LE3 .A278 2011

2011

Barr, Sandra
Murphy, Brendan

Acadia University

Master of Science

Masters

Geology

Earth & Environmental Sciences

The West Barneys River plutonic suite is an assemblage of medium- to coarse-grained syenite to alkali-feldspar granite and gabbro which outcrops over an area of about 100 km2 in the central part of the southern Antigonish Highlands. Based on mapping and petrological studies, the suite has been subdivided into three intermediate to felsic plutons (Laggan, Brora Lake, and Haggarts Lake), two mafic plutons (Garden River and Duck Ponds), and a large, heterogeneous pluton named the Poor Farm Brook composite pluton. The Poor Farm Brook composite pluton is dominantly mafic (~65%) but also includes the chemical equivalents of the intermediate and felsic rocks found in the other plutons. Magma mixing and mingling textures indicate a co-magmatic relationship between the intermediate-felsic and mafic lithologies. Intermediate to felsic rocks are hypersolvus and in some cases show interstitial granophyre indicative of shallow emplacement. Many of the intermediate and felsic rocks are highly magnetic but susceptibilities vary because of variability in opaque mineral composition and degree of alteration. Geochemically, intermediate and felsic rocks are dominantly alkaline and have characteristics of within-plate, A-type granitoid suites. Gabbroic rocks are dominantly inequigranular (locally porphyritic) and highly magnetic, attributed to the high percentage of opaque minerals although some samples have lower susceptibilities. The gabbroic rocks are transitional from tholeiitic to alkalic and have compositions characteristic of continental within-plate rocks. Emplacement of these rocks is probably related to ensialic back-arc extension associated with Tornquist sea oceanic lithosphere subduction beneath Avalonia throughout the Ordovician. The West Barneys River plutonic suite represents an important and previously unrecognized magmatic episode during the evolution of Avalonia.

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https://scholar.acadiau.ca/islandora/object/theses:220