Grenvillian magmatism in the eastern Grenville Province, Canada

Abstract

Abstract U-Pb age determinations on six newly identified examples of Grenvillian plutonism from eastern Labrador have yielded the following ages: Gilbert Bay granite 1132−6+7 Ma; Second Choice Lake pegmatite 1003±6 Ma; Southwest Pond granite 963±6 Ma; Chateau Pond granite 964±2 Ma; Riviere Bujeault headwaters quartz syenite 964±5 Ma; Upper St Lewis River (west) granite 956±1 Ma. In addition, eight new K-Ar and Rb-Sr hornblende and biotite ages ranging from 953 Ma to 811 Ma are reported for three of these plutons. In conjunction with three previous U-Pb determinations, it is concluded that the U-Pb dates reflect two periods of Grenvillian plutonism that occurred in separate areas of eastern Labrador, and which were also characterized by distinct emplacement styles. North of the Mealy Mountains terrane boundary (Lake Melville terrane and its border regions with the Mealy Mountains and Hawke River terranes) plutonism occurred between ∼1130 and 1080 Ma and consisted of sporadic, minor granitic intrusions. These intrusions had little structural effect on the host rocks and are inferred to have been emplaced at a high structural level, a model consistent with previous suggestions for tectonic stacking in the region. South of the Mealy Mountains terrane boundary (Mealy Mountains and Pinware terranes) plutonism was brief (966 to 956 Ma), but widespread. Typical magmatic products were circular (in plan) plutons, up to 20 km in diameter, having monzonite, quartz syenite and granite compositions. These plutons exerted marked structural influence on their host rocks and are interpreted to have been emplaced at intermediate structural levels. In a broader regional context, by utilizing a previously demonstrated correlation between positive magnetic anomalies and Grenvillian plutons together with reconnaissance geological mapping, a belt of Grenvillian plutons is inferred to exist across the southern half of the eastern Grenville Province. This zone of plutonism serves to emphasize a distinct difference between an exterior (northern) thrust belt and an interior (southern) magmatic belt. A similar bipartite division is apparent in other parts of the Grenville-Sveconorwegian Orogen. The mineral geochronological data probably do not reflect the cooling histories of individual plutons; alternative explanations include either a slow regional cooling event or the distal effects of younger plutonism/metamorphism to the south, as yet unidentified.