藏北羌塘中部上二叠统—下三叠统天泉山组的建立及意义——对龙木错-双湖-澜沧江洋构造演化的指示

The establishment and significance of the Upper Permian-Lower Triassic Tianquanshan Formation in central Qiangtang, northern Tibet: The constraint on the tectonic evolution of Lungmu Co-Shuanghu-Lancang River area

藏北羌塘中部改则县以北天泉山、屏风岭等地区大面积分布一套浅变质岩系,岩性以变质杂砂岩、千枚岩为主,夹变质玄武岩、变质安山岩等火山岩夹层,因缺乏化石依据时代不明。通过LA-ICP-MS锆石U-Pb同位素定年测得安山岩的^(206)Pb/^(238)U年龄为251.4±2.4Ma,同时结合该地区已测得的254Ma的^(206)Pb/^(238)U年龄,证实这套浅变质岩系的时代应属于晚二叠世—早三叠世,并非传统意义的展金组。在剖面测制和区域对比的基础上,将天泉山、屏风岭一带的浅变质岩系重新厘定为上二叠统—下三叠统天泉山组。天泉山组的发现和建立不仅进一步完善了该区的地层系统,而且是龙木错-双湖-澜沧江洋首次发现的晚二叠世—早三叠世洋盆地层记录,为进一步丰富和研究龙木错-双湖-澜沧江洋的演化提供了重要的地层证据。

There exist extensive epimetamorphic rocks in Tianquanshan and Pingfengling of Gaize County. These rocks are mainly metamorphic complex sandstone, phyllite and some interlayered volcanic rocks which are metamorphic basalt and andesite. Due to the lack of fossils, their formation epoch is unknown. There exists interlayered andesite, and the authors obtained the ^（206）Pb/^（238）U age of andesite（251.4±2.4Ma） by LA-ICP-MS zircon U-Pb isotope dating. Combined with the 254Ma^（206）Pb/^（238）U age obtained by Fan Jianjun in the region, it is confirmed that the epimetamorphic rocks in the region should belong to the Late Permian-Early Triassic rather than to the Zhangjin Formation held by the traditional opnion. Based on the profile measurement and regional correlation, the strata of Tianquanshan and Pingfengling along the shallow metamorphic rocks are redefined as the Upper Permian-Lower Triassic Tianquanshan Formation（P_3T_1t）. The discovery and establishment of Tianquanshan Formation（P_3T_1t） not only further improve the regional stratigraphic system but also provide the first oceanic basin record for Late Permian-Early Triassic strata in Lungmu Co-Sh-uanghu-Lancang River area. The results obtained by the authors provide important evidence for furthe study of the evolution of Lungmu Co-Shuanghu-Lancang River suture zone.