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