THE SUB-OPHIOLITIC METAMORPHIC ROCKS OF THE QUEBEC APPALACHIANS

Amphibolitic metamorphic rocks are associated with the Thetford, Asbestos and Orford ophiolites as well as the Mont Albert and Pennington Sheet peridotites of the Quebec Appalachians. An augmented compilation of the existing data on their field relations, mineralogy, geochronology, structural features and geothermobarometry is presented in order to help reconstruct the timing and processes of marginal basin closure and ophiolite emplacement during the Ordovician. A new, refined Ar-40/Ar-39 incremental release spectrum and isochron age is presented for the Thetford Mines ophiolite dynamothermal sole. The new 477 +/- 5 Ma age resolves the existing disparity between the crustal (plagiogranite) and sole ages. The sole was formed shortly after crustal formation, suggesting that the ophiolite was decoupled at or near a spreading centre. A hitherto undescribed ultramafic-mafic amphibolitic sole beneath the Asbestos ophiolite was decoupled and rotated during the continental emplacement of the overlying ophiolite. Dating of the sole at Asbestos was hampered by presence of low K2O amphiboles, but an Acadian (377 Ma) age was obtained from orthoclase. The Orford sole was dismembered and incorporated within a serpentinitic melange that contains other ophiolitic lithologies. Sheared amphibolites from alongside the Pennington Sheet in the Flintkote Mine are reinterpreted as a dynamothermal sole, rather than a metasomatically generated amphibole-bearing metasediment.