Geochemistry, zircon U-Pb ages and Hf isotopes of the Paleozoic volcanic rocks in the northwestern Chinese Altai: petrogenesis and tectonic implications

Abstract

Paleozoic intermediate-acidic volcanic rocks exposed in the northwestern Chinese Altai mainly consist of two different lithologic suites: rhyolite suite (RS) and dacite-rhyolite suite (DRS), which were both assigned to the late Ordovician Dongxileke Formation. Zircon U-Pb dating results demonstrate that the RS suite was formed in the late Ordovician (443±2 and 445±2Ma), while the DRS suite yielded much younger ages (411±4Ma) that are consistent with the early Devonian Kangbutiebao Formation of the Chinese Altai. Zircons from the two suites all have high ε Hf(t) values (mostly +2 to +9), indicating a source dominated by juvenile crustal materials. Rhyolites from the two suites are strongly peraluminous and have high SiO 2 (70.53-76.94wt.%) and ε Nd(t) values between -1.7 and -0.6, similar to those of the Paleozoic granitoids (>300Ma) but higher than the Early Paleozoic Habahe sediments, suggesting an origin by partial melting of a juvenile crustal source dominated by meta-igneous rocks. Dacite samples from the DRS suite possess high SiO 2 contents (64.71-67.77wt.%) and Mg # values (52). They have ε Nd(t) values similar to those of the Habahe sediments, which, in combination with their relative high Th/Yb (>3.5) and low Ba/La (<40) ratios, suggests the dacite was originated from sediment-derived melts modified by depleted mantle peridotite. The Mg-rich dacite formed in a critical period (around 410Ma), during which voluminous granitic intrusions and rhyolite were produced. Extensive partial melting of subducting sediments in the early Devonian is in agreement with a high geothermal gradient in the Chinese Altai. A hot subduction regime is proposed to interpret the early Devonian tectonic evolution of the Chinese Altai. © 2010 Elsevier Ltd.