Recycled oceanic crust in the form of pyroxenite contributing to the Cenozoic continental basalts in central Asia: new perspectives from olivine chemistry and whole-rock B–Mo isotopes

Abstract

Cenozoic continental basalts are widespread in central Asia. To explore their source nature and petrogenesis, this study presents an integrated study of olivine chemistry, bulk-rock 40Ar/39Ar age and geochemistry as well as Sr–Nd–Pb–B–Mo isotopes for the Miocene (ca. 15.5 Ma) Halaqiaola basalts in the Chinese Altai, central Asia. The Halaqiaola basalts mostly have basanite compositions with high total alkali (Na2O + K2O = 6.89–8.01 wt%) contents and high K2O/Na2O (0.87–1.39) ratios. Compared with partial melting products of mantle peridotite, the basaltic samples possess lower CaO and CaO/Al2O3 but higher TiO2, Zn/Mn and Zn/Fe values. Meanwhile, olivine phenocrysts from these basalts are characterized by lower Ca, Ni and Mn contents but higher Fe/Mn ratios than their counterparts in the peridotitic melts, suggesting a pyroxenite-rich source. Moreover, these rocks show OIB-like trace element patterns (e.g., spikes of Ba, Sr, Nb and Ta and troughs of Th and U), and constant Nd but variable Sr and EM1-like Pb isotopic compositions, and yield light δ11B (– 11.0 to – 8.1‰) and δ98Mo (– 0.40 to – 0.06‰) values. The above geochemical data suggest that secondary pyroxenite was likely produced by reaction of recycled oceanic crust with its ambient peridotite and subsequently became the main source for the basanite. Furthermore, their light and variable δ98Mo values probably reflect that recycled oceanic crust involved in such pyroxenite was altered with different degrees. In combination with available data from adjacent regions, we propose that the far-field effect of India–Eurasia collision was the first-order factor for the upwelling of dispersive asthenospheric mantle beneath central Asia, subsequent melting of which gave rise to the widespread Cenozoic volcanism.