The origin of Eo- and Neo-himalayan granitoids, Eastern Tibet

Abstract

Two distinctive modes of post-collisional magmatism are evident along a transect through the eastern Himalaya at ca. 92°E. The first comprises Eocene igneous complexes emplaced into the central Tethyan Himalaya Series (THS) (Dala granitoids, 44.1±1.2Ma) and the core of the Yala-Xiangbo dome (Yala-Xiangbo granitoids, 42±5Ma). Trace element characteristics and Rb–Sr and Sm–Nd isotopes indicate that the Dala granitoids formed from sub-equal mixtures of Gangdese-type magmas and rocks of the Greater Himalayan Crystallines and/or THS. The Yala-Xiangbo granitoids are geochemically complex but interpreted to be evolved equivalents of the Dala granitoid suite. The undeformed Dala plutons constrain the timing of deformation of THS metasediments to be >44Ma and their compositions indicates that components of the Indian foreland sequences had been accreted to the hanging wall of the main Himalayan decollement by that time. The second magmatic mode comprises Miocene intrusions (Arunachal- and Tsona-types) similar in most respects to the High Himalayan Leucogranites documented throughout the range. Although fluid-absent melting of metapelites appears to be the dominant origin of Miocene Himalaya leucogranites, thermometry and trace element data for the Arunachal-type intrusions suggest that they formed under fluid-present conditions. Sr and Nd-isotopes indicate that Lesser Himalayan metasediments were involved during the partial melting that produced the Arunachal-type leucogranites.