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 (met apelites and metagreywackes), orthogneisses (metaganites) and migmatit es. Melanosomes are generally biotite- (+/- muscovite)-bearing, but lo cally they may be tourmaline-rich when associated with boron-rich gran itic material, Leucosomes occur as lenses conformable with the foliati on, veins, patches, or as fillings in shear zones and extensional stru ctures, Field relationships, and mineralogical and chemical data show that three processes may have contributed to the formation of the Tibe tan Slab leucosomes: metamorphic differentiation or disequilibrium par tial melting (low-Zr tonalitic leucosomes), in-situ equilibrium partia l melting (high-Zr leucosomes and some granitic leucosomes) and inject ion of externally-derived melts (most granitic and some tonalitic leuc osomes). 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 fr om melting of the Tibetan Slab gneisses (Formation I). Phase relations hips, a more magnesian chemistry of the ferromagnesian minerals from t he Tibetan-Slab migmatites as compared to the Manaslu leucogranite, th e microtextures of accessory phases, and trace-element compositions (l ower U, Li, F and higher Sr, Eu, Y, Yb contents in the migmatite leuco somes) show that the in situ Himalayan migmatites, at the crustal leve l presently exposed, have not been produced under the same P-T-X(H2O) conditions as the Manaslu leucogranite magma. While the Formation I wa s the probable source for the Manaslu granite, migmatites within the f ormation are not the remanants of a melting process from which the Man aslu granite was derived, Both the Tibetan Slab migmatites and the Man aslu leucogranite may be considered as evidence of dehydration and mel ting at deeper crustal levels, and of percolation of melts and hydroth ermal fluids through the crust.