ABSTRACT The amalgamation of Laurentia’s Archean provinces ca. 1830 Ma was followed by ~700 m.y. of accretionary orogenesis along its active southeastern margin, marked by subduction of oceanic lithosphere, formation of arcs and back-arcs, and episodic accretion. This prolonged period of active-margin tectonic processes, spanning the late Paleoproterozoic and Mesoproterozoic eras, resulted in major accretionary crustal growth and was terminated by closure of the Unimos Ocean (new name). Ocean closure was associated with rapid motion of Laurentia toward the equator and resulted in continental collision that led to profound reworking of much of the accreted Proterozoic crust during the ca. 1090–980 Ma Grenvillian orogeny. The Grenvillian orogeny resulted in formation of a large, hot, long-duration orogen with a substantial orogenic plateau that underwent extensional orogenic collapse before rejuvenation and formation of the Grenville Front tectonic zone. The Grenvillian orogeny also caused the termination and inversion of the Midcontinent Rift, which, had it continued, would likely have split Laurentia into distinct continental blocks. Voluminous mafic magmatic activity in the Midcontinent Rift ca. 1108–1090 Ma was contemporaneous with magmatism in the Southwestern Laurentia large igneous province. We discuss a potential link between prolonged subduction of oceanic lithosphere beneath southeast Laurentia in the Mesoproterozoic and the initiation of this voluminous mafic magmatism. In this hypothesis, subducted water in dense, hydrous Mg-silicates transported to the bottom of the upper mantle led to hydration and increased buoyancy, resulting in upwelling, decompression melting, and intraplate magmatism. Coeval collisional orogenesis in several continents, including Amazonia and Kalahari, ties the Grenvillian orogeny to the amalgamation of multiple Proterozoic continents in the supercontinent Rodinia. These orogenic events collectively constituted a major turning point in both Laurentian and global tectonics. The ensuing paleogeographic configuration, and that which followed during Rodinia’s extended breakup, set the stage for Earth system evolution through the Neoproterozoic Era.

ABSTRACT Prolonged slow cooling (average 1–3 °C/m.y.) of Ottawan phase granulite-facies gneisses (peak temperature ≥850 °C ca. 1090–1080 Ma) through the argon closure temperatures ( T C ) of hornblende ca. 980–920 Ma and biotite ca. 890–820 Ma in the western Grenville Province and in an inlier in the central Appalachians is well established, but its tectonic setting has not been systematically investigated. Here, the case is made that this slow cooling occurred in the suprasolidus cores of large metamorphic core complexes that were exhumed during mid-Ottawan (ca. 1050 Ma) extensional orogenic collapse. The ductile midcrustal metamorphic cores of the large metamorphic core complexes are overlain across gently dipping extensional detachments by a brittle-ductile cover composed of upper orogenic crust, parts of which preserve evidence of relict pre-Ottawan fabrics and peak prograde Ottawan temperatures of <500 °C ( T C of Ar in hornblende), collectively implying thermal, structural, and rheological decoupling across the detachments. Slow average rates of cooling of the orogenic midcrust for >150 m.y. imply an anomalously hot upper mantle and mask short periods of more rapid cooling indicated by analyses of retrograde diffusional mineral zoning patterns. It is suggested that these slow average rates of cooling, coupled with slow average rates of exhumation of ≤0.1 km/m.y. modeled for one data set, were a result of decompression melting of rising asthenosphere and emplacement of voluminous mafic intrusions within or at the base of the crust, which reduced the buoyancy of the residual thinned lithosphere. This process is compatible with either delamination of subcontinental lithospheric mantle or slab rollback. The high-strain extensional detachments of the large metamorphic core complexes are sites of amphibolite-facies retrogression, suggesting a feedback between ingress of hydrous fluid, which was likely derived from beneath the detachment during crystallization of migmatite, and strain. Extensional juxtaposition of the hot midcrust ( T >850 °C) and cooler cover ( T <500 °C) across the detachments led to conductive heating of the base of the cover, locally raising its temperature above 500 °C, as recorded by amphibolite-facies metamorphism and young cooling ages. The slow cooling and exhumation of Grenvillian large metamorphic core complexes contrast with much faster rates in smaller metamorphic core complexes in other settings (e.g., North American Cordillera). The slow rates of these processes in large metamorphic core complexes are attributed to the prolonged high temperature and low viscosity of their metamorphic cores due to proximity of the asthenosphere, and to the intrusion of voluminous asthenospheric mafic magmas that both advected heat and reduced lithospheric buoyancy.

Abstract The proposition that the Grenville Province is a remnant of a large hot long-duration collisional orogen is examined through a comparative study of its present orogenic front, the Grenville Front, and a former front, the Allochthon Boundary Thrust. Structural, metamorphic and geochronologic data for both boundaries and their hanging walls from the length of the Grenville Province are compared. Cumulative displacement across the Grenville Front was minor (10 s of km) whereas that across the Allochthon Boundary Thrust was major (100 s of km), consistent with the observation that the latter boundary separates rocks with a different age, and P – T character, of metamorphism. On an orogen scale, Grenvillian metamorphism can be subdivided into two spatially and temporally distinct orogenic phases, a relatively high T Ottawan ( c . 1090–1020 Ma) phase in the hanging wall of the Allochthon Boundary Thrust, and a relatively lower T Rigolet ( c . 1000–980 Ma) phase in the hanging wall of the Grenville Front. It is argued that the structural setting and ≥50 My duration of Ottawan metamorphism are compatible with some form of channel flow beneath an orogenic plateau, with the Allochthon Boundary Thrust forming the base of the channel. Channel flow ceased at c . 1020 Ma when the Allochthon Boundary Thrust was reworked as part of a system of normal-sense shear zones, and following a hiatus of c . 20 My the short-lived Rigolet metamorphism took place in the former foreland and involved the development of a new orogenic front, the Grenville Front. Taken together, this suggests the Grenville Orogen developed as a large hot long-duration orogen during the Ottawan orogenic phase, but following gravitational collapse of the plateau the locus of thickening migrated into the foreland and active tectonism was restricted to a subjacent small cold short-duration orogen. The foreland-ward migration of the orogenic front from the Allochthon Boundary Thrust to the Grenville Front, the contrasting P–T–t character of the metamorphic rocks in their hanging walls, and the evidence for orogenic collapse followed by renewed growth, provide insights into the complex evolution of a long-duration collisional orogen.

The Cape Caribou River allochthon is a thick Grenvillian thrust sheet composed of Paleoproterozoic orthogneiss and Mesoproterozoic mafic dikes in which penetrative syn-thrusting deformation and recrystallization are largely restricted to the 1- to 2-km-wide basal shear zone, the Grand Lake thrust system. Grenvillian mylonitic fabrics in orthogneiss in the Grand Lake thrust system are characterized by the peak high-pressure (H P ) granulite-facies assemblages garnet-clinopyroxene-plagioclase-clinoamphibole-quartz (Grt-Cpx-Pl-Cam-Qtz) and orthopyroxene-clinopyroxene-plagioclase-garnet-clinoamphibole-quartz (Opx-Cpx-Pl-Grt-Cam-Qtz) in mafic and intermediate lithologies, respectively. Evidence for the prograde reactions Cam + Pl = Cpx + Grt + Qtz + H 2 O and Opx + Pl = Grt + Cpx + Qtz is preserved in some samples. Partial retrograde replacement of the subassemblage Grt-Cpx by Cam-Pl is widespread, but retrogression was domainal on both outcrop and thin-section scales. Thermobarometry applied to the syntectonic H P granulite-facies assemblage Grt-Cpx-Pl-Qtz-Cam ± Opx in the Grand Lake thrust system shear zone and adjacent hangingwall and footwall has yielded apparent peak pressure-temperature ( P-T ) estimates of ∼14 kilobars/875 °C that are considered to closely approximate the P at maximum T experienced by rocks, despite evidence in some samples for postpeak resetting of thermometers. The rocks are inferred to have followed a clockwise P-T path that involved quasi-isothermal decompression following peak T conditions, although details of the path remain only qualitatively constrained because of the unknown extent of postpeak reequilibration. The P-T results and tectonic setting suggest that the Cape Caribou River allochthon is part of the H P belt of the Grenville Province, and that the localized Grenvillian granulite-facies recrystallization in the Grand Lake thrust system was possibly partly a result of frictional heating.