FLUID-ROCK REACTIONS AND RESULTING CHANGE IN RHEOLOGICAL BEHAVIOR OF A COMPOSITE GRANITOID - THE ARCHEAN MOOSHLA STOCK, CANADA

The composite Mooshla stock displays clear evidence of variations in s tyle and intensity of strain that are closely related to its internal lithological heterogeneity. Gabbro-diorite, quartz diorite, and tonali te rocks are weakly foliated and characterized by brittle and brittle- ductile small-scale shear zones, whereas leucotonalitic rocks are stro ngly foliated and transected by numerous wide and extensive ductile sh ear zones. Increasing degrees of penetrative deformation and marked ch anges of strain style in the pluton, from the more mafic rocks to the more felsic ones, are interpreted to reflect metamorphism-related rheo logical contrasts, rather than differences in the physical conditions of deformation. Metamorphism of the stock is characterized by an inten sive hydration of the igneous rocks that has greatly enhanced their or iginal heterogeneities. Petrographic, microstructural, and chemical st udies show that the least deformed rocks are characterized by abundant albite-oligoclase (65-80%) with a matrix of minor quartz (5-10%) and actinolitic amphibole. The resistant plagioclase laths, although alter ed and replaced, form a stress-supporting framework that has protected the interstitial weak minerals, such as quartz, chlorite, and biotite , from deformation. However, the least deformed leucotonalites are cha racterized by low albite (35-45%) and high quartz contents (up to 65%) . Extensive metamorphic hydration of these rocks produces quartz and p hyllitic minerals that had enhanced significantly the ductility of the leucotonalites. Characterization of the chemical changes and the ther mochemical conditions of the fluid, using microstructure and measureme nt of stable isotopes, indicates that fluid-rock interactions during m etamorphism and syntectonic hydrothermal alteration have played an imp ortant role in creating the contrasting deformation of the composite g ranitoid.