequences were formed by the northward subduction of the new Tethys Ocean to the Lhasa block.Identification of the late Triassic Puxia oceanic island has important and positive significances on studies of the geological evolution of the Gangdise belt and the new Tethys Ocean.