MAFIC AND ULTRAMAFIC AMPHIBOLITES FROM THE NORTHWESTERN PONTIAC SUBPROVINCE - CHEMICAL CHARACTERIZATION AND IMPLICATIONS FOR TECTONIC SETTING

In the northwestern Pontiac Subprovince, metavolcanic rocks are expose d within a metagraywacke sequence that is intruded by metamorphosed ma fic dykes. The metavolcanics are Al-undepleted komatiites ([La/SM]N = 0.3, [Tb/Yb]N = 0.9) and tholeiitic Fe-basalts ([La/SM]N = 0.8 and [Tb /Yb]N = 0.8). The nearly flat chondrite-normalized distributions of hi gh field strength elements (HFSE), Ti and P, the constant Zr/Y, Nb/Th, Ti/Zr, and Ti/P ratios, and the lack of depletion of HFSE relative to rare-earth elements (REE) in both ultramafic and mafic metavolcanics, imply that crustal assimilation and magma mixing with crustal melts w ere not significant during differentiation and argue against the prese nce of subduction-related magmatic components. Contemporaneous volcani sm and sedimentation in the northwestern Pontiac Subprovince are unlik ely. The metavolcanics do not show any evidence of crustal contaminati on and likely represent a structurally emplaced, disrupted assemblage, chemically similar to early volcanics of the adjacent southern Abitib i Subprovince. Metamorphosed mafic dykes intruding the metagraywackes are not genetically related to the metavolcanics. The dykes have high CaO, P2O5, K2O, Ba, Rb, and Sr, intermediate Cr and Ni contents, and s trongly fractionated REE patterns ([La/Yb]N = 10.8). Normalized to the primitive mantle, they display pronounced negative Nb, Ta, Ti, Zr, an d Hf anomalies. These amphibolites are metamorphosed equivalents of Mg -rich calc-alkaline lamprophyre dykes, most likely derived from a hybr idized mantle source. Mantle metasomatism was probably related to a su bduction event prior to the peak of compressional Kenoran deformation in the Pontiac Subprovince.