VISCOSITY OF HIMALAYAN LEUKOGRANITES - IMPLICATIONS FOR MECHANISMS OFGRANITIC MAGMA ASCENT

The viscosity of the Gangotri Himalayan leucogranite has been experime ntally determined between 800 degrees and 1100 degrees C, 300 and 800 MPa, for meltwater contents of 3.98 and 6.66 wt%. The melt viscosity i s independent of pressure and shows an Arrhenian behavior relative to temperature within the range of conditions investigated. We present an empirical relation that can be used to determine leucogranite magma v iscosities knowing their meltwater content and temperature. This relat ion together with phase equilibria experiments constrain the viscosity of the Himalayan leucogranites to be around 10(4.5) Pa s during their emplacement. These viscosities and the widths of dikes belonging to t he feeder system are consistent with the theoretical relationship rela ting these two parameters and show that the precursor magma of the leu cogranite was at near liquidus conditions when emplaced within host ro cks with preintrusion temperatures around 350 degrees C. Calculated te rminal ascent rates for the magma in the dikes are around 1 m/s. Magma chamber assembly time is, on this basis, estimated to be less than 10 0 years (for a volume of 150 km(3)). In addition, the dynamical regime of the magma flow in the dikes was essentially laminar, thus allowing preservation of any chemical heterogeneity acquired in the source. Th ese results constrain the viscosity of melts formed during the first s teps of crustal anatexis, those involving muscovite breakdown, to be a lso around 10(4.5) Pa s. Thus compaction may not be the only mechanism of melt segregation in partially melted crustal rocks in view of the very short timescale inferred for magma ascent and emplacement.