COUPLED MICROPROCESSES AND RHEOLOGICAL EVOLUTION IN A REACTIVE SHEAR ZONE

Textural and petrologic data gathered across the sheared margin of the Lincoln Syenite in south-central Maine, USA are used here to evaluate processes involved in syn-metamorphic deformation. Microstructural interpretations and petrochemical data suggest that heterogeneous plastic strain and fluid infiltration enhanced metamorphic reactions and determined the degree to which textural and chemical reorganization were achieved. The effect of fluids on deformation mechanics and metamorphic reactions are of primary importance in understanding rheological changes in crustal rocks during deformation, and are the focus of this study.

The transition from a coarse-grained, granular, igneous rock (Cpx+Opx+Ksp+Bt) to a recrystallized and strongly foliated rock (Act+Ksp+Bt+Qz) is preserved both texturally and chemically across domains. Systematic variations in biotite composition track the progress of the bulk reaction, and show highest OH:(F+Cl) ratios in the most strongly foliated domain where recrystallization is complete. Thus, it is likely that the degree of recrystallization is a function of hydrous-fluid access which enhanced diffusion of chemical components along grain boundaries.

The fine-grained, interconnected product phases, biotite, actinolite, and quartz within the developing matrix foliation comprise weakened domains between feldspar megacrysts along which strain may be localized. Numerical modeling and experimental studies indicate that these types of processes may cause partitioning of higher strain-rates into biotite-rich, foliated zones, and result in a drastic weakening of the bulk rock. Thus, we suggest that the bulk effective viscosity of the Lincoln Syenite was decreased along its southeastern margin through strain-enhanced reactions and coalescence of elongate biotite and actinolite grains and grain aggregates.