渝北温泉镇地区三叠系“绿豆岩”特征及其地质意义

Characteristics and Geological Significance of the Triassic Mungbean Rocks in the Wenquan Town Area,Northern Chongqing

华南地区三叠系广泛分布蚀变火山灰(俗称"绿豆岩")。对渝北温泉镇地区平洪村剖面与谢家槽剖面的"绿豆岩"进行矿物学、全岩地球化学和锆石U-Pb测年及地球化学分析。X射线衍射分析表明,矿物组成均以石英和碱性长石为主,此外还含有伊利石、蒙脱石等黏土矿物。锆石岩相学、Th/U比值及锆石REE配分模式都显示典型的岩浆成因锆石特征,但样品和锆石的地球化学特征明显不同。锆石U-Pb测年结果揭示平洪村样品的形成年龄为(247.2±0.59) Ma(MSWD=2.3,n=30),与早、中三叠世的界线年龄(247.2 Ma)高度吻合,其锆石地球化学特征显示板内成因,推测是在峨眉山大火成岩省主要活动期之后板内火山活动过程中形成的。谢家槽剖面样品年龄为(225.9±1.4) Ma(MSWD=1.6,n=29),锆石地球化学特征显示岩浆弧成因,结合区域地质演化特征分析,特别是研究区北部(华北南缘)同时期火山灰的发现,认为谢家槽剖面"绿豆岩"很可能是勉略洋闭合过程中的产物。

Altered Triassic volcanic ash beds are widely distributed in South China,and is locally called"mungbean rocks".In the Pinghongcun and Xiejiacao sections in the Wenquan town(northern Chongqing),these"mungbean rocks"were sampled and X-ray diffraction(XRD)analyzed.Our results show these rocks contain mainly quartz and alkali feldspars,together with clay minerals such as illite and montmorillonite.Zircon morphology,Th/U ratio and zircon REE patterns indicate typical magmatic zircon features.However,geological interpretations yielded from zircon geochemistry of different samples are markedly different.Zircon U-Pb dating reveals that the Pinghongcun sample was formed at(247.2±0.59)Ma(MSWD=2.3,n=30),coeval with the T1/T2 boundary(247.2 Ma).The zircon geochemical characteristics indicate an intraplate origin,presumably resulted from the intraplate volcanism after the main-stage Emeishan large igneous province(LIP)activity.The Xiejiacao sample was formed at(225.9±1.4)Ma(MSWD=7.1,n=29),and the zircon geochemistry points to a magmatic arc origin.Combined with regional geology,especially the discovery of the coeval volcanic ash beds to the north(southern margin of North China Block),we inferred that the Xiejiacao sample was probably the product of the Mian-Lue Ocean closure.