Cordierite–gedrite rocks from the CentralMetasedimentary Belt boundary thrust zone(Grenville Province, Ontario): Mesoproterozoicmetavolcanic rocks with affinities to theComposite Arc Belt1
William H. Peck and Michael S. Smith
Abstract: Cordierite–gedrite rocks in the southern Grenville Province occur near the base of the Central MetasedimentaryBelt boundary thrust zone, interpreted by some as a crustal suture between the 1.29–1.24 Ga Composite Arc Belt and>1.4 Ga rocks of Laurentia. Major and trace-element compositions of these rocks are consistent with volcanic protolithsthat range in composition from basalt to dacite. These cordierite–gedrite rocks have low CaO (average 1.2 wt.%) andmajor element and oxygen-isotope ratios suggestive of hydrothermal alteration before metamorphism. Rare-earth element(REE) compositions also indicate igneous protoliths, although some REE patterns have been modified by local meltextraction. The trace-element compositions of cordierite–gedrite rocks, and neodymium-isotope systematics, are similarto those of metavolcanic rocks in the Composite Arc Belt and are consistent with the extension of the Composite ArcBelt to the base of the boundary thrust zone.
Résumé : Les roches à cordiérite–gédrite du sud de la Province de Grenville se trouvent près de la base de la zone dechevauchement à la limite de la ceinture métasédimentaire centrale, que certains interprètent comme une suture de lacroûte entre la ceinture composite d’arc, 1,29–1,24 Ga, et les roches de Laurentia, >1,4 Ga. Les compositions en élémentsmajeurs et traces de ces roches concordent avec les protolithes volcaniques dont la composition varie de basalte à dacite.Ces roches à cordiérite–gédrite ont une faible teneur en CaO (1,2 % poids) et les rapports d’éléments majeurs et desisotopes de l’oxygène suggérant une altération hydrothermale avant le métamorphisme. Les compositions en éléments desterres rares (ETR) indiquent aussi des protolithes ignés, bien que quelques patrons REE aient été modifiés par une ex-traction locale de roches en fusion. Les compositions en éléments traces des roches à cordiérite–gédrite, ainsi que lasystématique des isotopes du néodyme, sont semblables à celles des roches métavolcaniques dans la ceinture d’arccomposite et elles concordent avec l’extension de la ceinture d’arc composite jusqu’à la base de la zone de chevauche-ment limite.
[Traduit par la Rédaction] Peck and Smith 1828
Introduction
Enigmatic cordierite–gedrite rocks in Ontario, Canada, occupyan important structural position near the base of the CentralMetasedimentary Belt boundary thrust zone (CMBbtz), amajor terrane boundary in the southern Grenville Province(Fig. 1). The CMBbtz marks the western boundary of theComposite Arc Belt, which is interpreted by some as a collageof arcs and basins assembled away from Laurentia (e.g.,Carr et al. 2000) and by others as representing a marginalbasin environment developed at the southeastern edge of
Laurentia (e.g., Hanmer et al. 2000). The rheological propertiesof the cordierite–gedrite rocks and related lithologies mayhave helped control the location of the principle crystallinethrust sheets of the CMBbtz, which separates <1.3 Ga arcrocks and sediments from >1.4 Ga rocks of Laurentia (Hanmer1988; Hanmer and McEachern 1992).
Oxygen-isotope geochemistry by Peck and Valley (2000)suggests the cordierite–gedrite rocks at the base of the CMBbtzwere formed from a volcanic protolith that was hydrother-mally altered by seawater. This paper presents new majorand trace-element and Nd-isotope data for these rocks. The
Received 17 December 2004. Accepted 28 June 2005. Published on the NRC Research Press Web site at http://cjes.nrc.ca on22 December 2005.
Paper handled by Associate Editor J.D. Greenough.
W.H. Peck.2 Department of Geology, Colgate University, Hamilton, NY 13346, USA.M.S. Smith. Department of Earth Sciences, University of North Carolina, Wilmington, NC 28403, USA.
1This article is one of a selection of papers published in this Special Issue on The Grenville Province: a geological and mineralresources perspective derived from government and academic research initiatives.
cordierite–gedrite rocks have clear geochemical affinities tothe Composite Arc Belt and most likely represent a hydro-thermally altered component of the arc belt caught up duringthe imbrication of the CMBbtz.
Geological setting
The southern Grenville Province has been subdivided intothree main tectonic elements: (i) the parautochthonous +allochthonous components of Mesoproterozoic Laurentia (theCentral Gneiss Belt), (ii) the Composite Arc Belt, and (iii) theAdirondack Highlands – Frontenac terrane (Easton 1992;cf. Rivers et al. 1989; Carr et al. 2000). Rocks of the CentralGneiss Belt are for the most part 1.7–1.4 Ga amphibolite-and granulite-facies orthogneiss and have ca. 1.9 Ga Ndmodel ages proximal to the CMBbtz (Dickin 1998). TheAdirondack Highlands and Frontenac composite terrane ismade up of metaplutonic rocks with 1.3–1.1 Ga ages thatwere emplaced into Mesoproterozoic metasediments and wasjuxtaposed with the Composite Arc Belt at ca. 1.17 Ga (Corfuand Easton 1997).
Composite Arc BeltThe Composite Arc Belt of Carr et al. (2000) is a subdivision
of the southern Grenville Province that includes the Belmont,Grimsthorpe, and Harvey–Cardiff domains of the Elzevirterrane, the Mazinaw and Sharbot Lake terranes, and part orall of the Bancroft terrane (i.e., most of the Central Meta-sedimentary Belt; Easton 1992). These terranes containgreenschist- and amphibolite-facies metasedimentary and1.29–1.24 Ga metavolcanic and volcanoclastic rocks that areintruded by suites of gabbroic, tonalitic, and granitic plutons(Easton 1992). The volcanic rocks have arc geochemical
signatures and are dominated by mafic tholeiitic suites butinclude intermediate and felsic rocks, some with calc-alkalinesignatures (see Easton 1992; Carr et al. 2000; Smith et al.2001). Although not all of these volcanic suites are dated,there is an apparent age progression in geochemical signa-tures from early primitive arc to mature, and then to riftedarcs. The youngest crosscutting plutonic rocks may constrainterrane amalgamation in the Composite Arc Belt to 1.24–1.22 Ga (Carr et al. 2000).
The arc signature of the Composite Arc Belt is well estab-lished, but the tectonic interpretation of these rocks is con-troversial. Some workers have postulated that these arcsformed on oceanic crust offshore of Laurentia (e.g., Brownet al. 1975; Windley 1989; Carr et al. 2000), and others haveassigned these rocks to rifting and back-arc environmentsunderlain by continental crust and developed at the continen-tal margin. This second model is based on two lines of evi-dence: (i) trace-element signatures suggestive of crustalcontamination in volcanic rocks of the Composite Arc Belt(see Hanmer et al. 2000; Smith et al. 2001); and (ii) gabbrosfrom structurally above the CMBbtz with 1.44–1.30 Gainherited zircon ages, similar to ages from the Laurentianmargin (Pehrsson et al. 1996).
Central Metasedimentary Belt boundary thrust zone(CMBbtz)
The Composite Arc Belt of Carr et al. (2000) is boundedon the west by the CMBbtz, a mid-crustal shear zone thatseparates metasedimentary, metavolcanic, and metaplutonicrocks of the Composite Arc Belt from the orthogneiss-dominated Central Gneiss Belt of the Laurentian foreland.As defined by Hanmer (1988) and Hanmer and McEachern(1992), the upper part of the CMBbtz is equivalent to the
Fig. 1. (A) Location map of the Grenville Province. (B) The southern Grenville Province of Ontario, after Hanmer and McEachern(1992). CMBbtz, Central Metasedimentary Belt boundary thrust zone. The Composite Arc Belt is subdivided into the Harvey–Cardiff(H), Belmont (B), Grimsthorpe (G), Mazinaw (M), and Sharbot Lake (S) domains after Easton (1992).
Bancroft terrane (Easton 1992) and includes a structurallylower, more intensely deformed segment. The CMBbtz ismade up of annealed, heterogeneously deformed myloniticand marble mélange tectonites surrounding metaigneousthrust sheets (Hanmer 1988; Hanmer and McEachern 1992;McEachern and van Breemen 1993) (see Fig. 2). The timingof imbrication for the Composite Arc Belt is controversialbecause there is evidence for two deformation events in theCMBbtz at ca. 1.19 and 1.08–1.06 Ga (McEachern and vanBreemen 1993; Carr and McMullen 2000). The 1.08–1.06 Gaevent is associated with the most pervasive deformation, andCentral Gneiss Belt rocks structurally below the CMBbtzonly record the 1.08 Ga metamorphism, which is interpretedby Timmermann et al. (1997) as representing docking of theComposite Arc Belt.
Pehrsson et al. (1996) proposed that the Raglan gabbrobelt (Fig. 2; to the east of the CMBbtz) could have provideda rheological barrier that controlled the location of the top ofthe thrust zone. Carr et al. (2000) have questioned the long-held assumption that the CMBbtz should be grouped withthe Elzevir terrane in tectonic reconstructions. Carr et al.suggest that the metasediments of the CMBbtz may havebeen deposited on the Laurentian margin and that the suturebetween Laurentia and the Composite Arc Belt may lie withinthe CMBbtz, rather than being at the lower boundary of theCMBbtz. Hanmer (1988) and Hanmer and McEachern (1992)propose that the location of the lower boundary of the thrustzone was controlled by a weak (and probably discontinuous)horizon of aluminous gneisses that includes the cordierite–gedrite gneisses.
Cordierite–gedrite rocks of the CMBbtz
PetrologyCordierite–gedrite rocks have been reported at a number
of localities in the CMBbtz, most notably cropping out alongthe north shore of Fishtail Lake (Lal and Moorhouse 1969;Lal 1969a, 1969b) (see Fig. 2). These rocks have the meta-morphic mineral assemblage of garnet + cordierite + gedrite +quartz ± plagioclase (Table 1). More aluminous lithologieswith the metamorphic mineral assemblage of garnet +cordierite + sillimanite + quartz ± plagioclase are interca-lated on the scale of centimetres. Zircon, Fe–Ti oxides, andother accessory minerals are found in minor amounts, andstaurolite inclusions are preserved within the garnet (see Laland Moorhouse 1969; and Peck and Valley 2000 for detailedassemblages). The presence of retrograde kyanite and anda-lusite and calculated pressure–temperature (P–T) paths suggesta cooling path of �12 bar/°C (1 bar = 100 kPa) (Anovitz andEssene 1990).
Coarsely annealed cordierite–gedrite rocks are part of themylonite package bounding the rigid Redstone and Dysartthrust sheets (Culshaw 1986; Hanmer 1988, 1989). In theserocks the syntectonic mineral assemblage of sillimanite +garnet + cordierite + two feldspars + quartz is overprinted bythe post-tectonic assemblage of gedrite + cordierite + kyan-ite + biotite + garnet (Hanmer 1988, 1989). This assemblageprobably reflects cooling across the reaction of garnet + cor-dierite = aluminosilicate + orthoamphibole that moves theassemblage into the kyanite stability field. The �700 °C and�8 kbar P–T estimate for this region of the CMBbtz(Anovitz and Essene 1990) corresponds to the position ofthis reaction in P–T space (e.g., Cheney et al. 2004). Thepresence of abundant partial melting mineral segregations ofquartz + plagioclase + garnet provides additional support forthis P–T estimate (Culshaw 1986).
The northeasternmost sampled locality of the cordierite–gedrite unit is the group of outcrops near Hoare Lake describedby Millar (1983) and Breaks and Thivierge (2001). Otheraluminous rocks lacking gedrite to the east have been
Fig. 2. (A) Map of the Central Metasedimentary Belt boundary thrust zone after Hanmer and McEachern (1992). D, Dysart thrustsheet; F, Foymount thrust sheet; G, Grace thrust sheet; GL, Glamorgan thrust sheet; P, Papineau thrust sheet; R, Redstone thrust sheet;S, Stafford thrust sheet. (B) Distribution of orthoamphibole–cordierite units around the Redstone and Dysart thrust sheets, after Hanmer(1988). Sample localities are shown and described in the text (Table 1).
Ba 434.0 361.0 138.0 370.0 182.0 321.0 56.0 78.4 712.9
La 4.10 16.30 5.00 5.10 15.90 5.20 2.84
Ce 8.9 36.3 12.8 12.2 35.7 11.0 6.5
Pr 1.20 4.83 1.83 1.82 4.83 1.45
Nd 5.00 21.90 8.30 8.30 20.40 6.00 11.10
Sm 1.40 5.40 2.70 2.80 4.60 1.70 1.58
Eu 0.460 0.900 0.290 0.320 0.480 0.600 0.640
.Gd 2.98 5.90 4.08 4.60 5.21 3.86
Tb 0.98 0.94 0.85 0.97 0.87 1.11 0.72
Dy 8.86 5.75 6.05 6.60 5.88 9.21
Ho 2.31 1.33 1.35 1.37 1.34 2.14
Er 7.36 3.98 3.65 4.00 4.26 6.30
Tm 1.08 0.58 0.53 0.58 0.63 0.89
Yb 7.50 4.30 3.80 4.00 4.70 5.90 2.65
Lu 1.12 0.65 0.61 0.62 0.69 0.85 0.37
Hf 3.00 3.00 3.00 3.00 3.00 3.00 0.75
Ta 1.600
Pb 4.3 6.9
Th 1.20 4.00 1.80 1.00 3.00 0.90 0.13
U 0.42 2.24 0.57 0.30 1.65 0.99 0.33
Note: All samples are from Fishtail Lake unless otherwise noted. Sixteen cordierite–gedrite gneisses from the Central Metasedimentary Belt Boundaryin Smith et al. (1992a) for samples with the prefix CO88. The remainder were analyzed at XRAL Laboratories in Toronto, Ontario, Canada. X-ray fluo-copy (ICP–MS). Analytical methods for XRAL are described at < http://www.sgslakefield.com >. Bt, biotite; Crd, cordierite; Grt, garnet; Ky(t),LOI, loss on ignition.
Table 1. Whole-rock geochemistry of cordierite–gedrite rocks from the Central Metasedimentary Belt boundary thrust zone, Grenville
Zone (CMBBZ) were selected for this study. Major and trace-element compositions (including the REEs) were determined by the techniques describedrescence was used for major elements and Rb, Sr, Y, Zr, Nb, and Ba. Other trace elements were analyzed by inductively coupled plasma – mass spectors-trace kyanite; Op, orthopyroxene; Pl, plagioclase; Qtz, quartz; Sil, sillimanite. K-feldspar, zircon, Fe–Ti oxides, and staurolite inclusions are minor constituents.
correlated with this unit based on lithologic similarity andstructural position (Hanmer and McEachern 1992). TheHoare Lake gneisses contain the metamorphic mineral as-semblage orthopyroxene + gedrite + cordierite + garnet andhave enstatite-bearing leucocratic segregations. The presenceof metamorphic orthopyroxene reflects the higher metamor-
phic temperatures experienced by these rocks (�800 °C and�8.9 kbar; Carr and Berman 1997) relative to similar rocksto the southwest. The adjacent paragneiss experienced pene-trative deformation at ≤1.06 Ga (Carr and McMullen 2000),which constrains the age of deformation in the Hoare Lakegneisses.
Major element compositionThere has been little geochemical study of the rocks from
Fishtail Lake and other localities in the CMBbtz. Lal andMoorhouse (1969) and Lal (1969a) present wet-chemicalanalysis for three rocks from Fishtail Lake, Culshaw (1986)analyzed four rocks from the Drag Lake area, and Breaksand Thivierge (2001) analyzed three cordierite–gedrite rocksfrom the Hoare Lake area. This study incorporates their dataand provides new analyses to address the origin of theserocks (Table 1). In general, the major element geochemistryfor cordierite–gedrite and related rocks from Haliburton County(around the Redstone, Dysart, and Grace thrust sheets andincluding the Fishtail Lake rocks) is distinct from that of theHoare Lake samples.
The Haliburton County cordierite–gedrite rocks have highSiO2 contents that range from 53.5 to 72.1 wt.% (Fig. 3;Table 1). MgO contents of rocks from Haliburton Countyhave a negative correlation with SiO2 and range from 1.6 to11.7 wt.%. Fe2O3 (all Fe is reported as Fe2O3) ranges from3.4 to 13.4 wt.%, and some samples show iron enrichmentrelative to magnesium with increasing SiO2. Al2O3 contentsrange from 10.7 to 16.9 wt.%.
Cordierite–gedrite rocks are commonly described as havinghigh MgO and Al2O3 contents, but the CMBbtz rocks haveabundances of these oxides similar to those of intermediateto felsic igneous rocks. Alkali contents are in low to moderateabundance (average (avg.) Na2O = 2.2 ± 1.6 wt.%; avg.K2O = 1.5 ± 1.1 wt.%). CaO contents are very low, with anaverage of 1.2 ± 1.3 wt.%, and have no simple correlationwith the other major elements (Fig. 4).
At Hoare Lake, the SiO2 contents (avg. 52.2 ± 5.6 wt.%)are at the low end of the range of those from HaliburtonCounty. The Hoare Lake sample with the highest SiO2(58.2 wt.%) is from a leucocratic “mobilizate” assemblage(Breaks and Thivierge 2001). Conversely, MgO (13.1 ±4.2 wt.%), Fe2O3 (15.4 ± 1.1 wt.%), and Al2O3 (15.7 ±1.0 wt.%) contents are all high relative to those of rocksfrom Haliburton County. Sodium, potassium, and calciumare very low in abundance (avg. Na2O = 0.8 ± 0.2 wt.%;avg. K2O = 0.6 ± 0.3 wt.%; avg. CaO = 1.0 ± 0.5 wt.%).
In an alkali (Na2O + K2O) versus SiO2 magmatic evolutiondiagram (Le Maitre et al. 1989) (Fig. 3), the low abundancesof the alkali elements for the Hoare Lake samples are quitedistinct from those of the majority of Haliburton Countysamples. Furthermore, the Hoare Lake samples plot in thebasalt to basaltic andesite to andesite fields, whereas theHaliburton samples plot in the basaltic andesite to dacitefields.
Trace-element compositionTrace-element compositions of cordierite–gedrite rocks from
Haliburton County and Hoare Lake are rare in the literature(only partial analyses in Breaks and Thivierge 2001 and thisstudy). Large ion lithophile elements (LILE) in Haliburton
Fig. 3. Alkali versus SiO2 magmatic evolution diagram (LeMaitre et al. 1989) for cordierite–gedrite rocks. Samples fromthis study, Lal and Moorhouse (1969), Lal (1969a), Culshaw(1986), and Breaks and Thivierge (2001). A, andesite; B, basalt;BA, basaltic andesite; D, dacite; R, rhyolite.
Fig. 4. Alteration index of Lentz (1999) versus CaO, reflectingplagioclase dissolution and alteration mineral formation duringhydrothermal alteration of volcanic protoliths of cordierite–gedrite rocks from Haliburton County. Samples from this study,Lal and Moorhouse (1969), Lal (1969a), and Culshaw (1986).Average andesite/dacite after Gill (1981).
County samples are comparable to average continental crust(avg. Rb = 43 ± 46 ppm; avg. Cs = 1.3 ± 1.1 ppm; avg. Ba =281 ± 203 ppm). Sr, however, is low (72 ± 52 ppm) whencompared with an average value for continental crust (260 ppm;Taylor and McLennan 1985). If the anomalously metal-richsample (CO88.62) from south Haliburton Lake is excluded,the values for Cr, Ni, and Cu are all lower than averagecrustal abundance (17 ± 3, 12 ± 16, and 26 ± 31 ppm, re-spectively). The high field-strength element (HFSE) abun-dances are more variable. Th and Nb are low (1.8 ± 1.2 and5.4 ± 4.3 ppm, respectively), and Hf, Zr, and TiO2 are closer
to average crustal values (2.7 ± 0.8, 90 ± 20 ppm, and 0.6 ±0.2 wt.%, respectively).
In an Nb/Y versus Zr/Ti tectonic discrimination diagram(Fig. 5) (Winchester and Floyd 1977), the Haliburton Countysamples plot in the fields of evolved igneous rocks (e.g.,andesite to rhyodacite–dacite). In addition, the low Y/Zr ratios(avg. 3.2 ± 1.1) and the major element trends depicted inFig. 3 are consistent with tholeiitic differentiation (Robertset al. 2003).
Rare-earth element compositionRare-earth elements (REE) from Haliburton County and
Hoare Lake rocks have two characteristic patterns: light REE(LREE)-enriched and heavy REE (HREE) > LREE. The REEpatterns do not correlate with the mineral assemblage (Table 1).
The LREE-enriched samples (Fig. 6) can be separatedinto two groups: (i) four samples (96BZ20, 96FL10, 96FLA,and CO88.78) with 70–80 times chondrite LREE enrichment(Lan/Smn = 2.1 ± 0.2), prominent negative Eu anomalies,and relatively flat HREE at 50 times chondrite (Smn/Ybn =1.2 ± 0.1); and (ii) two other samples (96FL26 and 96FL13)with about 40 times chondrite LREE enrichment (Lan/Smn =1.15), prominent negative Eu anomalies, and slightly moreenriched HREE that are similar to the other four samples.
The four HREE > LREE samples (97CV2, 97CV3, 96FL9,and 96FL31) have much lower LREE (�5–25 times chondrite),very slight to no Eu anomalies, and increasing HREE (�15–70 times chondrite). Sample CO88.67 may also be part ofthis group (Fig. 7). The Hoare Lake rocks (samples 97CV2and 97CV3) are the least LREE enriched, concordant withtheir low SiO2. Other trace elements from the Hoare Lakelocality are also lower than those of the Haliburton Countysamples (e.g., Sr, Y, Zr); however, Zr/Y and Zr/Ti (Fig. 5)are similar.
Neodymium isotopesSm–Nd analyses were performed at Carleton University
(Cousens 1996, 2000). Powdered samples were leached in120 °C HCl, mixed with a 148Nd–149Sm spike, and dissolvedusing HF, HNO3, and lastly HCl before column chemistry(after Richard et al. 1976). Dissolved samples were loadedonto double filaments, and analyses were normalized to a146Nd/144Nd ratio of 0.72190. Laboratory averages are29.02 ppm Nd, 6.68 ppm Sm, and 143Nd/144Nd = 0.512668 ±20 (n = 4) for BCR-1. The long-term laboratory average forthe La Jolla standard is 143Nd/144Nd = 0.511876 ± 18 (n = 54).Average reproducibility for εNd values is approximately ±0.5.
The five CMBbtz samples yield a poorly constrained Sm–Nd isochron of 1.0 ± 0.2 Ga (1σ), probably reflecting meta-morphism (Fig. 8). If sample 96FL13 is excluded, depletedmantle model ages (DePaolo 1981) average 1.24 ± 0.08 Ga(Table 2). The depleted mantle model age for 96FL13 issomewhat older (1.79 Ga), but the near-chondritic Sm/Nd ra-tio of this sample compounds the uncertainty in εNd and thuserror in the mantle extraction age of this sample. Including thissample changes the average depleted mantle model age to1.35 ± 0.26 Ga. At 1.07 Ga (the age of deformation alongthe CMBbtz and the likely age of metamorphism of thecordierite–gedrite rocks), εNd for all analyzed samples is3.8 ± 0.9 (Fig. 8). Such a positive εNd suggests that theserocks were relatively juvenile additions to the crust. Intermineral
Fig. 5. Nb/Y versus Zr/Ti diagram (Winchester and Floyd 1977).Symbols as in Fig. 3. Ellipses represent 90% of andesite (A), basalticandesite (BA), and dacite (D) samples plotted and classified byPearce (1996) using a Le Maitre et al. (1989) diagram. See textfor discussion.
Fig. 6. Rare-earth elements from LREE-enriched HaliburtonCounty samples. These REE patterns are similar to those of un-altered metavolcanic rocks of the Composite Arc Belt, except forthe negative europium anomalies, which indicate plagioclase dis-solution during hydrothermal alteration. Chondrite REE normal-ization values from Nakamura (1974).
oxygen-isotope fractionations indicate that the cordierite–gedrite rocks have acted as closed systems at least since thepeak of metamorphism (Peck and Valley 2000). Althoughthe cordierite–gedrite rocks have variable Sm/Nd ratios(Table 2), εNd values are very similar at 1.07 Ga (Fig. 8)and suggest that the neodymium-isotope systematics forthese samples were broadly closed system.
Discussion
Genesis of cordierite–gedrite rocksThe apparent rarity of cordierite–gedrite and related rocks
is commonly ascribed to unusually Mg-rich, Al-rich, andCa-poor bulk compositions resulting from metasomatism, meltextraction, or some unusual protolith (see Robinson et al.1982; Schumacher 1988; Dymek and Smith 1990; Smith etal. 1992a, 1992b, and references therein). The CMBbtzcordierite–gedrite rocks have had several proposed protolithsthat include restites derived from the partial melting of meta-sediments (Lal and Moorhouse 1969; Breaks and Thivierge2001), soils (Culshaw 1986), and alkaline volcanic rocks(see Easton 1992). The protoliths for other worldwide occur-rences of cordierite–gedrite rocks have been ascribed tosimilar materials, as well as protoliths derived from hydro-thermally altered volcanic rock or ash. Although the protolithof cordierite–gedrite rocks can be more easily quantified involcanogenic massive sulfide (VMS) deposits (Bachinski 1978;Pan and Fleet 1995; Schandl et al. 1995; Zaleski and Peterson1995; Araujo et al. 1996; Roberts et al. 2003), or whereequivalent altered volcanic rocks are exposed at lower meta-morphic grade in the metamorphic terrane (Vallance 1967),it is more problematic in the regionally metamorphosed andstrongly deformed CMBbtz.
Oxygen-isotope analyses of CMBbtz rocks reveal δ18O(whole rock) values of 6‰–8‰, which is consistent with aseawater-altered volcanic protolith (Peck and Valley 2000).Hydrothermally altered basalts from ophiolites (Muehlenbachs1986), chlorite–quartz rocks from VMS deposits (Green etal. 1983), and cordierite–gedrite rocks from other localities(e.g., Burger et al. 2004) all commonly have oxygen-isotoperatios of 6‰–8‰. Although the field relations clearly showleucosome development in many of these rocks (Culshaw1986; Breaks and Thivierge 2001), melt extraction from peliticmetasediments alone cannot explain the measured oxygen-isotope ratios (Peck and Valley 2000).
GeochemistryThe cordierite–gedrite rocks of the CMBbtz have whole-
rock geochemical characteristics consistent with igneous rocksthat were hydrothermally altered and subsequently metamor-phosed in the upper amphibolite or granulite facies. Usingwhole-rock major element data, most rocks from HaliburtonCounty plot as andesites and dacites in a Le Maitre et al.(1989) total alkalis versus silica diagram (Fig. 3), and thecordierite–gedrite rocks from Hoare Lake plot as basalts andbasaltic andesites. Although these results may suggest potentialsources for the protolith, the possibility of element mobilityduring hydrothermal alteration is a factor that must be con-sidered (cf. Schandl et al. 1996). Thus, elements that aregenerally immobile during hydrothermal alteration must beevaluated to investigate the igneous affinity of these rocks.
Igneous protolithsImmobile elements in the cordierite–gedrite rocks are most
consistent with igneous protoliths and arc volcanic affinities,but many of the more mobile elements are still broadly con-sistent with this classification. These rocks plot in the andesiteand rhyodacite–dacite field (Fig. 5) in the Nb/Y versus Zr/Tidiagram (Winchester and Floyd 1977). In addition, in theNb/Y versus Zr/Ti diagram these rocks also plot within Pearce’s(1996) 90% ellipses of unaltered andesites and dacites clas-sified using a Le Maitre et al. (1989) total alkalis versussilica diagram (TAS; Fig. 3). The Haliburton County rocksplot in these fields in the TAS diagram, suggesting limitedsilica mobility during hydrothermal alteration. Compared withaverage andesites (e.g., Gill 1981) these rocks are also similarwith respect to the alkali elements (Na2O and K2O), but withlower Al2O3 and higher Mg/Fe, Fe2O3, and MgO. The moststriking characteristic of the cordierite–gedrite rocks is lowCaO. As in other cordierite–gedrite suites (e.g., Dymek andSmith 1990; Smith et al. 1992a, 1992b; Burger et al. 2004),coupled CaO, Sr, and Eu depletion is ascribed to plagioclasedissolution during hydrothermal alteration of the precursorrocks.
These rocks have FeO + MgO/K2O + Na2O values of upto 15, and one sample of Lal (1969a) has a value of 31(Fig. 4). The FeO + MgO/K2O + Na2O value, called thealteration index (AI) by Lentz (1999), shows a striking re-lationship with the CaO contents. In Fig. 4, the path ofplagioclase dissolution or chlorite formation during hydro-thermal alteration is tracked using this index. Rocks with thelowest CaO have the highest AI values. Plagioclase dissolutionand the formation of chlorite and other alteration products atthe expense of igneous minerals is commonly observedduring seafloor hydrothermal alteration of both mafic andfelsic volcanic rocks (Hajash and Chandler 1981; Shiraki et
Nd (ppm) Sm (ppm) 147Sm/144Nd 143Nd/144Nd εNd(1.07 Ga) TDM (Ga)
al. 1987; Thompson 1991) and is consistent with the majorelement characteristics of the cordierite–gedrite rocks.
Although Ba has the potential to be mobile during hydro-thermal alteration, the ratio Ban/Lan = 6.7 ± 5.0 is most similarto that of arc volcanic rocks, as are the overall low Nb contents(e.g., Perfit et al. 1980). The two groups of LREE-enrichedREE rocks and their broadly andesitic and tholeiitic chemistryare very similar to those of primitive arc rocks. The HREE >LREE enriched rocks are interpreted to have been modifiedduring anatexis and are discussed later in the paper.
Lack of a sedimentary signatureThe Al- and Mg-rich nature of some cordierite–gedrite
rocks has led some workers to suggest some form of sedi-mentary protolith (Culshaw 1986; Reinhardt 1987). Somediscrimination diagrams for distinguishing paragneiss fromorthogneiss use elements that appear to have been mobile inthe protoliths of the CMBbtz rocks (see Leyreloup et al.1977; Werner 1987). Immobile elements in these rocks arenot consistent with a sedimentary protolith, especially theREE. The HREE > LREE enriched samples are inconsistentwith the LREE-enriched patterns of sedimentary rocks. Thetwo groups of LREE-enriched cordierite–gedrite rocks aredissimilar from REE analyses of sediments in that thosematerials usually have pronounced slopes in the HREE, afeature not seen in the cordierite–gedrite rocks (Figs. 6, 9).For example, the North American Shale Composite (Grometet al. 1984) has Smn/Ybn = 2.4, whereas Smn/Ybn = 1.0 ±0.3 for LREE-enriched cordierite–gedrite rocks.
Reinhardt (1987) reported cordierite–gedrite rocks from
Queensland, Australia, that are associated with evaporiticmetasediments and appear to have a protolith of magnesianpelites. These rocks are distinct from most other cordierite–gedrite localities in that they are iron poor with FeO/MgO <0.4, similar to that of evaporitic sediments (Reinhardt 1987).Most other cordierite–gedrite rocks and altered volcanic rocksdescribed in the literature have intermediate FeO/MgO ratios(i.e., they plot near the center of AFM (Al2O3–FeO–MgO)projections from muscovite or plagioclase). The HaliburtonCounty rocks have FeO/MgO = 1.9 ± 0.7 and the HoareLake rocks FeO/MgO = 1.1 ± 0.3. These results indicate thatthe protolith of these rocks was quite distinct from magnesiansedimentary rocks.
Role of melt extractionEvidence for partial melting in the form of locally extensive
leucosome development has been described for many of thecordierite–gedrite rocks of the CMBbtz (Lal and Moorhouse1969; Culshaw 1986; Breaks and Thivierge 2001). Lal andMoorhouse (1969) and Breaks and Thivierge (2001) invokedmelt extraction from pelitic precursor rocks to explain thedepletion in CaO and the alkali elements. Several geochemicalcharacteristics are inconsistent, however, with melt extractionbeing the primary cause of the unusual bulk composition.
Many of the CMBbtz cordierite–gedrite rocks have SiO2contents higher than that of average shales (Gromet et al.1984), or of melt from pelites in equilibrium with cordierite +orthopyroxene in experiments conducted by Hoffer and Grant(1980). Closed-system melt extraction from pelitic rocks(typically with δ18O ≈ 10‰–15‰) cannot lower the δ18O ofthe residuum (restite) enough to produce the oxygen-isotoperatios measured (δ18O = 6‰–8‰). For example, 50% meltextraction at metamorphic temperatures shifts the restite<0.5‰ (Peck and Valley 2000).
Fig. 8. εNd versus time diagram showing the depleted mantle curve(DM) of DePaolo (1981) and isotopic evolution of cordierite–gedrite samples. Fields of ≥90% of samples from the CompositeArc Belt (Dickin and McNutt 2005), Central Gneiss Belt (Dickin1998), and 1.3 Ga tonalities (Daly and McLelland 1991) areshown for reference.
Fig. 7. Rare earth elements from HREE > LREE HaliburtonCounty and Hoare Lake samples. Symbols as in Fig. 3. Thesepatterns are suggestive of melt extraction. Open circles model a80% plagioclase 20% garnet residuum (restite) after 30% batchmelting. Distribution coefficients are from Irving and Frey(1978) and Fujimaki et al. (1984). Chondrite REE normalizationvalues are from Nakamura (1974).
There is evidence for some outcrop-scale modification ofprimary geochemistry by melt mobilization. The most dis-tinctive signature of melt extraction from these rock are theHREE > LREE enriched rocks at Fishtail and Hoare lakes(Fig. 7). Melt–mineral partition coefficients are poorly con-strained for cordierite and gedrite, but simplified models ofbatch melting produce broadly similar HREE > LREE en-riched patterns when granitic melts are removed from theother cordierite–gedrite rocks. A representative REE patternis shown in Fig. 7, which models 30% melt removal fromone of the cordierite–gedrite rocks, leaving a garnet–plagioclaserestite. Using the LREE-enriched samples as a precursor, orincreasing the amount of partial melting, only produces moresimilar HREE-enriched patterns. Note that HREE > LREE isonly found in a few samples (those with the lowest SiO2),and that this process has only modified the chemistry of therocks at the outcrop scale. Partial melting has left manyincompatible element ratios unchanged and has preservedthe δ18O values caused by hydrothermal alteration of theprotolith. Sm–Nd-isotope systematics have also not been mea-surably shifted. The εNd of sample 96FL9 (4.25 at 1.07 Ga) isindistinguishable from those of the cordierite–gedrite rockswith more typical igneous REE patterns (Fig. 8).
Correlations with the Composite Arc BeltMetavolcanic rocks of the Composite Arc Belt have been
the focus of many field geologic and geochemical studiesbut have received surprisingly little attention with respect togeochronology or radiogenic isotopes (Easton 1992; Carr etal. 2000). The arc affinities of these rocks have led totectonic models that typically involve either arc develop-ment on the margin of Laurentia (Hanmer et al. 2000; Smithet al. 2001) or formation off the Laurentian coast followedby later accretion (Carr et al. 2000). The presence of thecordierite–gedrite rocks in the CMBbtz and their affinitiesto the Composite Arc Belt should be taken into account ineither of these tectonic models.
Few Nd-isotope data are available for the Composite ArcBelt, and a direct correlation between a particular volcanicsequence and the cordierite–gedrite rocks is not possible atthis time. There is, however, an extensive database for variousorthogneiss units in the Central Gneiss Belt, which is theCMBbtz footwall (e.g., Dickin 1998). Rocks in the footwallhave depleted mantle extraction ages of 1.91 ± 0.06 Ga, andεNd(1.07 Ga) values of –7.6 to –3.6. These orthogneisses arequite distinct from the positive εNd values of the cordierite–gedrite rocks. The 1.3 Ga tonalitic rocks from the Adirondackshave been correlated with similar rocks in the CMBbtz(Hanmer and McEachern 1992) and Composite Arc Belt(Carr et al. 2000). The εNd(1.07 Ga) values of the tonalities are+0.7 to +1.4 (Daly and McLelland 1991) and are not asjuvenile as those of the cordierite–gedrite rocks. The Nd-isotope data presented here for the cordierite–gedrite rocksare not consistent with a Central Gneiss Belt affinity, but aremore similar to what would be expected for 1.29–1.22 Gaigneous activity in the Composite Arc Belt. Significantly,preliminary Nd-isotope data for the Composite Arc Belt(Dickin and McNutt 2005) indicate depleted mantle extrac-tion ages averaging 1.27 Ga, identical to the average of ana-lyzed cordierite–gedrite samples.
Fig. 9. (La/Sm)n versus (Sm/Yb)n for LREE-enriched HaliburtonCounty samples (i.e., with igneous patterns) for comparison withComposite Arc volcanic rocks. Fields are for Tudor Volcanics ofTudor township (Tud; Smith and Holm 1987), the TurriffVolcanics (Tur; Smith and Holm 1987), the Sharbot Lake volca-nic rocks (SL; Corfu and Easton 1997; Smith et al. 2001), andtonalites of the Glamorgan Gneiss Complex (G-ton; Easton1987). Other Composite Arc volcanic rocks are from Harnoisand Moore (1991), Corfu and Easton (1995), Smith and Harris(1996), and Smith et al. (1997). Shale composites are fromTurekian and Wedepohl (1961), Haskin and Haskin (1966), Vineand Tourtelot (1970), Gromet et al. (1984), and Taylor andMcLennan (1985). Chondrite REE normalization values are fromNakamura (1974).
Fig. 10. (La/Sm)n versus Sm for LREE-enriched HaliburtonCounty samples (i.e., with igneous patterns) for comparison withComposite Arc volcanic rocks. Fields are for Tudor Volcanics ofTudor township (Tud), the Turriff Volcanics (Tur), the SharbotLake volcanic rocks (SL), and tonalites of the Glamorgan GneissComplex (G-ton). References as in Fig. 9.
Rocks similar to cordierite–gedrite rocks of the CMBbtzare found in the Bondy Gneiss Complex of Quebec (Blein etal. 2004). These cordierite–orthopyroxene rocks are part of a1.4 Ga arc-related volcanic complex (Blein et al. 2003) andare in a region of Quebec interpreted to be an extension ofthe Ontario Frontenac terrane. The Bondy Gneiss Complexhas older Nd model ages than cordierite–gedrite rocks of theCMBbtz (�1.5 Ga versus �1.2 Ga), although the Bondycordierite–orthopyroxene rocks have similar δ18O values(8.4‰–8.9‰; Peck et al. 2004). These features point to similarstyles of hydrothermal alteration in the two suites, but in arcsof different ages in distinct parts of the southern GrenvilleProvince. In addition, the Bondy cordierite–orthopyroxenerocks show evidence for REE mobility during hydrothermalalteration (Blein et al. 2004), a feature not seen in the CMBbtz.
The evolved, but tholeiitic, geochemistry of the cordierite–gedrite rocks is similar to that of many volcanic sequencesin the Ontario Composite Arc Belt, although it is difficult tounequivocally point to volcanic protoliths. Hydrothermallyaltered volcanic rocks have been described from the Com-posite Arc Belt (e.g., Easton 1992; Smith and Harris 1996;Smith et al. 2001), but the alteration does not approach theextent of Ca depletion in the cordierite–gedrite rocks. Astriking characteristic of the cordierite–gedrite rocks is theirflat HREE patterns, suggestive of a lack of garnet in theirsource rocks during melting. This feature is not common inComposite Arc Belt volcanics, where Lan/Smn > 1 is usuallyaccompanied by increases in Smn/Ybn and REE contents(Figs. 9, 10). There are four rock suites in the CompositeArc Belt that have REE patterns and overall geochemistrysimilar to those of the cordierite–gedrite rocks (especiallythe LREE-enriched samples). These are the Tudor Volcanicsof Tudor Township (Smith and Holm 1987), the TurriffVolcanics (Smith and Holm 1987), the Sharbot Lake volca-nic rocks (Corfu and Easton 1997; Smith et al. 2001), andtonalitic rocks from the Glamorgan Gneiss Complex (Easton1987). All have similar REE patterns and evolved, tholeiiticchemistries, and three are in proximity to the CMBbtz.
The Sharbot Lake volcanic rocks are exposed in the SharbotLake terrane (Fig. 1). They are primarily mafic tholeiiticvolcanic rocks, with subsidiary felsic members and associ-ated carbonate rocks. The age of these rocks is constrainedto >1.22 Ga by crosscutting relations (Corfu and Easton1997). Smith et al. (2001) describe the Sharbot Lake rocksas representing a bimodal mafic to silicic suite, although themore evolved members have not been extensively sampled.The Sharbot Lake domain is interpreted as a rifted arc –back-arc environment (Corfu and Easton 1997; Smith et al.2001).
The Tudor and Turriff volcanics crop out in the BelmontDomain. The Turiff suite is made up of basalt, andesite, andminor rhyolite, and the Tudor suite is mostly basalt and minorrhyodacite. Both the Turriff and Tudor volcanics are undatedbut are thought to be in the range 1.29–1.28 Ga (Easton1992). Smith et al. (1997) have interpreted the tectonic affinitiesof the Belmont domain in the context of back-arc extension.
Tonalitic gneisses of the Glamorgan Gneiss Complex haveREE and trace-element compositions similar to those of thecordierite–gedrite rocks (Easton 1987). Tonalites from thegneiss complex have unpublished U–Pb ages of ca. 1.25 Ga(Easton 1987). The Glamorgan Gneiss Complex is part of
the Glamorgan thrust sheet (Hanmer and McEachern 1992)(GL in Fig. 2) in the CMBbtz.
The geochemical and isotopic similarities between thesevolcanic suites and the cordierite–gedrite rocks suggest tec-tonic continuity between the Composite Arc Belt and theCMBbtz. Carr et al. (2000) have suggested that parts of theCMBbtz may represent a part of the Laurentian margin,rather than being associated with the Composite Arc Belt.The presence of geochemically related volcanic rocks thatspan from the base of the CMBbtz into the Composite ArcBelt argues against this possibility.
Conclusions
Cordierite–gedrite rocks from the Central MetasedimentaryBelt boundary thrust zone have assemblages that reflecthydrothermal alteration of their volcanic protoliths. Oxygen-isotope ratios show that soils or pelitic sediments are notappropriate precursor rocks, but the isotope signature is verysimilar to that from volcanic rocks altered by seawater (Peckand Valley 2000).
Cordierite–gedrite rocks are located at the base of theCMBbtz and may have helped localize the location of thethrust because of their rheology (Hanmer and McEachern1992). Neodymium-isotope data, REE, and other immobiletrace elements suggest a strong affinity to unaltered metavol-canic rocks in the Composite Arc Belt and may necessitatethe extension of the Composite Arc Belt to the base of theCMBbtz (cf. Carr et al. 2000).
Acknowledgments
The Colgate Faculty Research Council and a Sigma XiGrant-In-Aid of Research to M.S.S. supported this research.We thank John Valley, Mike Easton, and Robert F. Dymekfor their encouragement in this project, and Dave Farber andMike DeAngelis for assistance in the field. M.S.S. wouldespecially like to thank Fred and Terra Breaks (Ontario Geo-logical Survey) for their Canadian hospitality, geologicalassistance, and good humor. Brian Cousens (Carleton Uni-versity) performed the samarium–neodymium isotope analyses,and Alan Dickin kindly supplied unpublished samarium–neodymium isotope data for Fig. 8. Reviews by Simon Hanmerand John Schumacher were very helpful for improving thismanuscript.
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