EARLY TRIASSIC A-TYPE FELSIC VOLCANISM IN THE XILINHAOTE-XIWUQI, CENTRAL INNER MONGOLIA: AGE, GEOCHEMISTRY AND TECTONIC IMPLICATIONS

Abstract

In this paper, we present the combined K-Ar geochronological, elemental and isotopic geochemical data for the felsic volcanics from Xilinhaote-Xiwuqi, Inner Mongolia. The whole-rock K-Ar dating gives an early Triassic age at around 245Ma and we interpret it as representing the intrusive age for these felsic volcanics. Geochemically, these rocks are high in silica and alkalis and low in calcium and magnesium and exhibit fractionated REE patterns with remarkably negative Eu anomalies. They are also high in Ga,Zr, Nb,Ce,Y and Rb/Sr and low in Ba and Sr and display relatively enriched (87Sr/86Sr)i(0.7068 ~ 0.7099) and moderately positive εNd(t) (+1.66 ~ +3.34) values. This indicates that these felsic volcanics share geochemical characteristics with aluminous A2-type granites, and probably generated by the direct partial melting of the newly-underplated lower crustal source heated by contemporaneous intrusion of mantle-derived basaltic magmas and the subsequent differentiations. They, together with the coeval tholeiite, constitute typical bimodal volcanic rock association developed within a post-orogenic or anorogenic extension setting. This not only provides further petrological evidences for establishing that the Inner Mongolia has seen the final amalgamation of terranes in the Central Asian Orogenic Belts (CAOB) during late Permian, but also has important implications for the predominant role played by contributions of mantle-derived magmas in vertical continental growth of the CAOB during post-orogenic period.