LITHOTECTONIC ELEMENTS OF THE GRENVILLE PROVINCE - REVIEW AND TECTONIC IMPLICATIONS

The Grenville Province formed the southeastern limit of Proterozoic La urentia and is composed of lithologic units that range in age from Arc hean to Late Mesoproterozoic. Reworked continuations of Archean, Early and Middle Paleoproterozoic orogens extend at the surface and at dept h well into the southwestern and central Grenville Province, but are a bsent in the northeast. There is evidence for the existence of an acti ve margin, with subduction-accretion and are formation, along southeas tern Laurentia for >400Ma from the Late Paleoproterozoic to the Late M esoproterozoic. Major juvenile crustal additions to the Laurentian mar gin, comprising Andean-style, calc-alkaline magmatic arcs and coeval i nboard backarc deposits, occurred between similar to 1.71 and 1.61, 1. 51 and 1.42, and 1.40 and 1.23 Ga. Backarc magmatic and (or) sedimenta ry products document variable degrees of backarc extension and basin f ormation. Examples include the coeval oceanic and continental backarc basin settings of Hastings/Frontenac and Wakeham/Seal Lake groups resp ectively during geon 12, and the incipient continental backarc extensi onal settings of the Michael-Shabogamo dyke swarm during geon 14 and o f alkali granite and anorthosite complexes during geons 13 and 12. Are magmatism was followed by accretionary orogenesis during the Labrador ian (similar to 1680-1660 Ma), Pinwarian(similar to 1500-1450 Ma) and Elzevirian(similar to 1250-1190 Ma) orogenies, respectively, resulting in substantial growth of Laurentia. A result of this growth is that t he ages of most major units tend to young towards the southeast of the Grenville Province, except for those that formed inboard of the conti nent margin in a backarc setting. The continent-continent Grenvillian Orogeny took place between similar to 1.19 and 0.98 Ga and comprised t hree distinct pulses of crustal shortening at similar to 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 w ere in the hinterland of the orogen, whereas the latest (Rigolet) puls e caused northwesterly propagation of the orogen into its foreland. Pe riods of crustal extension during the Grenvillian Orogeny were coeval with emplacement of mafic magmas and anorthosite complexes, implying t hat large quantities of mantle magma and heat had access to the base o f the previously thickened orogenic crust. This scenario is compatible with extensional collapse of the orogen following delamination or con vective removal of the lower lithosphere, as suggested previously by o thers. An inference from these conclusions is that anorthosite complex es formed in two contrasting extensional tectonic environments in sout heastern Laurentia during the Mesoproterozoic, that is, in areas of ba ckarc extension inboard from an active continental-margin magmatic are and within a collisional orogen during periods of tectonic collapse a nd rising isotherms. (C) 1997 Elsevier Science B.V.