RHEOLOGICAL TRANSITION DURING SYN-TECTONIC PARTIAL MELTING AND CRYSTALLIZATION OF METAPELITES: AN EXPERIMENTAL STUDY

This study investigates the rheological behaviour of metapelitic rocks at depth where they undergo synkinematic partial melting and subsequent crystallization during progressive deformation. Torsion experiments were performed on synthetic pelitic rock samples at 300 MPa confining pressure and 750^oC temperature with a constant strain rate ( 3x10^-4) for a range of finite shear strains (g = 0.5-15). Partial melting started a relatively low shear strains (g = 2-4), which was coupled with strong strain softening (~ 60%) in the creep behaviour. With further shearing (g = 4-10) the creep turned to be steady state flow with nucleation of tiny, new crystals. At higher shears (g = 10-15), new mineral phases started growing at the expense of partial melts, which resulted in weak strain hardening and finally, brittle failure of the sample at the same pressure-temperature.

The stress exponent values (n) were continuously increasing from 3 to 28 with progressive deformation indicating a transition from power to exponential flow law (power Law Breaking). Depending on the melt and solid proportions in the system the speculated four stages of percolation thresholds during partial melting and crystallization of lithosphere (Vigneresse et al., 1996) were established by the experimental data.