桂西南凭祥—崇左地区三叠系碎屑岩物源分析及其对印支期造山事件的启示

Disputes exists in whether there is a Triassic collision orogen in the southern- southwestern margin of South China Block, and also disputes on the relationship between Youjiang and Shiwandashan basins which are Triassic synorogenic sedimentary basins in southern-southwestern margin of the South China Block. Both Shiwandashan and Youjiang basins are filled with detrital material from the synorogenic belt in south of the basins in Triassic whose synorogenic detritus content increases toward the west. The two basins were transformed from deep-sea basin to foreland basin in Late Permian for Shiwandashan Basin and the later part of late Early Triassic for Youjiang Basin. In the transition zone between the two basins is the Pingxiang-Chongzuo region where a complete marine to submarine Triassic succession was preserved. In this study, we propose to conduct a thorough analysis of this succession that includes U-Pb age, geochemical composition and Hf isotope of the detrital zircon, and the geochemical composition of detrital Cr-spinel and rutile. In combining with the sedimentology, petrology, and whole-rock geochemistry of the succession, we aim to identify its sources and tectonic setting. Provenance comparisons against those of the Youjiang and Shiwandashan basins will help determine 1) whether they belong to one single basin system and 2) whether there was a diachronous collision between Indochina and South China blocks in Triassic. This study will provide new evidence for Mesozoic Paleogeography evolution of East Asian.

华南南缘-西南缘是否存在三叠纪碰撞造山事件、及其相关的同沉积盆地右江盆地和十万大山盆地之间的关系仍没有定论。右江盆地和十万大山盆地三叠系沉积物中都记录有盆地南侧同造山带火山碎屑物质，含量从东到西逐渐递增，且两个盆地分别从晚二叠世和早三叠世晚期进入前陆盆地阶段，演化具有不等时性。桂西南凭祥-崇左地区处于这两个盆地的过渡区，其三叠纪地层出露良好。本申请拟以凭祥—崇左地区三叠系碎屑岩为研究对象，通过分析其碎屑岩中锆石的U-Pb年龄、微量元素及Hf同位素组成，碎屑铬尖晶石和金红石的地球化学组成，结合沉积学、岩石学和全岩地球化学组成，揭示其潜在源区和源岩组合、构造属性。通过与右江盆地和十万大山盆地三叠系沉积物源区和盆地构造演化作对比，探讨研究区及其演化异同，并判断三个地区是否属于同一盆地体系。结合区域资料，探讨三叠纪华南与印支板块之间是否存在不等时碰撞造山事件，为重建东亚中生代古地理演化提供新证据。

华南西南缘凭祥－崇左地区三叠纪碎屑岩主要由石英、岩屑及少量长石组成，且岩屑比例从早三叠世沉积岩至中三叠世沉积岩中逐渐增多。早中三叠世样品记录有相同的碎屑锆石年龄谱，主要碎屑锆石年龄组为300-250 Ma、400-300 Ma（仅中三叠世样品）、480-420 Ma、900-700 Ma、1200-900 Ma，其中前泥盆纪碎屑锆石主要来自华南大陆前寒武基底和早古生代地层再旋回碎屑物质，泥盆纪-石炭纪、二叠纪-三叠纪碎屑锆石主要来自研究区南侧金沙江-哀牢山-Song Ma 洋盆闭合及板块碰撞产生的物质。海南岛二叠系碎屑岩中主要由石英和碎屑组成，其主要碎屑锆石年龄组为400-300 Ma、480-420 Ma、530-480 Ma、900-700 Ma 及1200-900 Ma，其中志留纪、奥陶纪、前寒武纪碎屑锆石主要来自华南大陆相关岩体和再旋回碎屑物质，530-480 Ma 和400-300 Ma 碎屑锆石主要来自其南侧古太平洋连续俯冲增生造山带。结合区域同时代地层物源组成、岩浆岩和变质事件得出：华南西南缘十万大山盆地、凭祥地区、右江盆地为同一个盆地，即大右江盆地，其演化受控于金沙江-哀牢山-Song Ma 洋盆的闭合及印支板块和华南板块的碰撞造山，海南岛与永安盆地具有相同的物源区，为古太平洋向华南东南内陆所形成的弧后前陆盆地，且海南岛为受该俯冲增生带影响的最西南端部分。