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Orientation-Dependent "Metamorphic Grade" in Phyllosilicates Belonging to a Slaty Cleavage Fabric

The intimate coexistence of two microstructurally defined phyllosilicate populations that have apparently achieved two different degrees of diagenetic/metamorphic maturity underscores the importance of chemical processes in slaty cleavage development. Petrographic observations and analytical data on phyllites from Ocoee Gorge, Tennessee, indicate that most of the cleavage-parallel phyllosilicates concentrated in thin lamellae grew during low-grade metamorphism. On the other hand, the bedding-plane parallel phyllosilicates found between lamellae are detrital or diagenetically altered grains that were in place before slaty cleavage developed. Electron microprobe and XRD analyses have been used to characterize the mica, interstratified mica/chlorite, and chlorite found in the two microstructurally defined populations. Cleavage-parallel phyllosilicates are homogeneous and have crystal chemical characteristics indicative of low-grade metamorphism. Bedding-parallel phyllosilicates, on the other hand, are more heterogeneous and appear to be detrital or diagenetic. Specific differences in the two groups include greater K content of cleavage-parallel mica and the greater chemical variability of bedding-parallel mica. Cleavage-parallel interstratified mica/chlorite is limited to a maximum of about 50% chlorite, whereas there is no limit on chlorite in bedding-parallel interstratified grains. In addition, the proportion of mica in the total phyllosilicates is higher parallel to cleavage than parallel to bedding. The differences between the cleavage-parallel and bedding-parallel phyllosilicates suggest that new grain growth and recrystallization of pre-existing grains were more important than passive concentration and rotation in the development of this fabric.