Meguma terrane orocline: U–Pb age and paleomagnetism of the Silurian Mavillette gabbro, Nova Scotia, Canada

Abstract

Paleomagnetic results and a U–Pb baddeleyite age from the Silurian Mavillette gabbroic sill in southwest Nova Scotia, Canada, provide new evidence about the Paleozoic tectonic evolution of the Meguma terrane. The Mavillette gabbro sill intruded ca. 440–430 Myr old bimodal rift-related metavolcanic and metasedimentary rocks of the White Rock Formation in the Silurian–Devonian Rockville Notch Group. The 426.4 ± 2.0 Ma Mavillette gabbro age is notably younger than the ca. 440 Ma magmatism, but is part of a geochemically defined suite of within-plate sills and volcanic rocks of the Rockville Notch Group with ages as young as Early Devonian. Paleomagnetic investigation of 13 sites distributed along the Neoacadian (ca. 390 Ma) synclinal limbs of the Mavillette sill reveal magnetization directions that fail a fold test and therefore postdate Silurian emplacement of the gabbro. The post-folding remanence has a mean direction of D = 153.4, I = 17.1°; α 95 = 6.5° (n = 12 sites), with corresponding paleopole 31.9 °S, 325.2 E; dp = 3.5°, dm = 6.7° that resembles a pervasive late Carboniferous Kiaman overprint magnetization in North America, but is rotated significantly 22.2° ± 8.1° counter-clockwise (CCW). Mavillette remanence acquisition likely occurred in concert with fluid mobilization related to Alleghanian deformation, recorded locally by ca. 320 Ma muscovite 40Ar–39Ar ages. Previously published paleomagnetic results from the Meguma terrane also have Carboniferous remanence directions with similar ∼24° CCW discordance. The regional CCW rotation of the southwest Meguma terrane post-dates this ca. 320 Ma tectonothermal remanence acquisition event, likely recording the development of an oroclinal bend of the Meguma terrane during the Alleghanian orogeny.