The Late Carboniferous to Early Permian high silica magmatism in the Southern Mongolia: Implications for tectonic evolution and continental growth

Abstract

Tremendous granites were produced during the late-stage evolution of the Central Asian Orogenic Belt and important to understanding the tectonic processes from subduction to collision. Here, we present zircon U-Pb-Hf, and whole-rock geochemical, and Sr-Nd isotopic data for a suite of Carboniferous-Permian granites from the Khan-Bogd area of the southern Mongolia. U-Pb age dating of zircons indicates that the alkaline feldspar granites and quartz monzonite-monzodiorites on the southwest of the Khan-Bogd town were formed at ~315–310 Ma and the alkaline feldspar granites on its south at ~290 Ma. The ~315–310 Ma alkaline feldspar granites possibly result from crustal anatexis of the Devonian-Carboniferous juvenile arc crust there, evidenced by their intensive continental geochemical signals and roughly similar Nd and Hf isotopes to these arc crust. The coeval quartz monzonite-monzodiorites with limited outcrop show arc affinity. Thus, these Late Carboniferous granites indicate a sharp decrease of mantle-crust interaction from earlier arc accretion. Combined with previous work, this change was possibly triggered by a low-angle subduction. The ~290 Ma alkaline feldspar granites show A-type petrological and geochemical features. In addition, they have more variable Nb/Ta (~10–18), Nd and Hf isotopes indicative of various continental crust involvement in their magma source. The lack of adakite and arc-type magma deprives the possibility of lithosphericdelamination, ridge subduction and slab rollback. As a significant mantle input evidenced by coeval bimodal magmatism, this crust-derived granite was likely induced by a slab breakoff (~305–290 Ma), rather than backarc extension. Combined with previous work, this slab breakoff probably indicates a tectonic transition of post collision (~315–290 Ma) from subduction-accretion and marks the closure of a wide secondary ocean (North Tianshan-Hegenshan ocean) of the Paleo-Asian Ocean. Moreover, it plays an important role for the vertical crustal growth of the CAOB.