The Four Plutonic Belts of the Transhimalaya-Himalaya: a Chemical, Mineralogical, Isotopic, and Chronological Synthesis along a Tibet-Nepal Section

The geochemistry (major, trace element, O- and Sr-isotope ratios) and petrology of the Transhimalaya, North Himalaya, High Himalaya and ‘Lesser Himalaya’ plutonic belts are compared based on the analyses of up to 492 samples. The composite Transhimalaya batholith is subalkaline or monzonitic in character rather than calc-alkaline. Its genesis was probably closely related to subduction processes associated with strike-slip movement. It was emplaced on both sides of the boundary between an earlier metavolcanic arc and a continental margin. Two principal periods of magmatic activity occurred: Upper Cretaceous and, particularly in this region, Eocene at the time of the India-Eurasia collision when sediments may have become involved in the subduction process. Magmatic differentiation, characterized by two superimposed stages of evolution, and hybridization processes, involving both basic and acidic magmas, can account for the genesis of the different plutonic units. Although a continental contribution is implied, the isotopic data (6.8 < σ18O < 9·2; 0·704 < 87Sr/86Sr, < 0·707) preclude a significant contribution from either old crust or surface derived sediments. The North, High and ‘Lesser’ Himalaya plutonic belts are fundamentally different and correspond to aluminous associations of two groups of ages (Lower Palaeozoic for the ‘Lesser Himalaya’ and part of the North Himalaya; Upper Cenozoic for the High Himalaya and part of the North Himalaya). They are all high-σ18O (9 < σ18O < 14) granites and adamellites with high initial 87Sr/86Sr, ratios (0·709 to < 0·740). The Lower Palaeozoic group was generated within the Gondwana continental crust, independent of any true orogenesis, with a probable but limited contribution from the mantle. High Himalaya and North Himalaya Cenozoic plutons are directly linked to the activity of the Main Central Thrust. They were derived by similar anatectic processes of the same continental source rocks. The small but distinct chemical and mineralogical differences among the plutons are related to the increase in the intensity of anatexis on going towards the north and the east.