LE3 .A278 2023

2023

Barr, Sandra

Acadia University

Bachelor of Science

Honours

Geology

Earth & Environmental Sciences

The Eastern Shore dykes are a swarm of mantle-derived spessartite (lamprophye) dykes in the Sheet Harbour area of eastern Nova Scotia. Two of the dykes, Pleasant Harbour-Borgles Island and Popes Harbour, contain abundant xenoliths and xenocrysts. Some of the xenoliths are granulite-facies metamorphic rocks that in previous studies were shown to contain high-pressure mineral assemblages and hence were interpreted to represent deeper crust of the Meguma terrane. This study was undertaken to investigate the xenocryst minerals in the dykes to determine their composition and provenance. Xenocrysts were studied in two samples from the Borgles Island - Pleasant Harbour dyke that were selected because they contain abundant and varied xenocrysts. About twenty thin section-sized chips were cut from those samples, and 14 chips with abundant xenocrysts were studied in polished sections and analyzed for major and trace elements by electron microprobe and LA-ICP-MS. Based on the petrologic evidence, the xenocrysts are divided into three groups: (1) silicate xenocrysts including amphibole, garnet, clinopyroxene, quartz, K-feldspar and kyanite; (2) unidentifiable altered xenocrysts completed replaced by chlorite and other secondary minerals; and (3) sulphide xenocrysts that consist of pyrite with small inclusions of chalcopyrite. Most silicate xenocrysts were likely derived from disaggregation of xenoliths in which similar minerals have been reported and which have textual and compositional similarities to the xenocrysts. Thermobarometric calculations based on clinopyroxene and amphibole xenocryst compositions indicate that they were derived from mid-crustal level (ca. 25 km) whereas the dykes were emplaced at depths of 11-14 km. Because of their large size the quartz xenocrysts may represent pieces of vein quartz. The origin of the pyrite xenocrysts remains uncertain, although their trace element compositions suggest that they are of low temperature hydrothermal origin.

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https://scholar.acadiau.ca/islandora/object/theses:4037