VOLCANIC ROCKS OF THE BAIYINGAOLAO FORMATION IN THE SOUTHWEST OF ZHALANTUN, INNER MONGOLIA:Geochemistry, Geochronology and Tectonic Implications

The volcanic rocks of Baiyingaolao formation in the southwest of Zhalantun are petrographically composed of rhyolite, with minor dacite. The zircon U-Pb dating results show that they were formed in Early Cretaceous from 125 to 129 Ma. The geochemical study suggests that the rhyolite is rich in silicon and alkali, belonging to peraluminous high-K calcalkaline series. The rocks are also characterized by obviously right-declined REE patterns and slightly negative Eu anomalies with high total REE contents. On the primitive mantle normalized multi-elements diagram, the volcanic rocks of Baiyingaolao Formation exhibit enrichments of Rb, Th and K and depletions of Ba and HFSEs such as Nb, Ta, Ti, Sr and P, similar to the characteristics of A-type granite. They have low Cr, Ni, Co and Mg^# concentrations, with Rb/Sr ratios of 0.58-2.06, Ti/Y ratios of 6.50-17.98 and Ti/Zr ratios of 7.49-40.87, indicating a crust magma source, instead of mantle source. From the above characteristics, it is proposes that the volcanic rocks of Baiyingaolao Formation were derived from the partial melting of the crust and formed in the non-orogenic intraplate extensional environment.