Deciphering mantle heterogeneity associated with ancient subduction-related metasomatism: Insights from Mg-K isotopes in potassic alkaline rocks

Abstract

Melts and fluids derived from subducting/subducted oceanic lithosphere contribute significantly to mantle compositional heterogeneity. Historically metasomatized lithospheric mantle modified by varying materials, especially volatile-rich slab fluids, has been understood to be crucial for the formation of giant ore deposits, thus providing potential prospective areas for mineral exploration. However, it is not straightforward to identify superimposed effects of slab melts and fluids on the sub-continental lithosphere (SCLM) that has experienced long-lived and/or multiple metasomatism. In this study, we present a novel study using Mg and K isotopes, together with radioactive isotopes on mafic post-collisional alkaline potassic rocks (PARs) with the aim of deciphering varying agents contributing to the heterogeneous metasomatism of the SCLM beneath the southeastern Tibetan Plateau. The PARs, formed at ∼36 Ma, are characterized by high abundances of large ion lithophile elements (LILE) but depleted in high field strength elements (HFSE) with high Rb/Sr ratios, indicating a phlogopite-bearing mantle source. We find that features of light Mg (δ26Mg = −0.52 to −0.25‰) and heavy K (δ41K = −0.56 to +0.08‰) isotopes observed in the PARs are largely inherited from their mantle sources with negligible effects of kinetic fractionation, fractional crystallization and crustal level processes. Our modelling reveals that the notable divergence of Sr–Nd–Pb–Mg–K isotope compositions between PAR suites in different tectonic blocks are best understood as resulting from varying contributions of slab-derived components into the overlying mantle lithosphere, including sediment-derived melts, ocean crust-derived supercritical fluids, and marine carbonates. Particularly, their high Th/U ratios and negative correlations between δ26Mg and δ41K values suggest that slab-derived supercritical fluids can dissolve and transport Mg-rich (dolomitic) carbonate into the mantle wedge.