CHANGES IN STRUCTURE AND COMPOSITION OF SERPENTINITES RELATED TO THE DEVELOPMENT OF DEFORMATION MICROSTRUCTURES IN FAULT ROCKS FROM THE BASE OF THE BAY OF ISLANDS COMPLEX, NEWFOUNDLAND

Fault rocks at different positions along WNW-ESE transects along thrust fault beneath the Bay of Islands ophiolite complex exhibit different characteristics. To the WNW, footwall sedimentary rocks are unmetamorphosed to weakly metamorphosed, and fault rocks exhibit little evidence of syntectonic recrystallization. To the ESE, both the fault rocks and the footwall rocks are fully recrystallized metamorphic tectonites. Near the center of WNW-ESE transects, serpentinized peridotites above the thrust are juxtaposed against weakly- to moderately-metamorphosed sedimentary rocks. Serpentinites in those locations exhibit a distinctive sequence of structures and optical microstructures as they accrue deformation. Undeformed to weakly deformed serpentinized peridotites, typically found farther from the footwall contact, exhibit pseudomorphic textures – mesh texture serpentine after olivine and bastite after pyroxene. Macroscopic veins with cross-fiber to parallel-fiber serpentine are common in these rocks. Where rocks are more deformed, typically closer to the contact with footwall rocks, the pseudomorphic textures are initially distorted and then obliterated due to recrystallization of serpentine. In nearly fully recrystallized serpentinites along the contact with the footwall rocks, angular remnants of deformed but less recrystallized serpentinite sit in a matrix of fine-grained, recrystallized serpentine that exhibits a strong dimensional preferred orientation. The angular remnants resemble angular wall rock fragments in cataclasites whereas the foliated matrix resembles the fine grained matrix in mylonites.

We have augmented optical microscopy with SEM analyses and X-ray diffraction studies. Pseudomorphic serpentine typically is lizardite with a chemical composition close to ideal serpentine. Chrysotile occurs in the veins that cut undeformed to weakly deformed serpentinized peridotite, and it sometimes contains small to significant amounts of aluminum. Recrystallized serpentine typically is lizardite, and it is often aluminous. The mineralogic changes and changes in chemical composition of serpentine provide confirmation that microstructural changes accompanied deformation.