Central Gneiss Belt Research Articles

Coeval migmatites and granulites, Muskoka domain, southwestern Grenville Province, Ontario

We present new field observations and petrologic and geochronological data from the Muskoka domain in the southwestern Grenville Province of Ontario in an attempt to constrain the relationship between amphibolite-facies and granulite-facies gneisses in areas of transitional metamorphic grade, and to examine their implication for tectonometamorphic models for the Grenville Province of Ontario. The predominant medium-grained amphibolite-facies migmatitic orthogneisses of the Muskoka domain contain several generations of leucosome, some of which are related to southeast-directed extensional structures. The amphibolite-facies granitoid gneisses contain numerous mafic enclaves with granulite-facies assemblages recrystallized from anhydrous precursors during Grenvillian metamorphism. Other associated granulites are characterized by their patchy occurrence and gradational contacts, similar to the charnockites in southern India. Patchy granulites, leucocratic vein networks in mafic enclaves, and crosscutting leucocratic granulite veins are interpreted to have formed as a result of local differences in reaction sequences and (or) fluid compositions. The U–Pb zircon lower intercept age of the patchy granulites overlaps with the previously determined range of 1080–1060 Ma for high-grade metamorphism in the Muskoka domain, while zircon and titanite from a crosscutting granulite vein crystallized at about 1065–1045 Ma, supporting a Grenvillian age for granulite formation. Peak metamorphic conditions of 750–850°C and 10–11.5 kbar (1 kbar = 100 MPa) were determined from the mafic enclaves, whereas the more felsic migmatites reequilibrated at somewhat lower temperatures. The high temperatures caused extensive migmatization and facilitated rheological weakening of the Muskoka domain 10–25 million years after the start of the Ottawan orogeny in the Central Gneiss Belt.

Amphibolites of the Shawanaga domain, Central Gneiss Belt, Grenville Province, Ontario: tectonic setting and implications for relations between the Central Gneiss Belt and Midcontinental USA

The Lighthouse gneiss association is an amphibolite-dominated assemblage situated in the allochthonous Shawanaga domain of the Central Gneiss Belt (CGB), Grenville Province. It consists of two units in which the amphibolites differ in chemical composition and associated lithologies. The upper unit consists of: (i) thick intervals of amphibolite interpreted as mafic flows and which form about half of the unit; (ii) a varied assemblage of thinly interlayered and laminated amphibolite and metasediment interpreted to have originated as mafic tuffs interlayered with sediment with a quartzofeldspathic source; and (iii) thick horizons of semipelite or regularly interlayered psammites and pelites interpreted to be metamorphosed greywacke-shale turbidites shed from a quartzofeldspathic source. The lower unit has a much higher proportion of thick amphibolite interpreted as mafic flows and the metasediments are semipelitic. Geochemically, both upper and lower amphibolites are olivine-normative tholeiites but have distinct trace element characteristics. The mantle-normalized trace element pattern of amphibolite in the upper unit lacks a subduction imprint and suggests an asthenospheric source, but that of the lower amphibolite has a subduction imprint (or an indication of eruption through attenuated continental crust). A back-arc environment is most consistent with the chemistry of the amphibolites and the nature of the associated metasediments. The recognition of the rocks of the Lighthouse gneiss association as juvenile material and their juxtaposition with another juvenile member of the Shawanaga domain, the metarhyolite-rich Sand Bay gneiss association, leads to a refinement of current models for the tectonic evolution of the CGB before the ≈1200–1000 Ma Grenville orogenic cycle. We also suggest that all elements of the Proterozoic geology of Midcontinental USA are present, although jumbled by Grenvillian tectonism, in the CGB. These elements include units interpreted as oceanward-younging juvenile arcs that are overlain by a granite–rhyolite layer and pierced by coeval plutons.

Corona microstructures between olivine and plagioclase of the Chenaux gabbro in the Grenville Province, Ontario.

A corona is a small-scale structure developed around a mineral grain by reaction with its surroundings. It could indicate a change in chemical and/or physical factors during a prograde or retrograde metamorphic event, or associated with cooling from igneous temperatures (Kretz, 1994). The olivine-plagiocalse corona is one of distinct and most common examples of them in metamorphosed rocks.In Ontario, the Grenville Orogen comprises the Central Gneiss Belt and the structurally overlying Central Metasedimentary Belt, divided into a northeastern Elzevir terrane and a southeastern Frontenac terrane. The Chenaux gabbro intrusion is one of plutons in the Raglan gabbro belt at the base of the Elzevir terrane hanging wall (Pehrsson et al., 1996) (Fig.1). It was emplaced during the late stage of the regional metamorphism and deformation, culminated at ca.1.1 Ga.The Chenaux Gabbro is partly deformed and recrystallized on the amphibolite facies condition. Intrusive contacts between the gabbro and surrounding carbonate rocks are observed at various places (Fig.2 (1)). It has occasionally igneous layering (Fig.2 (2)). The principal constituents of the gabbro are plagiolclase, olivine, clinopyroxene, orthopyroxene and calcic amphiboles, with small amounts of biotite, spinel and Fe-Ti oxides.

Crustal architecture and tectonic assembly of the Central Gneiss Belt, southwestern Grenville Province, Canada: a new interpretation

The Central Gneiss Belt, southwestern Grenville Province, is characterized by parautochthonous crust in the north and allochthonous lithotectonic domains in the south. Despite nearly two decades of study, the basal décollement to allochthonous domains transported from the southeast, known as the allochthon boundary thrust, has not been precisely located throughout much of the belt. Between Lake Nipissing and Georgian Bay where its surface trace is known, it separates 1.24 Ga Sudbury metadiabase in the footwall from eclogite remnants and 1.17-1.15 Ga coronitic olivine metagabbro confined to its hanging wall. On the premise that this relationship can be used to trace the allochthon boundary thrust elsewhere in the Central Gneiss Belt, we have sought to extend the known distribution of these mafic rock types, making use of field, petrographic, and geochemical criteria to identify them. New occurrences of all three mafic types are identified in a region extending from south of Lake Nipissing to western Quebec, and the mutually exclusive pattern of occurrence is maintained within this region. Structural trends and reconnaissance mapping of high-strain zones that appear to represent a structural barrier to the mafic suites suggest that the allochthon boundary thrust lies well to the north of its previously suggested location. Our preferred surface trace for it passes around the southern end of the Powassan batholith and through the town of North Bay before turning east to join up with the Lac Watson shear zone in western Quebec. This suggests that a large segment of "parautochthonous" crust lying north of, and including, the Algonquin domain is in fact allochthonous. The mutually exclusive distribution of the mafic suites points to significant separation of allochthonous and parautochthonous components prior to the Grenvillian orogeny, in accord with models of pre-Grenvillian continental rifting proposed by others. Despite a relative abundance of geological and geochronological data for the Central Gneiss Belt and a mafic rock distribution that appears to successfully locate a major tectonic boundary, we emphasize the need for additional field and laboratory work aimed at testing our structural model.

GENESIS OF CORDIERITE - GEDRITE GNEISSES, CENTRAL METASEDIMENTARY BELT BOUNDARY THRUST ZONE, GRENVILLE PROVINCE, ONTARIO, CANADA

Cordierite- and gedrite-bearing lithologies crop out along the base of the Central Metasedimentary Belt boundary thrust zone (CMBbtz) in Ontario, the boundary between the Central Metasedimentary Belt and the Central Gneiss Belt. It has been proposed that these rocks represent metamorphosed and partially melted Al- and Mg-rich sedimentary rocks. Whole-rock oxygen isotope values (δ18O in the range 6–8‰) from these rocks are not consistent with closed-system metamorphism of either soils or pelitic sediments, and cannot be explained solely by extraction of partial melt from pelitic rocks. The whole-rock δ18O and chemical composition of cordierite–gedrite rocks in the CMBbtz indicate that these rocks represent volcanic rocks hydrothermally altered by seawater. After hydrothermal alteration, these rocks were metamorphosed to the upper amphibolite facies, stabilizing the cordierite + gedrite assemblages. Diffusion modeling of quartz–garnet fractionations suggests intermineral exchange of oxygen in a closed system at moderate fugacity of H2O during slow cooling after regional metamorphism. We believe that cordierite–gedrite lithologies in the CMBbtz mark the site of Mid-Proterozoic rifting at the margin of Laurentia.

GRENVILLIAN METAMORPHISM OF MONOCYCLIC ROCKS, GEORGIAN BAY, ONTARIO, CANADA: IMPLICATIONS FOR CONVERGENCE HISTORY

The Parry Sound and Shawanaga domains of the Central Gneiss Belt along Georgian Bay, Ontario, contain monocyclic rocks that originated at or near the southeastern margin of Laurentia between ca. 1450 and 1120 Ma. Their deformation and metamorphism are entirely attributable to Grenvillian orogenesis. Metamorphic assemblages, fabrics, and P–T–t paths from these rocks therefore provide important constraints on Grenvillian thermal and tectonic history. Rocks in the interior and basal Parry Sound assemblages of the northern Parry Sound domain were metamorphosed to granulite-facies conditions during an early phase of Grenvillian tectonism, i.e. , at ca. 1161 and 1163 Ma, respectively. The most likely setting for high-P – high-T granulite-facies metamorphism in the interior Parry Sound assemblage was at or near the base of crust that was underplated by voluminous mafic magma. In contrast, heat advected from anorthosite could account for intermediate-P granulite-facies metamorphism in the basal Parry Sound assemblage. In the upper part of the basal Parry Sound assemblage, retrogression from the granulite to upper-amphibolite facies likely occurred in response to thrust emplacement of the interior Parry Sound assemblage onto this part of the basal Parry Sound assemblage at ca. 1159 Ma. In the lower part of the basal Parry Sound assemblage, thrust deformation and re-equilibration at lower-amphibolite-facies conditions took place at ca. 1120 Ma. In the southern Parry Sound domain, a highly attenuated sequence of quartzites and mafic rocks, deposited some time after 1140–1120 Ma, was affected by upper-amphibolite-facies metamorphism before or at 1080 Ma. Northwest of and structurally below the Parry Sound domain, rocks in the Shawanaga domain were affected by eclogite-facies metamorphism at ca. 1090–1085 Ma, suggesting deep burial or partial subduction of the Laurentian margin beneath the Central Metasedimentary Belt at this time. Widespread upper-amphibolite-facies metamorphism after ca. 1080 Ma was associated with a major phase of northwest-directed thrusting and crustal thickening. Sillimanite-grade conditions in the Shawanaga domain were maintained until at least ca. 1020 Ma, the time of major extensional deformation. Data on metamorphic grade and age are consistent with progressive northwest-directed juxtaposition of lithotectonic assemblages metamorphosed at progressively later times, and may thus record progressive or multistage convergence at the southeastern margin of Laurentia. Evidence for multiple phases of Grenvillian high-grade metamorphism in the Parry Sound and Shawanaga domains suggests that construction and interpretation of P–T–t paths from these rocks require careful assessment of timing and overprinting relationships. Derived P–T–t paths suggest that exhumation of interior Parry Sound granulites resulted from thrusting soon after peak metamorphism, whereas those for rocks of the Shawanaga domain suggest that exhumation was likely associated with both thrusting and extension.

SW Grenville Province, Canada: the case against post–1.4 Ga accretionary tectonics

Seven accretionary sutures, formed between 1.16 and 1.03 Ga, have been identified by different authors in the Ontario–Quebec–Adirondack (OQA) segment of the Mesoproterozoic Grenville orogen in Canada. With one exception, the inferred accretionary terrane boundaries lie within, or at the margins of the Central Metasedimentary Belt (CMB), located between the Central Gneiss Belt and the Adirondack Highlands (Central Granulite Terrane). However, geological, geochronological, and petrological data suggest that the Grenville orogen on both sides of the proposed terrane boundaries (sutures) preserves a common 1.4–1.03 Ga tectonomagmatic history, inconsistent with its origin as a post-1.4 Ga collage of exotic tectonic blocks. Features which straddle the proposed 1.16–1.03 Ga ‘sutures’, from the Central Gneiss Belt, via the Adirondack Highlands, to the Mauricie area, include: (1) Mesoproterozoic continental crust (1.5–1.4 Ga) forming the host and/or basement to younger magmatic and supracrustal suites. (2) A 1.35–1.3 Ga continental arc, remnants of which occur from the CMB boundary zone (CMBBZ) in Ontario to the Appalachians in the United States, built on the 1.5–1.4 Ga continental crust. (3) Intrusions of 1.17–1.13 Ga age in the Central Gneiss Belt (mafic suite), and the Adirondack Highlands and their Quebec extension (AMCG suite, i.e. anorthosite massifs and related granitoids). (4) Relics of 1.18–1.14 Ga sedimentary basins in the northwestern CMB and the Mauricie area. We propose that an alternative model can adequately account for the observed geology of this part of the Grenville orogen wherein, the rocks of the OQA segment were part of an Andean-type margin between 1.4 and 1.2 Ga. At 1.35–1.3 Ga, a continental magmatic arc was built upon the southeastern margin of Laurentia represented by the 1.5–1.4 Ga Mesoproterozoic continental crust. The arc split at 1.3 Ga forming an ensialic back arc basin, relics of which now occur in the northwestern part of the CMB, and the back arc basin was flanked to the southeast by an active 1.28–1.25 Ga arc. Collision between the Laurentian margin and another continent (Amazonia?) occurred at 1.2 Ga, resulting in closure of the back arc basin and initiation of thrusting along the CMBBZ. Post-collisional lithospheric shortening led to convective removal of thickened subcontinental lithosphere, upper mantle melting, and extension of the overlying crust, resulting in widespread magmatic activity at 1.17–1.13 Ga, including emplacement of the AMCG massifs. Crustal extension generated sedimentary basins now represented by the St Boniface sediments in the Mauricie area (1.18 to between 1.15 and 1.09 Ga), and the penecontemporaneous Flinton Group in the northwestern CMB. Renewed, post-collisional, granulite facies shortening commenced at 1.12 Ga, manifested as nappes in the Central Gneiss Belt, and thrusting in the Mauricie area. Continued post-collisional shortening at 1.08–1.05 Ga was more localised, resulting in reactivation of thrusting in the CMBBZ, and initiation of the kinematically compatible Tawachiche shear zone along the eastern border of the Quebec extension of the Adirondack Highlands. The characteristics of the OQA segment of the Grenville orogen can all be accounted for in the context of: (1) a 1.4–1.2 Ga, southeast facing Andean-type margin to a Laurentian upper plate, associated with northwest dipping subduction; (2) continental collision at 1.2 Ga; and (3) subsequent, continued, post-collisional shortening, without invoking accretion of exotic terranes between 1.4 and 1.0 Ga.

Crustal architecture and tectonic assembly of the Central Gneiss Belt, southwestern Grenville Province, Canada: a new interpretation

Crustal architecture and tectonic assembly of the Central Gneiss Belt, southwestern Grenville Province, Canada: a new interpretation

Structure and tectonic evolution of the transitional region between the central Appalachian foreland and interior cratonic basins

The transitional region between the central Appalachian foreland basin and Michigan and Illinois interior cratonic basins is characterized by first and second order structures related to plate convergent processes at the Laurentian plate margin. The first order structures are the Waverly and Cincinnati arches believed to be forebulges associated with the Taconic and Alleghanian orogenies, respectively. The second order structures are basement faults that exhibit a protracted history of recurrent displacements. First and second order structures are linked in that the basement faults have had a major effect in controlling the location and uplift history of the arches. The Waverly and Cincinnati arches share many similarities that suggest a common process of arch development. The Waverly arch developed over the low density rocks in the Central Metasedimentary belt of the Grenville province near its structural contact with the Central Gneiss belt. Uplift on the arch commenced during passive margin development in the Cambrian, but reached its maximum development at the end of the Middle Ordovician Taconic orogeny. Reactivation of faults along the arch continued as late as the Pennsylvanian. The Cincinnati arch as expressed in the Precambrian basement surface is coincident with the underlying Grenville front on a regional scale. The front is a major crustal suture which served to localize and control the history of the Cincinnati arch. Local deviations between the location of the arch and front are explained by the presence of Proterozoic rift basins filled with low density sedimentary rocks, by interactions between the developing Appalachian, Michigan, and Illinois basins, and by a northward decrease in Alleghanian tectonic loading. In a pattern similar to the Waverly arch, the Cincinnati arch began its history with a period of regional uplift as early as the Middle to Late Ordovician followed by maximum arch development in the Pennsylvanian in response to the Alleghanian orogeny. Neither arch showed significant migration following initial development as predicted by general models of forebulge development. We attribute this to the effect of preexisting basement anisotropies, such as lithotectonic boundaries, which served to control the initial location of the forebulge and affect its subsequent history. A third major arch, the Kankakee arch, occurs in the western part of the transitional region and is oriented at right angles to the other arches. Unlike the Waverly and Cincinnati arches, this arch owes its development to subsidence in the adjacent Michigan and Illinois basins rather than orogenic activity in the Appalachians.

Plutonic ages and tectonic setting of the Algonquin and Muskoka allochthons, Central Gneiss Belt, Grenville Province, Ontario

The Central Gneiss Belt comprises parautochthonous gneisses overlain by northwest-transported allochthonous terranes originating from the pre-Grenvillian Laurentian margin or from farther outboard as inferred for the Parry Sound allochthon. In the Huntsville region, orthogneisses of the Algonquin allochthon yielded U-Pb zircon igneous crystallization ages at 1444 +12-8, 1442 +9-8, and 1432 +54-98 Ma. In absence of direct evidence for older intrusions, the association of these plutonic rocks with gneisses giving Nd crustal residence ages of ca. 1.7 Ga sets the Algonquin allochthon apart from the underlying parautochthon, and from the overlying Muskoka allochthon. Orthogneisses from the latter also give zircon igneous ages at 1453 ± 6 Ma and 1427 +16-13 Ma, with no older memory. These ages closely correspond to the Nd model ages of associated gneisses, testifying to the juvenile nature of this terrane. Plutonic ages from the Algonquin and Muskoka allochthons are older than those of the Parry Sound allochthon to the northwest, thereby confirming its exotic nature. The importance, distribution, and nature of 1475-1410 Ma plutonism down structural section from the juvenile Muskoka allochthon, via the more mature Algonquin allochthon, into the parautochthon and Grenville foreland, testify to the development of an ensialic arc along that part of the Laurentian margin tectonically incorporated in the Central Gneiss Belt. Furthermore, the age and structural relationship require Grenvillian break-back thrust reactivation to account for the high structural position of the Muskoka allochthon, which was part of the Laurentian ramp during overthrusting of the younger and farther travelled Parry Sound allochthon.

Timing and thermal influence of late orogenic extension in the lower crust: a UPb geochronological study from the southwest Grenville orogen, Canada

In the southwestern Grenville Province, the Central Gneiss Belt consists of a belt of parautochthonous rocks in the north and a collage of allochthonous lithotectonic domains in the south. Near Pointe-au-Baril, Ontario, the allochthon-parautochthon boundary is marked by the Shawanaga shear zone, a northwest-directed thrust zone between the Britt and Shawanaga domains that was reactivated during ductile, top-to-the-southeast extensional shearing. UPb ages of 1042+4/−2, 1019±4 and 988±2 Ma for granitic pegmatite dykes that are pre-kinematic, late syn-kinematic, and post-kinematic with respect to top-side-down displacement constrain major extensional transport on the Shawanaga shear zone to ca. 1020 Ma. However, nearly all the zircons in the dykes are inherited from a pre-Grenvillian, ca. 1460 Ma granitic host rock to the dykes. Recognition of the inherited nature of these grains comes from the multi-dyke dating approach employed here, and illustrates a potential pitfall in dating shear zone movement using only single dykes interpreted as syn-kinematic. UPb ages of metamorphic titanite from 14 samples collected in a transect across the Shawanaga shear zone span a 93 m.y. segment of concordia. These ages do not correlate with titanite grain size or vary systematically from domain to domain. However, when sample microtexture, strain state, structural position, titanite morphology, and pegmatite UPb data are considered together, the titanite ages can be reasonably inferred to date: 1. (1) regional metamorphism (1049-1045 Ma); 2. (2) cooling below the titanite isotopic closure temperature (∼600°C) during extensional unroofing (1028-1018 Ma); 3. (3) recrystallization-controlled titanite growth and (or) isotopic resetting in high-strain zones (1008-1000 Ma); 4. (4) post-kinematic recrystallization, most likely in the presence of a late fluid phase (967-956 Ma). Each titanite age group is made up of samples from both the Britt and Shawanaga domains, indicating that these processes were not restricted to a single domain but rather occurred within localized regimes, some which were clearly structurally controlled. The 93 m.y. spread of concordant titanite ages within a 20 × 20 km area demonstrates that cooling through isotopic closure is only one of several possibilities to be considered when interpreting metamorphic titanite ages in high-grade orogenic terranes.

A regional paleomagnetic study of lithotectonic domains in the Central Gneiss Belt, Grenville Province, Ontario

We have made a regional paleomagnetic study of lithotectonic domains in the Central Gneiss Belt of the Grenville Province in Ontario along ten N-S and E-W traverses up to 200 km in length. Although originally intended to clarify the tectonic mechanism by which these exotic terranes were assembled and welded to the Archean Superior craton during the ∼1150-Ma Grenvillian orogeny, we actually learned much more about the timing of post-orogenic uplift of the various domains around 1000-900 Ma as they stabilized and became part of the Rodinia supercontinent. The normal (N) and reverse (R) natural remanent magnetizations (NRMs) of all domains, as well as those of reactivated regions flanking the Grenville Front (GF, the Superior-Grenville boundary) to the north and south, have paleomagnetic poles falling on the 980-920-Ma ( 40Ar/ 39Ar calibrated) portion of the Grenville apparent polar wander track for Laurentia. There is a general tendency for paleopoles to young with increasing distance of domains from the GF, implying that more southerly domains were uplifted and magnetized later, but two of the domains do not fit this pattern. Previously reported younging trends away from the GF, based on K/Ar thermochron maps and paleomagnetic ‘zone poles’, are untrustworthy because of hydrothermal alteration, which causes chemical remagnetization and anomalously old K/Ar ages near the GF. Another trend in our data is a regular increase in the R/N ratio with increasing distance south of the GF. In the reactivated zones flanking the GF, NRMs are overwhelmingly of N polarity, whereas well away from the GF, R/N is close to 50:50. Also, NRM intensities and susceptibility values increase 100-fold away from the GF, peaking ≈10 km south of the front, with a pulse-like pattern similar to that documented in anomalously high 40Ar/ 39Ar dates in the same region. Both the magnetic and Ar/Ar results are likely due to a ‘wave’ of hydrothermal alteration and remagnetization during which fluids were driven away from the GF. Since the remagnetized NRM near the GF has approximately the same direction as NRMs in domains away from the GF, hydrothermal activity along fault systems south of the GF must have continued until ∼1000 Ma, long after the collisional orogeny itself.

Plutonic ages and tectonic setting of the Algonquin and Muskoka allochthons, Central Gneiss Belt, Grenville Province, Ontario

Plutonic ages and tectonic setting of the Algonquin and Muskoka allochthons, Central Gneiss Belt, Grenville Province, Ontario

U-Pb Constraints on High-Pressure Metamorphism in the Southwestern Grenville Orogen, Canada

Garnet- and clinopyroxene-rich metabasites with mineralogical and textural evidence of eclogitefacies metamorphism are found in the 1.2-1.0 Ga Grenville orogen in central Ontario and western Quebec, Canada. Field and petrographic relationships indicate that this high-pressure event predated regional granulite- to amphibolite-facies metamorphism. Constraints on the timing of eclogitefacies metamorphism are critical for models of Grenvillian tectonic evolution. The eclogitic metabasites, which are derived from mafic igneous precursors, are found in allochthonous lithotectonic domains of the Central Gneiss Belt (Fig. 1). These metabasites are rare and mainly occur as tectonized bodies within high-strain zones marking domain margins. Identical metabasite occurrences in domain interiors indicate that high-pressure equilibration was not confined to potentially exotic blocks within shear zones, but instead affected a large region of crust (Davidson, 1990). Comparable evidence for eclogite-facies metamorphism has not been documented in the adjacent parautochthon (Fig. 1). The metabasites typically contain some or all of garnet, clinopyroxene, plagioclase, orthopyroxene, amphibole, kyanite, corundum, sapphirine, rutile, and titanite, most of which represent younger metamorphic products after the inferred eclogite-facies assemblage omphacite-garnet-kyanite. Omphacite has only been documented in an eclogitic lense in western Quebec (Indares and Dunning 1997), but augite-plagioclase symplectite in the other bodies is interpreted as a breakdown product of omphacitic clinopyroxene (Grant 1989; Davidson 1990).

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 >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.

Sand Bay gneiss association, Grenville Province, Ontario: a Grenvillian rift- (and -drift) assemblage stranded in the Central Gneiss Belt?

The Sand Bay gneiss association is an allochthonous assemblage of supracrustal rocks in the Central Gneiss Belt of the Grenville Province of Ontario. It comprises a predominantly potassic (pink) quartzofeldspathic migmatite, plagioclase-quartz-biotite schistose gneiss (Dillon schist), amphibolite with calc-silicate horizons and quartzite. Detailed mapping shows a compositionally asymmetric section that may represent original stratigraphy, preserved despite high ductile strains imposed during amphilbolite facies metamorphism. The major- and trace-element geochemistry of the potassic (pink) quartzofeldspathic migmatite and amphibolite are compatible with volcanic protoliths, and suggest they are members of a bimodal suite erupted on rifted continental crust. The potassic quartzofeldspathic migmatite is compositionally similar to rhyolite whereas the amphibolite resembles continental tholeiite. Other members are derived from sedimentary rocks ranging from relatively mature clastic rock (quartzite) to locally-derived, epiclastic sediment (Dillon schist). This assemblage may have been formed during opening of a Grenvillian ocean or in a back arc environment landward of an Andean-type margin. Thrusting during the Grenville orogeny moved the Sand Bay rocks from their original location near the continental margin to the interior of the orogen.

Time of metamorphism beneath the Central Metasedimentary Belt boundary thrust zone, Grenville Orogen, Ontario: accretion at 1080 Ma?

New U–Pb zircon and titanite data from the Muskoka domain, Grenville Province, Ontario, provide protolith and metamorphic ages for the southwestern Central Gneiss Belt. Discordant analyses from a migmatitic orthogneiss and its leucosome form a linear array with an upper intercept age of 1457 ± 6 Ma and a lower intercept age of 1064 ±18 Ma. U–Pb analyses on metamorphic zircon from an amphibolite yield a set of concordant analyses with an average 207Pb/206Pb age of 1079 ± 3 Ma. A weakly migmatitic granitoid rock and a transecting charnockitic vein in the immediate footwall of the Central Metasedimentary Belt boundary thrust zone yielded a discordant array of analyses wth an upper intercept age of 1394 ± 13 Ma and a lower intercept age of 1066 ± 8 Ma. The charnockitic vein yielded concordant zircon ages of 1077 ± 2 Ma. The upper intercept ages are interpreted in terms of protolith crystallization, and the concordant and lower intercept ages as Grenviilian high-grade metamorphism and associated anatexis. We have found no evidence for a ca. 1190–1160 Ma metamorphic event in these rocks, as required by some regional tectonic interpretations. We conclude that emplacement of the Central Metasedimentary Belt over the Central Gneiss Belt, which caused high-grade metamorphism in the Muskoka domain, occurred at or shortly before ca. 1080 Ma, and that this marks the time of accretion of the Central Metasedimentary Belt to the southeast margin of Laurentia.

The Parry Sound domain: a far-travelled allochthon? New evidence from U–Pb zicon geochronology

We report U–Pb zircon ages for metaplutonic and metasedimentary rocks from three lithotectonic assemblages within the Parry Sound allochthon of the Central Gneiss Belt, southwestern Grenville Orogen: the basal Parry Sound, interior Parry Sound, and Twelve Mile Bay assemblages. Magmatic crystallization ages for granitic to tonalitic gneisses from the basal Parry Sound assemblage fall in the range 1400–1330 Ma. Younger intrusions include the Parry Island anorthosite dated at 1163 ± 3 Ma and a crosscutting mafic dyke bracketed between 1151 and 1163 Ma. Dated at [Formula: see text] a tonalitic gneiss from the overlying interior Parry Sound assemblage is slightly younger than the older group of rocks from the basal Parry Sound assemblage. 207Pb/206Pb ages for zircons from a quartzite of the basal Parry Sound assemblage range from 1385 Ma to the Neoarchaean. An absolute maximum age for this quartzite is 1436 ± 17 Ma. In contrast, detrital zircons from a quartzite of the Twelve Mile Bay assemblage constrain the age of deposition at post-ca. 1140–1120 Ma. We speculate that Grenvillian-age zircons within this quartzite were derived from rocks in the Adirondack Highlands and Frontenac terrane, implying that part of the Parry Sound domain and these terranes were contiguous during deposition of the quartzite. Our data support previous interpretations that the Parry Sound domain is allochthonous with respect to its surroundings, and suggest that the most likely source region of the basal Parry Sound domain lies southeast of the Central Gneiss Belt, within the Central Metasedimentary Belt boundary thrust zone or the Adirondack Highlands. This implies the possibility of 100–300 km of displacement of the domain. Emplacement of the Parry Sound domain into its present position must have occurred relatively late in the orogen's history, by about 1080 Ma.

U–Pb geochronology of the Raglan gabbro belt, Central Metasedimentary Belt, Ontario: implications for an ensialic marginal basin in the Grenville Orogen

The Raglan gabbro belt of the Ontario Grenville Orogen is coincident with the top of the Central Metasedimentary Belt boundary thrust zone, a major mid-crustal shear zone separating the Central Gneiss Belt in the footwall from the Central Metasedimentary Belt in the hanging wall. It has been suggested that the gabbros making up the belt are coeval, that they formed in a marginal basin within the Central Metasedimentary Belt, and that they formed a horizon of Theologically stiff material that controlled the localization of the top of the boundary thrust zone during its initiation as the marginal basin closed at ca. 1190 Ma. U–Pb zircon dating of plutons within the Raglan gabbro belt was undertaken to test the coeval nature of intrusions in the belt. Magmatic crystallization ages for three of the gabbros fall in the range 1246–1227 Ma, and a fourth yields a minimum age of ca. 1175 Ma. The results are permissive of a common origin for the gabbros and allow that the Raglan gabbro belt may have been related to the marginal basin, at least with respect to the later stages of its evolution. Inherited 1440–1301 Ma zircons in the gabbros suggest interaction with underlying Central Gneiss Belt crust during magmatism and support an ensialic marginal-basin model, as opposed to an island-arc model, for the evolution of the northwestern part of the Central Metasedimentary Belt.

Tectonic and metamorphic events in the westernmost Grenville Province, central Ontario: new results from high-precision U–Pb zircon geochronology

The U–Pb isotopic ages of metamorphic zircons from deformed amphibolitized metadiabase dykes and associated unstrained anatectic pegmatites have been determined to constrain the times of postdyke ductile flow related recrystallization and deformation. Samples from the Central Gneiss Belt of the Grenville Province in the southern Georgian Bay region provide a means of linking areas formed during common episodes of ductile flow. Three zircon fractions from one attenuated dyke in the Moon River domain gave identical concordant ages of 1062, 1065, and 1064 ± 2 Ma, while those from another gave identical concordant ages of 1052, 1051, and 1050 ± 2 Ma for the time of metamorphism. Zircons from a folded dyke from the Go Home domain gave identical concordant ages of 1049, 1047, and 1046 ± 2 Ma, while a single overgrowth fragment from an unstrained anatectic pegmatite in the fold hinge gave a concordant age of 1047 ± 2 Ma, as if folding, metamorphic zircon growth and postfolding pegmatite are part of the same process. An anatectic pegmatite in a metadiabase boudin neck in the Ahmic domain gave two zircon fractions with identical concordant ages of 1079 and 1080 ± 2 Ma that match the concordant 1078 ± 2 Ma age of a single overgrowth type grain from the host migmatite. Zircons from the metadiabase have similar but less precise ages due to a low level of uranium and a low abundance. U–Pb results for the Love Lake granite situated at the Parry Sound–Ahmic domain boundary are equivocal but may imply folding of this structure after 1068 Ma, the age of contained titanite. Either the upper or lower end of the ca. 1050–1372 Ma zircon discordia may correspond to granite emplacement. Zircons from a dyke that postdates a granulite-grade migmatization in the Parry Sound domain record a metamorphism at 1114 ± 2 Ma but older inherited grains are present in other fractions. Ductile flow in the Go Home and Moon River domains occurred in the interval 1047–1064 Ma during which the latter was probably emplaced, whereas earlier (1080 Ma) ductile flow in the Ahmic domain may relate to the time of overthrusting of the Parry Sound domain. Parry Sound domain dyke metamorphic ages imply a lack of post 1114 Ma migmatization, and a predyke migmatization consistent with a more exotic derivation of this domain. A [Formula: see text] Ma age for a postmigmatite crosscutting pegmatite dyke sets an upper limit on the age of a late buckling event in the Ahmic domain.