Mid-Miocene high-temperature magmatism, a case study of the Sakya dome: Implications for the removal of the Himalayan orogenic root

Intense crustal anatexis, metamorphism, and structural deformation occurred throughout the collisional orogeny, preserving crucial records and constraints on the thermal and tectonic evolution. During the mid-Miocene, the Himalayan magmatism, including the leucogranites, adakite-like, and ultrapotassic rocks, culminated, coeval with (ultra)high-temperature metamorphism. However, the heat source and the deep process in the mid-Miocene Himalaya remain debated. This study presents results of U-(Th)-Pb dating of monazite and zircon, whole-rock geochemical and isotopic analyses, and three independent thermometric estimates for porphyritic granites and granite porphyries from the Sakya dome, northern Himalaya. The porphyritic granites and granite porphyries yield ages of 13.9−13.0 Ma and 12.6−11.6 Ma, respectively. Geochemical and isotopic analyses indicate the former were derived from the Greater Himalayan Crystalline (GHC), while the latter likely originated from a mixed source comprising ∼80% GHC and ∼20% Indian thickened lower crust materials. Estimated results from three thermometers consistently indicate elevated temperatures during both anatexis and subsequent magma evolution, implying an abnormally high-temperature background. Integrating evidence from the mid-Miocene Himalayan magmatism, (ultra)high-temperature metamorphism and regional tectonics, this paper adopts a model of the delamination of the Indian lithospheric mantle, which triggered the asthenospheric upwelling and additional heat input, thereby driving the (ultra)high-temperature magmatism and metamorphism, extensive anatexis at multiple depths, and diverse magmatism involving contributions from the GHC, Lesser Himalayan Sequence, Indian thickened lower crust, and the mantle in the mid-Miocene Himalaya. This delamination may signify the removal of the Himalayan orogenic root in the mid-Miocene, after which orogenesis persisted under continued convergence between the Indian and Asian plates.