天宝山组形成时代和形成环境——锆石SHRIMP U-Pb年龄和地球化学证据

Chronology and Tectonic Environment of the Tianbaoshan Formation: New Evidence from Zircon SHRIMP U-Pb Age and Geochemistry

川西会理一带出露的天宝山组以酸性火山岩为主,它们的SiO2含量为68.78%～76.96%,K2O含量明显大于Na2O含量,Na2O/K2O比值介于0.01～0.34之间,多属于流纹岩。酸性火山岩的轻重稀土元素分馏程度不高,Eu的负异常明显,Eu/Eu*=0.18～0.35。微量元素中Ba、Ta、Nb、Sr、Ti相对亏损。这些地球化学特征与A型花岗岩相似。酸性火山岩的εNd(t)值介于-5.32到-4.78之间,与地壳衍生的S型花岗岩相似,表明天宝山组酸性火山岩主要来自先存的地壳物质。锆石SHRIMPU-Pb定年结果表明,天宝山组酸性火山岩的形成年龄为1028±9Ma,属于中元古代晚期。

Volcanic rocks in the Tianbaoshan Formation, distributed in Huili area on western margin of Sichuan Province, are mainly felsic volcanics. These volcanic rocks are classified as rhyolite, riched in SiO2 from 68.78% to 76.96% and K2O, with ratio of Na2O/K2O between 0.01 and 0.34. They are mid fractionation in REEs, with strong negative anormality of Eu （Eu/Eu＊ =0. 18~0. 35）, meanwhile depleted in trace elements of Ba, Ta, Nb, Sr, Ti, so have some characteristics of A-type granite. εNd（t） in these volcanics is from --3.05 to --3.45, similar to the S-type granite from the melted crust, which indicates that the felsic volcanics mainly formed by the crust melting. Zircon SHRIMP U-Pb age dating of 1028 ± 9Ma indiacates that felsic volcanics in the Tianbaoshan Formation were erupted during the late stage of Mesoproterozoic.