LE3 .A278 2021

2021

Barr, Sandra

Acadia University

Master of Science

Masters

Earth and Environmental Science

Earth & Environmental Sciences

The Coldbrook Group is a unique suite of late Ediacaran volcanic and epiclastic rocks that together with related plutons formed during a short (<10 Myr) magmatic episode in the Avalonian Caledonia terrane of southern New Brunswick, Canada. In this study, petrographic and geochemical data combined with U-Pb zircon dating and zircon chemistry are used to provide new insight about the origin, evolution, and age of the voluminous silicic volcanic rocks of the Silver Hill Formation that dominate the upper part of the Coldbrook Group and clarify their previously postulated relationship to associated high-level granitic plutons of the Bonnell Brook suite. Based on textural criteria the silicic volcanic rocks are mainly eutaxitic pyroclastic rocks with less abundant lavas characterized by porphyritic and eutaxitic textures. Variations in whole-rock chemical characteristics, zircon chemistry and age show that silicic volcanic rocks of the Silver Hill Formation and the associated granitic plutons are not all cogenetic, and differences in zircon population morphology are used to investigate magma evolution processes among the dated samples. Rhyolite in the Silver Hill type area yielded a weighted mean 206Pb/238U age of 551.65 ± 0.15 Ma, similar within error to the age of 551.71 ± 0.15 Ma for a nearby fine-grained granite dome. Whole-rock and zircon chemistry suggest that the main body of the Bonnell Brook pluton was emplaced at a deeper level and is not cogenetic with the fine-grained granite and Silver Hill rhyolite. The main belt of rhyolite in the Fundy Trail Parkway area is younger at 549.18 ± 0.07 Ma, consistent with an age of 551.19 ± 0.20 Ma, obtained for an underlying lithic tuff unit. Transitional felsic I- to A-type chemical signatures suggest that magmatism occurred in an extensional within-plate setting, consistent with the tectonic regime at that time throughout Avalonia, but the cause of the exceptionally voluminous magmatism in the Caledonia terrane remains uncertain.

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https://scholar.acadiau.ca/islandora/object/theses:3681