老挝西北部二叠纪砂岩碎屑锆石U-b年代学、地球化学特征及其构造意义

老挝西北部的沉积演化历史一直未能很好地界定.因此，在老挝西北部地区出露较好的二叠纪地层中采集了4个砂岩样品，并系统开展了岩石学、地球化学和碎屑锆石U-Pb年代学的研究.岩相学表明该套样品主要包括了长石石英砂岩、岩屑砂岩和杂砂岩.全岩地球化学结果显示这些砂岩样品以高SiO₂（64.9%~91.2%）、高Al₂O₃（5.0%~17.4%）和高CIA（59.6~89.9）为特征，说明它们经历了中等至强的风化作用.另外，样品的微量元素地球化学特征也暗示其物源主要为来自岛弧环境和活动大陆边缘的长英质岩石，并伴有少量再循环沉积物.碎屑锆石U-Pb年代学结果显示出这4个样品具有1 880~1 870 Ma、1 470~1 450 Ma、890~860 Ma、450~415 Ma和275~252 Ma的5个主要年龄峰，其中275~252 Ma为最大的沉积年龄，因此，可以将研究区地层时代限定在不早于晚二叠世.综合前人研究及与邻近盆地碎屑锆石U-Pb年龄谱系的对比，认为前志留纪的锆石大多来自于扬子板块古老基底的再循环沉积物，志留纪至二叠纪的碎屑锆石主要来源于长山带、昌宁-孟连带、临沧-素可泰岩浆岩带和邻近金三角地区，证明了印支板块北缘与思茅地块在古特提斯演化时期为一个整体，在二叠纪时期具有扬子板块的亲缘性. 

Detrital Zircon U-Pb Geochronology and Geochemical Characteristics of Permian Sandstones in NW Laos and Its Tectonic Implications

The sedimentary evolution history of Northwest Laos has not been well defined. Therefore, it studied the petrology, geochemistry and detrital zircon U-Pb geochronology systematically by using four sandstone samples collected from the well-exposed strata of Permian in NW Laos. Petrography shows that this set of samples mainly includes feldspathic quartz sandstone, lithic sandstone and greywacke. Whole-rock geochemical analyses suggest that these samples are characterized by high SiO₂ (64.9%-91.2%), high Al₂O₃ content (5.0%-17.4%) and high CIA index (59.6-89.9), indicating that they have undergone moderate to strong weathering. Moreover, the trace elements of these samples reveal that the sediment provenance includes felsic rocks from an active continental margin or continental arc with minor amounts of recycled materials. Detrital zircon U-Pb ages suggest 5 significant peaks: 1 880-1 870 Ma, 1 470-1 450 Ma, 890-860 Ma, 450-415 Ma and 275-252 Ma. The youngest age spectrum of all the detrital zircons is 275-252 Ma, which constrains the deposited age to the Late Permian. Based on our studies and comparisons through the detrital zircons from adjacent basins, it proposes that the Pre-Silurian zircons were mainly originated from the recycled sediments of the Yangtze block, the Silurian to Permian detrital zircons were originated from the Truong Son, Changning-Menglian, Lincang-Sukhothai zones and Golden Triangle region. It is proved that the northern margin of the Indochina block and Simao block was a whole during the Paleotethys evolution period. Northern Indochina block showed affinity to Yangtze block in the Permian period.