Alkalic to transitional ferrogabbro magma associated with paleohelikian anorthositic plutons in the Flowers River area, southeastern Nain igneous complex, Labrador

Abstract

Fine grained gabbroic chilled margins and crosscutting dikes are associated in space and time with three ca. 1400 Ma anorthositic plutons in the Flowers River area, southeastern Nain igneous complex. Both the anorthositic and gabbroic rocks have distinctive compositions compared to rocks of similar age and lithology elsewhere in northcentral Labrador. The anorthositic rocks contain olivine and augite rather than orthopyroxene, and Fe-Ti oxides, apatite and orthoclase are unusually abundant. Cumulus plagioclase is abnormally enriched in incompatible elements. Most of the gabbroic rocks are uniform in composition, although the effects of contamination and fractionation are evident in some places. They define a transitional to alkalic ferrogabbro magma that is strongly enriched in K, P and incompatible trace elements. The chemical characteristics of the ferrogabbro magma imply derivation from enriched mantle or involvement of a significant crustal component. A parent-daughter relationship between the ferrogabbro magma and anorthositic rocks is suggested by their compositional similarities and the fact that the gabbroic chilled margins and plagioclase-rich pluton interiors appear to be completely gradational in composition and texture.