The role of subduction channel mélanges and convergent subduction systems in the petrogenesis of post-collisional K-rich mafic magmatism in NW Tibet

Abstract Post-collisional potassium-rich mafic magmatism occurred in the northwestern part of the Tibetan Plateau, close to the western syntaxis of the Himalayan orogen, from 8.3 Ma to the present. This magmatism is associated with a tectonic setting influenced by opposing N and S dipping subduction systems formed during the collision of India and Asia. It postdates continent–continent collision and has been linked to the onset of near vertical subduction of Indian continental lithosphere at ~ 8 Ma. The magmatic rocks have relatively high MgO (4.02–9.04 wt.%), SiO2 (46.15–57.49 wt.%), K2O (3.26–7.23 wt.%), Ba (1071–3210 ppm), Th (8.2–85.2 ppm), and Pb (18.6–54.8 ppm) contents, and relatively low Al2O3 (12.74–15.78 wt.%). Sr–Nd–Pb isotopic compositions range from: (87Sr/86Sr)i (0.7072–0.7131), (143Nd/144Nd)i (0.511953–0.512528) and (206Pb/204Pb)i (18.67–19.08). Chondrite-normalized rare earth element (REE) patterns are characterized by light REE (LREE) enrichment, flat heavy REE (HREE) patterns and slightly negative Eu anomalies in some of the magmatic rocks. Primitive mantle-normalized incompatible element patterns display strong enrichments in large ion lithophile elements (LILE) relative to high field strength elements (HFSE) and distinct negative Ta–Nb–Ti anomalies. The major and trace element and Sr–Nd–Pb isotope characteristics of the most primitive mafic igneous rocks are interpreted in terms of a mantle source region dominated by subduction channel-derived mélange material derived from both the Indian and Asian subduction systems. This mélange material was underplated below the lithosphere of the Songpan-Ganzi terrane of NW Tibet, probably within the past 25 Ma. Partial melting of the underplated mélange was induced by adiabatic decompression linked to the onset of near vertical subduction of the Indian slab at ~ 8 Ma.