COMPARISONS BETWEEN SOUTHERN QUEBEC AND ALBANIAN OOPHIOLITES : IMPLICATIONS FOR THE DEPOSITIONAL AND TECTONIC EVOLUTION OF IAPETAN OCEANIC CRUST

The Southern Quebec ophiolites, mainly made up of the Thetford-Mines, Asbestos and Mont-Orford complexes, was folded and faulted twice after its initial formation and subsequent obduction onto the Laurentian continental margin. The original relationships between the ophiolitic mantle, the overlying plutonic section, and onlapping Ordovician siliciclastic rocks are, therefore, frequently obscured by various generations of tectonic fabrics and structures. Comparisons with modern oceanic environments and well-preserved ophiolites in younger orogens, such as the Albanian Dinarides, can thus be used to better understand the stratigraphy of the polydeformed Appalachian ophiolites. The Jurassic Mirdita ophiolite in Albania extends for ca. 4000 km2 and was slightly affected by the Alpine orogeny of Eastern Europe during the Tertiary. Well-exposed sections of the ophiolite indicate that the oceanic mantle has been exhumed before obduction onto Apulia, and likely records the formation of oceanic core complexes as those documented in modern oceans. Mafic cumulates of the Mirdita ophiolite has been locally totally excised, and exhumation-related shear zones are preserved both within the ophiolitic mantle and at the interface of overlying basalts. The ophiolitic sedimentary sequence conformably overlies either basalts, sheeted dykes, or gabbros, and typically consists of debris flows intercalated and overlain by slumped oceanic sedimentary rocks. These characteristics of the ophiolite are consistent with active vertical tectonism, crustal excision, and mantle exhumation during oceanic crust formation and deposition of overlying sedimentary rocks. The Southern Quebec ophiolites shows similarities with lithological and structural features of the Mirdita ophiolite. Conformable contacts between the sedimentary cover sequence (debris flows and flysches) and both the mantle and crustal sections are locally exposed. In contrasts to Albanian ophiolites, metamorphic rocks fragments are locally abundant in ophiolitic debris flows of southern Quebec. This suggests that, although the ophiolites have experienced syn-oceanic exhumation, the uplift and erosion of both the oceanic crust and underlying continental margin metamorphic rocks continued during emplacement onto the Laurentian margin.