桂东南水汶盆地白垩系大坡组碎屑锆石U-Pb年龄及其地质意义

U-Pb dating age of detrital zircon from Dapo Formation of Cretaceous System in Shuiwen Basin, Southeast Guangxi and its geological significance

为讨论桂东南地区水汶盆地大坡组的物质来源和盆地属性,对大坡组红色砂岩开展LA-ICP-MS锆石U-Pb年代学研究。大坡组红色砂岩(样品NPM30)的碎屑颗粒呈棱角状、次棱角状,磨圆度较差,分选性中等,显示近源沉积的特征。碎屑锆石亦呈现自形程度较好的CL图像,其U-Pb年龄介于2781~105 Ma之间。碎屑锆石U-Pb年龄集中分布于180~145 Ma、270~200 Ma、480~390 Ma,少数介于570~540 Ma、780~720 Ma、1000~820 Ma、1300~1000 Ma、2000~1400 Ma和2800~2300 Ma。物源分析表明大坡组碎屑物质主体来自于云开地区燕山早期、华力西—印支期和加里东期岩浆岩,少量来自于扬子东南缘新元古代岩浆事件和华夏陆块格林威尔期和新太古代物质。大坡组碎屑锆石主体来自于沉积成岩之前的岩浆活动,与地层沉积时代一致的碎屑锆石含量极少;结合区域构造演化历史,判定水汶盆地为白垩纪伸展构造背景下的断陷拉分盆地。

In order to discuss the material sources and tectonic attribute of Shuiwen basin in Southeastern Guangxi, LA-ICP-MS zircon U-Pb geochronology of the red sandstone of Dapo Formation is researched. The clastic grains of the red sandstone(sample NPM30)of Dapo Formation are angular and subangular, with poor roundness and medium sorting, showing the characteristics of near source deposition. The detrital zircons also show CL images with well self-shape, and their U-Pb dating ages range from 2781 Ma to 105 Ma. The detrital zircon U-Pb dating ages are concentrated in 145-180 Ma, 200-270 Ma, 390-480 Ma, and a few ranging from 540-570 Ma, 720-780 Ma, 820-1000 Ma, 1000-1300 Ma, 1500-1900 Ma and 2500-2700 Ma. The material source analysis shows that the clastic materials of Dapo Formation are mainly derived from the pre-existing magmatic rocks around the Shuiwen basin, and a small amoun come from the products of Neoproterozoic magmatic rocks in the southeast margin of Yangtze block and Greenville orogenic events in Cathaysian block. The detrital zircons of Dapo Formation are mainly derived from magmatic activities before the sedimental diagenesis, and there are few detrital zircons consistent with the sedimentary age of strata. Combined with the history of regional tectonic evolution, Shuiwen Basin is determined as a depression pull-apart basin under the Cretaceous extensional tectonic background.