Lithotectonic elements of the Grenville Province: review and tectonic implications

Abstract

The Grenville Province formed the southeastern limit of Proterozoic Laurentia and is composed of lithologic units that range in age from Archean to Late Mesoproterozoic. Reworked continuations of Archean, Early and Middle Paleoproterozoic orogens extend at the surface and at depth well into the southwestern and central Grenville Province, but are absent in the northeast. There is evidence for the existence of an active margin, with subduction-accretion and arc formation, along southeastern Laurentia for >400 Ma from the Late Paleoproterozoic to the Late Mesoproterozoic. Major juvenile crustal additions to the Laurentian margin, comprising Andean-style, calc-alkaline magmatic arcs and coeval inboard backarc deposits, occurred between ∼ 1.71 and 1.61, 1.51 and 1.42, and 1.40 and 1.23 Ga. Backarc magmatic and (or) sedimentary products document variable degrees of backarc extension and basin formation. Examples include the coeval oceanic and continental backarc basin settings of Hastings/Frontenac and Wakeham/Seal Lake groups respectively during geon 12, and the incipient continental backarc extensional settings of the Michael-Shabogamo dyke swarm during geon 14 and of alkali granite and anorthosite complexes during geons 13 and 12. Arc magmatism was followed by accretionary orogenesis during the Labradorian (∼ 1680-1660 Ma), Pinwarian (∼ 1500-1450 Ma) and Elzevirian (∼ 1250-1190 Ma) orogenies, respectively, resulting in substantial growth of Laurentia. A result of this growth is that the ages of most major units tend to young towards the southeast of the Grenville Province, except for those that formed inboard of the continent margin in a backarc setting. The continent-continent Grenvillian Orogeny took place between ∼ 1.19 and 0.98 Ga and comprised three distinct pulses of crustal shortening at ∼ 1.19-1.14, 1.08-1.02 and 1.00-0.85 Ga, separated by periods of extension. The loci of the earlier (Shawinigan and Ottawan) pulses of crustal shortening were in the hinterland of the orogen, whereas the latest (Rigolet) pulse caused northwesterly propagation of the orogen into its foreland. Periods of crustal extension during the Grenvillian Orogeny were coeval with emplacement of mafic magmas and anorthosite complexes, implying that large quantities of mantle magma and heat had access to the base of the previously thickened orogenic crust. This scenario is compatible with extensional collapse of the orogen following delamination or convective removal of the lower lithosphere, as suggested previously by others. An inference from these conclusions is that anorthosite complexes formed in two contrasting extensional tectonic environments in southeastern Laurentia during the Mesoproterozoic, that is, in areas of backarc extension inboard from an active continental-margin magmatic arc and within a collisional orogen during periods of tectonic collapse and rising isotherms.