HIGH-GRADE METAMORPHISM IN THE WESTERN CAPE-BRETON HIGHLANDS, NOVA-SCOTIA, AND ITS RELATION TO TECTONISM

Geothermobarometry and detailed mapping in the western Cape Breton Hig hlands of Nova Scotia indicate that upper-amphibolite-facies (similar to 700 degrees C, >8 kilobars) supracrustal and plutonic rocks structu rally overlie Ordovician-Silurian supracrustal sequences that exhibit inverted metamorphic isograds. Maximum T-P values (similar to 640 degr ees C, 6.4 kilobars) in the latter occur at the boundary between the t wo suites. Geothermobarometry based upon garnet - biotite - amphibole - epidote (+/- clinopyroxene) assemblages in the overlying suite and g arnet - biotite - muscovite (rutile +/- ilmenite +/- staurolite) assem blages in the Ordovician-Silurian suite shows an abrupt increase in ca lculated P and T across the shear zone separating the two suites. The overlying rocks thus originated at considerably deeper levels in the c rust than the underlying supracrustal suite. It is thus unlikely that the two suites are correlative, as previously supposed. Extensive earl y to middle Devonian plutonism and uplift accompanied and followed the peak of metamorphism. Devonian greenschist-facies (similar to 400 deg rees C, 2.3 kilobars) shear zones give pressures similar to those of a ndalusite-cordierite hornfels aureoles around the late plutons. The ob served metamorphism can be modeled by late Silurian thrust emplacement of a thick (20 km), hot allochthon. In this model, similar to 14 kilo meters of denudation preceded development of the low-angle, extensiona l Margaree Shear Zone, along which a further 6 kilometers of cover was removed prior to Carboniferous sedimentation.