EXPERIMENTAL CRYSTALLIZATION OF LEUKOGRANITE MAGMAS

Both crystallization and melting experiments have been carried out on true natural, biotite-muscovite (DK) and tourmalin-muscovite (GB) High Himalayan leucogranites (HHL) at 4 kbar, logfO(2) = FMQ-0.5, aH(2)O = 1-0.03, and at five temperatures between 803 and 663 degrees C H2O co ntents of the quenched glasses mere analysed by ion microprobe. Plagio clase and biotite are the liquidus phases for reduced melt H2O content s and H2O-rich conditions, respectively. H2O saturation limits range f ront similar to 8 to 10 wt%. DK has a wider crystallization interval t han GB (150 vs 80 degrees C for conditions close to H2O saturation), a nd a slightly higher H2O-saturated solidus (645 compared with 630 degr ees C for GB). Tourmaline never crystallized spontaneously from the me lt. Tourmaline seeds always reacted out to biotite in the biotite-musc ovite sample, whereas they remained stable in the tourmaline-muscovite sample. Biotite is replaced by hercynite as the main ferromagnesian p hase at high temperature and reduced aH(2)O. Muscovite crystallization is restricted to near-solidus conditions. The compositions of plagioc lase, alkali feldspar, biotite and muscovite are given as a function o f bulk composition, temperature and aH(2)O. Glass compositions are ric her in normative quartz than the 4 kbar H2O-saturated Qz-Ab-Or eutecti c, and become more peraluminous and less mafic with increasing fractio nation. Biotite crystallization in peraluminous liquids is favoured by elevated Fe, Mg and Ti contents. Muscovite crystallization is not pro moted under H2O-saturated conditions. Tourmaline stability is strongly dependent on aH(2)O. For GB, tourmaline is present at elevated temper atures for intermediate values of aH(2)O (803 degrees c, similar to 0- 7), but not above 650 degrees C for H2O-saturated conditions. Comparis on of the natural crystallization sequence with experiments suggests i nitial water contents between 5 and 7.5 wt% for the DX magma, and > 7 wt% for the GB magma. Plagioclase core compositions give minimum tempe ratures of similar to 700 degrees C for GB and 750 degrees C for DK, c onsistent with an emplacement of these HHL as almost entirely liquid b odies. The restricted occurrence of biotite in the GB granite suggests that it reacted out during the magmatic evolution, owing to a marked change in fO(2) toward more oxidizing conditions. Tourmaline leucogran ites can be generated from Biotite leucogranites by fractional crystal lization under conditions of increasing degree of oxidation.