Hydrous ultrapotassic melts and trans-lithospheric weakening of Himalayan-Tibetan orogenesis

Abstract Ultrapotassic magmatism in orogenic systems has attracted much attention due to its genetic link with the deep geodynamics of mantle-crust interactions and its tendency to signal the ending of mountain formation. These magmas originate from a metasomatized lithospheric mantle that experiences partial melting triggered by geodynamic events such as slab tearing or delamination. We measured the electrical conductivity of hydrous ultrapotassic melt (0.01–6.31 wt% H2O) and established a general conductivity model that can be applied to interpret high-conductivity anomalies in tectonic settings, and to constrain magma reservoir properties beneath volcanoes erupting similar magmas worldwide. We integrated laboratory-measured electrical conductivity with geophysical observations of trans-lithospheric structures beneath the Himalayan-Tibetan orogen. Our results indicate that lower-crustal, high-conductivity anomalies correspond to zones containing 7–15 vol% hydrous ultrapotassic melt with 2.5–6 wt% dissolved H2O. We propose that tearing of the subducted Indian slab generated ultrapotassic melts that ponded and differentiated in the lower crust, accounting for the high-conductivity anomalies observed there. The thermal input induced partial melting of the overlying crust, producing granitic melts that dominate conductivity anomalies in the middle to upper crust. This ultrapotassic melt–driven weakening of the lithosphere promoted extensional deformation and likely signals the ending of the Himalayan-Tibetan orogenesis, providing a combined petrological and geophysical indicator of extension.