A Study of the Rare Earth Element Geochemistry and Mineral Chemistry of the Anorthosites and Related Rocks near Pleasant Bay, Cape Breton Island, Nova Scotia

Abstract

The anorthosites and related rocks of this study are located about 5 kilometres east of the village of Pleasant Bay, Cape Breton Island, Nova Scotia. The anorthosites outcrop as a thin (~900m), northeast-trending lens with smaller lenses also found outside the main body. The country rock consists mainly of gneiss, amphibolite, schist, gabbro and granitic intrusions with some diabase and aplite dykes cutting both the anorthosite and the country rock. The contact between the anorthosite and the country rock is not well known but there are some fault contacts noted and a mylonite zone is present on the southeast boundary of the main anorthosite lens.

The current study examines three main facets of these rocks. A study of the composition of the plagioclase in the anorthite was made to determine if any regular variation in the anorthosite content in the plagioclase occurred. It has been found that the anorthite content does change but in a irregular manner, perhaps due to localized metamorphic effects of shearing of an originally heterogeneous anorthosite body.

Associated with the anorthosite are gneissic rocks containing coexisting Ca-rich and Ca- poor pyroxenes and their compositions have been used as a geothermometer. The equilibration temperature for the pyroxenes is calculated to be between 870-930oC, in the pyroxene-granulite subfacies of regional metamorphism. This high grade of metamorphism has not previously been reported in this area.

The rare earth element (REE) content of the anorthosite was also examined in an attempt to calculate the nature of the parent magma. The anorthosite has a very low total REE content with a positive Eu anomaly. The calculated parent magma has a relatively enriched and less fractionated REE content. A comparison of the chondrite-normalized REE pattern for the calculated parent magma was made with the patterns developed by tholeiites, andesites and alkali basalts. The results of this comparison seem to indicate that the original parent magma of the anorthosite may have had a tholeiitic composition.

Textural features in the anorthosite, such as granulation of crystals, faulted contacts with the country rock and irregularly dispersed plagioclase compositions, and an associated mylonite zone all seem to indicate that the anorthosite was tectonically emplaced into its present position. Since the anorthosite is associated with a granulite gneiss, it was probably emplaced from depth as most of country rock around the anorthosite has only been metamorphosed to greenschist- amphibolite facies.

Keywords: Pages: Supervisors: D. B. Clarke