LE3 .A278 2020

2020

Barr, Sandra

Acadia University

Bachelor of Science

Honours

Geology

Earth & Environmental Sciences

The goal of this project was to provide new information about field relations, petrology, age, and tectonic significance of the enigmatic Gunshot Brook pluton in the eastern Cobequid Highlands of northern mainland Nova Scotia. Field mapping showed that pluton intruded the ca. 760-735 Ma Mount Ephraim plutonic suite on its southern margin and the >735 Ma Dalhousie Mountain Formation on its western, northern and eastern margins. Petrographic study revealed that the pluton consists mainly of granodiorite gradational to monzogranite and tonalite, with minor co-mingled dioritic rocks, all cut by mafic and felsic dykes. Whole-rock chemical data from about 40 samples display trends consistent with a comagmatic relationship among the tonalitic, granodioritic, and monzogranitic rocks, and a compositional gap between them and the mingled dioritic rocks. They are calc-alkalic and likely formed in a continental margin subduction zone together with the associated dioritic rocks. U-Pb zircon ages of 630.6 ± 2.7 Ma and 638.6 ± 2.5 Ma for granodiorite samples from the northern and southern parts of the pluton are consistent with petrological similarities which suggest that the cross-cutting Millsville fault does not have significant offset. Granitic and dioritic rocks from the southeastern part of the pluton have chemical characteristics which indicate that they are part of the Mount Ephraim plutonic suite rather than Gunshot Brook pluton, resulting in revision to the mapped extent of the Gunshot Brook pluton. Because these plutons differ in age from the ca. 620-600 Ma plutons of the Jeffers and Bass River blocks of the Cobequid Highlands, the Gunshot Brook pluton, Mount Ephraim plutonic suite, and ca. 735 Ma Six Mile Brook pluton, together with their host rocks, are assigned to a separate Mount Ephraim block, of as yet uncertain relation to the rest of the highlands, and to Avaloniain in general.

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https://scholar.acadiau.ca/islandora/object/theses:3426