The thermotectonic history of the Grenville Province of western Labrador

Connelly, James N. (1991) The thermotectonic history of the Grenville Province of western Labrador. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Grenville Orogen in western Labrador has been divided into three lithologically distinct terranes comprising, from the Grenville Front towards the southeast, the Gagnon, Molson Lake and Lac Joseph terranes. Of these terranes, only the Lac Joseph Terrane is devoid of lithologies that may be correlated with units to the north of the Grenville Front in the pre-Grenvillian, stable North American craton. -- In the Lac Joseph Terrane, high grade polydeformed migmatitic pelitic supracrustal rocks contain two distinct leucosomes which have been dated at 1658+36/-8 Ma and 1639 + 12/-7 Ma by U/Pb geochronology; a third migmatitic sample which is retrogressed yielded an age of 1611 + 13/-6 Ma. These ages indicate that the gneisses in the Lac Joseph Terrane all formed during the Labradorian Orogeny, which must have been a protracted event. High grade metamorphism and migmatization of the supracrustal rocks was synchronous with folding and the development of localized shear zones, one of which has yielded a U-Pb age of 1634 + 13/-4 Ma. Sillimanite and/or kyanite-bearing mineral assemblages in the restite of the pelitic migmatites occur throughout Lac Joseph Terrane, and also formed during the Labradorian Orogeny. They yield P-T estimates (garnet - biotite geothermometry and garnet - plagioclase - aluminosilicate - quartz geobarometry) between 575-800°C and 3-8 kbar in the southern Lac Joseph Terrane to 700-850°C and 6-10 kbar in the northwestern part of the terrane. Granitoid rocks were emplaced into the Lac Joseph Terrane throughout the Labradorian Orogeny; a gabbroic pluton of the Ossokmanuan Intrusive Suite has yielded a U/Pb age of 1623±7 Ma. -- Synchronous with Labradorian metamorphism in the Lac Joseph Terrane, granitoid rocks were emplaced into the adjacent Molson Lake Terrane, one body of which yielded a U-Pb zircon age of 1648±7 Ma. No evidence exists to suggest that these granitoid rocks, which are interpreted to represent a southwestern extension of the Trans Labrador Batholith, were tectonized during the Labradorian Orogeny. The Shabogamo Intrusive Suite, the only other unit in the Molson Lake Terrane, intruded into the granitoid rocks at 1431 ± 7 Ma during a period of south-southeast - north-northwest crustal extension; this unit is correlated, on the basis of geochemistry and age, with the Michael Gabbro of eastern Labrador. -- Retrogression of granulite facies basic gneisses in the Lac Joseph Terrane to amphibolite facies assemblages took place, at least locally, at about 1281 Ma, as recorded by U/Pb (titanite) and ⁴⁰Ar/³⁹Ar (hornblende) isotopic systematics. The tectonic significance of this event is not understood. -- The Molson Lake Terrane was extensively recrystallized and deformed during the Grenvillian Orogeny (1001 Ma - 990 Ma; U/Pb sphene and zircon data), locally under high P - moderate T conditions, between 8-12 kbar and 650-850°C (garnet - clinopyroxene geothermometry and gamet - clinopyroxene - plagioclase - quartz geobarometry). The near coincidence of Grenvillian U-Pb and ⁴⁰Ar/³⁹Ar ages from the Molson Lake Terrane suggests that this terrane was rapidly exhumed after peak metamorphism, probably along a crustal scale, frontal thrust zone. The Lac Joseph Terrane experienced only minor heating and non-penetrative deformation during the Grenvillian Orogeny; the preservation of a pre-Grenvillian ⁴⁰Ar/³⁹Ar amphibole age in the Lac Joseph Terrane while Ar was reset in biotite and muscovite, indicates that temperatures during this event were between about 350°C and 530°C. The Lac Joseph Terrane is thought to have been emplaced as a pre-assembled, coherent package over the Molson Lake Terrane late in the Grenvillian Orogeny along a decollement at, or near, the brittle-ductile transition. Rocks in the Lac Joseph Terrane preserving Labradorian, deeper level P-T conditions were thus emplaced over freshly exhumed rocks, recording Grenvillian P-T conditions, in the Molson Lake Terrane.

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