Geochronology and Geological Significance of Late Triassic Volcanic Rocks in B.Xiang-Nou Area, North Laos

The B.Xiang-Nou area of northern Laos is located in the Jinghong-Sukhothai volcanic arc of the western Nan-Uttaradit suture tectonically, where the volcanic rocks are developed. The tuff accompanied with volcanic conglomerate is occurred in the bottom of Late Triassic Nan-tuan Formation. A 1:200 profile measurement had been carried out for the area of exposed volcanic rocks, the samples were collected from every formation of strata for geochemical analysis and zircon U-Pb dating. Their SiO₂ contents range from 56.13% to 74.58%, belonging to intermediate-acidic magmatic rocks series (dacitic tuffs), while their Al₂O₃ and MgO vary from to 13.9% to 25.38% and 0.43% and 0.86%, respectively, showing the features of island arc environment characterized by high aluminum and low magnesium; their Na₂O+K₂O contents change from 4.73% to 5.74%,presenting a typical feature of alkaline-calc-alkaline series. The U-Pb age of volcanic rocks is (215±1.4 Ma), indicating that these rocks were formed in Late Triassic Norian age, and providing an approach to ensure the collision time and tectonic setting of Burmatama block and Lanping-Simao block.