The Early Cretaceous bimodal volcanic suite from the Yinshan Block, western North China Craton: Origin, process and geological significance

Abstract

We present geochemical and age data for coeval basalt and rhyolite in the Yinshan Block, western North China Craton, to study the origin of the ‘Daly Gap’ in this bimodal volcanic suite. SHRIMP zircon U-Pb dating for felsic rocks and bulk rock 40Ar/39Ar dating for mafic rocks yield their ages of 107–133 Ma. The basalts show elevated abundances of incompatible elements with [La/Sm]N = 2.65–3.54 and enriched Sr-Nd-Hf isotopic signatures (87Sr/86Sr(t) = 0.7054–0.7084, 143Nd/144Nd(t) = 0.5119–0.5123, 176Hf/177Hf(t) = 0.2824–0.2826), consistent with their derivation from low degree melting of previously metasomatized lithospheric mantle. The rhyolites display similar enrichment in alkalis and other incompatible elements, but have obvious Nb-Ta depletion. Combined with their more enriched Nd-Hf isotopic compositions (εNd(t) = -12 – -18, εHf(t) = -2 – -14), these rhyolites are inferred to have been generated by mixing of the mantle-derived basaltic magma with felsic melts derived from lower continental crust anataxis induced by the basaltic magma. Furthermore, the extremely low Ba, Sr, Eu contents, high Rb/Sr ratios and unexpectedly high 87Sr/86Sr (0.7920-0.7159) in these felsic samples also suggests that their parental magma experienced protracted fractional crystallization of plagioclase (K-feldspar to a lesser extent) en route to the surface. This rock association and the unique geochemical characteristics of these volcanic rocks offer a new view on the petrogenesis of bimodal volcanism: the mafic rocks originated from partial melting of the ancient lithospheric mantle. Underplating of the basaltic magmas triggered melting of lower continental crust. The extensive fractional crystallization of the hybridized magma resulted in the highly fractionated rhyolites. This mechanism explains the “Daly Gap” exhibited in this rock association in a continental setting and also the observation that both the lithospheric mantle and crust in East China are thinner than that in West China.