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.
Han et al. (Mon,) studied this question.