Manaslu leucogranite: A collision signature of the Himalaya: A model for its genesis and emplacement


  • Patrick Le Fort


The Manaslu pluton is one of 10 leucogranites that formed in the overthrusted Higher Himalaya after the Indo-Eurasian collision. Field and analytical data indicate that the underlying migmatites of the Tibetan Slab may be where the leucogranitic melts were generated. The Himalayan crustal thrusting of a hot slab over a rather cold volcano-sedimentary pile [Le Fort, 1975a] provides the necessary release of fluids. These fluids cross the Main Central Thrust (MCT), induce the partial anatexis of the overheated Tibetan Slab, and produce a leucogranitic magma. The emplacement of the magma at first is located along the main disharmonic plane above the MCT, between the infrastructure and the superstructure. There it generates a convective hydrothermal system extending very far laterally according to the stratification of permeabilities. Progressive emplacement of the granite proceeds as the convected fluids, including the fluids released by the saturated magma, dissolve the mainly calcareous host rocks of Tibetan sedimentaries. A frozen image of this ‘caving out’ progression is given by the extensive network of granitic dikes outside the pluton. The two quite independant fluid cycles of the generation and of the emplacement were triggered or guided by the tectonics due to collision. This two-fold model, dominated by fluid activity, may be of importance for other leucogranites and granites.