The Twillingate granite and its relationships to surrounding country rocks

Abstract

The volcanic rocks of the Twillingate area are divided into two groups separated by the intrusion of the Twillingate Granite. The Sleepy Cove Group, comprising mixed mafic and silicic volcanic rocks, is intruded pre-tectonically by the Twillingate Granite, which is an unusual, homogeneous, trondhjemite pluton. These two units are cut post-tectonically by mafic dykes of the Herring Neck Group which is composed of mafic volcanic rocks. All three units are cut by a variety of mafic, acidic and alkaline dykes of different ages. -- The one major deformation in the area produced a fabric in the Sleepy Cove Group and the Twillingate Granite that is very intense in the south of the area but generally becomes faint and absent to the north. The Herring Neck Group is post-tectonic with respect to this fabric. Earlier refolded fabrics are present in some mafic inclusions in the Twillingate Granite. -- The distinctive chemistry of the Twillingate Granite when compared to other trondhjemites and experimental data suggests that it was produced by partial melting of oceanic lithosphere on a subduction zone. Both groups of volcanic rocks are considered to be island arc tholeiites having a consanguinous and nearly coeval origin with the granite. -- A model is postulated of a late Cambrian to early Ordovician island arc developed on older ocean crust. The sequence of mafic and acidic magmatism and deformation is thought to be fairly continuous and occupy a short time span.