Experimental Crystallization of Leucogranite Magmas

Abstract Both crystallization and melting experiments have been carried out on two natural, biotite-muscovite (DK) and tourmaline-muscovite (GB) High Himalayan leucogranites (HHL) at 4 kbar, logfO2 = FMQ−0·5, aH2O = 1−0⋅03, and at five temperatures between 803 and 663°C H2O contents of the quenched glasses were analysed by ion microprobe. Plagioclase and biotite are the liquidus phases for reduced melt H2O contents and H2O-rich conditions, respectively. H2O saturation limits range from ∼8 to 10 wt%. DK has a wider crystallization interval than GB (150 vs 80°C for conditions close to H2O saturation), and a slightly higher H2O-saturated solidus (645 compared with 630°C for GB). Tourmaline never crystallized spontaneously from the melt. Tourmaline seeds always reacted out to biotite in the biotite-muscovite sample, whereas they remained stable in the tourmaline-muscovite sample. Biotite is replaced by hercynite as the main ferromagnesian phase at high temperature and reduced aH2O. Muscovite crystallization is restricted to near-solidus conditions. The compositions of plagioclase, alkali feldspar, biotite and muscovite are given as a function of bulk composition, temperature and aH2O. Glass compositions are richer in normative quartz than the 4 kbar H2O-saturated Qz–Ab–Or eutectic, and become more peraluminous and less mafic with increasing fractionation. Biotite crystallization in peraluminous liquids is favoured by elevated Fe, Mg and Ti contents. Muscovite crystallization is not promoted under H2O-saturated conditions. Tourmaline stability is strongly dependent on aH2O. For GB, tourmaline is present at elevated temperatures for intermediate values of aH2O (803° C, ∼0–7), but not above 650°C for H2O-saturated conditions. Comparison of the natural crystallization sequence with experiments suggests initial water contents between 5 and 7·5 wt % for the DK magma, and > 7 wt% for the GB magma. Plagioclase core compositions give minimum temperatures of ∼700°C for GB and 750°C for DK, consistent with an emplacement of these HHL as almost entirely liquid bodies. The restricted occurrence of biotite in the GB granite suggests that it reacted out during the magmatic evolution, owing to a marked change in fO2 toward more oxidizing conditions. Tourmaline leucogranites can be generated from biotite leucogranites by fractional crystallization under conditions of increasing degree of oxidation.