Mafic and ultramafic amphibolites from the northwestern Pontiac Subprovince: chemical characterization and implications for tectonic setting

Abstract

In the northwestern Pontiac Subprovince, metavolcanic rocks are exposed within a metagraywacke sequence that is intruded by metamorphosed mafic 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 high 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 were not significant during differentiation and argue against the presence of subduction-related magmatic components. Contemporaneous volcanism and sedimentation in the northwestern Pontiac Subprovince are unlikely. The metavolcanics do not show any evidence of crustal contamination and likely represent a structurally emplaced, disrupted assemblage, chemically similar to early volcanics of the adjacent southern Abitibi 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 strongly fractionated REE patterns ([La/Yb]N = 10.8). Normalized to the primitive mantle, they display pronounced negative Nb, Ta, Ti, Zr, and Hf anomalies. These amphibolites are metamorphosed equivalents of Mg-rich calc-alkaline lamprophyre dykes, most likely derived from a hybridized mantle source. Mantle metasomatism was probably related to a subduction event prior to the peak of compressional Kenoran deformation in the Pontiac Subprovince.