LE3 .A278 2010

2010

Barr, Sandra

Acadia University

Bachelor of Science

Honours

Geology

Earth & Environmental Sciences

Mafic dykes occur throughout ca. 560 Ma granitoid plutons and their host rocks in the Boisdale Hills of south- central Cape Breton Island. Based on this study of the petrology and chemistry of 28 samples from throughout the area, the dykes are divided into four types: ( 1) clinopyroxene- bearing, accompanied by plagioclase and in some cases embayed quartz; ( 2) amphibole- bearing, accompanied by plagioclase and small amounts of relict pyroxene and quartz; ( 3) plagioclase- rich, typically with sericitized plagioclase phenocrysts and amygdales filled with secondary chlorite and calcite; ( 4) intensely altered ( sericitized) dykes, likely related to the plagioclase- rich dykes. All four types contain secondary minerals including both Fe- and Mg- rich chlorite, epidote, calcite, sericite and saussurite. Mineral analyses by electron microprobe in types 1- 3 yielded plagioclase compositions in the range An13- 45 and showed that the pyroxene is diopside in type 1 and augite in type 2 with about 2% TiO2. In amphibole- bearing dykes, the amphibole includes both magnesio- hornblende and secondary actinolite. Whole- rock chemical analyses show some scatter, especially in mobile elements as a result of alteration, but most data suggest tholeiitic affinity and emplacement in a within- plate tectonic setting. The dykes are petrographically and chemically similar to Middle Cambrian volcanic rocks of the adjacent fault- bounded Bourinot belt, and were likely related to that volcanic episode. Hence the Bourinot belt is not allochthonous relative to the Bras d’Or terrane as has been previously suggested. The dykes show less similarity to mafic dykes in ca. 620 Ma volcanic and plutonic rocks of the Sporting Mountain area in the Mira terrane, which have more arc- like characteristics. However, they are similar to Devonian mafic dykes of the Creignish Hills, probably because they formed in the same tectonic setting.

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https://scholar.acadiau.ca/islandora/object/theses:703