GRANITE RHEOLOGY - MAGMA FLOW AND MELT MIGRATION

Granite magma rheology has long been studied by rock physicists, petro logists and experimentalists. New data on viscosity of dry and H2O-bea ring melts and glasses in a wide range of temperatures and H2O content changed drastically the general view on the Arrhenian behaviour of gr anitic melts. Recent studies on the deformation of partially molten gr anite rocks at up to 50 vol% of melting show the failure of the RCMP ( rheologically critical melt percentage) concept for granites. We prese nt new direct measurements of melt migration from a combination of cen trifuge experiments, falling sphere viscometry of partially molten gra nites assisted by a centrifugal field and viscoelastic behaviour of pa rtially molten granites in torsion oscillation apparatus. The results suggest that the extrapolation of the Einstein-Roscoe equation for the viscosity of partially molten rocks does not give adequate results fo r granites with 50 vol% of melt phase. Rheology of granite magmas is v ery sensitive to chemical composition, and viscosity of the melt phase . Adding H2O to partially molten granites results in a decrease in the viscosity of the melt phase, as well as dissolution of quartz crystal s. In turn, this produces more felsic and viscous melt. Thus, these fa ctors smear out the contrast between partially and completely molten g ranites, and between anhydrous and H2O-bearing magmas at moderate H2O contents (1 to 2 wt%).