Amphibolites of the Shawanaga domain, Central Gneiss Belt, Grenville Province, Ontario: tectonic...

Precambrian Research 113 (2002) 65–85

Amphibolites of the Shawanaga domain, Central GneissBelt, Grenville Province, Ontario: tectonic setting and

implications for relations between the Central Gneiss Beltand Midcontinental USA

Nicholas Culshaw a,*, Jaroslav Dostal b

a Department of Earth Sciences, Dalhousie Uni�ersity, Halifax, NS, Canada B3H 3J5b Department of Geology, Saint Mary’s Uni�ersity, Halifax, NS, Canada B3H 3C3

Received 29 August 2000; accepted 1 June 2001

Abstract

The Lighthouse gneiss association is an amphibolite-dominated assemblage situated in the allochthonousShawanaga domain of the Central Gneiss Belt (CGB), Grenville Province. It consists of two units in which theamphibolites differ in chemical composition and associated lithologies. The upper unit consists of: (i) thick intervalsof amphibolite interpreted as mafic flows and which form about half of the unit; (ii) a varied assemblage of thinlyinterlayered and laminated amphibolite and metasediment interpreted to have originated as mafic tuffs interlayeredwith sediment with a quartzofeldspathic source; and (iii) thick horizons of semipelite or regularly interlayeredpsammites and pelites interpreted to be metamorphosed greywacke-shale turbidites shed from a quartzofeldspathicsource. The lower unit has a much higher proportion of thick amphibolite interpreted as mafic flows and themetasediments are semipelitic. Geochemically, both upper and lower amphibolites are olivine-normative tholeiites buthave distinct trace element characteristics. The mantle-normalized trace element pattern of amphibolite in the upperunit lacks a subduction imprint and suggests an asthenospheric source, but that of the lower amphibolite has asubduction imprint (or an indication of eruption through attenuated continental crust). A back-arc environment ismost consistent with the chemistry of the amphibolites and the nature of the associated metasediments. Therecognition of the rocks of the Lighthouse gneiss association as juvenile material and their juxtaposition with anotherjuvenile member of the Shawanaga domain, the metarhyolite-rich Sand Bay gneiss association, leads to a refinementof current models for the tectonic evolution of the CGB before the �1200–1000 Ma Grenville orogenic cycle. Wealso suggest that all elements of the Proterozoic geology of Midcontinental USA are present, although jumbled byGrenvillian tectonism, in the CGB. These elements include units interpreted as oceanward-younging juvenile arcs thatare overlain by a granite–rhyolite layer and pierced by coeval plutons. © 2002 Elsevier Science B.V. All rightsreserved.

Keywords: Tectonic setting; Grenville Province; Central Gneiss Belt; Metavolcanic rocks; Midcontinental USA; Geochemistry

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* Corresponding author. Tel.: +1-902-494-3501; fax: +1-902-494-6889.E-mail address: [email protected] (N. Culshaw).

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N. Culshaw, J. Dostal / Precambrian Research 113 (2002) 65–8566

1. Introduction

The emerging lithotectonic framework for thehistory of the Grenville Province (Fig. 1) prior tothe �1200–1000 Ma Grenville orogenic cyclepoints to the existence of a long-lived active mar-gin. Many of the lithological components of theprovince are interpreted as components of thethrust-imbricated southern margin of Laurentiaconstructed from �1800 to �1400 Ma continen-tal arcs (Gower and Tucker, 1994; Gower, 1996;Culshaw et al., 1997; Rivers, 1997; Carr et al.,2000; Hanmer et al., 2000; Rivers and Corrigan,2000). As such, the Laurentian margin may havebeen one of the earliest margins of Pacific rim-scale (Culshaw et al., 2000), and it is in partbecause of its potentially unique role in the his-tory of continental evolution that it is important

to establish the original tectonic setting of litho-logical assemblages in the Grenville Province.

This paper is aimed at determining a possibleoriginal tectonic setting (geodynamic environ-ment) for amphibolites of the Lighthouse gneissassociation in the Shawanaga domain of the Cen-tral Gneiss Belt (CGB) of the Grenville Provincein Ontario. We also discuss the amphibolites, andother members of the CGB, within the context ofrocks of the Proterozoic Laurentian margin ofMidcontinental United States. CGB lithologiesare in all likelihood, equivalents of those of theMidcontinent, although the latter were unaffectedby the Grenville orogenic cycle (Easton, 1986;Van Schmus et al., 1996).

A practical difficulty in determining geody-namic environments in the Grenville Provincestems from the fact that much of it consists of

Fig. 1. The Grenville Province (Grenville) in Laurentian context (after Hoffman (1989), Van Schmus et al. (1996)). The location ofthe section of the CGB along Georgian Bay, Lake Huron (Fig. 2) is indicated. The CMB, south of the CGB is indicated. Pre-1.8Ga Laurentia consists of Archean microcontinents assembled along Paleoproterozoic collisional orogenic belts. Orogens that addedjuvenile material to the southern margin of Laurentia in the Paleoproterozoic–Mesoproterozoic include Yavapai (Y), Mazatzal (M),CPO, and Penokean (P). An extensive terrain of undeformed, 1470–1370 Ma granite–rhyolite (southern- and eastern-, SGR, EGR)underlies Midcontinental USA in subcrop; heavy dashed line is Nd model-age boundary of Van Schmus et al. (1996) (see text forexplanation). MCR, Mid Continental Rift. Gv indicates expected subcrop extent of Grenville Province. Light dashed line, limits ofGrenvillian and Appalachian deformation, adapted from Hoffman (1989), Van Schmus et al. (1996).

N. Culshaw, J. Dostal / Precambrian Research 113 (2002) 65–85 67

Fig. 2. CGB along Georgian Bay. GFTZ, Grenville Front Tectonic Zone. Legend: Parautochthon, 1, Britt, Britt domain (thepolyorogenic rocks are unornamented) and, L.GH, Lower Go Home domain; 2, �1.45 Ga granitoids (including Pointe-au-Barilcomplex, situated next to village of same name). Allochthons: 3, SH, Shawanaga domain (Dillon Schist, white), MR, Moon Riverdomain and, U.GH, Upper Go Home domain; 4, bPS, iPs, basal and interior Parry Sound domain. 5, Mappable bodies ofanorthosite and minor bodies of anorthosite. 6, Larger bodies of pink leucocratic gneiss mapped by Culshaw et al. (1988, 1989, 1990)in the parautochthon and allochthon. Structures: bold lines are boundaries interpreted as major thrusts (rectangles on hanging wall);closely spaced dashes labelled SSZ, is the extensional Shawanaga shear zone; the across-strike extent of the PSSZ is indicated witharrows (its northwestern, lowermost, boundary is the thrust reworked as an extensional shear zone illustrated in Fig. 3). The mapis bounded on the east side by Highway 69.

amphibolite- and granulite-facies gneisses. The in-herent interest of the deep-crustal processesrecorded by these high-grade rocks has histori-cally resulted in an emphasis on structural, meta-morphic, and geochronological studies (see e.g.,Culshaw et al. (1997), Carr et al. (2000) andreferences therein). Perhaps more importantly,studies of original geodynamic environment arediscouraged by penetrative ductile deformationthat eradicated most small-scale primary volcanicand sedimentary features.

To overcome these difficulties, we documentboth the lithostratigraphic association (by meansof a measured section) and the geochemistry ofthe amphibolites of the Lighthouse gneiss associa-tion. In these rocks, measured sections are veryuseful because primary variations in composition,unlike most smaller scale primary features, arenot eliminated by ductile deformation. Thus, thepattern of lithological variation displayed by asection may be suggestive of a primary stratigra-phy and is complementary to the geochemicaldata for diagnosis of tectonic environment.

2. Regional geology

2.1. Central Gneiss Belt along Georgian Bay

The CGB as exposed along the shoreline ofGeorgian Bay in Lake Huron, together with theGrenville Front Tectonic Zone (GFTZ), is part ofthe structurally lowest segment of the GrenvilleProvince exposed in Ontario (Fig. 2). The CGBstructurally overlies the GFTZ (Fig. 2), whichmarks the northwest boundary of the orogen. Tothe southeast, the Central Metasedimentary Belt(CMB; Fig. 1) represents post-1400 Ma magmaticarcs overthrust onto the CGB during theGrenville orogenic cycle (Culshaw et al., 1997;Carr et al., 2000).

The CGB comprises allochthonous rocks (e.g.,Shawanaga, Parry Sound, Upper Go Home andMoon River (sub-) domains) and underlying pa-rautochthonous rocks (e.g., Britt domain in thenorth and the comparable Lower Go Home do-main in the south) (Fig. 2; Culshaw et al., 1997).These were tectonically stacked and penetrativelydeformed in the period 1160–1020 Ma (Culshaw


et al., 1997). Whereas allochthonous rocks havebeen affected only by Grenvillian tectonism, pa-rautochthonous rocks show evidence for high-grade metamorphism both before and during theGrenville orogenic cycle.

The Shawanaga shear zone, the lower boundaryof the allochthonous rocks in the north (Fig. 2),represents the extension, in the CGB, of theprovince-wide Allochthon Boundary Thrust(ABT), a major detachment active during theGrenville orogenic cycle (Rivers et al., 1989; Cul-shaw et al., 1994, 1997; Ketchum et al., 1998;Carr et al., 2000; Ketchum and Davidson, 2000).The Shawanaga shear zone was reworked duringupper-amphibolite-facies extension at �1020 Ma(Ketchum et al., 1998).

The parautochthonous southern Britt domain,immediately underlying the Shawanaga shearzone, consists of a �1602–1800 Ma (Krogh,personal communication) polyorogenic granitoidplutonic suite that is cut by 1430–1460 Ma(Krogh, personal communication) megacrysticgranitoids of the Pointe-au-Baril complex (Fig. 2).The polyorogenic nature of the older suite isshown, where diabase dykes metamorphosed onlyduring the �1200–1000 Ma Grenville orogeniccycle cut reworked pre-Grenville metamorphicleucosomes dated at �1450 Ma (Ketchum et al.,1994). Comparable field relations are present, butless widespread, in the Lower Go Home domainin the southern part of the Georgian Bay transect(Fig. 2).

South of the Shawanaga shear zone, bodies ofanorthosite occur mainly close to boundaries ofthe allochthonous domains (Fig. 2). These rangein size from mappable bodies, such as that withinthe basal Parry Sound domain, to fist-sized frag-ments, most likely derived from tectonically disag-gregated bodies. They are accompanied at severallocalities by small metabasite bodies that containgarnet- and clinopyroxene-rich high-pressure as-semblages (Davidson, 1990).

The Shawanaga domain is tectonically overlainby the allochthonous basal Parry Sound assem-blage of the Parry Sound domain. The boundarybetween these two domains is precisely marked inthe vicinity of Sandy Island (Fig. 3) by an un-named extensional shear zone that reworks the

base of the Parry Sound Shear Zone (PSSZ), awide thrust-related deformation zone that coin-cides with most of the basal Parry Sound assem-blage (Fig. 2). The basal Parry Sound assemblagecontains marble and post-�1436 Ma quartziteinterlayered with 1400–1330 Ma granitoid or-thogneiss and �1350 and �1160 Ma gneissicanorthosite (van Breemen et al., 1986; Wodicka,1994; Wodicka et al., 1996). Grenville-age amphi-bolite-facies mineral assemblages are predominant(locally granulite facies) and all lithologies are cutby metamorphosed mafic dykes. High-grademetamorphism and deformation related to thrust-ing took place at �1120 Ma in the lower part ofthe basal Parry Sound assemblage (Culshaw et al.(1997) and references therein). The allochthonousParry Sound domain is thought to be far-travelledwith respect to all underlying units and geneticallyunrelated to any of them (Wodicka et al., 1996).

2.2. Tectonostratigraphy of Shawanaga domain

The Shawanaga domain consists of the SandBay and Ojibway gneiss associations and therocks featured in this study, the Lighthouse gneissassociation. The distinctive compositions of themetasupracrustal rocks of the Sand Bay gneissassociation and the lack of crosscutting maficdykes in the domain distinguish the Shawanagadomain from the Britt and Parry Sound domains.

The Ojibway gneiss association is mostly amigmatitic gneiss of granodioritic to tonaliticcomposition that is in places very leucosome-rich.The most widespread lithology in the Sand Baygneiss association is quartzofeldspathic migmatite,including voluminous pink leucocratic gneiss(Culshaw and Dostal, 1997). Other important, butless voluminous, units of the Sand Bay gneissassociation are amphibolites, a plagioclase-quartz-biotite schist (Dillon Schist) and ‘quartzite’(metaquartzarenite). Field relations and composi-tion demonstrate that the Sand Bay rocks are allsupracrustal in origin. The chemistry of the am-phibolites and the pink leucocratic gneisses indi-cate they likely originated as, respectively,continental tholeiites and rhyolites (Culshaw andDostal, 1997). The bimodal suite was interpretedas originating in a continental rift environment


(ocean opening or back-arc in an Andean-typesetting, Culshaw and Dostal, 1997).

The Sand Bay and Ojibway gneiss associationsoriginated between �1450 Ma, the crystallizationage of the Ojibway gneiss (Krogh, personal com-munication), and �1364 Ma, the age of theyoungest detrital zircon in the Dillon Schist mem-ber of the Sand Bay gneiss association (Krogh,personal communication). A granodioritic or-thogneiss was emplaced at �1350 Ma (van Bree-men et al., 1986) into the Shawanaga domainalong strike from the rocks featured in this study.

The earliest structures in the Shawanaga do-main are major refolded nappe-like folds (F1) andrelated fabrics (LS1) interpreted to have formedduring thrust transport over the subjacent pa-rautochthonous members of the CGB (Culshaw etal., 1997). Upper amphibolite facies metamorphicconditions during thrusting are estimated at 700–800 °C and 10–12Kb (Tuccillo et al., 1990;Ketchum et al., 1994; Wodicka et al., 2000). Thepervasive high-grade metamorphism and defor-mation related to thrusting took place between1082 and 1050 Ma (see Wodicka et al. (2000) and

Fig. 3. Map of Sandy Island–Bateau Island area. The location of the upper and lower units of the Lighthouse gneiss associationin the measured section is indicated. Lithologies are keyed to text. Structures: lithological boundary separating basal Parry Sounddomain from the Shawanaga domain is marked by an extensional shear zone (tick with ball on hanging wall) that reworked a thrustfault; S1, gneissosity developed in PSSZ; S2, gneissosity of Shawanaga shear zone. Location of map shown in Fig. 2.


Fig. 4. Measured section of the upper and lower units of the Lighthouse gneiss association, Shawanaga domain. Location shownin Fig. 3.

references therein). The F1 structures are over-printed by transverse folds (F2) and fabrics (LS2)that accompanied high-grade, ductile extensionalong the Shawanaga shear zone at �1020 Ma(Ketchum et al., 1998). These events are inferredto have accompanied deep burial of theShawanaga domain within a thrust stack trans-ported from close to the edge of Laurentia andsubsequent reworking by extension (Culshaw etal., 1997).

2.3. Lighthouse gneiss association

Rocks of the Lighthouse gneiss association aresituated at the structural top of the Shawanagadomain immediately below the extensional shearzone at the base of the Parry Sound domain (Fig.3). The Lighthouse gneiss association, as definedby Wodicka et al. (1996), Culshaw and Dostal(1997), Wodicka et al. (2000), is thickest near

Sandy Island, where two amphibolite-dominatedunits are separated by a sheet of gneissicanorthosite (Fig. 3). Although not shown as aseparate unit on regional maps, it is regionallyextensive, and attenuated intervals occur north-eastward along strike at Highway 69 (Fig. 2). Theunits are strongly foliated (S1) owing to thrust-re-lated deformation associated with formation ofthe PSSZ. They contain intrafolial isoclinal folds(F1), although there are no large F1 isoclinalfolds comparable to that affecting the Ojibwayand Sand Bay gneiss associations. All composi-tional layering and tectonic foliation dip moder-ately steeply to the southeast. As with rocks in theShawanaga Bay section described by Culshaw andDostal (1997), tectonic thinning may have beensevere, thus the thicknesses of units, measuredperpendicular to S1 and L1 and recorded in themeasured section (Fig. 4), must be regarded asminimum estimates.


We define two spatially distinct units within theLighthouse gneiss association. In them, the am-phibolites have a distinctive outcrop appearanceand the nature of the interlayered metasedimen-tary rocks have differences. It will be shown in alater section that amphibolites of the two units arealso geochemically distinct. The field descriptionsbelow are keyed to the measured section (Fig. 4),although relevant off-section observations areincluded.

2.3.1. Upper unitThe upper unit, situated immediately below the

bounding extensional shear zone (Fig. 3), consistsof amphibolite and metasedimentary rocks inter-layered in several different styles. Where mea-sured, the unit is 370 m thick, inclusive of outcropgaps. A key feature shown by the measured sec-tion of the upper unit is that amphibolite and twometasedimentary rock types, metasemipelite andmetapsammite–pelite (henceforth simplified tosemipelite and psammite–pelite, respectively)form subunits separately or in combination witheach other (Fig. 4). These subunits are importantfor interpretation of the upper unit.

Amphibolite, which occurs as layers from a fewcentimetres to up to 30 m thick, forms more thantwo thirds of the upper unit. There is one compo-sitional variety, migmatitic garnet amphibolite, inwhich leucosome-bearing varieties are interlayeredwith uniform, granoblastic, leucosome-free inter-vals. Some parts are subtly layered with variationsin proportions of amphibole and garnet.

Metasedimentary rocks form just less than athird of the upper unit. Most of this total (almosta quarter of the section) consists of a few horizonsmore than 3 m thick that are shown as individualhorizons on Fig. 4. There are many more bedsthinner than 2 m that are interlayered withequally thin amphibolite and define discrete lay-ered or laminated subunits. Apart from two iso-lated subunits at the southeastern (structurallyhighest) part of the upper unit, there are twoprincipal metasedimentary rock types. The first islayered psammite–pelite that has regularly alter-nating psammitic and leucosome-bearing peliticlayers 10–20 cm thick. The second is semipelite(including amphibole-bearing calcareous schists)

that forms uniform, unlayered horizons.Semipelite and psammite–pelite both occur in thesoutheast (top) of the upper unit, but semipelite isthe predominant metasedimentary rock type inthe bottom third.

The most voluminous subunit is migmatitic up-per amphibolite that forms 17 relatively thick(5–30 m) layers. The thick layers comprise abouthalf of the rocks in the upper unit and about twothirds of the amphibolite. Laminated amphibolite-metasediment contains garnet amphibolite andsemipelite that are interlayered on 10–40 cmscale. These form minor intervals up to 10 m thickin the structurally lowest part of the upper unit.In layered amphibolite-metasediment, garnet am-phibolite layers about 40 cm to 1.5 m thick alter-nating with 1–2 m wide horizons of metasedimentform a major interval about 30 m thick in thestructurally lower part of the upper unit andseveral other thinner intervals elsewhere in thisunit. The metasediment in the layered amphibo-lite-metasediment subunit, which may be eithersemipelite or psammite–pelite, locally containssome pods of garnet amphibolite. Layered psam-mite–pelite and semipelite both form several ofthe thicker horizons shown as individual subunitsof metasedimentary rock in Fig. 4. Off the sectionline, psammite–pelite is, in places, up to severaltens of metres thick.

There is a single horizon of layered pink leuco-cratic gneiss in the upper unit. This subunit liesabout a third of the way up the upper unit andseparates an upper interval in which thick hori-zons of amphibolite are predominant from one inwhich there are more layered and laminated am-phibolite-metasediment members. In the latter,semipelite interlayers and laminae predominate.This part of the upper unit may represent atransition to the lower unit, where a similarsemipelite is interlayered with amphibolite.

2.3.2. Lower unitOutcrops of the lower unit are dispersed among

relatively widely separated islands (Fig. 3). How-ever, projection along strike suggests that theoutcrop gaps are underlain by rocks similar tothose on the islands and that the section is repre-sentative. About one third of the estimated 400 m


total thickness of the lower unit was measured(Figs. 3 and 4).

Amphibolite forms as much as 90% of thelower unit. The lower amphibolite contrasts inoutcrop appearance with amphibolite of the upperunit. It forms several horizons each about 30 mthick that are made up of uniform, granoblasticgarnet amphibolite layers each a metre or morewide and separated by thin amphibole-rich lami-nae paired with thin leucosome. Within thesehorizons, there are a few thin layers of semipelitecontaining amphibolite pods. The semipelite hasthe same compositional variation as the memberwith the same name in the upper unit. It is theonly quartzofeldspathic lithology in this part ofthe lower unit. Lower (northwest) in the measuredsection, the metasedimentary rocks form morefrequent layers and, off section, there are signifi-cant intervals several metres thick of interlayeredamphibolite and semipelite that resemble the lam-inated amphibolite-metasediment of the upperunit.

2.4. Interpretation of field relations

Although the upper unit contains scattered iso-clinal folds, there is no repetition of members ineither the upper or lower units that can be at-tributed to the presence of large-scale tight orisoclinal folds. Rather, both units display anasymmetric disposition of members that is in-ferred to be related to the original depositionalsuccession that was either a ‘layer cake’ or alaterally variable apron, now thrust-stacked andthinned.

In both upper and lower units, the thick amphi-bolite members lack relict plutonic textures andthey are uniform in composition. We interpretthese features to reflect derivation from fine-grained mafic volcanic rocks. The protoliths ofthe layered and laminated amphibolite-metasedi-ment members of the upper and lower units areinterpreted as aqueously reworked mafic tuffsmixed with detritus from a compositionally dis-tinct quartzofeldspathic source. The amphibole-bearing schist suggests a similar but morecalcareous composition than the spatially associ-ated semipelite, and thus mixing of mafic volcanic

detritus with material from the quartzofeldspathicsource. The mafic pods scattered within some ofthe metasedimentary horizons in the upper unitmay represent large clasts of depositional ratherthan tectonic origin because they lack the periodicalignment of boudinaged layers nor is there anyevidence that they are tectonically ‘stoped’ frag-ments from the margins of thick layers.

Many of the semipelite horizons contain coarsefeldspar grains which, in contrast to the maficpods, can be shown to have been derived bytectonic disaggregation of pegmatites and shouldnot be mistaken for primary clasts. Once theseclasts have been subtracted, the dominance ofplagioclase-biotite in the matrix of the semipeliteshows that the mineralogy is similar to that of theDillon Schist of the Sand Bay gneiss association.The latter is interpreted to have an immatureepiclastic sedimentary protolith (Culshaw andDostal, 1997).

The thickness and composition of individuallayers of the psammite–pelite members of theupper unit are uniform. The layer pairs mostlikely represent metamorphosed turbiditicgreywacke-shale couplets, consistent with thecomposition of the layers and the similarity oftheir physical appearance to greywacke-shales atsimilar metamorphic grade in other orogens (Cul-shaw and van Breemen, 1990). We infer that boththe layered psammite–pelites and the metasedi-mentary components of some of the laminatedand layered subunits of the upper unit werederived from a quarztofeldspathic source and de-posited in a high-energy environment.

The pink leucocratic gneiss of the upper unit isvery similar to voluminous members of the SandBay gneiss association interpreted as metarhyolite(Culshaw and Dostal, 1997). Accordingly, this isthe favoured interpreted protolith for the singleinterval in the upper unit of the Lighthouse gneissassociation.

3. Geochemistry

Thirty samples of amphibolite were collectedfrom the upper and lower units, i.e., from withinthe thick horizons shown as individual subunits


Fig. 5. Variations of Fe* (total Fe) (wt.%), Zr (ppm), Cr (ppm), Al2O3 (wt.%), TiO2 (wt.%), Ni (ppm) relative to Mgc(100×MgO/[MgO+FeO] in mol%) for the amphibolites illustrating the effects of fractional crystallization (after Miyashiro (1974)).

(and, by inference, individual flow units) in Fig. 4.All are compositionally homogeneous amphibolitelacking leucosome.

3.1. Analytical methods

The samples of amphibolite were analysed formajor and some trace elements (Rb, Sr, Ba, Ga,Zr, Y, Nb, V, Ni and Cr) by X-ray fluorescencespectrometry in the Regional Geochemical Centreat Saint Mary’s University, Halifax. Sixteen repre-sentative samples were selected from this set foranalysis of other trace elements (rare-earth ele-ments (REE), Th, Nb and Ta) by ICP-MS in theGeoscience Laboratories of the Ontario Geologi-

cal Survey in Sudbury. Precision and accuracy ofthe X-ray data are reported by Dostal et al. (1994)and for ICP-MS data by Ayer and Davis (1997).The analytical error of the trace element determi-nations is 2–10 and �5% for major elements.

3.2. Results

Representative analytical results for amphibo-lites of the upper and lower units (upper andlower amphibolite, respectively) are presented inTable 1. All the amphibolites have a basalticcomposition. The major and trace element com-positions of the amphibolites are similar to thoseof modern volcanic rocks. This suggests that the

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Table 1Major and trace element composition of selected representative samples from the Lighthouse gneiss association

Upper amphibolite Lower amphibolite

4A 4C 4D 4E 4G 13.2 13.4 13.7 13.8 13.1a 13.1b 14.8 14.9Sample 2E2D 3B

46.90 45.41 45.85 48.37SiO2 (wt.%) 47.4545.79 46.54 48.40 47.77 47.95 47.45 47.2550.23 45.84 46.02 46.201.84 1.94 1.23 1.54 1.45 1.55 1.58 2.181.21 1.962.16 1.65 1.38TiO2 0.961.271.77

15.8013.87 14.78 14.61 16.59 13.81 13.39 14.13 13.71 12.62 13.26 13.39 13.9314.12 16.96 17.52Al2O3

11.50 12.55 14.83 14.96 12.27 13.56 14.03 13.30 13.65 15.76 15.52 14.17 13.4815.02Fe2O3* 13.46 12.420.23 0.22 0.18 0.22 0.24 0.21 0.21 0.250.20 0.240.20 0.21 0.230.17MnO 0.21 0.24

7.976.97 7.23 7.28 8.35 7.20 8.17 7.61 6.94 6.09 6.25 7.01 7.775.61 7.42 8.63MgO10.9311.04 10.34 10.66 10.98 11.15 10.57 10.88 10.86 10.22 10.37 9.69 11.119.91 10.86 10.42CaO

2.84 2.69 2.24 1.92 2.39 2.38 2.30 2.112.49 1.86Na2O 2.52 2.752.582.772.392.030.43 0.48 0.40 0.64 0.49K2O 0.500.63 0.50 0.84 0.52 1.09 0.820.59 0.45 0.33 0.370.23 0.20 0.10 0.13 0.13 0.15 0.15 0.230.11 0.180.25 0.17 0.13P2O5 0.28 0.090.12

0.410.3 0.39 0.34 0.59 0.41 0.48 0.81 0.61 0.48 0.49 0.97 0.830.19 0.56 0.56LOI

100.03 98.79 98.79 98.95 98.80 98.06 98.92Total 98.5698.31 98.60 98.33 99.6798.76 98.87 98.78 98.2449 49 57 51 54 53 50 4356 4448 49 53Mgc 605445

146109 84 88 121 253 276 260 252 87 146 181 32579 86 83Cr (ppm)50 70 54 51 103 54 93 82 49 16 28 51 6912 67Ni 88

47 53 54 51 51 52 46 4956 5054 46 4657Co 46 37273347 330 337 239 379 336 312 396 427 424 380 345361 264 220V7267 79 11 72 88 41 95 87 90 111 56 3431 18 43Cu

78 62 84 121 128 158 154 11092 117Zn 100 11572911061127 6 8 6 9Rb 86 8 11 5 20 33 4 4 6

588 534 435 29 349 368 318 61334 16914 275 349327Ba 290 135261161 321 293 253 142 162 158 174 168 154 130 159347 327 273Sr

14 14 15 14 13 14 14 12Ga 1313 13 1414 14 16 146 7.4 3.6 2.3 2.3 2.7 2.7 4.03.2 5.45.9 4.2 2.1Nb 3.53.66.0

1.913.38 2.84 3.12 1.86 2.42 2.32 2.57 2.59 3.81 3.30 2.72 2.083.78 2.17 1.81Hf70118 108 117 69 87 83 92 92 137 118 100 79136 76 63Zr

32 33 21 33 31 32 35 4420 4321 34 2916Y 36 370.370.53 0.67 0.81 0.26 0.46 0.53 0.47 0.62 0.88 1.15 0.91 0.490.55 0.34 0.26Th4.708.79 7.98 9.26 4.29 6.30 5.56 5.59 6.37 9.10 7.73 6.85 4.9011.7 4.40 3.64La

21.38 23.95 11.05 15.79 14.11 14.99 16.09 23.7211.79 20.50Ce 17.68 12.939.9211.9228.7422.873.36 3.84 1.85 2.58 2.27 2.47 2.61 3.83 3.29Pr 2.783.62 2.094.30 1.88 1.58 1.87

17.46 18.61 8.98 12.54 11.40 12.07 12.86 18.769.55 16.0919.29 13.44 10.39Nd 7.829.7120.572.705.16 4.67 5.16 2.72 3.79 3.57 3.79 3.94 5.77 4.90 4.10 3.275.30 2.82 2.29Sm

1.61 1.72 1.06 1.36 1.27 1.34 1.40 1.94Eu 1.641.78 1.35 1.171.69 1.14 0.90 1.020.84 0.97 0.56 0.85 0.78 0.83 0.90 1.230.52 1.080.94 0.87 0.72Tb 0.410.550.94

3.546.26 5.41 6.04 3.63 5.64 5.25 5.50 5.76 8.04 6.82 5.63 4.866.32 3.64 2.73DyHo 1.121.30 1.28 0.77 1.23 1.15 1.21 1.24 1.75 1.51 1.22 1.061.33 0.76 0.57 0.69

3.21 3.60 2.20 3.47 3.26 3.40 3.58 4.962.03 4.402.14 3.52 3.081.62Er 3.79 3.810.290.52 0.45 0.51 0.31 0.49 0.47 0.49 0.52 0.72 0.63 0.51 0.440.53 0.29 0.21Tm

Yb 2.963.46 3.22 1.90 3.13 3.04 3.11 3.30 4.57 4.13 3.40 2.883.64 1.90 1.51 1.820.46 0.51 0.30 0.49 0.48 0.49 0.53 0.71 0.640.28 0.53Lu 0.460.53 0.55 0.30 0.22

Mgc =100×MgO/(MgO+FeO*) in mol%; LOI – loss on ignition.


amphibolites are the metamorphic equivalent ofsuch rocks and that they retain to a large degreetheir magmatic composition. This conclusion isalso supported by high Cr/Th (�100) and lowTh/La (�0.1) ratios, which differ from those ofsedimentary rocks (Taylor and McLennan, 1985)but are typical of magmatic suites (Rollinson,1996), and by the relative homogeneity of themajor element compositions (Table 1).

Covariations of high-field-strength, rare-earthand transition elements and Mgc (100×MgO/[MgO+FeOtot] in mol%) suggest that these ele-ments were immobile during metamorphism assecondary processes are unlikely to produce suchconsistent trends (Fig. 5, Table 1). Thus, theseelements can be used to constrain the origin andtectonic affinities of these rocks.

The amphibolites from both units have a com-position corresponding to subalkaline basalts(Fig. 6) with SiO2 (LOI-free) ranging between 47and 52 wt.% and Mgc between 43 and 60 (Table1). The upper amphibolite (LH-2, 3 and 4) has onaverage higher Mgc but lower SiO2 and Crcontents (Figs. 5 and 6) than those of the loweramphibolite.

Both the upper and lower amphibolites aremostly olivine-normative tholeiites (others are hy-persthene and/or quartz normative in the CIPWscheme). Both amphibolites display a distinct cor-relation of major and trace elements with differen-tiation (Fig. 5). Elongated variation trendsindicate that the rocks of each unit might repre-sent a cogenetic suite and were modified by afractional crystallization process. Both amphibo-lites exhibit a decrease of Mg, Ni and Ca but anincrease of Fe, Ti and incompatible trace elementsincluding Zr (Figs. 5 and 6, Table 1) with differ-entiation. There are, however, some differences inthe fractionation trends between the two amphi-bolites. The lower amphibolite shows a decreasein Cr with decreasing Mgc whereas in the upperamphibolite, Cr does not show an appreciablechange (Fig. 5). On the other hand, in the upperamphibolite, Al decreases with differentiation butremains largely constant in the lower amphibolite.These trends reflect differences in mineral assem-blages that crystallized from the magmas. Frac-tional crystallization in the lower amphibolite was

Fig. 6. Zr/TiO2 versus SiO2 diagram (Winchester and Floyd,1977) illustrating subalkaline basaltic composition of bothamphibolite units.

Fig. 7. Variations of TiO2 (wt.%) and Zr (ppm) relative to Y(ppm) illustrating the differences between the upper and loweramphibolites.


Fig. 8. Chondrite-normalized REE abundances in the upper and lower amphibolites. BCR-1, a Columbia River continental floodbasalt is shown for comparison (Govindaraju, 1994). N-type MORB and E-type MORB and normalizing values after Sun (1982).Lighthouse gneiss association data shown for simplicity as shaded fields; Sand Bay gneiss association data, dashed outline.

dominated by clinopyroxene, whereas plagioclaseand olivine were dominant fractionating phases inupper amphibolite. An increase of Fe, Ti (Fig. 5)and V with differentiation argues against crystal-lization of Fe–Ti oxides. There are also smalldifferences between the two suites in the distribu-tion of incompatible trace elements including Zr,Y, Nb (Fig. 7, Table 1) and their ratios, e.g.,Zr/Y, TiO2/Y (Fig. 7, Table 1), probably reflect-ing differences in the mantle sources of the twosuites.

The chondrite-normalized REE patterns of theupper amphibolite have a mostly flat light REEsegment with some fractionation of heavy REEaccompanied by (La/Yb)n ranging from 1.3 to 2(Fig. 8). The abundances of light REE and the(La/Yb)n ratios increase with differentiation. Thelower amphibolite has REE patterns (Fig. 8) thatare intermediate between N-type and E-typeMORB with flat heavy REE profiles and (La/Yb)n between 1 and 1.3. The absolute concentra-tion of REE increases with differentiation in both


Fig. 9. Mantle-normalized incompatible trace element abundances in the upper and lower amphibolites. BCR-1, a Columbia Rivercontinental flood basalt is shown for comparison (Govindaraju, 1994). N-type MORB, E-type MORB and normalizing values afterSun and McDonough (1989). Lighthouse gneiss association data shown as shaded field; Sand Bay gneiss association data, dashedoutline.

amphibolites although the shape of the patternsremains the same. These variations are consistentwith the crystallization in both the amphibolitesof mineral assemblages involving clinopyroxene,plagioclase and olivine.

The mantle-normalized trace element patternsof the upper amphibolite (Fig. 9) show an overallminor enrichment in incompatible elements, withnormalized concentrations decreasing with de-

creasing incompatibility from La towards heavyREE. Likewise, they have low concentrations ofNb and Th compared to La although their pat-terns do not show a significant Nb anomaly withrespect to La and Th. This suggests that thesource of these basalts has not been significantlymodified by subduction-related fluids, and thatthe rocks have not been contaminated by crustalmaterial. The patterns of the lower amphibolite


Fig. 10. Ti (ppm) versus Cr (ppm) plot of Pearce (1975) for the amphibolites. The figure shows the compositional fields for islandarc tholeiites (IAT) and ocean-floor basalts (OFB).

(Fig. 9) are even less enriched in strongly incom-patible elements than the upper amphibolite.However, the lower amphibolite displays a smallnegative Nb anomaly relative to La and Th, sug-gesting that its source was affected by subductionprocesses, but, the relatively high contents of Tiand Cr (Fig. 10) as well as high Ti/V (�20) andZr/Y (�3) ratios (Shervais, 1982; Rollinson,1996), imply that the lower amphibolite is notdirectly arc-related.

3.3. Interpretation of geochemistry

Distinctive trace-element distributions in maficvolcanic rocks have been frequently used to re-construct ancient geodynamic settings (Rollinson,1996). The application of such schemes is basedon an assumption that the relationship betweentrace element distributions and tectonic settinghas not changed with time. Both amphiboliteunits of the Lighthouse gneiss association share asub-alkaline (Fig. 6) and tholeiitic (e.g., increaseof TiO2 with decreasing Mgc /increasing frac-tionation, Fig. 5) character that is probably rift-related (Fig. 10). Further, discrimination betweenpossible sources of the two units is made bymeans of REE and trace element diagrams. The

upper amphibolite has REE and trace elementpatterns that resemble MORB (Figs. 8 and 9).Thus, the upper amphibolite could be MORB,primitive continental tholeiite or back-arc basalt(BAB). However, the absence of a Nb anomalyand a low Th/La ratio on the trace element plot(Fig. 9) suggest an asthenospheric or primitivemantle source without any suprasubduction im-print or crustal contamination, severely limitingthe continental tholeiite option. The lower amphi-bolite cannot be directly related to the upperamphibolite, because, although its REE patterndoes suggest it could be MORB, primitive conti-nental tholeiite or BAB (Fig. 8), the trace elementpatterns show some evidence for subduction im-print or very modest amounts of crustal contami-nation (Fig. 9). However, the high Ti (Figs. 5 and9) and Cr (Fig. 5) content rules out formation inan arc.

At this point, it is worth comparing the Light-house amphibolites with those of the Sand Baygneiss association (Figs. 8 and 9), also of theShawanaga domain, which have been interpretedas continental tholeiites erupted onto rifted conti-nental crust together with rhyolite (pink leuco-cratic gneiss) during bimodal volcanism (Culshawand Dostal, 1997). For example, if the lower


amphibolite of the Lighthouse gneiss associationrepresents continental tholeiite, its less pro-nounced LILE enrichment suggests it was em-placed on thinner continental crust than the SandBay amphibolites (metabasalts).

4. Discussion

The following sections contain a synthesis ofthe field relations and geochemical data and anattempt to place the rocks of the Lighthousegneiss association in a plate tectonic framework.This is followed by a discussion of the significanceof the Lighthouse rocks with respect to the evolu-tion of the Proterozoic Laurentian margin.

4.1. Plate tectonic perspecti�e

The subunits of the upper unit are interpretedas volcanic flows (thick horizons of amphibolite)and as reworked mafic tuffs mixed with materialderived from a quartzofeldspathic source and de-posited in a subaqueous environment (laminatedand layered amphibolite-metasediment). Both therelatively thick intervals of layered psammite–pelite, interpreted as greywacke-shale, and thoseforming the metasedimentary components ofsome of the laminated and layered subunits wereclearly derived from a quartzofeldspathic sourceand deposited in a high-energy environment.These rocks make up a significant and characteris-tic fraction of the upper unit, indicating thatsedimentary and eruptive processes were of com-parable importance in the upper unit and that thesite of mafic volcanism may have been locatedclose to an elevated, subaerially exposed area.With respect to the greywacke-shale, it is notablethat evidence for deposition from turbidity cur-rents is very common in modern back-arc deposits(Carey and Sigurdsson, 1984) as well as inPhanerozoic and Precambrian rocks interpretedto have formed in back-arcs (Condie, 1986; Strat-ford and Aitchison, 1996).

Although similarly layered amphibolite-metasediment subunits are present in the lowerunit, they make up far less of the lithostrati-graphic section than in the upper unit, and the

metasedimentary component is entirely semipelite.Thus, for the lower unit, eruptive processes weremore important than sedimentary processes, rela-tively small amounts of detritus were deliveredfrom outside the system, and there is little evi-dence for deposition in a high-energy environmentsuch as indicated by the presence of layered psam-mite–pelite in the upper unit.

The contrast between the upper and lower unitsthat is evident from their metasedimentary com-ponents is also shown in the chemistry of theamphibolites. The upper amphibolite is moreprimitive and MORB-like with no evidence ofcrustal contamination or subduction signature.The lower amphibolite cannot have had the samesource as the upper amphibolite, because it has asubduction imprint best explained by crustal con-tamination or metasomatised mantle (Dupuy andDostal, 1984). Such a contrast between spatiallyassociated metabasalts is not unexpected becausedata from the Archean through to the Cenozoicshow that spatially associated (meta-) basalts of-ten have varying chemical signatures, usually in-terpreted as indicative of evolving mantle sourcesbeneath a single site. Indeed, evolving mantlesources for mafic volcanic rocks may have accom-panied a variety of tectonic environments rangingfrom plume-related magmatism, through conti-nental rifting to back-arc rifting (e.g., Archean:Tomlinson et al., 1996, 1998, 1999. Proterozoic:Condie, 1986; Knoper and Condie, 1988; Zhao,1994; Stern et al., 1995. Mesozoic: Fodor andVetter, 1984; Wever and Storey, 1992. Cenozoic:Saunders and Tarney, 1984; Price et al., 1990;Rocholl et al., 1995). Thus, a combination ofchemical signatures such as observed in the Light-house gneiss association may be diagnostic of thesame tectonic setting for both units if they formeda single terrane that originated at the same placeand time. Alternately, spatially associatedmetavolcanic rocks within an orogen may be un-related multiple terranes, tectonically juxtaposedat an active margin. Unfortunately, at present,these scenarios for the Lighthouse gneiss associa-tion cannot be distinguished without independentevidence (e.g., detrital zircons from metasedi-ments). Although both hypotheses are consistent


with current models for the development of theGrenville Province as an active margin in whichthe ages of juvenile additions decrease monotoni-cally with distance from the craton (Culshaw etal., 1997; Rivers, 1997), the single terrane hypoth-esis is attractive for its simplicity.

There are several plausible single terrane-evolv-ing source models that fit the chemistry of theLighthouse amphibolites (metabasalts). The SandBay amphibolites (metabasalts) and pink leuco-cratic gneiss (metarhyolites) can be included in themodels with varying success.

In a plume model, the upper unit of the Light-house gneiss association represents the plumesource, the lower unit, a mixture of plume andlithosphere, and the Sand Bay amphibolite (meta-basalts) a variably crustally contaminated mixtureof more lithospheric mantle component and lessplume component than the lower unit of theLighthouse gneiss association. The pink leuco-cratic gneiss of the Sand Bay gneiss associationrepresents rhyolite produced by crustal meltingtriggered by plume heat. The distinctive metasedi-ments of the Lighthouse upper unit have no obvi-ous significance in this model. This model haslittle appeal without the inclusion of the Sand Baygneiss association and is thus critically dependenton the ages of units of the Lighthouse gneissassociation.

In a rifted continent model, the upper unitrepresents the most advanced rifting of the conti-nental margin such that asthenospheric melts wereformed. The metabasalts of the Sand Bay gneissassociation (Culshaw and Dostal, 1997) and thoseof the lower unit of the Lighthouse gneiss associa-tion represent, respectively, eruption through pro-gressively thinner continental crust. Again, theage of the Lighthouse units is crucial. The relativeproportions and depositional environment ofmetasedimentary rocks from the upper unit againhave no obvious place in this hypothesis. Like-wise, rift-related sediments have not beenrecognized.

In a back-arc hypothesis, the metabasalts of thelower and upper units represent, respectively, pro-gressive rifting. The relatively voluminousmetasediments of the upper unit would representarc-derived detritus. A possible role for the

metarhyolites of the Sand Bay gneiss associationis suggested by the inference of Condie (1986) thatrhyolites, common in Paleoproterozoic assem-blages in the southwest USA, may have beendeposited in back-arcs. More specifically, theSand Bay rocks could represent the early riftingof an Andean-type back-arc, in keeping both withthe volume of metarhyolites and the continentaltholeiitic nature of the metabasalts (Culshaw andDostal, 1997). However, inclusion of the SandBay rocks is not crucial to either an oceanic or anAndean back-arc hypothesis.

4.2. Gren�ille Pro�ince perspecti�e

Some workers suggest that the GrenvilleProvince was an Andean-type arc, with no accre-tion of terranes, for some or all of its historybefore initiation of the Grenville orogenic cycle at�1.2 Ga (Rivers, 1997; Hanmer et al., 2000;Rivers and Corrigan, 2000). Although the preciseages of the rocks of the Lighthouse gneiss associa-tion are unknown, the available 1450–1350 and1436–1330 Ma age brackets for the contiguousOjibway and Sand Bay gneiss associations and thebasal Parry Sound assemblage, respectively, sug-gest that the age of the Lighthouse rocks is com-parable and therefore that they formed when thehypothetical Andean-type arc was active. Anorigin in an Andean-type back-arc for the Light-house gneiss association (whether or not the SandBay rocks were cogenetic) would be consistentwith this hypothesis. Carr et al. (2000) acknowl-edge the existence of a long-lived Andean-type arcon the Laurentian margin but note that the 1400–1190 Ma history is poorly constrained. However,as mentioned previously, the continental oroceanic affinity of the Lighthouse rocks is anopen question.

4.3. Midcontinental perspecti�e

The potential linkage of the CGB with rocks ofthe Midcontinent region of the USA may behelpful in giving a context for the Lighthousegneiss association. The Midcontinent region isphysically continuous with and is similar in age to(Easton, 1986) the CGB (Fig. 1), and direct corre-


lations with Grenvillian elements have been sug-gested (Rivers (1997) and references therein; Riv-ers and Corrigan, 2000).

In Midcontinental USA, Paleoprotoerozoic(1800–1600 Ma) juvenile rocks of the CentralPlains Orogen (CPO) give way southward andeastward to the �1370 and �1470 Ma southernand eastern granite–rhyolite provinces (Fig. 1). Inaddition, plutons of the same age as the granite–rhyolite province rocks occur throughout theCPO. The granite–rhyolites are thought to havebeen derived by partial melting of subjacent juve-nile material equivalent to rocks of the CPO(Bickford, 1988; Bickford and Anderson, 1993;Van Schmus et al., 1996). Van Schmus et al.(1996) pointed out that, in detail, the Nd modelages of the �1370 and �1470 Ma granite–rhyo-lites of the Midcontinent indicate they werederived from slightly older subjacent juvenilerocks. The model-ages define a northwestern

group that are �1.55 Ga and a �1.55 Gasoutheastern group (Fig. 1), and the two agegroups are interpreted to reflect the presence be-neath the granite–rhyolites of Paleo- and Meso-proterozoic juvenile terranes, respectively. Thetwo groups of Midcontinent model-ages are simi-lar to those in the CGB of Ontario, where ages�1.6 Ga predominate in the parautochthon andthose �1.6 Ga are most common in the al-lochthonous units (Dickin and McNutt, 1990).These features suggest that removal of displace-ment on the thrusts would restore the spatialdistribution of model-ages in the CGB to a pat-tern closely comparable to that inferred beneaththe granite–rhyolites of the Midcontinent andsimilar to that on current restored cross sectionsof the CGB (Fig. 11).

An extension of the comparison between theCGB and the Midcontinent region would predictan upper-crustal granite–rhyolite layer in the

Fig. 11. Schematic cross-sections of the Grenville Province in Ontario (after Culshaw et al. (1997)). Unit abbreviations are as in Figs.1 and 2; in addition: PSD, Parry Sound domain and CMBBZ, Central Metasedimentary Belt Boundary Zone. (a) Cross-section at980 Ma (end of Grenville orogenic cycle). Top and base of section are, respectively, present erosion surface (es) and Moho (M). (b)Parautochthonous and autochthonous units are restored to a template at 1160 Ma (beginning of Grenville orogenic cycle) on whichages decrease from left to right (NW–SE, i.e., towards inferred oceanward edge of active margin) and units with similar maficplutonic bodies are grouped together. The latter include �1235 Ma Sudbury diabase (Dudas et al., 1994), �1160 Ma coroniticmetagabbro (Davidson and van Breemen, 1988; van Breemen and Davidson, 1990; Heaman and LeCheminant, 1993) andanorthosite. The boundary between allochthonous and parautochthonous domains, ABT is shown with a heavy line. It separatesallochthonous rocks with maximum Nd model-ages of �1.6 Ga from parautochthonous domains with predominant Nd model-agesof �1.6 Ga (Dickin and McNutt, 1990). This boundary may be comparable to that inferred to lie within juvenile terranes belowthe granite–rhyolite provinces of Midcontinental USA (Van Schmus et al., 1996).


CGB. Such a layer does not figure on currentrestored cross sections of the CGB (Culshaw etal., 1997; Carr et al., 2000). However, units ofpink leucocratic gneiss that occur in the GeorgianBay section in several parautochthonous and al-lochthonous domains (Culshaw et al., 1988, 1989,1990) provide evidence for remnants of a layer(s)of granite–rhyolitic composition thrust-telescopedand locally affected by F1 fold nappes in contactwith rocks of a variety of ages, with the exceptionof the Parry Sound domain (Fig. 2). The abun-dant pink leucocratic gneiss of the Shawanagadomain (Culshaw and Dostal, 1997) is a primeexample of such a unit.

This Midcontinental template suggests anotherscenario, in addition to the models discussedabove, in which the Lighthouse and Sand Baygneiss associations may have been originally re-lated. In this scenario, the upper and lower unitsof the Lighthouse gneiss association are equiva-lent to the Mesoproterozoic juvenile crust inferredto lie beneath the 1370–1470 Ma granite–rhyo-lites of the Midcontinent region. The Lighthouserocks most likely together form a single unit in anAndean-type or oceanic back-arc. The volumi-nous pink leucocratic gneiss and subordinate am-phibolites of the Sand Bay gneiss association arepart of the Midcontinental granite–rhyolite ‘up-per deck’, which has been tectonically mixed (F1folds) and then overridden by the ‘lower deck’ ofslightly older juvenile rocks during Grenvillianthrusting. The dominance of pink leucocraticgneiss over amphibolite in the Sand Bay gneissassociation is analogous to the granite–rhyolitesof the Midcontinent region in which mafic rocksare sparse (Bickford and Anderson, 1993). Thecontinental tholeiitic chemistry of the Sand Baymetabasalts is also consistent with the within-plate setting implied by models for genesis of thegranite–rhyolites (Van Schmus et al., 1996). The�1450 Ma granites that intrude the Britt andLower Go Home domains would be analogous toMesoproterozoic plutons (Easton, 1986; van Bree-men et al., 1986) of the same age as the granite–rhyolite veneer in the Midcontinent that intrudedthe exposed Paleoproterozoic juvenile terranes ofthe CPO. The occurrence of both minor andrelatively large gneissic anorthosite bodies in the

Georgian Bay section (Fig. 2) makes sense withinthis framework insofar as it has been suggestedthat Proterozoic anorthosites may represent themantle component that supplied heat to producethe granite–rhyolites from the Proterozoic juve-nile terranes (Bickford, 1988; Bickford and An-derson, 1993).

In the CGB cross-section of Culshaw et al.(1997) (Fig. 11(b)), the candidate granite–rhyo-lites (units of pink leucocratic gneiss) occur inevery unit of the restored Laurentian margin. TheParry Sound domain is shown on the cross-sec-tion as part of a belt of ‘composite arcs’ whoserelationship with respect to the Laurentian marginis not known at the beginning of the Grenvilleorogenic cycle. The apparent absence of pinkleucocratic gneiss from the Parry Sound domainmay thus signify that it was not attached toLaurentia when the granite–rhyolites were pro-duced. A further feature emerging from this inter-pretation of the cross-section is that the ABT is aboundary within Proterozoic juvenile terranesthat was reworked in the Grenville orogenic cycle.

5. Conclusions

We suggest on the basis of field relations, geo-chemistry, protolith composition and known agesof rocks in spatially associated domains that rocksfrom the upper and lower units of the Lighthousegneiss association probably originated in the sametectonic environment rather than as separate ter-ranes. They were likely components of a back-arcthat formed in a Mesoproterozoic active margin.Whether this margin was an Andean-type marginis not entirely clear.

Within a Midcontinent-Grenville context (i.e.,the southern Laurentian margin in the Protero-zoic), it is suggested that all elements of Midconti-nent Proterozoic geology, albeit shuffled byGrenvillian tectonism, are present in outcrop inthe CGB. These include Proterozoic juvenile rocks(e.g., Lighthouse gneiss association), an overlyinggranite–rhyolite unit (e.g., widespread pink leuco-cratic gneiss), Mesoproterozoic plutons of thesame age as different elements of the granite–rhy-olite unit, and anorthosites.


Proof or modification of these related hypothe-ses critically depend on future U–Pb dating ofigneous protoliths and detrital zircons.

Acknowledgements

Trond Slagstad is thanked for lively discussionsof Grenville tectonics. Many thanks are due toNatasha Wodicka and Phil Thurston for theirextremely helpful reviews. The work was com-pleted with the help of NSERC Operating Grantsto NC and JD.

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