Protoliths and petrogenesis of Archean gneisses from the Kenora area, English River Subprovince, northwest Ontario

Precambrian Research, 17 (1982) 245--274 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

245

PROTOLITHS AND PETROGENESIS OF ARCHEAN GNEISSES FROM THE KENORA AREA, ENGLISH RIVER SUBPROVINCE, NORTHWEST ONTARIO

CHARLES F. GOWER

Department of Mines and Energy, Newfoundland and Labrador, P.O. Box 4750, St.John's, Nfld. A I C 5T7 (Canada) DALLM K. PAUL

Geochronology Division, Geological Survey of India, 29, J.L. Nehru Road, Calcutta 16 (India) JAMES H. CROCKET

Department of Geology, McMaster University, Hamilton, Ont. L8S 4M1 (Canada) (Received November 5, 1980; second revision accepted December 9, 1981 )

ABSTRACT Gower, C.F., Paul, D.K. and Crocket, J.H., 1982. Protoliths and petrogenesis of Archean gneisses from the Kenora area, English River Subprovince, northwest Ontario. Precambrian Res., 17 : 245--274. Archean gneissic rocks in the Kenora area can be divided into three groups: (1) remnants of a dominantly mafic supracrustal sequence; (2) biotite tonalite gneisses; (3) granitic pegmatoid gneisses. From field, petrographic and compositional data, the biotite tonalite gneisses are interpreted as plutonic rocks derived by partial melting of the supracrustal sequence. The granitic pegmatoid gneisses are considered to be the product of fractional crystallization of the tonalitic magma. Attempts were made to reconstruct this scheme by computer-assisted trace element modelling using REEs. With the source rock constrained to one having a bulk composition similar to that of the fine-grained amphibolites within the supracrustal sequence (comparable to Archean tholeiite elsewhere), the most satisfactory source rock was found to be a two-pyroxene garnet granulite (quartz 5%, plagioclase 35--25%, clinopyroxene 30--40%, orthopyroxene 25--15%, garnet 5--15%). This, when 10% melted under equilibrium, non-modal conditions, will yield a model melt with a REE pattern similar to that observed i n t h e tonalites. Quartz eclogite, granulite without garnet, and amphibolite are not acceptable source rocks under the modelling conditions used. The mineralogy of the o p t i m u m model source rock is similar to that expected in a quartz tholeiite melting between 45 and 50 km depth. The granitic pegmatoid gneisses have very low REE content, especially LREEs, and their patterns defied conventional modelling. They may result from REE loss in residual fluids or the end result, after precipitation of REE-rich phases, such as allanite.

0301-9268/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

246 INTRODUCTION

The purpose of this paper is to report field, petrographic and chemical data for Archean gneissic rocks from the Kenora area, northwest Ontario. These data are used to draw comparisons with similar rocks elsewhere and to discuss the protoliths and petrogenesis of the gneisses. GENERAL GEOLOGY

The Kenora area is located immediately north of the town of Kenora at the southern margin of the English River Subprovince and forms part of the Winnipeg River plutonic complex. The area can be subdivided into three lithological groupings, namely: (1) Wabigoon Subprovince greenstone beltgranite terrain; (2) Dalles granodiorite and other plutons intruding the gneisses; and (3) the gneissic rocks (Fig. 1). In the Kenora area the Wabigoon Subprovince is composed mostly of mafic volcanic and intrusive rocks with minor felsic volcanics and sediments. These have been mapped by H.L. King (in preparation) for the Ontario Geological Survey. The Wabigoon Subprovince is separated from the English River Subprovince in the Kenora area by a major fault (Kenora fault), along which a sheet of megacrystic granodiorite has been intruded (marginal granodiorite). The Dalles granodiorite is a medium-grained, K-feldspar-poor pluton which exerted a strong control on the present disposition and fabric of the gneisses. Within the gneisses, four granitoid bodies have been mapped. From field relationships the Melick tonalite is the oldest. Compositionally, it is very similar to the biotite tonalite gneisses, but geochronologicaUy it appears to be an early member of the ca. 2630 Ma plutons (Gower and Clifford, 1981; Wooden and Gower, in preparation). The Austin intrusion is a coarse-grained, K-feldspar megacrystic biotite granite and is probably an offshoot of the Lount Lake Batholith (defined by Breaks et al., 1978). The Lulu granodiorite is a medium-grained, K-feldspar megacrystic granodiorite and is intruded by the Muriel diorite/granodiorite. The latter is a coarse-grained biotite and hornblende-bearing unit but also includes minor hornblendite and some K-feldspar megacrystic granodiorite phases. The petrogenesis of the plutons is discussed in a later paper (Gower et al., in preparation). The Kenora gneisses are classified in terms of three end-members (Fig. 2): (1) amphibolite; (2) biotite tonalite gneiss; and (3) granitic pegmatoid gneiss. These, and transitional types, are mappable, non-genetic lithological associations. The geological map amalgamates spatial and compositional groups; these are shown separately by Gower (1978), however. The end-members are regarded as the key lithologies in understanding the petrogenesis of the Kenora area and are discussed in turn in the following sections. Minor lithologies mapped within the gneisses include white-weathering leucopegmatoid gneiss, meladiorite/amphibolite, dioritic intrusion breccia and miscellaneous dioritic

GEOLOGY OF THE KENORA AREA, ENGLISH RIVER SUBPROVINCE ~

GEOLOGY BY C.F.GOWER

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251

BIOTITE TQNALITE GNEISS /~TONALITE, LEUCOTONALITE GNEISS / . . ' \ MINOR AMPHIBOLITE AND ANITIC GNEISS

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rocks. These are not discussed here, but detailed field, petrographic and geochemical data are reported by Gower (1978). The detailed geological history and R b - Sr geochronology is given by Gower (1978), Gower and Clifford (1981}, Wooden (1978} and Wooden and Gower (in preparation). ANALYTICAL TECHNIQUES

Major and trace element data reported as averages in subsequent tables were determined using a Phillips 1450 X-Ray fluorescence spectrometer. Accuracy, precision and sample homogeneity can be gauged by comparing accepted and mean analysed values for U.S.G.S. standards BCR-1 and GSP-1 and rock sample G886 in Table I. FeO was determined using standard potassium dichromate titration methods;accuracy was monitored using in-house standards and is better than 0.5%. For samples for which REEs are available, A1203, FeO t, MgO, CaO, Na20, K20, Rb, Sr and Ba were also determined (whole rocks and essential minerals), using atomic absorption spectrometry on a PerkinElmer 603 spectrometer. Average departure of the atomic absorption values from the XRF data were as follows; A1203 3.4%, FeO t 6.7%, MgO 8.4%, CaO 5.9%, Na20 5.8%, K20 4.6%, Rb 4.7%, Sr 7.9% and Ba 6.5%, excluding values at detection limits. REEs, Ba and Th were determined using neutron activation techniques. Also, Rb and Sr were determined by Wooden on samples used for Rb--Sr geochronology (Wooden and Gower, in preparation) and the agreement was usually within 2%. The L.O.I. was also determined for samples investigated in detail. Further details of sample preparation procedures, analytical techniques and more extensive documentation of accuracy, precision and detection limits are given by Gower (1978).

252 TABLE I Accuracy and precision data for Kenora area rocks

SiO~ TiO 2 Al~O3 Fe~O3 t MnO MgO CaO Na~O K20 P~O s Rb Sr Ba Ce (XRF) Zr Y Th Pb

1

2

3

4

5

54.72 2.21 13.65 13.49 0.19 3.48 6.96 3.28 1.68 0.33

54.57 2.27 13.91 13.16 0.18 3.55 7.01 3.24 1.75 0.37

67.91 0.67 15.32 4.37 0.04 0.97 2.04 2.82 5.58 0.28

68.03 0.69 15.52 4.38 0.06 1.04 2.04 2.87 5.44 0.27

74.18 0.11 14.18 0.69 0.04 0.19 1.66 4.40 4.13 0.01

47 330 680 54 185 37? 6 15

48 339 642 46 178 36 6 15

250 230 1300 390? 500 32 105 53

253 59.7 230 359.3 1340 1834.5 309 nd 531 13.6 32 nd 105 nd 55 23.1

6

7 0.16 0.02 0.10 0.01 0.00 0.01 0.02 0.10 0.07 0.00

0.5 3.1 15.9 -1.7 --0.7

74.55 0.99 14.24 0.08 0.04 0.09 1.68 4.39 4.12 0.01 59.0 361.9 1838.4 nd 12.4 nd nd 23.5

8 0.08 0.00 0.06 0.01 0.00 0.02 0.01 0.14 0.02 0.00 0.4 0.6 12.1 5.7 1.5 1.8 1.4 0.6

(1) BCR-I accepted value (Abbey, 1977); (2) BCR-I mean analysed value (three analyses, single determinations for Ba and Ce); (3) GSP-1 accepted value (Abbey, 1977); (4) GSP-1 mean analysed value (three analyses for major elements, four analyses for trace elements; lo for Ce, Y and Th given in column 8); (5) G 8 8 6 mean analysed value of ten splitsof sample taken prior to pulverisation;(6) la for column 5; (7) G 8 8 6 mean analyses value of ten analysed of a single pellet;(8) la for column 7 or where below detection limit for column 4. For R E E analysis of the fine-grained amphibolites and tonaliticrocks instrumental neutron activation techniques were used and the following errors, based on repeated analyses of U.S.G.S. standards A G V - 1 and GSP-1, are applicable (95% confidence level): La +8%, Ce +-12%, Nd +30%, Sm +9%, Eu -+15%, Tb +30%, Yb +-13%, Lu +12%. The granitic pegmatoid gneisses were analysed b y radiochemical neutron activation methods w i t h significantly l o w e r error.

FIELD AND PETROGRAPHIC

CHARACTERISTICS

Fine-grained amphibolites and associated lithologies Gray-black weathering, fine-grained, mafic--mafelsic amphibolites with an a v e r a g e g r a i n s i z e o f ca. 1 m m o c c u r as m a p p a b l e u n i t s i n t e r l e a v e d w i t h o t h e r gneissic rocks. They may be massive, foliated or banded, with banding defined b y m i n e r a l o g i c a l a n d g r a i n s i z e h e t e r o g e n e i t i e s , h o r n b l e n d e - r i c h v e n e e r s , calcsilicate lenses and concordant leucogranitoid veins. Severely deformed, ellipsoidal shapes believed to be former pillows are p r e s e n t in a f e w l o c a l i t i e s ( e x a c t l o c a t i o n s g i v e n b y G o w e r , 1 9 7 8 ) . T h e m a f i c v e n e e r s a r e i n t e r p r e t e d a s v e s t i g e s o f f o r m e r p i l l o w m a r g i n s a n d t h e calc-sili-

253 cate lenses ( c l i n o p y r o x e n e + quartz +calcic plagioclase +grossularite/andradite +calcite) may represent calcareous sedimentary interpillow material t h a t has been modified and mobilized during high-grade metamorphism. In support of the latter interpretation it is n o t e d t h a t the calc-silicate lenses are c o m m o n l y b o u n d e d by mafic veneers and t h a t the lenses closely resemble calcareous interpiUow material seen in some pillowed basalts in the Wabigoon Subprovince. The mineralogy o f the fine-grained amphibolites is hornbl ende + plagioclase + quartz + almandine + c l i n o p y r o x e n e + c u m m i n g t o n i t e with accessory apatite, sphene and pyrite (rimmed by magnetite and altered t o hematite). Retrograde phases include actinolite, clinozoisite/epidote, chlorite, biotite, sphene, white mica, albite and quartz. Averaged analyses of h o r n b l e n d e and plagioclase f r o m the t w o fine-grained amphibolites for which REE data are available are included in Table VI. T h e fine-grained amphibolites are accompanied by medium-grained amphibolites, ultramafic, mafelsic and m e t a s e d i m e n t a r y rocks. As no f u r t h e r reference is made to these associated lithologies compositional data are r e p o r t e d and discussed at this stage (Table II). Apart f r o m grain size, the mediumTABLE II Major and trace element data for the fine-grained amphibolites and associated lithologies

SiO2 TiO~ Al~O3 Fe~O~ FeO MnO MgO CaO Na20 K20 P2Os

1

2

3

4

5

6

49.93 1.13 14.21 3.35 9.57 0.21 7.18 11.39 2.43 0.53 0.09

50.19 1.12 14.34 3.66 8.12 0.20 7.74 9.92 3.25 1.25 0.20

52.55 0.59 5.95 3.31 7.28 0.26 17.94 8.89 0.93 2.23 0.07

52.60 0.55 8.11 2.97 5.34 0.18 13.19 14.38 1.98 0.59 0.10

78.62 0.06 2.03 5.04 8.20 0.40 2.41 2.64 0.38 0.19 0.03

56.88 0.88 17.80 1.79 7.76 0.20 4.18 4.37 3.29 2.73 0.11

Rb Sr Ba Ce Zr Y Pb

5 111 63 6 49 22 nd

31 176 185 29 71 22 6

114 31 235 16 38 19 3

7 302 158 25 29 11 3

3 6 53 4 7 10 3

92 132 435 30 110 22 11

n

15

11

6

4

4

5

nd = not detected, n = no. of analyses. All analyses normalized to 100% on an anhydrous basis. (1) fine-grained amphibolites; (2) medium-grained amphibolites; (3) biotite~linoamphibole schists; (4) mafic hornblende 'porphyry'; (5) banded metachert; (6) meta-volcanoclastite/graywacke.

254 grained amphibolites differ in that they lack calc-silicate lenses, mafic veneers and banding. Also, leucogranitoid veinlets form an agmatitic network rather than being concordant to banding and in outcrop the medium-grained amphibolites occur as angular, irregular blocks rarely exceeding a few metres extent. They are mineralogically and chemically similar to ~he fine-grained amphibolites (except for alkalis) and are interpreted as co-genetic intrusions or the centres of thick flows. The higher alkali contents in the medium-grained amphibolites are attributed to the agmatitic leucogranitoid veinlets, some of which were impossible to exclude during sample preparation. The ultramafic rocks include a possible ultramafic sill, biotite~linoamphibole (cf. cummingtonite) schists and an unusual rock type composed mainly of hornblende porphyroblasts in an interstitial matrix made of plagioclase, epidote, clinopyroxene and actinolite, referred to here as mafic hornblende ' porphyry '. Excluding the possible ultramafic sill, the closest compositional analogue for these rocks is basaltic komatiite. Any textural clues for this protolith, for example spinifex texture, if ever present, have been obliterated by metamorphism. Two types of mafelsic rock are present, inhomogeneous and homogeneous, and both are rare lithologies in the area. The inhomogeneous type is classified as tholeiitic basalt using the classification of Irvine and Baragar (1971} and could be pillow breccia from its field relationships. The homogeneous type is classified as andesite and may be extrusive. However, both rock types have suffered metamorphism and possibly compositional modification, so that any speculation, beyond the suggestion that they are probably volcanic, is unwarranted. The most common metasedimentary rock types are banded metachert and volcanoclastite/graywacke but other rare lithologies include possible metaarkose and meta-argillite. The banded metachert can be unequivocally identified in the field and confirm a supracrustal origin for the associated finegrained amphibolites. The volcanoclastite/graywacke rocks can be chemically and isotopically distinguished from the remainder of the gneissic rocks in the Kenora area (Longstaffe and Gower, in preparation). The $180 range from the volcanoclastite/graywacke is 6.15--7.99 in contrast to 7.34--8.61 for the biotite tonalite gneiss to which it is compositionally most similar. The major and trace element and isotopic composition of the volcanoclastite/graywacke indicates a mafic provenance. This conclusion is in keeping with its association with a predominantly mafic supracrustal sequence. Biotite tonalite gneiss

The fabric of the biotite tonalite gneisses can be variously described as foliated, foliated to gneissic, or gneissic. Banding is defined by mineralogical heterogeneities, leucogranitoid layers, hornblende and biotite schlieren and granitic veins. Excluding the leucogranitoid veins, schlieren and enclaves,

255 these pale gray, medium-grained tonalitic gneisses are remarkably uniform texturally with no hint of a primary heterogeneous fabric. Plagioclase (An43-27 ,average An28), quartz and biotite are essential phases. Occasionally hornblende and epidote are also essential minerals. Accessory minerals include microcline, garnet (only in rocks occurring in the deepest structural level), aUanite, zircon, sphene, apatite and opaque minerals. Retrograde minerals are white mica, albite, carbonate, clinozoisite/epidote, sphene and opaque minerals. Morin (1977) has reported microprobe data on allanite from biotite tonalite gneiss in the Kenora area. Analytical data for essential minerals from tonalites for which REE analyses are available are reported by Gower (1978). The tonalitic gneisses are part of a gradational series; with increasing hornblende content (correlated in outcrop with increasing abundance of amphibolite enclaves) the rock grades into hornblende--biotite tonalite; with increasing alkali feldspar there is a gradation into mixed biotite tonalite-granitic pegmatoid gneiss. Figure 2 illustrates this gradational nature and is the empirical basis on which the gneisses were mapped. The amphibolitic part of the amphiboliteleucogranitoid association shows many characteristics of the fine-grained amphibolites and occurs as rafts, bands and lenses separated and/or injected by anastamosing leucogranitoid veinlets. Interfaces between amphibolite and leucogranitoid material may be sharp or gradational and there is evidence for both external injection and in situ derivation for the latter. Typical features of the hornblende--biotite tonalite gneiss include leucogranitoid veins, biotite schlieren, hornblende--epidote screens and boudins of fine to coarse-grained amphibolite and hornblendite.

Granitic pegrnatoid gneiss The granitic pegmatoid gneiss rarely occurs as a mappable unit (at 1:15 000 scale) and on Fig. 1 it has been grouped with the mixed biotite tonalite-granitic pegmatoid igneiss into which it grades (Fig. 2). The rock occurs in irregular bands, lenses and patches which can be ascribed partly to original shape, partly to later deformation and partly to the presence of large K-feldspar megacrysts commonly exceeding 20 cm in diameter. The granitic pegmatoid gneisses are interpreted as deformed pegmatites injected both concordantly and discordantly into pre-existing tonalitic rocks. Margins discordant with an earlier fabric are rare, however. The essential minerals are quartz, poorly-twinned plagioclase (An15-21), microcline perthite and biotite. Myrmekite is abundant and epidote and muscovite are major accessory phases. Minor accessory phases include carbonate, chlorite, hematite, allanite (rare), pyrite and magnetite. The first three minor accessory phases are alteration products. Analyses of essential minerals from the two granitic pegmatoid gneisses for which REEs are available are reported by Gower (1978).

256

CLASSIFICATION, PROTOLITHS, P E T R O T E C T O N I C A N A L O G U E S A N D FETROGENESIS

Fine-grained amphibolites Using the scheme of Irvine and Baragar (1971) and the TiO2 -- Zr/P205 and P2Os- Zr diagrams of Floyd and Winchester (1975) the present chemistry of the fine-grained amphibolites Consistently classifies the rocks as tholeiitic. In terms of a possible modern-day petrotectonic environment using discriminant diagrams involving immobile{?) trace elements the following statements can be made. (1) The TiO2 -- Zr and P2Os -- Zr diagrams of Floyd and Winchester {1975) suggest an oceanic parentage. (2) In the Ti/100- Zr- Y × 3 and T i - Z r diagrams of Pearce and Cann (1973), the fine-grained amphibolites plot within the low-K tholeiite and/or ocean floor basalt fields. The average fine-grained amphibolite of Table II disguises the fact that the fine-grained amphibolites fall into two groups with no overlap in TiO~, P2Os, Zr and Y abundances. In Table III we show the correspondence of the low and high-Ti amphibolites with the island-arc tholeiite (IAT) and ocean floor basalt (OFB) groupings of Pearce and Cann (1973). Although this may be due to insufficient data it was a factor in selecting one member of each group for REE analysis. T A B L E III T e n t a t i v e division o f fine-grained a m p h i b o l i t e s i n t o low and high-Ti groups and comparison w i t h IAT and OFB L o w Ti

High Ti

IAT

OFB

TiO~ P~O 5 Zr Y

0.70--1.25 0.05--0.08 26--48 17--22

1.41--1.87 0.10--0.14 48--91 26--35

0.65--1.12 -33--68 16--22

1.02--1.99 -64--129 22--47

n Example

11 G867A

4 G835A

46 72 Pearce and Cann ( 1 9 7 3 )

n = no. o f analyses.

The REE patterns for G835A and G867A are shown in Fig. 3. Both samples have flat, parallel patterns 9-14 × chondrite, both fall within the field for common Archean tholeiite (TH1) and the envelope for MORB-arc modern tholeiite (Condie, 1981). In common with Archean tholeiites elsewhere the amphibolites show low Al:O3 suggesting feldspar fractionation. Secondly, high FeOt/FeOt+MgO and low TiO: (in most) suggests olivine and ilmenite fractionation. Thirdly, lack of HREE depletion precludes amphibole or garnet fractionation. These

257 I00

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Yb Lu

Fig. 3. REE patterns for fine-grained amphibolites.

features are consistent with an origin involving shallow-level magma equilibration and fractionation after partial melting of an anhydrous peridotite source. Petrogenetic modelling of Archean tholeiite has been attempted by Arth and Hanson (1975) and Condie and Harrison (1976). Arth and Hanson adequately model tholeiite REE patterns by 25% melting of a plagioclase peridotite source rock. The model lherzolite source rock of Condie and Harrison, from which model tholeiite is derived by 30% partial melting, has similar mineralogy and REE concentrations to the peridotite of Arth and Hanson. Either of these models appears an adequate explanation for the Kenora fine-grained amphibolites on present data. Biotite tonalite gneiss Three possible protoliths have been suggested for Archean tonalitic gneisses. These are: (1) metasediments (Sutton and Watson, 1951; Sutton, 1967; Kalsbeek, 1970; Andrews , 1973; Cheney and Steward, 1975); (2) acid volcanic rocks (Sheraton, 1970; Bowes et al., 1971; Goldich et al., 1972; Barker and Peterman, 1974}; (3) plutonic rocks (Bridgewater et al., 1974; Tarney, 1976; Windley and Smith, 1976). Although an igneous, rather than sedimentary, protolith has received greater support, the latter continues to have its adherents. With reference to the Kenora area, however, there is no convincing evidence for a metasedimentary origin. The points in favour of an igneous origin are as follows.

258 (1) The tonalitic gneisses show remarkable uniformity compared to most sedimentary sequences and there is no evidence for rock types such as conglomerates or breccias which, even in an extremely deformed state, are comparatively easy to recognize. With reference to the Apsley gneiss, Shaw (1972, p. 23) wrote that: "If the protolith was graywacke, these circumstances are unusual, because in younger rocks it is c o m m o n to find graywacke interbedded with shale. " A similar objection can be made with reference to the Kenora area tonalitic gneiss. (2) Compositional igneous-metasedimentary discrimination criteria all favour an igneous protolith, in contrast to those rocks identified in the field as possible sediments. Some of the better evidence is as follows: (i) all tonalitic gneisses score positive (igneous) values on Shaw ~ (1972) discriminant function, DF3; (ii) all tonalitic gneisses fall on the igneous trend of de la Roche (1972); (iii) all tonalitic gneisses fall in the igneous field (as defined by Shaw's screened analyses) on the Weisbrod (1969) plot. (3) The above criteria do not exclude graywackes. However, compared to graywackes and gneisses believed to be derived from graywackes (Table IV) the tonalites have higher CaO, SiO2 and Na20 and lower TiO2, FeO t, MgO, K20 and Rb. On the basis of high SiO2 it could be argued that the biotite tonalite gneiss represents a more mature, quartz-enriched graywacke. Removing the SiO2 difference and renormalizing reduces (but does n o t eliminate) TiO2, FeO t, MgO, K~O and Rb contrasts but also has the effect of accentuating Na20 and CaO. The low A1203/Na20 of the tonalitic gneisses, used as an index of sediment maturity (Pettijohn, 1957), is an igneous value (3.2, in contrast to 'mature' values of 10 or more). This further militates against suggestions that the tonalites could be mature graywackes. (4) The whole-rock 5180 values of 7.34--8.61 (Longstaffe and Gower, in preparation) for eight biotite tonalite gneisses compares closely with the plutonic Melick tonalite in the Kenora area (7.51--8.85), but contrasts to a range of 8.83--11.69 determined in Archean turbidites and paragneisses (Longstaffe and Schwarcz, 1977). (5) The low initial Sr ratio in the biotite tonalite gneiss (0.7009, Wooden and Gower, in preparation) imposes severe constraints on any suggested graywacke source: it would have to be one with a short crustal residence time and derived from a short-lived precursor which came from a low initial Sr-ratio source, such as oceanic crust or mantle. (6) Tonalitic rocks derived by large-scale melting of graywackes can also be eliminated. Arth and Hanson (1972) have noted that this is unlikely because, if the parent is in the plagioclase liquidus volume of the A b - - A n - - O r - Q z quaternary, partial melting will yield a K-enriched magma but Archean tonalites have much lower K and Rb contents than Archean graywackes. To distinguish between extrusive and intrusive igneous protoliths is more difficult. Interpretation of the biotite tonalite gneiss as a felsic volcanic sequence (which would probably include fragmental rocks) is difficult to accept as it is necessary to argue that all primary fabric has been destroyed; an untenable argument in view of obvious primary fabrics in associated (earlier)

259 TABLE IV Major and trace element whole-rock data comparing biotite tonalite gneiss and Melick tonalite, and contrasted to Archean graywacke and Archean gneisses derived from graywacke

SiO s TiO s AI20 s Fe~O s FeO MnO MgO CaO Na~O K~O P~O. Rb Sr Ba Ce Zr Y Th Pb n

1

2

3

4

5

6

7

69.65 0.44 15.55 1.06 2.21 0.06 1.05 3.84 4.72 1.30 0.12

69.93 0.38 15.73 1.35 1.53 0.06 0.93 3.68 4.93 1.38 0.I0

66.04 0.69 15.47 1.16 4.86 0.11 3.11 2.74 3.27 2.40 0.15

65.40 0.63 15.70 1.12 5.54 -3.16 2.24 3.77 2.44 --

69.10 0.54 10.60 1.25 6.05 -4.70 2.09 1.88 1.67 --

67.40 0.90 16.20 1.50 3.75 0.08 2.70 3.28 2.24 1.83 0.12

63.54 0.66 17.32 5.46 -0.07 2.27 3.78 3.86 2.68 0.36

49 372 481 35 197 10 4 9

50 524 567 41 140 5 4 7

18

15

------. . 20

88 54 424 134 -319 56 46 --16 -. . . . . . 6

6

68 -

-

207 -103 -. . . 21

92 861 1278 79 ---

8

n ffino. of analyses. All normalized anhydrous to 100%; M n O and P~O s ignored for normalization where not reported. Total Fe as F % O 3 where F e O not reported. (1) Biotite tonalite gneiss, Kenora area; (2) Melick tonalite,Kenora area; (3) Graywacke, Slave Province (Henderson, 1975); (4) Graywacke, W y o m i n g (Condie, 1976); (5) Graywacke, Sheba Formation, Barberton (Condie, 1976); (6) Wacke palaeosome, Ear Falls--Manigotagan Gneiss Belt (Breaks and Bond, 1977); (7) Twilight (metasedimentary) gneiss, Cedar Lake area, English River Subprovince (Westerman, 1978). s u p r a c r u s t a l r o c k s . T h e p r i n c i p l e f a b r i c f e a t u r e in t h e b i o t i t e t o n a l i t e gneiss is b a n d i n g , w h i c h is d e m o n s t r a b l y t e c t o n i c a n d b e s t d e v e l o p e d in t h e d e e p e s t s t r u c t u r a l level ( G o w e r , 1 9 7 8 ) . I n t u i t i v e l y o n e m i g h t e x p e c t b o t h i n t r u s i v e and extrusive rocks which, anticipating chemical similarity, would only be distinguishable on textural criteria. As no primary textures remain the problem c a n n o t be r e s o l v e d . S o m e c r o s s - c u t t i n g r e l a t i o n s h i p s i n v o l v i n g b i o t i t e t o n a l i t e gneiss a g a i n s t a f a b r i c in t h e a m p h i b o l i t e s a r e seen. T h i s f u r t h e r p o i n t s t o an igneous intrusive origin b u t the possibility of tectonic t r u n c a t i o n due to transposition cannot be excluded completely. T h e c o m p o s i t i o n o f A r c h e a n t o n a l i t i c r o c k s d e s c r i b e d in t h e l i t e r a t u r e h a s led to the following statements: (1) high A12Os, l a c k o f a n e g a t i v e E u a n o m a l y , h i g h CaO, S r / B a , S r / C a , S r / K a n d l o w R b / S r suggest t h a t p l a g i o c l a s e was n o t a r e s i d u a l o r p r e c i p i tating phase;

260 (2} low Y and low HREEs suggest that garnet is a residual phase; (3) Low LIL and a low initial Sr ratio suggest an origin from the mantle or from undifferentiated crustal rocks with a short crustal residence time. Features similar to those noted above led Arth and Hanson, on the basis of quantitative REE modelling, to conclude that quartz diorite (IUGS tonalite} could be generated by partial melting of quartz eclogite or amphibolite at depths > 45 km. Glikson {1976) and Condie and Hunter {1976) have also suggested quartz eclogite. However, Barker and Arth {1976) propose amphibolite as an alternative, and Green (1975) considers that Archean geothermal gradients were too high for eclogite stability. Further discussion of these models is given by Arth (1979), Barker (1979} and Condie {1981). In the REE modelling presented in this paper, a similar source rock composition to that selected by the authors above is assumed {but for independent reasons). However, both quartz eclogite and amphibolite are rejected as acceptable rock types from which to generate the tonalitic rocks of the Kenora area. Instead, a mafic garnet granulite is preferred. The trace element modelling presented here also makes some attempt to impose additional constraints, utilizing major element chemistry. Because there are departures in the modelling approach and the conclusions drawn, details of the modelling procedure are given below. Modelling parameters are summarized in Table V. The assumed source rock is fine-grained amphibolite; the only abundant earlier rock type in the area. The mineral assemblage in the source rock will vary depending on whether melting occurs in the upper amphibolite, granulite or eclogite facies. Stability fields for the various phases have been summarized by Carmichael et al. (1974), Ringwood (1975), Wyllie et al. (1976) and Green {1977). Utilizing these data, a selection of five minerals was choosen for a given P--T regime but mineral proportions in the source rock were selected by computer, which calculated in 5% intervals (0--100%) all the possible combinations of a fivephase assemblage that, when melted according to specified criteria, yielded a model REE pattern that matched the observed pattern seen in the tonalites within a pre-determined narrow range of chondrite-normalized abundances. Fyfe (1970) has suggested that the only water present in the rock after the peak of metamorphism is contained in the hydrous phases. We have, therefore, assumed that vapour is absent. The only hydrous phase modelled is hornblende, as the lack of phase equilibria and/or distribution coefficient data for other potential hydrous phases, such as serpentine minerals or phlogopite, prevents their effects from being evaluated. The tonalitic melt must be a primary magma by the terms of the theoretical modelling equations. Thus, given a source rock mineralogy and knowing the composition of the minerals present (discussed shortly), it is possible to calculate the proportions of minerals it is necessary to melt to yield a liquid of tonalitic bulk composition; 10% melting has been assumed, an arbitrary but, we submit, reasonable value. Less than 5% melting has difficulty in extracting the melt from the source and more than ca. 20% melting has difficulty

261 TABLE V Modelling aarameters used in modelling tonalite genesis Source rock

One having the composition of Kenora area finegrained amphibolite (comparable to Archean tholeiite elsewhere)

P'--T conditions

To be inferred from model source-rock mineralogy

Partition coefficients for dacitic magmas

From Arth (1976) with additional information from Condie and Harrison (1976) and Condie and Hunter (1976)

Major element composition of minerals

See Table VI

Relative proportions of minerals melted

Obtained by recalculating the composition of the daughter rock to a mineral assemblage stable under given P--T conditions

Fraction of melting

10%

Nature of melting

Equilibrium, non-modal

Equation

CL/C o

= I / ( D 0 + F (l--P))

CL = trace element concentration of the liquid; C O= initial trace element concentration of the solid; D O= X a K alz + X b K b / l + X c K c/1 + . . . (bulk distribution coefficient); X a ffi fraction of phase a in the source rock; K az ffi partition coefficient for trace element in question (crystal/liquid); P = P o K a/z + P b K b / l + P c K c / l + • • • ; P a ffi fraction of liquid contributed by phase a; F = fraction of melting.

in generating a tonalitic melt from a tholeiite, particularly with respect to SiO2 concentrations. Any combination of minerals in the source rock suggested for trace element modelling must be compatible with the assumed major element composition of the source rock. This was checked by mass balance calculations using assumed mineral compositions. Major element mineral compositions were taken from the literature for high-pressure phases in rocks of roughly similar bulk composition to tholeiite, except for hornblende and plagioclase, which were analysed from two Kenora fine-grained amphibolites (Table VI). Though the composition of the hornblende and plagioclase will differ at higher P and T, they were preferred here to literary data for unrelated rocks. This aspect of the modelling was an acknowledged weakness, as only one composition for each phase (two for garnet and clinopyroxene) was/were used in the calculations. In reality many of the minerals are solid solutions and considerable departures from the assumed compositions may occur. FeO t and MgO were grouped in an attempt to accommodate one aspect of mineral chemistry flexibility. A computer-selected acceptable model source rock bulk composition was taken as one that fell within + 4% of the major element composition of average fine-grained amphibolite for the following components: SiO2, FeOt+MgO, A1203, CaO, Na20.

262 TABLE VI Major element compositions of minerals used in calculating bulk compositions of model source rock mineralogies and model melts

SiO=

Qtz.

Kyn.

Plag.

Hbl

Opx

Cpx I

Gnt,

Cpx2

Gnt2

100.0

40.6

37.5

54.3

44.6

50.1

48.6

40.8

54.1

TiO 2

--

--

--

1.0

0.6

0.9

0.3

0.3

0.2

Al=O 3

--

62.0

28.7

9.1

1.2

11.1

20.6

11.0

22.4

0.5 ---

0.4 0:2 10.6

18.8 10.2 11.1

30.0 15.8 1.4

6.3 12.0 19.4

14.3 14.9 6.4

3.2 11.3 15.7

12.3 13.4 10.0

5.7

1.3

FeO t

--

MgO

--

CaO

--

Na, O

--

K20

.

-.

.

.

--

.

2.0

0.i

4.4

0.1

0.i

0.I

--

0.1

Sources: (qtz.) quartz assumed; (Kyn.) kyanite: Deer et al. (1971); (plag.) plagioclase; Hbl: average of two analyses for each mineral from Kenora area fine-grained amphibolites; Orthopyroxene (Opx): granulite facies orthopyroxene (Deer et al.,1971); clinopyroxene (Cpx,), garnet (Gnt~): average of 12 clinopyroxenes, nine garnets from granulite facies basaltic rocks (Lovering and White, 1969); clinopyroxene (Cpx=), garnet (Gnt 2): average of 24 clinopyroxenes, 19 garnets from eclogite (Reid et al.,1976).

Equilibrium, non-modal melting processes are preferred to fractional crystallization models because field and structural data do not indicate the compositional continuum that might be expected if fractional crystallization were the case. Modal melting models can also be rejected, as such models ignore the whole fabric of experimental petrology which clearly demonstrates that modal melting applies to special situations. Equilibrium melting is preferred • BIOTITE TONALITE GNE~

I IO0

IOC

oc

~

(b)

MODEL REE PATTERNS

,~

......

n~

LaCe . Nd . . .Srn. Eu. . Tb "

'

" ' Yb Lu

Lo Ce

Nd

Sm Eu

Tb

Yb Lu

Fig. 4. (a) REE patterns for biotite tonalite gneiss and Meliek tonalite. (b) Model REE patterns. All model liquids are derived by 10% melting of source rock under equilibrium non-modal conditions. Models 1 and 2 from anhydrous two-pyroxene garnet granulites. Models 3 and 4 from hornblende-bearing orthopyroxene garnet granulite. Model 5 from hornblende-bearing clinopyroxene garnet granulite. Details of modelling are given in Table VII.

263 t o f r a c t i o n a l m e l t i n g ( t h o u g h , strictly, o n e s h o u l d c h o o s e neither) because, as A r t h ( 1 9 7 6 ) has p o i n t e d o u t , c o n t i n u o u s r e m o v a l o f small a m o u n t s o f liquid ( p e r f e c t f r a c t i o n a l melting) is p h y s i c a l l y unlikely. T h e small c o m p o sition range in t h e K e n o r a area tonalites f u r t h e r leads us t o p r e f e r e q u i l i b r i u m f u s i o n models. H o w e v e r , it is w o r t h n o t i n g t h a t , f o r t h e small p e r c e n t a g e s o f m e l t i n g a s s u m e d here, results d o n o t d i f f e r greatly w h i c h e v e r m e l t i n g s c h e m e is a d o p t e d . T h e R E E p a t t e r n s f o r t w o b i o t i t e t o n a l i t e gneisses a n d t w o Melick t o n a lites are s h o w n in Fig. 4a. T h e Melick tonalite, t h o u g h later, has a very simi lar bulk c o m p o s i t i o n (Table IV) a n d it is possible t h a t b o t h units were deTABLE VII Mafic garnet granulite models for generating tonalite 1

2

3

4

5

Mineralogy of model source rock Qtz Cpx Gnt ~ag Opx Hbl

5 30 5 35 25 --

5 40 15 25 15 --

5 5 45 40 5

5 15 40 30 10

5 55 15 20 -5

52.3 15.8 21.24 6.7 2.4

51.0 15.4 16.0 14.3 2.3

35 0 55 3 0

35 0 55 3 7

33 2 58 0 7

68.6

Bulk composition of model source rock SiO 2

53.2

51.7

53.7

AI~O3

14.7

14.9

14.9

FeO t + MgO CaO Na20

18.7 10.2 2.4

18.7 11.6 2.2

21.4 6.2 2.6

Fraction of liquid contributed by each phase Qtz Cpx Plag Opx Hbl

33 2 58 7 0

33 2 58 7 0

Bulk composition of model liquid SiO2

A1203 FeO t + MgO CaO Na20

69.0 16.9

69.0 16.9

69.5 16.8

69.5 16.8

3.9 6.6 3.3

3.9 6.6 3.3

3.7 7.0 4.0

3.7 7.0 4.0

17.4 2.7 7.3 3.4

1, 2 = anhydrous two-pyroxene garnet granulite models; 3,4 = hornblende orthopyroxene garnet granulite models; 5 ffi hornblende clinopyroxene garnet granulite model. Model REE patterns are illustrated in Fig. 4b. Bulk composition of the model liquid may be compared with biotite tonalite gneiss reported in Table IV and bulk composition of model source rock with fine grained amphibolite reported in Table II.

264

rived from a common source. However, the Melick tonalite has slightly lower HREEs. Models are displayed in Fig. 4b with numerical data given in Table VII, and embrace the best-fitting patterns of about 125 000 computercalculated models. It can be seen that the models all have broadly similar patterns and suffer from the same deficiencies, namely low La and Tb and high Sm and Lu. The model La and Lu are contrary to the slope of the rest of the model pattern and do not appear to be particularly meaningful. Despite these deficiencies the following features are evident. (1) Even in those rocks showing the least HREE depletion some garnet (viz. 5%) is required in model source rocks (models 1 and 3) because without it HREE depletion in the melt (with respect to the source rock) can only be achieved by leaving large proportions of clinopyroxene in the residue. However a clinopyroxene-rich source rock precludes a residue with high plagioclase proportions (which are necessary to suppress Eu-enriched melts) and also precludes high orthopyroxene (which helps to generate LREEenriched melts). (2) More garnet (i.e. 15%) exerts an overwhelming influence on the HREE content of the melt (models 2, 4 and 5). To prevent overdepletion, it is necessary to retain plagioclase and orthopyroxene or large quantities of clinopyrox. ene in the residue. (3) Very large quantities of garnet (>20%) are not permitted in the residue because other minerals cannot compensate for the severe HREE depletion that garnet alone causes. (4) In the case of hydrous model source rocks (models 3, 4 and 5) it is apparent that large quantities of hornblende (> 10%) in the residue are unlikely. Hornblende is far less efficient than garnet as a means of generating HREE depletion (using partition coefficient data for hornblende BP-1; Arth and Barker, 1976). Thus to generate adequate HREE depletion large proportions of hornblende must remain in the residue. However, as D-values for LREEs on hornblende BP-1 are > 1.0 (with the exception of Ce) this means that the LREEs are also depleted in the melt, an unwelcome side-effect. The high proportion of hornblende in the residue also excludes adequate amounts of other minerals to counteract these effects. (5) Plagioclase is necessary in all model source rocks to suppress Eu enrichment in the melt. (6) Orthopyroxene, or large proportions of clinopyroxene, are necessary in the model source rock to assist in generating LREE enrichment. From a bulk composition standpoint (Table VII), the acceptability of any model source rock depends largely on the clinopyroxene/orthopyroxene ratio. Without clinopyroxene the bulk composition of the model source rock is low in CaO and high in MgO+FeO and slightly high in SiO2. Without orthopyroxene the model source rocks are high in CaO and low in MgO+FeO. The most compatible model source rock contains both. Table VII also gives the fraction of liquid contributed by each phase and the model liquid composition that results. The model liquids are all high in

265

CaO and A1203 compared to the Kenora tonalites but are reasonable approximations. It is important to appreciate that, with the low percentage of melting considered here, manipulation of the fraction of liquid contributed by each phase makes very little difference to the model REE pattern. Computer modelling also included parameters for Rb, Sr and Y. Average values for these, combining the biotite tonalite gneiss and Melick tonalite, are Rb 49 ppm, Sr 443 ppm and Y 8 ppm. Model melt values are in the ranges of Rb 36--39 ppm, Sr 120--170 ppm and Y 5--10 ppm. The model Sr enrichment is grossly inadequate but it should be noted that Sr in the Kenora area fine-grained amphibolites is low (111 ppm) compared to many other Archean tholeiites, especially mafic rocks in the adjacent Wabigoon Subprovince (217 ppm; Baragar and Goodwin, 1969). We suggest that Sr was depleted during metamorphism prior to melting in the fine-grained amphibolites (and perhaps Rb slightly also). All the model source rocks are some type of garnet granulite. Eclogitic source rocks can be excluded for the Kenora tonalites because no more than 20% garnet is permitted. The remainder of any model eclogite must be made of quartz, kyanite and clinopyroxene leading to model source rocks that are too rich in SiO2, A1203 and CaO. None of the resultant model source rocks mineralogies compare favourably with natural eclogites (58% garnet, 42% clinopyroxene; Reid et al., 1976). It is stressed, however, that eclogitic source rocks are not excluded for tonalites showing both lower LREEs and HREEs (e.g. Saganaga tonalite). Amphibolites are also excluded because sufficient HREE depletion in the melt can only be achieved by large proportions of residual hornblende if garnet is absent. However, this suppresses LREE enrichment. A garnetiferous amphibolite source rock allows the proportion of hornblende to be reduced, but the amount of garnet cannot exceed 10--15% without causing overdepletion of HREEs. If the proportion of residual plagioclase to hornblende is increased, HREE depletion is less severe but the amount of residual plagio° clase permitted from major element considerations is inadequate for this to be effective. Figure 5, adapted from Ringwood (1975), shows the variation of mineralogy with depth of a rock of quartz tholeiite composition. The changing mineralogy with pressure reflects the reaction plagioclase + orthopyroxene = clinopyroxene +garnet+quartz This reaction is divariant over several kilobars, throughout which interval the five-phase assemblage is stable. Garnet and clinopyroxene forming at lower pressures are Ca and Fe-rich, but as pressures increase the garnet becomes more Mg-rich as orthopyroxene is consumed. Clinopyroxene becomes Na and A1 rich as plagioclase is consumed (Green and Ringwood, 1972; Ringwood, 1975; Hansen, 1981). The left-hand side of the shaded area indicates the proportions of minerals in Model 1 source rock and the right-hand side is for Model 2 source rock. From Ringwood's scale a pressure of 15--17.5 kbar is indicated (roughly equivalent to 45--50 kin).

266 "o*"'

DIOI~ID[

~/~/~//~~/~//////~~

_ D I O I ~ I D E '~ JAOEITE

ib i~ ~o ~ Kb Fig. 5. Changing mineralogy with depth of a rock with quartz tholeiite composition, adapted from Ringwood (1975). The left-hand side of the shaded area indicates the proportions of minerals in Model 1 source rock and the right-hand side for those in Model 2 source rock.

As only 10% melting is envisaged and the assemblage is almost anhydrous, the intersection of this pressure range with the dry quartz tholeiite solidus can be used as a point on the geothermal gradient. P - T conditions determined during high-grade metamorphism (650-750°C, 4--7 kbar) can be used as a second point and thus indicate the geothermal gradient during generation of the tonalites. The resultant gradient (25-30°C/km) is slightly higher than the range of gradients believed to be c o m m o n at the present time.

Granitic pegmatoid gneiss The major element composition of the granitic pegmatoid gneiss compares closely with Archean pegmatites elsewhere {Table VIII). The 'meta-arkose' (Barooah, 1970), actually a pegmatite, and the gneissose pegmatites of Sheraton et al. {1973) have both suffered granulite facies metamorphism whereas the Drumbeg pegmatite escaped that metamorphism and the post-tectonic pegmatites in the Kenora area are unmetamorphosed. It may be possible to correlate alkali contents with grade of metamorphism, noting that pagmatites metamorphosed to granulite facies have lower K20 and Rb contents than unmetamorphosed examples. The granitic pegmatoid gneiss, which has been metamorphosed -- but to amphibolite rather than granulite facies -- shows intermediate values. Many of the metamorphic reactions inferred for the Kenora area (Gower, 1978) require K to be introduced and this is one possible source. A further point worth noting is that the granitic pegmatoid gneisses show high K/Rb (360--624) in contrast to normal crustal values seen in the Kenora tonalites {234). Biotite fractionation from a tonalite magma could produce the high K/Rb seen but, in view of the high K/Rb in analysed Kenora tonalite biotite (149--178) in contrast to 'normal' values around 80 (Shaw, 1968) and the probable co-precipitation of much larger proportions of plagioclase (with higher K/Rb than the tonalite), this is not a likely mechanism.

267 TABLE VIII Granitic p e g m a t o i d gneiss and s o m e comparisons

SiO 2 TiO~ AI:O 3 F%O3 FeO MnO MgO CaO Na20 K~O P2Os Rb Sr Ba Zr Th Pb n

1

2

3

4

74.41 0.09 14.54 0.33 0.36 0.04 0.15 1.71 4.16 4.20 0.01

76.04 0,07 12,69 0.82 -0.05 0.23 0.26 1,68 8.15 0.01

76.76 0.00 14.20 0.19 0.33 0.02 0.17 1.38 3.51 3.42 0.01

5 72.10 0.29 15.20 1.46 0.76 0.03 1.00 1.78 4.70 2.68 0.09

74.00 0.03 14.60 0.28 0.05 0.01 -0.53 3.30 7.13 0.01

74 326 1390 33 nd 26

205 223 -74 ---

-------

32 505 1618 188 2 17

245 141 870 10 5 38

11

2

2

18

1

n = no. o f analyses. (1) Granitic p e g m a t o i d gneiss, K e n o r a area; (2) Post-tectonic pegm a t i t e dikes, K e n o r a area ( Festeryga, 1977 ); (3) 'Meta-arkose ', southeast o f Scourie (Barooah, 1970); (4) Microcline gneiss (gneissose pegmatites), Assynt ( S h e r a t o n e t al., 1973); (5) Scourie p e g m a t i t e ( S h e r a t o n et al., 1973).

An alternative explanation, suggested by Condie and Lo (1971) for the anomalously high K/Rb in aplites from the Louis Lake Batholith, involves loss of late-stage magmatic fluids highly enriched in Rb. Low Ba, Cs and total REEs in some aplites is attributed to the same mechanism. Low Ba and total REEs are also characteristics of the granitic pegmatoid gneisses. The Ab--An--O1--Qz--H20 system at 5.0 kbar is used to discuss the crystallization and/or fusion history of the granitic pegmatoid gneiss. The system is considered adequate as the average content of mesonormative Ab+An+Or+Qz for the Kenora tonalitic rocks is 89% and for the granitic pegmatoid gneiss it is 93%. The saturated system is used because there are indications that biotite is a refractory phase (in skialiths and as mafic borders to leucogranitoid veins), which argues against severe undersaturation (Maaloe and Wyllie, 1975). Further, following Jahns and Burnham (1969) the appearance of coarse pegmatire is equated with the development of an aqueous fluid phase. Five kbar is chosen because the granitic pegmatoid gneiss intrudes strongly metamorphosed rocks but has itself been metamorphosed. The assumption is made that crystallization or fusion took place during high-grade metamorphism (4--7 kbar).

268

The rocks are plotted in A b - - O r - Q z projection in Fig. 6. The tonalitic rocks plot in t h e quartz--plagioclase solidus volume or at the edge of the quartz--plagioclase--K-feldspar solidus volume. They are in a band roughly normal to the quartz--plagioclase cotectic surface. The granitic pegmatoid gneisses plot on the quartz--plagioclase liquidus cotectic surface. Very few samples plot outside the plagioclase liquidus volume; thus plagioclase is assumed to be the liquidus phase. • • •

GRANITIC PEGMATOID GNEISS BIOTITE TONALITE GNEISS MELICK TONALITE

MF'SONORMATWE

FRACTIONALLY CRYSTALLIZED MAGMA DEPLETED IN PLAGIOCLASE PRIMARY TONALITIC MAGMA ~ FRACTIONALLY ~;',~;,;' CRYSTALLIZED......-..~.. :-"" MAGMA / ENRICHED IN PLAGIOCLASE/

AB ,

"

.L~....... • ' "" *" ,"4- "" *', "".

• RESIDUAL LIOUID

,

OR

Fig. 6. Mesonomative Ab--Or--Qz plot of biotite tonalite gneiss, Melick tonalite, and granitic pegmatoid gneiss. Cotectic line is a projection onto the surface for 5 kbar.

Detailed discussion of various fractionation schemes based on the work of Presnall and Bateman {1973) is given by Gower (1978). The model that best fits the data is as follows. Some of the primary tonalite magma crystallizes without fractionation and falls within the indicated area on Fig. 6. Some of the magma that did fractionally crystallize is enriched in early-precipitating plagioclase and forms a 'tail' towards the Ab--An face. The fractionally crystallized magma depleted in plagioclase forms a trend towards, and partly across, the quartz--plagioclase cotectic surface. The granitic pegmatoid gneisses are liquids which crystallized on the cotectic line. The tail associated with the granitic pegmatoid gneiss can be interpreted as either due to the residual crystals which were incorporated from the previous crystallization segment or to the metamorphic K-depletion suggestion made earlier. The REE patterns of the two granitic pegmatoid gneisses examined in detail are shown in Fig. 7. The REE depletion in these patterns is extreme and, to our knowledge, has not been reported elsewhere. Condie and Lo

269

GRANITIC PEGMATOID GNEISS

'10 C 0 ..C

1

i

Lo Ce

i

Nd

I

I

I

Sm Eu

I

t

I

I

I

Tb

i

I

Yb Lu

Fig. 7. REE plot for granitic pegmatoid gneiss.

(1971) and Chou et al. (1976) report aplite and pegmatite respectively which show REE depletion relative to the inferred parent rock, but n o t to the extent shown by the granitic pegmatoid gneisses. Three possible interpretations of the patterns are: (1) REE depletion during metamorphism; (2) REE removal with escaping magraatic fluids; and (3) fractional crystallization of LREE-bearing phases, especially accessory phases, causing preferential depletion of LREE. If the first mechanism has been operative one wonders why the granitic pegmatoid gneisses have been affected and not the other rocks in the area. Evidence for or against the second mechanism is lacking but the work of Taylor et al. (1980) on the effects of complexing on REEs may offer a viable explanation. With reference to the third alternative, the granitic pegmatoid gneisses show low Zr, P2Os and TiO2 compared to the tonalitic rocks, suggesting zircon, apatite and sphene (or possibly ilmenite or futile) fractionation. Allanite, c o m m o n in the tonalitic rocks, is rare in the granitic pegmatoid gneisses indicating that it too may be a fractionating phase. The differences in P2Os, Zr and TiO2 concentrations between the tonalitic gneisses and the granitic pegmatoid gneiss are inadequate to allow sufficient amounts of zircon, apatite or sphene to cause effective fractionation. Unlike the other accessory minerals, no single c o m p o n e n t resides almost entirely within aUanite so it is more difficult to impose constraints on the maximum allowable amount that can be precipitated. Gower (1978) calculated model depleted REE patterns that approximated those observed, assuming precipitation of small quantities of allanite with concentrations for most REEs between those reported for allanite by Lee and Bastron (1967) and Lyakhovich (1962).

270 CONCLUSIONS

The earliest rocks in the Kenora area were mafic volcanic rocks (some pillow-form), cogenetic sills and minor ultramafic, intermediate and sedimentary rocks. The mafic rocks are chemically classified as tholeiite and, in terms of modern-day tectonic setting, are similar to basalts found in ocean floor or island arc environments. Major and trace elements, including REEs, are compatible with an origin by shallow-level fractionation of magma derived from a peridotite source. Rare associated ultramafites are compositionally similar to komatiites and a few intermediate rocks could be andesite. Some inhomogeneous basaltic/intermediate rocks are interpreted as pillow breccias or pyroclasts. Metasediments, though rare, provide proof of the supracrustal origin of the associated fine-grained amphibolites. Metachert and volcanoclastite/graywacke are the most common rock types. The supracrustal rocks are typical of those found in the lower parts of Archean greenstone belt sequences, differing only in their high grade of metamorphism, deformation and fragmentary preservation. The biotite tonalite gneiss, for which a sedimentary origin can be rejected, is the next major addition to the newly formed crust in the Kenora area (see Fig. 8). The tonalitic gneisses are interpreted as plutonic rocks derived by partial melting of rocks compositionally equivalent to the Kenora area finegrained amphibolites. From quantitative REE and major element modelling, a two-pyroxene garnet granulite is found to be the most satisfactory source rock. When 10% melted under equilibrium, non-modal conditions, this source rock will yield model melts with REE patterns similar to those seen in the tonalites. Quartz eclogite, granulite without garnet (or with more than 10% hornblende) and amphibolite are not acceptable source rocks under the modelAIl~ite rk pre©i~totion ? 2(3O

150 Z REE ppm I00

FRACTIONAL CRYSTALLIZATION

PARTIAL MELTING (after ©anverlion to mafic 9ornef gromdi

TONALITE

50

I ~iSOURCE THOLEIITE "l~lv (from mantle) 40

5b

&~

i sioz

Tb

~ GRANITIC ~"~'~ (~IEIssPE~ATOID s

wt,/.

so

Fig. 8. S u m m a r y o f t h e p e t r o g e n e t i c s c h e m e p r o p o s e d f o r t h e K e n o r a a r e a g n e i s s i c r o c k s . R E E = L a + Ce + N d + S m + E u + T b + Y b + L u .

271

ling conditions used. The optimum source rock mineralogy is similar to that expected in a rock of quartz tholeiite composition melting between 45 and 50 kin. The granitic pegmatoid gneisses are interpreted from field evidence as deformed pegmatites injected discordantly and concordantly into tonalitic gneisses and amphibolites. From relationships in the An--Ab--Or--Qz--H20 system for 5 kbar the granitic pegmatoid gneisses are regarded as the product of fractional crystallization of a tonalitic magma. The granitic pegmatoid gneisses have very low REE concentrations. They may be the result of loss of REEs in residual fluids or the end result after precipitation of REE-rich phases, such as allanite.

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