黔南马坡脚上二叠统吴家坪组凝灰岩成因及其地质意义

The Origin and Geologic Significance of the Tuff within the Upper Permian Wuchiaping Formation at Mapojiao in the Southern Guizhou,China

为确定贵州南部马坡脚上二叠统海相碳酸盐岩夹凝灰岩的形成年代、成因及其与峨眉山大火成岩省的关系,对其进行了全岩地球化学、锆石U-Pb年龄、锆石微量元素及Hf同位素分析。结果显示,马坡脚凝灰岩具有较高的Al_(2)O_(3)/TiO_(2)比值和显著的Eu负异常。2个样品的锆石U-Pb年龄分别为(259.1±1.5)Ma和(257.5±1.8)Ma,与云南宾川酸性熔结凝灰岩的年龄相近。马坡脚凝灰岩中锆石的ε_(Hf)(t)值为1.1~8.4,且Th/Nb比值较低,具有板内岩浆岩的地球化学性质。锆石年龄和地球化学特征指示马坡脚凝灰岩可能来自峨眉山大火成岩省的酸性火山活动。结合已发表的高精度CA-TIMS年龄与牙形石生物地层年龄,可以认为峨眉山酸性火山活动可能至少持续至晚二叠世吴家坪早期,且峨眉山酸性火山喷发可能是导致吴家坪期气候变冷的重要影响因素之一。

In order to explore the formation age and origin of the tuff within the upper Permian marine limestones at Mapojiao site in the southern Guizhou and its possible relationship with the Emeishan Large Igneous Province(LIP),a study on the bulk rock geochemistry,U-Pb dating,trace elements and Hf isotopic geochemistry of zircon grains of the tuff has been carried out in this paper.The results show that samples of the tuff at Mapojiao display relatively high Al_(2)O_(3)/TiO_(2)ratios and obvious negative Eu anomalies.The U-Pb ages of zircon grains from two samples are(259.1±1.5)Ma and(257.5±1.8)Ma,respectively.They are similar to that of the felsic ignimbrite in the Binchuan area.Theε_(Hf)(t)values of zircons from the tuff samples vary from+1.1 to+8.4,and the Th/Nb ratios of those zircons are relatively low,indicating that they have geochemical characteristics of the intraplate magmatic rocks.The above U-Pb ages and geochemical characteristics of zircons imply that the tuff at Mapojiao could be sourced from the felsic volcanic activity related to the Emeishan LIP.Combined with the published ages of high-precision CA-TIMS zircon U-Pb dating and conodont biostratigraphy,it is believed that the Emeishan felsic volcanism could at least last to the Early Wuchiapingian epoch of the Late Permian.The Emeishan felsic volcanic eruption could be an important factor resulting in the climate cooling in the Wuchiapingian epoch.