MESOPROTEROZOIC EVOLUTION OF THE LAURENTIAN MARGIN; INSIGHTS FROM THE GRENVILLE PROVINCE (Invited Presentation)

Despite pervasive high-grade metamorphism and deformation during the Grenvillian orogeny (1.09 to 0.98 Ga), the Grenville Province in Canada preserves important record of the growth and evolution of the SE Laurentian margin (present day coordinates) throughout the Mesoproterozoic. Margin-wide crust-forming events are linked to the development of a continental arc system at 1.71–1.65 Ga (Labradorian), contemporaneous with the Mazatzal province in the US, and a more diverse tectonic setting at 1.50–1.35 Ga (the 1.50–1.45 Ga part of it known as Pinwarian), active through most of the presently exposed Grenville, and extending to the SW into the Granite-Rhyolite province. The 1.50–1.35 Ga event is manifested by two continental arc systems, separated by a composite arc belt (Quebecia) in the central Grenville. The later preserves a record of rifting at 1.5 Ga, building of island arcs on rifted pericratonic crustal slivers at 1.5–1.45 Ga, and re-amalgamation of these slivers at 1.4–1.35 Ga.

Pinwarian continental and island arc volcanic sequences are exposed in the Low-P belt in SE part of the province that reached mostly amphibolite facies conditions during the Grenvillian orogeny, and show no evidence of earlier metamorphism. In contrast, high-grade Pinwarian-age metamorphism is reported in some lithotectonic domains farther north. Differences between Quebecia and continental arcs elsewhere in the Grenville at 1.5–1.35 Ga attest to lateral variations in subduction dynamics under Laurentia at that time, comparable to the modern-day Andean system.

In addition, the SW part of the province exposes 1.3 –1.2 Ga peri-Laurentian packages re-amalgamated between 1.25 and 1.16 Ga. Crustal growth involving continental arcs and docking of pericratonic island arcs through most of the Mesoproterozoic, was followed by intermittent crustal extension episodes from 1.25 Ga to the onset of the Grenvillian orogeny. This extension was manifested by localized development of supracrustal sequences and widespread mafic dyke swarms at 1.25 Ga, followed at 1.1 Ga by orogen-scale but variably distributed mantle-derived plutonism. The extension-dominant tectonic regime from 1.25 to 1.1 Ga is not well understood, but it is clear that at least part of the Laurentian-side lithosphere was hot and weak at the onset of the Grenvillian orogeny.