STRATIGRAPHIC AND GEOCHEMICAL CONSTRAINTS ON THE RELATIVE AGE OF THE MARGAREE SHEAR ZONE IN WESTERN CAPE-BRETON ISLAND, WITH IMPLICATIONS FOR THE EARLY EVOLUTION OF THE MARITIMES BASIN

The Margaree Shear Zone is interpreted to be a thick, brittle-ductile low-angle extensional fault which was active in the Mid to Late Devoni an at the time of initiation of the Maritimes Basin. In western Cape B reton Island, the fault separates two distinct basalt assemblages: low -grade to non-metamorphosed intra-continental basalts of the Upper Dev onian Fisset Brook Formation occur in the hangingwall, whereas more pr imitive Ordovician-Silurian basalts, which were metamorphosed in Early Devonian time, to upper greenschist grade occur in the footwall. Majo r and trace element geochemistry from mylonitic basalt near the top of the shear zone demonstrate that the Fisset Brook Formation has been a ffected by the shearing. In particular the mylonites of Fisset Brook a ffinity are characterized by higher Ti, Zr, P, Y, and Ga contents, and lower Ni, Cr, and Mg values compared to the older basalts. Elemental plots are represented as ratios to eliminate enrichment or dilution fa ctors caused by mobile elements such as Si, Ca, K, and Na. The age of the shear zone is further constrained in the region by an unconformity , which displays Tournaisian conglomerate of the Horton Group resting directly on the flat-lying mylonite, the former composed of clasts inc luding mylonite as well as non-deformed vesicular basalt of the Fisset Brook Formation.