Abstract

The Shabogamo intrusive suite, a predominantly gabbroic magma series intrusive into a variety of Archean, Aphebian, and Helikian units in the Churchill and Grenville Provinces of western Labrador, is reliably dated at circa 1375 Ma using both the Rb/Sr and Sm/Nd methods on whole rocks and mineral separates. The suite is thus synchronous with Elsonian magmatism in Labrador, which is characterized by the intrusion of large volumes of gabbroic, anorthositic, and associated magma, and so invites petrogenetic correlations on a regional scale.Gabbros of the Shabogamo intrusive suite are emplaced into volcanics and volcaniclastics of the Blueberry Lake group, which are provisionally dated at 1540 ± 40 Ma. The volcanic rocks are therefore of similar age to, and probably correlative with, the upper Petscapiskau Group and Bruce River Group felsic volcanics, which occur further east in a narrow belt within the Grenville Foreland zone. The linear disposition of centres of felsic volcanism in the Grenville Foreland zone about 1500 Ma ago is suggestive of the development of a major ensialic rift at least 300 km in length at that time. A twofold magmatic history during the Helikian of this part of Labrador is now emerging from the field mapping and geochronological studies. Early extrusive felsic volcanism about 1500 Ma ago confined to a linear belt immediately north of the Grenville Province was followed by voluminous mafic magmatism (with emplacement of gabbroic, anorthositic, and associated rocks) occurring over a wide area both within and outside of the present location of the Grenville Province.Rb/Sr dating of Aphebian quartzofeldspathic schists from within the Grenville Province near Wabush – Labrador City shows that the high-grade metamorphism and development of a penetrative schistosity were Grenvillian features formed about 1000 Ma ago. This result effectively precludes the possibility of a Hudsonian metamorphic imprint, a feasible interpretation that was raised during regional mapping of the area.

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