ORDOVICIAN OPHICALCITES OF SOUTHERN QUEBEC APPALACHIANS - A PROPOSED EARLY SEA-FLOOR TECTONOSEDIMENTARY AND HYDROTHERMAL ORIGIN

The Riviere des Plantes ophiolitic Melange (Middle Ordovician) in southern Quebec contains carbonatized ophiolites known as ophicalcites. Pervasive carbonatization of ultramafic material followed serpentinization and shearing. Carbonates are also found as discrete void-filling phases in the ophicalcites. The first phase to develop is a laminated and graded micrite (delta(18)O, -5.3%; delta(13)C, -0.1%) that is dull under cathodoluminescence. Major fracturing followed and was synchronous with calcite cementation. Initial isopachous crusts of nonluminescent palisade calcite (delta(18)O, -13.0%; delta(13)C, 0.8%) are followed by blocky, bright(delta(18)O, -14.4%; delta(13)C, -0.6%) and then dull (delta(18)O, -13.1%; delta(13)C, -0.9%) luminescent calcites. The isotopic values of the micrite fall in the range of recently published values for Early to Middle Ordovician seawater (delta(18)O, -4% to -6.5%; delta(13)C, 0% to -1.5%). Crosscutting relationships between internal sediments, cements, and clasts of cemented ophicalcite breccias provide evidence for complex early seafloor fracturing, cementation, and fluid circulation. Seawater-driven serpentinization of ultramafics supplied the Ca+2 needed for extensive calcite cementation. The close to normal marine delta(13)C values for cements (-1.4% to + 1.0%) suggest significant involvement of marine waters with some volcanic-derived CO2. The oxygen isotopic composition of the carbonate cements argues for precipitation from heated seawater; temperature of precipitation probably reached a maximum of 80 degrees C. We propose a seafloor hydrothermal vent system to explain the synchroneity of micrite sedimentation, fracturing and cementation.