EVOLUTION OF MICROSTRUCTURE AND MELT TOPOLOGY IN PARTIALLY MOLTEN GRANITIC MYLONITE: IMPLICATION ON THE RHEOLOGY OF PARTIALLY MOLTEN LOWER CRUST

The effect of syn-deformation melting and melt infiltration on the rheology of orogenic and middle crust is presented. High grade orthogneisses (granulite to amphibolites facies) show extreme viscous deformation of K-feldspar and plagioclase aggregates compared to weakly elongated quartz grains. The original weakening of alkali feldspar is generally achieved by decomposition into albite chains and K-feldspar resulting from heterogeneous nucleation process. Subsequent collapse of alkaline feldspar and development of mono-mineralic layering is attributed to onset of syn-deformational melting associated with production/infiltration of ~2% melt. The common metamorphic reaction producing the melt in the rock took place in plagioclase Mu-Bi rich bands and the melt infiltrated into the K-feldspar bands. At this stage the plastically deforming K-feldspar aggregate contains numerous interstitial grains of albite and quartz grains that reflect the topology of crystallized melt with melt pockets oriented perpendicular to the stretching lineation. The melt concentrated in the feldspar aggregates is characterised as an interconnected network within a framework of dilated isometric grains with highest permeability in the lineation direction. Final deformation stage is marked by mixing of feldspars which is explained by higher melt production due to introduction of external water or infiltration of silicate melts. Already small amount of melt is responsible for extreme weakening of feldspar due to Melt Connectivity Threshold effect triggering grain boundary sliding deformation mechanisms. Grain boundary sliding controls diffusion creep at small melt fraction and evolves to particulate flow at high melt fractions. Strong quartz shows dislocation creep deformation mechanism throughout the whole deformation history marked by variations in activity of slip systems, which are attributed to variations in stress and strain rate partitioning with regard to changing rheological properties of deforming feldspars. The mechanisms described above is discussed in terms of melt transport through deforming rocks and the concept of reactive porous flow through continental crust is intorduced?