Late Cretaceous post-orogenic delamination in the western Gangdese arc: Evidence from geochronology, petrology, geochemistry, and Sr-Nd-Hf isotopes of intermediate-acidic igneous rocks

Late Mesozoic uplift of the Gangdese arc occurred prior to the India-Asia collision and was important in the evolution of the Tibetan plateau, but the geodynamic mechanisms that drove this process are unclear. In this paper, we report geochronological, petrological, geochemical, and Sr-Nd-Hf isotopic data for intermediate-acidic igneous rocks from the western Gangdese arc. These igneous rocks were emplaced at 87-67 Ma and are characterized by enrichments in light rare earth and large-ion lithophile elements, depletions in high-field-strength elements, high (Sr-87/Sr-86)(i) ratios (0.7074-0.7116), and low epsilon(Nd)(t) (-7.5 to -4.1) and epsilon(Hf)(t) values (-16 to 0.8). These magmas were most likely derived by high-temperature (960-1200 degrees C) partial melting of the thickened crustal materials, with minor mantle input, related to post-orogenic lithospheric delamination. The increasing epsilon(Nd)(t) and epsilon(Hf)(t) values and decreasing initial Sr isotope ratios from the ca. 87 Ma diorites to ca. 67 Ma diorites and rhyolites indicate an increasing mantle contribution as delamination proceeded. Estimated pressure conditions for the high-Sr/Y gabbroic diorites indicates the magmas were derived from crust at pressures of up to 19 kbar (i.e., a depth of similar to 66 km). Therefore, the western Gangdese arc had been thickened during the Late Cretaceous, possibly due to the flat-slab subduction of Neo-Tethyan oceanic lithosphere and coeval collision of the Qiangtang-Lhasa terranes.