Granite-migmatite genetic link: The example of the Manaslu granite and Tibetan Slab migmatites in central Nepal

In central Nepal, the Tibetan Slab is made up of biotite-gneisses (metapelites and metagreywackes), orthogneisses (metaganites) and migmatites. Melanosomes are generally biotite- (+/- muscovite)-bearing, but locally they may be tourmaline-rich when associated with boron-rich granitic material, Leucosomes occur as lenses conformable with the foliation, veins, patches, or as fillings in shear zones and extensional structures, Field relationships, and mineralogical and chemical data show that three processes may have contributed to the formation of the Tibetan Slab leucosomes: metamorphic differentiation or disequilibrium partial melting (low-Zr tonalitic leucosomes), in-situ equilibrium partial melting (high-Zr leucosomes and some granitic leucosomes) and injection of externally-derived melts (most granitic and some tonalitic leucosomes). The Manaslu pluton belongs to the High Himalayan leucogranite belt and was emplaced at the top of the Tibetan Slab, It corresponds to a muscovite-biotite leucogranite that has been assumed to derive from melting of the Tibetan Slab gneisses (Formation I). Phase relationships, a more magnesian chemistry of the ferromagnesian minerals from the Tibetan-Slab migmatites as compared to the Manaslu leucogranite, the microtextures of accessory phases, and trace-element compositions (lower U, Li, F and higher Sr, Eu, Y, Yb contents in the migmatite leucosomes) show that the in situ Himalayan migmatites, at the crustal level presently exposed, have not been produced under the same P-T-X(H2O) conditions as the Manaslu leucogranite magma. While the Formation I was the probable source for the Manaslu granite, migmatites within the formation are not the remanants of a melting process from which the Manaslu granite was derived, Both the Tibetan Slab migmatites and the Manaslu leucogranite may be considered as evidence of dehydration and melting at deeper crustal levels, and of percolation of melts and hydrothermal fluids through the crust.