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

The Riviere des Plantes ophiolitic Melange (Middle Ordovician) in sout hern Quebec contains carbonatized ophiolites known as ophicalcites. Pe rvasive carbonatization of ultramafic material followed serpentinizati on and shearing. Carbonates are also found as discrete void-filling ph ases in the ophicalcites. The first phase to develop is a laminated an d graded micrite (delta(18)O, -5.3%; delta(13)C, -0.1%) that is dull u nder cathodoluminescence. Major fracturing followed and was synchronou s with calcite cementation. Initial isopachous crusts of nonluminescen t 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 dul l (delta(18)O, -13.1%; delta(13)C, -0.9%) luminescent calcites. The is otopic values of the micrite fall in the range of recently published v alues for Early to Middle Ordovician seawater (delta(18)O, -4% to -6.5 %; delta(13)C, 0% to -1.5%). Crosscutting relationships between intern al sediments, cements, and clasts of cemented ophicalcite breccias pro vide evidence for complex early seafloor fracturing, cementation, and fluid circulation. Seawater-driven serpentinization of ultramafics sup plied 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 CO 2. The oxygen isotopic composition of the carbonate cements argues for precipitation from heated seawater; temperature of precipitation prob ably reached a maximum of 80 degrees C. We propose a seafloor hydrothe rmal vent system to explain the synchroneity of micrite sedimentation, fracturing and cementation.