Source diversity in controlling the compositional diversity of Cenozoic granites in the Tethyan Himalaya

Cenozoic granites in the Himalayan orogen show compositional diversity. Yet the importance of source diversity in causing the compositional diversity is underestimated. Here we studied whole-rock Sr–Nd isotope geochemistry and relict zircon U–Pb geochronology of three suites of granites in the Himalayan orogen. Combing our new data and previous data, a new source component, namely metasedimentary rocks from the Triassic Langjiexue Group in the Tethyan Himalaya, was identified. Such a component is characterized by 200–400 Ma-aged zircons and relatively less enriched Sr–Nd isotopic compositions. These signatures have been imprinted on both the Eocene and Miocene granites from the Yalaxiangbo area. As a comparison, geochemical features and relict zircon U–Pb age spectrum of the Miocene Cuona granites both suggested they were derived from partial melting of metapelites of the Higher Himalayan Sequence. Phase equilibrium modeling results suggest that both the Langjiexue metapelite and the Higher Himalayan metapelite have comparable fertility. Given their similar lithology and major-trace element compositions, the resultant melts will have no significant distinction in lithochemistry. This explains why many reported granites in the Tethyan Himalaya have undistinguishable major-trace elemental compositions to the Higher Himalayan leucogranites, but have less enriched Sr–Nd isotopic compositions. However, if metamafic rocks were involved, the resultant melts will depart from the Higher Himalayan leucogranites in both elemental and isotopic compositions, exemplified by the Miocene adakites in the Tethyan Himalaya. This study highlights the importance of source diversity in causing the compositional diversity of the Cenozoic granites in the Tethyan Himalaya.