Petrology and geochemistry of the Nimish Formation, western Labrador, Newfoundland.

The Nimish Formation comprises an accumulation of Early Proterozoic (1879 Ma) volcanic, intrusive, and volcaniclastic rocks which occur in the second cycle supracrustal succession of the Knob Lake Group, New Quebec Orogen. The volcanic and intrusive suites are predominantly composed of basalt and gabbro with minor intermediate and felsic differentiates. The latter suite also includes previously unrecognized sill-like bodies of peridotite and pyroxenite. The basalts and gabbros exhibit a lower greenschist mineral assemblage with clinopyroxene as the only remaining primary phase. Thin lenses of volcaniclastic strata occur in many parts of the study area ranging from mafic tuff and agglomerate, to volcanic and jasper clast-rich conglomerates. The flows, sills, and volcaniclastic rocks are coeval with Lake Superior-type iron formation (Sokoman Formation) and occur in allochthons of a foreland fold and thrust belt that has been transported westwardly over the Archean Superior Craton. The Nimish volcanic suite ranges from alkali basalt to trachyte in composition and has alkaline to transitional geochemical affinities based on major and trace element abundances. Further, the relatively evolved chemistry of the basalts indicates that they are not the direct products of primary melts. The gabbros also have alkaline affinities as shown by their high field strength element abundances and clinopyroxene mineral chemistry. The overall similarities in elemental abundances, geochemical affinities, and REE spectra between the basalts and gabbros indicate that the suites are comagmatic. In contrast, the petrology and primitive geochemistry of the peridotites and pyroxenites reveals that they are cumulates representative of crystallization processes by which the primitive Nimish alkaline magma became differentiated prior to the formation of the basalts and gabbros. The strong incompatible trace element enrichment patterns of the Nimish basalts demonstrate that the basalts are similar to other alkaline volcanic rocks from oceanic and continental settings. As such, they indicated derivation from a relatively undepleted or enriched mantle source. (Abstract shortened by UMI.