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

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. eNd(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.