Geochronology and Geological Significance of Late Triassic Volcanic Rocks in B.Xiang-Nou Area, North Laos
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摘要: 老挝北部仙诺地区大地构造位置为难河-程逸-沙缴缝合带西侧的景洪-素可泰火山弧带，在该区中三叠世晚期南团组底部发现有一套火山岩组合，通过1：200剖面测量，在凝灰岩中采集样品进行主量元素、微量元素、稀土元素分析，以及锆石U-Pb定年。SiO2含量为56.13%~74.58%，Al2O3含量为13.9%~25.38%，MgO含量为0.43%~0.86%，Na2O+K2O含量为4.73%~5.74%，属中-酸性系列火山岩（英安质凝灰岩），反映出该火山岩具高铝低镁，典型的碱性-钙碱性岩系列特征。U-Pb测年为（215±1.4）Ma，属晚三叠世诺利期产物，为缅泰马地块与兰坪-思茅地块碰撞时间的确定提供了依据，同时探讨了形成的构造背景。Abstract: 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.
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Key words:
- B.Xiang-Nou Area in Laos /
- Late Triassic /
- volcanic rocks /
- formation age
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