| Summary: | New wholerock lithogeochemical results for eleven Cambrian mafic rocks from western Avalon Peninsula (Newfoundland), including seven pillow basalt lavas, three mafic tuffs and one gabbro, are compared to available data on Cambrian mafic rocks from a previous study in the same area. The basalts are interbedded, and, locally, in apparent tectonic contact, with black shale and minor carbonate layers and lenses of the Manuels River Formation (Harcourt Group) at Chapel Arm, southern Trinity Bay. At Placentia Junction, 15 km to the south, mafic tuff is in fault contact with darkgrey to black shale probably belonging to the Miaolingian Chamberlain¿s Brook¿Manuels River transition. Contact relationships of the gabbro have not been observed, but it either crosscuts, or is in fault contact with, red shales of the Terreneuvian to Cambrian Series 2, Adeyton Group. Previously documented fossil assemblages of the Manuels River Formation place the basalts in the biostratigraphic Paradoxides davidus Zone of the Drumian Stage (absolute age of 504.5¿500.5 Ma). All of these Cambrian mafic rocks have moderate Mg#¿s, Zr/Ti ratios, and Ni and Cr concentrations indicating they are primitive magmas that experienced limited differentiation prior to emplacement. They are light rareearthelementenriched, alkaline, OIBlike basalts, having high Nb concentrations and high Nb/Yb and Ti/Y ratios. They likely formed as lowdegree partial melts from a garnet lherzolite mantle source, as indicated by their high TiO2/Yb, (Sm/Yb)MN and (Tb/Yb)CN ratios. The Cambrian basalts occur in a narrow northtrending linear (half)graben, parallel to a northtrending, possibly Acadian thrust fault. The OIBlike chemical affinity of the magmas, the elongated orientation of the sedimentary basin in which they occur, and the presence of an adjacent (half)grabenparallel fault are consistent with their eruption under extensional conditions, possibly along preexisting, Neoproterozoic structures Chris Finch and staff at the Geological Survey’s geochemical ...
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