Paleomagnetism and rock magnetism of Early Silurian Dunn Point volcanics, Avalon Zone, Nova Scotia

Abstract

Paleomagnetic results have been obtained from the subaerial volcanics (andesites, rhyolites, ignimbrites) from the Dunn Point Formation exposed in the Antigonish Highlands of the Avalon Zone, Nova Scotia. An Early Silurian age of 430 ± 15 Ma is assigned to these volcanic rocks. The age is in agreement with the fossil evidence from conformably overlying sedimentary rocks. The volcanic units were tightly folded and metamorphosed to sub-greenschist facies during one major and one secondary phase of deformation probably associated with the Mid-Late Devonian Acadian orogeny. This paleomagnetic study is based on 45 samples collected from nine sites. The site-mean directions of the natural remanent magnetization (NRM) make a large deviation from the present Earth's field with NRM intensities ranging from 10 −2 to 10 Am −1. The most commonly observed remanence direction, A( D=153°, I=−28°), obtained from both alternating fields (30–75 mT) and thermal (500–560°C) treatments yields a corresponding direction A′( D=0°, I=−63°) after correction for geological tilt. A large scatter in direction (up to = 60°) is observed in A′ and the fold test is not significantly positive. A second and less common remanence component, B ( D=146°, I=+54°), which presumably postdates folding, is also observed; this component is mainly isolated by thermal treatment in the 540–630°C range. Room-temperature isothermal experiments and optical examination of polished sections indicate the presence of magnetite and hematite as the carriers of both A and B directions. The corresponding reversed paleomagnetic poles fall at A (163°E, 52°N) and B (146°E, 4°N) in close agreement with cratonic North American Upper Jurassic and Late Devonian poles, respectively. In the Mid-Late Devonian time interval, the Avalon Zone was situated slightly to the south but close to the North American eastern margin.