藏东左贡地区碧土蛇绿岩:古特提斯主洋盆的地质记录

Bitu ophiolite in the Zogang area,eastern Tibet:Geological records of Paleo-Tethyan major oceanic basin

藏东碧土一带的玉曲流域保存有丰富的古特提斯主洋盆的地质记录,包括洋底、海山和大陆边缘的火山-沉积岩。蛇绿岩由超镁铁岩、辉长岩、辉绿岩(席状岩墙群的残块)和玄武岩等组成。岩石化学特征表明其属低钾的大洋拉斑玄武岩,形成的构造背景是大洋中脊,稀土配分型式和部分微量元素指示碧土洋盆以P-型MORB和N-型MORB同时发育为特征。洋盆发育到石炭纪时达到鼎盛,于二叠纪和晚三叠世经历了2次消减。碧土缝合线南延是滇西的昌宁-孟连缝合线,西北延则接青海南部的各拉丹冬和藏北的龙木错-双湖缝合线,代表消失了的古特提斯主洋盆。

Abundant geological records of the Paleo-Tethyan oceanic basin, including submarine, seamount and continentalmargin volcanic-sedimentary rocks, are preserved along the Yuqu River valley in the Bitu, Zogang area, eastern Tibet. The Bitu ophiolitic mélange consists of ultramafic rocks, gabbro, diabase （relict masses of sheeted dike swarms） and basalt. Petrochemical characteristics show that the mafic rocks are low-K oceanic tholeiite formed in the mid-ocean ridge. The REE patterns and part of trace elements indicate that the Bitu oceanic basin is characterized by the development of both P-type MORB and N-type MORB. The development of the Bitu oceanic basin reached its culmination in the Carboniferous and the oceanic crust experienced two subductions in the Permian and Late Triassic respectively. The southward extension of the Bitu suture in eastern Tibet is the Changning-Menglian suture in western Yunnan, while its northwestward extension is connected with the Longmu Co-Shuanghu suture in northern Tibet and Geladaindong in southern Qinghai. It represents the extinct Paleo-Tethyan major oceanic basin.