Miocene Alkaline Basaltic Magmatism in Northeastern Tibetan Plateau: Implications for Mantle Evolution and Plateau Outward Growth

Abstract

The widespread Cenozoic alkaline magmatism within and around the Tibetan Plateau offers a prime opportunity to probe the nature of the mantle at the depths where basalt magmas originate. The close temporal and spatial relationship between volcanism and regional strike-slip fault systems also helps better understand the geodynamics of outward growth of the plateau in response to the continued India-Asia convergence. We present a comprehensive study of the deeply sourced alkaline basalts formed along the Kunlun strike-slip fault with the aim of understanding their petrogenesis and the composition of mantle sources beneath the northeastern Tibetan Plateau. High Nb/U and Ce/Pb ratios and relatively depleted bulk-rock Sr-Nd-Pb isotope compositions corroborate the mantle origin of these alkaline basalts. Homogeneous and low ⁸⁷Sr/⁸⁶Sr of clinopyroxene indicates negligible crustal contamination during magmatic evolution. Low δ²⁶Mg in the alkaline basalts and positive correlations with Hf/Sm and Ti/Ti* indicate that the basalts were derived from mantle that was metasomatized by melts derived from sedimentary carbonates during the Paleo-Tethyan seafloor subduction. Based on ⁴⁰Ar/³⁹Ar dating results, it appears that the alkaline basaltic magmatism in the northeastern Tibetan Plateau occurred simultaneously with Kunlun strike-slip faulting. These observations suggest that the India-Asia convergence must have reactivated ancient subduction plate boundaries and resulted in strike-slip faulting along these suture zones within and around the Tibetan Plateau. The eruption of low-volume and deeply rooted alkaline basalts may have been controlled by fractures associated with the strike-slip fault systems.