Zircon U-Pb ages and geochemistry of the potassic and sodic rhyolites of the Kangbutiebao Formation in the southern margin of Altay, Xinjiang
新疆阿尔泰南缘康布铁堡组钾-钠质流纹岩锆石U-Pb年龄和地球化学

There are large volumes of the Late Paleozoic volcanic rocks in the Kangbutiebao Formation on the southern margin of Altay, in which host Fe, Cu and Pb-Zn deposits. The potassic and sodic rhyolites in the Mazi and Kelang basins are 396.7±1.4Ma and 394.0±6.0Ma respectively, by using LA-ICP-MS zircon U-Pb dating, which show they were originated from Early Devonian magmatism. Our geochronology results, combining with latest published data, prove that the Altay orogenic volcanic rocks were formed in early Late Paleozoic, with a zircon U-Pb peak age of ca. 400Ma. The potassic-sodic rhyolites are characterized by high-silicon(SiO2 content ranges from 73%~82%), high-alkali(total alkalis content ranges from 4%~7%)and peraluminous (high A/CNK>1), with occurrence of biotite and muscovite. Additionally, Sr and Nd isotopic characteristics indicate that the two types of rhyolites were generated from partial melting of an juvenile crust, with 87Sr/86Sr=0.7074~0.7144, 143Nd/144Nd=0.512072~0.512252. Combined with the regional geodynamic evolution in Late Paleozoic, it is plausible that the two types of rhyolites were formed in a continental marginal arc related with subduction. Therefore, we infer that the source magma of the potassic and sodic rhyolites in the Mazi and Kelang basins was a kind of high-silicon, high-alkali granitic magma, which are products of low degree partial melting of juvenile upper crust near-solidus, derived by underplating of high-level basaltic magma in the continental crust.