Deformation mechanisms and rheology inversion of jointly deformed marble and quartzite in natural thermal gradient

Amphibolite facies marble surrounded by locally pure but mostly impure quartzite with -10-14% of dispersed white mica and crosscut by granite veins were jointly deformed within an extensional shear zone. Both marble and quartzite show three microstructural stages, M1m-M3m and M1q-M3qm, respectively, correlated with the onset of extension Stage 1 at -600 degrees C, and retrograde extension Stage 2 at -380 degrees C and Stage 3 at -300 degrees C. The coarse-grained microstructures M1m (1200 & mu;m) in marble and M1q (300 & mu;m) in pure quartzite showed the activity of basal (a) slip and prism (a) - rhomb (a) slip, respectively. The finer grained M2m (-50 & mu;m) microstructure in marble is associated with strain localization, subgrain rotation recrystallization and persistence of basal (a) slip. The M2q (175 & mu;m) microstructure in pure quartzite shows a crossed girdle c-axis pattern, while the M2qm (120 & mu;m) in impure quartzite shows locally c-axes parallel to the lineation, indicating the possible activity of prism [c] slip. Stage 3 in marble is characterised by minor recrystallization M3m that occurs mostly along microcracks formed subparallel to foliation. The impure quartzite M3qm microstructure shows further reduction in quartz grain size (80-90 & mu;m) related to pining of well-dispersed white mica. The microstructure M1m-M1q and M2m-M2q formed by dislocation creep, while the M2qm and M3qm suggest the Rachinger grain boundary sliding of quartz grains along micas. Piezometric calculations suggest differential stresses of 1-5 MPa and -20 MPa for the M1m and M2m microstructures and 7 MPa and 11 MPa for the M1q and M2q microstructures, respectively. Comparison of these data with experimental flow laws confirmed that during Stage 1 marble is weaker than quartzite. However, during Stages 2 and 3, mica in impure quartzite became more dispersed but also locally aggregated; thus, the bulk strength of quartzite became governed by mixture of quartz and weak mica behaviour. This change in microstructure and the resulting deformation mechanism facilitated fluid transport across quartzite, which resulted in embrittlement of dominantly coarse-grained and impermeable competent marble. This process is accompanied by the formation of stretching faults developed parallel to weak layers of altered granite dykes in the vicinity of the marble-quartzite contact. The rheology inversion thus results from a mixture of deformation mechanisms in the weak polyphase quartzite and fluid-induced drop of effective pressure of strong and brittle marble at late stages of the extensional deformation.