Microstructural and petrological data suggest that a single episode of syn- to post-tectonic metamorphism affected the boundary region between the Clements Markham fold belt and Pearya, a postulated Caledonian terrane, during a mid-Paleozoic orogenic event in northern Ellesmere Island. The sedimentary rocks of the Clements Markham fold belt pass from chlorite to biotite to garnet grade over a distance of about 10 km as the contact with the Mitchell Point belt gneisses of Pearya is approached from the south. Foliation development and chevron-style folding was followed by the growth of the index minerals chlorite, biotite, chloritoid, garnet, staurolite, and kyanite in semipelitic rocks in four metamorphic zones. Thermobarometry of garnet porphyroblasts indicates peak metamorphic conditions of about 600 °C and 600 MPa in the highest grade rocks. Chloritoid-involving phase relations define an invariant point at 540 °C and 500 MPa only 2 km away from the highest grade zone. It may be concluded from the calculated pressure and temperature differences over this short distance that the isogradic surfaces of the post-chevron-folding metamorphism are steeply oriented. Much of the observed metamorphic pattern can, therefore, be explained as the result of a significant post-chevron-folding differential uplift (overthrusting) of the hot Mitchell Point belt gneisses relative to the Clements Markham fold belt. This indicates that the Mitchell Point belt forms a thrust sheet which overlies the Clements Markham fold belt and that the accretion of Pearya predates the Late Silurian.

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