Abstract

The Gilbert River Belt, in the Grenville Province in southeastern Labrador, is a distinctive, west–northwest-trending zone of locally intense deformation and voluminous granitoid plutonism, up to 30 km in width. In an attempt to directly quantify the timing of deformation in ductile shear zones within the belt, rocks interpreted as having been intruded synchronously with ongoing deformation were sampled for U–Pb isotopic analysis. Three of these samples are <2 m wide granitic veins that have sharp intrusive contacts that truncate ductile deformation fabrics, but are themselves deformed at metamorphic conditions similar to their host rocks and are therefore interpreted as having intruded after the initiation of deformation and fabric development, but prior to cessation of this deformation. The first vein is syntectonic with respect to amphibolite-facies deformation and yielded a zircon age of graphic. The second vein intruded synchronously with the development of a zone of amphibolite-facies straight gneisses, which defines the southern limit of the Gilbert River belt at graphic. The third vein is syntectonic with respect to greenschist-facies deformation and yielded a zircon age of graphic and a monazite age of 1078 ± 2 Ma. A sample of the K-feldspar megacrystic granite that underlies much of the belt and is interpreted as having intruded during ongoing amphibolite-facies deformation yielded a zircon age of graphic; a mildly deformed granitic vein that crosscuts the megacrystic granite at the same location contained zircon that indicate a graphic crystallization age. Monazite from a granodioritic gneiss yielded a concordant age of 1077 ± 3 Ma, interpreted as the time of final cooling during gneiss formation. These results indicate that much of the amphibolite-facies deformation (1664 – 1644 Ma) in the Gilbert River Belt is correlative with the regionally extensive Labradorian orogenic event, whereas greenschist-facies deformation (1113 – 1062 Ma) and monazite growth (1078 Ma) are the result of renewed tectonomagmatic activity during Grenvillian orogenesis.

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