Geochemistry and petrogenesis of Archean mafic volcanic rocks of the southern Abitibi Belt, Québec

Precambrian Research, 57 (1992) 207-241 Elsevier Science Publishers B.V., Amsterdam

207

Geochemistry and petrogenesis of Archean mafic volcanic rocks of the southern Abitibi Belt, Qurbec M.R. Lafl~che a, C. Dupuy a and H. Bougaultb aC.N.R.S., Centre G~ologique et G~ophysique, Universit~ de Montpellier II, Place Eugkne Bataillon, 34095 Montpellier Cedex 5, France blFREMER, Centre de Brest, B.P. 70, 29263 Plouzank, France (Received July 18, 1991; accepted after revision December 18, 1991 )

ABSTRACT Lafl~che, M.R., Dupuy, C. and Bougault, H., 1992. Geochemistry and petrogenesis of Archean mafic volcanic rocks of the southern Abitibi Belt, Qurbec. Precambrian Res., 57:207-241. The Blake River Group of the late Archean Abitibi Greenstone Belt is characterized by the abundance of differentiated lava series as well as by the presence of coeval tholeiitic and calc-alkaline eruptives. Differentiated tholeiites from this group are interpreted as the product of magma mixing between asthenospheric tholeiites and subduction-related calcalkaline basaltic andesites. From the lower to the upper southern Abitibi Belt, chemical variations recorded in tholeiites are indicative of a major change in the geodynamic regime. Tholeiites from the lower volcanic sequence were produced by a high degree of partial melting of adiabatically rising diapirs below a rifted lithosphere. Uncontaminated tholeiites from the upper part of the belt reflect a lower degree of partial melting probably due to decreasing lithospheric stretching. This may be related to the closure of the rift-related volcanic basins at the begining of the Kenorean deformational events in the Southern Abitibi Belt.

Introduction

The late Archean Abitibi Belt (2.7 Ga) is the world's largest Archean greenstone belt covering an area of more than 85,000 km 2. In Qurbec, the southern Abitibi Belt (SAB) is dominated by a thick succession of metavolcanic rocks. The presence of large synclinoriums and anticlinoriums as well as the good preservation of supracrustal rocks and of primary textures, permit a detailed study of the volcanic stratigraphy and of the chemical evolution of the SAB through more than 40 Ma of volcanic activity (e.g. Corfu et al., 1989). Mafic volcanic rocks of the SAB in Qurbec Correspondence to: Dr. M.R. Laflrche, Centre Groscientifique de Qurbec, 2700 rue Einstein, C.P. 7500, Ste-Foy, Qurbec, Canada, G1V 4C7.

can be broadly subdivided in a lower tholeiitic sequence (Pichr, Malartic, Kinojrvis Groups) and an upper sequence of mixed tholeiitic and calc-alkaline rocks (Blake River Group ). Here we present new analytical and field data and a comprehensive geochemical study of volcanic rocks from the Blake River Group in the Rouyn-Noranda area (western Qurbec). The Blake River Group which has been previously studied in a pionering work by Baragar ( 1968 ) and further by Grlinas et al. ( 1977 ) is characterized by coeval tholeiitic and calc-alkaline volcanism, a feature rarely encountered in Archean greenstone belts. In a second section, the present study is extended to the lower tholeiitic sequences of the SAB, and a petrogenetic model with geotectonic implications is proposed in order to explain the secular evolution

0301-9268/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

208

of magmatism in this part of the Abitibi Greenstone Belt.

Geological setting The E-W trending Abitibi Belt is about 800 km long and 240 km wide. It is the largest Archean plutono-metavolcanic belt of the Superior Province (Card, 1990) and of the Canadian Shield (Goodwin and Ridler, 1970). The Abitibi Belt is bounded to the north by the Archean Quetico gneisso-plutonic Subprovince, to the west by the Kapuskasing Structural Zone (e.g. Percival, 1986 ), and to the south and east by the Archean Pontiac Subprovince and the Proterozoic Grenville Province. The southern part of the Abitibi Belt (SAB, Fig. 1 ) is composed of low-grade metamorphic volcanic, sedimentary and plutonic rocks (e.g. G61inas et al., 1977) intruded by Early and Late Proterozoic dyke swarms. The SAB is bounded to the south by the Larder Lake-Cadillac Fault Zone separating the greenstone belt from the Archean metasedimentary and plutonic rocks of the Pontiac Subprovince (Goulet, 1978; Dimroth et al., 1983; Fig. 1 ). In northwestern Qu6bec, the volcanic rocks can be essentially subdivided in 5 groups (OGS-MERQ, 1984; Jensen, 1985)which locally overlap. ( 1 ) The bimodal calc-alkaline rocks of the Hunter Mine Group (HMG) (Lafl~che and Ludden, 1991 ) is the oldest volcanic sequence of the SAB (2730 to 2713 Ma, Mortensen, 1987; Corfu et al., 1989). (2) In eastern Ontario and Qu6bec, the ultramafic (komatiitic) and mafic tholeiitic volcanic rocks of the Stroughton-Roquemaure and Malartic Groups overly the calc-alkaline Hunter Mine Group. Rhyolites, in the lower part of the Stroughton-Roquemaure Group are dated at 2714_+2 Ma (Corfu et al., 1989). (3) The Kinoj6vis Group overlies the previous groups and is composed of a monotonous series of tholeiitic basalts and rare komatiitic-basaltic flows. It is intruded, in its

M.R. LAFLECHE ET AL.

lower part, by numerous sub-volcanic gabbroic dykes and sills. (4) The Blake River Group (BRG) (27032698 Ma, Mortensen, 1987) which is investigated in the present study is the youngest volcano-plutonic group and overlies the Kinoj6vis Group in the center of an E-W trending faulted synclinorium (Jensen, 1985). It is located between the Larder Lake-Cadillac Fault Zone to the south and the Porcupine-Destor Fault Zone to the north (Ambrose, 1941 ). The BRG in northwestern Qu6bec is characterized by subaqueous differentiated felsic lavas interlayered with dominantly mafic tholeiitic and subordinate calc-alkaline volcanic rocks (Baragar, 1968; G61inas et al., 1977; G61inas and Ludden, 1984). In eastern Ontario, the Blake River Group (Misema sub-Group) (not investigated in this study) is characterized by the predominance of calc-alkaline mafic and rhyolitic volcanic rocks over tholeiitic rocks (Jensen, 1985 ). Volcanic rocks of the BRG are intruded by a paraconcordant metamorphosed trondhjemite-tonalite suite and by younger discordant granodioritic and ultrapotassic intrusions (e.g. Lafl~che et al., 1991 ). (5) The Timiskaming volcano-sedimentary Group (Cooke and Moorhouse, 1969) unconformably overlies the Kinoj6vis and Blake River Groups. It is dated at 2680+5 Ma (Corfu et al., 1991 ) and contains numerous shoshonitic and trachytic flows and pyroclastic rocks (Ujike, 1985) associated with alluvial-fluvial sediments (Ojakangas, 1985 ). Supracrustal rocks of the SAB were affected by the Kenorean metamorphic and deformational events mainly expressed in the area by N-S shortening (Goulet, 1978; Trudel, 1979).

Petrographic characteristics of the BRG mafic lavas Basaltic and andesitic rocks are dominantly pillowed or massive lavas suggestive of extrusion in a subaqueous environment. Field observations suggest that, in the upper part of the

209

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Fig. 2. Simplified lithological map of the northeastern Blake River Group (after Lafl~che, 1991 ). Informal formation names are 1: South Dufault Rhyolite; 2: North Lake Dufault tholeiitic; 3: South J6vis Rhyolite; 4: Cl6ricy Highway tholeiitic; 5 and 7: J6vis Rhyolitic complex; 6: White Hill calc-alkaline; 8: Big Bear tholeiitic; 9: Thunder Hill Rhyolite; I0: Lake Dufresnoy calc-alkaline; 11 and 13: Mobrun Rhyolite Complex; 12: Copper Hill calc-alkaline; 14: Mobrun tholeiitic; 15: Dufresnoy River sedimentary unit; 16: Variolitic tholeiitic marker unit; 17: Bassignac Creek tholeiitic; 18: Kewagama metasedimentary Group; 19: Malartic Group; 20: Kinoj6vis Group.

volcanic lava pile, rate of volcanic accumulation exceeded subsidence. Some emergent (Lafl~che, 1986) to sub-aerial pyroclastic landforms (Tass6 et al., 1978) were probably formed and later eroded. Bathymetric variations inferred from the volcanic facies changes suggest that cyclic subsidence was related to an extensional event which occurred contemporaneously with volcanism (e.g. Dimroth et al., 1985, Lafl~che, 1991 ). Hydrothermal alteration was widespread in this part of the BRG and sulphide deposition occurred in the vicinity of major syn-volcanic faults related to subsidence,

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Calc-alkaline and tholeiitic mafic lavas of the northeast BRG are distinguishable in the field. The calc-alkaline rocks form some relatively thin units, less than 30 m thick within rhyolitic complexes, although their thickness may reach 300 m (Fig. 2). Morphological features such as thin chilled margins (0.5-1.5 cm), small pillow-sizes ( 30-100 cm long) high vesicularity (15-35%), and increasing brecciation with stratigraphic elevation characterize these units. Imbrications, reverse and normal grading are preserved in flow breccias and locally in phreatomagmatic breccias and hyalotuffs. Tholeiitic flows represent relatively thick and laterally extensive mappable units (Figs. 2 and 3 ). They are essentially pillowed with or without interpillow hyaloclastic accumulations. Morphological features such as thick chilled margins (2.5 to 8 cm), tubes and megapillows, (up to 2.5 m long) and low vesicularity (0 to 5%) are characteristic of these units. Porhyritic to sub-

ARCHEAN MAFIC VOLCANIC ROCKS OF THE SOUTHERN ABITIB1 BELT, QUEBEC

ophitic comagmatic sills and dykes ( 1 to 40 m thick) make up some 10-15% of the tholeiitic rocks. Some variolitic pillowed and massive flows are also present (Lafl~che, 1991 ). Calc-alkaline and tholeiitic flows show some common textural and mineralogical features. Rapid cooling of flows is indicated by the frequent presence of sub-spherulitic textures. Mafic lavas are generally aphyric and display microlitic plagioclase and rare pseudomorphs of clinopyroxene and olivine phenocrysts. The groundmass is an assemblage of albite, quartz, actinolite, epidote, chlorite, sericite, with or without calcite, pumpellyite, sphene and leucoxene.

Geochemistry Methodology A total of 600 samples were analysed for major element and volatile contents (H20-CO 2) (Lafl~he, 1991 ). This wider set of data is used below to discuss the major element mobility and is plotted in Fig. 4. About 55 samples were selected as representative of the least altered rocks and altered samples. They were crushed in an agate mortar in order to avoid metallic contamination, and analysed for major, rare earth (REE) and trace elements. Zr, Y, Nb, Rb, Sr were determined on a SIEMENS SRS 303 (X-ray fluorescence) at the IFREMER Institute of Oceanography (Brest, France) with an operating voltage of 60 kV and 50 mA using a LIF 100 crystal and a Nal detector. Limit of detection was 0.2 ppm for the special routine for Nb determinations with a standard deviation of 0.1 ppm for concentrations between 0 and 5 ppm. Values for international standards are taken from Jochum et al. (1990). Li, Rb, Sr, V, Cr, Co, Ni, Cu, and Zn were determined using an atomic absorption technique at the "Laboratoire de g6ochimie des ~l~ments traces du C.G.G." of the C.N.R.S. at Montpellier. Routine precision for major elements is better than 1% and for most minor components, bet-

211

ter than 5%. Precision for Co, Sc, Ta, Hf, Th, La, Ce, Sm, Eu, and Yb is better than 5%, whereas it is between 5 and 10% for Nd, Tb and Lu (obtained by I.N.A. at Montpellier). Ta has been reanalysed under a special I.N.A. cadmium irradiation routine (samples irradiated at the Grenoble nuclear reactor). Details of the technique and precision data for individual elements are given by Dostal et al. (1986).

Influence of alteration Before consideration of igneous processes can be attempted, the influence of seafloor alteration and/or low-grade metamorphism must be considered.

Major elements The alteration index of Hashiguchi et al. (1983): A.I.= [MgO + K20/MgO-t- K20 + N a E O + C a O ] X 100 was used for our selection of least altered rocks. Rocks anomalous in normative minerals (i.e. corundum) were considered altered. The Hashiguchi index is reliable for most mafic volcanic rocks. For example, the A.I. obtained from compiled data from fresh MORB and arc related mafic volcanic rocks give respectively 36 _+8 and 34 _+ 10. Chloritization and/or sericitization produce higher A.I. values ( > 50) whereas A.I. values are much lower than 30 in albitized samples. This alteration index, however, cannot be used in strongly olivine cumulative rocks or komatiites where it may reach values of 85 _ 10, close to the average value of 90_+ 2 for mantle lherzolites. In the present study, rocks which display high A.I. and peraluminous index (P.I.) in Fig. 4 are all characterized by high FeO*+TiO2/AI203 ratios (Fig. 5 ), high incompatible trace element contents and low Cr and Ni abundances (Table 1 ) which preclude signifcant olivine accumulation in these rocks.

212

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AJ. Fig. 4. Peraluminous index (P.I.), MgO, SiO2, CaO, FeO', and F e O ' / M n O as a function of alteration index (A.I.) as defined by Hashiguchi et al. (1983). A.I.= [(MgO + KEO)/(Na20 + K20 + CaO + MgO)] × 100. P.I.= A1203 / (Na20 + K20 + CaO) (molecular). Mafic volcanic rocks from the northeast BRG; dots: tholeiites; dashed line: calcalkaline field. On this diagram, typical fresh MORB plot near A.I. = 35 and P.I. = 0.80. Note the strong MgO enrichment and CaO depletion of highly altered tholeiites.

Low-grade regional metamorphism and/or spilitization may have produced the observed mineralogical assemblages. Samples classified as least altered were all microlitic, with good preservation of primary igneous fabric and low amygdule content, and consequently the present mineralogy is believed to be a product of low-grade metamorphism. Chemical alteration is more pronounced in tholeiitic than calc-alkaline rocks, but also decreases in a broad manner in the tholeiites toward the top of the volcanic sucession. This may be in relation to the presence of a large subvolcanic differentiated sill (Cl6ricy sill; Fig. 2 ) at the base of the volcanic sequence which may have produced sufficient heat to induce

convection of hydrothermal fluids in the volcanic pile. The most altered tholeiites display an increase in MgO, S, K20 and to a lesser degree of SiO2 and a decrease in Na20, CaO (Fig. 4, Table 1 ). On the other hand, FeO, A1203, TiO2 and P205 were not significantly affected. In moderately altered samples, the breakdown of plagioclase is reflected by a decrease in CaO and Na20, with subsequent crystallization of epidote and actinolite. Further alteration is marked by an enrichment in K20 and sericite crystallization. In the most altered samples, MgO and H20 contents increase reflecting complete replacement of primary ferromagnesian minerals and secondary actinolite by

ARCHEAN MAFIC VOLCANICROCKS OF THE SOUTHERN ABITIBIBELT, QUEBEC

FeO* +Ti02

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213

vine and Baragar, 1971 ). This suggests that, for least altered samples, major element mobility was relatively weak.

FeO *

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Fig. 5. Compositions of least altered mafic rocks projected onto (FeO" + TiO2)-A1203-MgO diagram (after Jensen, 1976) and FeO'-(Na20 + K20)-MgO diagram (after Irvine and Baragar, 1971) discriminatingbetween volcanic rocks of tholeiitic (Th) and calc-alkaline (Ca) affinity. chlorite. This is characteristic of high-temperature hydrothermal alteration observed in the vicinity of volcanogenic sulphide deposits (e.g. Riverin and Hodgson, 1980). The calc-alkaline rocks display low H20 ( < 3%) and CO2 ( < 2 . 5 % ) contents (Table 1). In areas near shear zones, CO2 contents increase. This is accompanied by a small increase in MgO and K20 associated with a decrease in Na20 and CaO (Fig. 4, Table 1 ). Altered BRG volcanic rocks display unusual fractionation of the FeO*/MnO ratio (25 to 200 in Fig. 4). It is interpreted as a consequence of the different chemical behavior of these elements during fluid/rock interactions with strongly reducing sea waters probably at the vicinity of active hydrothermal vents. Anoxic hydrothermal solutions increased the Mn 2+ solubility in circulating seawater as compared to Mn 4+. This lead to reduced Mn contents of the altered rocks. The least altered rocks are plotted in Fig. 5. Despite the varying, weak alteration, igneous variation trends permit recognition of both tholeiitic and calc-alkaline units of the northeast BRG. Jensen's (1976) cationic plot subdivides the mafic volcanic rocks into tholeiitic and calc-alkaline units. Both rock series are distinguished on the AFM diagram (Fig. 5; Ir-

Trace elements Trace elements are potentially mobile during low-grade metamorphism (Hellman and Green, 1979) or spilitization (e.g. Staudigel et al., 1981; Lesher et al., 1986). Several geochemical studies (e.g. Condie et al., 1977 ) have demonstrated the mobility of large ion lithophile elements (LILE) and the relative stability of high field strength elements (HFSE) and heavy rare earth elements (HREE). On the other hand, Ludden et al. (1982) and Nystr6m (1984) demonstrated the potential mobility of elements usually regarded as immobile such as the light rare earth elements (LREE) during extreme epidotization and carbonatization. To evaluate the chemical influence of alteration we have chosen, in the tholeiitic units, eight altered samples in addition to least altered rocks. Five altered samples are characterized by the predominance of chlorite and/or sericite (e.g. No. 8733 in Table 1 ) whereas the other three are carbonatized (e.g. Nos. 8729 and 8707). The carbonatized samples contain vesicules filled with calcite and are enriched in CaO, Sr and Eu (Eu* > 0.97 ) (Table 1 ). The presence of bivalent Eu in relatively reducing hydrothermal solutions is inferred from the Eu enrichment (Fig. 9). Carbonatized rocks display weak major and trace element mobility in comparison with the chloritized and sericitized samples, and they do not show preferential incorporation of LREE over MREE and HFSE, being comparable to the least altered tholeiites in this respect. To evaluate major and trace element mobility in the most chloritized and sericitized sampies, composition-volume calculations were performed following the method of Gresens (1967) and Grant (1986) and are sumarized in Fig. 6. Using measured specific gravities and chemical analyses of altered and least altered rocks (assumed protolith ), chemical gains and

214

M.R. LAFLECHE ET AL.

TABLE 1

Representative whole rock major, trace, and rare earth analyses of mafic volcanic rocks and associated syn-volcanic Clericy Sill of northeast BRG Group:

I

No:

8704

SiO2 TiO2 A1203 Fe203* MnO MgO CaO Na20 KzO P205 H20 CO2

Li Rb Sr Sc V Cr Co Ni La Ce Nd Sm Eu Tb Yb Lu Zr Y Hf Th Nb Ta

II 8711

8705

58.6 1.61 12.83 12.81 0.20 4.87 6.52 2.51 0.16 0.16 2.1 2.21

49.9 1.18 15.28 10.92 0.24 7.77 10.1 3.16 1.12 0.1 1.18 1.4

50.80 1.23 15.80 14.18 0.21 7.95 8.14 1.28 0.04 0.08 2.24 2.77

10 3 149 37 350 60 30 46 10.3 28.8 22.7 7.6 1.94 1.68 7.03 1.10 182 66 5.0 0.8 9.4 0.58

24 45 649 45 352 265 44 77 5.4 13.4 18.0 4.1 1.27 1.07 3.84 0.63 72 37 2.0 n.d. 4.7 0.23

21 1 146 41 358 247 49 109 2.8 8.0 6.3 2.2 0.82 0.60 2.25 0.38 57 22 1.5 0.2 2.5 0.15

8712

8729

56.6 56.4 0.89 0.99 1 7 . 4 1 14.74 7.17 9.34 0.14 0.16 4.2 2.75 12.3 13.2 1.6 1.65 0.25 0.07 0.06 0.16 1.1 0.19 1.29 6.69 14 5 210 39 265 356 47 157 2.8 7.7 5.7 2.1 0.76 0.46 1.57 0.24 50 16 1.3 0.3 2.8 0.15

8699

II1 8702

8696

57.3 55.0 1.82 1.32 12.33 13.80 1 4 . 1 5 13.14 0.2 0.2 4.1 6.05 5.68 7.66 3.73 3.1 0.2 0.22 0.19 0.13 0.02 0.29 1.38 1.47

57.5 1.18 12.80 13.14 0.24 6.11 10.7 1.48 0.11 0.13 1.17 1.06

10 4 6 1 2 6 142 95 187 26 40 42 207 333 335 210 27 36 39 43 43 147 6 52 2.8 16.4 9.5 7.1 44.1 24.7 4.8 28.9 17.9 1.6 7.8 4.9 0.68 2.03 1.15 0.37 1.71 1.10 1.19 6,53 4.16 0.21 1.05 0.68 44 191 120 13 63 40 1 5.2 3.2 0.2 1.9 1.t 2.5 12.6 7.0 0.16 0.81 0.50

6 2 158 38 337 102 35 29 8.2 21.4 14.7 4.4 1.01 1,14 3.68 0.61 111 37 2.5 0.6 6.6 0.42

losses have been calculated. Chloritized samples whith A.I. > 95 show an enrichment in Li and Rb correlated positively with the amount of sericite. On the other hand, CaO, Sr, and to a lesser degree Eu 2 ÷ are depleted in the altered samples. This is ascribed to the complete transformation of Ca-rich phases (plagioclase, epidote and actinolite) to more hydrated phases, chlorite and sericite, which cannot readily accommodate the doubly charged LILE (Dickin and Jones, 1983). The Rb/Sr ratio, a good indicator of secondary alteration, in-

8683 57.5 1.64 12.89 13.16 0.22 3.41 7.87 1.89 0.49 0.17 0.26 0.32

Clericy Sill

IV 8684 55.5 1.7 13.15 16.2 0.26 3.57 6,17 2.37 0.51 0.2 2.12 0.23

8733 63.7 1.78 11.69 15.82 0.15 8.34 0.21 0.03 0.06 0.19 3.9 0.9

8707 59.2 2.06 13.9 9.89 0.18 2.41 8.53 3.54 0.59 0.33 0.6 5.45

8734 54.6 1.8 16.22 16.9 0.17 8.47 0.3 0.21 1.2 0.23 3.83 1.15

8724 51.2 0.43 16.6 9.56 0.17 9.38 11.88 1.18 0.05 0.03 1.69 1.08

8725 55.0 0.24 21.38 6.29 0.1 4.89 10.24 2.33 0.1 0.04 1.47 1.27

4 11 21 20 24 7 7 6 10 1 20 20 1 1 196 126 2 177 4 155 182 37 4l 35 32 36 32 16 394 455 357 268 373 159 136 4 21 4 18 50 432 242 40 52 18.6 41 20 44 28 9 11 6 12 24 93 80 16.4 15.8 15.0 9.8 6.5 1.8 1.7 40.7 37.5 36.4 23.3 16.3 4.4 3.9 25.6 26.7 22.2 16.2 9.7 2.0 2.7 6.9 6.9 6.6 4.7 2.8 1.0 0.8 1.7 1.7 1.05 1.63 1.03 3.57 0.62 1.62 1.62 1.53 0.96 0.58 0.26 0.23 6.20 6.20 5.86 3.89 2.08 1.06 0.78 t.04 1.00 0.95 0.64 0.37 0.19 0.12 197 181 181 107 71 20 16 64 59 54 35 21 11 7 n.d. 4.8 4.9 2.9 1.7 0.5 0,3 n.d. 1.8 1.8 0.7 0.8 0,2 0.2 12,9 11.7 11.7 6.6 3.9 0.6 0.8 0.82 0.81 0.78 0,39 0.21 0.04 0.03

creases from 0.007 in the least altered samples to 8 in highly altered samples. Among the transition elements, chalcophile elements (e.g. Cu and Zn) are enriched during alteration (Fig. 6). This enrichment is positively correlated with the sulphur content of the rocks. Cr and Co are released during the breakdown of the primary igneous assemblage. This behavior may reflect their low partitioning in chlorite. The Ni content is similar in altered and least altered samples, and V, TiO2 and Sc are apparently not affected by alteration.

ARCHEAN MAFIC VOLCANIC ROCKS OF THE SOUTHERN ABITIBI BELT, QUEBEC

C1 8721 SiO2 TiO2 A1203 Fe203* MnO MgO CaO Na20 K20 P205 HzO CO2

Li Rb Sr Sc V Cr Ni La Ce Nd Sm Eu Tb Yb Lu Zr Y Hf Th Nb Ta

215

C2 8720

8689

8719

8708

C3 8693

8685

8717

68.0 0.74 13.18 5.55 0.1 2.00 5.24 4.37 1.20 0.25 0.65 0.92

55.0 1.36 16.1 9.44 0.2 5.60 5.46 4.98 1.10 0.26 1.69 2.09

55.0 1.13 17.44 11.05 0.2 7.10 6.89 1.96 0 0.18 2.37 3.25

55.0 0.82 19.83 8.24 0.1 4.20 8.75 2.85 0.70 0.13 1.99 1.71

56.0 1.03 16.91 9.12 0.3 3.10 6.47 2.6 1.90 0.31 0.43 0.92

60.0 1.03 15.97 8.49 0.1 4.40 6.74 2.39 1.00 0.20 1.74 1.13

53.0 0.97 21.07 7.98 0.1 7.70 5.54 3.49 0.80 0.20 2.52 1.94

56.0 0.69 17.35 8.14 0.1 5.90 6.77 4.63 0 0.11 2.35 3.34

7 26 96 20 97 86 28 17.9 41.2 22.7 5.1 1.27 0.98 3.93 0.65 207 35 4.8 2.5 8 0.58

22 17 48 32 275 47 35 14.7 34.8 19.4 4.8 1.00 0.84 3.02 0.5 114 29 2.8 2.2 5.8 0.42

27 2 208 35 283 148 45 6.5 16.1 10.3 2.5 0.79 0.44 1.54 0.26 53 16 1.2 0.8 3.1 0.18

16 22 225 27 184 97 55 7.2 16.7 9.1 2.2 0.78 0.40 1.42 0.23 78 14 1.9 0.8 3.5 0.28

27 59 193 21 145 29 12 23.5 52.3 26.8 6.4 1.62 0.97 3.4 0.56 134 32 3.1 3.3 8.5 0.47

22 32 348 19 193 51 45 14.5 34.3 18.2 3.9 1.14 0.63 1.96 0.33 157 21 3.6 1.6 6.2 0.48

41 25 237 25 196 183 81 12.9 29.2 13.8 3.2 1.08 0.56 1.66 0.28 121 18 2.7 1.1 5.1 0.36

22 2 176 22 195 60 112 11.4 23.4 11.3 2.7 0.99 0.47 1.56 0.26 99 16 2.4 1.7 4.1 0.35

8718 54.0 0.68 17.19 8.81 0.1 6.40 8.26 4.70 0.10 0.113 2.13 2.10 18 2 155 25 173 156 110 8.5 19.3 9.4 2.4 0.76 0.43 1.39 0.23 86 15 2 0.8 3.6 0.26

8710

8716

8713

8709

63.0 0.98 17.0 5.89 0.2 1.90 5.46 4.18 1.20 0.35 0.63 0.69

60.0 0.70 15.95 8.06 0.1 4.40 9.19 1.59 0.10 0.14 1.71 2.93

51.0 1.26 18.97 8.42 0.2 3.60 11.70 2.76 0.3 0.17 0.6 1.79

50.0 0.85 12.41 12.03 0.2 12.0 10.1 1.51 0.7 0.17 1.36 1.27

15 34 40 21 89 6 9 34.3 73.5 37.8 7.9 2.41 1.21 3.41 0.54 174 38 4.3 5.5 12 0.76

26 2 395 21 183 68 58 15.9 38.6 20.5 4.8 1.19 0.68 1.75 0.27 68 19 1.6 0.7 3.2 0.23

16 9 283 18 328 143 123 10.4 23.8 12.3 2.7 0.92 0.36 1.02 0.17 43 10 1 1.2 3.1 0.18

32 19 363 22 210 745 470 9.1 21.4 10.1 2.1 0.67 0.26 0.77 0.13 33 9 0.7 1.2 2.4 0.13

n.d. = not determined. *Fe203 as total iron.

Unlike alkaline and alkaline earth elements, the trivalent REE and the HFSE (Th, Hf, Nb and Ta) are not significantly affected by the alteration (Fig. 6). This is shown by the constancy of HFSE/REE ratios, such as Th/La, Hf/Sm, N b / C e and Zr/Sm, between altered and least altered samples. All these elements are correlated with Zr and are apparently immobile. Even in the case of strong alteration (A.I.> 90), some major elements (e.g. TiO2, A1203, FeO*, P205) and most trace elements

(REE (exept Eu in carbonatized samples), HFSE, and Sc, V, Ni ) offer a suitable basis for petrogenetic discussion. Geochemical characterization Tholeiitic mafic volcanic rocks Major elements •

Subalkaline rocks are usually characterized by their SiO2 contents, but SiO2 is, in the rocks

216

M.R. L A F L E C H E E T AL.

and compared to average MORB (e.g. Viereck et al., 1989 ), BRG tholeiites have significantly lower FeO* and TiO2 abundances.

fv

8699/8732 ~._o

:'J-~o oz

o~ o

d8

0-

-t-

o<

-2

L

Fig. 6. Enrichment and depletion of selected elements in altered andesite (No. 8732) relative to least altered andesite (No. 8699). The relative enrichment/depletion, - l o g fv represents the density weighted ratio of the element abundances in protolith and altered rock (equations from Lesher et al. 1986).

under study, slightly affected by alteration and cannot be used alone for classification of the different rock types (basalte, basaltic andesite and andesite). However, in the present study, tholeiitic rocks which display chemical and textural (thin section) evidence of silicification are not taken into consideration in the following major elements characterization oftholeiites. In the same way, the use of the Mg number (Mg# = Mg/Mg + Fe 2÷ ) as a differentiation index is hazardous since MgO is mobile. Consequently, Zr, which is among the most stable elements, is used in addition to SiO2 for the determination of the various rock types (e.g. Fig. 7). Tholeiitic volcanic rocks are basalt (3040%), basaltic andesite (40-50%) and andesite (15-20%). These rocks are quartz and hypersthene normative and locally, in the lower BRG, some Mg-rich tholeiites display olivine in their norm. The high A1203 contents of the BRG tholeiitic rocks resemble those of some tholeiites encountered in modern rift environments and also in marginal and back-arc basins (e.g. Saunders and Tarney, 1979; Hochstaeder et al., 1990). At a given MgO content

R E E distribution On the basis of their REE patterns and La/ Yb ratios, tholeiitic rocks have been subdivided into 4 groups (Fig. 8 ). Group I, basalts with one andesite (sample No. 8704 in Table 1 ) outcrops mostly in the lower part of the volcanic pile in the study area (units 2, 16 and 17 in Figs. 2 and 3 ). Group I is characterized by [ La/Yb ] N (normalized to chondrites) ratios between 1.1 and 1.7. The basalts display a weak fractionation of the HREE ([Tb/Yb]N from 1.10 to 1.42). This feature is not observed in modern island-arc tholeiites or MORB, but characterizes some Archean tholeiites in the Kolar schist belt of South India (Rajamani et al., 1989). The variation of the [La/Sm]N ratio (0.82 to 1.13 ) in these tholeiites indicates variable degrees of LREE enrichment or depletion. The group I andesite has a similar REE pattern but presents a relative REE increase and a negative Eu anomaly compared to basalt. Group II (unit 4 in Fig. 2 ), stratigraphically overlies Group I. It comprises basaltic andesites and rare andesite (No. 8699 in Table I ). It is marked by a LREE enrichment with [La/ Yb]N ratios between 1.5 and 1.8 and [Tb/ Yb]N ratios between 1.14 and 1.23. These rocks display a pronounced Eu anomaly (Eu* 0.59-0.80). Group Ill tholeiites (unit 8, Fig. 2) mark the end of the tholeiitic volcanic activity in the northern part of the BRG. This group is composed exclusively of Fe- and Tirich andesites with the highest REE contents and Eu negative anomalies (Eu* from 0.43 to 0.65 ). They have also higher [La/Yb]N ratios ( 1.48 to 2.2 ). The Eu negative anomalies increase toward the most differentiated Fe-Ti andesites (Figs. 8 and 9). Although Eu anomalies may be enhanced by alteration processes, the regular decrease of the Eu* ratio (as well a s A I 2 0 3 and Sr contents) from basaltic to andesitic rocks, sug-

ARCHEAN MAFIC VOLCANIC ROCKS OF THE SOUTHERN ABITIBI BELT, QUEBEC

80

SiO 2

2.0

Ti02

zo. /

(g /

70 60

._."

//e/"j ,,,,'/ " !111 1

"''.~5'

217

A1203

/

. ,,'~...... .

I.~

~5. , I/

1.0

i I

~' :.:"

""

,L"

~wt ; 0.,5

50.

I0,

,~o ~6o ,~ 26o Zr

~

,~o ,,~o 2~o Zr

~o i5o ,@o ~o Zr

Fig. 7. Plots of SiO2-Zr, TiO2-Zr, and AI203-Zr illustrating the limited variation in TiO2 and A1203 contents of calcalkaline rocks (dots) as compared to tholeiites of Groups I, II and III.

,50

,50

I0

I0

.5

5

w

tx 8 7 1 1 • 87C~ 0 8712

o7

•

a

i

T

V 0 0 rY

i

868'5 8684

•

9733

o 8707

8729

Z 0T" 5 0

•

i

i

i

,

i

rr

•

8734

i

r

I

i

i

i

r

r

CLERICY S I L L

.'50

I0,

I0

5

5 •

8699

o

8702

o

8724

•

8696

•

8725

i

1

i

i

t_,,c, ~

i

i

S~Eu ~;

LoCe

| Nd

i

;

SmEu

u

Tb

i

•

YbLu

Fig. 8. Chondrite-normalized rare earth element patterns for mafic volcanic rocks of Groups I, II, III, and IV tholeiites, and for cumulative gabbroic rocks of the Clericy Sill. Chondrite normalizing values from Nakamura (1974).

gests possible plagioclase fractionation. To test the hypothesis of plagioclase fractionation in the tholeiitic series, two gabbroic cumulates were sampled in the lower part of the syn-volcanic C16ricy Sill (Nos. 8724 and 8725 in Table 1 ). These samples display positive Eu (normalized) anomalies reflecting their plagioclase-rich mineralogy. This implies that fractional removal of plagioclase took place in the sub-volcanic magma chambers during volcanism. Although fractionation of plagioclase is likely in most Proterozoic and m o d e m tholeiitic series (e.g. Basaltic Volcanism Study Project, 1981; Condie, 1989; Viereck et al.,

1989), Eu anomalies are rarely encountered and, when present, are very small. This difference between Phanerozoic and possible Archean analogues may reflect a difference in redox conditions in the magma chamber and/or in the mantle source during melt extraction processes. Under relatively lowfo2, Eu, like Sr, is dominantly bivalent, and is preferentially concentrated in plagioclase (Philpotts, 1970; Drake and Weill, 1975). For this reason, the observed increase of Eu* anomalies with differentiation in the BRG tholeiites may represent plagioclase crystallization under especially low fo2. Low fo2 during plagioclase

218

M.R. LAFLECHE ET A L TABLE 2

1.5-

Average ratios of some incompatible elements from representative samples of BRG tholeiitic rocks

Eu* 1.0 _

- '_~oe %F ....

' . . . . . . . . . . .P.C. . . . .

0\~Oo o II o ~. 0 " ~o o II

o;.'~.,

o°,

2 ,°

0.5 []

0 0

60

D

260 ZF

Fig. 9. Eu'-Zr diagram showing the effect of plagioclase fractionation in least altered tholeiites of northeast BRG (dots), carbonated (solid squares) and chloritized (squares) tholeiites, and central BRG tholeiites (circles). Central BRG data were compiled from Trudel (1979), Grlinas et al. (1984), Lafl~che(1986), Ujike and Goodwin (1987), and Camir6 (1989). Eu° is chondritenormalized measured Eu divided by interpolated Eu between chondrite-normalized Sm and Tb. fractionation is also suggested by the lack of crystallization of F e - T i oxides during differentiation of the tholeiites (Table 1 ). Group IV tholeiites (unit 14, Fig. 2) are mainly composed of TiO2-rich andesites of high A1203 and P205, and low FeO ° contents (e.g. sample 8707, Table 1 ). These rocks display [La/Yb]N ratios between 1.46 and 2.2, and have some HFSE anomalies characteristic of calc-alkaline rocks.

High field strength elements distribution Like REE, Nb, Ta, Hf, and Th show increasing abundances in the tholeiites with magmatic differentiation (Table I ). They are positively correlated among themselves and with Zr. The average ratios among several incompatible elements are calculated for each group of tholeiites previously characterized on the basis of their REE, and reported in Table 2 and in Fig. 10. Some of these ratios, such as Zr/Y, Z r / N b , and T i / Z r , are correlated with the L a /

Ta/La Nb/Th Th/La Nb/Ta Zr/Hf Hf/Sm La/Yb Zr/Y Zr/Nb Ti/Zr Ti/V Sc/Yb reO*/TiO2 TiO2/PzO5 AIzO3/TiO2 CAO/AIzO3 Mg#

Group IA s.d.

Group 11 Group III s.d. s.d.

0.05 0 10.5 1.7 0.07 0.1 18.2 1.7 37.6 1 0.61 0.1 1.6 0.3 Z74 0.6 18.6 3.1 108 15 20 0.3 19.5 6 9.25 0.9 12.01 2.7 12.53 0.5 0.59 0.1 0.51 0

0.50 7.8 0.1 15.3 40 0.64 2.2 3 17 03 21 10.8 9.16 9.37 10.41 0.63 0.44

0 0.05 1.5 6.7 0 0.1 0.7 14.9 3.5 37.4 0.1 0.63 2.6 0.1 3.3 I 15.4 5 55 0 27 3.2 6.2 1 8 0.4 9.07 1.6 7.8 0.1 0.54 0 0.3

Group IV s.d.

0.003 0.05 0.006 0.28 13.8 1.8 0.05 0.07 0.007 0.5 17.2 2.2 1 44 0.3 0.56 0.09 0.2 2.17 0.26 0.28 2.99 0.56 0.2 16.1 2 3 158 32 3 28 1 0.4 22.7 1.2 0.7 7.43 (1.2 0.57 13.63 1.2 0.06 19.88 0.32 0.07 0.69 0.02 0.02 0.52 0.01

standard deviation. Group IA, n = 3 : group 1B, n = 3 ; group II, n = 7 : group 111, n = 3 . s.d. =

Yb ratio and vary regularly from group I to group III tholeiites. Such a relationship might suggest the influence of mineral fractionation, in agreement with partition coefficients for Zr, Y, Nb and Ti in clinopyroxene (Irving and Frey, 1984). The N b / T h values, which decrease from group I to group III may suggest, in a first approximation, that DNb < DTh. Since the ratios of these elements are considered constant during magmatic differentiation (e.g. Saunders et al., 1988 ) their variation suggests the influence of other processes (e.g. crustal contamination, source heterogeneity). The N b / T a average value of 17.6_+ 2.2 in group I basalts is close to the estimate of Sun and McDonough ( 1989 ) for the primitive mantle. Differentiated tholeiites in groups II and III, however, display lower average values (15.3 + 0.7 and 14.9 + .5 respectively). These values are lower than the average value for MORB (16.5 in Jochum et al., 1986) and for late Archean basaltic rocks (17 in Condie, 1989 ). Although, the variation between groups I, II, and III is small, the difference cannot be

ARCHEANMAFICVOLCANICROCKSOF THE SOUTHERNABITIBIBELT,QUEBEC

T •

15

Zr/Nb

I

•

"~

I0

•

Nb/Th

oo

219

o

5 0

t

I

I Th~L °

t

I

I

"."

0.1 0.05

T

.4-"

....

I

0

I

I

I

T

0.8

I

Zr/Y"

#'

;

I

1

I

TI/Zr

Hf/,Sm _,~

.

.

o

~o

_ ~ f

0.6

I 0.4

j

~o

t

t

0

Zr/Hf

40

LaAYb

I

. . . .

35 30 21

I

I

I

•

I

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NblTa

25

17

0

I

I

I

50

~oo

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I

zoo

250

(ppm)

o

T

i•

5

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I

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~,\ \

15

•

I

e,

20 \

I

0

50

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150

Zr

•

200

250

(ppm)

Fig. 10. Ratios of highly incompatible ( N b / T h , T h / L a , Hf/Sm, Z r / H f and Z r / N b ) and moderately incompatible trace elements (Zr/Y, Ti/Zr, La/Yb and Sc/Yb plotted against Zr abundances for northeast BRG tholeiites. Star: primitive mantle (from Sun and McDonough, 1989); dots: Group I; circles: Group II; solid triangles: Group III. Note the unusual fractionation of Hf/Sm, Nb/Ta, and N b / T h in tholeiitic rocks and the scatter of data in basaltic compositions (Zr < 65 ppm) which may reflect source heterogeneity a n d / o r variable extent of contamination. Calculated error bars for trace elements ratios are indicated on the left sight of each diagram.

explained by simple mineral fractionation processes. Ratios such as Th/La, H f / S m and Zr/ H f remain nearly constant in the three groups. However, group I basaltic rocks are heterogeneous with respect to these ratios as shown by the scatter of data in Fig. 10. Group I basaltic rocks (Nos. 8728 and 8729 in Table l, and unit 2 in Fig. 2 ) which outcrop in the lower part of

the northeast BRG have the highest N b / T h and the lowest T h / L a ratios (Table 2). Group IV differs from the previous ones in having weak but distinct negative anomalies of Nb and Ta. They may thus represent a transition between the first three groups oftholeiites and the calc-alkaline rocks characterized by more pronounced Nb and Ta anomalies.

220

Calc-alkaline mafic volcanic rocks' Major elements Zr is used, in addition to SiO2, in order to individualize the different rock types. Calc-alkaline basaltic andesites are characterized by Z r < 85 ppm whereas andesites display Zr values comprised between 85 and 150 ppm (Table 1, Fig. 7 ). Dacitic rocks, which are present but subordinate, have Z r > 170 ppm. On average, calc-alkaline rocks have generally higher Mg# (0.58+0.07) than tholeiites, and the most primitive calc-alkaline basaltic andesites have olivine in their CIPW norms. The main difference between tholeiitic and calc-alkaline mafic rocks is the relative constancy of the FeO*, TiO2 and A1203 contents in the latter (Fig. 7 ). Compared to modern oceanic arc andesite (e.g. Woodhead, 1989), least altered BRG calc-alkaline mafic rocks display slightly lower A1203, FeO, MnO and K20, but higher MgO and Na20 contents. REE distribution BRG calc-alkaline mafic rocks display subparallel REE patterns, and unlike tholeiites, they rarely show Eu" anomalies (Fig. 11). Their normalized HREE contents do not show the concave-upward patterns typical of many Archean calc-alkaline plutonic and volcanic rocks (Jahn et al., 1981; Brooks et al., 1982; Shirey and Hanson, 1986). From their REE contents and La/Yb ratios, the calc-alkaline rocks of the northeast BRG (e.g. unit 6 in Fig. 2) are subdivided into 3 groups. Group C1 outcrops in the lower part of the volcanic pile in northeast BRG and has the lowest [La/ Yb]N ratios (2.87-3.41). These rocks show slight fractionation of the HREE ( [Tb/Yb]N: 1.06-1.19 ) but display strong variation in REE abundances (20-60 times chondrites for LREE) reflecting the presence of differentiated rocks (e.g. sample 8721 in Table 1 ). On the basis of their HFSE contents, rocks of group C1 are further subdivided in 2 subgroups. Group C1A is characterized by samples dis-

M.R. LAFLECHE ET AL.

playing high Hf/Sm and Z r / N b ratios and Group C1B by low Hf/Sm and Z r / N b ratio. Dacitic rocks of group C 1A have higher REE contents but display Eu* anomalies (0.60) which suggest plagioclase fractionation, supported by the decrease of A1203 and Sr abundances. Group C2 calc-alkaline rocks are present in the intermediate part of the volcanic pile (unit 10 in Fig. 2) where they are interstratifled with Na-rich rhyolitic complexes. Their REE patterns show intermediate fractionation of the LREE/HREE ( [La/Yb]N: 3.95-5.22) which essentially reflects increasing HREE fractionation ([Tb/Yb]N: 1.19-1.45). This group is also characterized by Mg# values between 0.60 and 0.64, and Ni contents near 100 ppm, but it displays strong variations in its overall REE abundances (e.g. LREE: 22 to 45 times chondrite ). Group C3 calc-alkaline rocks outcrop in the northern part of the Blake River Group (unit 12 in Fig. 2), and in the vicinity of the Porcupine Destor Fault Zone (within unit 17, but not shown in Fig. 2 due to scale), where they are part of a volcanic formation tectonically interlayered with the Kewagama sediments (Fig. 2 ). C3 calc-alkaline rocks display the highest LREE contents with [La/ Yb]N values comprised between 6.1 and 7.9 and [ T b / Y b ] N between 1.45 and 1.66. Compared to modern arc series such as the Mariana or Aleutian arcs (Woodhead, 1989), the BRG calc-alkaline mafic rocks have higher La/Yb ratios, which seems to be a characteristic of Archean calc-alkaline volcanic (e.g. Condie, 1989)and plutonic rocks (e.g. Bickle et al., 1983). It is not, however, unique to the Archean, since highly fractionated basaltic andesites and andesites are also present in modern continental margins such as Central Chile (Hildreth and Moorbath, 1988). Furthermore, in recent oceanic arc environments, where young and hot oceanic lithosphere has been subducted, "adakitic" type basaltic andesires (Defant and Drummond, 1990) display low Y, Yb and HFSE contents. These rocks shared common features with BRG calc-alka-

ARCHEAN MAFIC VOLCANIC ROCKS OF THE SOUTHERN ABITIBI BELT, QUEBEC

221

b)

o)

O1

50

I0 5

I

• 8720 0 8689 • 8719 • •

, •

, ,

, ,

, ,

, ,

, ,

, ,

, ,

, ,

, ,

t i

, ,

, ,

i ,

.I,1,t,I

I: ::

::

C2

50.

I0

Ld 5

r~ E3 Z 0

I

& • 0 •

8695 8685 8717 8718

I

p::;:

I I

I I I I

L) 5 0 Y (.3 0 cr

o

I0

5. • 8709 i LaCe

i

i

, Nd

i

' SmEu'

[ T'b

i

i

i Y;I'u

Th I I..a I (~e I Z'r r , s m l & "l"b ~" Ta Nb Nd Hf Ti

Ybl'u

Fig. 11. (a) Rare earth element patterns of mafic calc-alkaline volcanic rocks from the northeast BRG. Chondrite normalizing values from Nakamura (1974). Samples 8721 and 8720 (from Group CI) are of dacitic composition. (b) Primitive mantle normalized element distributions of mafic calc-alkaline rocks. Note the progressive decrease in Nb, Ta, Zr, and Hf abundances from Group CI to Group C3.

line rocks. "Adakitic" volcanic rocks are interpreted as being derived directly from the melting of the subducted oceanic lithosphere ( D r u m m o n d and Defant, 1990), a model which has been often suggested to explain the origin of many Archean tonalitic suites (e.g. Arth and Baker, 1976; Martin, 1987). Archean calc-alkaline basaltic andesites lack the Ce-Eu negative anomalies characteristic of some m o d e m oceanic arc series (e.g. Woodhead, 1989). These anomalies are usually explained, in the recent arc environment, by incorporation, via subduction, . of pelagic sediments or of a sufficiently oxidizing slab (White and Patchett, 1984). Given the more reducing nature of the Archean oceans, sub-

duction of Archean pelagic sediments need not have produced a selective Ce fractionation.

High field strengh elements distribution HFSE positively correlate with REE and behave as incompatible trace elements during the igneous differentiation processes in the calc-alkaline rocks, as for the tholeiites. BRG calc-alkaline rocks are relatively depleted in Ti, Nb and Ta with or without Zr and Hf depletion (Fig. 11 b). These strong anomalies are not observed in BRG tholeiites, but are characteristic of m o d e m oceanic and continental arc volcanism (e.g. Wood et al., 1979; Briqueu et al., 1984). Compared to BRG tholeiites, calc-alkaline rocks display lower N b / T h , N b / T a , T i /

222

M.R. LAFLECHE ET AL.

Zr, and Ta/La ratios, and higher Th/La, Zr/ Hf, Hf/Sm, Z r / Y and Z r / N b ratios (see Tables 2 and 3). Meaningful variations of Nb/ .Th, Ya/La, Hf/Sm and Z r / N b ratios are observed among the groups of calc-alkaline rocks (Table 3). Nb/Th, Hf/Sm, Zr/Nb, and Z r / Y are higher in group C 1A and C2 calc-alkaline rocks than in group C1B and C3 (Table 3 ). The high values of Th/La in group C1B and C3 (0.11-0.13 ) may suggest the assimilation of a crustal component during their genesis. For comparison, this ratio is as low as 0.048 in NMORB (Sun and McDonough, 1989) and may reach values of 0.23 to 0.28 in crustal materials which may be approximated by the average Archean middle and upper crust (Taylor and McLennan, 1985; Wedepohl, 1991). On the other hand, these ratios are not substantially fractionated during differentiation;. hence not correlated to La/Yb as it is the case for BRG tholeiites. This implies that the variation of these ratios is related to source heter-

ogeneities and/or to additional processes such as assimilation prior to differentiation. It is noteworthy that most incompatible trace element ratios do not display chondritic and/ or primitive mantle values. This is particularly true for Ya/La, Nb/Th, Ti/Zr, and Nb/Ta ratios which are usually lower (Table 3 ) and for Zr/Nb and Z r / Y ratios which display higher values. High Zr/Nb and Z r / Y ratios result from impoverishment in Nb and Y. The relatively low abundances in Y, compared to modern island-arc calc-alkaline rocks, suggest the presence of garnet in the residual source. Furthermore, as is the case for some Andean calcalkaline rocks, the low Y / T b in the BRG calcalkaline rocks (Table 3) also suggests the influence of garnet, since Dv>DTb in garnet (Pearce and Norry, 1979; Baxter et al., 1985 ). This ratio displays values near 42 in the primitive mantle, as well as in Abitibi Komatiites and modern MORB (e.g. Barnes, 1985; Sun and McDonough, 1989 ) whereas Y / T b values

TABLE3 Average ratios of i n c o m p a t i b l e e l e m e n t s from r e p r e s e n t a t i v e s a m p l e s of calc-alkaline rocks C 1A

C2

C3 s.d.

Ta/La Nb/Th Th/La Nb/Ta Zr/Hf Zr/Sm Hf/Sm La/Yb Tb/Yb Y/Tb Zr/Y Zr/Nb Ti/Zr Ti/V Sc/Yb TiO2/P205 AI203/TiO2 CaO/A1203 Mg# Ta/La

0.039 4.38 0.11 12.5 41 35.9 0.88 5.07 0.29 35 5.57 22.3 63 26.7 18.8 6.31 24.2 0.44 0.50 0.039

0.035 4.22 0.11 13.39 43.6 36 0.85 6.63 0.28 35 6.34 23.1 43 27.2 13.3 4.99 21.9 0.43 0.59 0.035

0.01 0.8 0.03 2.06 2.8 3 0.07 0.62 0.03 0.3 0.8 2.35 6.4 2.9 2.7 0.49 3.43 0.14 0.05 0.01

C 1B s.d.

0.017 2.85 0.11 16.48 43.3 18.2 0.39 10.3 0.38 28.4 4.03 15.73 106 34 16 5.05 17.44 0.58 0.51 0.017

0.003 1.01 0.04 1.67 2.43 6 0.09 0.98 0.02 2.6 0.42 3.19 60 18 8.3 1.63 3.25 0.17 0.10 0.003

C2B s.d.

0.026 3.26 0.13 15.52 42.4 23 0.53 5 0.27 38 3.62 18.38 100 27 16.7 5.75 13.6 0.37 0.55 0.026

-

Dacite s.d.

0.02 4.45 0.099 18.09 43.41 21 0.47 6.42 0.29 34 3.79 15.91 80 35 13 3.96 15.91 0.43 0.5 0.02

s.d. = s t a n d a r d deviation. G r o u p C I A , n = 1; group C2, n = 13; group C3, n = 4 ; group C1B, n = 2 ; group C2B; n = 2 ; dacites, n = 2 .

-

s.d. 0.034 3.32 0.15 14.58 43.3 41 0.95 4.41 0.26 34.5 6.09 25.3 22 47 5.33 3.26 17.28 0.41 0.46 0.034

ARCHEAN MAFIC VOLCANIC ROCKS OF THE SOUTHERN ABITIBI BELT, QUEBEC

in calc-alkaline rocks average 35, and are as low as 27 in group C3. Average Nb/Ta is 12.5 in group C1A and 13.4 in group C2, whereas it increases to up to 18.1 in rocks of group C 1B and C3 (Table 3 ). Conversely, Hf/Sm is higher in group C1A and C2 and lower in group C3 and C 1B (Table 3 ). According to Wolff (1984) and Green and Pearson (1987), rutile and sphene are the only likely minerals in the melting region or mixing region having suitable partition coefficients which can efficiently fractionate Nb/Ta in such an extent. Discussion

Differentiation of tholeiitic rocks The Blake River Group tholeiites display increases in SIO2, TiO2, FeO*, MnO and P205 with decreases in A1203, MgO and CaO during differentiation (Fig. 7, Table 1 ). To explain the origin of the evolutionary trends, we formulate and test a model of fractional crystallization, then modify the model to include magma mixing and assimilation. The observed chemical variations are typical of fractionation of a gabbroic assemblage; olivine, plagioclase and clinopyroxene. The variation of the A1203/TiO2 ratio from 18 in Mg-rich basalt to 7 in FeTirich andesite (Table 2) implies that plagioclase was a fractionating phase during differentiation. Such plagioclase influence can also be inferred from the variation of the Eu* ratio (Fig. 9) and Sr abundance (Table 1 ). The low A1203/TiO2 ratio of basaltic tholeiitic rocks (Table 2 ) reflects the lack of plagioclase accumulation, supported by petrographic observations. The CaO/A1203 ratio, which decreases from 0.64 in basalt to 0.45 in andesite, reflects clinopyroxene fractionation. This fractionation is also shown by the systematic variation of the Sc/Yb and La/Yb ratios (Fig. 10). Especially high FeO* and TiO2 contents in andesites (Table 1 ) preclude significant separation of Fe-Ti oxides, and imply relatively low fo2. The consistency of the TiO2/P205 ratio in ba-

223

saltic andesites and andesites (Table 2) supports the lack of Fe-Ti oxide crystallization and also argues against the involvement of sphene, rutile or apatite. On the other hand, the variation of the Ti/V ratio, which increases from 20 in basaltic andesite to 28 in andesites, does permit a minor amount of Fe-Ti oxide fractionation in late stages of differentiation. This is caused by the combined effect of decreasing temperature and increasing fo2. Cobalt content, nearly constant in basaltic andesites and andesites, confirms that oxide fractionation was not significant in tholeiitic rocks. To test the fractional crystallization hypothesis for the chemical evolution of the tholeiites, we used Nielsen's (1988) CHAOS5 lowpressure fractional crystallization program jointly with mass balance considerations. Before modeling, some a priori have to be taken into consideration. The composition of the parental magma (starting composition: Co) is critical, since this controls the calculated liquidus phases, and thus the liquid path during differentiation. Cumulative and altered samples must be avoided. Two potential parental rocks have been chosen on the basis of petrographic and chemical criteria. The first has olivine (pseudomorph) and plagioclase as liquidus phases whereas the other has olivine, plagioclase and clinopyroxene on the liquidus. During differentiation, both produced strong FeO ° and TiO2 enrichment coupled with a decrease in A1203 (Fig. 12, Appendix II). These differentiation trends are typical of low-pressure fractional crystallization in modern MORB and Hawaiian tholeiitic series (e.g. Perfit and Fornary, 1983; Defant and Nielsen, 1990). In the present case, such a process may explain the generation of Fe-rich basalt from the Abitibi lower volcanic sequence ( Fig. 12, trend A) as also shown by least-squares calculations (Appendix Ia). Low-pressure liquid lines of descent, calculated for a Mg-tholeiite undergoing pure fractional crystallization as well as for steady-state magma chambers undergoing recharge and periodic eruption do not match the

224

M . R . L A F L E C H E E T AL.

2O

Fe©* 15 2

v/~L'~'-L"

I0

ei

©II 0

50

I 0

150

!

i

200

250

Zr Fig. 12. FeO ° versus Zr illustrating the different FeO" enrichment trends for tholeiites of the SAB. Groups I (dots), II (circles), and III (triangles) are BRG tholeiites (upper volcanic sequence of the SAB). Volcanic rocks from the lower volcanic sequence of the SAB; Field 1: Cadillac Formation; field 2:Pich6 Group; field 3: Malartic Group; field 4: Jacola Group; field 5: H6va Formation (compiled from Babineau 1982 and Parent 1985 ). C.A.: calc-alkaline rocks of the BRG, shown for comparison. Curves A and B are the calculated fractional crystallization trends for Mg-rich tholeiite (A) and Fe-rich tholeiite (B). Curve C is the fractional crystallization trend for BRG basalt sample 8705 (Table 1). In Nietsen's CHAOS.5 program, fo2 is fixed at - 2 log QFM. Curve D is an AFC trend calculated for the basalt No. 8705 assimilating a calc-alkaline melt with composition of average Group C2 calc-alkaline andesite. See text for discussion.

chemical compositions of the differentiated BRG tholeiites. In Fig. 12 it is noteworthy that BRG tholeiites define a particular evolutionary trend characterized by FeO" contents between those of the lower tholeiitic cycles of the southern Abitibi Belt (e.g. Kinojevis, Malartic, Pich6 Groups ) and those of the BRG calcalkaline rocks (Fig. 12). BRG tholeiites display relatively high A1203 contents which have also intermediate values between those of AIrich calc-alkaline rocks and Al-poor tholeiites of the lower tholeiitic cycles. It is suggested, based on the most incompatible trace element

ratios (e.g. Nb/Th, Nb/Ta) and on La/Yb (Fig. 13a, b), that BRG tholeiites are contaminated by a crustal component. Given the geological setting of this part o f the SAB, there may be three contaminants: ( 1 ) sedimentary rocks, (2) rhyolite and trondhjemite, and (3) calcalkaline magmas. AFC trends (e.g. Nielsen, 1988) calculated for tholeiites assimilating sedimentary rocks or rhyolite/trondjhemite clearly indicate that assimilation of these components may yield residual liquids with FeO* contents comparable to those of BRG tholeiites, but the A1203 contents of calculated derivative melts are too low to match the composition of BRG basaltic andesites and andesites (Appendix II ). The increase in A1203 is an effect of incremental assimilation of aluminous materials. It is readily buffered by increasing plagioclase fractionation until the residual liquids regain the cotectic. Trace elements in BRG trondjhemitic and rhyolitic rocks (Ujike and Goodwin, 1987; Paradis et al. 1989) are characterized by low La/Yb ( < 4.0), chondritic Hf/Sm ratios, and high Y values. Contamination of asthenospheric basalts by trondjemites and/or rhyolites cannot explain the chemistry of BRG tholeiites since it is obvious from Fig. 13a, that BRG tholeiites do not plot in the vicinity of the mixing line (line D) between typical end-members (asthenospheric basalts and average rhyolites). Furthermore, the density and viscosity contrasts between basaltic liquids and rhyolites make it unlikely that magma mixing produced hybridized tholeiites (e.g. Grove and Baker, 1983 ). Mixing in crustal magma chambers of mafic tholeiitic and calc-alkaline melts in relatively large amounts may explain the variation shown in Figs. 12 and 13 (lines B and C). Contamination of tholeiitic basalts by calc-alkaline andesitic magma may also explain variations of HFSE ratios such as N b / T h and N b / Ta as well as the increasing La/Yb ratios from group I to group III tholeiites which is reported in Fig. 13a, b. To test this possibility with major elements, we have performed least-squares

225

A R C H E A N M A F I C V O L C A N I C R O C K S O F T H E S O U T H E R N A B I T I B I BELT, Q U E B E C 20

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150 200

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Fig. 13. (a) ( La/Yb )- (Nb/Th ) diagram showing calculated partial melting line (A) of a garnet lherzolitic source ( open

star), and mixing lines between asthenospheric depleted tholeiites (high Nb/Th ratios) and (B and C) extreme compositions of calc-alkaline basic andesite (from Group C3), and (D) rhyolite (square; average BRG rhyolite composition from Lafl6che 1986). I, II, III, and C 1, C2 and C3 refer respectively to tholeiitic and calc-alkaline groups of Table 1. Field I corresponds to tholeiitic basalts from the lower volcanic sequence of the SAB. Solid star: primitive mantle estimate of Sun and McDonough ( 1989). Garnet lherzolite composition from Bodinier et al., (1989) (cpx: 0.15, opx: 0.25, olivine: 0.55, garnet: 0.05; La: 0.32 ppm, Yb: 0.49 ppm, Nb: 0.33 ppm, Th: 0.02 ppm). Partition coefficients for garnet, cpx, opx, and olivine are taken from Irving and Frey (1984), and Pearce and Norry (1979). (b) (Nb/Th)-Zr and (Nb/Ta)-Zr diagrams showing assimilation and fractional crystallization (AFC; DePaolo, 1981 ) trends calculated for a group I tholeiitic basalt (sample 8705 ) assimilating a calc-alkaline andesite (No. 8715 ). Assimilation rate is 0.30. Relative amount of magma solidified (F) graduated from 30 to 80% at every 10%. Zr bulk distribution coefficient fixed at 0.60. Star: primitive mantle. Ia, Ib, II, and III are tholeiite groups of Table 1. Dashed arrow is pure fractional crystallization trend w i t h D T h ~ DNb~ D T a .

calculations which are presented in Appendix lb. These calculations indicate that major element contents o f B R G tholeiitic andesite can be reproduced by the addition o f 20% o f calcalkaline melt to Fe-rich tholeiitic basalts and subsequent removal by fractional crystallization o f an assemblage of approximately 10% olivine, 30% plagioclase and 11% clinopyroxene. On the other hand, some low F e O ° tholeiitic basaltes (e.g. No. 8711 in Table I ) m a y b e produced by m a g m a mixing o f Mg-rich B R G tholeiites with calc-alkaline andesites (Appendix Ic). Using the proportions determined by the least-squares calculation, the progressive increase of the L a / Y b ratio from basaltic to andesitic compositions can be explained by simple fractional crystallization. However, this process cannot explain the variations o f the more incompatible trace element ratios such as

N b / T h and N b / T a (Fig. 13b). The variation of these ratios during differentiation o f the B R G tholeiites corroborates the hypothesis of m a g m a mixing which is further sustained by the model reported in Fig. 13a, b. Among the tholeiitic rocks, the Fe-rich basalts are the only rocks that can be explained by fractional crystallization o f the assemblage olivine-plagioclase-clinopyroxene (Appendix Ia). The petrogenesis o f the other tholeiites, and especially of the tholeiitic andesites, requires both differentiation and mixing with the calc-alkaline magmas.

Comparison of BRG with southern Abitibi Belt tholeiitic rocks Figure 12 (FeO* vs Z r ) separates (1) the lower volcanic sequence marked by a strong FeO* enrichment trend, (2) the B R G tholei-

226 ites which display an intermediate FeO* enrichment trend, and (3) the calc-alkaline rocks of the BRG marked by nearly constant FeO* contents. In this figure, tholeiites with Z r > 130 p p m correspond to andesitic rocks, and the calculated liquid line of descent from a primitive Mg-tholeiite with redox conditions fixed at - 2 l o g QFM is reported (trend a). This trend remarkably matches the composition of the lower volcanic sequence of the Southern Abitibi Belt, and reflects plagioclase fractionation (without oxides) which allows very strong FeO* and TiO2 enrichment (e.g. Michael and Chase, 1987; Klein and Langmuir, 1987).

Differentiation of calc-alkaline rocks BRG calc-alkaline rocks are characterized by

high SiO2 contents ( 5 8 + 4 wt% in Table 1) which vary in a relatively small range compared to tholeiites. The low FeO ° and associated high M g # (0.58_+0.07), contrast with their SiO2-rich nature. On average, these calcalkaline rocks also display low K20 (0.7 _+0.6 wt%). Given the low variation in major element contents, it seems that the calc-alkaline suite has undergone only limited fractional crystallization. This is supported by the low phenocryst contents (5%) of these rocks (by comparison, differentiated modern arc lavas show up to 60% phenocrysts, Woodhead, 1989 ). The high normative plagioclase (Ab + An + Or = 6 5 _+ 10%) of the rocks suggests, with the limited variation of A1203 and Sr, that plagioclase fractionation was not important in the petrogenesis of the calc-alkaline mafic rocks. The BRG calc-alkaline lavas may have differentiated at intermediate pressure (e.g. 5 kbar, 2% H20), since rock samples plot near the 5 kbar cotectic on the pseudoternary olivine-quartz + orthoclase-plagioclase projection of Baker and Eggler (1987) (Fig. 14). High PH2o delays plagioclase crystallization to lower temperatures, and thus decreases its pro-

M,R. LAFLECHE ET AL.

Plag

dry~

IQtm

/ ~

OI

~ k t l : r(2%H20)

SiOr

Fig. 14. Effectof Pr~2oon the phase volume in the olivine (O1)-plagioclase (Plag)-Qtz + orthoclase ( SiOr ) pseudoternary after Baker and Eggler (1987). The 1 atm dry saturation surface is shown as a continuous line whereas at 5 kbar (and 2% H20) the surface is shownas a discontinuous line. Dots are least altered calc-alkaline basaltic andesites and andesites from the Blake River Group. See text for discussion.

portion in the crystallizing assemblage (Michael and Chase, 1987; Sinton and Fryer, 1987 ). This process likely explains the plagioclase-saturated nature of BRG calc-alkaline rocks. The production of dacites by fractional crystallization however, requires a relatively high proportion of plagioclase crystallization to explain the impoverishment in A1203 and Sr. This is to be expected if dacites are generated in the upper levels of the crust (subsidiary magma chambers) following the ascent of mafic calc-alkaline magmas. In this respect, least squares calculations (Appendix Id) indicate that dacites can reasonably be produced by removal of a mineralogical assemblage of 36% plagioclase, 6.4% clinopyroxene 8.6% olivine and 4% titanomagnetite from an calc-alkaline andesitic parental composition (Appendix Id). Fractionation of titanomagnetite is supported by the strong fractionation of the T i / V (and TiO2/P2Os) ratio which varies from 26 in andesite to 47 in dacite, as well as by the decreasing Co (Tables 1 and 3 ).

ARCHEAN MAFIC VOLCANIC ROCKS OF THE SOUTHERN ABIT1BI BELT, QUEBEC

Origin of tholeiitic and calc-alkaline rocks

BRG tholeiitic rocks do not display the characterisitic depletion in Nb and Ta of modem arc tholeiites (Table 2). BRG tholeiites have undergone intensive fractional crystallization, probably at intermediate and/or low pressure. Differentiation of the tholeiites is inferred to have been accompanied by assimilation of calc-alkaline magma. The calc-alkaline rocks do not display a broad range of compositions as do the tholeiites. BRG calc-alkaline rocks show smaller HFSE anomalies than modern orogenic andesites (Fig. 1 lb). The BRG represents the uppermost volcanic group of the Southern Abitibi Belt and overlies a thick basaltic to komatiitic sequence (lower tholeiitic sequence). To discuss the origin of the BRG tholeiitic and calc-alkaline volcanism and to understand the evolution of the mafic volcanism in the Southern Abitibi Belt, available data from the BRG and the lower volcanic groups are plotted and compared on figures 15 and 16. Altered samples were rejected from the data base according to criteria discussed before. Lower versus upper tholeiitic sequence Constraints on the source composition and physical conditions during melting can be inferred from major element behavior. Other variables are temperature and pressure conditions (e.g. Jaques and Green, 1980) and the degree of partial melting (e.g. Fuji and Scarfe, 1985; Klein and Langmuir, 1987). Given that Na20 variations cannot be applied to low-grade metamorphic rocks (Klein and Langmuir, 1987), we have focussed on the systematic variation of FeO°/TiO2, a ratio which is only weakly affected by moderate hydrothermal alteration and seafloor weathering and metamorphism. In Fig. 15a, komatiites of the Abitibi Belt display the highest FeO*/TiO2 ratios (25 to 33 ) whereas BRG tholeiites have the lowest (7 to 10). Tholeiites from the lower volcanic sequence display FeO*/TiO2 varying from 21 to

227

16, whereas other tholeiites have FeO*/TiO2 between 15 and 10.5. Similar variations are recognized in the western part of the Abitibi Belt (data of Goodwin and Smith, 1980 where FeO*/TiO2 varies from 16 in the lower volcanic sequence to 11 in the upper one). FeO*/ TiO2 remains relatively constant during anhydrous differentiation of mafic magma as indicated by the limited variation of FeO*/TiO: after 50% of fractional crystallization (from 10.5 to 8.6 in Appendix II). Conversely, during partial melting of mantle peridotite, this ratio will vary in derivative melts. This is in agreement with the fact that TiO2 is more incompatible than FeO* in mantle mineral phases Experimental data of Fuji and Scarfe ( 1985 ) plotted in Fig. 15a, indicate an increase of FeO*/TiOE from 9 at 1250°C to 17 at 1310°C in basaltic melts. This relation between increasing temperature and FeO*/TiO2 suggests that the variation observed in the SAB is related to the degree of partial melting. Klein and Langmuir ( 1987 ) highlighted the positive correlation between CaO/A1203 ratio and partial melting ofperidotite. The CaO/A1203 ratio increases with increasing degree of partial melting until clinopyroxene is no longer a residual phase (Fuji and Scarfe, 1985 ). In view of these concepts, CaO/A1203 was plotted against FeO*/TiO: for tholeiites of the Southern Abitbi Belt. The good correlation of these ratios with experimental results of Fuji and Scarfe ( 1985 ), confirms that FeO*/TiO: systematics for basaltic tholeiites are a good indicator of the extent of partial melting (Fig. 15b). This relationship further suggests that the Abitibi basaltic tholeiites can be related by a decrease in the degree of partial melting from a single mantle parental source. To evaluate the possible fractionation of the FeO*/TiO2 and CaO/A1203 ratios during assimilation coupled with crystal fractionation, AFC calculations were undertaken for assimilation of calc-alkaline andesitic melts. With Mg-tholeiite as the parental magma and calcalkaline andesite as contaminant, AFC calcu-

228

M.R, LAFLECHE

ET AL,

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Fig. 15. (a) (FeO*/TiO2)-MgO diagram showing the progressive melting trend of a synthetic lherzolite at 10 kbar pressure (from Fuji and Scarfe, 1985). Arrow indicates direction of compositional change with incremental melting. Note that Abitibi tholeiites define a trend subparallel to the experimental one. Shaded field: BRG tholeiites; fields I to 5: same as in Fig. 12; field 6: Abitibi komatiites (from Barnes, 1985 ). Star: pyrolite. See text for discussion. (b) (CaO/A1203 ) (FeO*/TiO2) diagram and lherzolite melting trend from Fuji and Scarfe ( 1985 ). Increased melting produces melts with high CaO/A1203 and FeO*/TiO2 ratios. (c) (FeO*/TiO2)-(Zr/Y) diagram showing the systematic variation of FeO*/ TiO2 with Zr/Y suggesting that tholeiites from the southern Abitibi Belt can be related by different degrees of partial melting of a parental source of approximate lherzolite composition. Fields: as in (a).

lation yields FeO*/TiO2 and CaO/A1203 varying from 10.5 to 8.7 and 0.74 to 0.89, respectively (Appendix II). The limited variation suggests that for the modelled rock compositions, these ratios are only weakly affected by AFC processes. The conclusions inferred from the behavior of the major elements, are corroborated by trace elements. Tholeiites from the lower vol-

canic sequences of the southern Abitibi Belt have La/Yb ratios between 0.7 and 1.5 (Fig. 16 ). BRG tholeiites display higher La/Yb ratios (1.7 to 2.5 ); this supports a lower degree of partial melting or different source compositions. On the other hand, Group IV tholeiites (Table 1) and Central BRG tholeiites (e.g. G61inas et al., 1984; Ujike and Goodwin, 1987; Camir6, 1989) have La/Yb ratios higher than

ARCHEANMAFICVOLCANICROCKSOF THE SOUTHERNABITIBIBELT,QUEBEC

5

L__~Q Yb

I I I

4

io _

I

,,-e

.a. v,

I

I

B

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• I ©~ A~

o

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Fig. 16. ( L a / Y b ) - Y b diagram for tholeiitic rocks from the BRG and the lower volcanic sequences of the SAB. Fields I to 6: as in Figs. 12 and 15; field 7: Kinojrvis tholeiites (Smith, 1980); field 8: BRG group IV tholeiites; field 9: Central BRG tholeiites (as in Fig. 9); star: primitive mantle. Trends A and B (graduated at 5, 10, 20, 30, 40) are derivative melts from modal partial melting of respectively a garnet lherzolite (from Bodinier et al., 1989) and a spinel iherzolite (from Viereck et al. 1989). Partition coefficients for La and Yb in garnet and spinel are taken from Irving and Frey (1984), and Pearce and Norry (1979). La/Yb in the source is average La/Yb of Abitibi LREE-depleted komatiites (La/Yb = 0.65, Barnes 1985 ); Yb value (0.49) from the primitive mantle o f Sun and McDonough (1989). Fractional crystallization (F.C.) trends are calculated with Nielsen's CHAOS 5 program, and graduated at 10%.

2.7. These "enriched" tholeiites are mainly the product of magma mixing with calc-alkaline andesitic magma. Compositions of liquids produced by partial melting of a LREE depleted mantle source (La/Yb--- 0.70) were calculated. The choice of

229

a LREE-depleted source is supported by available REE and Nd isotopic data on basalts and komatiites from the Abitibi Greenstone Belt (Cattell et al., 1984; Dupr6 et al., 1984; Machado et al., 1986; Barrie and Shirey, 1989). The observed range of end ratios ( + 2 to + 4 in tholeiites and komatiites) implies an origin from a long time LREE-depleted asthenospheric mantle. The composition of the liquids produced by partial melting of a LREE-depleted spinel and/or garnet lherzolite are plotted in Fig. 16. The La/Yb fractionation trend which results from partial melting of spinel lherzolite (trend b) cannot explain the observed variation recorded in tholeiites from the greenstone belt. Conversely, varying degrees of partial melting of a garnet lherzolitic source (trend a) can produce a large variation in La/ Yb ratios and this may explain some of the trace element characterisitics of the Abitibi tholeiites. From these considerations, the lower volcanic sequence, where Mg-rich tholeiites are abundant, may be produced by a relatively high degree of partial melting (20 to 40%) whereas BRG type tholeiites may be generated by a low degree of partial melting (e.g. 10 to 15%). Tholeiites displaying high La/Yb ratios ( > 3 ) can still be obtained by a lower degree of partial melting but this model cannot explain their FeO*/TiO2 ratios and Nb (and Ta) negative anomalies. Thus, rather than low-degree partial melting of a garnet lherzolite source, we suggest that the increase in La/Yb in tholeiites with La/Yb > 3, resulted from assimilation of a crustal component during magmatic differentiation. Calc-alkaline volcanic rocks Grlinas et al. (1984) and Grlinas and Ludden (1984) suggested that calc-alkaline rocks of the BRG were generated by mixing and hybridization of tholeiites with crustal melts represented by the BRG rhyolites. Available rhyolite analyses (Laflrche, 1986; Ujike and Goodwin, 1987, Camir6 and Watkinson, 1990) have an average La/Yb ratio of 3.5, fiat and

230

nearly chondritic HREE patterns and, strong Eu negative anomalies. The rhyolites are also characterized by nearly chondritic Hf/Sm and low Z r / Y ratios ( < 4). From these incompatible element ratios, simple mixing between rhyolitic melts and tholeiites cannot produce the large range of compositions observed in the calc-alkaline rocks. This is shown in Fig. 13a where a calculated mixing line (D) between tholeiites and rhyolites is presented. Smith (1980) and Capdevila et al. ( 1982 ) suggested that the calc-alkaline rocks of the BRG were generated by a low degree of partial melting of an unmodified upper mantle source containing garnet. The low Y, Yb and Sc contents (Table 1 ) as well as low Y/Tb (34 _+2 ) and high La/Yb (6.8_+1.8) and Tb/Yb (0.31 _+0.03) ratios (Table 3) imply the presence of garnet in the residual source of the calc-alkaline basaltic andesites. The high La/Yb ratio of the calc-alkaline andesites can be explained by less than 5% partial melting of a garnet lherzolite source (Fig. 16 ). The calculated model shows a weak but distinct increase of Yb abundance in derivative melts with decreasing degree of partial melting. Such an increase in Yb abundances with increasing La/Yb values is not observed in BRG calc-alkaline andesites. In order to produce a slight decrease in Yb abundances, such as observed in the calc-alkaline rocks, the calculated model should involve a bulk distribution coefficient for Yb higher than 1. Assuming that Kd (gt/liq Yb) = 4 and Ko (cpx/liq Y b ) = 0 . 6 (Philpotts et al., 1972; Irving, 1980; Shimizu and Kushiro, 1975 ), this implies that at least 20% of garnet in addition to 33% clinopyroxene should be present in the source assemblage. Such a large amount of garnet is not characteristic of mantle assemblages. Furthermore, partial melting of an unmodified mantle garnetiferous source cannot explain the observed variations of several ratios of incompatible trace elements such as N b / T h (3.8_+1), Nb/Ta (14.2_+2) and Zr/Sm (31 _+ 1 ) (e.g. Table 3). The decrease of the Ta/La ratio may suggest that the parental

M.R. LAFLECHE ET AL.

mantle source has suffered some sort of subduction influence prior to melting, but this is unlikely since interlayered basaltic tholeiites do not show chemical evidence of such a process. Furthermore, there is no evidence, at present, that subduction processes fractionate ratio such as Nb/Ta. Indeed, Davidson and Wolff ( 1989 ) have demonstrated that the average Nb/Ta ratio of island arc rocks approximates 16 and is similar to other oceanic magmas and to chondrites. Titanate phases such as rutile and sphene readily accomodate Nb and Ta (e.g. Wolff, 1984; Green and Pearson, 1987). The presence of these phases in the source may fractionate the Nb/Ta, Ta/La and Nb/Yb ratios because Ka (rutile/liq Ta) is higher than for Nb, La, and Yb. From the experimental studies of Tatsumi and Nakamura (1986), typical subduction related processes do not fractionate ratios such as N b / Y b which remain close to unity in residuum and/or derivative melts. The particular chemical behavior of these elements (low solubility) is also corroborated by the variation in N b / Y b ratios (0.82_+0.2) in subduction related lavas from oceanic arcs (McCulloch and Gamble, 1991 ). The strong fractionation of this ratio in the BRG calc-alkaline rocks (from 1.75 in Group C1 to 3.8 in group C3 basaltic andesites) also argues against a typical "modern type" subduction related origin and outlines the strong influence of garnet in the residuum during partial melting. A crustal origin for calc-alkaline rocks is inferred from the requirements of abundant garnet and the presence of titanates (rutile) in the parental source (e.g. Hellman and Green, 1979). Rutile is often recognized in garnet granulites, eclogites as well as in garnet amphibolites (e.g. Sorensen and Grossman, 1989 ) but it has been demonstrated that, for mantle compositions and pressure-temperature conditions, the high solubility of Tirich accessory phases in basaltic melts prevents their retention in the source region during partial melting (Green and Pearson, 1986 ). To evaluate the origin of BRG-type calc-al-

ARCHEANMAFICVOLCANICROCKSOF THE SOUTHERNABITIBIBELT,QUEBEC

kaline rocks, compositions produced by partial melting of metabasaltic crust under various P - T conditions are illustrated in Fig. 17. In these models, liquids produced by partial melting of garnet-bearing rocks (A: plagioclase amphibolite; B: garnet amphibolite; C: garnet granulite; D: garnet eclogite) display an enrichment in La/Yb associated with a systematic decrease in Yb abundance. BRG lavas are obviously not related to a plagioclase amphibolitic source (trend A), nor to DIIc II II

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~

Yb

Fig. 17. ( L a / Y b ) - Yb diagram for BRG calc-alkaline rocks; hexagons: group CI; squares: group C2; triangles: group C3. Tholeiitic rocks plotted for comparison; dots: BRG group I; shaded fields: BRG groups II and III; field 1: SAB tholeiites from the lower sequence. Trends A to D defined by the composition of derivative liquids produced by partial melting of a metabasaltic source (half circle) under different P-Tconditions. A: plagioclase amphibolite (45% plag + 55% am); B: garnet amphibolite (10% garnet + 90% am); C: garnet granulite (25% opx + 35% cpx + 30% garnet + 10% plag); D: garnet eclogite (50% cpx + 50% garnet). Kd for am, garnet, plag, cpx, opx taken from Irving and Frey (1984) and Martin (1987). Fractional crystallization trend (F.C.) calculated with Nielsen's CHAOS. 5 program.

231

a typical garnet amphibolitic source (trend B), as has been suggested for the origin of Archean calc-alkaline rocks (e.g. Jensen, 1985). According to experimental petrology (e.g. Beard and Lofgren, 1989), a degree of partial melting as low as 10% (estimated on the basis of the La/Yb ratios (Fig. 17 ) ) of a garnet amphibolite source, yields low Mg# and SiO2-rich tonalitic melts which are not encountered in BRG calc-alkaline rocks. BRG calc-alkaline rocks do not display concave-up REE patterns (Fig. 1 l a). The absence of concave-up patterns is mainly due to the presence of garnet and to the absence of amphibole in the residuum (e.g. Hildreth and Moorbath, 1988). The meta-aluminous nature of the BRG calcalkaline rocks is attributed to the presence of Al-rich phases, such as garnet, in the residuum. According to Beard and Lofgren (1989), residual amphibole leads to per-aluminous magmas during water-saturated or dehydration partial metling of a metabasaltic source. Given the particular composition of BRG calc-alkaline rocks, amphiboles were probably not present in their residual source in major proportions. According to the particular behavior of the REE and HFSE as well as the meta-aluminous nature of the BRG rocks, the origin of BRG calc-alkaline magmatism may be related to a pyroxene-rich mafic garnet granulite or garnet eclogite crustal sources. Such metamorphic conditions in Archean Greenstone belts suggest the presence of an underplated thickened crust or the subduction of oceanic basaltic crust.

Geodynamic considerations and concluding remarks Two broad groups of tectonic models have been proposed to explain the evolution of the southern Abitibi belt. The first has Phanerozoic plate tectonic analogues (e.g. Condie and Baragar, 1974; Dimroth et al., 1983; G61inas and Ludden, 1984; Ujike and Goodwin, 1987 ).

232

The second series is unique to the Archean and invokes the cyclical development of gravitational instabilities producing a geodynamic regime dominated by sagging of dense ultramafic to mafic crust, partial melting of the lower crust, and tonalitic diaperism (e.g. Gorman et al., 1978; Goodwin and Smith, 1980; Jensen, 1985). Unlike early Archean greenstone belts (e.g. Campbell and Jarvis, 1984) the late Archean SAB is characterized, in its lower stratigraphic part by abundant felsic volcanism. In Quebec, the first 15 to 20 Ma of volcanism (2735-2720 Ma), which has been considered elsewhere (Lafl6che and Ludden, 1991 ), was dominated by bimodal calc-alkaline magmatism (the Hunter Mine Group). Low K20 felsic volcanic rocks, which dominate this group, display chemical compositions typical of Archean aluminous tonalites and trondjhemites (Barker and Arth, 1976; Martin, 1987). Hunter Mine group rhyodacites and rhyolites are interpreted has being derived from partial melting of metabasaltic crust followed by low- pressure differentiation prior to erruption. The absence of herited zircon in these felsic rocks and the depleted mantle Nd isotopic signature of co-magmatic plutons (Mortensen, 1987; Corfu et al., 1989; Pintson et al., 1991 ) do not favour processes of underplating and partial melting of sialic crust (e.g. Hildreth and Moorbath, 1988; Bergantz, 1989 ). Subduction of hot, young Archean oceanic lithosphere, in the central part of the Abitibi belt, may explain the origin of calc-alkaline magmatism in the Hunter Mine Group. Following Bickle ( 1978 ), Martin (1987), Arkani-Hamed and Jolly (1989), partial melting of the basaltic part of the oceanic lithosphere, under the high Archean mantle geotherm, inhibited dehydration processes before the beginning of partial melting of the subducted plate. This led to tonalitic melts instead of initiating hydrous partial melting of overlying mantle peridotites. The lack of major plate dehydration processes and subsequent hydrous metasomatism (LILE en-

M.R. LAFLECHE ET AL.

richment) of the overlying mantle wedge, may explain why typical modern arc tholeiites and calc-alkaline rocks are generally observed neither in the Abitibi Greenstone belt or elsewhere in the Superior Province. Hunter Mine Group calc-alkaline magmatism was followed in the SAB by 15 Ma of voluminous tholeiitic and komatiitic volcanism (Mortensen, 1987; Corfu et al., 1989). The tholeiites and komatiites are uncontamined by a sialic component (Dupr6 et al., 1984; Machado et al., 1986). There is no chemical evidence of modern subduction-type processes during development of tholeiitic magmatism in the SAB; trace elements in basaltic rocks do not show HFSE and Ti anomalies typical of Phanerozoic tholeiites erupting in island-arc or back-arc basins (e.g. Hochstaeder et al., 1990) and magmatic differentiation did not occur under the high PH2oand fo2 conditions of typical arc magmatic series (e.g. Ballhaus et al., 1990). Komatiite, Mg and Fe tholeiites are interpreted as partial melts of adiabatically upwelling asthenospheric mantle beneath a rift. Decrease of lithospheric stretching and strong convective cooling of mantle diapirs in the rift through time, may explain the presence of MgO-rich basaltic tholeiites (high FeO*/TiO2 and CaO/A1203 ratios) associated with komatiites in the lower volcanic sequence. The compositions of these rocks suggest high degree partial melting of lherzolites. On the other hand, basaltic tholeiites from the upper volcanic sequence display chemical characteristics impling low degrees of partial melting of a lherzolitic source (low FeO*/TiO2 and CaO/ A1203 ratios) which suggest cooling of the mantle diapirs. Volcanic facies observed in the upper volcanic sequence of the SAB suggest significant rheological modification of the crust through time. Large, high-level differentiated magma chambers, which are not present in the lower volcanic sequence, are inferred from highly differentiated volcanic series and the presence of cauldrons in the SAB (Gibson, 1990). The

ARCHEAN MAFIC VOLCANIC ROCKS OF THE SOUTHERN ABITIBI BELT, QUEBEC

presence of numerous differentiated felsic synvolcanic sills and plutons within the low-grade metamorphic rocks (e.g. Jolly, 1980) of the BRG argues for a relatively thick simatic crust at the time of extrusion. It is noteworthy that such siliceous magma chambers are not observed on normal modern oceanic crust. Abundant injection of tonalitic magmas at the base of the Abitibi crust during subduction of oceanic lithosphere may explain the increased thickness of the crust and its accelerated stabilization. Decrease in the rate of lithospheric stretching during the last 5 Ma of volcanism in the SAB (BRG volcanism ) marked the beginning of closure of the volcanic basins and a change from an extensional to a new compressional tectonic regime. The new tectonic environment, which culminated 10 Ma latter in the major Kenorean orogeny, was characterized by calc-alkaline mafic volcanism which is interpreted as evidence of subduction processes (without the involvement of hydrous metasomatism of the mantle produced by slab dehydration ). Increasing rate of subduction just before deformational events (~2680 Ma; Jemielita et al., 1990; Corfu et al., 1991 ) may have permitted subduction of metasedimentary rocks (greywackes) which probably yields fuids enriched in LILE elements in order to explain the production of shoshonitic ultrapotassic volcanism and plutonism (Lafl~che et al., 1991). The shoshonitic magmatism, less voluminous than precedent volcanism in the SAB, postdades by 10 to 15 Ma volcanism in upper volcanic sequence of the SAB. It may be related to translithospheric shear zones (dextral wrenching) which probably initiate partial melting of strongly metasomatised mantle lithosphere beneath the SAB crust. Such interpretation is in agreement with structural observations (e.g. Lafl~che, 1991 ) which demonstrate the relation of ultrapotassic plutons and dykes to the vicinity of major dextral transcurent faults. Given the particular rheological and ther-

233

mal characteristics of early Archean crust, lithosphere and asthenosphere (England and Bickle, 1984; Hargraves, 1986; Richter, 1988), "sagduction" models may explain the geodynamic evolution of early Archean terranes such as present in Rhodesia and South Africa (e.g. Gorman et al., 1978; Vlaar, 1986). However, when these models are applied to the evolution of late Archean terranes such as encountered in the southern Superior Province, major geological problems make such models unlikely. For example, and contrary to "sagduction" models, major deformational events and metamorphism are not contemporaneous with volcanism and usually postade by 10 to 15 Ma the end of volcanic activity in the greenstone belt. On the other hand, the tectonic style recorded in deformed supracrustal rocks gives information about the geodynamic regime which prevailed soon after the end of volcanism. The low volume of pre to syn-kinematic plutons in the SAB (OGS-MERQ, 1984) and the fact that kinematic indicators demonstrated strong flatening, reverse faulting and locally late dextral horizontal movements (no normal faulting in the vicinity of pluton margins as predicted for classical diaperism) support the idea that gravity tectonics was not the major mechanism controling the geodynamic evolution of the greenstone belt. According to recent structural (Robert, 1989; Daigneault and Archambault, 1990), metamorphic (Jolly, 1980; Camir6 and Burg, 1991 ) and geophysical (Green et al., 1990) studies, a model of large scale horizontal shortening (dominantly N-S) in the Abitibi Belt with formation of nappe structures and barovian metamorphism in the southern extension of the Abitibi Belt (Camir6 and Burg, 1991 ) suggests a primitive form of plate collisional tectonics. Finally, such a model suggested for the SAB may be incorporated in a more general tectonic framework of accretion tectonics recently proposed for the entire Superior Province of the Canadian Shield (Card, 1990).

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Aknowledgements This study was conducted as part of the Ph.D. thesis requirements of M.R. Lafl6che at the Universit6 de Montpellier II (France). In Canada, the research was supported and financed by Cambior Inc.. Sincere appreciation is expressed to M.A.J. Ouellet, director of exploration and his team of field geologists in Abitibi. A first review of the manuscript was completed by G. Camir6, N.T. Arndt and T.C. Birkett and J.N. Ludden. M.R. Lafl6che aknowledges informative discussions with J.L. Bodinier, Ph. Vidal, C. Coulon and J.P. Bard. We acknowledge financial support from C.N.R.S. (France) and F.C.A.R. (Qu6bec).

References Ambrose, J.W., 1941. Rdgions de Cldricy et de la Pause, Qudbec. Can. Geol. Survey Mem., 233. Arkani-Hamed, J. and Jolly, W.T., 1989. Generation of Archean tonalites. Geology, 17:307-310. Arth, J.G. and Baker, F., 1976. Rare earth partitioning between hornblende and dacitic liquid and implications for the genesis of trondjhemitic tonalitic magmas. Geology, 4: 534-536. Babineau, J., 1982. Evolution g6ochimique et p6trologique des sdries volcaniques de la r6gion de CadillacMalartic. M6moire de mahrise, Univ. Montr6al, 130 PP. Baker, D.R. and Eggler, D.H., 1987. Compositions of anhydrous and hydrous melts coexisting with plagioclase, augite, and olivine or low-Ca pyroxene from 1 arm to 8 kbar: application to the Aleutian volcanic center of Atka. Am. Mineral., 72: 12-28. Ballhaus, C., Berry, R.F. and Green, D.H., 1990. Oxygen fugacity controls in the Earth's upper mantle. Nature, 348: 437-440. Baragar, W.R.A., 1968. Major element geochemistry of the Noranda volcanic belt, Quebec, Ontario. Can. J. Earth Sci., 5: 773-790. Barker, F. and Arth, J.G., 1976. Generation of trondhjemitic-tonalitic liquids and Archean bimodal trondhjemite-basalt suites. Geology, 4: 596-600. Barley, M.E., Sylvester, G.C. and Groves, D.I., 1984. Archean calc-alkaline volcanism in the Pilbara Block, Western Australia. Precambrian Res., 24, 295-319. Barnes, S.J., 1985. The petrography and geochemistry of Komatiite flows from the Abitibi greenstone belt and a model for their formation. Lithos, 18: 241-270. Barrie, C.T. and Shirey, S.B., 1989. Geochemistry and Nd-

M.R. LAFLECHEETAL Sr isotope systematics of the Kamiskotia area, western Abitibi Subprovince, Canada: implications for mantle processes during the formation of the Southern Superior Craton. Abstract from Lunar and Planetary Institute, Houston, TX, Workshop on the Archean Mantle, pp. 11-13. Basaltic Volcanism Study Project, 1981. Basaltic Volcanism on the Terrestrial Planets, Pergamon, New York. Baxter, A.N, Upton, B.G.J. and White, W.M., 1985. Petrology and Geochemistry of Rodrigues Island, Indian Ocean. Contrib. Mineral. Petrol., 8: 90-101. Beard, J.S and Lofgren, G.E., 1989. Effect of water on the composition of partial melts ofgreenstone and amphibolite. Science, 244:195-197. Bergantz, G.W., 1989. Underplating and partial melting: Implications for melt generation and extraction. Nature, 245: 1093-1095. Bickle, M.J., 1978. Heat loss from the Earth: a constraint on Archean tectonics from relation between geothermal gradients and the rate of plate production. Earth Planet. Sci. Lett., 40:301-315. Bickle, M.J., Bettenay, L.F., Barley, M.E., Chapman, H.J., Groves, D.I., Campbell, I.H. and Laeter, J.R., 1983. A 3500 Ma plutonic and volcanic calc-alkaline province in the Archean East Pilbara block. Contrib. Mineral. Petrol., 84: 25-35. Bodinier, J.-L., Dupuy, C. and Dostal, J., 1989. Geochemistry and petrogenesis of Eastern Pyrenean peridotites. Geochim. Cosmochim. Acta., 52: 2893-2907. Briqueu, L., Bougault, H. and Joron, J.L., 1984. Quantification of Nb, Ta, Ti and V anomalies in magmas associated with subduction zones: petrogenetic implications. Earth Planet. Sci. Lett., 68: 297-308. Brooks, C.K., Ludden, J.N., Pigeon, Y. and Hubregtse, J.J.M.W., 1982. Volcanism of shoshonite to high-K andesite affinity in an Archean arc environment, Oxford Lake, Manitoba. Can. J. Earth Sci., 19: 55-67. Camir6, G. E., 1989. Volcanic stratigraphy in the Hunter Creek Fault area, east of the Flavrian Pluton, RouynNoranda, Quebec. M.Sc. Thesis, Carleton Univ., Ottawa, Canada, 213 pp., unpubl. Camir6, G. and Burg, J.P., 1991. Tangential tectonics and inflating diapirism in the Western Pontiac Subprovince of the Canadian Shield. Precambrian Res., submitted. Camir6, G. and Watkinson, D.H., 1990. Volcanic stratigraphy and structure in the Hunter Creek Fault area, Rouyn-Noranda, Qu6bec. Can. J. Earth Sci., 27:13481358. Campbell I.H. and Jarvis, G.T., 1984. Mantle convection and crustal evolution. Precambrian Res., 26:15-56. Capdevila, R., Goodwin, A.M., Ujike, O. and Gorton, M.P., 1982. Trace element geochemistry of Archean volcanic rocks and crustal growth in southwestern Abitibi belt, Canada. Geology, 10:418-422. Card, K.D., 1990. A review of the Superior Province of

ARCHEAN MAFIC VOLCANIC ROCKS OF THE SOUTHERN ABITIBI BELT, QUEBEC

the Canadian Shield, a product of Archean accretion. Precambrian Res., 48:99-156. Cartel, A., Krogh, T.E. and Amdt, N.T., 1984. Conflicting Sm-Nd whole rock and U-Pb zircon ages for Archean lavas from Newton Township, Abitibi, Ontario. Earth Planet Sci Lett., 70: 280-290. Condie, K.C., 1989. Geochemical change in basalts and andesites aross the Archean-Proterozoic boundary: identification and significance. Lithos, 23: 1-18. Condie, K.C. and Baragar, W.R.A., 1974. Rare-earth element distributions in volcanic rocks from the Archean greenstone belts. Contrib. Mineral. Petrol., 45: 237246. Condie, K.C., Viljoen, M.J. and Kable, E.J.D., 1977. Effects of alteration on element distributions in Archean tholeiites from the Barberton greenstone belt, South Africa. Contrib. Mineral. Petrol., 64: 75-89. Cooke, D.L. and Moorhouse, W.W., 1969. Timiskaming volcanism in the Kirkland Lake area, Ontario, Canada. Can. J. Earth Sci., 6:117-132. Corfu, F., Krogh, T.E., Kwok, Y.Y., and Jensen, L.S., 1989. U-Pb zircon geochronology in the southwestern Abitibi Greenstone Belt, Superior Province. Can. J. Earth Sci., 26: 1747-1763. Corfu, F., Jackson, S.L. and Sutcliffe, R.H., 1991. U-Pb ages and tectonic significance of late Archean alkalic magmatism and nonmarine sedimentation: Timiskaming Group, southern Abitibi belt, Ontario. Can. J. Earth Sci., 28: 489-503. Daigneault, R. and Archambault, G.,1990. Les grands couloirs de d6formation de la sous-province de l'Abitibi. In: M. Rive, P. Verpaelst, G. Riverin, J.M. Lullin and A. Simard (Editors), C.I.M. Symposium on the Abitibi Polymetallic Belt. Canadian Institute of Mining, Spec. Vol. 20: 43-64. Davidson, J.P. and Wolff, J.A., 1989. On the origin of the Nb-Ta "anomaly" in arc magmas. EOS: 1989. Defant, M.J and Drummond, M.S., 1990. Deri~cation of some modern arc magmas by melting of young subducted lithosphere. Nature, 347: 662-665. Defant, M.J. and Nielsen, R.L., 1990. Interpretation of open system petrogenetic processes: Phase equilibria constraints on magma evolution. Geochim. Cosmochim. Acta, 54: 87-102. DePaolo, D.J., 1981. Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization. Earth Planet. Sci. Lett., 53: 189-202. Dickin, A.P. and Jones, N.W., 1983. Relative elemental mobility during hydrothermal alteration of a basic sill, Isle of Skye, N.W. Scotland. Contrib. Mineral. Petrol., 82: 147-153. Dimroth, E., Imreth, U, Goulet, N. and Rocheleau, M., 1983. Evolution of the south-central segment o f the Abitibi Belt, Quebec. Part 3: Plutonic and metamorphic evolution and geotectonic model. Can. J. Earth Sci., 20: 1374-1388.

235

Dimroth, E., Imreth, L., Cousineau, P., Leduc, M., and Sanschagrin, Y., 1985. Paleogeographic analysis of mafic submarine flows and its use in the exploration for massive sulphide deposits. In: L.D. Ayres, P.C. Thurston, K.D. Card and W.Weber (Editors), Evolution of Archean Supracrustal Sequences. Geol. Assoc. Can. Spec. Pap., 28: 203-222. Dostal. J., Baragar, W.R.A. and Dupuy, C., 1986. Petrogenesis of the Natkusiak continental basalts, Victoria Island, N.W.T. Can. J. Earth Sci., 23: 622-632. Drake, M.J. and Weill, D.F., 1975. The partition of Sr, Ba, Ca, Y, Eu 2+, Eu 3+ and other REE between plagioclase feldspar and magmatic silicate liquid: an experimental study. Geochim. Cosmochim. Acta., 39: 689712. Drummond, M.S. and Defant M.J., 1990. Trondhjemitetonalite-dacite (TTD) genesis via partial melting of the subducted slab. A uniformitarianism model for crustal growth. EOS, 71: 43. Dupr6, B., Chauvel, C. and Arndt, N.T., 1984. Pb and Nd isotopic study of two Archean komatiitic flows from Alexo, Ontario. Geochim. Cosmochim. Acta, 48:19651972. England, P. and Bickle, M.J, 1984. Continental thermal and tectonic regimes during the Archean. J. Geol., 92(4): 353-367. Fuji, T. and Scarfe, C.M., 1985. Composition of liquids coexisting with spinel lherzolite at 10 kbar and the genesis of MORBs. Contrib. Mineral. Petrol., 90:1828.

G61inas, L. and Ludden, J.N., 1984. Rhyolitic volcanism and the geochemical evolution of an Archaean central ring complex: the Blake River Group volcanics of the southern Abitibi belt, Superior province. Phys. Earth Planet. Int., 35: 77-88. G61inas, L., Brooks, C., Perrault, G., Carignan, J., Trudel, P. and Grasso, F., 1977. Chemo-stratigraphic divisions within the Abitibi Volcanic belt, Rouyn-Noro anda District, Quebec. In: W.R.A. Baragar, L.C. Coleman and J.M. Hall (Editors), Volcanic Regimes in Canada. Geol. Assoc. Can. Spec. Pap., 16: 265-295. G61inas, L., Trudel, P. and Hubert, C., 1984. Chemostratigraphic division of the Blake River Group, RouynNoranda area, Abitibi, Qu6bec. Can. J. Earth Sci., 21: 220-231. Gibson, H.L., 1990. The mine sequence of the central Noranda volcanic geology, alteration, massive sulphide deposits and volcanological' reconstruction. Ph.D. Thesis, Carleton Univ., Ottawa, Ont., unpubl. Goodwin, A.M. and Ridler, R.H., 1970. The Abitibi orogenic belt. Symp. on Basins and Geosynclines of the Canadian Shield. Geol. Surv. Can. Pap., 70-40: 1-24. Goodwin, A.M. and Smith, E.M., 1980. Chemical discontinuities in Archean volcanic terrains and the development of Archean Crust. Precambrian. Res., 10: 301311.

236

Gorman, B.E., Pearce, T.H. and Birkett, T.C., 1978. On the structure of Archean Greenstone belts. Precambrian Res., 6: 23-41. Goulet, N., 1978. Stratigraphy and structural relationship accross the Cadillac-Larder Lake Fault, Rouyn-Beauchastel area, Quebec, Ph.D. thesis, Queen's Univ., Kingston, Ont. Grant, J.A, 1986. The isocon diagram. A simple solution to Gresens' equation for metasomatic alteration. Econ. Geol., 81: 1976-1982. Green, A.G., Milkereit, B., Mayrand, L.J., Ludden, J.N., Hubert, C., Jackson, S.L., Sutcliffe, R.H., West, G.F., Verpaelst, P. and Simard, A., 1990. Deep structure of an Archaean greenstone terrane. Nature, 344: 27-330. Green, T.H. and Pearson, N.J., 1986. Ti-rich accessory phase saturation in hydrous mafic-felsic compositions at high P, T. Chem. Geol., 54: 185-201. Green, T.H. and Pearson, N.J., 1987. An experimental study of Nb and Ta partitioning between Ti-rich minerals and silicate liquids at high pressure and temperature. Geochim. Cosmochim. Acta, 51: 5562. Gresens, R.I_., 1967. Composition-volume relationships of metasomatism. Chem. Geol., 2: 47-65. Grove, T.L. and Baker, M.B., 1983. Phase equilibrium controls on the tholeiitic vs calc-alkaline differentiation trends. J. Geophys. Res., 89: 3253-3274. Hargraves, R.B., 1986. Faster spreading or greater ridge length in the Archean? Geology, 14: 750-752. Hashigushi, H., Yamada, R. and Inoue, T., 1983. Practical application of low Na20 anomalies in footwall acid lava for delimiting promising areas around the Kosaka and Fukazawa Kuroko deposits, Akita Prefectural, Japan. Econ. Geol., 387-394. Hellman, P.L. and Green, T.H., 1979. The role of sphene as an accessory phase in the high-pressure partial melting of hydrous mafic compositions. Earth Planet. Sci. Lett., 42: 191-201. Hildreth, W. and Moorbath, S., 1988. Crustal contributions to arc magmatism in the Andes of Central Chile. Contrib. Mineral. Petrol., 98: 455-489. Hochstaeder, A.G., Gill, J.B., and Morris, J.D., 1990. Volcanism in the Sumisu Rift, 1I. Subduction and nonsubductin related components. Earth Planet. Sci. Lett., 100: 195-209. Irvine, T.N. and Baragar, W.R.A., 1971. A guide to the chemical classification of the common volcanic rocks. Can. J. Earth Sci., 8: 523-548. Irving, A.J., 1980. Petrology and geochemistry of composite ultramafic xenoliths in alkalic basalts and implications for magmatic processes within the mantle. Am. J. Sci., 280-A: 389-426. Irving, A.J. and Frey, F.A., 1984. Trace element abundances in megacrysts and their host basalts: Constraints on partition coefficients and megacryst genesis. Geochim. Cosmochim. Acta., 48:1201-1221. Jahn, B.M., Gruau, G.~ and Glikson, A.Y., 1981. Koma-

M.R. LAFLECHE ET AL.

tiites of the Onverwacht Group, S. Africa: REE geochemistry, S m / N d age and mantle evolution. Contrib. Mineral. Petrol., 80: 25-40. Jaques, A.L. and Green, D.H., 1980. Anhydrous melting of peridotite at 0-15 kb pressure and the genesis of tholeiitic baslats. Contrib. Mineral. Petrol., 73: 287310. Jemielita, R.A., Davis, D.W. and Krogh, T.E., 1990. U Pb evidence for Abitibi gold mineralization postdating greenstone magmatism and metamorphism. Nature, 346:831-834. Jensen, L.S., 1976. A new cation plot for classifying subalkalic volcanic rocks. Ont. Div. Mines, Misc. Pap., 66: 22 pp. Jensen, L.S., 1985. Stratigraphy and petrogenesis of Archean metavolcanic sequences, southwestern Abitibi Subprovince, Ontario. In: L.D. Ayres, P.C. Thurston, K.D. Card, and W. Weber (Editors) Evolution of Archean Supracrustal Sequences. Geol. Ass. Can. Spec. Pap., 28: 65-87. Jochum, K.P., Seufert, H.M., Spettel, B. and Palme, H., 1986. The solar system abundances of Nb-Ta and Y, and the relative abundances of refractory lithophile elements in differentiated planetary bodies. Geochim. Cosmochim. Acta, 50:1173-1183. Jochum, K.P., Seufert, H.M. and Thirlwall, M.F., 1990. High-sensitivity Nb analysis by spark-source mass spectrometry (SSMS) and calibration o f X R F Nb and Zr. Chem. Geol., 81 : t - l 6. Jolly, W.T., 1980. Development and degradation of Archean lavas, Abitibi area, Canada, in light of major element geochemistry. J. Petrol., 21: 323-363. Klein, E.M. and Langmuir, C.H. 1987. Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness. J. Geophys. Res., 92:8089-8115. Eaflbche, M.R., 1986. Pdtrochimie et volcanologie du complexe rhyolitique de Don, Rouyn-Noranda, Qudbec. M.Sc. Thesis, Universit6 de Montr6al, Montrdal, Que., 189 pp., unpubl. gaflbche, M.R., 1991. Evolution du magmatisme archden de la partie sud de la ceinture de l'Abitibi. Ph.D. Thesis, Universit6 de Montpellier It, unpubl. Laflbche, M.R. and Eudden, J.N., 1991. The petrogenesis ofArchean felsic magmas from the Southern Superior Province: Implications for Late Archaean felsic magma genesis. GAC-MAC 1991, Toronto, Program with Abstracts, Vol. 16, p. 70. Lafl~che, M.R., Dupuy, C. and Dostal, J.. 1991. Archean Orogenic Ultrapotassic Magmatism: an example from the Southern Abitibi Greenstone Belt. Precambrian Res., 52:71-96. Lesher, C.M., Gibson, H.L. and Campbell, I.H., 1986. Composition-volume changes during hydrothermal alteration of andesite at Buttercup Hill, Noranda District, Quebec. Geochim. Cosmochim. Acta, 50: 26932705.

ARCHEANMAFICVOLCANICROCKSOF THE SOUTHERNABITIBIBELT,QUEBEC Ludden, J.N., G61inas, L. and Trudel, P., 1982. Archean metavolcanics from the Rouyn-Noranda district, Abitibi Greenstone Belt, Quebec. 2. Mobility of trace elements and petrogenetic constraints. Can. J. Earth Sci. 19: 2276-2287. Machado, N., Brooks, C. and Hart, S.R., 1986. Determination of initial STSr/865r and ~43Nd/~44Nd in primary minerals from mafic and ultramafic rocks: experimental procedure and implications for the isotopic characteristics of the Archean mantle under the Abitibi greenstone belt, Canada. Geochim. Cosmochim. Acta., 50: 2335-2348. Martin, H., 1987. Petrogenesis ofArchean trondhjemites, tonalites, and granodiorites from eastern Finland: major and trace element geochemistry. J. Petrol., 28:921953. McCulloch, M.T. and Gamble, J.A., 1991. Geochemical and geodynamical constraints on subduction zone magmatism. Earth Planet. Sci. Let., 102: 358-374. Michael, P.J. and Chase, R.L., 1987. The influence of primary magma composition, H20 and pressure on midocean ridge basalt differentiation. Contrib. Mineral. Petrol., 96: 245-263. Mortensen, J.K., 1987. Preliminary U - P b zircon ages for volcanic and plutonic rocks of the Noranda-Lac Abitibi area, Abitibi Subprovince, Quebec. In: Current Research, Part A, Geol. Surv. Canada, Pap. 87-1a, pp. 581-590. Nakamura, N., 1974. Determination ofREE, Ba, Mg, Na, and K in carbonaceous and ordinary chondrites. Geochim. Cosmochim. Acta, 38: 757-775. Nielsen, R.L., 1988. A model for the simulation of combined major and trace element liquid line of descent. Geochim. Cosmochim. Acta, 52: 27-38. Nystr(Sm, J.O., 1984. Rare earth element mobility in vesicular lava during low-grade metamorphism. Contrib. Mineral. Petrol., 88:328-331. O.G.S.-M.E.R.Q., 1984. Lithostratigraphic map of the Abitibi Subprovince, Ontario Geological Survey / Minist~re de 1' Energie et des Ressources du Qu6bec, Map 2484 in Ontario, D.V. 83-16 in Qu6bec. Ojakangas, R.W., 1985. Review of Archean clastic sedimentation, Canadian Shield: major felsic volcanic contributions to turbidite and alluvial fan-fluvial associations. In: L.D. Ayres, P.C. Thurston, K.D. Card and W. Weber (Editors), Evolution of Archean Supracrustal Sequences. Geol. Assoc. Can. Spec. Pap., 8: 2348. Paradis, S., Ludden, J.N. and G61inas, L., 1989. Evidence for contrasting compositional spectra in comagmatic intrusive and extrusive rocks of the late Archaean Blake River Group, Abitibi, Quebec. Can. J. Earth Sci., 25: 134-144. Parent, G., 1985. G6ochimie du groupe de Malartic, d'~ge arch6en, r6gion de l'Abitibi. M.Sc. Thesis, Univ. de Montr6al, Montr6al, Qu6., unpubl.

237

Pearce, J.A. and Norry, M.J., 1979. Petrogenetic implications of Ti, Zr, Y, and Nb variations in volcanic rocks. Contrib. Mineral. Petrol., 69: 33-47. Percival, J.A., 1986. A possible exposed Conrad discontinuity in the Kapuskasing uplift, Ontario. In: Reflection Seismology: The Continental Crust. Geodyn. Ser., 14. Am. Geophys. Union, Washington, DC, pp. 135141. Perfit, M.R. and Fornary, D.J., 1983. Geochemical studies of abyssal lavas recovered by DSRV Alvin from Eastern Galapagos Rift. Inca Transform and Ecuador Rift. 2. Phase chemistry and crystallization history. J. Geophys. Res., 88: 10530-10550. Philpotts, J.A., 1970. Redox estimation from a calculation of Eu 2+ and Eu 3+ concentrations in natural phases. Earth Planet. Sci. Lett., 9: 257-268. Philpotts, J.A. and Schnetzler, C.C., 1970. Phenocrystmatrix partition coefficients for K, Rb, Sr and Ba with applications to anorthosite and basalt genesis. Geochim. Cosmochim. Acta., 34: 307-322. Philpotts, J.A., Schnetzler, C.C. and Thomas, H.H., 1972. Petrogenetic implications of some new geochemical data on eclogitic and ultrabasic inclusions. Geochim. Cosmochim. Acta., 36:1131-1166. Pintson, H., Ludden, J.N., Jahn, B.M. and Rive, M., 1991. S m - N d constraints on the origin of late Archean granitoid rocks, central Abitibi and Pontiac subprovinces, Superior Province, Canada. GAC-MAC 1991, Toronto. Program with Abstracts, Vol. 16. Rajamani, V., Shirey, S.B., and Hanson, G.N., 1989. Ferich Archean tholeiites derived from melt-enriched mantle sources: Evidence from the Kolar schist Belt, South India. J. Geol., 97:487-501. Robert, F., 1989. Internal structure of the Cadillac tectonic zone southeast of Val d'Or, Quebec. Can. J. Earth Sci., 26: 2661-2690. Richter, F.M., 1988. A major change in the thermal state of the Earth at the Archean-Proterozoic boundary: consequences for the nature and preservation of continental lithosphere. J. Petrol. Spec. Vol. Oceanic and Continental Lithosphere: Similarities and Differences., pp. 39- 53. Riverin, G. and Hodgson, C.J., 1980. Wall-rock alteration at the Millenbach Cu-Zn Mine, Noranda, Quebec. Econ. Geol., 75: 424-444. Saunders, A.D. and Tarney, J., 1979. The geochemistry of basalts from a back-arc spreading centre in the east Scotia sea. Geochim. Cosmochim. Acta., 43: 555-572. Saunders, A.D., Norry, M.J. and Tarney, J., 1988. Chemically-depleted mantle reservoirs: trace element constraints. In: M.A. Menzies and K.G. Cox (Editors), Special Volume 1988, Oceanic and Continental Lithosphere: Similarities and Differences. J. Petrol., pp. 415-445. Shirey, S.B. and Hanson, G.N., 1986. Mantle heterogeneity and crustal recycling in Archean granite-green-

238 stone blets: Evidence from Nd isotopes and trace elements in the Rainy Lake area, Superior Province, Ontario, Canada. Geochim. Cosmochim. Acta., 50: 2631-2651, Shimizu, N. and Kushiro, I., 1975. The partitioning of rare earth elements between garnet and liquid at high pressures: preliminary experiments. Geophys. Res. Lett.. 2: 413-416. Sinton, J.M. and Fryer, P., 1987. Mariana trough lavas from 180 N: Implications for the origin of back arc basin basalts. J. Geophys. Res., 92: 12782-12802. Smith, I.E.M., 1980. Geochemical evolution of the Blake River Group, Abitibi greenstone belt, Superior Province. Can. J. Earth Sci., 17: 1292-1299. Sorensen, S.S. and Grossman, J.N., 1989. Enrichment of trace elements in garnet amphibolites from a paleosubduction zone: Catalina Schist, southern California. Geochim. Cosmochim. Acta, 53:3155-3177. Staudigel, H., Muehlenbachs, K., Richardson, S.H. and Hart, S.R., 1981. Agents of low temperature ocean crust alteration. Contrib. Mineral. Petrol., 77:150-157. Sun, S.S. and Mc Donough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: A.D. Saunders and M.J. Norry (Editors), Magmatism in the Ocean Basins. Geol. Soc. Spec. Publ., 42: 313-345. Tatsumi, Y. and Nakamura, N., 1986. Composition of aqueous fluid from serpentinite in the subducted lithosphere. Geochem. J., 20: 191-196. Taylor, S.R. and McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, Oxford, 312 pp. Tass6, N., Lajoie, J. and Dimroth, E., 1978. The anatomy and interpretation of Archean volcaniclastic sequence, Noranda region, Qu6bec. Can. J. Earth Sci., 15: 874888.

M.R. LAFLECHE ET AL.

Trudel, P., 1979. Le volcanisme arch6en et la g6ologie structurale de la r6gion de C16ricy, Abitibi, Quebec. Th~se D.Sc. A., Ecole Polytechnique de Montr6al, 307 PP. Ujike, O, 1985. Geochemistry ofArchean alkalic volcanic rocks from the Crystal Lake Area, east of Kirkland Lake, Ont., Canada. Earth Planet. Sci. Lett., 73: 333344. Ujike, O. and Goodwin, A.M., 1987. Geochemistry and origin of Archean metavolcanic rocks, central Noranda area, Quebec, Canada. Can. J. Earth Sci., 24: 2551-2567. Viereck, L.G, Flower, M.F.J., Hertogen, J., Schmincke, H.V. and Jenner, G.A., 1989. The genesis and significance of N-MORB sub-types. Contrib. Mineral. Petrol., 102: 112-126. Vlaar, N.J., 1986. Archean Global Dynamics. Geol. Mijnbouw, 65: 91-101. Wedepohl, K.H., 1991. Chemical composition and fractionation of the continental crust. Geol. Rundsch., 80(2): 207-223. White, W.M. and Patchett, J., 1984. H f - N d - S r isotopes and incompatible-element abundances in island arcs: implications for magma origins and crust-mantle evolution. Earth Planet. Sci. Lett., 67: 167-185. Wolff, J.A., 1984. Variation in N b / T a during differentiation of phonolitic magma, Tenerife, Canary Islands. Geochim. Cosmochim. Acta, 48: 1345-1348. Wood, D.A., Joron, J.L. and Treuil, M., 1979. A reappraisal of the use of trace elements to classify and discriminate between magma series erupted in different tectonic settings. Earth Planet. Sci. Lett., 45: 326-336. Woodhead, J.D., 1989. Geochemistry of the Mariana arc (Western Pacific): source composition and processes. Chem. Geol., 76, 1-24.

239

ARCHEAN MAFIC VOLCANIC ROCKS OF THE SOUTHERN ABITIBI BELT, QUEBEC

Appendix I. Crystal fractionation and AFC calculations (a) Differentiation of the Mg-rich tholeiites

Element

Daughter

Ol

Pl

Cpx

Parent

Model parent

SiO2 TiO2 A1203 FeO* MnO MgO CaO Na20 KzO P205 Wt. prop.

50.9 1.25 15.59 I 1.54 0.22 7.43 9.32 2 0.43 O. 11 73.4

40 0 0 12 0 47 0 0 0 0 6.4

50 0 32 0 0 0 15 3 0 0 12.8

52.6 0.95 2.3 9.2 0 15.5 19 0 0 0 6.9

50.0 0.89 15.69 9.8 0.17 9.52 10.06 2.27 0.12 0.08

49.97 0.99 15.76 9.86 0.16 9.53 10.10 1.86 0.32 0.08

S(residuals) 2:0.2315. The daughter rock is the average group Ib Fe-rich basalts (see Table 1 ) whereas the parent is the average of 11 Mg tholeiites of the Lac Pelletier lower units of the BRG (e.g. G61inas et al., 1984). Mineral composition for olivine is in agreement with a distribution coefficient (Ko= Mg/Fe olivine: Mg/Fe liquid of 0.30 with total Fe calculated as FeO*. Plagioclase and Cpx compositions were determined by CHAOS 5 (e.g. Defant and Nielsen, 1990). (b) AFC calculations for the BRG Fe-rich basalts

Element

Daughter

O1

P1

Cpx

C.A.

Parent

Model parent

SiO2 TiO2 A1203 FeO* MnO MgO CaO Na20 K20 P205 Wt. prop.

56.0 1.7 13.15 14.6 0.26 3.57 6.17 2.37 0.51 0.2 68

39.5 0 0 19.3 0 41 0 0 0 0 9.6

50 0 32 0 0 0 15 3 0 0 29.7

52.6 0.95 2.3 9.2 0 19 19 0 0 0 11.1

58.2 0.77 16.21 6.59 0.12 5.33 7.85 4 0.27 0.16 (-20)

50.9 1.25 15.59 11.54 0.22 7.43 9.32 2 0.43 0.11

50.94 1.11 15.58 11.49 0.15 7.41 9.26 1.7 0.3 0.1

S(residuals) 2: 0.14. The daughter rock is the average least altered group III andesite (see Table 1 ) whereas the parent is the average group Ib Fe-rich basalt (see Table 1 ). Mineral compositions were calculated as for Appendix la. The following abbreviations are used in the table P1= plagioclase, Cpx = clinopyroxene, C.A. = average calc-alkaline andesite of the BRG (Table l ). According to the present calculation, addition of 20% calc-alkaline melt to the tholeiitic parent can produce (after subsequent fractionation of olivine, plagioclase and clinopyroxene) the Fe-Ti rich tholeiitic andesitic daughter.

240

M.R. LAFLECHE ET AL.

(c) AFC calculations for the BRG Mg-rich tholeiites

Element

Daughter

Ol

P1

C.A.

Parent

Model parent

SiO2 TiO2 A1203 FeO* MnO MgO CaO P205 WI. prop.

50 1.18 15.28 9.8 0.24 7.77 10.1 0.1 97

40.4 0 0 11.8 0 47.2 0 0 4.8

50 0 32 0 0 0 15 0 5.3

58.2 0.77 l 6.21 6.59 0.12 5.33 7.85 O. 1 ( -6.1

49.97 0.99 15.76 9.86 0.16 9.53 10.1 0.08

49.98 1.11 15.68 9.76 0.23 9.54 10.22 0.09

S(residuals)2: 0.05. The daughter rock is a basaltic rock (No. 8 711, Table 1) whereas the parent is a Mg-rich tholeiite (same as Appendix Ia). Mineral compositions were calculated as for Appendix la. Abbreviations as for Appendix Ib. According to the present calculation, contamination by 6% of calc-alkaline melt of the Mg-rich tholeiitic parent can produce (after fractionation of olivine, plagioclase and clinopyroxene) the composition of group Ia basaltic rocks. (d) FC calculations for the BRG calc-alkaline andesites.

Element

Daughter

P1

Cpx

Ti-mgt

Ol

Parent

Model parent

SiO2 TiO2 A1203 FeO* MnO MgO CaO K20 P205 Wt. prop.

63.00 0.82 13.73 7.17 0.10 3.80 5.85 1.10 0.23 46

53.14 0 29.9 0 0 0 12.3 0.02 0 36

52.14 1.17 2.3 9 0 14.38 19.7 3 0 6.4

0 13.3 3.5 75 0.59 3.2 0.03 0 0 4

40 0 0 14 0 45.3 0.23 0 0 8.6

56.9 0.72 18.1 8.36 0.11 6.8 8.71 0.11 0.14

56.9 1.02 18.2 8.30 0.07 6.8 8.73 0.53 0.11

S(residuals)2: 0.28. The daughter rock is an average of the calc-alkaline dacitic rocks (see Table 1 ) whereas the parent is the average group C2 calc-alkaline andesite. Mineral compositions were calculated as for Appendix Ia. According to the present calculation, low pressure fractional crystallization of a calc-alkaline andesite can produce the calc-alkaline dacites.

ARCHEANMAFICVOLCANICROCKSOF THE SOUTHERNABITIBIBELT,QUEBEC Appendix II. CHAOS.5

SiO2 TiO2 A1203 FeO* MnO MgO CaO Na20 K20 P205 CaO/AI203 FeO*/TiO2

241

r e s u l t s for 50% c r y s t a l l i z a t i o n

1

2

3

4

5

6

7

8

9

10

11

12

13

50.3 0.95 16.35 10.02 0.17 8.83 11.8 1.60 0.02 0.08 0.72 10.54

52.2 1.71 13.4 14.7 0.24 5.65 10.2 1.63 0.03 0.16 0.76 8.57

52.2 1.69 13.4 14.7 0.24 5.65 10.2 1.63 0.03 0.16 0.75 8.67

52.0 1.70 13.3 15.0 0.24 5.73 10.0 1.63 0.04 0.16 0.72 8.81

55.2 1.55 12.1 12.8 0.21 5.61 10.1 1.87 0.28 0.18 0.80 8.30

56.6 1.49 11.9 12.5 0.21 5.15 9.5 1.84 0.67 0.15 0.89 8.43

53.4 1.46 12.5 12.7 0.21 6.41 11.3 1.77 0.06 0.14 0.52 8.70

52.3 1.27 16.3 11.9 0.22 8.15 8.4 1.32 0.04 0.08 0.62 9.37

56.4 2.42 11.1 17 0.29 3.76 6.8 1.87 0.07 0.16 0.62 7.02

56.1 2.41 10.8 17.4 0.29 3.78 6.8 1.82 0.07 0.16 0.63 7.22

55.8 2.41 10.6 17.8 0.29 3.87 6.7 1.77 0.07 0.16 0.53 7.39

57.0 2.04 12.6 14.7 0.27 5.0 6.8 1.24 0.06 0.13 0.45 7.23

55.0 1.7 15.7 13.9 0.24 4.74 6.4 1.94 0.08 0.15 0.40 7.94

1: The starting composition is a Mg-tholeiite from the Lac Pelletier area, southern BRG. 2, 3 and 4: fractional crystallization under respectively - 2, 0 and + 1 log QFM redox conditions. 5, 6 and 7: Assimilation and fractional crystalization (AFC) with respectively; Kewagama sediments, Hunter Mine group trondhjemite and BRG calc-alkaline andesite as contaminants. 8: The starting composition is a Fe-rich basalt from the northern BRG ( # 8705). 9, 10 and 11: fractional crystalization under respectively - 3, 0 and + 1 log QFM. 12: AFC with BRG calc-alkaline andesite as contaminant.