Petrogenesis of Mesozoic volcanic rocks in the Zha'ertai area, Inner Mongolia: Constraints from trace elements and Sr–Nd–Pb isotopes

Abstract

The Mesozoic acid volcanic rocks of the Zha'ertai area, located in Inner Mongolia, were formed in the late Early Cretaceous and resulted in polymetallic deposits of gold, silver, and other precious metals with huge commercial value. The more developed acid volcanic rocks include subvolcanic and pyroclastic rocks and volcanic lava, with SiO2 of 73.18–83.82%, Na2O + K2O of 6.17–9.03%, and K2O/Na2O of 1.28–70.27%. These rocks are enriched in large‐ion lithophile elements and high‐field strength elements. They are also enriched in light rare earth elements (REEs), with light to heavy REE ratios of 2.91–14.30; light to heavy REE fractionation is significant, and there are obvious negative Eu anomalies (δEu = 0.02–0.25). The acid volcanic rocks have extremely low TiO2, MgO, P2O5, and TFe2O3 contents, exhibit significant enrichment in Th, U, and Pb, and are strongly depleted in Ba and Sr, showing the trace elements feature of A‐type rhyolite. The rocks have smaller (87Sr/86Sr)i, lower εNd (t), younger TDM2, lower (206Pb/204Pb)i,(207Pb/204Pb)i, and (208Pb/204Pb)i. The acid volcanic rocks differ considerably from the intermediate to basic volcanic rocks in geochemical composition, and there is a compositional hiatus present in the succession. This indicates that neither endmember of these Mesozoic bimodal volcanic rocks is cogenetic and that rhyolitic rocks might not be a product of basaltic magma crystallization differentiation. Instead, they may result from partial melting of crust‐derived rock, stemming from partial melting of rocks in the pre‐existing crust underplated by mantle‐derived magma. This suggests that the acid volcanic rocks were formed in a lithospheric extensional setting during the Early Cretaceous.