四川广元地区朝天剖面上二叠统吴家坪组凝灰岩成因及其地质意义

四川广元地区朝天剖面上二叠统吴家坪组凝灰岩位于茅口组顶部王坡层黏土岩之上,其成因可能对认识峨眉山大火成岩省火山活动时限具有重要意义。对朝天剖面吴家坪组2个凝灰岩样品进行全岩矿物含量、地球化学特征、锆石U-Pb定年、锆石微量元素和Hf同位素分析,探讨凝灰岩的成因,进而讨论其对峨眉山大火成岩省火山活动时限的启示。结果表明:广元地区朝天剖面上二叠统吴家坪组凝灰岩主要由黏土矿物组成,锆石U-Pb年龄为(260.1±2.8)Ma,并具有较高的Al2O3/TiO2值(分别为37.90和31.49)、明显的Eu负异常(Eu/Eu*值分别为0.13和0.06)及缺失Nb、Ta负异常;凝灰岩样品中,锆石具有较高的εHf(t)值(7.8~11.4)和较低的Th/Nb值(5.4~14.9),与宾川剖面峨眉山火山序列酸性熔结凝灰岩及上寺剖面上二叠统下部凝灰岩基本一致,而明显不同于清音阁王坡层黏土岩锆石εHf(t)值和Th/Nb值。综上所述,四川广元地区朝天剖面上二叠统吴家坪组凝灰岩的形成与峨眉山大火成岩省酸性火山活动有关,且峨眉山大火成岩省的火山活动时限可能一直持续到晚二叠世早期。

Origin of Tuffs from Upper Permian Wujiaping Formation in Chaotian Section of Guangyuan Area,Sichuan, China and Its Geological Significance

The tuffs from Upper Permian Wujiaping Formation in Chaotian section of Guangyuan area,Sichuan are located above Wangpo claystone beds at the top of Maokou Formation,the origin of tuffs may provide a good chance to constrain the age of Emeishan large igneous province volcanism.Mineral contents,whole-rock geochemical characteristics and zircon U-Pb age,trace element and Hf isotopic composition for two tuff samples from Wujiaping Formation in Chaotian section were analyzed to discuss the sources and its geological significance for the age of Emeishan large igneous province volcanism.The results show that the tuffs from Upper Permian Wujiaping Formation in Chaotian section of Guangyuan area with zircon U-Pb age of(260.1±2.8)Ma,are dominated by clay minerals,and have high Al2O3/TiO2 ratios(37.90 and 31.49),strong negative Eu anomalies(Eu/Eu*ratios are 0.13 and 0.06),and no negative Nb and Ta anomalies;zircons from the tuffs have highεHf(t)(7.8-11.4)and low Th/Nb ratios(5.4-14.9);the geochemical characteristics are similar to the lower tuffs from Upper Permian in Shangsi section and the acid ignimbrites of Emeishan volcanic sequence in Binchuan section,but different from theεHf(t)and Th/Nb ratios of Wangpo claystone beds in Qingyinge.In general,the tuffs from Upper Permian Wujiaping Formation in Chaotian section of Guangyuan area,Sichuan are related to the acid volcanism of Emeishan large igneous province,and the age of Emeishan large igneous province volcanism may extend to the early period of Late Permian.