The southwestern Grenville Province in Ontario consists of Paleoproterozoic and Mesoproterozoic rocks that formed along the southeastern margin of Laurentia during a significant period of growth of the North American continent. The area investigated consists of high-grade ortho- and paragneisses whose protoliths formed between 1500 and 1350 Ma. Geochemical and geochronological data from the rocks investigated point to a period of arc and back-arc magmatism along the Laurentian margin between ca. 1500 and 1450 Ma. A-type charnockites and granites are temporally and spatially associated with the arc rocks, and have compositions interpreted to indicate derivation from a tholeiitic basaltic underplate and from crustal melting, respectively. This interpretation implies that the arc underwent extension during part of its history. Between ca. 1450 and 1430 Ma, magmatism apparently ceased in the study area and was temporally associated with high-grade metamorphism in the back-arc region, possibly suggesting a change from an extensional to a compressional tectonic regime, although other interpretations are equally likely. Few data exist for the period after ca. 1430 Ma; however, a number of geochronological studies from nearby parts of the Grenville Province suggest that arc magmatism may have moved to a more outboard position on the continental margin. At ca. 1360 Ma, bimodal rhyolitic and basaltic magmatism suggests a return to an extensional tectonic regime. The geochemical data generally support, but suggest refinements to, previously proposed models for the evolution of the southeastern Laurentian margin in the Grenville Province that may be applicable to other parts of the margin.

No abstract available.

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.

The supercontinent Rodinia is thought to have been formed by 1.2 to 1.0 Ga continent-continent collisions and to have dispersed between 0.75 and 0.6 Ga. The existence of Rodinia implies the presence of a Panthalassalike peri-Rodinian ocean between ca. 1.0 and 0.75 Ga within which juvenile crust developed. Although the vast majority of this crust was later subducted, vestiges are preserved in terranes that accreted to the leading edges of the dispersing continents following the breakup of Rodinia. These terranes are recognized by their ca. 1.2 to 0.75 Ga Sm-Nd depleted mantle (T DM ) model ages, coeval with the life of Rodinia. They include ca. 0.9 to 0.8 Ga ophiolites and ensimatic arc complexes, and ca. 0.8 to 0.6 Ga recycled mafic to fel-sic arc complexes. Formed within the peri-Rodinian ocean, the terranes were accreted to their respective continental margins in the Late Neoproterozoic. For orogens in which subduction culminated in Late Neoproterozoic continental collision (e.g., the Southern Yangste margin, Brasiliano, and Trans-Saharan orogens), vestiges of peri-Rodinian crust became cratonized within the suture zones between the colliding cratons. In accretionary orogens, in which subduction was not terminated by continental collision (e.g., the Arabian Shield and peri-Gondwana orogens), the terranes were subsequently involved in Paleozoic orogenesis. More generally, crustal formation in Panthalassa-type oceans and the subsequent recycling of this crust can be recognized by Sm-Nd T DM model ages that overlap the life span of the supercontinent.

The Lac Volant Ni-Cu-Co-PGE prospect, located 75 km northeast of Sept-Iles in the Grenville Province of Québec, is an example of magmatic sulfide mineralization associated with mafic magmas in a high-grade metamorphic terrain. Disseminated and massive sulfides occur within a 20- to 25-m-thick zone in a gabbronorite dike intruding the gabbronorite of the host Matamec Complex. The average composition ( n = 29) of the sulfides is estimated to be 2.0% Cu, 1.5% Ni, 0.12% Co, 67 ppb Pt, and 256 ppb Pd. The age of the dike is 1351 ± 6 Ma (U-Pb zircon age), which is identical to the age of the Rivière-Pentecôte anorthosite (1354 ± 3 Ma) located 130 km to the southwest. The dike originated by means of multiple injections of sulfide-bearing, tholeiitic magmas derived from a depleted, N-type mid-ocean ridge basalt (N-MORB)-type source. The dike is chemically similar to the gabbronoritic host, suggesting similar parental magmas. Geochemical variation within the dike was caused by fractional crystallization of silicates (orthopyroxene, clinopyroxene, and plagioclase) and by ingestion of crustal rocks. Assimilation is suggested by the presence of xenoliths of granite and metasediment within the dike or narrow branching dikes; enrichment of the dike in Rb, Th, Ba, and light rare earth elements; and a negative Ta anomaly with respect to Th. The observed composition of the dike can be explained by 15% crustal contamination, which would have occurred at depth before the magma began to crystallize mafic minerals. Massive, matrix, and disseminated sulfides, interpreted to be of magmatic origin, are composed of pyrrhotite (the principal species, 75%), pentlandite (altered to bravoite and violarite), chalcopyrite, and pyrite. Magmatic breccia structures are common in gabbronorite containing disseminated sulfides. The presence of metasedimentary xenoliths and a high S/Se ratio (9,000–16,000) suggest that sulfide saturation was caused by contamination of the magma. The sulfide liquid interacted with a small volume of magma (R = silicate liquid/sulfide liquid = 200). Impoverishment of the sulfides in platinum-group elements (PGE) relative to Ni and Cu may have been caused by the loss of PGE during an earlier and deeper episode of sulfide separation. Limited sulfide fractionation is suggested by the chemical similarity of the three types of sulfide. Metamorphic recrystallization produced xenomorphic pyrrhotite and idiomorphic pyrite crystals up to 25 cm and 4 cm in size, respectively. Alteration by meteoric water caused the transformation of pentlandite into bravoite and violarite. The Lac Volant prospect is similar in terms of composition, style of emplacement, magma type, and age to certain Norwegian deposits (the Ertelian and Flåt deposits in the Sveconorwegian Province) and to the Voisey's Bay deposit in Labrador.

Granitic rocks related to 1.18 to 1.13 Ga anorthosite-mangerite-charnockitegranite plutonism stitch three terranes in the southwestern Grenville Province (Adirondack Highlands–Morin terrane, Frontenac terrane, Elzevir terrane). Because of the refractory nature of zircon (Zrn), analysis of oxygen-isotope ratios of dated igneous zircon from these rocks allows calculation of δ 18 O values of original magmas even if the rocks were subjected to late magmatic assimilation, postmagmatic alteration, or metamorphism. Documented variability in δ 18 O(Zrn) for these granitic rocks corresponds to their geographic location. Seven plutons from the central Frontenac terrane (Ontario) have a high average δ 18 O(Zrn) = 11.8 ± 1.0‰, which corresponds to δ 18 O magma values of 12.4–14.3‰. In contrast, twenty-seven other plutons and dikes of this suite (New York, Ontario, and Québec) average δ 18 O(Zrn) = 8.2 ± 0.6‰, with a typical igneous range of 8.6 to 10.3‰ for δ 18 O magma values. High δ 18 O values in the Frontenac terrane are some of the highest magmatic oxygen-isotope ratios recognized worldwide, but these plutons are not unusual with respect to whole-rock chemistry or radiogenic isotope compositions. Such high δ 18 O values can result from mixing between paragneiss (δ 18 O ≈ 15‰) and hydrothermally altered basalts and/or oceanic sediments (δ 18 O ≈ 12‰) in the source region. We propose that high-δ 18 O, hydrothermally altered basalts and sediments were subducted or underthrust to the base of the Frontenac terrane during closure of an ocean basin between the Frontenac terrane and the Adirondack Highlands at or prior to 1.2 Ga.

Many vertical sections across deeper parts of collisional orogens contain gently dipping thrusts that have the geometric style of shallow-crustal, dip-slip faults. In the southwestern Grenville Province (central and southeastern Ontario), however, structural geologists have found it difficult to document northwest-directed thrusting within large, polydeformed complexes of medium- to high-grade metamorphic rocks. This applies, in particular, to the Mazinaw domain of the eastern Composite Arc Belt, where narrow north- to east-northeast-striking (“longitudinal”) zones of high strain have been linked tentatively to ductile thrusting at the sole of Elzevirian (1230–1190 Ma) or Early Ottawan (1085–1015 Ma) fold nappes. During subsequent northwest-southeast shortening, the apparent thrusts behaved as structural detachment horizons or were subhorizontally folded, together with their wallrocks. Gravity modeling and reflection-seismic profiling have shown that in the present shallow crust most tabular rock bodies and coplanar structural surfaces (including high-strain zones and ductile faults) are horizontal or dip <45°SE. This seems incompatible with the idea that Elzevirian and Early Ottawan thrusts acquired strong curvature in the northwest-southeast vertical plane during upright buckling of fold nappes and other southeast-dipping lithotectonic units. The longitudinal high-strain zones and their ductilely deformed wallrocks are characterized by steeply inclined to vertical planar fabrics (S) such as geometric systems of strained mineral constituents, pebbles, and volcanic fragments. Linear fabrics (L) coexist with the planar fabrics at many localities, but are commonly weak or seemingly absent. The prevalence of planar fabrics is difficult to explain by shallow-crustal-style ductile thrusting, but these fabrics may have been produced by large-scale strains associated with the formation of midcrustal stretching thrusts. For a variety of thrust-dip options, we use L < S fabrics to derive the local sense of tangential shear strain within and adjacent to six high-strain zones. The sense of accumulated shear strain proves to be reverse for most realistic options, even if one presumes that the zones rotated during superimposed buckle folding. Mesoscopic Z-folds are common in one high-strain zone, and attest to dextral-reverse shearing and shortening of thrust walls during Mid- to Late Ottawan deformation (1015–980 Ma). The combined results of structural measurement and modeling of shear-strain sense suggest that the high-strain zones and their walls were stretched or shortened progressively in the direction of simultaneous tangential shearing and/or irrotational slip. This means that the longitudinal dislocations of the Mazinaw domain qualify as stretching thrusts that were active in a midcrustal regime of northwest-directed distributed shearing, northwest-southeast pervasive shortening, and associated vertical thickening.

The 1.4 Ga volcanic-plutonic arc-related Bondy gneiss complex in the western Grenville Province of Québec, Canada, comprises a Cu-Au-Fe oxides hydrothermal system characterized by a series of showings among aluminous and magnesian litho-facies, such as tourmalinite, aluminous sillimanite-bearing gneiss, garnetite, and cordierite-orthopyroxene white gneiss. The latter has a mineral assemblage and a major element chemistry comparable to those of cordierite-orthoamphibole gneiss found in alteration pipes associated with volcanogenic massive sulfide (VMS) deposits. Supporting evidence for a hydrothermal origin for the Bondy cordierite-orthopyrox-ene gneiss is given by mass-balance calculations that record a loss in Ca and gain in Mg, K, and Si with respect to the adjacent nonaltered rhyolitic protolith, compatible with feldspar destruction, sericitization, and chloritization. These lithofacies present birdwing-shaped rare earth element (REE) profiles, reflecting a mass change of REE and high field strength elements (HFSE) involving relative leaching of light rare earth elements (LREE) and enrichment in heavy rare earth elements (HREE), with a general calculated mass increase from Gd to Yb. These trace element characteristics differ from those of known, Cl-driven, VMS-associated hydrothermal cordierite-orthopyroxene or orthoamphibole rocks, but share similarities with F-driven hydrothermal signatures. The decoupling between major element and trace element signatures suggest that the type of fluids driving the postvolcanic or premetamorphic hydrothermal alteration and mineralization event in the Bondy gneiss complex evolved through time from seawater related to orthomagmatic.

We report sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages from high-grade gneisses of the Bondy gneiss complex, a volcano-plutonic arc and back-arc edifice hosting a Cu-Au-Fe oxides hydrothermal system in the Central Metasedimentary Belt, Grenville Province, Québec. Samples of quartzofeldspathic gneiss gave broadly similar results, with zircon cores indicating ages between 1.59 and 1.21 Ga and mantles or whole new zircon crystals giving Grenvillian metamorphic ages. A few analyses indicate minor involvement of Paleoproterozoic and Archean crustal material. Zircon cores are interpreted to have grown during crystallization of the quartzofeldspathic gneiss protoliths, but precise ages could not be determined for any of the samples. The spread of ages is attributed mainly to isotopic disturbance owing to hydrothermal alteration, high-pressure-temperature ( P-T ) metamorphism, and/or recent Pb loss. Only a sample of tonalitic gneiss yielded a well-defined, igneous crystallization age of 1386 ± 10 Ma. Younger zircons from the dated lithologies provide evidence for two episodes of Grenvillian metamorphism: a period of high- P-T granulite-facies metamorphism and partial melting at 1.21–1.18 Ga, and a younger, albeit localized, metamorphic overprint at ca. 1.15–1.13 Ga. The new SHRIMP U-Pb data indicate that the composite volcano-plutonic edifice likely formed, at least in part, at ca. 1.39 Ga. Together with recently published geochemical and Nd data, the new data (1) extend the known distribution of ca. 1.39 Ga arc-related magmatism in the Grenville Province, and (2) suggest that a major portion of the source region to the Bondy gneiss complex was produced by the addition of voluminous juvenile Mesoproterozoic material.

Zircon grains extracted from anatectites in the New Russia gneiss complex have been dated to establish the timing of very high-temperature metamorphism in the northeastern Adirondack Highlands, southwestern Grenville Province. The isotopic and chemical systematics of the zircons studied indicate that (1) zircon grains extracted from anatectic segregations in metagabbro are isotopically homogeneous, implying that only zircon that grew during anatexis is present, and (2) zircons from an anatectic charnockite are complex and preserve evidence of a multistage thermal evolution for the gneiss. This complex history can be deciphered by combining knowledge acquired from backscatter electron images and electron microprobe chemical analyses to guide selection of grain components for isotopic dilution thermal ionization mass spectrometry (ID-TIMS) U-Pb geochronology. The analysis indicates that melt from the metagabbro crystallized at 1054 ± 0.4 (2σ) Ma at T = 915 ± 50 °C. Zircon from the charnockitic gneiss yielded ages that date an inherited component at 1143 ± 2 Ma ( 207 Pb/ 206 Pb); the emplacement of the protolith at 1112 ± 2 Ma; and the crystallization of the anatectic melt at 1040 ± 1 Ma ( T ≥ 825 ± 50 °C). Late, retrograde metamorphic zircon yielded a range of ages including a concordant age at 1003 ± 4 Ma. The method described allows precise dating of anatexis that typically marks the metamorphic peak in granulite-facies terranes and so can be used to constrain model pressure-temperature-time ( P-T-t ) paths. The timing of anatexis and a previously reported age of 1026 Ma for andraditic garnets from skarn adjacent to the Marcy meta-anorthosite establish rapid, nearly isobaric cooling from 915–750 °C in ∼30 m.y.