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 Soun d 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 upperamphibolite facies likely occurred in response to thrust emplacement of the interior Parry Sound assemblage onto this part of t he basal Parry Sound assemblage at ca. 1159 Ma. In the lower part of the basal Parry Sound assemblage, thrust deformation and reequilibration 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-amphibolitefacies 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 thrust ing soon after peak metamorphism, whereas those for rocks of the Shawanaga domain suggest that exhumation was likely associated with both thrusting and extension.


SOMMAIRE
Les domaines de Parry Sound et Shawanaga de la Ceinture gneissique centrale le long de la baie Georgienne, en Ontario, contiennent des roches monocycliques qui ont pris naissance à (ou près de) la marge sud-est de Laurentia entre environ 1450 et 1120 Ma. Leur métamorphisme et déformation sont dûs entièrement à l'orogenèse grenvillienne. Ainsi, les assemblages métamorphiques, les textures et les trajectoires P-T-t provenant de ces roches permettent d'établir des contraintes importantes quant à l'histoire thermique et tectonique grenvillienne. Dans la région nord du domaine de Parry Sound, les roches provenant de l'intérieur et de la base des assemblages de Parry Sound ont été métamorphisées au faciès granulite pendant une phase précoce du tectonisme grenvillien, soit vers 1161 et 1163 Ma, respectivement. Le milieu le plus probable pour le métamorphisme du faciès granulite de pression et température élevées à l'intérieur de l'assemblage de Parry Sound était à (ou près de) la base de la croûte sous-plaquée par des venues volumineuses de magma mafique. D'autre part, la chaleur émise durant la mise en place d'une anorthosite pourrait être la cause du métamorphisme au faciès granulite de pression intermédiaire à la base de l'assemblage de Parry Sound. Dans la partie supérieure de la base de l'assemblage de Parry Sound, la rétrogression du faciès granulite au faciès INTRODUCTION Important advances have been made in understanding the relationships between tectonics and metamorphism within orogenic belts through the use of quantitative pressure -temperature -time (P-T-t) data (e.g., Spear et al. 1984 and the application of one-and two-dimensional thermal-kinematic models (e.g., Oxburgh & Turcotte 1974, England & Richardson 1977, England & Thompson 1984, Ruppel & Hodges 1994, Huerta et al. 1998. However, interactions between thermal and tectonic processes, and their consequences for regional metamorphism, remain poorly understood (Jamieson et al. 1998). In ancient orogenic belts, these problems are compounded by difficulties associated with the construction and interpretation of P-T-t paths, especially in high-grade rocks (e.g., Jamieson 1988, Frost & Chacko 1989, Vernon 1996, and difficulties in understanding exhumed crustal sections, which are typically overprinted by later effects, in terms of fundamental metamorphic processes.
The deeply eroded Grenville orogen of the Canadian Shield offers an excellent opportunity to examine the metamorphic and structural evolution of the lower continental crust during the development of a collisional orogen of Himalayan proportions (e.g., Dewey & Burke 1973, Windley 1986. As a result of late-to post-Grenvillian extensional and erosional unroofing, middle to deep levels of crust are exposed throughout most of the Grenville orogen, as shown by the prevalence of granulite-and upper-amphibolite-facies rocks (e.g., Davidson 1986, Rivers et al. 1989, Anovitz & Essene 1990). In the Ontario segment of the Grenville orogen, metamorphic and geochronological studies have provided thermobarometric and thermochronological constraints on pre-Grenvillian (>1190 Ma) and Grenvillian (ca. 1190-980 Ma) metamorphism. Some authors have presented P-T-t paths that have been interpreted in terms of Grenvillian thermal and tectonic processes (e.g., Anovitz & Chase 1990, Tuccillo et al. 1990, Jamieson et al. 1995. However, the well-documented superposition of multiple pre-Grenvillian and Grenvillian high-grade metamorphic and tectonic events in many parts of the Grenville orogen (e.g., Jamieson et al. 1992, Tuccillo et al. 1992, Ketchum et al. 1994, Culshaw et al. 1997) means that constructing and interpreting P-T-t paths from these rocks requires careful assessment of timing and overprinting relationships, and the degree to which maximum P-T estimates really reflect peak P-T conditions (e.g., Frost & Chacko 1989, Frost et al. 1998, Pattison 1998. Only then can the metamorphic data provide quantitative constraints on the tectonic processes operating during the Grenvillian orogenesis. In this paper, we report metamorphic data from the Parry Sound and Shawanaga domains along the wellexposed eastern shore of Georgian Bay (Figs. 1, 2, and 3). This region contains rocks that were affected only by Grenvillian metamorphism, and therefore potentially provides the best opportunity to study links between metamorphic and tectonic processes during Grenvillian orogenesis. Our metamorphic data, combined with data on timing of metamorphism, are consistent with the progressive northwest-directed juxtaposition of lithotectonic assemblages metamorphosed at progressively later times. However, we also show that derivation of tectonically meaningful P-T-t paths for these high-grade rocks is limited by variable overprinting and partial re-equilibration of mineral assemblages, fabrics, and thermobarometers. The extent of overprinting can be linked to strain, fluid access, and high-temperature residence time.

Regional framework
In the southeastern Canadian Shield (Fig. 1), the Grenville orogen is widely regarded as a Himalayanscale, deeply eroded, convergent orogen developed as a result of collision between the SE-facing Laurentian margin and arc assemblages and continental terranes lying to the southeast (e.g., Rivers et al. 1989, Easton 1992, McLelland et al. 1996. The Ontario segment of the Grenville orogen comprises ( Fig. 1; Wynne-Edwards 1972): (1) the Grenville Front Tectonic Zone, which marks the northwestern boundary of the orogen, (2) the Central Gneiss Belt (CGB), which consists largely of reworked high-grade rocks (pre-1400 Ma) of  Davidson (1984) and Davidson & van Breemen (1988), with minor modifications by Culshaw et al. (1997). Geology of the Parry Sound and Pointe-au-Baril areas is shown in Figures 2 and 3, respectively. Inset map shows location of study area in Grenville orogen of the southeastern Canadian Shield.
the Laurentian craton and younger supracrustal sequences deposited on its margin, and (3) the Central Metasedimentary Belt (CMB) interpreted as a post-1400 Ma composite system of magmatic arcs and marginal basins (e.g., Windley 1989, Easton 1992, Davidson 1995, accreted to or accumulated on the Laurentian FIG. 2. Geological map of (a) the northern Parry Sound and southern Shawanaga domains, and (b) the southern Parry Sound domain along Georgian Bay (after Davidson 1984, Culshaw et al. 1989, Wodicka 1994. The boundaries of the Lighthouse assemblage differ slightly from those of the Lighthouse gneiss association of Culshaw et al. (1989;their Fig. 2). PI: Parry Island anorthosite, W: Whitestone anorthosite, I: Isabella Island granitic gneiss. Location of samples used for thermobarometry is shown.  Krogh 1994). During and following the later stages of convergence, much of the Ontario segment of the Grenville orogen underwent extension (e.g., Carlson et al. 1990, Jamieson et al. 1995, Busch & van der Pluijm 1996, Busch et al. 1996, Ketchum et al. 1998. Whereas the CGB appears to have extended by distributed, subhorizontal ductile flow in an extremely weak lower crust (Culshaw et al. 1997), narrow, steeply dipping, late-to post-orogenic extensional faults characterize the CMB (Carlson et al. 1990, Busch & van der Pluijm 1996, Busch et al. 1996. Contrasts in geological, metamorphic, and structural histories have been used to distinguish a number of lithotectonic units in the CGB (Davidson & Morgan 1981, Davidson et al. 1982, Culshaw et al. 1983, 1988, margin during more than 300 m.y. of tectonic activity. The CMB overlies the CGB along the crustal-scale Central Metasedimentary Belt boundary thrust zone ( Fig. 1; Hanmer & McEachern 1992).
Contractional deformation that culminated in continent-continent collision led to the progressive juxtaposition, internal imbrication, and northwestward translation of Mesoproterozoic allochthonous assemblages of the CGB and CMB, and reworking of Paleoand Mesoproterozoic parautochthonous units of the CGB (e.g., Rivers et al. 1989, Easton 1992, Jamieson et al. 1992, Davidson 1995, Culshaw et al. 1997. Thrust displacement began as early as ca. 1190 Ma (McEachern & van Breemen 1993) and ended at ca. 980 Ma along the Grenville Front Tectonic Zone (e.g., Haggart et al. FIG. 3. Geological map of the Shawanaga domain in the vicinity of Pointe-au-Baril (after , Ketchum 1995 1989,1990,1994). A NW-SE cross-section of the CGB and its bounding shear zones shows a crustal-scale duplex with contrasting structural styles in the upper and lower levels of the crust ( Fig. 4a; Culshaw et al. 1997 (Wodicka et al. 1996), is now structurally overlain by lithotectonic units that have Laurentian affinities (i.e., Moon River and Muskoka domains). The Moon River domain is interpreted to have been emplaced onto the Parry Sound domain along an out-of-sequence ductile thrust zone (Culshaw et al. 1997). The allochthonous rocks of the CGB are separated from the underlying parautochthonous units by a regional décollement, the Shawanaga shear zone ( Fig. 4b; , Ketchum 1995. Most of the tectonic boundaries shown in Figure 4 are interpreted to have originated as NW-directed thrusts, but many of them have been partly or pervasively overprinted by SE-directed extensional fabrics (see below; e.g., Culshaw et al. 1994, Jamieson et al. FIG. 4. (a) Crustal-scale cross-section extending from the Grenville Front to the CMB (Fig. 1), based on geological and structural data and seismic reflection profiles (AGL-31, 32, 33 and GLIMPSE-J; Green et al. 1988, White et al. 1994cf. Culshaw et al. 1997). Abbreviations: PSD: Parry Sound domain, uGH: upper Go Home domain, GFTZ, CMBBZ, and CMB as in Figure 1; foreland corresponds to Southern and Superior Provinces in Figure 1. Circles with dots indicate NE-trending lineation. Details of the construction techniques are given by Culshaw et al. (1997). (b) Schematic cross-section along A-A' of Figure 1 (after Culshaw et al. 1997) showing (1) approximate time of thrusting and extension as determined by U-Pb and 40 Ar/ 39 Ar geochronology for the basal Parry Sound boundary shears and Shawanaga shear zone, (2) distribution of relict eclogite and coronitic metagabbro in allochthonous units, and (3) U-Pb data for granulite-facies metamorphism (boxes with square edges), eclogite-facies metamorphism (lozenge-shaped boxes), and amphibolite-facies metamorphism (boxes with rounded edges) in the study area. Solid lines represent the boundaries between lithotectonic units shown in Figure 1. Arrows show shear sense (normal and thrust), and hooked arrow represents an inferred folded thrust. References for age dates are given in the text.  1995). The superposition of late orogenic extension on the ductile thrust belt is considered to be responsible for the subhorizontal structural attitude and NW-trending folds dominant along Georgian Bay .
In this paper, we focus on allochthonous monocyclic rocks of the Parry Sound and Shawanaga domains, which preserve no evidence for pre-Grenvillian orogenesis (e.g., Wodicka 1994, Ketchum 1995, Culshaw et al. 1997. Their metamorphic mineral assemblages are considered to be entirely of Grenvillian age. Thus, these rocks provide the best available constraints on the thermal and tectonic evolution of this part of the Grenville orogen. The following section summarizes the most important features of these allochthonous domains.
Throughout this paper, we use the term "monocyclic" for units affected only by Grenvillian orogenesis (e.g., Rivers et al. 1989). The term "allochthonous" is used to describe any far-travelled unit, whether or not it is inferred to have originally formed part of Laurentia (e.g., Rivers et al. 1989, Culshaw et al. 1997, Ketchum & Davidson 2000. The term "Grenvillian orogenesis" is used here for events (tectonic, metamorphic, and plutonic) within the range ca. 1190 to 980 Ma (e.g., Rivers et al. 1989, Rivers 1997.

The Parry Sound domain
Geological and geochronological data suggest that the Parry Sound domain (Davidson & Morgan 1981) comprises at least three distinctive lithotectonic assemblages. From highest to lowest structural level, these include the: (1) interior Parry Sound, (2) basal Parry Sound, and (3) Twelve Mile Bay assemblages ( Fig. 2; Culshaw et al. 1989, Wodicka 1994, Wodicka et al. 1996. The interior Parry Sound assemblage  consists of ca. 1400-1300 Ma (van Breemen et al. 1986, Wodicka et al. 1996 granulite-facies metaplutonic rocks ranging from gabbro to granite, with subordinate metasupracrustal rocks including pelitic gneiss, calc-silicate, and marble (Culshaw et al. 1989, Wodicka 1994. The upper part of the basal Parry Sound assemblage  consists of the Whitestone anorthosite (1350 ± 50 Ma; van Breemen et al. 1986) intruded by the 1383 ± 14 Ma Isabella Island granitic gneiss (Wodicka et al. 1996), garnet amphibolite of probable igneous origin, and minor pelitic and quartzofeldspathic gneiss. The lower part of the assemblage comprises ca. 1400-1330 Ma tonalitic to granodioritic orthogneiss, the Parry Island anorthosite intruded at 1163 ± 3 Ma, and post-ca. 1436 Ma quartzite interlayered with pelitic and semipelitic gneiss and schist, quartzofeldspathic gneiss, amphibolite, and marble (Wodicka et al. 1996). In contrast to the uppermost units, these rocks are cut by variably deformed and metamorphosed mafic dykes emplaced at some time between 1163 and 1151 Ma (Wodicka et al. 1996), suggesting that the basal Parry Sound assemblage could be composite (Wodicka 1994, Wodicka et al. 1996. The Twelve Mile Bay assemblage (Wodicka et al. 1996), which structurally underlies the interior Parry Sound assemblage in the south (Fig. 2b), comprises a thin sequence of supracrustal rocks (quartzite, pelitic gneiss, and amphibolite) in tectonic contact with tonalitic to granodioritic orthogneiss and anorthosite cut by mafic dykes. These rocks resemble those of the basal Parry Sound assemblage, but the quartzite is younger (deposited after ca. 1140-1120 Ma; Wodicka et al. 1996).
The Parry Sound domain has clear lithological and age affinities with rocks of the Central Metasedimentary Belt boundary thrust zone and eastern CMB (Wodicka et al. 1996). It is now completely detached from its presumed southeastern source, implying at least 100 km of displacement of the allochthon (Fig. 4a). Field evidence and U-Pb geochronology suggest that the different lithotectonic elements of the Parry Sound domain were assembled early in the convergence history prior to their final emplacement onto the Laurentian craton (Wodicka et al. 1996, Culshaw et al. 1997.

The Shawanaga domain
The Shawanaga domain (Ketchum 1995), which structurally underlies the Parry Sound domain, can be divided into four lithotectonic assemblages (Figs. 2a and 3; Culshaw et al. 1988Culshaw et al. , 1989Culshaw et al. , 1997. In descending structural order, these are the (1) Lighthouse assemblage, (2) Sand Bay assemblage, (3) Ojibway assemblage, and (4) Shawanaga pluton. The uppermost Lighthouse assemblage comprises interlayered pelitic and psammitic gneiss (metagreywacke-shale), quartzofeldspathic gneiss, amphibolite, and calc-silicate ( Fig. 2a). Originally interpreted as belonging to the basal Parry Sound assemblage (Culshaw et al. 1989, Wodicka 1994, the Lighthouse assemblage has been re-assigned to the Shawanaga domain primarily on the basis of the absence of mafic dykes, which contrasts with the overlying units of the basal Parry Sound assemblage (Culshaw & Dostal 1997). The structurally underlying Sand Bay assemblage is characterized by migmatitic quartzofeldspathic gneiss and schist, quartzite, calcareous gneiss, amphibolite, and rare marble ( Fig. 3; Davidson et al. 1982. Single-crystal zircon dating of a quartz-rich paragneiss indicates deposition after ca. 1417 Ma (T. Krogh, unpubl. data). On the basis of field relations and geochemical data, paragneisses of the Sand Bay assemblage are interpreted to mark the opening of a Grenvillian ocean or a marginal basin landward of an Andean-type margin (Culshaw & Dostal 1997). A unit of granodioritic orthogneiss lying above this supracrustal assemblage has been dated at 1346 +69 -39 Ma (U-Pb on zircon; van Breemen et al. 1986). The Ojibway assemblage is dominated by migmatitic granodioritic to tonalitic orthogneiss (1466 ± 11 Ma; T. Krogh, unpubl. data) in which leucosomes become progressively more abundant toward the southeast, close to the contact with the Parry Sound domain. It also contains isolated concordant layers of granitic gneiss and amphibolite, as well as pelitic gneiss, which occurs as enclaves in orthogneiss and in a large exposure of supracrustal rock (Fig. 3) previously mapped as a tectonic window of the Britt domain . The lowermost Shawanaga pluton, a granitic to granodioritic orthogneiss, forms an elongate body between the parautochthonous Britt domain and the Ojibway assemblage (Fig. 3). Dated at ca. 1460 Ma (T. Krogh, unpubl. data), this pluton is similar in age to both the Britt domain and the Ojibway granitoid orthogneisses. The monocyclic rocks of the Shawanaga domain are interpreted to represent the remnants of an active, post-1450 Ma Laurentian margin (Culshaw & Dostal 1997, Culshaw et al. 1997. In the Shawanaga domain, rare layers of garnet-bearing amphibolite interpreted as mafic dykes are confined to the Sand Bay assemblage (Ketchum 1995, Culshaw & Dostal 1997. These dykes are texturally distinct from mafic dykes in both the underlying Britt and overlying Parry Sound domains, and may represent syn-volcanic intrusions (Culshaw & Dostal 1997). Minor mafic intrusions in the Ojibway assemblage and Shawanaga pluton include small podiform bodies of metagabbro, and garnet-clinopyroxene-rich rocks interpreted as retrograded eclogites ( Fig. 4b; Davidson 1990Davidson , 1991. The metagabbro bodies belong to the ca. 1170-1150 Ma (Davidson & van Breemen 1988, van Breemen & Davidson 1990, Heaman & LeCheminant 1993 coronitic olivine metagabbro suite that occurs throughout the CGB, with the exception of the Parry Sound and Britt domains (e.g., Grant 1987, Davidson 1991. Though similar in age, the coronitic metagabbros do not have the same chemical or physical attributes as the 1163-1151 Ma mafic dykes of the basal Parry Sound assemblage (N. Wodicka, unpubl. data, A. Davidson, pers. commun., 1998).

Structure
Rocks throughout the Parry Sound and Shawanaga domains were deformed by penetrative ductile flow during the Grenvillian orogeny. Lithological boundaries are generally parallel to the dominant fabric, a gneissic foliation (dominantly S ≥ L), with a shallow to locally steep, southeast-dipping enveloping surface associated with a southeast-plunging lineation. Although locally overprinted by extensional shear in high-strain zones (see below), the dominant gneissic fabric in both the Parry Sound and Shawanaga domains formed during northwest-directed thrusting, as suggested by sparse kinematic indicators and extension lineations (e.g., Davidson 1984, Wodicka 1994, Ketchum 1995. In the basal Parry Sound assemblage, a northeast-trending lineation is preserved within the Parry Island anorthosite and paragneisses in an adjacent low-strain area (Fig. 4a;Gower 1992, Wodicka 1994. Locally, deformed 1163-1151 Ma mafic dykes, which themselves carry a northeast-plunging lineation, cut at a moderate angle the orogen-parallel fabric in the ca. 1163 Ma Parry Island anorthosite. Orogen-parallel deformation in the basal Parry Sound assemblage thus appears to have been broadly coeval with emplacement of the Parry Island anorthosite and mafic dykes (Wodicka 1994).
Boundaries separating lithotectonic assemblages are distinguished by the presence of kinematic indicators, localized zones of high strain, truncation of structures in adjacent rocks, and contrasts in tectonostratigraphy , Wodicka 1994, Ketchum 1995. The interior Parry Sound -basal Parry Sound contact is a thrust-sense shear zone that coincides with a sharp lithological break (Fig. 4b). The lower boundary of the basal Parry Sound assemblage, i.e., the Parry Sound -Shawanaga contact, is interpreted as a cryptic thrust that is overprinted by a discrete extensional shear zone developed within thick, older tectonites , Wodicka 1994. Geochronological data suggest that displacements along the thrusts were not contemporaneous. The thrust separating the basal Parry Sound assemblage from the overlying interior Parry Sound assemblage has been dated at ca. 1159 Ma (van Breemen et al. 1986). Though not dated directly, thrust displacement along the Parry Sound -Shawanaga contact probably occurred, at least in part, at ca. 1120 Ma (Wodicka 1994).
The allochthonous Shawanaga domain is separated from the parautochthonous Britt domain by the shallowly to moderately dipping upper-amphibolite-facies Shawanaga shear zone (Ketchum 1995). The shear zone coincides with an important seismic reflector that dips gently beneath the Parry Sound domain (White et al. 1994). It displays dominantly top-to-the-southeast extensional kinematic indicators, but there is evidence that the extensional shear zone reactivated an older thrust with opposite vergence , Ketchum 1995, Ketchum et al. 1998). Geochronological constraints indicate that major extensional displacement along the Shawanaga shear zone occurred at ca. 1020 Ma (Ketchum et al. 1998).
Large, recumbent, nappe-like folds in the Shawanaga domain (Culshaw et al. 1989 and isolated, reclined folds in the interior Parry Sound assemblage (Culshaw et al. 1989), and possibly also in the basal Parry Sound assemblage (Gower 1992), are interpreted to have formed during thrust-related deformation (Culshaw et al. , 1997. Extension-related deformation produced regional-scale, upright, north-northwest-trending folds with subhorizontal enveloping surfaces in both the Shawanaga domain and its footwall (Britt domain) ( Fig. 3; e.g., Culshaw et al. , 1997.

PETROGRAPHIC RELATIONS
The mineralogical and textural characteristics of monocyclic rocks from the Parry Sound and Shawanaga domains are outlined below and in Tables 1 and 2. Abbreviations for mineral names are mainly after Kretz (1983).

The Parry Sound domain
Mineral assemblages in the Parry Sound domain document conditions ranging from lower amphibolite to granulite facies (Fig. 5). In the interior Parry Sound assemblage, metamorphism generally occurred in the granulite facies (Anovitz & Essene 1990), although ret-    , Culshaw et al. 1989. Sillimanite is the ubiquitous aluminosilicate in pelitic granulites (N122b, N125b, N130a, and N131a) from the northern interior Parry Sound assemblage (Table 1), although kyanite has been reported from several localities north and east of the study area (Anovitz & Essene 1990, Davidson, pers. commun., 1990. In most pelitic granulites, porphyroclastic fabrics are prevalent (Fig. 6a): thermal peak min-eral phases, including garnet (Alm 45-53 Prp 44-51 Grs 1-4 Sps 1-4 ), perthite, orthopyroxene (En 69 Fs 22 ), and spinel, occur as porphyroclasts wrapped by a foliation defined by aligned crystals of prismatic sillimanite, biotite, and ribbon quartz. Biotite compositions are Mg-rich (0.19 < X Fe < 0.31, i.e., phlogopite; 4-5% TiO 2 ) and are heterogeneous on the scale of a thin section. In most samples, there is no distinct correlation between Fe/(Fe + Mg) and TiO 2 content of the biotite with its color, morphology, or location with respect to garnet in a single section. Recrystallized plagioclase in the matrix is generally sodic (An 10-21 ), and there is no significant compositional variation within individual samples except N130a (An 2-12 ). Sillimanite is either entirely enclosed within garnet porphyroclasts or defines curved trails of inclusions that are continuous with the thrust-related fabric in the matrix. Sillimanite prisms in the matrix are aligned parallel to the extension lineation. Locally, minor retrogression led to secondary growth of biotite, and to the recrystallization of perthitic feldspar into discrete albite and K-feldspar grains.
In mafic gneiss and granulite (N144 and N171) from the northern interior Parry Sound assemblage, garnet (Alm 58-63 Prp 11-12 Grs 20-21 Sps 5-8 ), orthopyroxene, amphibole, clinopyroxene, and plagioclase form porphyroclasts in a fine-grained matrix of plagioclase (An 23-27 ), quartz, amphibole (ferropargasite), orthopyroxene, clinopyroxene, biotite, and ilmenite. In strongly deformed samples, hornblende, plagioclase, and orthopyroxene porphyroclasts are recrystallized to finergrained equivalents along their margins. The alignment of orthopyroxene, clinopyroxene, amphibole, and biotite crystals within the main foliation documents the synmetamorphic nature of the thrust-related fabric. Late alteration is restricted to minor growth of late amphibole around pyroxene, late biotite around and within the cleavage planes of hornblende, and titanite around ilmenite. Overall, the textural relationships outlined above for the pelitic and mafic rocks from the northern interior Parry Sound assemblage suggest that thrust-related deformation occurred under granulite-facies conditions (Table 1), which caused recrystallization of pre-existing assemblages without retrogression. Both rock types also show evidence for minor, late-stage retrogression. In some cases, retrogression of the granulite-facies rocks can be related to the emplacement of late pegmatitic dykes, as indicated by the presence of retrograde haloes adjacent to the dykes.
Rocks exposed in the southern interior Parry Sound assemblage are characterized by upper-amphibolite-facies assemblages (Fig. 5b) and are texturally distinct from rocks in the northern part of the assemblage (Table  1). In the study area, evidence for the retrograded nature of the amphibolite-facies assemblages includes (1) the presence of numerous patches of granulite-facies rocks scattered throughout the predominantly grey to black gneisses, (2) relict clinopyroxene and orthopyroxene grains rimmed by biotite ± hornblende, and (3) the presence of epidote and titanite. Field relationships indicate that extensive hydration and re-equilibration of the granulites occurred in response to fluid infiltration along the boundary of the Parry Sound and Moon River domains (Culshaw et al. 1997). In sample N192a, compositionally homogeneous garnet (Alm 64 Prp 8 Grs 23 Sps 5 ) forms strikingly poikiloblastic grains, with inclusions of quartz, plagioclase, and amphibole. Amphibole (ferropargasite) and plagioclase (An 28 ) also occur as recrystallized grains in the matrix.
Pelitic rocks from the upper part of the basal Parry Sound assemblage are texturally distinct from those from the lower part of the assemblage (Table 1) and are here referred to as Group-I pelitic gneisses. In these gneisses (N75c and N76b), large garnet porphyroclasts (Alm 70-73 Prp 19-24 Grs 4-5 Sps 2-3 ) contain fine-grained inclusions in their core, and fewer but very coarse-grained inclusions in their rim. A second generation of smaller, inclusion-poor, idioblastic garnet crystals is common in the matrix. Biotite is found both as laths that define the matrix foliation (0.52 < X Fe < 0.54; 3.5-4.0% TiO 2 ) and as coarse grains together with quartz and kyanite in the pressure shadows and resorption embayments around garnet porphyroclasts (Fig. 6b). Plagioclase occurs as granoblastic to xenoblastic matrix grains (An 26 ), and locally forms a thin moat around garnet. Kyanite occurs as corroded porphyroclasts aligned parallel to the main matrix foliation, whereas sillimanite defines inclusion trails in the rim of garnet that pass continuously into the matrix foliation (Fig. 6b). As indicated by its highly embayed morphology, kyanite probably ceased crystallizing prior to sillimanite.
Garnet amphibolite (N74c), spatially associated with Group-I pelitic gneiss, contains garnet (Alm 52 Prp 16 Grs 27 Sps 5 ) typically rimmed by plagioclase coronas. The composition of the matrix plagioclase (An 48 ) is uniform at the thin-section scale. Clinopyroxene occurs as xenoblastic porphyroclasts, as inclusions in garnet, and as a matrix phase (Wo 45 En 35 Fs 15 ). Amphibole (magnesiopargasite) variably replaces clinopyroxene. These textural features suggest that the plagioclasehornblende assemblage may be the consequence of the decompression-hydration reaction: (1) (Hicks 1992). For consistency, all reactions are written with the higher-T assemblage on the right-hand side of the equation.
In the lower part of the basal Parry Sound assemblage, granulite-facies assemblages are well preserved only in the core of the mildly deformed Parry Island anorthosite and its cross-cutting mafic dykes (Table 1, Figs. 5a, 6c). Mafic dykes (N153c) contain texturally well-equilibrated orthopyroxene (En 61 Fs 34 ), amphibole (magnesiopargasite), plagioclase (An 53 ), and garnet (Alm 51 Prp 26 Grs 20 Sps 3 ). Peak-grade assemblages in the anorthosite are characterized by secondary orthopyroxene, clinopyroxene, hornblende ± garnet coronas around relict igneous orthopyroxene. Locally, the orogen-parallel, NE-plunging lineation in the anorthosite is defined by the alignment of orthopyroxene crystals. Toward the margin of the anorthosite body, the mafic dykes and the fabric in the anorthosite are transposed into parallelism. Anorthosite is mostly characterized by upper-amphibolite-facies Pl + Hbl ± Scp ± Cpx ± Grt ± Qtz ± Bt ± Ttn ± Cc assemblages that developed synkinematically. Pervasive, thrust-related deformation transformed the mafic dykes within the margin of the anorthosite and within the host rocks to amphibolites (Hbl + Pl ± Grt ± Cpx ± Qtz ± Bt ± Ttn). Similarly, the para-and orthogneisses flanking the anorthosite show penetratively overprinted fabrics and upper-amphibolite-facies assemblages (Fig. 5a). Pelitic gneisses are characterized by Grt + Bt + Qtz + Pl + Kfs + Ky + quartzofeldspathic pods ± Sil ± Rt ± Ilm assemblages, whereas tonalitic and granitic gneisses contain Bt + Pl + Qtz + quartzofeldspathic pods ± Kfs ± Grt ± Hbl ± Cpx ± Ttn assemblages. Rare occurrences of relict metamorphic orthopyroxene in the strongly deformed orthogneisses suggest that the flanking gneisses share the same metamorphic history as the Parry Island anorthosite and mafic dykes. These features combine to indicate that (1) rocks in the lower part of the basal Parry Sound assemblage underwent an earlier granulite-facies metamorphism during orogen-parallel deformation, (2) growth of the upper-amphibolite-facies minerals was synchronous with thrust-related deformation, and (3) the upper-amphibolite-facies para-and orthogneisses were formerly granulite-facies rocks.
Locally, pelitic rocks from the lower part of the basal Parry Sound assemblage contain lower-amphibolite-facies assemblages. Here referred to as Group-II pelitic schists (N59, N73, and N138), these coarsely recrystallized rocks occur directly east of the Parry Island anorthosite (Fig. 5a). In this area, the highest-grade minerals (garnet, K-feldspar, and kyanite) are typically corroded and altered, and the new assemblage comprises garnet, plagioclase, sillimanite, staurolite, and muscovite (Fig. 6d). Locally, garnet porphyroblasts are partially rimmed by intergrowths of sillimanite and biotite. The presence of a staurolite rim around kyanite suggests the following low-temperature reaction: These relationships suggest that the lower-amphibolitefacies assemblages are retrograde and formed at the expense of the upper-amphibolite-facies assemblages. In a manner analogous to pressure shadows with retrograde assemblages adjacent to porphyroblasts, the restriction of the retrograde lower-amphibolite-facies assemblages to a low-strain area adjacent to the Parry Island anorthosite suggests an origin related to a "megapressure-shadow" effect (Wodicka 1994). The Parry Island anorthosite is considered to have behaved as a rigid body during penetrative thrust-related deformation. Group-II rocks in the strain shadow of the anorthosite would have been shielded from maximum compressive stress by their proximity to the rigid anorthosite. Retrogression and coarse recrystallization of these rocks were most likely contemporaneous with both thrust deformation and the related upper-amphibolite-facies metamorphic overprint in the gneisses flanking the anorthosite. The matrix foliation in Group-II pelitic schists is defined by biotite (0.41 < X Fe < 0.44; 2-4% TiO 2 ), sillimanite, graphite, and locally kyanite. Plagioclase occurs either as a matrix phase (An 23-31 ) or as idioblastic porphyroblasts (An 33 ) that overgrow matrix crenulations. Sillimanite outlasted growth of kyanite, but is itself overprinted by muscovite in highly retrograded samples. Garnet forms several morphological populations. In samples N59 and N138, garnet (Alm 76-78 Prp 14-15 Grs 6 Sps 2-3 ) occurs as porphyroblasts up to 2 cm in diameter with few to abundant inclusions in their cores that are either crystallographically oriented or define straight to curved trails of inclusions oriented at a high angle to the external foliation. In contrast, sample N73 contains a second generation of small (ca. 1 mm) and inclusion-free garnet grains (Alm 79 Prp 14 Grs 3 Sps 4 ).
The Twelve Mile Bay assemblage is characterized by mostly upper-amphibolite-facies rocks (Fig. 5b). Pelitic gneisses (N205a) contain the assemblage Grt + Bt + Qtz + Pl + Kfs + Sil + quartzofeldspathic pods, with the notable absence of kyanite. Garnet (Alm 66 Prp 26 Grs 5 Sps 3 ) either occurs as large, embayed poikiloblasts ( Fig. 6e) or as smaller, subidioblastic, and inclusionpoor grains in the matrix. Garnet and feldspar porphyroclasts are wrapped by the matrix foliation defined by aligned prismatic to acicular sillimanite, biotite, and ribbon quartz. Compositions of the matrix biotite show a considerable variation in Fe/(Fe + Mg) (0.40 < X Fe < 0.48). A correlation exists between a high ratio of garnet to biotite (V garnet /V biotite > 1) and a low Fe/(Fe + Mg) value in biotite.
Although most mafic rocks of the Twelve Mile Bay assemblage have been converted to amphibole, these rocks locally preserve granulite-facies assemblages (Table 1). Evidence for retrogression includes the presence of a hornblende ± biotite rim around pyroxene grains. The alignment of hornblende and biotite within the main foliation suggests that growth of these lowertemperature minerals was synchronous with deformation related to thrusting. Amphibolitic rocks (N204b) contain garnet porphyroblasts (Alm 53 Prp 15 Grs 28 Sps 4 ), amphibole (ferropargasite), plagioclase (An 54 ) ± quartz ± biotite ± titanite. The textural features in the pelitic and mafic rocks of the Twelve Mile Bay assemblage combine to indicate upper-amphibolite-facies conditions during thrust-related deformation.

The Shawanaga domain
In contrast to the Parry Sound domain, migmatitic upper-amphibolite-facies assemblages are characteristic of the Shawanaga domain (Figs. 5a, 7), although orthopyroxene is present in coronitic metagabbro and relict eclogite (Table 2). Pelitic gneiss (N7f, N31a, and N43b) from the structurally highest Lighthouse assemblage (Fig. 2a) contains the assemblage Grt + Ky + Kfs + Pl + Bt + quartzofeldspathic pods ± Rt ± Ilm (Table 2, Fig. 8a). Garnet (Alm 76-77 Prp 13-16 Grs 7-8 Sps 2-3 ) forms fractured porphyroclasts with abundant, generally finegrained inclusions in its cores that commonly define straight to weakly sigmoidal trails of inclusions oriented at a high angle to the matrix fabric. Rare inclusions in the rim are of similar size to the matrix. Plagioclase compositions are homogeneous within a given sample (An 28-32 ). The penetrative thrust fabric is defined by coarse quartz-aggregate ribbons, kyanite, and biotite (0.44 < X Fe < 0.46; 3-4% TiO 2 ). These minerals are deformed around the garnet porphyroclasts. These features suggest that the high-pressure phases grew prior to or during thrust-related deformation. Leucosomes of coarse-grained plagioclase + quartz + K-feldspar ± garnet are oriented parallel to the main foliation and are commonly rimmed by kyanite and biotite.
At the Parry Sound -Shawanaga contact (Figs. 2a,  4b), late extensional shearing transformed the pelitic gneisses and amphibolites of the Lighthouse assemblage into mylonites (Wodicka 1994). The pelitic mylonites (N1d, N71b, and N58a) differ from the gneisses by their finer grain-size, the presence of sillimanite, and lower modal proportions of biotite. The latter two features suggest relatively high-temperature conditions during mylonitization. Garnet, kyanite, and perthite occur as highly fractured and disaggregated (i.e., kyanite) porphyroclasts in a mylonitic matrix consisting of recrystallized quartz, K-feldspar, plagioclase, sillimanite, biotite (0.38 < X Fe < 0.46; 4-5% TiO 2 ), and rutile (Fig.  8b). Garnet is almandine-pyrope-rich (Alm 67-73 Prp 21-28 Grs 5 Sps 1 ) with significant zoning from core to rim (from Alm 67 Prp 20 Grs 9 Sps 1 to Alm 77 Prp 17 Grs 5 Sps 1 ) detected in one sample (N71b). Coarse inclusions of plagioclase in the core of the zoned garnet are less sodic (An 33 ) than matrix plagioclase (An 26-27 ). Both kyanite and sillimanite are also present as inclusions in the rim of garnet porphyroclasts, and sillimanite locally defines inclusion trails that are continuous with the matrix foliation. The restriction of sillimanite and of garnet rims with kyanite inclusions to the pelitic mylonites suggests that these phases grew during late extensional deformation. In some places, inclusion trails in garnet cores are abruptly truncated against kyanite, which itself contains inclusion trails oriented parallel to the matrix foliation. This suggests that kyanite formed after growth of garnet cores, possibly as a result of a continuous reaction of the type: Bt + Ky + Qtz = Grt + Kfs + H 2 O Kyanite along garnet margins is deformed, suggesting operation of this reaction before or during extensional shear. Ketchum (1995) observed similar relationships in pelitic rocks within the Shawanaga shear zone. Pelitic mylonites of the Lighthouse assemblage at the Parry Sound -Shawanaga contact lack retrograde muscovite, whereas most pelitic gneisses in the immediate footwall show evidence for minor retrogression of kyanite, garnet, and K-feldspar to muscovite. The coarse laths of muscovite locally form foliation "fish" with an extensional sense of shear. These features are consistent with juxtaposition of cooler basal Parry Sound hanging-wall rocks on hotter Lighthouse footwall rocks along an extensional shear zone. Mylonitized garnet amphibolite (N1c) within the extensional shear zone contains garnet (Alm 58 Prp 17 Grs 21 Sps 4 ), plagioclase, and hornblende porphyroclasts in a fine-grained mylonitic matrix consisting of recrystallized hornblende (ferropargasite) and plagioclase (An 34 ). Garnet porphyroclasts are highly resorbed and typically rimmed by a plagioclase corona.
Although strongly deformed by extensional shear fabrics, all lithologies of the Sand Bay assemblage (Fig. 3) show granoblastic polygonal textures, suggesting widespread recovery and postkinematic growth of the grains. With few exceptions, all units contain abun-dant leucosome hosting porphyroblasts of muscovite, magnetite, amphibole, and clinopyroxene. Paragneiss contains the assemblage Qtz + Kfs + Pl + Bt + Ms + Ep ± Hbl ± Ttn ± Chl ± Rt ± Ilm (Fig. 8c). Plagioclase ranges from fresh to strongly sericitized, with overgrowths of muscovite, calcite, and epidote. Other overgrowth relationships include muscovite on biotite, epidote on allanite, and titanite on ilmenite. In contrast to the Ojibway gneisses, muscovite and chlorite are ubiquitous in the Sand Bay gneisses. The differences in mineral assemblages and textures between the Ojibway and Sand Bay assemblages (Table 2) may be attributed to variations in composition of the bulk rocks or metamorphic fluid (or both).

Summary and interpretation
The mineral assemblages presented above, together with evidence for partial melting (i.e., quartzofeldspathic pods), place first-order constraints on the P-T conditions during thermal peak metamorphism and subsequent overprinting events in the Parry Sound and Shawanaga domains. In the interior Parry Sound assemblage, the presence of orthopyroxene in pelitic granulites (Table 1) suggests that temperature conditions were above the lower-T limit of the dehydration-melting reaction: Bt + Qtz ± Pl = Opx + Kfs + L For typical pelitic rocks, this reaction is crossed at 800°C (e.g., Spear 1993). The local occurrence of the peak assemblage Opx-Sil-Qtz in these granulites (Table 1; Davidson et al. 1990) constrains pressures to more than about 9 kbar at 800°C (e.g., Carrington 1995, Aranovich & Berman 1996. Rocks from the upper part of the basal Parry Sound assemblage are extensively retrograded to upper-amphibolite-facies assemblages (Fig. 5a). Textural features within Group-I pelitic gneisses (see above) are consistent with a P-T path that evolved from the stability field of sillimanite, then toward and probably across the Ky-Sil boundary, and back into the stability field of sillimanite. The coexistence of these aluminosilicates with K-feldspar indicates temperature conditions above the dehydration melting of muscovite according to the reaction: Ms + Qtz ± Pl = Ky or Sil + Kfs + L In addition, the presence of leucosomes with garnet in these gneisses indicates that the reaction: Bt + Ky or Sil + Qtz ± Pl = Kfs + Grt + L also occurred. For plagioclase-bearing pelitic rocks, reactions (6) and (7) occur at 680-740°C and ca. 750°C (for pressures between 6 and 10 kbar), respectively (LeBreton & Thompson 1988, Vielzeuf & Holloway 1988. The absence of orthopyroxene implies temperatures less than about 800°C.
A tectonic enclave of fine-grained, garnet-clinopyroxene-rich metabasite, hosted by straight gneiss of the Ojibway assemblage in the Shawanaga shear zone, contains a similar assemblage of minerals as found in the recrystallized coronitic metagabbro. The unfoliated core of this body, characterized by abundant clinopyroxeneplagioclase symplectite, is reduced to foliated amphibolite near the margins. Orthopyroxene is a relict matrix phase in symplectite. Reaction textures and mineral composition suggest that mafic bodies identical to this enclave throughout the Central Gneiss Belt are re-equilibrated relics of eclogite (Davidson 1990). Clinopyroxene-plagioclase symplectite is interpreted as the product of breakdown of omphacite during decompression from eclogite facies. Mineral assemblages in coronitic metagabbro and relict eclogite indicate eclogite-facies metamorphism in the Ojibway assemblage prior to initial re-equilibration in the granulite facies and final equilibration in the amphibolite facies (see also Needham 1992). extensively overprinted by upper-to lower-amphibolitefacies assemblages (Fig. 5a). The coexistence of kyanite ± sillimanite and K-feldspar in the flanking upperamphibolite-facies pelitic gneisses indicates that metamorphic temperatures exceeded those required for the dehydration-melting reaction (6) (i.e., >680-740°C between 6 and 10 kbar). In the lower-amphibolite-facies pelitic schists (Group II) east of the Parry Island anorthosite, growth of sillimanite and biotite at the expense of garnet may be the result of back-reaction of the subsolidus equilibrium (3) (but with sillimanite as a product) or of the partial melting reaction (7).
In the Shawanaga domain, migmatized pelitic rocks provide some constraints on the P-T conditions during upper-amphibolite-facies metamorphism. In pelitic gneisses of the Lighthouse assemblage (Fig. 2a), the occurrence of biotite and kyanite in melanosomes surrounding leucosomes suggests the partial melting reaction (7), implying temperatures of ≥750°C. The presence of kyanite places a lower-P limit of ca. 8-10 kbar. Textural features within the pelitic gneisses suggest that these P-T conditions were reached during thrust-related deformation. Furthermore, the presence of sillimanite in pelitic mylonites at the Parry Sound -Shawanaga contact indicates that P-T conditions in the Lighthouse assemblage entered the stability field of sillimanite during extensional shearing.
In the Ojibway assemblage, local persistence of texturally stable muscovite in kyanite-sillimanite-bearing pelitic gneisses suggests that the dehydration-melting reaction (6) did not go to completion. As noted above, this reaction implies temperatures of ca. 680-740°C between 6 and 10 kbar. Furthermore, textural features within these gneisses suggest that Al 2 SiO 5 growth history was similar to that in pelitic rocks of the Lighthouse assemblage, implying a P-T path that progressed from the stability field of kyanite toward that of sillimanite.

P-T ESTIMATES
Twenty-seven samples of pelitic and mafic gneiss, garnet amphibolite, and coronitic metagabbro with granulite-or amphibolite-facies mineral assemblages appropriate for thermobarometric work were selected from the Parry Sound and Shawanaga domains for analysis. Sample sites (Figs. 2, 3) were chosen in order to determine the P-T conditions of the thermal peak of metamorphism and subsequent overprinting events and (2) constrain possible P-T paths for the Parry Sound and Shawanaga domains. The mineral compositions used for P-T estimates (Appendix I) are based on results of 8 to 93 electron-microprobe analyses per sample. In each polished thin section, compositional data were collected from two or more microdomains to check for consistency of mineral compositions. Where a given mineral composition was consistent within a sample, the data were averaged to yield a representative composition for use in thermobarometry. Where chemical heterogeneity was detected, averaged compositions from texturally well-equilibrated subdomains or results of individual analyses were used. Core-inclusion and rim compositions from strongly zoned minerals (e.g., sample N71b) were treated separately. To minimize the potential effects of retrograde Fe-Mg exchange, all analyzed ferromagnesian minerals were separated by a neutral phase (e.g., quartz, feldspar).

Approach
All P-T estimates were calculated with the TWQ software program of Berman (1991) using an internally consistent thermodynamic dataset for end members and solution properties. Activities of end-member mineral components were calculated using the models of Berman (1990) for garnet, Aranovich (1991) for plagioclase, McMullin et al. (1991) for biotite, Newton (1983) for pyroxenes, and Mäder et al. (1994) for amphibole. The Aranovich (1991) solution model for plagioclase was favored over the widely used Fuhrman & Lindsley (1988) model for the following reasons: (1) for those samples that contain plagioclase with An content <20%, pressures estimated using the Fuhrman & Lindsley (1988) model were 3-4 kbar higher than those calculated using the Aranovich (1991) solution model, and P-T estimates fell in the kyanite stability field, which is not consistent with the observed assemblage of minerals, and (2) for those samples with An content >20%, good agreement (pressure differences ≤0.5 kbar) was obtained using either model. Unless stated otherwise, all other minerals were treated as pure phases.
For all samples, P-T estimates were computed from subsets of the most robust linearly independent equilibria for pelitic and mafic rocks (Table 3; e.g., Mäder et al. 1994). The thermobarometric results and assemblages used in TWQ analysis are listed in Table 4. The P-T results are also presented graphically in Figures 9  and 10. We adopted a conservative uncertainty of ±1 kbar and ±50°C for our results, which probably closely reflects the maximum P-T uncertainty (e.g., Essene 1989). We also report the standard deviations of the P-T intersections as calculated by Berman's (1991) INTERSX program (Table 4). These error bars allow an assessment of the overall consistency of the results, but do not represent statistically valid uncertainties in P-T, as they do not include errors in the electron-microprobe data, thermodynamic properties or solution models. The P-T estimates quoted below have been rounded to the nearest 5°C and 0.5 kbar.

The Parry Sound domain
The highest P-T estimates from the Parry Sound domain were obtained from interior Parry Sound assemblage rocks (Figs. 5,9, Table 4). The mafic gneiss and granulite samples (N144 and N171) from the northern part of the assemblage (Fig. 2a) yielded P-T estimates of 12.5-13.0 kbar and 885-975°C, and plot within error of the stability field of the appropriate aluminosilicate (Fig. 9). We interpret these results as representing peak or near-peak granulite-facies conditions in the interior Parry Sound assemblage. These high pressures and temperatures are consistent with the presence of Opx + Sil + Qtz in nearby pelites and of Spr + Qtz in pelitic granulites to the northeast of the study area (Davidson et al. 1990). Sillimanite-bearing pelitic granulites (N122b, N125b, and N130a) from the northern part of the interior Parry Sound assemblage gave somewhat lower P-  T estimates than the mafic rocks, in the range 10-12 kbar and 760-875°C. Since none of these samples contains rutile in equilibrium with ilmenite, these P-T determinations are based on only two independent equilibria involving garnet -plagioclase -biotite -sillimanite -quartz (Table 4). With the exception of N125b, the estimates plot within or close to the stability field of sillimanite and are similar to previously determined P-T estimates for this area (Anovitz 1991 ; Fig. 9). For these reasons, two of the three results (i.e., 10-12 kbar, 870-875°C) are regarded as reliable and may reflect partial re-equilibration. The orthopyroxene-bearing pelitic granulite N131a yielded the lowest P-T estimate, 7.5 kbar and 730°C, and may reflect re-equilibration at a later stage. Sample N192a, a garnet amphibolite from the retrograded area in the southern interior Parry Sound assemblage (Figs. 2b, 5b), gave the highest P-T estimate of 15 kbar and 1010°C. This estimate appears high for upper-amphibolite-facies metamorphism, but there are insufficient data from this area to determine the significance of this result.
In the upper part of the basal Parry Sound assemblage, Group-I pelitic gneisses (N75c and N76b) yielded P-T estimates of 7-8 kbar and 780-870°C (Figs. 5a, 9, Table 4). The partially hydrated garnet amphibolite (N74c) yielded a slightly higher P of 8.5 kbar and a lower T of 725°C. All three estimates straddle the Ky-Sil boundary or plot within the stability field of sillimanite, consistent with sillimanite outlasting kyanite in the pelitic gneiss samples. However, sample N75c records a temperature well above the dehydration melting reaction (5), inconsistent with the absence of orthopyroxene in this pelitic rock. Given the petrogenetic constraints outlined above, we suggest that the results from samples N74c and N76b may closely reflect the P-T conditions during the upper-amphibolitefacies overprinting event in this part of the basal Parry Sound assemblage.
In the lower part of the basal Parry Sound assemblage, the granulite-facies mafic dyke N153c (Fig. 2a) that cuts the Parry Island anorthosite yielded a P-T estimate of 7.0 kbar and 660°C (Fig. 9, Table 4). We interpret this P-T  N7f/N31a result as recording peak or near-peak granulite-facies conditions during orogen-parallel deformation within the basal Parry Sound assemblage, although minor re-equilibration during cooling cannot be ruled out.
Group-II pelitic schists (N59, N73, and N138), located in the low-strain area east of the Parry Island anorthosite (Fig. 2a), yielded the lowest P-T estimates (Figs. 5a,9,Table 4), consistent with the observation of lower-amphibolite-facies retrogression in this area. P-T determinations for these samples are in the range 5.0-6.5 kbar and 615-690°C. Although sample N73 shows a large degree of scatter in P (±1.7 kbar), it yielded a P-T estimate that falls within error of the other two samples ( Table 4).
The sillimanite-bearing pelitic gneiss N205a from the Twelve Mile Bay assemblage yielded a P-T estimate of 7.0 kbar and 825°C, whereas the garnet amphibolite N204b gave a much higher P of 13.0 kbar and T of 940°C (Figs. 5b,9,Table 4). Both results fall within the stability field of the appropriate aluminosilicate. However, the garnet amphibolite records a pressure and temperature well above the dehydration-melting reaction (7), inconsistent with the mineral assemblage observed in the pelitic gneiss (see above). It is thus possible that the garnet amphibolite escaped re-equilibration to upper-amphibolite-facies conditions and records P-T conditions during granulite-facies metamorphism. Thermobarometric data from this area are currently insufficient to test this hypothesis. In contrast, although the pelitic gneiss shows a large scatter in P (±1.4 kbar), its P-T estimate is consistent with mineral equilibria and may thus reflect the P-T conditions during the upperamphibolite-facies overprinting event.

The Shawanaga domain
TWQ results for the pelitic gneisses and mylonites from the Lighthouse assemblage show remarkable intersample consistency (Figs. 5a, 10, Table 4), suggesting that local equilibrium was achieved in the immediate footwall to and at the Parry Sound -Shawanaga contact. P-T determinations for the kyanite-bearing pelitic gneiss samples (N7f, N31a, and N43b) are in the range 7.0 kbar and 700-725°C. All three points fall within the stability field of sillimanite, suggesting that the calculated pressures and temperatures represent minimum estimates of upper-amphibolite-facies conditions. Data from the pelitic mylonites (N1d and N58a), including matrix and rim compositions from sample N71b, yielded lower P-T estimates of 5.5-6.5 kbar and 660-690°C that may reflect ambient P-T conditions during extensional shearing along the Parry Sound -Shawanaga contact. In contrast, the garnet core -inclusion assemblage in the pelitic mylonite N71b and the mylonitized garnet amphibolite N1c yielded much higher P-T estimates, 10.5-11.0 kbar and 830°C (Fig. 10, Table 4). These two samples may thus record an earlier point on the P-T path for the Lighthouse assemblage rocks.
In the structurally lower Ojibway assemblage, the highest pressure estimate (8.5 kbar, 685°C; Figs. 7, 10, Table 4) was obtained from recrystallized domains within coronitic metagabbro (sample 125; Fig. 8d) in the Shawanaga pluton. The amphibolite sample 208b yielded a lower P of 7.5 kbar and a slightly higher T of 700°C. These P-T estimates suggest that peak conditions of metamorphism in the Ojibway assemblage may have reached at least 8.5-7.5 kbar and 700-685°C. Similar results were reported from the northern Shawanaga domain by Anovitz & Essene (1990) for coronitic metagabbro (<10.2 kbar, 700-710°C) and by Tuccillo et al. (1992) for garnet metagabbro and pelite (~7-9 kbar at 700°C). Pelitic gneisses yielded lower P-T estimates than the coronitic metagabbro and amphibolite, consistent with the greater resistance of mafic rocks to fluid influx and retrograde reaction (e.g., Brodie & Rutter 1985). The pelitic gneiss sample D47a, collected from the large exposure of supracrustal rock (Fig. 3), yielded an estimate of 6.5 kbar and 670°C, whereas the pelitic gneiss enclave 129b gave a lower P-T estimate, 5.0 kbar and 580°C (Figs. 7, 10, Table 4). Both results fall within error of the Ky-Sil univariant curve, consistent with the presence of both phases in these gneisses. We suggest that P-T estimates from the pelitic gneiss reflect partial re-equilibration during cooling. Figure 11 shows possible P-T-t paths for Parry Sound and Shawanaga domain rocks. The paths were generated from P-T data from various samples that (1) shared a common metamorphic history and (2) were either sufficiently closely spaced or from the same structural level that significant differences in P-T-t history between outcrops are unlikely (cf. Jamieson et al. 1995). Furthermore, we assume that the P-T estimates used to construct the P-T-t paths reflect either conditions approaching the thermal peak or re-equilibration at a later stage. For these reasons, we believe it to be unlikely that the paths shown in Figure 11 represent metamorphic field-gradients. P-T-t paths could not be reconstructed for the basal Parry Sound or Twelve Mile Bay assemblages because of insufficient data.

P-T-t PATHS
The Parry Sound domain P-T data from the interior Parry Sound assemblage lie on a P-T-t path suggesting initial near-isothermal decompression over the range 13-10 kbar and 950-870°C, followed by decompression (10→8 kbar) accompanied by more cooling (870 →730°C) (Fig. 11a). The higher-P segment of the P-T-t path resembles that proposed by Anovitz (1991), whereas the lower-P segment has a much steeper slope than the one obtained by Anovitz & Essene (1990). These investigators interpreted their P-T-t path to reflect a composite history involving a significant period of pre-peak unloading followed by isobaric cooling (Anovitz 1991). The isobaric cooling path was modeled by postulating a postthrusting period of extension at or near the peak of metamorphism (Anovitz & Chase 1990). In contrast, our data suggest that the post-peak P-T-t evolution of interior Parry Sound assemblage rocks involved decompression and cooling which, as discussed below, can be attributed to thrusting.
The final part of the P-T-t path followed by interior Parry Sound assemblage granulites overlaps with P-T data from the upper part of the basal Parry Sound assemblage, when realistic uncertainties in P-T estimates are taken into account (Fig. 11a). Retrograde amphibolite-facies minerals in rocks from the upper part of the basal Parry Sound assemblage define the tectonic fabric associated with thrust-related deformation below the interior Parry Sound -basal Parry Sound contact (Fig. 4b). On the basis of these observations, we sug-gest that granulite-facies assemblages in this part of the basal Parry Sound assemblage were re-equilibrated to upper-amphibolite-facies conditions in response to overthrusting by interior Parry Sound assemblage rocks. P-T data from Group-II pelitic schists from the lower part of the basal Parry Sound assemblage record the lowest pressures and temperatures from the Parry Sound domain (5.0-6.5 kbar, 615-690°C; Fig. 11a), and likely reflect conditions during the lower-amphibolite-facies metamorphic overprint. Textural evidence for an earlier, high-grade Ky-Kfs assemblage in these schists does not justify construction of a path that directly links these data with those from the granulite-facies mafic dyke N153c. Unfortunately, there are currently no thermobarometric data for the Ky-Kfs assemblage, but petrogenetic constraints suggest that Group-II pelitic schists likely reached T ≥ 680-740°C and P ≥ 8-10 kbar (see above) following granulite-facies metamorphism at intermediate P and T (7.0 kbar and 660°C) and prior to re-equilibration to lower-amphibolite-facies conditions (5.0-6.5 kbar and 615-690°C).

The Shawanaga domain
Core and rim thermobarometric data from the pelitic mylonite N71b of the Lighthouse assemblage lie on a P-T-t path showing decompression (11→6.5 kbar) accompanied by cooling (830→685°C) (Fig. 11b). Several features suggest that the relative shape of this path in P-T space is correct: (1) P-T points from the other Lighthouse assemblage rocks within and adjacent to the Parry Sound -Shawanaga contact all fall along the same path defined by the core and rim P-T estimates, and (2) the path progresses from the kyanite field toward the stability field of sillimanite, consistent with the observed sequence of aluminosilicate polymorphs. However, given the magnitude of P-T uncertainties, the possibility that the Ky-Sil transition occurred at a lower P than that shown in Figure 11 cannot be ruled out.
Three of the four P-T points from the Ojibway assemblage lie on a P-T-t path suggesting decompression over the range 7.5-5.0 kbar and 700-580°C (Fig. 11b). A steeper P-T-t path segment is defined by linking these data with the fourth P-T point, coronitic metagabbro 125, which lies at a higher pressure (8.5 kbar). The initial part of the P-T-t path crosses from the kyanite field to the sillimanite field, consistent with mineral textures in Ojibway pelitic gneisses, which indicate sillimanite growth after kyanite. The middle part of the path overlaps with the final P-T-t path segment followed by Lighthouse assemblage rocks, suggesting that these rocks had, at least in part, a similar exhumation history as the Ojibway assemblage rocks. However, final equilibration in the Ojibway assemblage occurred at a lower P and T than in the Lighthouse assemblage (Fig. 11b).
In general, the P-T-t paths shown in Figure 11 for the interior Parry Sound, Lighthouse, and Ojibway assemblages closely resemble those previously determined by Wodicka (1994) and Ketchum (1995) for the same areas (see Culshaw et al. 1997). Minor differences, particularly in the location of the paths in P-T space, probably result from the use of (1) a different thermobarometric approach and (2) different solutionmodels for plagioclase. Wodicka (1994) and Ketchum (1995) generally used all possible equilibria (i.e., the multi-equilibrium approach of Berman 1991) and the Fuhrman & Lindsley (1988) solution model to calculate P and T in the Parry Sound and Shawanaga domains, whereas in this paper we used only the most robust linearly independent equilibria and the Aranovich (1991) model. Our results tend to be up to 1.5 kbar lower for pelitic rocks and about 3 kbar higher for mafic rocks than those estimated by Wodicka (1994) and Ketchum (1995).

The Parry Sound domain
Geochronological data show that the granulite-and amphibolite-facies assemblages in the northern Parry Sound domain formed as a result of more than one metamorphic event (Fig. 4b). High-P -high-T granulitefacies metamorphism in the interior Parry Sound assemblage has been dated at 1161 ± 3 Ma ( Fig. 11a; van Breemen et al. 1986). This metamorphism was approximately coeval with the upper-amphibolite-facies overprinting event in the upper part of the basal Parry Sound assemblage, which is constrained by a 1157 ± 3 Ma kyanite-bearing pegmatitic leucosome from a Group-I kyanite-sillimanite pelitic gneiss and by metamorphic zircon from a nearby garnet amphibolite, which yielded a slightly younger, minimum age of 1148 ± 6 Ma ( Fig. 11a; Wodicka 1994). Tuccillo et al. (1992) obtained an identical 207 Pb/ 206 Pb age of 1157 ± 1 Ma on monazite from a metasedimentary rock lying at the same structural level as Group-I rocks. Thrust deformation along the interior Parry Sound -basal Parry Sound contact has been dated at ca. 1159 Ma (Fig. 4b;van Breemen et al. 1986), consistent with the suggestion that conversion of granulite-facies rocks to amphibolite within this part of the basal Parry Sound assemblage was a consequence of overthrusting by interior Parry Sound assemblage rocks (see above).
In the lower part of the basal Parry Sound assemblage, field relationships indicate that intermediate-P granulite-facies metamorphism was coeval with emplacement of the 1163 ± 3 Ma Parry Island anorthosite (Figs. 4b, 11a;Wodicka 1994). The lower-amphibolitefacies metamorphic overprint in this part of the basal Parry Sound assemblage appears to postdate both kyanite-grade metamorphism at higher structural levels and granulite-facies metamorphism in the basal Parry Sound assemblage by ca. 40 m.y. Monazite from a Group-II pelitic schist from the low-strain area east of the Parry Island anorthosite yielded an age of 1116 +16 -10 Ma (Wodicka 1994), and a comparable 207 Pb/ 206 Pb garnet age of ca. 1123 Ma was reported from another pelitic unit from the lower part of the basal Parry Sound assemblage (Tuccillo et al. 1992). These data are interpreted to date the time of thrust-related, upper-to lower-amphibolite-facies metamorphic overprint at ca. 1120 Ma in the lower part of the basal Parry Sound assemblage ( Fig. 11a; Wodicka 1994). This interpretation is consistent with recently reported U-Pb zircon ages of ca. 1129-1104 Ma for boudin infills in amphibolitized rocks from the lower part of the basal Parry Sound assemblage (Krogh 1997). At higher structural levels within the interior Parry Sound assemblage, evidence for minor deformation and associated amphibolite-facies metamorphism at about the same time comes from an 1121 ± 5 Ma late tectonic pegmatite (van Breemen et al. 1986) and 1114 ± 2 Ma metamorphic zircon from a deformed and amphibolitized mafic dyke (Bussy et al. 1995). Thus, these data combine to suggest a significant time-gap between granulite-facies metamorphism (ca. 1163 Ma) and deformation and associated hydration (ca. 1120 Ma) in the lower part of the basal Parry Sound assemblage, i.e., retrogression did not occur during cooling of the granulite-facies rocks soon after ca. 1163 Ma.
U-Pb titanite data from the basal and northern interior Parry Sound assemblages record ages between ca. 1120 and 1080 Ma ( Fig. 11a; Tuccillo et al. 1992, Wodicka 1994. These data likely reflect cooling through ≥600°C following high-grade metamorphism (Culshaw et al. 1997), although the possibility that the ages represent the times of late shearing or recrystallization (or both) (e.g., Tuccillo et al. 1992, Wodicka 1994) cannot be excluded. Decompression in the interior Parry Sound assemblage must have occurred before 1120-1080 Ma, as all the metamorphic temperatures exceed the closure temperature of titanite (Fig. 11a). Textural observations, together with U-Pb data on timing of thrusting along the interior Parry Sound -basal Parry Sound contact, suggest that decompression of rocks of the interior Parry Sound assemblage can be linked to thrusting soon after peak granulite-facies metamorphism.
Cooling through closure temperatures of hornblende (ca. 500°C) took place between ca. 1068 Ma and 1018 Ma ( Fig. 11a; Wodicka 1994). 40 Ar/ 39 Ar muscovite ages are considerably younger and indicate cooling of the basal Parry Sound domain through ca. 300°C by ca. 890 Ma (Reynolds et al. 1995).
Overall, the geochronological data from the northern Parry Sound domain indicate that the rocks experienced polyphase metamorphism and deformation. With increasing structural depth, i.e., from the northern interior Parry Sound assemblage to the lowest structural level of the basal Parry Sound assemblage, there appears to be a systematic younging of amphibolite-facies metamorphism, associated deformation, and subsequent cooling through ≥600-500°C (Wodicka 1994, Krogh 1997. This pattern is consistent with northwestward propagation of the Grenville orogen into its foreland (e.g., Jamieson et al. 1992, Haggart et al. 1993. In the Twelve Mile Bay assemblage, the absolute time of metamorphism is currently unknown. However, metamorphism must have postdated the deposition of quartzites at ca. 1140-1120 Ma (Wodicka et al. 1996), and either predated or was synchronous with widespread upper-amphibolite-facies metamorphism in the CGB at ca. 1080-1035 Ma (e.g., Culshaw et al. 1997).

The Shawanaga domain
U-Pb zircon dates derived from garnet-clinopyroxene-rich rocks within the Shawanaga domain and its bounding shear zones indicate a major phase of eclogitefacies metamorphism at ca. 1090-1085 Ma ( Fig. 4b; Ketchum & Krogh 1997). An older age of ca. 1120 Ma has also been obtained from these rocks, but the significance of this age is currently not very well constrained.
Though not yet dated, granulite-facies metamorphism must be younger than ca. 1170-1150 Ma, the age of coronitic metagabbro bodies that preserve granulitefacies assemblages. On the basis of petrographic considerations, this metamorphism is likely also younger than the age of the eclogite-facies event. Upper-amphibolite-facies metamorphism in the Shawanaga domain is distinctly younger than that recorded in the Parry Sound domain (Fig. 4b). Migmatite in the immediate footwall of the Parry Sound domain has recently been dated at ca. 1082 Ma (Krogh 1997), and a somewhat younger metamorphic zircon age of ca. 1050 Ma (Heaman & LeCheminant 1993) and a discordant titanite age of ca. 1058 Ma (Tuccillo et al. 1992) have been obtained from rocks at lower structural levels within the Shawanaga domain (Fig. 11b). It is not yet clear whether upper-amphibolite-facies metamorphism between about 1080 and 1050 Ma was continuous or episodic. Sillimanite-grade conditions were maintained until at least ca. 1020 Ma, the age of a late-syntectonic granitic pegmatite within the extensional Shawanaga shear zone (Ketchum et al. 1998). 40 Ar/ 39 Ar data on hornblende suggest that extensional displacement along the Parry Sound -Shawanaga contact occurred some time between ca. 1020 and 970 Ma (Wodicka 1994), i.e., possibly at about the same time as along the Shawanaga shear zone. A ca. 990 Ma post-tectonic pegmatite indicates that ductile deformation in the northern Shawanaga domain ceased prior to this time (Ketchum et al. 1998).
U-Pb titanite data from the Shawanaga domain define two distinct age-populations. The older group of ages (1028-1018 Ma; Fig. 11b) is interpreted to mark regional cooling through the closure temperature of titanite during or immediately following extensional unroofing, whereas the younger group of ages (967-956 Ma) is considered to date postkinematic recrystallization (Ketchum et al. 1998). 40 Ar/ 39 Ar data indicate that the Shawanaga domain cooled through the closure temperature of hornblende at 970 ± 10 Ma ( Fig. 11b; Culshaw et al. 1991, Wodicka 1994, some 50-80 m.y. later than the Parry Sound domain. Together with 40 Ar/ 39 Ar muscovite ages of 900 +10 -20 Ma, the hornblende data indicate slow, uniform cooling of the domain at 2-4°C/m.y. (Culshaw et al. 1991, Reynolds et al. 1995 following extensional deformation at sillimanite-grade conditions.
Since most metamorphic temperatures in both the Lighthouse and Ojibway assemblages exceed the closure temperature of titanite, the initial stages of decompression must have occurred before 1028-1018 Ma (Fig. 11b), i.e., prior to extensional displacement along the Shawanaga shear zone and the Parry Sound -Shawanaga contact. In the case of the Lighthouse assemblage, structural and petrological data suggest that initial decompression can be attributed to northwestdirected thrusting. In contrast, textural observations and U-Pb data suggest that final decompression in the Light-house and Ojibway assemblages can be linked to extension on the Parry Sound -Shawanaga contact (Wodicka 1994, Ketchum 1995.

IMPLICATIONS CONCERNING THE CONTROLS OF METAMORPHISM
As geochronological data provide evidence for multiple phases of Grenvillian high-grade metamorphism in the Parry Sound and Shawanaga domains, construction and interpretation of P-T-t paths from these rocks require careful assessment of timing and overprinting relationships, and the degree to which estimates of maximum P-T really reflect peak P-T conditions (e.g., Frost & Chacko 1989, Frost et al. 1998, Pattison 1998. Only then can the metamorphic data provide quantitative constraints on the tectonic processes operating during Grenvillian orogenesis. We argue here that the extent of overprinting of mineral assemblages, thermobarometers, and fabrics in the polymetamorphosed rocks of the Parry Sound and Shawanaga domains can be linked to three important factors: strain, fluid access, and high-temperature residence time. The importance of deformation and associated infiltration of fluid in metamorphism is well known (e.g., Brodie & Rutter 1985, Rubie 1990, Erambert & Austrheim 1993, Moecher & Wintsch 1994, St-Onge & Lucas 1995. In the Parry Sound domain, the preservation of granulite-facies assemblages and associated fabrics appears to have been largely dependent upon fluid availability and strain intensity. Both the northern interior Parry Sound assemblage and the Parry Island anorthosite of the basal Parry Sound assemblage remained relatively strong during Grenvillian deformation, preserving granulite-facies assemblages and structures that formed during an early phase of Grenvillian tectonism. Although these rocks are deformed, deformation was not pervasive and occurred under relatively anhydrous conditions (see above). Mafic rocks from these lithotectonic assemblages were the most resistant to resetting of thermobarometers, and thus appear to preserve a record of peak or near-peak P-T conditions.
In contrast, penetration of fluid, particularly along high-strain zones, facilitated weakening of the granulite-facies rocks by retrograde hydration. For example, rocks along the flanks of the Parry Island anorthosite and within the upper part of the basal Parry Sound assemblage underwent partial to complete compositional change as a result of interaction with a fluid during highstrain, thrust-related deformation. Any pre-existing structures were completely transposed into the dominant SE-dipping foliation in these highly strained, upperamphibolite-facies rocks. In the southern interior Parry Sound assemblage, conversion of granulite-facies rocks to amphibolite likely occurred in response to thrust emplacement of the overlying Moon River domain, with fluid flow focused along the Parry Sound -Moon River boundary (Culshaw et al. 1997). Re-equilibration of Group-II rocks to lower-amphibolite-facies conditions was accompanied by extensive growth of muscovite, suggesting infiltration of aqueous fluids in the strain shadow of the Parry Island anorthosite. We believe that retrogression in this low-strain area took place at the same time as re-equilibration at upper-amphibolite-facies conditions in the adjacent high-strain zones flanking the anorthosite. Extensive growth of muscovite in the strain shadow of the Parry Island anorthosite presumably reflects both greater influx of fluid and further localization of intense retrogression in this area, and the influence of X(H 2 O) on reaction positions in P-T space.
In the Shawanaga domain, relict eclogite-and granulite-facies assemblages are only preserved in rocks of suitable composition in low-strain areas. Between ca. 1080 and 1050 Ma, penetrative ductile deformation at upper-amphibolite-facies conditions largely overprinted these mineral assemblages. Variable re-equilibration also occurred during high-temperature extensional deformation at ca. 1020 Ma. Ductile extensional flow along both the Shawanaga shear zone and the Parry Sound -Shawanaga contact overprinted earlier thrust fabrics. The widespread occurrence of retrograde muscovite and chlorite in rocks of the Sand Bay assemblage provides direct evidence for fluid infiltration.
Extended residence time in the deep crust may have also led to significant resetting of thermobarometers in both Parry Sound and Shawanaga domain rocks. Although rocks of the Parry Sound domain may have cooled slightly following ca. 1160 Ma granulite-facies metamorphism, final cooling through the closure temperature of titanite did not take place until ca. 1120-1080 Ma (Fig. 11a). Similarly, U-Pb and Ar-Ar data indicate that high-grade metamorphism in the Shawanaga domain probably took place over an interval of ca. 40 m.y., and that the crust remained above ca. 500°C for another 40-50 m.y. following high-temperature extensional deformation at ca. 1020 Ma (Fig. 11b).

INTERPRETATION OF TECTONIC HISTORY
A tectonic interpretation for high-grade metamorphism and assembly of the Parry Sound and Shawanaga domains during the Grenvillian orogeny is shown in Figure 12. The reconstruction is largely based on geological, geometrical, and chronological constraints from the CGB along Georgian Bay (Culshaw et al. 1997, and references therein).
At ca. 1163-1161 Ma, granulite-facies metamorphism affected both the interior and basal Parry Sound assemblages. This metamorphism was coeval with the late stages of upper-amphibolite-facies metamorphism and thrust deformation in the Central Metasedimentary Belt boundary thrust zone (McEachern & van Breemen 1993), with the waning stages of the "Elzevirian" orogeny in the CMB and Adirondack Highlands (e.g., Moore & Thompson 1980, McLelland et al. 1996, and with widespread anorthosite -mangerite -charnockite -  granite (AMCG) magmatism between ca. 1180 and 1125 Ma in the southeastern, central, and eastern parts of the Grenville orogen (e.g., Emslie & Hunt 1990, McLelland & Chiarenzelli 1991, Higgins & van Breemen 1996. In contrast, with the exception of the emplacement of gabbro at ca. 1170-1150 Ma, there is no tectonic activity of this age known from the Laurentian craton and margin. Thus, at the time of highgrade metamorphism, the Parry Sound domain cannot have been contiguous with the Laurentian rocks that now underlie it (Wodicka et al. 1996, Timmermann et al. 1997. Our data do not show whether the convergence responsible for ca. 1163-1161 Ma high-grade metamorphism and thrusting in the Parry Sound domain occurred at a distal margin of Laurentia which is not preserved, or within some part of the composite arc system now represented by the Central Metasedimentary Belt. Peak metamorphic conditions of ca. 12-13 kbar and 885-975°C have been estimated for the interior Parry Sound assemblage rocks. The high pressures require either substantial thickening of the crust or exhumation of these rocks from synorogenic lower crust corresponding to depths of 35-40 km. Although there is no compelling evidence one way or the other, the latter hypothesis may be favored by the lack of clear evidence for continental collision in this part of the orogen, and by the P-T-t path segment suggesting near-isothermal decompression (Fig. 11a). In addition, if granulites of the interior Parry Sound assemblage reached peak conditions in the middle part of a thick orogenic crust, Moho temperatures must have been substantially above 1000°C; this seems unlikely by comparison with modern orogenic settings, and should have led to widespread melting in the lower crust at this time, for which there is no evidence.
Possible explanations for high temperatures of metamorphism in the interior Parry Sound assemblage include tectonic accretion of heat-producing material, heat transfer from coeval anorthosite, or underplating by voluminous mafic magma. Lateral accretion or tectonic underplating of heat-producing crustal material can produce the necessary temperatures (Huerta et al. 1998, Jamieson et al. 1998, but this part of the Parry Sound domain is dominated by mafic to intermediate metaigneous rocks, with relatively little granitic or metasedimentary material. If accretion of heat-producing material was responsible, this material is now gone. The interval 1180-1125 Ma coincides with a major episode of AMCG magmatism in much of the Grenville orogen. Heat advected from anorthosite could account for intermediate-P (≥7.0 kbar and ≥660°C) granulitefacies metamorphism in the lower part of the basal Parry Sound assemblage (Fig. 12a), but the absence of synmetamorphic anorthosite or any other intrusive rocks in the interior Parry Sound assemblage suggests that another mechanism is required. It has recently been suggested that this AMCG magmatism was associated with widespread mafic underplating and lithosphere delamination in the CMB and Adirondack Highlands (Corrigan 1995, McLelland et al. 1996, Corrigan & Hanmer 1997. Although the proposed mechanism, convective removal of over-thickened suborogenic lithospheric mantle, seems incompatible with the lack of evidence for collision in the study area, underplating by voluminous mafic magma, whatever its origin, is likely to have accompanied AMCG magmatism and is also likely to have transferred a significant amount of heat to the immediately overlying crust. The postulated underplated material, which on cooling would have been both denser and stronger than the overlying crust, is unlikely to have been incorporated into the orogenic crust during subsequent tectonic shortening. We conclude that the most likely setting for 1161 ± 3 Ma high-P -high-T granulite-facies metamorphism in the interior Parry Sound assemblage was at or near the base of crust underplated by voluminous mafic magma at or shortly before 1161 Ma (Fig. 12a). In contrast, barometric data suggest that rocks of the basal Parry Sound assemblage lay at much higher levels in the crust (ca. 20-25 km, 7 kbar) at the time of emplacement of the 1163 ± 3 Ma Parry Island anorthosite (Fig. 12a). At ca. 1159 Ma, tectonic emplacement of the interior Parry Sound assemblage onto the upper part of the basal Parry Sound assemblage probably resulted in conversion of these rocks to amphibolite (≥7.0-8.5 kbar, 725-775°C; Fig. 12b). The position of rocks of the Parry Sound domain relative to Laurentia at the time is unknown.
At ca. 1120 Ma, amphibolite-facies metamorphism and thrust deformation affected the lower part of the basal Parry Sound assemblage (Fig. 12c). We suggest that at this time, the pre-assembled interior Parry Sound assemblage and upper part of the basal Parry Sound assemblage were transported northwestward onto rocks of the lower part of the basal Parry Sound assemblage. Petrological and structural data from the lower part of the basal Parry Sound assemblage suggest that thrusting and concomitant erosion brought the Parry Sound domain into the middle to upper crust (5-6.5 kbar, 15-20 km) at ca. 1120 Ma. At about the same time, the Twelve Mile Bay quartzite was deposited from a varied source that included AMCG and Laurentian rocks (Wodicka et al. 1996;Fig. 12c). As suggested by Culshaw et al. (1997), this period may mark the initial encounter of the Parry Sound domain and correlative rocks (i.e., Central Metasedimentary Belt boundary thrust zone allochthons) with the outer margin of Laurentia.
Apart from ca. 1100 Ma syntectonic granitic pegmatites near the boundaries of the Moon River and Seguin domains (van Breemen & Davidson 1990, Nadeau 1990, few data indicate that the interval 1120-1090 Ma represents a major phase of thrust deformation in the southwestern CGB or CMB, which suggests that convergence slowed down or stopped. Whether this was due to aborted subduction of the buoyant Laurentian margin or to a far-field change in plate motions, perhaps associated with the formation of the 1109-1087 Ma Midcontinent rift (Van Schmus 1992, Cannon 1994, remains unknown. The recent recognition of eclogite-facies metamorphism at ca. 1090-1085 Ma in the Shawanaga domain and other allochthonous domains of the CGB (Ketchum & Krogh 1997) suggests deep burial or partial subduction of the Laurentian margin beneath the CMB at this time (Fig. 12d). Seismic and geological evidence indicating that the CGB extends beneath the CMB (e.g., Culshaw et al. 1983, Hanmer & McEachern 1992, Milkereit et al. 1992, White et al. 1994) supports this interpretation. Heating of this deeply buried or partially subducted material formed leucosome-rich migmatites in the Shawanaga domain approximately 10 m.y. after eclogite-facies metamorphism (Culshaw et al. 1997). At some time between ca. 1120 and 1080 Ma, the Twelve Mile Bay assemblage was affected by upper-amphibolite-facies metamorphism and was overridden by the Parry Sound domain.
Eclogite-facies metamorphism in the Shawanaga domain broadly coincided with ca. 1090-1080 Ma thrusting and high-grade metamorphism in the Central Metasedimentary Belt boundary thrust zone, suggesting renewed convergence near the Laurentian margin at this time (McEachern & van Breemen 1993, Timmermann et al. 1997Fig. 12e). Between ca. 1080 and 1050 Ma, the Shawanaga domain was affected by upper-amphibolite-facies metamorphism and deformation, which coincided with widespread metamorphism in the CGB, with the exception of the Parry Sound domain and the Grenville Front Tectonic Zone. Metamorphic conditions recorded by our data suggest that rocks of the Lighthouse assemblage lay at crustal depths of ca. 30-35 km (10.5-11.0 kbar) and temperatures of ca. 830°C, whereas rocks of the Ojibway assemblage lay at depths of at least ca. 22 km (7.5-8.5 kbar and 700-685°C) at this time. Upper-amphibolite-facies metamorphism is interpreted as a response to the ca. 1090-1080 Ma thrusting (Culshaw et al. 1997). Transport of the Shawanaga and Parry Sound allochthons over the Laurentian craton may have been facilitated by the Shawanaga domain migmatite, acting as a thin, extremely weak, intra-crustal décollement (i.e., the thrust stage of the Shawanaga shear zone; Jamieson et al. 1992, White et al. 1994, Ketchum 1995, Culshaw et al. 1997. Transport of eclogite to higher levels in the crust, accompanied by recrystallization to upper-amphibolitefacies assemblages, was probably associated with thrusting along the Shawanaga shear zone or similar structures. Subsequent out-of-sequence thrusting in the Moon River domain caused reworking and retrogression of the southern interior Parry Sound assemblage (Culshaw et al. 1997).
At ca. 1020 Ma, extension on the Shawanaga shear zone (Ketchum 1995, Ketchum et al. 1998) generally postdated thrusting in the CGB, and resulted in SE-di-rected transport of allochthonous rocks (Fig. 12f). Ductile extensional fabrics at the Parry Sound -Shawanaga contact may have developed at about the same time (Wodicka 1994). Extensional displacement along these shear zones postdated peak metamorphism, although growth of sillimanite parallel to the extension lineation indicates that these rocks were hot during extensional shear (Wodicka 1994, Ketchum 1995. Our data suggest that extensional unroofing brought rocks of the Lighthouse and Ojibway assemblages into the middle to upper crust (5-6.5 kbar; 15-20 km) at ca. 1020 Ma. Extension could have been driven by reduced rates of convergence, excess gravitational potential related to the development of thick orogenic crust and high topography, or reduced strength of the lower crust associated with high temperatures and partial melting (Culshaw et al. , 1997; it is not clear which of these mechanisms was most important in the study area. In summary, the metamorphic and age data are consistent with progressive NW-directed juxtaposition of lithotectonic assemblages metamorphosed at progressively later times, and appear to record progressive or multi-stage convergence at the southeastern margin of Laurentia. The earliest phase of convergence took place southeast of the Laurentian craton between ca. 1163 and 1120 Ma, and resulted in assembly and granulite-to amphibolite-facies metamorphism of lithotectonic assemblages within the Parry Sound domain. The second phase of convergence, which began at or shortly before 1090-1080 Ma, involved deep burial or partial subduction of the Laurentian margin beneath the CMB. Continued convergence resulted in thrust imbrication and upper-amphibolite-facies metamorphism of the weakened Laurentian craton (including the Shawanaga domain) beneath the Parry Sound domain, transport of garnet-clinopyroxene-rich rocks to mid-crust levels, and assembly and transport of CGB lithotectonic units over the craton. A major episode of extension at ca. 1020 Ma followed convergence and resulted in SE-directed transport of allochthonous rocks. CONCLUSIONS 1. The Parry Sound and Shawanaga domains contain monocyclic rocks that originated at or near the southeastern margin of Laurentia between ca. 1450 and 1120 Ma. Their metamorphism and deformation are entirely attributable to Grenvillian orogenesis.
2. In the Parry Sound domain, both the interior and basal Parry Sound assemblages preserve granulite-facies assemblages and structures that formed during an early phase of Grenvillian tectonism. The most likely setting for ca. 1161 Ma high-P -high-T (ca. 12-13 kbar and 885-975°C) 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 ca. 1163 Ma intermediate-P (≥7 kbar and 660°C) granulite-facies metamorphism in the basal Parry Sound assemblage. In the upper part of the basal Parry Sound assemblage, retrogression of granulitefacies assemblages to upper-amphibolite-facies conditions (≥7-8.5 kbar and 725-775°C) likely occurred in response to thrust emplacement of the interior Parry Sound granulites onto this portion 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 (5-6.5 kbar and 615-690°C) took place at ca. 1120 Ma. In the Twelve Mile Bay assemblage, 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.
3. 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. Our data suggest that rocks from the uppermost Lighthouse assemblage lay at depths of ca. 30-35 km in the crust (10.5-11.0 kbar) and temperatures of ca. 830°C, whereas those from the structurally lower Ojibway assemblage lay at depths of at least ca. 22 km (7.5-8.5 kbar and 700-685°C) at this time. Sillimanite-grade conditions in the Shawanaga domain were maintained until at least ca. 1020 Ma, the time of major extensional deformation. 4. P-T-t paths for the Parry Sound and Shawanaga domains generally show decompression accompanied by some cooling. Petrological and structural data suggest that decompression of the interior Parry Sound granulites can be linked to thrusting soon after peak metamorphism, whereas exhumation of Shawanaga domain rocks was likely associated with both thrusting and extension. 5. Evidence for multiple phases of Grenvillian highgrade 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. Only then can the metamorphic data provide quantitative constraints on the tectonic processes operating during Grenvillian orogenesis. In the study area, the extent of overprinting of mineral assemblages, thermobarometers, and fabrics can be linked to strain, fluid access, and high-temperature residence time.
6. The metamorphic and age data are consistent with progressive NW-directed juxtaposition of lithotectonic assemblages metamorphosed at progressively later times, and appear to record progressive or multistage convergence at the southeastern margin of Laurentia.