The Gander and Davidsville groups : major techtonostratigraphic units in the Gander Lake area, Newfoundland

McGonigal, Michael Henry (1972) The Gander and Davidsville groups : major techtonostratigraphic units in the Gander Lake area, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Detailed investigations in the Gander area have revealed the presence of two previously unrecognised terranes with contrasting structural, metamorphic and intrusive histories. These are named the metasedimentary terrane and the sedimentary and volcanic terrane. Older pre-Middle Ordovician polydeformed and metamorphosed rocks, mainly sediments of the Gander Group, are assigned to the metasedimentary terrane. The Gander Group is informally subdivided into the amphibolite-psammite member, the mixed member, and the pelite-mafic volcanic member. These members structurally overlie undifferentiated, predominantly psammitic metasediments of the group. The sedimentary and volcanic terrane includes the lower and upper members of the Davidsville Group. Mafic agglomerates occur at the base of this mid-Ordovician greywacke-slate sequence. -- Intrusive into the Gander Group are schistose, pre- or early-kinematic garnetiferous leucocratic granite, mafic and ultramafic bodies, as well as late-kinematic 'porphyritic granite' and apparently undeformed gabbro and diabase. Early gabbro, pyroxenite and serpentinite, and post-tectonic diabase dykes intrude the Davidsville Group. -- The Gander Group contains southeast facing, major recumbent F₂ folds, with two later crenulation fabrics and associated folds. Major structures are refolded into steep attitudes in the East near the contact with the basement, and in the West close to the Davidsville Group. This step-like refolding is considered to be Acadian (Devonian) in age. Rocks of the Gander Group have undergone greenschist to low amphibolite facies metamorphism, and local thermal overprinting. Within the Davidsville Group, upright open to close folding and later crenulation or kinking are attributed to the Acadian deformation. This terrane is much less recrystallized and of lower metamorphic grade (sub-greenschist facies). -- Greywackes of the Davidsville Group contain metamorphic detritus derived from the metasedimentary terrane, indicating deformation, meta-morphism, uplift and erosion of the Gander Group before mid-Ordovician time. A fault now separates rocks of the metasedimentary terrane and volcanic terrane in the Gander area.

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