滇西马登地区晚二叠世-早三叠世地层组合及年代学:火山岩锆石U-Pb定年证据

在西南"三江"造山带中段的兰坪盆地内,由于露头状况不好,盆地基底岩石出露状况不详,导致地层划分、归属相当混乱。详细的野外地质调查揭示,盆地东缘马登地区出露的基底岩石主要由2个构造地层单元组成,上部为火山-沉积序列,下部为浅海相泥岩、灰岩及生物碎屑灰岩。上部火山-沉积序列出露厚约1200m,可分为4个喷发-沉积韵律,由英安质熔岩、流纹质熔岩与晶屑凝灰岩、火山集块岩、火山角砾岩、流纹质凝灰岩、火山碎屑岩及少量泥岩相间组成。火山岩锆石LAICP-MS U-Pb法测年数据显示,岩浆活动始于250Ma,持续至244Ma,总体处于早三叠世,构成江达-维西-云县弧火山岩带的一部分。强烈变形的海相地层与火山岩二者呈断层接触,其时代老于250Ma。结合砂岩中碎屑锆石年龄结果(大于260Ma)判定,这套沉积岩应属于晚二叠世,其与早三叠世-中三叠世火山岩一起组成兰坪盆地的基底岩石。

Late Permian to Early Triassic tectonostratigraphy of Madeng area, northwestern Yunnan, SWChina: Volcanics zircon U-Pb dating

Mainly due to the bad outcrops, the basement composition of the Lanping Basin is not very clear, which has led to confusion in tectonostratigraphic division. Detailed field study along the eastern margin revealed two tectonostratigraphic units for the basement of the Lanping basin:a volcanic-sedimentary sequence in upper, and a shallow marine facies unit in bottom. The total thickness of the volcanic-sedimentary unit is (1200m, consisting of rhyolitic lava, dacitic lava, dacitic or rhyoltic conglomerate and tuff, and minor sedimentary rocks, which form four volcanic sequences. LA-ICP-MS U-Pb dating results of the zircons from the volcanic rocks demonstrate that, the volcanism started at 250Ma and continued to 244Ma, i.e., from Early Triassic. We infer that the volcanic rocks are part of the Paleotethyan Jomda-Weixi-Yunxian arc belt. The shallow marine facies and the volcanic rocks showed the fault contact relationship, and the former overlaid on the latter. So the shallow marine facies must be older than Early Triassic. LA-ICP-MS U-Pb dating of detrital zircons indicates that, the sedimentary rocks are younger than 260Ma. Thus, the shallow marine facies rocks belong to Late Permian, which along with the Early Triassic volcanic rocks form the basement of the Mesozoic Lanping Basin.