广西东攀剖面粘土岩的火山岩特征及LA-ICP-MS锆石U-Pb年龄

广西扶绥县柳桥镇东攀地区出露1套上二叠统大隆组和下三叠统罗楼组,大隆组合Ophiceras sp.,罗楼组合Ophicerastingi Tien生物化石,二者为整合的连续沉积地层.生物特征显示,在第12-3层和第13-1A层之间为二叠系—三叠系的界线,界面附近存在较厚的粘土岩层.从结构上看,粘土岩具有较明显的火山岩碎屑结构和沉凝灰构造,岩石地球化学研究也显示其具有钙碱性中酸性火山岩的特征,沉积环境为大陆边缘环境.在粘土岩中选取自形程度较高的火山成因锆石进行激光剥蚀法U-Th-Pb同位素测年研究,岩石中锆石的结晶年龄为254.7Ma±3.7Ma,表明本区二叠纪—三叠纪之交存在着火山活动.

Volcanics characteristics and LA-ICP-MS zircon U-Pb ages of clay rocks along Dongpan section of Guangxi

There exist Permian Dalong Formation and Lower Triassic Luolou Formation along Dongpan section crossing PTB. Dalong Formation contains Ophiceras sp., and Luolou Formation contains Ophiceras tingi Tien, suggesting that they are continuous. The evidence from paleontology indicates the PTB lies between 12-3 and 13-1A. There exists several claystone layers near the Permian-Triassic boundary, and the layers have obvious volcanic debris and tuffite structure. Geochemical data also show that claystones have volcanogenic features. U-Th-Pb zircon isotope data show a weighted mean 206Pb/238U age of 254.7Ma±3.7Ma, which represents the crystallization age of the protolith of the clays, roughly identical with the zircon U-Pb age 253.4Ma±0.2Ma of the bottom boundary of the Changhsingian period. It is also shown that strong volcanic eruptions occurred in this area.