E L S E V I E R Precambrian Research 85 (1997) 97-113

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

Gneiss Belt?

Nicholas Culshaw a,,, Jarda Dostal b a Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada, B3H3J5

b Department of Geology, Saint Mary's University, Halifax, Nova Scotia, Canada, B3H3C3

Received 31 October 1996; received in revised form 30 June 1977; accepted 30 June 1997

Abstract

The Sand Bay gneiss association is an allochthonous assemblage of supracrustal rocks in the Central Gneiss Belt of the Grenville Province of Ontario. It comprises a predominantly potassic (pink) quartzofeldspathic migrnatite, plagioclase-quartz-biotite schistose gneiss (Dillon schist), amphibolite with calc-silicate horizons and quartzite. Detailed mapping shows a compositionally asymmetric section that may represent original stratigraphy, preserved despite high ductile strains imposed during amphibolite facies metamorphism. The major- and trace-element geochemistry of the potassic (pink) quartzofeldspathic migmatite and amphibolite are compatible with volcanic protoliths, and suggest they are members of a bimodal suite erupted on rifted continental crust. The potassic quartzofeldspathic migmatite is compositionally similar to rhyolite whereas the amphibolite resembles continental tholeiite. Other members are derived from sedimentary rocks ranging from relatively mature clastic rock (quartzite) to locally-derived, epiclastic sediment (Dillon schist). This assemblage may have been formed during opening of a Grenvillian ocean or in a back arc environment landward of an Andean-type margin. Thrusting during the Grenville orogeny moved the Sand Bay rocks from their original location near the continental margin to the interior of the orogen. © 1997 Elsevier Science B.V.

Keywords." Geodynamic setting; Grenville Province; Metasedimentary rocks; Metavolcanic rocks; Protoliths

1. Introduction

The collisional Grenville orogeny took place between ,-~ 1200 and 1000 Ma. The last ~ 1 Ga of erosion of the Grenville orogen (Fig. 1A) has exposed huge areas of high grade gneisses, ana- logues of the deep, presently inaccessible parts of modem orogens. Much of this high grade rock

* Corresponding author.

0301-9268/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0301-9268 (97) 00036-3

may have originated in earlier orogenic cycles with the result that in the Grenville orogen, unlike more recent or less deeply eroded orogens, there is fragmentary preservation of assemblages related to development of a continental margin and ocean prior to the orogeny. Perhaps only a few percent of the orogen is composed of such rift-and-drift assemblages, most concentrated in a single bel t - - the Central Metasedimentary Belt of southwest Ontario and Quebec that likely formed at a conti- nental margin during the first stages of the

98 N. Culshaw, J. Dostal / Precambrian Research 85 (1997) 97-113

~bo hr. /

Fig. 1. (A) Location map. (B) Sh, Shawanaga domain; PS, Parry Sound domain; MS, Muskoka domain (Moon River domain is the western lobe); CMB, Central Metasedimentary Belt. Heavy lines, domain boundaries (selected thrust sense shown); heavy lines with crosses, synform traces.

Grenville cycle (McEachern and van Breemen, 1993) or as several accreted terranes (Easton, 1992). In contrast, much of the exposed Grenville orogen, from Labrador to Ontario, contains rocks added to Laurentia and involved in orogeny during ca 1800-1400 Ma (Gower and Tucker, 1994). Any remaining potential Grenvillian rift-and-drift assemblages may be preserved as belts of monocy- clic (showing evidence of GrenviUian metamor- phism only) supracrustal rocks hidden among older polycyclic (Grenvillian and older metamor- phism) gneisses.

This situation is exemplified in the Central

Gneiss Belt (CGB) of Ontario where polycyclic rocks related to the development of Laurentia well before the Grenville cycle (Dickin and McNutt, 1989) are abundant (Fig. 1B). These occur at the lowest structural levels and characteristically con- tain evidence for two or more ages of granitic plutonism and pre-Grenvillian high grade meta- morphism. The Britt domain is a good example, having plutonism at ca 1.7 and 1.45 Ga (van Breemen et al., 1986; Corrigan et al., 1994) and granulite facies metamorphism at ca 1.45 Ga (Ketchum et al., 1994). More southerly polycyclic domains have similar field relations to the Britt domain (e.g. Culshaw et al., 1983, 1990). Monocyclic units in the CGB include the small Sand Bay gneiss association, an assemblage of gneisses of supracrustal origin in the Shawanaga domain [Fig. 2; Culshaw et al. (1989, 1994)). These, together with the overlying monocyclic Parry Sound domain [also containing monocyclic supracrustal and plutonic rocks; Davidson et al. (1982); Culshaw et al. (1989); Wodicka (1994); Wodicka et al. (1996)] and Moon River domain, forms an allochthonous stack that is itself overthrust by the Central Metasedimentary Belt (Fig. 1).

One result of the residence of these gneisses in the deep orogenic crust is that deformation at high metamorphic grade has modified them to such an extent that mapping tectonostratigraphically sig- nificant groupings of gneiss is a challenge. In such circumstances it is tempting to be pessimistic about assessing the geodynamic significance of such rocks. Our method of identifying significant pack- ages of supracrustal gneiss is, however, quite straightforward. In part it relies on recognition of sequences in which unmistakeably supracrustal compositions (e.g. pelitic, quartzitic) are interlay- ered with rock compositions that could have origi- nated in the plutonic or surficial domains (e.g. quartzofeldspathic gneiss, amphibolite). Assessment of the protoliths of the interlayered ambiguous compositions is straightforward because deformation at high metamorphic grade typically removes all vestiges of primary textures from supracrustal gneisses, in contrast to plutonic gneisses. Thus, once the recognition of a grouping of supracrustal gneisses has been made with some

N. Culshaw, J. Dostal / Precambrian Research 85 (1997) 97-113 99

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%

s ~ o ~ r gnaum~m~n ~ a~wn)

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Fig. 2. Locality map. GF, Grenville Front; B, Britt domain; N1SZ, Nares Inlet shear zone; Sh, Shawanaga domain (Sand Bay gneiss association, patterned; Ojibway gneiss association, no pattern); SSZ, Shawanaga shear zone; PSSZ, Parry Sound shear zone; bPS, basal Parry Sound assemblage; iPS, interior Parry Sound assemblage; MR and Mrb, sub-assemblages of the Moon River domain (part of the Muskoka domain); L.GH, U.GH, lower and upper Go Home domains; TMB, Twelve Mile Bay assemblage.

confidence there is no reason why hypotheses concerning their geodynamic setting cannot be put forward and tested--which is what is attempted here.

The confident recognition of monocyclic supracrustal gneisses in the Shawanaga domain raises several questions. In what plate tectonic environment did they form? Are they related to Proterozoic events in mid-continental U.S.A.

and/or elsewhere in the Grenville Province? Or are they a remnant of a Grenvillian margin, kin to rocks of the Central Metasedimentary Belt? Wodicka et al. (1996) suggested a comparable origin for the superincumbent Parry Sound domain rocks that is, formed at or close to the Laurentian margin and during the early stages of the Grenville cycle. To begin to clarify these questions we have attempted to constrain the composition and origin

100 N. Culshaw, J. Dostal / Precambrian Research 85 (1997) 97-113

of the monocyclic supracrustal gneisses of the Sand Bay gneiss association by documenting a represen- tative section and geochemistry of selected units.

2. Regional geology

In the CGB shear zones separate lithological domains that are interpreted as thrust sheets assembled during the Grenvillian orogenic event (Davidson and Morgan, 1981; Davidson et al., 1982; Culshaw et al., 1983; Rivers et al., 1989). The Shawanaga shear zone separates polycyclic assemblages from the overlying predominantly monocyclic assemblages [Figs. 1 and 2; Culshaw et al. (1994); White et al. (1994)].

Geochronological data show that the polycyclic CGB is a collage of gneisses with an older plutonic component (ca 1600 Ma), followed by granitoid magrnatism at ca 1450-1340Ma. This bimodal age-division and further groupings within it have correlatives in the mid-continental U.S.A. The similar timing of granitic magmatism in the Grenville Front area near Killarney, Ontario at ca 1740 and 1470Ma [Fig. 1; van Breemen and Davidson (1988)] provides a link between the Grenville and the mid-continent, suggesting the polycyclic CGB to have been derived from rocks equivalent in age to those within the mid-continent (Easton, 1986; van Breemen et al., 1986; van Breemen and Davidson, 1988; Nadeau, 1990; Dickin and McNutt, 1989; Dickin et al., 1990). Metasediments [mostly (semi-) pelitic, quartzitic] comprise a small percentage of the polycyclic CGB and are constrained by cross-cutting relations to be older than 1450 Ma (with most older than 1600 Ma).

In contrast, the range in U-Pb ages for metaplu- tonic rocks within the monocyclic Parry Sound domain is more restricted. Igneous crystalliza- tion age of the protolith of granitoid gneisses [ca 1425Ma, van Breemen et al. (1986); 1380-1300Ma, Wodicka (1994); Wodicka et al. ( 1996)] overlapped with emplacement of anortho- site [ca 1350Ma, van Breemen et al. (1986); 1163 Ma, Wodicka (1994); Wodicka et al. (1996)] and quartzite deposition [< 1385 Ma, and < 1120 Ma for northwest and southeast of Parry

Sound domain, respectively, Wodicka (1994); Wodicka et al. (1996)]. These ages broadly coin- cide with the 1.35-1.25 Ga range for ages of older volcanism and sedimentation in the Central Metasedimentary Belt. As pointed out by Wodicka (1994) and Wodicka et al. (1996) younger sedi- mentation in the Central Metasedimentary Belt (Frontenac Terrane 1.31-1.18 Ga; Flinton Group, ca 1.16 Ga) has age correlatives in allochthonous monocyclic rocks of the CGB.

Most U-Pb age estimates for metamorphic min- eral growth (zircon, monazite, titanite etc) in the recognized parautochthon, lie within the interval ca 1030-1070Ma (van Breemen et al., 1986; Davidson and van Breemen, 1988; Corrigan et al., 1994; Bussy et al., 1995). In the Parry Sound domain estimates for the age of metamorphism lie in the older interval, ca l120-1160Ma (van Breemen et al., 1986; Tuccillo et al., 1992; Wodicka, 1994). Culshaw et al. (in press) presented a model in which all domains of the CGB and the Central Metasedimentary Belt were assembled in an essentially piggy-back fashion at or shortly before ca 1080 Ma. An important feature of this model, is that on their restored section, rock ages decrease away from the craton; some of the youn- gest rocks (e.g. Parry Sound domain) have already been metamorphosed before this phase of convergence.

3. Tectonostratigraphy of Shawanaga domain and basal Parry Sound domain

Culshaw et al. (1994) described assemblages within those segments of the Britt, Shawanaga (that contains the Sand Bay gneiss association-- the subject of this study), and Parry Sound domains that are contiguous along Georgian Bay.. The assemblages are briefly described below and more detailed descriptions are contained in Culshaw et al. (1994). We note, in our description below, a change in classification of some units that has resulted from more detailed mapping since Culshaw et al. (1994) and has significant implica- tions for understanding of the tectonostratigraphy.

The southern Britt domain [Bayfield-Nadeau Island assemblage of Culshaw et al. (1994)] con-

N. Culshaw, J. Dostal / Precambrian Research 85 (1997) 97-113 101

tains an older, predominantly granitoid plutonic suite (1602-1800 Ma; Krogh, personal communi- cation) and minor supracrustal rocks intruded by younger K-feldspar megacrystic granitoids of the Pointe-au-Baril complex at 1430-1460Ma (Krogh, personal communication). Mafic dykes cut pre-Grenvillian leucosomes and foliation, and serve as markers separating Grenvillian from older metamorphism and deformation. Pre-Grenvillian granulite facies mineral assemblages, dated at ca 1450 Ma [U-Pb on zircon; Ketchum et al. (1994)], persist locally, but are generally overprinted by syntectonic, upper amphibolite facies GrenviUian assemblages.

The Shawanaga domain overlies the Britt domain and is separated from the latter by a cryptic thrust reactivated during extension along the Shawanaga shear zone [see Culshaw et al. (1994)]. Along Georgian Bay the Shawanaga domain includes the gray, migmatitic, granodio- ritic to tonalitic gneiss of the Ojibway gneiss association (crystallization age 1450 Ma; Krogh, personal communication) that grades southeast- ward into a leucosome-rich migmatite (Fig. 2). Supracrustal units of the Sand Bay gneiss associa- tion include uartzofeldspathic migmatite, plagio- clase-quartz-biotite schist (Dillon schist), amphibolite and quartzite. Their contact with the Ojibway gneiss is folded (Fig. 2) and may be either a deformed unconformity or a decollement (Culshaw et al., 1994). An important feature of the Shawanaga domain is that it lacks the crosscut- ting mafic dykes that are prevalent in assemblages above and below (Culshaw et al., 1994). In this regard we have recently recognized the significance of the absence of mafic dykes from a sequence that overlies the Sand Bay gneiss association in the southwest and was formerly assigned (Culshaw et al., 1994) to the basal Parry Sound assemblage. The unit comprises two sequences of amphibolite: one with micaceous quartzofeldspathic paragneiss; and another with metapelite-arenite (metagrey- wacke-shale) interlayers. Because it lacks mafic dykes this sequence is excluded from the basal Parry Sound assemblage [contrast Culshaw et al. (1994)]. A late extensional shear zone, reactivates the cryptic thrust that separates the overlying

dyked units of the basal Parry Sound assemblage and the structurally lower dyke-free units.

All rocks of the Shawanaga domain preserve upper amphibolite facies mineral assemblages. A Grenvillian age of metamorphism is consistent with the absence of metamorphosed mafic dykes cutting older metamorphic fabrics. The assemblage contains, in contrast to the Britt domain, members of a ca 1170-1150 Ma coronitic olivine metagab- bro suite (Davidson and van Breemen, 1988; van Breemen and Davidson, 1990; Heaman and Le Cheminant, 1993) and, near the contact of the assemblages, garnet- and clinopyroxene-rich meta- basite. The latter record a high pressure metamor- phism and do not occur at lower structural levels indicating that they and their host rocks must have originated from deep crustal levels before being emplaced at their present site.

The basal Parry Sound assemblage, the structur- ally lowest unit of Parry Sound domain contains marble and quartzite [deposition age less than ca 1385 Ma, Wodicka (1994); Wodicka et al. (1996)] interlayered with ca 1380-1300Ma granitoid orthogneiss (Wodicka, 1994; Wodicka et al., 1996) and gneissic anorthosite intruded at ca 1160 Ma (Wodicka, 1994; Wodicka et al., 1996). Syntectonic Grenville age upper amphibolite facies mineral assemblages occur within much of the basal Parry Sound assemblage. Mafic dykes cut the ca 1380-1300 Ma orthogneisses, primary composi- tional layering in quartzite and a metamorphic fabric in the anorthosite, and distinguish the Parry Sound domain from immediately subjacent units along Georgian Bay.

The Shawanaga shear zone (Fig. 1) that sepa- rates polycyclic from monocyclic assemblages is a wide extensional shear zone that, according to seismic reflection profiling, dips gently southeast- ward beneath the CGB (White et al., 1994). Extensional shear occurred under upper amphibo- lite facies conditions and was preceded by thrusting (Culshaw et al., 1994). This extension is similar kinematically to the localized extensional shear along the base of the Parry Sound domain. Culshaw et al. (1994) postulated a cryptic thrust separating dyke-free from overlying dyked rocks of the Parry Sound domain. High strain ductile fabrics throughout the units are a composite of

102 N. Culshaw, J. Dostal / Precambrian Research 85 (1997) 97--113

strains accumulated during thrusting and exten- sion; large folds with axes trending northwest, parallel to the orogenic transport direction, formed in all units during extension while earlier nappe- like isoclines are restricted to allochthonous, monocyclic units (Culshaw et al., 1994).

4. Lithology

trending synform. The column is presented with the structural base (west side) shown as the strati- graphic base. Compositional polarity across the section and a lack of obvious repetition that could be interpreted as a result of early folding or thrusting, suggests original stratigraphy, albeit severely thinned and with stratigraphic top unknown. In contrast, no primary structures or textures are preserved in any of the units.

Essentially continuous sections of the Sand Bay gneiss association are exposed at several locations along the excellent shoreline exposure of Georgian Bay. The section measured at Shawanaga Bay is representative of the others, differing in minor details of lithology but having the advantage of being one of the sections least complicated by folding. The section is presented in the form of a stratigraphic column in Fig. 3. This is a convenient way of presenting the measured thicknesses and lithological data because it illustrates the relative proportions of rock types and demonstrates our contention that the sequence is not significantly duplicated at this scale by isoclinal folding or thrusting. However, ductile deformation must have severely thinned the Shawanaga Bay section which has been affected by strain related to both thrusting and extension (Culshaw et al., 1994). There is no reliable estimate of the amount of strain, although, the presence of sheath folds nearby implies that the layers may have been thinned by as much as ten to one hundred times from their original thicknesses. The thicknesses shown in the section, which was measured in, or close to, the XZ section of the bulk strain ellipsoid, are probably therefore much smaller than original values. Layers are continuous over tens of metres and boudinage is relatively minor, thus it is likely that the thinning is, to a first approximation, homogeneously distrib- uted between amphibolitic, quarzofeldspathic and quartz-rich units.

4.1. Shawanaga bay section

The section in Fig. 3 is almost 750 m wide (including 150m of outcrop gaps) and extends from the western contact with the Ojibway gneiss association to the axial trace of a late northwest

4.1.1. Pink leucocratic gneisses Pink leucocratic gneisses are restricted to the

bottom half of the section. There are two major horizons (each ca 50 m thick) and several thinner intervals, including some thin ridges in ribbed units. These are bright pink, equigranular, gra- noblastic, moderately migmatitic gneisses with a notably uniform composition, generally lacking layering other than some discontinuous mafic (amhibole-biotite) bodies. Microcline, albite-oli- goclase and quartz with minor biotite and magne- tite (accessory aUanite) are ubiquitous. This rock type is voluminous elsewhere in the Sand Bay gneiss association and very similar rocks are encountered in other domains along the Georgian Bay transect.

4.1.2. Amphibolites Amphibolites are concentrated within the top of

the basal half of the section, sandwiched between pink leucocratic gneisses (Fig. 3). The total thick- ness of ca 100 m includes several units separated by narrow horizons of calcsilicate and quarzofelds- pathic gneiss. Most units are combinations of layered, homogeneous and migmatitic amphibo- lite, all of which are granoblastic and medium grained. The topmost units are distinctive, being coarser grained and in places containing amphi- bole-rich layers (these are referred to below as 'heterogeneous amphibolites'). Layered amphibo- lite has parallel-sided, colour bands or lenses defined principally by variations in proportions of amphibole and plagioclase or biotite. At the west side, an amphibolite dyke cuts the layering. Such cross-cutting dykes are absent in the remainder of the section and from other lithologies in the Shawanaga domain (Culshaw et al., 1994). Migmatitic amphibolite, developed within homo-

N. Culshaw, J. Dostal / Precambrian Research 85 (1997) 97-113 103

WIm

w ~ t

Fig. 3. Lithological column from Shawanaga Bay.

~ / l r /

a0o

4o0

2o0

geneous amphibolite, has leucosomes parallel to axial surfaces of late folds and in extensional pull- aparts. Relict plutonic textures, such as occur in many other mafic bodies in the CGB, are absent. Amphibole, plagioclase (andesine or andesine-lab- radorite) and biotite dominate, clinopyroxene is sporadic throughout the section whereas alman- dine appears to be restricted to a short interval near the base; ilmenite or titanite and apatite and zircon are accessories.

4.1.3. The calcsilicate gneiss The calcsilicate gneiss intervals that separate the

amphibolite units are thin (from 15 cm to 4 m), compositionally layered and are composed of vari- ous combinations of clinopyroxene, amphibole,

K-feldspar, plagioclase, quartz, epidote, titanite, calcite and garnet. These resemble gneisses else- where in the Sand Bay gneiss association that outcrop close to amphibolite and are interlayered with marble.

4.1.4. The Dillon schist The Dillon schist dominates the east half of the

section but also occurs in the west, as noted above, interlayered with migmatitic gneiss and quartzite forming intervals of ribbed gneiss. The east part of the section is ca 300 m thick (including interlay- ered quartzites etc.), although this thickness may be exaggerated by minor, extension-related fold- ing. The Dillon schist is light to dark grey, medium grained, equigranular rock. The principal mineral

104 N. Culshaw, J. Dostal / Precambrian Research 85 (1997) 97-113

constituents are: plagioclase (oligoclase, oligo- clase-andesine), quartz, phlogopitic biotite, amphibole, epidote and titaniferous magnetite; and some varieties have microcline (1.5% BaO), mus- covite or scapolite as major constituents. Allanite and apatite are common accessories. The rock often has horizons < 1 m to several metres thick differentiable on the basis of slight colour changes due to variations in proportions of the primary constituents, in rare cases the horizons are amphib- olitic. The horizons may themselves display a faint, inch-scale, colour banding. Many horizons contain characteristic spindles of quartz that define linear fabric. Some of these are demonstrably derived from reoriented pull-apart fillings, boudined veins or parts of pegmatites. While some leucosomes in the Dillon schist are compositionally simple (K-feldspar-quartz-amphibole) other outcrops contain distinctive, mineralogically complex, leucosome-like veins (e.g. K-feldspar-quartz- carbonate-chlorite-ilmenite +_ amphibole, apatite, tourmaline, sphene, epidote, garnet).

There are seven occurrences of quartzite (Culshaw et al., 1989) concentrated at the east and west ends of the section. These are thin units that occur as isolated layers (30 cm-2 m wide) or as clusters of several (up to 17) layers, each 5-30 cm wide, interlayered with Dillon schist. In the basal part they form intervals of ribbed gneiss along with grey or pink migmatitic gneiss and Dillon schist. The quartzites are in fact somewhat imma- ture, being in places migmatitic and contain- ing minor microcline, plagioclase, biotite, +muscovite, titanite and ilmenite and accessory allanite and apatite.

4.1.5. Migmatitic grey and grey-pink gneisses Migrnatitic grey and grey-pink gneisses are con-

centrated near the west side of the section. They occur as horizons thick enough (6-15 m) to be shown as units on the column and thin intervals (5-75 cm) interlayered with other compositions (primarily Dillon schist) forming intervals of ribbed gneiss. Most are compositionaUy homoge- nous but locally display colour banding corre- sponding to variations of K-feldspar and

plagioclase. Plagioclase, quartz and K-feldspar dominates biotite and/or amphibole and/or epidote and accessory titanite, allanite and ilmenite.

5. Geochemistry

5.1. Analytical techniques

Forty-two samples of amphibolite and pink leu- cocratic gneiss were collected along the Shawanaga Bay section. These were selected to test if they had a volcanic protolith--as suggested by their miner- alogical composition and field relations. The samples were analysed by X-ray fluorescence for major and several trace elements (Rb, Sr, Ba, Zr, Nb, Y, Zn, V, Cr and Ni). Additional trace elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er,Tm, Yb, Lu, Nb, Th and U) were determined by inductively-coupled plasma-mass spectrometer (ICP-MS) in 16 selected samples. The precision and accuracy of the ICP-MS and X-ray fluores- cence data were reported by Dostal et al. (1986, 1994), respectively. Analytical errors for trace ele- ments are generally <10 rel.%. The analyses of representative samples are given in Table 1.

5.2. Validity of approach

Chemical compositions of volcanic rocks meta- morphosed under high-grade conditions have been extensively used to determine the tectonic setting in which they were extruded. An interpretation of the chemical data assumes that the concentrations of elements under consideration remained essen- tially unchanged during metamorphism and defor- mation, although secondary processes might have modified the abundances of some elements. In the case of the Sand Bay rocks, there are several lines of evidence to suggest that most major elements such as A1, Fe and Mg, as well as the relatively 'immobile' elements including high-field-strength- elements (HFSE-Ti, P, Zr, Hf, Y, Nb), rare earth elements (REE) and the transition elements (Cr, Ni and V), retained their original distribution. When these elements are plotted against Zr, which is considered to be a good indicator of fraction-

N. Culshaw, J. Dostal / Precambrian Research 85 (1997) 97-113 105

Table 1 Major and trace element compositions of selected representative samples from the Shawanaga Bay section

Sample Amphibolites Leucocratic gneisses

Group 1 Group 2 Group 3

DS-2-29 DS-2-31 DS-2-12 DS-2-14 DS-2-16 DS-2-5 DS-2-8 DS-2-20 DS-2-23 DS-2-55 DS-2-52 DS-2-34

Si02 (%) 47.99 47.56 49.75 49.02 49.19 50.04 5 1 . 4 8 51.76 53.43 71.16 77.07 75.19 Ti02 1.14 1.13 1.89 2.14 1.77 2.99 2.27 2.44 2.63 0.41 0.08 0.28 A1203 16.58 15.99 13.35 13.28 13.67 12.02 12 .22 11.91 12.22 14.71 12,29 11.60 Fe20 ~ 11.05 11.81 15.03 15.27 14.86 18.39 15 .45 16.24 16.41 2.01 1,10 2.81 Mn0 0.19 0.20 0.22 0.22 0,21 0.24 0.20 0.21 0.22 0.03 0.01 0.05 Mg0 7.29 7.30 6,13 5.63 6.30 3.57 3.85 3.58 3.56 0.22 0.01 0.01 Ca0 10.40 10.82 9,99 10.15 10.07 8.56 8.15 8.25 7.88 0.85 0.53 0.43 Na20 3.19 3.08 2.92 2.80 3.09 2.91 3.17 3.00 3.12 4.03 3.21 2.39 Kz0 1.20 1.15 0.89 0.86 1.05 1.17 1.64 1.55 1.00 5.71 4.75 6.36 Pz0~ 0.29 0.29 0.22 0.25 0.22 0.47 0.30 0.34 0.40 0.05 0.02 0.01 LOI 0.40 0.20 0.30 0.60 0.20 0.10 0.90 0.20 0.10 0.40 0.20 0.10

Total 99.72 99.53 100 .69 100 .22 100 .63 100.46 9 9 . 6 3 9 9 . 4 8 100.97 99.58 99.26 99.23

Mg ~ 0.57 0.55 0.45 0.42 0.46 0.28 0.33 0.30 0.30

Cr (ppm) 198 189 80 81 77 5 16 9 - - 8 6 13 Ni 140 147 49 41 51 23 22 24 15 - - 5 - - V 214 214 380 417 358 489 459 424 445 10 5 - - Zn 129 132 128 149 144 166 141 158 163 31 6 48 Rb 23 16 I1 12 15 21 56 42 9 127 144 173 Ba 122 110 180 150 192 183 344 410 145 1581 49 993 Sr 574 558 247 322 292 297 289 288 350 119 40 82 Ga 19 18 21 22 20 25 21 24 22 19 17 18 Nb 5.3 4.8 5.9 5.6 5.5 11.7 8.7 10.3 9.9 11.0 6.2 15.0 Hf 2.63 2.65 3.51 3.85 3.30 6,11 4.64 5,51 6.03 13.10 4.71 14.30 Zr 80 83 115 129 108 206 157 178 200 455 160 483 Y 21 20 32 36 31 51 38 45 42 36 17 66 Th 0.68 0.91 1.17 1.40 1.20 5.19 4.02 4.78 4.52 9.57 15.21 11.42 La 9.44 11.97 11.63 12.68 11.46 27.73 2 0 . 6 0 24.87 25.68 93.47 29.87 57.53 Ce 24.58 27.55 28.93 32.03 27.67 64.65 46.69 57.08 56.29 190.3 70.02 128.2 Pr 3.63 3.75 4.21 4.72 4.12 8.77 6.35 7.75 7.52 23.20 9.48 16,34 Nd 16.96 16.41 19.38 22.86 18.07 37.09 2 8 . 2 8 34.04 32.78 89.56 37.23 66.49 Sm 4.22 4.17 5.12 5.77 5.13 9.22 7.04 8.21 8.18 13.78 8.50 14.82 Eu 1.31 1.28 1.76 1.91 1.78 2.36 1.85 2.24 1.80 2.37 0.13 2.04 Gd 4.36 4.11 6.21 7.05 5.87 9.49 7.26 8.89 8.54 9.43 6.69 13.65 Tb 0.67 0.64 0.98 1.11 0.97 1.55 1.17 1.36 1.32 1.21 0.93 2.16 Dy 4.18 3.86 6.05 7.10 6.20 9.71 7.33 8.80 8.47 7.31 4.80 13.24 Ho 0.87 0.81 1.28 1.48 1.22 2.02 1.53 1.85 1.69 1.39 0.82 2.75 Er 2.40 2.30 3.72 4.17 3.68 5.75 4.44 5.19 4.68 4,18 1.96 8.07 Tm 0.35 0.33 0.53 0.61 0.55 0.81 0.60 0.76 0.64 0.60 0.25 1.20 Yb 2.46 2.07 3.56 3.75 3.12 5.35 3.97 4.72 4.2l 3.90 1.63 7.56 Lu 0.36 0.35 0.49 0.55 0.51 0.74 0.58 0.73 0.66 0.64 0.23 1.13 Ti/V 32 32 30 31 30 37 30 35 35 246 90 Zr/Y 3.8 4.1 3.6 3.6 3.5 4.0 4.1 4.0 4.8 12.6 9A 7.3 (La/Yb), 2.33 3.50 1.98 2.05 2,23 3.14 3.14 3.19 3.70 14~53 11.11 4.61 Th/La 0.07 0.08 0.10 0.11 0.10 0.•9 0.20 0.19 0.18 0.10 0.51 0.20

aMg, Mg/(Mg+ F e); n, chondrite-norrnalized.

106 N. Culshaw, J. Dostal / Precambrian Research 85 (1997) 97-113

ation and is apparently immobile under most metamorphic conditions (e.g. Winchester and Floyd, 1977), they display distinct correlations (Fig. 4). Remobilization during metamorphism is unlikely to produce such a consistent result. The consistency of the trends and their similarities to those of modern volcanic rocks suggest that the distribution of the elements was not significantly modified. The elongated trends suggest that the amphibolites might represent a cogenetic suite. On the other hand, a scatter of K, Rb, Ba, Sr and Li when plotted against Zr indicates that these elements have been redistributed (not shown; Table 1 ).

The amphibolites show close compositional sim- ilarities and variation trends comparable to modern basaltic rocks. On several discrimination diagrams (Leake, 1964; Van de Kemp, 1969) including Cr-TiO z and Ni-TiO2 (Fig. 5), the varia- tions of the mafic rocks parallel those of basaltic suites and exceed the field for para-amphibolites and pelites. The homogeneity of the amphibolites, including the relatively restricted range of SiOz accompanied by high FeO* (total Fe as FeO), MgO and CaO, argues also that the precursors were not sediments but mafic igneous types.

300

1 0 0

30

10

! \

..... ;"1 ................ '":"..,,, l | \ "'. I \ "

• \ :

Pelite ~ ." ~ . Y, Field ,, • . ~" • .." ~

\

CRB Field \

\ •

\ • \ •

% \ •

Cr (ppm) \ " \~•

n ~ L e

t 3 TiO2 (%)

• \ e"

\ . . . . ' e #

Polite \,."" • " F eld ..'" \ ~,

. . ' * \ . . " \ \ !

\\ CRB Ni (ppm) \ \ Field

\ \

\ i I i

1 2 3 TiO2 (%)

Fig. 5. Discrimination diagrams illustrating the igneous origin of the amphibolites. The pelite fields after Johnson et al. (1987). CRB (Columbia River continental flood basalts), fields from data of Hooper (1988) and Hooper and Hawkesworth (1993).

5.3. Amphibolites

The Zr/TiOz-Nb/Y and ZrTiO2-SiO 2 diagrams (Figs. 6 and 7) confirms the distinctly bimodal nature of the Sand Bay suite and suggests that the amphibolites were originally subalkalic basalts. The amphibolites have an SiO2 range (Fig. 7)

60

50

40

30

200

150

100

50

0

Y (ppm) • 8

• 0 • 7

¢ • •• 6

• ~1. ° e

• ¢ : 5

• 4

• 3 n n u

Cr (ppm) 4

3

2

1 "¢ ; .

i i e I ' ~ e l •

100 150 200 Zr (ppm)

MgO (%)

# •

• ¢~ ••

i i i

FeO*/MgO • 12 •

o~

• , ~

i i i

0 5 0 1 0 0 1 5 0 2 0 0

Zr (ppm)

0.3

o.1

0.03

0.01

0.003

Corn/Pant t Phonolite

Rhyolite ~ /i,,.,,,,, ( ~ (3D " ' ' ' " - .

o (~ " "~ , / Trachyte ..... o ..... o-..o. ..... . /

R h y o d a c i t e / o - ......... Dacite TrachyAnd " ~

A n d ° s i t e / o ,"" I B a s a l t ~ . . . . . I Bsn/Nph

Sub-Alkaline B a s a l t

, l , i , J , , , ) , , , , I . . . . . . . . I

0.1 0.3 1 3 10 Nb/Y

Fig. 4. Y (ppm), Cr (ppm), MgO (%) and FeO*/MgO versus Zr diagrams illustrating covariations of these elements in the amphibolites.

Fig. 6. Zr/TiO2 versus Nb/Y diagram (Winchester and Floyd, 1977) illustrating the bimodal nature of the suite and the subal- kalic basaltic composition of the amphibolites.

N. Culshaw, J. Dostal / Precambrian Research 85 (1997) 97-113 107

80 o

Rhyolite 0 o 75 o 0

70 •m/Pan R h y o d a c i t e / D a c i t e

,-- 65 , / / /

55 ~ / / Phonolite

, o

Sub-AB .." AB/,/ B~/Traoh/Neph

45 . . . . . . . . i . . . . . . . . i J 0 ' 3 ' ,~=,,I 0.003 0.01 0.03 0.1 . 1

Zr / TiO 2

Fig. 7. Zr/TiO2 versus SiO: diagram (Winchester and Floyd, 1977) illustrating the bimodal nature of the suite and the subal- kalic composition of the amphibolites.

between 45 and 55% (LOI-free) and display a typical tholeiitic trend of an increase of TiO2 content with decreasing (Mg) number (Fig. 8). Compared to island arc tholeiites, the rocks have higher Ti/V [> 20; Shervais (1982)] and Zr/Y [> 3; Pearce and Norry (1979)] ratios; similar values are found in intraplate basalts, including continental tholeiites. The rocks display a large range of (Mg) numbers [Mg/Mg + Fe*], which distinctly correlate with several major and trace elements (Fig. 8). The variations include an increase of several incompatible trace elements such as Zr, La and Th but a decrease in AI, Ca, Mg, Cr and Ni with the decrease of (Mg) numbers. However, the ratios of A12Oa/CaO, Zr/Y and La/Yb remain relatively constant. These variation trends are typical of tholeiitic low pressure fractional crystallization involving olivine, pyroxenes and plagioclase. The process is also consistent with the behaviour of REE which show an increase of the absolute abundances with fractionation while the shape of the REE patterns remains essentially unchanged. The patterns show moderate light REE enrich- ments (Fig. 9). By virtue of variation in (Mg) numbers, the tholeiites can be subdivided into

18

16

14

12

AI203 (%)

• e •

Ti02 (%) $,0, • •

e O e e •

• l °

150 : •

• " 100

50

• e •

I

Ni (ppm)

e •

• 0 #

0 '

i f ~ A1203/CaO

- s .o % ' * • • l l ~ Q •

o •

| $

I I I I I

t I • Th (ppm) 25 La (ppm)

20 f " 15 • e • e l

e I 10

' ' 0 1 4 ' 0 1 5 ' ' 0J4 ' ' 03 0.3 0.5 (Mg) (Mg)

I .

I

Fig. 8. Variations of AlzO3(%), TIO2(%), Th (ppm), Ni (ppm), La (ppm) and AI:O3/CaO ratio relative to (Mg) number (Mg/Mg +Feto0 for the amphibolites.

three groups (Table 1). The rocks of group 1 are the most primitive with (Mg) ratios ca 0.55 (0.53-0.57), the rocks of the second group have (Mg) numbers in the range of 0.38-0.47, whereas the rocks of group 3 have (Mg) between 0.27 and 0.33.

The rocks of group 1 are relatively rare and occur at the top of the sequence (amphibolites with heterogeneous textures). They have the high- est contents of transition elements, including Ni and Cr, and the lowest concentrations of the incompatible trace elements. Compared to the other two groups, they have higher contents of A1203, C a • and Sr. The amphibolites have rela- tively fiat REE patterns with (La/Yb), of 2-4. Their patterns are similar to those of continental flood basalts such as those from Madagascar (Fig. 9) and Deccan Traps [the Ambenali Formation; Lightfoot and Hawkesworth (1988)] and relatively primitive continental tholeiites such as a USGS standard rock W-1 (Fig. 9), a Meso- zoic tholeiitic diabase from Virginia, U.S.A. (Govindaraju, 1994).

108 N. Culshaw, J. Dostal / Preeambrian Research 85 (1997) 97-113

100

oo 30 L U

I-- nr" CI 10 Z O "1" O

O O ~ 100

30

10

A AMPHlaOUTE- GROUP 1 100

~ . . . D S - 2 - 2 9 30 w'-.~-.. ................... W-I - ' ~ 10

J i i i = = I i , i i I , i

B AMPHIBOLITE- GROUP 2

DS-2-14

DS-2-12

300

C AMPHIBOLITE- GROUP 3 ioo

..2.~:~.~.

" ~ ' ~ ~ 30

BCR-I . . . . 2'~" '-' '"'-".2 "-'"'-" ' 2"2 ' •

10

I , & i I I I = , * * I I I i 1 I i

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce

D LEUCOCRATIC GNEISS

DS-2-34 ......., ..........

.~.g- -, ..........................................

I I i i i | i i t t = i

Pr Nd Sm Ea Gd Tb Dy Ho Er Tm Yb Lu

Fig. 9. Chondrite-normalized REE abundances in the amphibolites and leucocratic gneisses. (A) Amphibolites of group 1 (samples DS-2-29, and DS-2-31). USGS standard rock W-I, a Mesozoic continental tholeiite from Virginia, U.S.A. (Govindaraju, 1994) is shown for comparison. (B) Amphibolites of group 2 (DS-2-12 and DS-2-14). Cretaceous continental flood basalts from Madagascar [sample 58; Dostal et al. (1994)] is shown for comparison. (C) Amphibolites of group 3 (samples DS-2-5 and DS-2-8). USGS standard rock BCR-1, a Columbia River continental flood basalt (Govindaraju, 1994) is shown for comparison. (D) Felsic rocks (samples DS-1-55, DS-2-34 and DS-2-52). Normalizing values after Sun (1982).

The rocks of group 2 comprise all of the homo- geneous amphibolite in the middle of the amphibo- lite succession (Fig. 3, inset) and some interspersed with group 3 rocks in the large interval of migmati- tic and layered amphibolites situated immediately below the heterogeneous amphibolite unit. These have higher incompatible trace elements but lower transition elements and have similar shaped REE patterns (Fig. 9). Their mantle normalized incom- patible element patterns show gradual increase from Lu to Th, accompanied by a distinct negative Nb anomaly similar to those of continental tholei- ites (e.g. W-1 and BCR-1, Fig. 10).

The rocks of group 3, with the exception men- tioned above, form the structurally lowest intervals in the succession (west side of the section). They have low MgO but high FeO*, TiO 2 and PzOs.

The low abundances of the compatible transition elements, Cr and Ni, confirm that the rocks underwent extensive fractional crystallization. They have the highest concentrations of REE and their patterns display a small negative Eu anomaly (Fig. 9) consistent with extensive fractional crys- tallization. Their primitive mantle-normalized incompatible element patterns (Fig. 10), marked by negative anomalies of Nb, are typical of conti- nental tholeiites (Thompson et al., 1982; Erlant 1984). They have compositions similar to that of Columbia River flood basalts [e.g. USGS standard rock BCR-1 from Washington, U.S.A., Fig. 10; Govindaraju (1994)]. In addition to extensive frac- tional crystallization, the rocks of group 3 appear to be also affected by crustal contamination. The Th/La (Table 1), which is frequently considered

N. Culshaw, J. Dostal / Precambrian Research 85 (1997) 97-113 109

100

30

10 LU . d I- z <

o 0 100= i1"

30

10

1

0.3

A AMPHIBOLITE - GROUP 1

\W- I \ ... DS-2-31 ,:~.' .......

~ ~ - ~ ¢ ~ , ~ DS-2-29

i 1 i i i | i I I * i * , i J i 1 i

100

30

10

B AMPHIBOLITE - GROUP 3

~ l ~ DS-2-5 ~_ + " ' ~ . _ . 2-8

BCR- I .e:.' ."'.=~ .........

l l l l l l l l l l p I I 1 1

,;-..,, C LEUCOC r,c GNE,SS ,0o

" ~ DS-2-34 30

"%--.. . . . . . . . . . . . 10

3

1

03 I I I | I I I I I I I I I 1 | i i i

Th Nb La Ce Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Er Tm Yb Lu

D LEUCOCRATIC GNEISS

' \ \ V RUM- 1 , . ~ ~

i , i i 1 i * , , i i , , J i , 1 L

Th Nb La Ce Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Er Tm Yb Lu

Fig. 10. Primitive mantle-normalized incompatible element abundances in the amphibolites and leucocratic gneisses. (A) Amphibolites of group 1 (samples DS-2-29 and DS-2-31). USGS standard rock W-1 [diabase; Govindaraju (1994)] is shown for comparison. (B) Amphibolites of group 3 (samples DS-2-5 and DS-2-8). USGS standard rock BCR-1 (Columbia River plateau basalt) is shown for comparison. (C) Leucocratic gneisses (samples DS-2-34 and DS-1-55). (D) Leucocratic gneiss (DS-2-52). USGS standard rock RGM-1 [calc-alkalic rhyolite from Glass Mountain, CA, U.S.A.; Govindaraju (1994)] is shown for comparison. Normalizing values after Sun and McDonough (1989).

to be a sensitive indicator of crustal contamination, is high in group 3 ( ~ 0.2), significantly higher than the mantle values (0.12). The high Th/La ratios in group 3 suggest that the fractional crystalliza- tion was probably accompanied by crustal contamination.

The compositional trends of the amphibolites within each group and also among various groups can be modelled by low-pressure fractional crystal- lization of pyroxenes, plagioclase and olivine. In addition, the rocks of group 3 were also affected by crustal contamination which probably accom- panied fractional crystallization. It is possible, but unlikely, that such intimately interlayered rocks might have been derived from different sources in a heterogeneous mantle.

5. 4. Pink leucocratic gneiss

Felsic rocks have mostly rhyolitic composition with SiO2 > 75% and A1203 > 11%. They are K-rich mostly with >5% K20 and typically with K20 > Na20. Compositionally they resemble calc- alkalic rhyolites. The relationships among Rb, Nb and Y show that the felsic rocks straddle the boundary between within-plate and volcanic arc types (Fig. 11). The REE patterns of the felsic rocks are distinctly enriched in light REE and some of them have also strongly fractionated, heavy REE (Fig. 9). Their (La/Yb), ratios range from 5 to 15. The patterns also display variable negative Eu anomalies. Their mantle-normalized incompatible element patterns (Fig. 10) are char-

110 N. Culshaw, .1.. Dostal / Precambrian Research 85 (1997) 97-113

v

100

30 !

WPG ,- ,....-'"""

10 O

O

3 VAG + syn-COLG

1

O

O ORG

, , , , I , , , , , , , , t . . . . . . . . I

10 30 100 300 1000 Y (ppm)

Fig. 11. Variations of Nb (ppm) versus Y (ppm) in the felsic rocks. The fields after Pearce et al. (1984). VAG, volcanic arc granites; syn-COLG, syn-collision granites; WPG, within-plate granites; ORG, ocean ridge granites.

acterized by negative Nb, Eu and Ti anomalies, and resemble modern rhyolites including those associated with continental rifting and arc environ- ments (Fig. 10). Assuming that the rocks are rhyo- lites, most of these rocks do not appear to be directly related to the associated amphibolites.

6. Discussion and conclusions

The chemistry shows that the amphibolites and the pink leucocratic gneisses have compositions compatible with a volcanic origin. The amphibo- lites display typical tholeiitic characteristics, and closely resemble continental tholeiites from conti- nental flood basalt or rifted margin environment. A flood basalt environment is unlikely because they are not voluminous compared to other rocks in the Sand Bay gneiss association. The presence of Nb depletion in all of the rocks suggests that they were formed in an environment that had been above a subduction zone (e.g. subcontinental litho- sphere previously fluxed by subduction fluids) or that they were affected by crustal contamination. Neither of these scenarios can be readily distin- guished. The compositions of the pink leucocratic gneisses resemble those of rhyolites. The amphibo-

lites and the pink leucocratic gneisses have a different petrogenesis, as would be expected for a bimodal suite. A volcanic origin for these two members of the Sand Bay gneiss association is also suggested by field relations. A volcanic protolith for the pink leucocratic gneisses is consistent with their uniform composition and lack of relict plu- tonic textures. They could be meta-arkoses, but their uniform composition, evident in field and geochemically, would in any case imply a nearby igneous source. The amphibolites also lack relict plutonic textures and a volcanic origin is compati- ble with interpretation of the interlayered, thin calcsilicate gneisses as low energy, partly chemical inter flow sediments. Homogeneous amphibolite and migmatitic amphibolite developed from it are interpreted as derived from flows. The layered varieties may be the highly strained derivatives of flows originally displaying any of a number of features, for example, pillows and breccias. It is less likely that these have been derived from aque- ously reworked material because of a lack of a siliciclastic component. The amphibolite dykes cut- ting the amphibolite may have been formed during construction of the volcanic pile.

The field evidence also suggests a supracrustal origin for all members of the suite. The Shawanaga Bay section has a polarity suggestive of evolution in time, as expected in a supracrustal pile. This is apparent on the large scale (e.g. leucocratic pink gneisses and amphibolites in west, Dillon schist in east) and at a smaller scale (e.g. within amphibo- lites, dyke and garnets to west, heterogeneous amphibolite to east). The quartzites clearly have a sedimentary protolith. Their appreciable feld- spathic component indicates they were not com- pletely mature. Detrital zircons have a spread of ages, from older than 2000 Ma to a slightly discor- dant grain 1382+_ Ma old (Krogh, personal com- munication), that is indicative of a Laurentian source. The mineralogical composition and prelim- inary chemical data from the Dillon schist are compatible with derivation from an immature, somewhat calcareous, epiclastic sediment. Detrital zircons suggest in part a proximal volcanic source probably ca 1364 Ma old that supplied most of the detritus (Krogh, personal communication). The localized, repeated interlayering with quartzite

N. Culshaw, J. Dostal / Precambrian Research 85 (1997) 97-113 111

suggests periods of cyclic variation in the energy of the depositional environment. Interlayering with Dillon schist is also compatible with supracrustal origin for the gray gneisses. Their composition suggests acidic to intermediate volcanic rocks or reworked volcanic rocks.

The evidence outlined above shows that the Sand Bay gneiss association in the Shawanaga Bay section is likely a supracrustal succession in which the stratigraphy is probably preserved in its origi- nal order, although the stratigraphic top cannot be determined. The present thickness of ca 600 m represents a tectonically thinned succession. An original thickness of several kilometres is plaus- ible. More specifically, the allochthonous Sand Bay gneiss association may represent a Meso- proterozoic rift succession originally laid down on Laurentian crust. The comparative thinness of the quartzites in comparison to the other major units and their interlayering with lithologies not typical of a platform environment, for example, the Dilon schist militates against the idea that this is a rifted platform environment. It may be comparable to other rift-related successions of comparable age but better preservation (lower metamorphic grade) on and adjacent to the eastern Grenville Province. These include the 1271-1239 Ma Aguanus Group of the Wakeham Supergroup (Martignole et al., 1994) and the 1250-1224 Ma Seal Lake Group (Romer et al., 1994). It could mark the opening of a Grenvillian ocean or back-arc rifting landward of an Andean arc. Either of these options is compatible with the cross-section of Culshaw et al. (in press) that shows the Sand Bay gneiss associa- tion restored to a position near the feather-edge of early-Grenvillian Laurentia before GrenviUian shortening. It would have been transported onto the polycyclic domains together with the superin- cumbent Parry Sound domain which likely also originated at a location close to the Laurentian margin (Wodicka et al., 1996). The deposition age of ca 1364 Ma for the Sand Bay gneiss associa- tion is somewhat older than that established for any supracrustal component of the Central Metasedimentary Belt [ e .g . 1300-1280 Ma, Grimsthorpe Domain volcanism; 1300-1240 Ma, sedimentation (.9) in Frontenac Terrane; Easton (1992)]. It is tempting to suggest that because rift-

related rocks should be older than assemblages offshore of a margin, that the Sand Bay gneiss association is part of the rift assemblage formed during opening of the Grenvillian ocean. However, they could have formed landward of an older, yet unidentified, arc. The abundant grey orthogneisses within the Muskoka domain would be good candi- dates for such an arc (Fig. 1B).

We conclude with a point of general interest: hypotheses of the geodynamic significance of pack- ages of high grade, ductilely deformed gneisses can be made on the basis of field relations and com- position and tested geochemically. A further exam- ple of such a hypothesis is that the entire allochthonous Sand Bay gneiss association (includ- ing the succession--not dealt with in this account--of amphibolite with metapelite-arenite that lies at the structurally highest levels) is part of the evolving Mesoproterozoic Laurentian margin (and possibly offshore material) that has been structurally telescoped and thrust over the older, more inboard Laurentian interior. This hypothesis awaits geochemical and geochronologi- cal testing.

Acknowledgment

N.C. thanks Neil Burns for able field assistance. N.C. and J.D. acknowledge financial assistance from NSERC.

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