U-Th systematics of dispersed young volcanoes in NE China: Asthenosphere upwelling caused by piling up and upward thickening of stagnant Pacific slab

Young (300 to 5500years old) dispersed intraplate volcanism characterized by small volume of melt and large compositional variability, occurred over NE China at Jingbohu, Long-gang, Tianchi and Wudalianchi. These young volcanic rocks provide ideal opportunities for investigating the mechanism of dispersed continental volcanism using short-lived uranium-series isotopes. We present systematic U-series, Nd, Sr, Pb isotope and trace element analyses of the young lavas from NE China. Jingbohu lavas display variable extents of Th-230 excesses (10 to 28%) and moderately depleted Nd isotopic compositions (epsilon(Nd) = + 1.5 to + 3.3). Long-gang lavas have pronounced (33 to 35%) Th-230 excesses and slightly depleted Nd isotopic compositions (epsilon(Nd) = + 0.5 to + 0.7). The Tianchi lavas display moderate (12%) Th-230 excesses and slightly enriched Nd isotopic compositions (epsilon(Nd) = -1.0 to -1.1). Th-230 enrichments in all these volcanic rocks are uncharacteristic of melts generated by subduction. The strongest evidence against the involvement of subduction-related fluids is derived from the Long-gang lavas having more than 30% (230)Ch excesses, low U/Th, Sr/Th and Ba/Th ratios, and positive Nb and Ta anomalies. The fluids released from the subduction of the Pacific plate under the Eurasian plate might have already been completely lost before the slab reached the mantle beneath NE China. We postulate that, in addition to the convective circulation process, the continuous piling up and upward thickening of stagnant Pacific slab in the mantle transition zone may help drive a broad asthenosphere upwelling far from the trench, leading to mantle partial melting and the formation of the young volcanoes in NE China. In addition, our U-Th disequilibrium data provide constraints on magma evolution time. The magma evolution time from low SiO2 (47.5%) basalt to higher SiO2 (50.4%) basalt at Long-gang is within 10 thousand years (10ka). The magma evolution time from trachyte to rhyolite at Tianchi volcano is also less than 10ka. If the Tianchi silicic rocks were evolved from Long-gang basaltic magmas with minor crustal assimilation, then the magma evolution time from basaltic magma to silicic magmas is less than 110 thousand years. (C) 2008 Elsevier B.V. All rights reserved.