Experimental Crystallization of Leucogranite Magmas

Both crystallization and melting experiments have been carriedout on two natural, biotite-muscovite (DK) and tourmaline-muscovite(GB) High Himalayan leucogranites (HHL) at 4 kbar, logfO₂ =FMQ–05, aH₂O = 1–0•03, and at five temperaturesbetween 803 and 663C H₂O contents of the quenched glasses wereanalysed by ion microprobe. Plagioclase and biotite are theliquidus 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 80Cfor conditions close to H₂O saturation), and a slightly higherH₂O-saturated solidus (645 compared with 630C for GB). Tourmalinenever crystallized spontaneously from the melt. Tourmaline seedsalways 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 ferromagnesianphase at high temperature and reduced aH₂O. Muscovite crystallizationis restricted to near-solidus conditions. The compositions ofplagioclase, alkali feldspar, biotite and muscovite are givenas a function of bulk composition, temperature and aH₂O. Glasscompositions are richer in normative quartz than the 4 kbarH₂O-saturated Qz–Ab–Or eutectic, and become moreperaluminous and less mafic with increasing fractionation. Biotitecrystallization in peraluminous liquids is favoured by elevatedFe, Mg and Ti contents. Muscovite crystallization is not promotedunder H₂O-saturated conditions. Tourmaline stability is stronglydependent on aH₂O. For GB, tourmaline is present at elevatedtemperatures for intermediate values of aH₂O (803 C, 0–7),but not above 650C for H₂O-saturated conditions. Comparisonof the natural crystallization sequence with experiments suggestsinitial water contents between 5 and 75 wt % for the DK magma,and > 7 wt% for the GB magma. Plagioclase core compositionsgive minimum temperatures of 700C for GB and 750C for DK,consistent with an emplacement of these HHL as almost entirelyliquid bodies. The restricted occurrence of biotite in the GBgranite 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 leucogranitesby fractional crystallization under conditions of increasingdegree of oxidation. KEY WORDS: leucogranite; melting experiments; crystlization experiments; Himalayas; phase relations ^*Corresponding author