The interaction between chemical and mechanical processes during metamorphism: a microstructural and petrologic study of amphibolite shear zones, Cheyenne Belt, Southeastern Wyoming

Shear zones which deform margins of amphibolite boudins in the Cheyenne Belt, SE Wyoming, record a full strain transition from relatively undeformed amphibolite which has relict igneous textures to mylonitic amphibolite with a strongly developed L-S tectonic fabric. The strain transition is marked by the rotation of amphibole and plagioclase aggregates into parallelism with the shear zone boundary and progressive grain size reduction. These observations indicate that strain magnitude increases across the shear zone. Detailed petrologic and microstructural analysis of a single amphibolite shear zone has been conducted in order to: 1) document the petrologic and microstructural evolution of the shear zone and 2) investigate the interrelationships between mechanical and chemical processes associated with shear zone formation.

Amphibolites throughout the shear zone consist of amphibole + plagioclase with only minor amounts of quartz + chlorite + epidote + sphene + ilmenite. Within the relatively undeformed amphibolite, amphibole and, to a lesser extent, plagioclase has wide compositional variation. Amphibole compositions vary from actinolitic hornblende to magnesio-hornblende which involves increases in Al, Fe, Na and K contents and decreases in Si and Mg. Plagioclase compositions vary from Angp in cores of plagioclase grains to Anjo within grain boundary domains. With increasing strain magnitude across the shear zone variation of amphibole composition decreases and become predominantly magnesio-hornblende. Plagioclase compositions also decrease in range although grain boundary domains still have higher albite content. The observed variation of amphibole compositions indicate that shear zone formation occurred during prograde metamorphism although compositional changes may also be a function of changing grain boundary fluid composition.

These petrologic data indicate that shear zone metamorphism was in part controlled by the magnitude of strain during deformation. Scanning electron microscope back-scattered images and color enhanced X-ray compositional maps indicate that compositional variation in plagioclase and amphibole occurs along margins of highly angular grains of various sizes. These textural observations have been interpreted to indicate that chemical reactions occurred by a dissolution and reprecipitation processes following or during cataclastic deformation. Transmission electron microscope (TEM) images show local zones of high dislocation density adjacent to microcracks suggesting that work hardening may have been an important processes during cataclasis. Alternatively, microcracks may have acted as source for development of dislocations.

The importance of deformation in assisting shear zone chemical processes is evidenced by: 1) the observation of new mineral overgrowth along grain boundaries and 2) TEM images of amphibole which show that actinolitic hornblende has a high defect density whereas magnesio-hornblende overgrowths are relatively defect free. This observation suggests that strain energy associated with dislocations may have contributed to the chemical process. Thermodynamic modelling of reaction progress within the shear zone using the Gibbs Method indicates that observed modal and compositional changes can occur isothermally if strain energy is added to the system. Increases in reaction progress with deformation may have also been due to increases in fluid infiltration or diffusion due to grain size reduction. The general conclusion of this study is that in order to apply petrologic, geochemical and isotopic data to understanding geochemical and tectonic processes, microstructural information on the magnitude of strain and the type of deformation mechanism must be evaluated, quantitatively if possible.