Grenville Front Tectonic Zone Research Articles

In-situ Lu-Hf garnet dating of Archean deep crust granulites from the polymetamorphic Grenville Front Tectonic Zone

Abstract Recent advances in geochronological techniques now allow the ability to efficiently decipher the timing and duration of geological processes in complex high-grade polymetamorphosed orogenic terranes. This is the case of the Grenville Front Tectonic Zone, which truncates the Superior Craton to the southeast. The zone exposes parautochtonous Archean rocks that underwent mid- to high-pressure granulite facies metamorphism of uncertain age. The metamorphic assemblages have been either interpreted as Archean and associated with the final stages of the Superior craton assembly, or as the result of Mesoproterozoic Grenvillian metamorphism, based on cross-cutting relationships and traditional geochronology methods such as U-Pb zircon and 40Ar-39Ar mica dating. Herein, we revisit the extent of the Grenvillian metamorphic overprint in the parautochtonous domain and provide new age constraints for granulite-facies metamorphic assemblages through in-situ garnet dating within migmatitic paragneiss, migmatitic orthogneiss, and mafic granulites, combined with in-situ trace element mapping. Six samples, which show bell-shaped and occasionally sharp and oscillatory lutetium growth zoning in garnet, yield garnet Lu-Hf isochrons with identical Archean dates of c. 2.6 Ga. Sparce analyses of material trend toward Grenvillian ages (c. 1 Ga) in one sample from which garnet shows lutetium zoning consistent with post-growth fluid-assisted disturbance. Overall, our results indicate that the widespread granulite-facies metamorphism within the Grenville Front Tectonic Zone is dominantly late Neoarchean in age, unveiling a rare exposure of Archean lower crust in the southeastern Superior Craton. Our results also point towards a limited Grenvillian metamorphic overprint though the spatial extent and precise thermal conditions of this metamorphism are still unknown. The results presented herein demonstrate the potential of in-situ isotopic geochronology on rock-forming minerals like garnet in polymetamorphic terranes.

Unraveling 800 Ma of pegmatite formation in the western Grenville Front tectonic Zone near Sudbury, Canada

The origin of pegmatites has been at the core of petrological research for decades, but despite much effort their origin remains controversial – protracted fractionation of a causative felsic magma versus anatexis of an appropriate protolith. To address this controversy, we present the results of U-Pb analyses (zircon (n = 774), monazite (n = 230), xenotime (n = 295)) for 32 well characterized pegmatites from an extensive swarm of felsic dike rocks outcropping, along a 12 km transect ca. 12 km south of Sudbury (Ontario, Canada) in the westernmost part of the Grenville Front Tectonic Zone (GFTZ). Collectively these rocks exhibit a remarkably large morphological, textural and mineralogical variation related to the crystallization of three distinct pegmatite-forming events that span 800 Ma. The derived concordant U-Pb ages of the dated phases coincide with the late Paleoproterozoic (ca. 1740 Ma) Yavapai orogeny, the Mesoproterozoic (ca. 1450–1430 Ma) Chieflakian event, and the early Neoproterozoic (ca. 1000–970 Ma) Grenville orogeny. Field observations and related geochemical data suggest the two older events are genetically related to regional magmatism (i.e., fractionation of a progenitor), whereas the Neoproterozoic pegmatites were generated via partial melting of an appropriate protolith at depth. The data therefore support a duality of models for pegmatite genesis. We emphasize the important role of imaging (CL and BSE) as an integral part of our protocol, as this revealed widespread coupled dissolution-precipitation (CDP) replacement textures and metamictization in both zircon and xenotime, the latter due to high U contents (i.e., >1000 pm). Interestingly, the Neoproterozoic pegmatites show similar U(±Th), Nb(±Ta) and REE enrichments like their contemporaneous Sveconorwegian counterparts. Many zircons record U-Pb system resetting at ca. 220–200 Ma which is possibly related to the far-field effects of the onset of the Central Atlantic Magmatic Province (CAMP) related to the opening of the Atlantic Ocean. Finally, this study shows that anatexis is a viable process of rare-element pegmatite formation, even in areas rich in potential parental granites, which suggests that anatectic rare-element pegmatites might be globally under-recognized.

Detrital zircon U-Pb geochronology of the Precambrian Middle Run Formation (Eastern North America Basement): Implications for Grenvillian foreland basin evolution and Midcontinent Rifting

The Grenvillian orogeny and formation of the Midcontinent Rift system (MRS) are the primary tectonic events that modified Mesoproterozoic basement of eastern Laurentia prior to Neoproterozoic breakup of Rodinia. Trends of the Grenville Front Tectonic Zone (GFTZ), the cratonward limit of Grenvillian crustal reworking, and the East Continent Rift, an extension of the MRS, appear to converge in the subsurface of central Kentucky and southwestern Ohio. The Middle Run Formation (MRF) is a terrestrial clastic sequence that accumulated in the region of convergence and was interpreted as either a foreland basin or rift-fill deposit. Detrital zircon U-Pb geochronology for MRF feldspathic litharenites from three locations with varying proximity to the GFTZ provides the following constraints on the origin of the MRF and potential GFTZ-MRS interaction. (1) Youngest detrital zircon ages of 1.00 to 1.04 Ga limit the maximum depositional age of the Middle Run Formation to ~1.0 Ga, requiring the MRF to be a latest Grenvillian foreland deposit and not a syn-rift deposit. (2) The abundance of Midcontinent Rift System-age detrital zircon grains attests to the presence of rift-related felsic volcanism in the East Continent Rift and/or recycling of rift-fill sediments from the East Continent Rift into the MRF basin. (3) Detrital zircon age distributions reflect an actively unroofing Grenville orogen as represented by crustal components corresponding to Shawinigan (Geon 11), Elzevirian (Geon 12), and pre-Elzevirian (Geon 13) phases of the Grenville Orogenic Cycle, with significant Eastern Granite-Rhyolite Province basement (Geon 14) input. (4) The presence of only a minor Ottawan (Geon 10) detrital age component indicates that deeper Ottawan crust in the Grenville orogen had not been significantly exhumed. (5) Age modes in four of five samples correlated with the East Continent Rift magmatic interval (1117–1110 Ma) are only slightly older than the early phase of felsic Midcontinent Rift System magmatism (1109–1106 Ma) indicating that magmatism in the East Continent Rift was contemporaneous with the early phase of magmatism in the MRS. The MRF is confirmed to constitute the most proximal region of a Grenvillian clastic wedge that is only preserved or exposed in more distal regions of Laurentia.

Perceiving the Crust in 3‐D: A Model Integrating Geological, Geochemical, and Geophysical Data

AbstractRegional characterization of the continental crust has classically been performed through either geologic mapping, geochemical sampling, or geophysical surveys. Rarely are these techniques fully integrated, due to limits of data coverage, quality, and/or incompatible data sets. We combine geologic observations, geochemical sampling, and geophysical surveys to create a coherent 3‐D geologic model of a 50 × 50 km upper crustal region surrounding the SNOLAB underground physics laboratory in Canada, which includes the Southern Province, the Superior Province, the Sudbury Structure, and the Grenville Front Tectonic Zone. Nine representative aggregate units of exposed lithologies are geologically characterized, geophysically constrained, and probed with 109 rock samples supported by compiled geochemical databases. A detailed study of the lognormal distributions of U and Th abundances and of their correlation permits a bivariate analysis for a robust treatment of the uncertainties. A downloadable 3‐D numerical model of U and Th distribution defines an average heat production of µW/m3, and predicts a contribution of TNU (a Terrestrial Neutrino Unit is one geoneutrino event per 1032 target protons per year) out of a crustal geoneutrino signal of TNU. The relatively high local crust geoneutrino signal together with its large variability strongly restrict the SNO+ capability of experimentally discriminating among BSE compositional models of the mantle. Future work to constrain the crustal heat production and the geoneutrino signal at SNO+ will be inefficient without more detailed geophysical characterization of the 3‐D structure of the heterogeneous Huronian Supergroup, which contributes the largest uncertainty to the calculation.

Timing and anatomy of granitic strain gradients in the Grenville Front tectonic zone, Ontario, Canada

Timing and anatomy of granitic strain gradients in the Grenville Front tectonic zone, Ontario, Canada

Seismites within Ordovician–Silurian carbonates and clastics of Southern Ontario, Canada and implications for intraplate seismicity

A regional survey of Lower Paleozoic sedimentary strata (Ordovician–Silurian) exposed across some 7000km2 of Southern Ontario, Canada has identified the sedimentary record of paleo-seismicity at a time of known faulting and rapid subsidence of Appalachian foreland basins. Shallow marine shoreface sandstones and shales of the Lower Silurian Thorold Formation show laterally extensive beds of large (<2m) synsedimentary ball-and-pillow structures outcropping on the face of the Niagara Escarpment near Hamilton. It is argued that these are seismites produced by ground shaking caused by a moderate intracratonic earthquake. Other comparable deformation structures that can be attributed to seismic activity also occur within sandstone–shale shoreface successions of the Upper Ordovician Georgian Bay, Lower Silurian Grimsby and the dolostone DeCew Formations indicating recurring activity. Later seismic activity is recorded in carbonate strata by deformed rhythmically laminated lagoonal dolostones of the mid-Silurian Eramosa Member of the Guelph Formation consisting of intraformational syndepositional reverse faults and horizons of brecciated laminites indicating compressional stresses and mass flow. Deformation structures are exposed in several quarries and can be correlated in drill cores 15km distant indicating widespread extent. Deformed dolostones are attributed to co-seismic slumping of lagoonal facies from bioherms and are similar to seismites of the same age in closely adjacent parts of the US. These were attributed to reactivation of underlying Precambrian basement structures such as the Grenville Front Tectonic Zone at times of rapid basin subsidence. Paleozoic seismites in Ontario occur close to basement structures that are seismically active at the present day supporting the emerging model for mid-continental seismicity involving reactivation of Precambrian structures.

Earthquake-triggered slumps (1935 Timiskaming M6.2) in Lake Kipawa, Western Quebec Seismic Zone, Canada

The Western Quebec Seismic Zone (WQSZ) of eastern North America is characterised by frequent moderate magnitude intracratonic earthquakes (e.g., 1732, M5.8; 1935, M6.2 and 1944, M5.2). The WQSZ is centered along the Timiskaming and Ottawa-Bonnechere grabens, which form part of a complex aulacogen (St.Lawrence Rift) within the Canadian Shield. The WQSZ includes the urban areas of Montreal and Ottawa but seismic risk analysis is challenged by short instrumental records and long recurrence intervals. The M6.2 1935 Timiskaming Earthquake is the largest recorded to date and was felt over some 1.3 million km 2 of eastern North America with many aftershocks of magnitude 4 to 5. Its epicenter lies below the western margin of Lake Kipawa, Quebec in the area where a major Proterozoic crustal boundary (the Grenville Front Tectonic Zone) crosses the Timiskaming Graben. A high-resolution ‘chirp’ seismic reflection survey of the lateglacial and postglacial sediment infill of Lake Kipawa reveals a clear record of recent ground shaking that is attributed to the 1935 earthquake. Widespread large slumps record down slope failure of the lateglacial and postglacial sediment fill indicating that the 1935 temblor was the largest in this area since deglaciation some 8000 years ago. Systematic mapping of landslides shows that they extend across an area of 600 km 2 around the earthquake's epicenter. Lakes cover a large area of eastern Canada; a regional-scale survey of lake floors could constrain historic epicenters and postglacial seismic history of the heavily populated WQSZ.

Proterozoic mafic dykes as monitors of HP granulite facies metamorphism in the Grenville Front Tectonic Zone (western Quebec)

Metamorphosed mafic dykes and pods in the Grenville Front Tectonic Zone (GFTZ) are correlated, on the bases of map patterns and geochemical trends, with Paleo- and Mesoproterozoic dykes that cross-cut Archean rocks of the foreland, NW of the Grenville Front. The GFTZ consists of a SE-dipping 30 km thick slab of quartzo-feldspathic granulites and biotite–garnet (±opx) gneisses overriding, along the McLaurin thrust, a 4 km thick zone of muscovite–sillimanite micaschists that abuts on the Grenville Front. The granulite slab in turn dips under amphibolite-grade migmatites of the Réservoir Dozois terrane along the Dorval detachment. According to published U–Pb and new U–Th–Pb ages on monazite, the granulites, the gneisses and the schists of the GFTZ were metamorphosed at ca. 2.7–2.8 Ga and reworked at ca. 1.0 Ga (Grenvillian). Proterozoic mafic dykes and pods cross-cutting the GFTZ are thus monocyclic, i.e. they went through the Grenvillian metamorphic event only. Several of these metamorphosed mafic dykes and pods have preserved evidence of emplacement in a brittle regime such as intersection of the gneissic foliation, chilled margins, magmatic layering, this in spite of a strong metamorphic overprint attested by coronitic fabrics. In particular, metamorphosed mafic dykes of the granulite slab, contain large pyroxenes rimmed by garnet coronas with minute quartz inclusions, suggesting the reaction pl + opx = grt + qtz. These garnet coronas, in turn, are locally replaced by symplectites of opx, hbl, pl and qtz, probably formed through back-reactions grt + cpx + qtz = pl + opx and grt + cpx = hbl + qtz. Thermobarometric calculations using the above assemblages indicate peak equilibration at about 1.2–1.5 GPa at temperatures of about 800 °C followed by a quasi-isothermal decompression down to 0.9 GPa at temperatures around 700 °C. Proterozoic mafic dykes thus record a Grenvillian HP granulite metamorphism resulting from burial of upper crustal levels down to sub-Moho depths with concomitant heating. This event was followed by rapid exhumation of the GFTZ zone probably controlled by normal displacement along the Dorval detachment.

U–Pb evidence for polymetamorphic history of Huronian rocks within the Grenville front tectonic zone east of Sudbury, Ontario, Canada

This paper presents U–Pb geochronological data for zircon, titanite, monazite, apatite, allanite, and rutile from Street Township east of Sudbury that help unravel two main facets of the geology of the Grenville Front Tectonic Zone (GFTZ). They document the presence of extensive Huronian Supergroup magmatic (ca. 2480–2460 Ma) and sedimentary rocks inside the GFTZ, and highlight the significance of bimodal magmatism in the genesis of the Huronian Supergroup. The data also provide further insights into the complex orogenic evolution of the GFTZ, showing in particular that amphibolite facies metamorphism, migmatization and deformation during Grenvillian compression at 995–980 Ma overprinted, but did not totally erase, the memory of earlier metamorphic events at ca. 1720 and 1470 Ma. From the point of view of U–Pb systematics, the results emphasize the nearly ubiquitous state of isotopic disequilibrium in these rocks. Most of the minerals investigated comprise two or more generations and are generally in a state of partial resetting because of the superimposed effects of younger events. The preservation of pre-Grenvillian ages in titanite and allanite demonstrates that processes related to final Grenvillian collision and exhumation occurred at great speed. Only apatite and to some degree rutile yield ages that are clearly post-tectonic, thereby reflecting the final stages of exhumation and cooling of the region.

00/02425 A process and apparatus for treating solid fuel materials

LITHOPROBE is Canada's national, collaborative, multidisciplinary geoscience project directed toward an enhanced understanding of how the North American continent has evolved. Research in its eight transects, which span the country and geological time, are spearheaded by seismic reflection studies. These, combined with other studies, provide new insight into the varied tectonic processes that have been active in forming the continent. The Archean Kapuskasing Structural Zone, a prominent feature which cuts obliquely across east-west Superior Province structures, has been imaged as a low-angle thrust sheet above a major décollement, along which 55–70 km of crustal shortening has occurred to bring mid-crustal rocks to the surface. The shortening was accomplished by brittle faulting and erosion at levels above 20 km and ductile shear and flow in the lower crust. The GLIMPCE transect in Lake Huron shows a spectacular series of east-dipping crustal-scale reflections that coincide with the Grenville Front Tectonic Zone. A two-stage model involves terrane collision with the cratonic margin in the Early Proterozoic followed by crustal-scale stacking and ramping associated with the Middle Proterozoic Grenvillian orogeny. In Lake Superior, GLIMPCE data reveal the structure of the Middle Proterozoic mid-continent rift that almost splits North America. The Lithoprobe East transect covering the Paleozoic Newfoundland Appalachians reveals three lower crustal. blocks, defined seismically on the basis of reflectivity characteristics imaged along the seismic profiles. Reflective upper crust can be related to geological structures at the surface; the thin-skinned tectonics implied have yet to be reconciled with the block tectonics of the lower crust. Results from the southern Canadian Cordillera show that the Mesozoic-Cenozoic east-west shortening in the eastern and central Cordillera occurred by compressional deformation forming crustal-scale antiforms that overrode a westward thinning cratonic North America; and in the western Cordillera by subduction processes that have tectonically eroded the lower lithosphere of accreted terranes and left underplated sediments and oceanic lithosphere. The reflection Moho has highly variable characteristics as imaged within transects and among different transects. Crustal and Moho reflectivity is caused by a wide range of features, including fault/shear zones, lithologie contacts, compositional layering, fluids in zones of high porosity, and anisotropy.

A seismic-based cross-section of the Grenville Orogen in southern Ontario and western Quebec

A schematic crustal cross-section is presented for the southwestern Grenville Province based on reprocessed Lithoprobe near-vertical incidence seismic reflection data and compiled seismic refraction - wide-angle velocity models interpreted with geological constraints. The schematic crustal architecture of the southwest Grenville Province from southeast to northwest comprises allochthonous crustal elements (Frontenac-Adirondack Belt and Composite Arc Belt) that were assembled prior to ca. 1160 Ma, and then deformed and transported northwest over reworked rocks of pre-Grenvillian Laurentia and the Laurentian margin primarily between 1120 and 980 Ma. Reworked pre-Grenvillian Laurentia and Laurentian margin rocks are interpreted to extend at least 350 km southeast of the Grenville Front beneath all of the Composite Arc Belt. Three major structural boundary zones (the Grenville Front and adjacent Grenville Front Tectonic Zone, the Central Metasedimentary Belt boundary thrust zone, and the Elzevir-Frontenac boundary zone) have been identified across the region of the cross-section based on their prominent geophysical signatures comprising broad zones of southeast-dipping reflections and shallowing of mid-crustal velocity contours by 12-15 km. The structural boundary zones accommodated southeast over northwest crustal stacking at successively earlier times during orogeny (ca. 1010-980 Ma, 1080-1060 Ma, and 1170-1160 Ma, respectively). These shear zones root within an interpreted gently southeast-dipping regional décollement at a depth of 25-30 km corresponding to the top of a high-velocity lower crustal layer.

Pb isotope mapping of differentially uplifted Archean basement: a case study from the Grenville Province, Ontario

Uranium mobility during regional metamorphism typically causes the continental crust to become stratified in the U/Pb ratio, and hence, over time, Pb isotope ratio. This means that Pb isotope analysis of old gneiss terranes might be used as a means of estimating the original burial depth of the crust and its subsequent uplift history. The approach would be useful if other measures of burial depth, such as mineral chemistry, have been erased by younger metamorphic events, a situation found in the NW Grenville Province. To test the usefulness of this approach, whole-rock Pb isotope data were collected for Grenville gneisses in order to investigate their burial-uplift history. Results were compared with Levack gneiss data from Sudbury, immediately north of the study area. Several Grenville gneisses have Pb isotope ratios close to the least radiogenic Levack gneiss signature, believed to be exhumed from >20 km depth. Hence, it is concluded that gneisses immediately SE of the Grenville Front Tectonic Zone represent deeply eroded Archean basement, exhumed in the Grenville orogeny. On the other hand, high Pb isotope ratios near the Archean–Proterozoic suture suggest the preservation of Archean upper crustal rocks in this vicinity, consistent with the burial of the craton margin under an accreted Paleoproterozoic arc. It is concluded that Pb isotope mapping is a useful tool to study the exhumation of Archean crust in highly metamorphosed orogenic belts such as the Grenville Province.

Transect across the northwestern Grenville orogen, Georgian Bay, Ontario: Polystage convergence and extension in the lower orogenic crust

The Grenville orogenic cycle, between ∼ 1190 and 980 Ma, involved accretion of magmatic arcs and/or continental terranes to the Laurentian craton. A transect across the western Central Gneiss Belt, Georgian Bay, Ontario, which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20–30 km, offers some insights on the thermal and mechanical behavior of the lower crust during the development of the Grenville orogen. Prior to Grenvillian metamorphism, this part of Laurentia consisted largely of Meso‐proterozoic (∼ 1450 Ma) granitoid orthogneisses, granulites, and subordinate mafic and supracrustal rocks. Grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed (∼ 1160 Ma) Parry Sound domain over the craton sometime between 1120 Ma and 1080 Ma. This stage of transport was followed by out‐of‐sequence thrusting and further convergence along successively deeper, foreland‐propagating ductile thrust zones. A major episode of extension at ∼ 1020 Ma resulted in southeast directed transport of allochthonous rocks along the midcrustal Shawanaga shear zone. The final stage of convergence involved deformation and metamorphism in the Grenville Front Tectonic Zone at ∼ 1000–980 Ma. Peak metamorphism along most of the transect at 1065–1045 Ma followed initial transport of allochthonous rocks over the craton by 15–35 m.y. Regional cooling, which postdated peak metamorphism by &gt;70 m.y., was probably delayed by the combined effects of late‐stage extension and convergence. Transport of allochthons at least 100 km over the craton was accomplished along a weak, migmatitic decollement; further propagation of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement. Extensional deformation was associated with distributed ductile flow, the formation of regional transverse folds with axes parallel to the stretching direction, and reactivation of the allochthon‐parautochthon thrust boundary as an extensional decollement. The extensional lower crustal flow was likely the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the Central Gneiss Belt.

Crustal velocity structure of the Superior and Grenville provinces of the southeastern Canadian Shield

The 1992 Lithoprobe Abitibi–Grenville Seismic Refraction Experiment was conducted using four profiles across the Grenville and Superior provinces of the southeastern Canadian Shield. Delay-time analysis and tomographic inversion of the data set demonstrate significant lateral and vertical variations in crustal velocities from one terrane to another, with the largest velocity values occurring underneath the Central Gneiss and the Central Metasedimentary belts south of the Grenville Front. The Grenville Front Tectonic Zone is imaged as a southeast-dipping region of anomalous velocity gradients extending to the Moho. The velocity-anomaly maps suggest an Archean crust may extend, horizontally, 140 km beneath the northern Grenville Province. Near-surface velocity anomalies correlate well with the known geology. The most prominent of these is the Sudbury Structure, which is well mapped as a low-velocity basinal structure. The tomography images also suggest underthrusting of the Pontiac and Quetico subprovinces beneath the Abitibi Greenstone Belt. Wide-angle PmP signals, indicate that the Moho varies from a sharp discontinuity south of the Grenville Front to a rather diffuse and flat boundary under the Abitibi Greenstone Belt north of the Grenville Front. A significant crustal thinning near the Grenville Front may indicate post-Grenvillian rebound and (or) the extensional structure of the Ottawa–Bonnechere graben. Crustal thickening resulting from continental collision may explain the tomographic images showing the Moho is 4–5 km deeper south of the Grenville Front.

The Grenville Orogenic Cycle (ca. 1350-1000 Ma): an Adirondack perspective

The Adirondack Mountains are characterized by three major events that took place during the interval ca. 1350-1000 Ma. The earliest of these is the arc-related Elzevirian Orogeny (ca. 1350-1185 Ma) during which substantial volumes of juvenile calc-alkaline crust were added to the Adirondacks as well as to the northwest segment of the Central Metasedimentary Belt. Data from the southwestern United States as well as from Ireland and Baltica indicate that Elzevirian magmatism and orogeny were of global dimensions. Within the southwestern sector of the Grenville Province, the Elzevirian Orogeny culminated at ca. 1185 Ma when accretion of all outboard terranes was completed. Compressional orogeny related to this convergence resulted in overthickened crust and lithosphere which subsequently delaminated giving rise to orogen collapse and AMCG magmatism that swept southeastward from the Frontenac Terrane into the Adirondack Highlands during the interval ca. 1180-1130 Ma. Localized compressional events within neighboring parts of the Grenville Province emphasize the continued existence of contraction during this interval, although crustal extension caused local in sedimentary basins in which were deposited the Flinton and the St. Boniface Groups. The Adirondacks have not yet provided any record of events within the interval ca. 1125-1100 Ma, although there is evidence of contraction elsewhere in the southwestern Grenville Province at that time. At 1100-1090 Ma the northern Adirondack Highlands were invaded by mildly A-type hornblende granites (Hawkeye suite) that are interpreted to be the result of local crustal thinning contemporaneous with rifting and mafic magmatism taking place in the Midcontinent rift. Immediately following, at ca. 1090 Ma, the global-scale continental collision of the Ottawan Orogeny was initiated. Strong convergence, deformation, and metamorphism continued to at least ca. 1070 Ma, and rocks older than this are profoundly affected by this event. During the waning stages of the Ottawan Orogeny, overthickened crust and lithosphere delaminated and the orogen underwent collapse. Large extensional faults such as the Carthage-Colton-Labelle shear zone developed and rapidly exhumed granulite facies rocks in the mobile core of the orogen which centers on the Adirondack-Morin terranes and extends southeastward into the New York-New Jersey Highlands. Extensional faulting along the Carthage-Colton mylonite zone dropped the amphibolite facies Lowlands down to the west and into juxtaposition with granulite facies rocks of the Highlands. UPb cooling ages from garnet, monazite, and titanite exhibit a sufficiently broad spectrum to accommodate an initial rapid rate of rebound-related cooling followed by a slower, erosion-controlled cooling history. During delamination, late- to post-tectonic granites of the Lyon Mt. Gneiss (ca. 1070-1045 Ma) were emplaced. The youngest member of this suite is an undeformed fayalite granite dated at ca. 1045 Ma which crosscuts all older rocks and fabric. High-potassium, post-tectonic granites of similar age are common in other parts of the southwestern Grenville Province. Renewed contraction and metamorphism at ca. 1030 Ma demonstrate that the Ottawan Orogen was still experiencing convergence well after the peak of orogeny. However, most of the manifestations involve reactivation of older thrust faults, including the Grenville Front Tectonic Zone. The intrusion of small bodies of anorthosite at ca. 1015 Ma (i.e., Labrieville) provide further evidence for the emplacement of these rocks within collisional orogens, albeit in their collapsing phase.

Differential stress control on the growth and orientation of flame perthite: A palaeostress-direction indicator

Albite flames result from the replacement of K-feldspar by albite through alkali exchange. In flame perthites from deformed granites within the Grenville Front Tectonic Zone, replacement is driven by retrograde breakdown of plagioclase and hydration of K-feldspar. Within the strain fabric of the host rock, albite flames preferentially form subparallel to the inferred maximum compression direction and also develop at high-stress points such as grain-to-grain contacts. Flame tips are generally parallel to the ‘normal’ perthite crystallographic direction i.e. the Murchison plane, which is the orientation of the plane of minimum crystallographic lattice misfit between the albite and K-feldspar. We propose that the morphology of an albite flame is controlled by the coherent propagation of its tip into the K-feldspar crystal lattice, and that the orientation of the flames with respect to the rock fabric is controlled by the differential stress imposed during metamorphism. This type of replacement requires dry conditions in which there would be inefficient dissolution-reprecipitation along completely incoherent interfaces (normal replacement), and increased strength of feldspar. Under these conditions coherency is maintained to avoid a very large kinetic barrier. The differences in lattice misfit along the Murchison plane between K-feldspar and albite requires either high elastic strain energy (if the fit is coherent), or lattice dislocation energy (if the fit is semi-coherent). Such energy might be expected to inhibit the replacement growth of albite lamellae in a host feldspar. However, compression parallel to the Murchison plane reduces the lattice misfit along the compression direction. This results (1) in an increase in the Helmholtz free energy of the K-feldspar, and (2) in a reduction in the Helmholtz energy of the albite lamellae or of the semi-coherent interface. Thus, albite replacement allows a decrease in net free energy without the destruction of the alumino-silicate framework and is therefore favoured under conditions of high differential stress. Since flame growth is controlled by the imposed differential stress and not by strain in the host rock, flame perthite has potential as a palaeostress-direction indicator.

The velocity structure of the Britt Domain, southwestern Grenville Province, from laboratory and refraction experiments

The Britt Domain in the southwestern Grenville Province of the Canadian Shield is believed to be an exposure of high-grade (upper amphibolite facies) mid-crustal rocks of predominantly granitic and granodioritic composition. A 270 km refraction line was conducted by Lithoprobe across the Britt Domain and the Grenville Front Tectonic Zone to the north in order to determine the deep velocity structure under the region. This data set demonstrates a uniform velocity structure to a depth of 15 km in the central Britt Domain, with near-surface P- and S-wave velocities of 6.15 and 3.55 km/s, respectively, and linear vertical gradients at depth of 0.02 and 0.01 s−1, respectively. The data also show that the Grenville Front Tectonic Zone is strongly anisotropic at shallow depths. To determine the acoustic properties of rocks from the Britt Domain, 80 velocity samples representing different lithologies in this area were measured at confining pressure up to 600 MPa. These studies show that P-wave velocities at 600 MPa range from 6.29 km/s (granitic gneiss) through 6.51 km/s (intermediate gneiss) to 6.90 km/s (mafic rocks), and have an area-weighted mean of 6.36 ± 0.08 km/s. S-wave velocities for selected samples range from 3.54 km/s (paragneiss) through 3.62 km/s (granitic gneiss) to 4.04 km/s (amphibolite). P-wave velocity anisotropy is weak in the granitic rocks (1.1% on average), but stronger in paragneisses and amphibolites from the region (5.2–5.9%). Comparison of laboratory and refraction data suggests that the Britt Domain is granodioritic in composition to mid-crustal levels. The lower crust beneath the Grenville Front Tectonic Zone in the central Britt Domain appears to be composed of diorite at the top, but becomes increasingly mafic toward the Moho.

La zone tectonique du front de Grenville à l'est de Louvicourt, Québec : exhumation de la croûte archéenne pendant l'orogénie grenvillienne

The Grenville Front tectonic zone, East of Louvicourt in Quebec, defines a tectonometamorphic domain marked by the Grenville Front oriented North 50°, that is locally crosscut by a North 30° reverse fault, the Matchi-Manitou Fault. These tectonic accidents separate the central parts of the Superior Province to the northwest from the Grenville Province to the southeast. The Archean age, high-grade polymetamorphic sequences of the Grenville Province consist of paragneisses and migmatitic quartzofeldspathic gneisses associated to coarse-grained, anatectic pegmatitic veins, the whole containing shreds of two pyroxenes-bearing mafic gneisses. The metamorphic disequilibrium textures, the chemical analyses of the mineral phases, and the whole-rock chemistry of these sequences indicate that a strong crustal thickening, marked by the incorporation of shreds of volcanic rocks in the metasedimentary sequences, led to the granulitization of the lithologies and the quartzofeldspathic pegmatitic vein formation by fluid-absent partial melting of the surrounding migmatitic quartzofeldspathic gneisses under 750–825 MPa and 675–745 °C conditions. Subsequently, a higher pressure and slightly lower temperature episode (920–990 MPa and 625–750 °C) is at the origin of a development of secondary coronitic garnet around primary garnet. A late fluid is responsible for final retrograde reequilibrations in all the lithologies. All these successive events probably occurred during the Archean ages. During the Grenvillian orogeny (1.0 Ga), the thrusting movement with a strong sinistral component of the Grenvillian Province onto the Superior Province is characterized: (i) to the east of the Grenville Front, by the exhumation and the rotation of 9–12 km thick sequences of the Grenville Province; (ii) to the west of the Grenville Front, by the development of a 705–845 MPa and 570–605 °C medium-grade metamorphism that overprints the 400–565 MPa and 600–660 °C regional metamorphism of Kenorean age. Both metamorphic episodes define a 1–6 km width band made of biotite and garnet-bearing paragneisses.

North‐west propagation of the Grenville orogen: Grenvillian structure and metamorphism near Key Harbour, Georgian Bay, Ontario, Canada*

Abstract We report structural and metamorphic data from a c. 25‐km transect across the eastern Grenville Front Tectonic Zone (GFTZ) to the Britt domain at the northern end of Georgian Bay near Key Harbour, Ontario. Constrasting Grenvillian structural and fabric elements characterize the eastern GFTZ, northern Britt domain and a narrow Transition Zone between them.Moderately to steeply dipping foliations with strong down‐dip lineations in all three divisions appear to be associated with NW‐directed thrusting. In the Transition Zone and northern Britt domain, early S = L fabrics with steep lineations are overprinted by younger structures (S &gt; L) with shallow, SE‐SSE‐plunging lineations in which sparse, dominantly (but not exclusively) normal‐sense kinematic indicators are recognized. Pressure and temperature estimates from Grenvillian metamorphic assemblages in metadi‐abase indicate that conditions of P ± 12 kbar and T c 800° C were achieved before or during the thrust‐related deformation, with P‐T‐t paths that indicate near‐isothermal decompression to P c. 4 kbar and T c. 700° C. Correlation of fabric elements with points on the P‐T‐t paths suggests that exhumation occurred during two stages, the first associated with thrusting (≥1035 Ma) and the second with extension and thrusting (pre‐1003 Ma).The GFTZ contains steeply to moderately dipping, thrust‐related fabrics and lacks shallow, extensional structures; the latest episode of thrusting in the GFTZ is inferred to have taken place at 990‐980 Ma. The data are interpreted in terms of a tectonic model involving two stages of propagation of the Grenville orogen towards its foreland (≥1035 Ma and ≥980 Ma), with an intervening period of extension, although the tectonic regime probably remained compressional on the scale of the orogen.

40Ar/39Ar traverse — Grenville Front Tectonic Zone to Britt Domain, Grenville Province, Ontario, Canada*

Abstract 40Ar/39Ar data (on hornblende, muscovite and K‐feldspar) are presented for samples from the western Grenville Province taken along a 140‐km traverse from the Grenville Front into the Britt domain. Our interpretation is based on 28 new analyses, synthesized with 20 previously reported from the traverse area.In regions where comparisons are possible, muscovite and (large domain) K‐feldspar apparent ages appear similar (at c. 920–930 Ma), but throughout the traverse, these are c. 60–70 Myr younger than the hornblende ages. The inferred cooling rate over the c. 350–500°C temperature range, c.2°C Myr‐1, is appropriate for exhumation controlled by post‐orogenic erosional unroofing. At the Grenville Front Tectonic Zone (GFTZ) — Britt domain boundary there is a c. 25‐Myr offset in both hornblende and muscovite/K‐feldspar ages. We interpret the lower ages in the Britt domain to reflect variations in crustal thickness and geothermal gradient between the flank and interior of a thick orogen.The argon data from the GFTZ are interpreted in the context of an asymmetric crustal‐scale antiformal structure developed during a late episode of convergence. Hornblende from rocks on either side of the core of the antiform has an apparent age of c. 990 Ma, our estimate of the age of the compressional event. In the west, we infer that these date the short‐lived thermal event associated with the development of the crustal‐scale antiform previously postulated. In the east, the ages reflect the cooling of material brought toward the surface in the flank of the antiform. Hornblendes from the antiform core appear to contain excess radiogenic argon. We suggest that this was the ambient argon in rocks transported from depth that was subsequently trapped when the rocks cooled rapidly.