Petrology and geochemistry of the Jeffers Brook Pluton, Cobequid Highlands, Nova Scotia
LE3 .A278 2017
Barr, Sandra M.
Bachelor of Science
Environmental & Sustainability Studies
The Cobequid Highlands of northwestern mainland Nova Scotia are part of the microcontinent of Avalonia, formed on the periphery of Gondwana. The highlands consist of two fault-bounded crustal blocks – the Jeffers block in the north and the Bass River block in the south. Field mapping, sampling, and petrological study of the Jeffers Brook pluton in the Jeffers block show that it consists mainly of dioritic and tonalitic rocks with abundant dioritic inclusions, as well as minor syenogranite, cut by mafic dykes. An age of ca. 605 Ma is inferred from previous 40Ar/39Ar dating of hornblende. Chemical analyses of 15 samples representing all the rock types in the pluton, combined with previously reported chemical data, display a wide range in SiO2 and trends consistent with a cogenetic relationship. They exhibit light rare earth element (REE) enrichment relative to heavy REE, with approximately parallel sloping patterns that lack europium anomalies. The syenogranite has lower REE abundance overall and a negative Eu anomaly indicating plagioclase fractionation. The rocks are calc-alkalic and likely formed in a continental margin subduction zone. They display chemical similarities to the calc-alkalic volcanic host rocks of the Jeffers Group, supporting interpretations that the pluton is the intrusive equivalent of the group. The pluton contains a similar range of rock types as occur in plutons of similar age in the Bass River block, and the rocks also show overall chemical similarity and formed in a similar tectonic setting. Hence the plutons in these two areas could be related, even though they occur in different host rocks. In contrast, the mafic dykes are tholeiitic and likely formed in a within-plate setting, part of the widespread bimodal magmatism that occurred in the Cobequid Highlands during the Late Devonian and Early Carboniferous.
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