Characterization of Calcite Amygdules in Devonian Basalts, in the McArras Brook Formation, Nova Scotia

Abstract

Along the North Shore of Nova Scotia, Canada vesicular basalts in the McArras Brook Formation are host to carbonate amygdules and veins. These basalts are interbedded within a well characterized Devonian strata but have not been directly dated. Through previous studies with U-Pb dating, the ages for calcite amygdules have been shown to form shortly after the eruption of the host basaltic lava. Secondary processes lead to the formation of overgrowths on existing calcite amygdules and low-T veins. These may form during tectonic emplacement and thus, multiple generations of calcite can also be found in the same suite of basaltic rocks. Before the U-Pb ages of both volcanism (amygdule formation) and later tectonic activity (secondary calcite) can be accurately measured and used as a proxy for the age of the basalt flows, the growth history of the calcite amygdules and viens must be carefully characterized. In this study we have used basic textural relationships and UV fluorescence to examine calcite growth history, both in the field and in collected samples. In-situ UV-fluorescence shows the growth of several generations of calcite amygdules and veins in the basalts from McArras Brook. We will use the UV characterization to help target analysis of different generations of calcite using a scanning electron microscope (SEM) to analyse the relationships between the fluorescence and the major elements. The results are expected to identify activator elements in the calcite which correspond to the differences in fluorescence colour, and correlate element compositions to the growth history. This will allow us to target the best areas for subsequent U-Pb dating of calcite to determine the absolute (volcanic) ages of these basalts, and the ages of secondary processes.