Petrographic and chemical characteristics of mafic dykes and sills in the Antigonish Highlands, Nova Scotia
LE3 .A278 2015
Barr, Sandra M.
Bachelor of Science
Earth & Environmental Sciences
The Antigonish Highlands of northern mainland Nova Scotia have a long and complex tectonic history involving several episodes of magmatism. Mafic dykes and sills mapped at almost 200 locations throughout the Antigonish Highlands are presumably related to one or more of these magmatic events. Wide variations in petrographic, magnetic, and chemical characteristics suggest multiple episodes of dyke/sill emplacement. Based on the petrography of 62 samples, four main types of sills and dykes are recognized: intergranular, coarse-grained clinopyroxene-bearing, amphibole-bearing, and plagioclase-porphyritic. Whole-rock chemical analysis of 32 of these dykes/sills shows that the majority are mafic (<52% SiO2) but a few are intermediate with up to 55% SiO2. Loss on ignition is typically high (ca. 5-10%) consistent with extensive alteration in the rocks, including saussuritization of plagioclase and replacement of pyroxene by chlorite. Both major and trace elements show wide variation, and little correlation is apparent between petrographic features and chemical characteristics. The dykes/sills appear to fall into four distinct groups based on chemical affinity and tectonic setting discrimination diagrams using mainly immobile elements: volcanic-arc, within-plate transitional to alkalic, within-plate tholeiitic, and alkalic. Samples in the subalkalic volcanic-arc group show chemical characteristics similar to those of the Neoproterozoic volcanic and plutonic units of the Antigonish Highlands, generally interpreted to have formed in a subduction zone. The alkalic samples show similarities to the Arbuckle Brook Formation. Both the within-plate transitional to alkalic and tholeiitic samples show chemical similarities to the within-plate Ordovician plutonic and volcanic units of the Antigonish Highlands.
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