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On the nature of greenstone belts in the precambrian
Tectonophysics, 73 (1981) 195-212 Elsevier Scientific Publishing Company,
195 Amsterdam
- Printed
in The Netherlands
Geological Contributions: Extinct Rift Valleys ON THE NATURE OF GREENSTONE BELTS IN THE PRECAMBRIAN
A.F. GRACHEV
and V.S. FEDOROVSKY
Institute of Physics of the Earth, Academy of Sciences, Moscow (U.S.S.R.) Geological Institute of the Academy of Sciences, Moscow (U.S.S.R.) (Received
July 31, 1980)
ABSTRACT Grachev, A.F. and Fedorovsky, V.S., 1981. On the nature of greenstone cambrian. In: J.H. Illies (Editor), Mechanism of Graben Formation. 73: 195-212.
belts in the PreTectonophysics,
At present there is considerable speculation concerning the origin of greenstone belts. They are compared with Phanerozoic geosynclines, fold belts, modern island arcs or with the rift structures formed above subduction zones in marginal seas. Four stages in greenstone-belt development can be determined (3.8-3.0, 3.0-2.6, 2.6-1.9, and 1.9 b.y. and younger). Sections of early greenstone belts contain all the essential features of the modern oceanic sequences. Their peculiarities are: great thickness, presence of jaspilites and scarce occurrence of very coarse elastics. On the contrary, the latter type of sediment is widespread in younger greenstone belts and in those of the last stage which are completely formed in continental environment. Whole rock chemistry of Precambrian volcanics (more than 1800 analyses all over the world) confirms that alkali-olivine basalta are absent till 1.9 b.y. and andesites are generally rare. Widespread type of Early Precambrian basalts are 01-Hy and Q normative ones, showing close similarity to typical abyssal tholeiites. The presence of basaltic komatiites is not a unique petrochemical feature of volcanism in the Early Precambrian as one thinks. Now similar rocks have been found in Phanerozoic rift zones. There is no evidence suggesting significant shortening of initial crustal complexes and greenstone belts. These data along with slight metamorphism and above features can only be explained by the formation of greenstone belts in rifting environment. During the early stages (3.8-1.9 b.y.) the thickness of the protocrust was not more than lo-20 km. It allows us to understand the rare occurrence of intermediate volcanics at early stages of the Earth’s evolution. In summary it is argued that in the Early Precambrian the spread of primitive oceanic lithosphere was not compensated by the consumption in subduction zones. Consequently, plate tectonics cannot explain the formation of the crust in the early history of the Earth.
INTRODUCTION
The origin and evolution of greenstone belts in the Precambrian is the most popular and debatable problem of present day geology. The rapid growth of papers all over the world is the best illustration of such a phenomenon. 0040-1951/81/0000-0000/$
02.50 @ 1981 Elsevier Scientific
Publishing
Company
196
Before the appearance of the plate tectonics doctrine greenstone belts have been considered as alpinotype fold belts, specific type of geosynclines unknown in the Phanerozoic. Now when plate tectonics is widely used the greenstone belts are compared with rift zones, island arcs or the rift structures formed above subduction zone in marginal seas. The aim of the present paper is to synthesize available data with emphasis on the origin of greenstone belts. GENERAL
FEATURES
OF GR~ENSTON~
BELTS
Four stages in greenstone-belt development can be determined: 3.6-3.0, 3.0-2.6, 2.6-1.9, 1.9 by. and younger. Apparently these time spans as well as a number of stages reflect the state of the Precambrian geochronology at present. In time the number of stages would increase and in the first place it should be expected to reveal greenstone belts older than 3.6 b.y. The general Early Precambrian section is illustrated in Fig. 1. Independent of age all greenstone belts have similar structure, volcano-sedimentary sequences, magmatism and metamorphism. The greenstone belts are several hundred kilometers long and 50-60 km wide (Figs 2 and 3). In plan they form a system of strips, sometimes broad zones, branching off from the main belt. In some cases the overall length exceeds 2000 km (Fig. 3). In spite of the fact that Archaean shields are widely spaced they have a similar sequence (Anhaeusser et al., 1969). The Barberton greenstone belt is a tectonotype of the first and second stages. Rocks of this belt have been subdivided into lower volcanic (the Onverwacht Series) and upper volcanoelastic and sedimentary formations (Fig Tree and Moodies Groups, Fig. l), the total thickness of which is about 21000 m. The lower volcanic pile consists of ultramafic and mafic material; the upper formation (Fig Tree) includes greywacke, shale, chert at the base and trachytic tuff in the upper part. The sequence is terminated by conglomerate, quartzite, and shale interbedded with jaspilite and lavas. Rhythmic bedding, ~thologic~ composition and such sedimentary features as graded and convolute bedding, ripple marks, slump bedding suggest that the Fig Tree Group formed in the abyssal zone (Viljoen and Viljoen, 1969). Kuenen (1963) regarded this succession as a result of turbidity flow activity. Rocks of the Belingwe greenstone belt (South Africa) are ~on~orne~~s, banded iron quartzite, marble, locally stromatolitic in lower part, grading UP -__-.-..Fig. 1. Comparison of the volcano-sedimentary stratigraphy and isotope dates of Early Precambrian greenstone belts. 1 = basement (granulites, gneisses, tonalites), 2 =: ultramafic lavas, 3 = mafic laws, 4 = intermediate and felsic laws, 5 = mafic (a) and felsic (b) pyroclasts, 6 = quartzites, cherts, 7 = iron banded formation, 8 = conglomerates, 9 = sandstones, 10 = siltstones, graphite shales, 11 = limestones, 12 = dotomites. The stratigraphic columns are compiled after: A = Dimroth et al. (1970); B = Zagarodny
197
1.E
2.c 2qso
2.2 32003400
2.4
2.6
2.8
3.0
3.2
et al. (1964); C = this paper; D = Riley (written communication, 1979); E = this paper; F = Wilson et al., 1978; G = Naldrett and Turner, 1977; H = Viljoen and Viljoen (1969). Age in million years to columns: B = 1880 - ultrabasic and basic rocks intruded the Pechenga Series; 2900-3000 - metamorphism of basement; E = 1850-2000 - metamorphism of trough sequence; 3000-3500 - metamorphism of basement; F = the Great Dyke of Rhodesia, 2651, 2690 - granites, intruded by the Belingwe greenstone sequence, 2884, 2970 - basement gneisses and tonalites: G = 2670 - granite conglomerate pebbles; 2612 - granites cutting the Yakabindie Complex, 2665 - age of metamorphism; H = 2980 -shales of the Fig Tree Series, 3200-3400 - granites intruding the Onverwacht Series, 3360 - Onverwacht lavas, 3375 - cherty shales of the Middle Marker horizon, 3600 - lavas at the base of the Onverwacht Series,
Fig. 2. Early Precambrian greenstone belt in southern East Siberia 1 and 2 = greenstone belt (I -at an initial stage of separation - 3.0 to 2.6 b.y.; 2 -at a maximum stage of separation - 2.6 to 2.0 b.y.), 3 = basic to ultrabasic melanocratic basement (more than 3.0 b.y.); 4 = complexes of a strongly metamorphosed volcanie+edimentary cover (more than 3.0 b.y.) 5 = sedimentary cover (2.6-2.0 b.y.); 6 = volcano-plutonic complex and molasse (1.8-1.6 b.y.); 7 = sedimentary cover of the Siberian platform; 8 = master faults. Letters in circles show the position of columns in Fig. 1.
the section into massive and ultramafic pillow lavas and mafic rocks (Fig, 1). The upper and lower parts of the sequence are similar, except for ultramafic lavas (Wilson et al., 1978). The Yakabindie belt (Western Australia) has two rock sequences (Fig. 1). The lower part consists of ultramafic and mafic rocks (over 90%) and less than 10% felsic rocks, while the upper part is characterized by felsic rocks interbedded with basalts, volcanoclasts and chert shales (Naldrett and Turner, 1977). The structural analogs of greenstone belts have been known south of the Siberian platform where the Aldan shield extends for about 2200 km from the Baikal Lake to the Sea of Okhotsk (Fig. 2). These features have been described as suture troughs (Fedorovsky and Leytes, 1968) and later from the point of view that their development was rift controlled (Grachev and Fedorovsky, 1970). Volcano-sedimentary sequences contain greenschists and amphibolites derived from mafic lavas and tuffs, sandstones, iron quartzites and associated bodies of ultramafic rocks and gabbros (Kodar and Udokan Ranges). The lower part of the trough succession in the Olokit-Synnyr region consists of quartzites, marbles and shales interbedded with mafic and ultramafic volcanics overlain by massive and pillow basalts, black chert shales, conglomerates and sandstones. In the southwestern Aldan shield
199
Fig. 3. The main greenstone belts of the Kola Peninsula and Karelia. 1 = greenstone belts; 2 = oldest gneisses and schists; 3 = platform cover. Letters in circles: A = Parandov; B = Pechenga; C = Hautavara troughs.
(Priolchonie) the trough sequence is represented by three sedimentary cycles. Each cycle commences with marbles and mafic lavas, followed by pyroelastics and terminated by marbles (Fig. 1). And, lastly, the Ust-Gilyuy trough section is a succession of basalt flows (2500 m), greywacke (3000 m) and mafic volcanics locally with greywacke (2000 m). Similar suture troughs have been established in the eastern Baltic shield (Konkin et al., 1975; Robonen et al., 1978, Fig. 3). The lava succession of the Parandov trough consists of pillow and massive lavas interbedded with lava-breccia, tuffs and quartzites. The upper portion includes quartzites, graphitic shales, marbles, and iron pyrites. The total thickness of the entire section exceeds 2600 m. The Hautavara trough is filled in by tholeiitic and picrite basalts, rhyo-dacites, pyroclastics, quartzites, shales and iron pyrites with a maximum thickness of 4000 m. Among greenstone belts belonging to the third stage we include the Pechenga-Imandra-Varzuga (in further Pechenga) belt in the Baltic shield and
200
the Labrador trough on the Canadian shield. The former stretches through the Kola peninsula for a distance of 600 km and is about 145 km wide (Fig. 3). In the northwest the Pechenga succession consists of four sequences (Zagorodny et al., 1964). The first one contains in ascending order conglomerates, gravelites, shales and thick layers of diabases. The second sequence commences with quartzites, sandstones, dolomites and limestones and is capped by mafic and felsic lavas. The third member includes conglomerates, sandstones, quartzites, dolomites and diabases with tuff layers. The fourth part is characterized by rhythmically layered conglomerates, sandstones, siltstones, phyllites and pyroclastics which in turn grade into pillow-basalts, picrites and rhyolites. Four main cycles of sedimentation are easily determined in the Pechenga succession. They begin with elastic rocks and are terminated by shales or limestones. The lower cycles are shallow-water sediments (red beds, limestones and dolomites), the upper cycles are characterized by turbidites. The Labrador trough succession changes from place to place (Dimroth et al., 1970). Metasediments and magmatic rocks dominate at the western and eastern sides, respectively. The section in Fig. 1 shows the structure on the eastern side. At the base it consists of shallow-water sediments (conglomerates, sandstones, shales, dolomites). Cycle I of Dimroth (1972) and mafic lavas and greywackes, sandstones, quartzites and shales including the Sokoman iron formation occur in the upper portion of cycle II. The sequence is capped by basic lavas and felsic pyroblastics. It is suggested that all the above mentioned sequences have some common features independent of time span development. They include: (1) great thicknesses of volcano-sedimentary rocks formed in a characteristic sequence; (2) the presence of ultramafic lavas, usually in the lower portions and (3) if any, of intermediate lavas and pyroclastics. The disrare occurrences, appearance of ultramafic lavas in the greenstone belts of the third stage (2.6-1.9 b.y.) is a peculiar feature. The comparison of structure and metamorphism of different greenstone belts reveals their great similarity. As it was shown by Viljoen and Viljoen (1969) for the Barberton belt and confirmed later by studying other belts, most of them are characterized by broad development of large synclinal folds. Usually the anticlinal folds between the adjacent synclinal folds are absent and the latter are separated by a system of faults. Only in the vicinity of basement contact, the isoclinal folds, are widespread indicating the local compression (Barberton, Pechenga, Labrador and other belts). All the successions of greenstone belts have suffered greenschist facies metamorphism; in some cases there are signs of the greenstone regeneration. Up-grade of metamorphism to amphibolite facies occurs around the periphery of the belt. At these localities along contacts of granite-gneiss and migmatite domes with volcano-sedimentary assemblages show good evidence of sharp metamorphic changes and deformation. Generally the structure pattern resembles the outlines of diapiric domes. These data imply the lack of compression.
The mineral composition of formations, peculiar structure and metamorphism of greenstone belts and their place in the evolution of the shields suggest that these belts developed in an extension environment of primordial crust. BASIC PECULIARITIES
OF
VOLCANIC EVOLUTION
The Early Precambrian greenstone belts may be divided into two groups taking into account the structure of volcanic piles. To the first group we assign “Barber-ton types” belts (Belingwe, Selukwe and others in South Africa; Schebandowan, Kagakie Lake, Stormy Lake and others in Canada; Yakabindie, Monger, Scotia, Kalgoorly in Western Australia). The belts of Pechenga type form the second group (Kodar-Udokan and Olokit-Synnyr areas in eastern Siberia, Parandova, Hautavara and others in Karelia, Labrador, Yellowknife, Grenville in North America, and others). The former were commonly developed during early Archaean time (to 3.0 b.y.), the latter include the Late Archaean-Early Proterozoic (to 1 S-2.0 b.y.) belts. The belts of the first group are characterized by sections which we called the complete ones. They consist of three volcanic sequences: (1) ultramaficmafic; (2) mafic (tholeiitic) with subordinate felsic pyroclastics; and (3) bimodal basalt-(rhyodacite) rhyolite association. In sections of the second group the lower ultramafic-mafic member is absent, and the third member is not always present. Such sequences we refer to as incomplete types. The ultramafic-mafic volcanic pile consists of interlayered ultramafic and mafic lava. Tholeiitic basalts being significant in mafic sequence in upper portion grade into felsic volcanics and pyroclastics. It is necessary to emphasize the rhythmical layering of ultramafic-mafic and mafic piles of the greenstone belts. In the first case it is a successive change of peridotite komatiites by basalt komatiites and felsic tuffs. In the second case the cycle consists of tholeiitic basalts-rhyolites (rhyodacites) chert shales - banded iron formation. Following Viljoen and Viljoen (1969) many authors have paid attention to peculiar features of Early Precambrian volcanism. It is marked by great thicknesses of volcanic rocks (lo-15 km), presence of ultramafic and mafic lavas unknown in younger suites and defined as komatiites; the latter are characterized by a high CaO/A120J ratio and spinifex textures. Indeed, in many belts thicknesses of only the ultramafic-mafic complex is about 5-10 km and together with the second one amount to lo-19 km. But if we compare the thickness of oceanic crust of 5-7 km (2 and 3 layers together) with the crustal thickness in Iceland (up to 11 km), their uniqueness of the Early Precambrian thickness becomes doubtful. Let us consider the chemistry of basalts in greenstone belts which are under discussion. Table I shows small variation of averages. All basalts contain normative quartz (0.35-5.3s) and there is no significant difference in CIPW norms between tholeiites and komatiites. For the former CaO/A.1203 ratio ranges between 0.5 and 0.8 compared with komatiite averages of 0.9-1.35.
202 TABLE I Averages
of Early Precambrian
bassIts (wt. %) -
1
2
3
4
5
6
7
50.40 0.63 7.59 4.10 6.68 14.84 10.27 1.32 0.16 1.35
50.84 1.05 13.21 4.20 7.41 6.45 9.09 3.00 0.28 0.65
49.85
47.99
49.38
0.76 12.96 1.96 8.71 8.52 10.57 2.30 0.24 0.80
0.50 10.50 3.22 8.92 15.20 9.00 1.20 0.70 0.89
0.87 14.99 2.70 8.36 7.94 9.99 2.06 0.63 0.66
48.86 0.95 14.50 3.09 9.39 7.49 10.48 2.33 0.62 0.71
50.92 0.62 15.14 1.35 8.67 6.19 12.47 2.04 0.56 0.81
8
9
10
11
12
13
14
49.09
49.83 0.90
49.44 0.98 14.14 2.35 9.52 10.89 7.21 1.96 0.34 0.57
53.10 1.04 14.01 3.68 8.13 5.53 8.19 3.12 0.92 0.57
50.29
14.48 2.50 7.32 6.35 10.57 1.84 0.48 0.71
46.12 2.17 13.44 7.05 7.39 6.92 8.27 2.55 0.50 0.61
47.80 1.54 12.77 3.26 10.42 7.17 10.26 2.15 0.18 0.70
15
16
17
18
19
20
SiOz
47.09
TiO2
1.61 12.30 3.58 10.84 7.02 9.21 2.25 0.34 0.75
48.80 1.07 14.05 2.36 9.78 7.28 10.07 2.52 0.10 0.78
48.57 1.14 13.96 3.24 9.02 7.36 9.67 2.74 0.77 0.69
46.08 1.65 7.16 3.01 10.65 15.82 9.75 0.66 0.12 1.35
46.52 2.50 8.21 3.36 12.51 14.69 7.78 2.05 0.94 0.94
47.12 1.25 14.26 2.30 8.49 12.58 11.70 1.80 0.22 0.82
----_____ SiOz TiOz 4203 Fe203
Fe0 MgG CaO NazO K2O
CaO/AlzOa
SiOz TiOz 403 Fe203
Fe0 MgG CaO NazO K2O
CaO/AlzOa
-41203 Fe2
03
Fe0 MgG CaO Na20 K20
CaO/AIzOs
1.01 14.47 2.81 10.02 6.50 11.02 2.16 0.39 0.77
1.73 13.55 6.76 7.13 5.53 6.47 3.47 0.97 0.48
South Africa, Barberton: 1 = basaltic komatiite, 2 = tholeiitic basalt (Viljoen and Viljoen, 1969); 3 = South Africa, Bulawayo and Que Que basahs (Hawkesworth and O’Nions, 1977); Central Karelia, Suma-Semch: 4 = komatiitic basalts, 5 = metadiabase, 6 = metadiabase, Palaya Lamba - Iron Gate (Kratz, 1978); India, Dharwar: 7 = basalts (Srinivasan, Screenivas, 1972), 8 = basalts of ChitaIdrug (Naqvi, 1972); east Siberia, Olokit-Synnyr: 9 = basalt of lower and middle sequence, 10 = basalts of upper sequence; 11 = basalts of Parandov and Hautavara (data from Ruchkin); 12-l 5 = Kola Peninsula, Pechenga, basalts of first, second, third and upper sequences (data from petrochemical bank of NorthWestern Geological Survey, Leningrad); Canada, Labrador trough: 16 = basaits, 17 = gabbro (Dimroth et al., 1972), 18 = Pechenga, high Mg-basalt+ 19 = Pechenga, olivine gabbro (data from the same source as in point 12-15), 20 = high Mg-basalts, Iceland (Polyakov, 1978).
203
Fig. 4. Early Precambrian basalt6 plotted on AFM diagram. For numbers 1-19cf. Table I. Insert: a, b, c; trends of Hawaii tholeiites (Irvine and Baragar, 1971), mid-oceanic basalts and Archaean basaltic komatiites (present paper).
The AFM values for all groups, presented in Table I, are plotted in Figs 4 and 5. It is evident that there is no gap between the tholeiitic and komatiitic fields, on this diagram they all together show a common trend of differentiation. Our data demonstrate the existence of tholeiitic series of greenstone belts which are different from the Hawaii and mid-oceanic tholeiitic suites (insight in Fig. 4). In the MgO-CaO-A1203 diagram (Fig. 5) there are two fractionation lines. The first trend (olivine fractionation) is reflected by a line going away from the MgO apex of the diagram, the second one corresponds to the curve which extends towards the A1203 comer (clinopyroxene fractionation). These trends are apparently characteristic of all mafic Early Precambrian series (Fig. 6).
204
Fig. 5. AFM diagram showing fields of Early Precambrian basaltic komatiites (1) and tholeiites (Z), mid-oceanic basalts (3), abyssal volcanic glass of Atlantic (4), Pacific (5), and India (6) Oceans (Melson et al., 1977).
More than 1800 chemical analyses from various greenstone belts (with the age close to 2.0 b.y.) were used for the definition of basic chemistry type of basalts. The cluster analysis has shown the existence of two main groups of averages and CIPW norms given in Table II. The first group contains hypersthene tholeiites, and the second includes the quartz tholeiites. It is very important that Ne-normative basalts are absent in the Early Precambrian and are widely developed in the Late Precambrian and Phanerozoic. Another important feature which has been revealed by clustering procedure is the lack of evidence suggesting the existence of basaltic komatiites as a definite chemistry group. The latter is well in accord with the proposal (Williams, 1972) to use the term high Mg basalts instead of basaltic komatiites.
1.88
3.95 -
0.57 2.81 20.63 27.90, 20.55 21.72 -
2.14
2.38 23.44 26.82 19.95 18.07 2.02 4.15 -
1.09 14.34 2.77 9.58 6.87 9.97 2.68 0.39 -
0.96
14.28 2.64 8.90 7.30 10.35 2.36 0.46
49‘04
49.57
2
2.71
1.59 17.97 26.42 21.97 23.54 1.56 4.24 -
47.76 1.38 13.00 2.82 11.18 7.75 10.34 2.05 0.26 -
3
2.45
3.88 -
1.70 5.52 25.17 23.88 16.62 21.77 -
51.57 1.26 14.29 2.61 8.58 6.69 8.69 2.90 0.91
4
2.06
4.77 -
0.57 3.06 22.49 26.67 17.23 23.15 -
1.05 14.22 3.18 8.40 7.80 9.33 2.57 0.50
49.64
AVr.
4.48
8.83
2.34 6.66 25.29 20.53 16.04 15.83 -
48.57 2.28 13.21 5.89 8.67 6.23 7.87 2.89 1.09
II
4.34
8.18 -
1.92 9.02 25.68 21.32 17.04 13.48 -
2.21 13.78 5.45 7.30 6.28 8.29 2.82 1.47
49.98
1
2.83
2.04 23.62 29.98 19.55 14.62 3.79 3.58 -
1.46 15.63 2.42 8.11 7.89 10.67 2.74 0.34 -
49.30
2
Oceanic basalis
3.72
2.34 20.53 28.66 22.23 13.39 4.85 4.27
48.35 1.93 14.70 2.90 8.55 8.14 11.14 2.39 0.39 -
3
5.33 3.92 2.87
4.51 2.04 21.82 29.57 19.95 9.09 -
15.22 7.48 2.97 7.61 10.96 2.54 0.34
49.89 1.49
4
4.79
3.22 20.53 26.86 22.81 16.00 2.31 3.48 -
48.58 2.49 14.27 2.37 10.86 6.49 10.88 2.40 0.54 -
5
1.98
6.64 -
0.39 7.93 24.62 29.86 15.15 13.42 -
49.88 1.02 16.77 4.47 5.55 6.17 9.60 2.84 1.31
1
2.44
2.84 32.01 27.68 14.09 8.65 8.52 3.76 -
49.14 1.23 16.16 2.48 6.83 7.07 8.70 3.62 0.46 -
2
Island-arc basalts
1.86
16.98 4.88 -
27.03 17.35 30.22 1.08 -
48.21 0.95 17.99 3.26 6.40 6.28 6.16 2.11 4.40 -
3
1.39 30.17 26.83 17.04 12.55 0.41 1.64 7.39 2.61
49.06 1.34 15.54 8.30 1.70 8.23 9.57 3.47 0.23 -
4
4.46
6.78 -
0.48 5.99 29.51 22.21 16.75 13.81
48.68 2.25 14.34 4.48 7.68 5.74 8.29 3.34 0.97
5
Precambrian basalts: type I-subtypes 2, 2, 3, 4; abyssal basalts: 1 and 2 (Grachev, 1977); 3 and 4 (present paper); 5 = Iceland (Grachev, 1977); island arcs: 1,2,3,4,5 = main types of basalts (present paper).
Or Ab An Ix HY 01 Mt Hm Ilm
Q
K2O
FeG MgQ CaO NaaO
Fe203
A1203
SiGz TiOz
1
I
Early Precambrian basalts
Averages and CIPW norms of main types of Early Precambrian and oceanic basahs (cluster analysis)
TABLE II
206
r,g
4.--.-
AM
Fig. 6. CaO-MgO-A1203 diagram for Early Precambrian basalts. Symbols are the same as in Fig. 4; o, b, c = basalt trends of eastern Finland (Blais et al., 1977), Botswana, South Africa (Key et al., 1976) and Yakabindie, Western Australia (Naldrett and Turner, 1977).
The Early Precambrian felsic volcanism is a peculiar problem because acid lavas and pyroclastics have great thicknesses of about 3-4 km. However, very often pyroclastics are well developed in one place but in another they are almost absent. A fine example is the Shebandowan area in Canada (Smith, 1978). A similar feature is common for belts of the Kola peninsula, West Australia, South Africa. The common feature of pyroclastics is an exclusively broad development of volcano-elastic rocks (lapilly and psammite tuffs, breccias, agglomerates, lahar deposits). Usually they are rhythmically bedded when mafic lavas in the lower part grade upward into pyroclastic accompanied by thin acid lava layers.
Gee et al. (1976) suggest that felsic volcanic sequences may be formed from a number of separate central volcanoes or volcanic centres. Such a conclusion allows us to understand the disappearance of pyroclastics in the individual parts of the same greenstone belt. Many authors suggest that activity of such central volcanoes in the Early Precambrian were in conditions similar to recent island arc systems. It has been based upon the occurrence of andesites and dacite-rhyolites in upper volcanic sequences. However, the comparative analysis of frequency distribution and chemistry of these rocks has shown that they have nothing to do with island-arc volcanics. It is possible to note the following main differences: (1) Intermediate volcanic rocks (strictly andesites) are absent from most greenstone belts with the apparent exception of Canadian ones. In some cases, for example in the Sturgeon Lake volcanic belt in Canada, andesites are referred to as rocks with SiOz range between 51-53% (Franklin, 1978) in other regions (Barberton, Bulawayo in South Africa) the rocks described as andesites (Harrison, 1970) in fact are dacites (SiOz > 63%) or basaltic andesites (Si02 < 57%). The true andesites are scarce, but in island arcs they constitute more than 60% of the entire rock-volume (Erlich, 1966). It is necessary to take into account the paucity of inte~ediate rocks in Cenozoic rift zones despite their occurrence in Iceland, Ethiopia and Chara rifts. (2) The felsic rocks (rhyolites and rhyo-dacites) together with basalts from the bimodal contrasting basalt-rhyolite association. The latter is an important feature of all the greenstone belts where acid volcanism took place. So Goldich and Peterman (1978) pointed out that “this bimodal distribution is a problem common to the Archaean as well as to younger rock suites” (p. 231). The equivalent of the Early Precambrian basalt-rhyolite association within young complexes is the basalt-trachyte (rhyolite) formation of recent oceanic islands with well developed Daly gap, falling to andesites. With all the reasons we may suppose that in the Early Precambrian the basaltrhyolite association was also caused by the activity of the central type volcanoes located on proto-oceanic crust. The pattern of the differentiation trend on an AFM diagram differs from the field of Kuril-Kamchatka zone, the typical Cal-alkaline series (Fig. 7). One cannot but admit that the pattern of three volcanic belts is considered as an example of typical island-arc systems formed due to different mechanism and under different conditions. (3) The sign~ic~t difference between the chemists of greenstone belts and island arcs basalts (Table II) is shown by A1203, CaO, Na,O and K,O contents. Gottini-Rittmann’s diagram (Fig. 8) confirms this conclusion. Furthermore, Hawkesworth and O’Nions (1977) have shown that Archean greenstone belt volcanics and those of modem island arcs are distinguished by REE and trace elements. (4) And finally, the difference lies in the absence of lateral zonality in greenstone belts volcanics. As is well known, the latter is an important character of modem island arc volcanism. As a whole, all above data suggest
208
”
\i
i.
”
v
”
\,
'M
”
Fig. 7. Fields and trends of volcanic series of present island arcs, rifts, and Precambrian greenstone belts; 1 = Kuril-Kamchatka (Erlich, 1966); 2 and 3 = basalt-trachyte association; (2) Udokan Range, Baikal Rift (present paper), and (3) the Azores (White et al., 1979); 4 = Rainy Lake, Canada, and 5 = Vermillion, Canada (Goldlich and Peterman, 1978).
1.5 b 49 1.0
0.5 -0.5 01
02
0.
0.5
l3
4 (’ 0
&l&o 5 [-‘\,6 k
\7
209
that greenstone belt volcanism in the Early Precambrian is very similar to that of the tholeiites series of modern oceanic and continental rift zones. In belts of the first and second stages it has the oceanic trend and in the third stage belts, the features of transitional rift volcanism take shape (Grachev, 1977). TECTONIC STYLE OF GREENSTONE
BELT DEVELOPMENT
It is evident that any attempt to find modern structure equivalents of greenstone belts must include a comparative analysis of both Archaean and recent volcanism. Tectonically one may only speak about two main types of volcanics. The first one includes volcanics of extension areas (continental and oceanic rifts), the second one is referred to as volcanics of early erogenic (island arc) and late erogenic stages of fold belts. As mentioned above, the greenstone belt volcanism has nothing to do with that of island arcs as well as with fold belts (young and mature). Moreover, a comparative analysis of basalts of all tectonic regimes throughout the earth’s history suggests that greenstone belt volcanics formed in areas of extension of earlier protocrust, i.e. in rifting environments. That is why it is possible to determine the greenstone belts as rift structures. Because of the absence of the island arcs in Early Precambrian, there is no sense in using the marginal basin model for greenstone belts origin. Noting the similarity between the greenstone belts and modern rifts one takes into account the apparent features of distinctions. In the Early Precambrian greenstone belts the olivine alkaline (Ne-normative) basalts and other alkaline rocks are unknown, at the same time they are widespread in modern continental rifts. The chemistry of mafic volcanics and sedimentary sequences of greenstone belts led to the conclusion about the existence of all three stages of rift development: continental, transitional and oceanic (Grachev, 1977). Although all the presently known greenstone belts formed on pre-existing sialic crust, the latter differed greatly from the Phanerozoic continental crust in thickness. A primary crust being very thin, only tholeiitic lavas would be generated under low pressure conditions. During the course of granite-greenstone terrain development the early crust may have thickened bringing into effect the mechanism of the formation of the mature continental crust. The main part of the present continental crust has already been created as a result of Early Precambrian tectogenesis about 1.8-2.0 b.y. ago. There are two cycles in the evolution of greenstone belts. The first cycle is characterized by graben formation and syntectonic volcanic pile accumulaFig. 8. Early Precambrian basalts on Rittmann-Gottini diagram. 1 = airerage compositions of Archaean basalts (cf. Table I), 2 = averages of abyssal tholeiites and abyssal volcanic glasses, 3 = averages of main types of island arc basalts (for numbers cf. Table II), 4,5, 6 = fields of average basalt compositions of Archaean, mid-oceanic ridges and island arcs, 7 = tholeiite = alkaline basalt - hawaiite trend (Rittmann, 1973).
210
tion. Sedimentation took place within the graben. The next cycle was marked by the growth of granitic diapir uplift and erosion resulting in the formation of mountain relief and appearance of coarse-grained sediments containing granite material. Similar stages are known in Cenozoic continental rift development (Grachev, 1977). The lower sequence consists of fineg-rained rocks, and upper portion is referred to as a molasse. In oceanic rifts at the initial stage of spreading the second stage is marked by the deposition of turbidites. Such a similarity of sections of upper portions of greenstone belts sequences and Cenozoic rift filling was associated with mountain-relief formation due to erosion of uplifted rift shoulders. In greenstone belts the reason for the growth of granite diapirs is an isostatic rise (Ramberg, 1967) in the Cenozoic continental rifts; the uplift of shoulders reflects the gravity instability of the anomalous mantle. CONCLUSIONS
The comparison of structure, volcano-sedimentary filling and magmatism of both greenstone belts and modern rifts shows that all of them have many features in common. Therefore it is possible to conclude that rifting is a principle process of the oceanic crust formation in the geological history of the earth. Besides, the morphology and structure of sequences of greenstone belts indicates that in the Early Precambrian rifting no basins comparable with modern oceans were formed. However the numbers of greenstone belts established in the Precambrian shields is so large that their total width is commensurable with, for example, the Atlantic Ocean. In the Early Precambrian subduction did not take place since no significant volumes of andesites have been found. This suggests that plate tectonics did not operate during most of Precambrian time. ACKNOWLEDGMENTS
The authors are grateful to Drs. M.S. Markov and A.A. Saveljev for discussions. We also thank Mr. V.I. Mishin for developing the computer program and Mr. N.B. Phillipov for technical assistance. REFERENCES Anhaeusser, C.R., Mason, R., Viljoen, M.J., and Viljoen, R.P., 1969. A reappraisal of some aspects of Precambrian shield geology. Geol. Sot. Am. Bull., 80: 2175-2200. Blais, S., Auvray, B., Capdevila, R and Hameurt, I., 19’77. Les series komattques et tholeiitiques des uintures archeennes de roches vertes de Finlande orientale. Bull. Sot. Geol. Fr., XIX: 965-970. Dimroth, E., 1972. The Labrador geosyncline revisited. Am. J. Sci., 272: 487-506. Dimroth, E., Baragar, W.R.A., Bergeron, R. and Jackson, G.D., 1970. The filling of the Circum-Ungava geosyncline. In: A.J. Boer (Editor), Symposium on Basins and Geosynclines of the Canadian Shield. Geol. Surv. Can. Pap., 70-40: 45-142.
211 Erlich, E.N., 1966. The petrochemistry of Cenozoic Kuril-Kamchatka volcanic province Nauka, Moscow, 280 pp. (in Russian). Fedorovsky, V.S. and Leytes, A.M., 1968. On the Early Proterozoic geosynclinal troughs in the Olekma-Vitim Mountain Land. Geotectonica, 4: 46-61. (in Russian). Franklin, J.M., 1978. Petrochemistry of the South Sturgeon Lake volcanic belt. Proc. 1978 Archean Geochemistry Conf. Univ. Toronto Press, Toronto, Ont., pp. 161-180. Gee, D.R., Groves, D.J. and Fletcher C.J., 1976. Archaean geology and mineral deposits of the Eastern Goldfields. Guideb. Excursion 42A, 25th Int. Geol. Congr., Sydney. Goldich, S.S. and Peterman, Z.E., 1978. Geology and geochemistry of the Rainy Lake area. Proc. 1978 Archaean Geochemistry conference, Toronto Univ. Press, Toronto, Ont., pp. 209-234. Grachev, A.F., 1977. The rift zones of the Earth. Nauka, Leningrad, 248 pp. (in Russian). Grachev, A.F. and Fedorovsky, V.S., 1970. On unified nature of rifts, aulacogenes and geosynclinal troughs. Soviet Geology: 12, 121-122 (in Russian). Also in: Int. Geol. Rev., 2,197l. Harrison, N.M., 1970. The geology of the country around Que Que. Bull. Geol Sure. Rhodesia, 67: 125. Hawkesworth, C.I. and O’Nions, R.K., 1977. The petrogenesis of some Archaean volcanic rocks from Southern Africa. J. Petrol., 18: 487-520. 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. Key, R.M., Litherland M. and Hepworth I.V., 1976. The evolution of the Archaean crust of northeast Botswana. Precambrian Res., 3: 375-413. Konkin, V.D., Ruchkin, G.V. and Fedorovsky, V.S., 1975. The comparative analysis of the Precambrian suture structures of Karelia and BaikaI area. Geotectonika, 3 : 15-26 (in Russian). Kratz, K.O. (Editor), 1978. The geology and petrology of the Archaean granite greenstone terrain in Central Karelia. Nauka, Leningrad, 264 pp. (in Russian). Kuenen, Ph.H., 1963. Turbidites in South Africa. Trans. Geol. Sot. S. Afr., 66: 191-195. Melson, W.G., Byerly, G.R., Nelen, J.A., O’Hearn, T., Wright Th.1. and Vallier, T., 1977. A catalog of the major element chemistry of abyssal volcanic glasses. In: B. Mason (Editor), Mineral Sciences Investigations 1974-1975, Smithson. Contrib. Earth Sci., 19: 31-60. Naldrett, A.I. and Turner, A.R., 1977. The geology and petrogenesis of a greenstone belt and related nickel sulfide mineralization at Yakabindie, Western Australia. Precambrian Res., 5: 43-103. Naqvi, S.M., 1972. The petrochemistry and significance of Jogimsrdi traps, Chitaldrug schist belt, Mysore. BuII. Volcanol., XXXY,: 1069-1093. Polyakov, A.I., 1978. The chemical composition of magmatic rocks. In: V.V. Beloussov, V.I. Gerasimovsky (Editors), Iceland and Mid-Oceanic Ridge. Geochemistry, 1978: 7-54. Ramberg, H., 1967. Gravity, Deformation and the Earth’s Crust as Studied by Centrifuged Models. Academic Press, London, 214 pp. Rittmann, A., 1973. Stable Mineral Assemblages of Igneous Rocks. A Method of CaIculation. Springer-Verlag, Berlin, 353 pp. Robonen, V.J., Rybakov, S.J., Ruchkin, G.V., Konkin, V.D., Svetova, A.J. and Sergeeva, N.E., 1978. Iron Pyrites of Karelia. Nauka, Leningrad, 192 pp. (in Russian). Smith, I.E.M., 1978. Volcanic and Plutonic Rocks of the Lake She Bandowan Area. Proc. 1978 Archaean Geochemistry Conference. Toronto Univ. Press, Toronto, Ont., pp. 185-191. Srinivasan, R. and Sreenivss, B.L., 1972. Flood basal& from Dharwars of Mysore, India. Bull. Volcanol., XXXY: 824-840. Viljoen, M.I. and Viljoen, R.P., 1969. Introduction to the geology of the Barberton granite-greenstone terrain. Geol. Sot. S. Afr. Spec. Publ., 2: 9-28.
212 White, W.M., Tapia, M.P.M. and Shilling, J.G., 1979. The petrology and geochemistry of the Azores islands. Co&rib. Miner. Petrol., 69: 201-203. Williams, D.A.C., 1972. Archean ultramafic, mafic and associated rocks, Mt. Monger, Western Australia, J. Geol. Sot. Aust., 19: 163-188. Wilson, I.F., Bickle, MI., Hawkesworth, C.I., Nisbet, E.G. and Orpen, I.L., 1978. Granitegreenstone terrain of the Rhodesian Archaean craton. Nature, 271: 23-27. Zagorodny, V.G., Mirskaya, D.D. and Suslova, S.N., 1964. Geological structure of the Pechenga volcano-sedimentary sequence. Nedra, Leningrad, 84 pp. (in Russian).
195 Amsterdam
- Printed
in The Netherlands
Geological Contributions: Extinct Rift Valleys ON THE NATURE OF GREENSTONE BELTS IN THE PRECAMBRIAN
A.F. GRACHEV
and V.S. FEDOROVSKY
Institute of Physics of the Earth, Academy of Sciences, Moscow (U.S.S.R.) Geological Institute of the Academy of Sciences, Moscow (U.S.S.R.) (Received
July 31, 1980)
ABSTRACT Grachev, A.F. and Fedorovsky, V.S., 1981. On the nature of greenstone cambrian. In: J.H. Illies (Editor), Mechanism of Graben Formation. 73: 195-212.
belts in the PreTectonophysics,
At present there is considerable speculation concerning the origin of greenstone belts. They are compared with Phanerozoic geosynclines, fold belts, modern island arcs or with the rift structures formed above subduction zones in marginal seas. Four stages in greenstone-belt development can be determined (3.8-3.0, 3.0-2.6, 2.6-1.9, and 1.9 b.y. and younger). Sections of early greenstone belts contain all the essential features of the modern oceanic sequences. Their peculiarities are: great thickness, presence of jaspilites and scarce occurrence of very coarse elastics. On the contrary, the latter type of sediment is widespread in younger greenstone belts and in those of the last stage which are completely formed in continental environment. Whole rock chemistry of Precambrian volcanics (more than 1800 analyses all over the world) confirms that alkali-olivine basalta are absent till 1.9 b.y. and andesites are generally rare. Widespread type of Early Precambrian basalts are 01-Hy and Q normative ones, showing close similarity to typical abyssal tholeiites. The presence of basaltic komatiites is not a unique petrochemical feature of volcanism in the Early Precambrian as one thinks. Now similar rocks have been found in Phanerozoic rift zones. There is no evidence suggesting significant shortening of initial crustal complexes and greenstone belts. These data along with slight metamorphism and above features can only be explained by the formation of greenstone belts in rifting environment. During the early stages (3.8-1.9 b.y.) the thickness of the protocrust was not more than lo-20 km. It allows us to understand the rare occurrence of intermediate volcanics at early stages of the Earth’s evolution. In summary it is argued that in the Early Precambrian the spread of primitive oceanic lithosphere was not compensated by the consumption in subduction zones. Consequently, plate tectonics cannot explain the formation of the crust in the early history of the Earth.
INTRODUCTION
The origin and evolution of greenstone belts in the Precambrian is the most popular and debatable problem of present day geology. The rapid growth of papers all over the world is the best illustration of such a phenomenon. 0040-1951/81/0000-0000/$
02.50 @ 1981 Elsevier Scientific
Publishing
Company
196
Before the appearance of the plate tectonics doctrine greenstone belts have been considered as alpinotype fold belts, specific type of geosynclines unknown in the Phanerozoic. Now when plate tectonics is widely used the greenstone belts are compared with rift zones, island arcs or the rift structures formed above subduction zone in marginal seas. The aim of the present paper is to synthesize available data with emphasis on the origin of greenstone belts. GENERAL
FEATURES
OF GR~ENSTON~
BELTS
Four stages in greenstone-belt development can be determined: 3.6-3.0, 3.0-2.6, 2.6-1.9, 1.9 by. and younger. Apparently these time spans as well as a number of stages reflect the state of the Precambrian geochronology at present. In time the number of stages would increase and in the first place it should be expected to reveal greenstone belts older than 3.6 b.y. The general Early Precambrian section is illustrated in Fig. 1. Independent of age all greenstone belts have similar structure, volcano-sedimentary sequences, magmatism and metamorphism. The greenstone belts are several hundred kilometers long and 50-60 km wide (Figs 2 and 3). In plan they form a system of strips, sometimes broad zones, branching off from the main belt. In some cases the overall length exceeds 2000 km (Fig. 3). In spite of the fact that Archaean shields are widely spaced they have a similar sequence (Anhaeusser et al., 1969). The Barberton greenstone belt is a tectonotype of the first and second stages. Rocks of this belt have been subdivided into lower volcanic (the Onverwacht Series) and upper volcanoelastic and sedimentary formations (Fig Tree and Moodies Groups, Fig. l), the total thickness of which is about 21000 m. The lower volcanic pile consists of ultramafic and mafic material; the upper formation (Fig Tree) includes greywacke, shale, chert at the base and trachytic tuff in the upper part. The sequence is terminated by conglomerate, quartzite, and shale interbedded with jaspilite and lavas. Rhythmic bedding, ~thologic~ composition and such sedimentary features as graded and convolute bedding, ripple marks, slump bedding suggest that the Fig Tree Group formed in the abyssal zone (Viljoen and Viljoen, 1969). Kuenen (1963) regarded this succession as a result of turbidity flow activity. Rocks of the Belingwe greenstone belt (South Africa) are ~on~orne~~s, banded iron quartzite, marble, locally stromatolitic in lower part, grading UP -__-.-..Fig. 1. Comparison of the volcano-sedimentary stratigraphy and isotope dates of Early Precambrian greenstone belts. 1 = basement (granulites, gneisses, tonalites), 2 =: ultramafic lavas, 3 = mafic laws, 4 = intermediate and felsic laws, 5 = mafic (a) and felsic (b) pyroclasts, 6 = quartzites, cherts, 7 = iron banded formation, 8 = conglomerates, 9 = sandstones, 10 = siltstones, graphite shales, 11 = limestones, 12 = dotomites. The stratigraphic columns are compiled after: A = Dimroth et al. (1970); B = Zagarodny
197
1.E
2.c 2qso
2.2 32003400
2.4
2.6
2.8
3.0
3.2
et al. (1964); C = this paper; D = Riley (written communication, 1979); E = this paper; F = Wilson et al., 1978; G = Naldrett and Turner, 1977; H = Viljoen and Viljoen (1969). Age in million years to columns: B = 1880 - ultrabasic and basic rocks intruded the Pechenga Series; 2900-3000 - metamorphism of basement; E = 1850-2000 - metamorphism of trough sequence; 3000-3500 - metamorphism of basement; F = the Great Dyke of Rhodesia, 2651, 2690 - granites, intruded by the Belingwe greenstone sequence, 2884, 2970 - basement gneisses and tonalites: G = 2670 - granite conglomerate pebbles; 2612 - granites cutting the Yakabindie Complex, 2665 - age of metamorphism; H = 2980 -shales of the Fig Tree Series, 3200-3400 - granites intruding the Onverwacht Series, 3360 - Onverwacht lavas, 3375 - cherty shales of the Middle Marker horizon, 3600 - lavas at the base of the Onverwacht Series,
Fig. 2. Early Precambrian greenstone belt in southern East Siberia 1 and 2 = greenstone belt (I -at an initial stage of separation - 3.0 to 2.6 b.y.; 2 -at a maximum stage of separation - 2.6 to 2.0 b.y.), 3 = basic to ultrabasic melanocratic basement (more than 3.0 b.y.); 4 = complexes of a strongly metamorphosed volcanie+edimentary cover (more than 3.0 b.y.) 5 = sedimentary cover (2.6-2.0 b.y.); 6 = volcano-plutonic complex and molasse (1.8-1.6 b.y.); 7 = sedimentary cover of the Siberian platform; 8 = master faults. Letters in circles show the position of columns in Fig. 1.
the section into massive and ultramafic pillow lavas and mafic rocks (Fig, 1). The upper and lower parts of the sequence are similar, except for ultramafic lavas (Wilson et al., 1978). The Yakabindie belt (Western Australia) has two rock sequences (Fig. 1). The lower part consists of ultramafic and mafic rocks (over 90%) and less than 10% felsic rocks, while the upper part is characterized by felsic rocks interbedded with basalts, volcanoclasts and chert shales (Naldrett and Turner, 1977). The structural analogs of greenstone belts have been known south of the Siberian platform where the Aldan shield extends for about 2200 km from the Baikal Lake to the Sea of Okhotsk (Fig. 2). These features have been described as suture troughs (Fedorovsky and Leytes, 1968) and later from the point of view that their development was rift controlled (Grachev and Fedorovsky, 1970). Volcano-sedimentary sequences contain greenschists and amphibolites derived from mafic lavas and tuffs, sandstones, iron quartzites and associated bodies of ultramafic rocks and gabbros (Kodar and Udokan Ranges). The lower part of the trough succession in the Olokit-Synnyr region consists of quartzites, marbles and shales interbedded with mafic and ultramafic volcanics overlain by massive and pillow basalts, black chert shales, conglomerates and sandstones. In the southwestern Aldan shield
199
Fig. 3. The main greenstone belts of the Kola Peninsula and Karelia. 1 = greenstone belts; 2 = oldest gneisses and schists; 3 = platform cover. Letters in circles: A = Parandov; B = Pechenga; C = Hautavara troughs.
(Priolchonie) the trough sequence is represented by three sedimentary cycles. Each cycle commences with marbles and mafic lavas, followed by pyroelastics and terminated by marbles (Fig. 1). And, lastly, the Ust-Gilyuy trough section is a succession of basalt flows (2500 m), greywacke (3000 m) and mafic volcanics locally with greywacke (2000 m). Similar suture troughs have been established in the eastern Baltic shield (Konkin et al., 1975; Robonen et al., 1978, Fig. 3). The lava succession of the Parandov trough consists of pillow and massive lavas interbedded with lava-breccia, tuffs and quartzites. The upper portion includes quartzites, graphitic shales, marbles, and iron pyrites. The total thickness of the entire section exceeds 2600 m. The Hautavara trough is filled in by tholeiitic and picrite basalts, rhyo-dacites, pyroclastics, quartzites, shales and iron pyrites with a maximum thickness of 4000 m. Among greenstone belts belonging to the third stage we include the Pechenga-Imandra-Varzuga (in further Pechenga) belt in the Baltic shield and
200
the Labrador trough on the Canadian shield. The former stretches through the Kola peninsula for a distance of 600 km and is about 145 km wide (Fig. 3). In the northwest the Pechenga succession consists of four sequences (Zagorodny et al., 1964). The first one contains in ascending order conglomerates, gravelites, shales and thick layers of diabases. The second sequence commences with quartzites, sandstones, dolomites and limestones and is capped by mafic and felsic lavas. The third member includes conglomerates, sandstones, quartzites, dolomites and diabases with tuff layers. The fourth part is characterized by rhythmically layered conglomerates, sandstones, siltstones, phyllites and pyroclastics which in turn grade into pillow-basalts, picrites and rhyolites. Four main cycles of sedimentation are easily determined in the Pechenga succession. They begin with elastic rocks and are terminated by shales or limestones. The lower cycles are shallow-water sediments (red beds, limestones and dolomites), the upper cycles are characterized by turbidites. The Labrador trough succession changes from place to place (Dimroth et al., 1970). Metasediments and magmatic rocks dominate at the western and eastern sides, respectively. The section in Fig. 1 shows the structure on the eastern side. At the base it consists of shallow-water sediments (conglomerates, sandstones, shales, dolomites). Cycle I of Dimroth (1972) and mafic lavas and greywackes, sandstones, quartzites and shales including the Sokoman iron formation occur in the upper portion of cycle II. The sequence is capped by basic lavas and felsic pyroblastics. It is suggested that all the above mentioned sequences have some common features independent of time span development. They include: (1) great thicknesses of volcano-sedimentary rocks formed in a characteristic sequence; (2) the presence of ultramafic lavas, usually in the lower portions and (3) if any, of intermediate lavas and pyroclastics. The disrare occurrences, appearance of ultramafic lavas in the greenstone belts of the third stage (2.6-1.9 b.y.) is a peculiar feature. The comparison of structure and metamorphism of different greenstone belts reveals their great similarity. As it was shown by Viljoen and Viljoen (1969) for the Barberton belt and confirmed later by studying other belts, most of them are characterized by broad development of large synclinal folds. Usually the anticlinal folds between the adjacent synclinal folds are absent and the latter are separated by a system of faults. Only in the vicinity of basement contact, the isoclinal folds, are widespread indicating the local compression (Barberton, Pechenga, Labrador and other belts). All the successions of greenstone belts have suffered greenschist facies metamorphism; in some cases there are signs of the greenstone regeneration. Up-grade of metamorphism to amphibolite facies occurs around the periphery of the belt. At these localities along contacts of granite-gneiss and migmatite domes with volcano-sedimentary assemblages show good evidence of sharp metamorphic changes and deformation. Generally the structure pattern resembles the outlines of diapiric domes. These data imply the lack of compression.
The mineral composition of formations, peculiar structure and metamorphism of greenstone belts and their place in the evolution of the shields suggest that these belts developed in an extension environment of primordial crust. BASIC PECULIARITIES
OF
VOLCANIC EVOLUTION
The Early Precambrian greenstone belts may be divided into two groups taking into account the structure of volcanic piles. To the first group we assign “Barber-ton types” belts (Belingwe, Selukwe and others in South Africa; Schebandowan, Kagakie Lake, Stormy Lake and others in Canada; Yakabindie, Monger, Scotia, Kalgoorly in Western Australia). The belts of Pechenga type form the second group (Kodar-Udokan and Olokit-Synnyr areas in eastern Siberia, Parandova, Hautavara and others in Karelia, Labrador, Yellowknife, Grenville in North America, and others). The former were commonly developed during early Archaean time (to 3.0 b.y.), the latter include the Late Archaean-Early Proterozoic (to 1 S-2.0 b.y.) belts. The belts of the first group are characterized by sections which we called the complete ones. They consist of three volcanic sequences: (1) ultramaficmafic; (2) mafic (tholeiitic) with subordinate felsic pyroclastics; and (3) bimodal basalt-(rhyodacite) rhyolite association. In sections of the second group the lower ultramafic-mafic member is absent, and the third member is not always present. Such sequences we refer to as incomplete types. The ultramafic-mafic volcanic pile consists of interlayered ultramafic and mafic lava. Tholeiitic basalts being significant in mafic sequence in upper portion grade into felsic volcanics and pyroclastics. It is necessary to emphasize the rhythmical layering of ultramafic-mafic and mafic piles of the greenstone belts. In the first case it is a successive change of peridotite komatiites by basalt komatiites and felsic tuffs. In the second case the cycle consists of tholeiitic basalts-rhyolites (rhyodacites) chert shales - banded iron formation. Following Viljoen and Viljoen (1969) many authors have paid attention to peculiar features of Early Precambrian volcanism. It is marked by great thicknesses of volcanic rocks (lo-15 km), presence of ultramafic and mafic lavas unknown in younger suites and defined as komatiites; the latter are characterized by a high CaO/A120J ratio and spinifex textures. Indeed, in many belts thicknesses of only the ultramafic-mafic complex is about 5-10 km and together with the second one amount to lo-19 km. But if we compare the thickness of oceanic crust of 5-7 km (2 and 3 layers together) with the crustal thickness in Iceland (up to 11 km), their uniqueness of the Early Precambrian thickness becomes doubtful. Let us consider the chemistry of basalts in greenstone belts which are under discussion. Table I shows small variation of averages. All basalts contain normative quartz (0.35-5.3s) and there is no significant difference in CIPW norms between tholeiites and komatiites. For the former CaO/A.1203 ratio ranges between 0.5 and 0.8 compared with komatiite averages of 0.9-1.35.
202 TABLE I Averages
of Early Precambrian
bassIts (wt. %) -
1
2
3
4
5
6
7
50.40 0.63 7.59 4.10 6.68 14.84 10.27 1.32 0.16 1.35
50.84 1.05 13.21 4.20 7.41 6.45 9.09 3.00 0.28 0.65
49.85
47.99
49.38
0.76 12.96 1.96 8.71 8.52 10.57 2.30 0.24 0.80
0.50 10.50 3.22 8.92 15.20 9.00 1.20 0.70 0.89
0.87 14.99 2.70 8.36 7.94 9.99 2.06 0.63 0.66
48.86 0.95 14.50 3.09 9.39 7.49 10.48 2.33 0.62 0.71
50.92 0.62 15.14 1.35 8.67 6.19 12.47 2.04 0.56 0.81
8
9
10
11
12
13
14
49.09
49.83 0.90
49.44 0.98 14.14 2.35 9.52 10.89 7.21 1.96 0.34 0.57
53.10 1.04 14.01 3.68 8.13 5.53 8.19 3.12 0.92 0.57
50.29
14.48 2.50 7.32 6.35 10.57 1.84 0.48 0.71
46.12 2.17 13.44 7.05 7.39 6.92 8.27 2.55 0.50 0.61
47.80 1.54 12.77 3.26 10.42 7.17 10.26 2.15 0.18 0.70
15
16
17
18
19
20
SiOz
47.09
TiO2
1.61 12.30 3.58 10.84 7.02 9.21 2.25 0.34 0.75
48.80 1.07 14.05 2.36 9.78 7.28 10.07 2.52 0.10 0.78
48.57 1.14 13.96 3.24 9.02 7.36 9.67 2.74 0.77 0.69
46.08 1.65 7.16 3.01 10.65 15.82 9.75 0.66 0.12 1.35
46.52 2.50 8.21 3.36 12.51 14.69 7.78 2.05 0.94 0.94
47.12 1.25 14.26 2.30 8.49 12.58 11.70 1.80 0.22 0.82
----_____ SiOz TiOz 4203 Fe203
Fe0 MgG CaO NazO K2O
CaO/AlzOa
SiOz TiOz 403 Fe203
Fe0 MgG CaO NazO K2O
CaO/AlzOa
-41203 Fe2
03
Fe0 MgG CaO Na20 K20
CaO/AIzOs
1.01 14.47 2.81 10.02 6.50 11.02 2.16 0.39 0.77
1.73 13.55 6.76 7.13 5.53 6.47 3.47 0.97 0.48
South Africa, Barberton: 1 = basaltic komatiite, 2 = tholeiitic basalt (Viljoen and Viljoen, 1969); 3 = South Africa, Bulawayo and Que Que basahs (Hawkesworth and O’Nions, 1977); Central Karelia, Suma-Semch: 4 = komatiitic basalts, 5 = metadiabase, 6 = metadiabase, Palaya Lamba - Iron Gate (Kratz, 1978); India, Dharwar: 7 = basalts (Srinivasan, Screenivas, 1972), 8 = basalts of ChitaIdrug (Naqvi, 1972); east Siberia, Olokit-Synnyr: 9 = basalt of lower and middle sequence, 10 = basalts of upper sequence; 11 = basalts of Parandov and Hautavara (data from Ruchkin); 12-l 5 = Kola Peninsula, Pechenga, basalts of first, second, third and upper sequences (data from petrochemical bank of NorthWestern Geological Survey, Leningrad); Canada, Labrador trough: 16 = basaits, 17 = gabbro (Dimroth et al., 1972), 18 = Pechenga, high Mg-basalt+ 19 = Pechenga, olivine gabbro (data from the same source as in point 12-15), 20 = high Mg-basalts, Iceland (Polyakov, 1978).
203
Fig. 4. Early Precambrian basalt6 plotted on AFM diagram. For numbers 1-19cf. Table I. Insert: a, b, c; trends of Hawaii tholeiites (Irvine and Baragar, 1971), mid-oceanic basalts and Archaean basaltic komatiites (present paper).
The AFM values for all groups, presented in Table I, are plotted in Figs 4 and 5. It is evident that there is no gap between the tholeiitic and komatiitic fields, on this diagram they all together show a common trend of differentiation. Our data demonstrate the existence of tholeiitic series of greenstone belts which are different from the Hawaii and mid-oceanic tholeiitic suites (insight in Fig. 4). In the MgO-CaO-A1203 diagram (Fig. 5) there are two fractionation lines. The first trend (olivine fractionation) is reflected by a line going away from the MgO apex of the diagram, the second one corresponds to the curve which extends towards the A1203 comer (clinopyroxene fractionation). These trends are apparently characteristic of all mafic Early Precambrian series (Fig. 6).
204
Fig. 5. AFM diagram showing fields of Early Precambrian basaltic komatiites (1) and tholeiites (Z), mid-oceanic basalts (3), abyssal volcanic glass of Atlantic (4), Pacific (5), and India (6) Oceans (Melson et al., 1977).
More than 1800 chemical analyses from various greenstone belts (with the age close to 2.0 b.y.) were used for the definition of basic chemistry type of basalts. The cluster analysis has shown the existence of two main groups of averages and CIPW norms given in Table II. The first group contains hypersthene tholeiites, and the second includes the quartz tholeiites. It is very important that Ne-normative basalts are absent in the Early Precambrian and are widely developed in the Late Precambrian and Phanerozoic. Another important feature which has been revealed by clustering procedure is the lack of evidence suggesting the existence of basaltic komatiites as a definite chemistry group. The latter is well in accord with the proposal (Williams, 1972) to use the term high Mg basalts instead of basaltic komatiites.
1.88
3.95 -
0.57 2.81 20.63 27.90, 20.55 21.72 -
2.14
2.38 23.44 26.82 19.95 18.07 2.02 4.15 -
1.09 14.34 2.77 9.58 6.87 9.97 2.68 0.39 -
0.96
14.28 2.64 8.90 7.30 10.35 2.36 0.46
49‘04
49.57
2
2.71
1.59 17.97 26.42 21.97 23.54 1.56 4.24 -
47.76 1.38 13.00 2.82 11.18 7.75 10.34 2.05 0.26 -
3
2.45
3.88 -
1.70 5.52 25.17 23.88 16.62 21.77 -
51.57 1.26 14.29 2.61 8.58 6.69 8.69 2.90 0.91
4
2.06
4.77 -
0.57 3.06 22.49 26.67 17.23 23.15 -
1.05 14.22 3.18 8.40 7.80 9.33 2.57 0.50
49.64
AVr.
4.48
8.83
2.34 6.66 25.29 20.53 16.04 15.83 -
48.57 2.28 13.21 5.89 8.67 6.23 7.87 2.89 1.09
II
4.34
8.18 -
1.92 9.02 25.68 21.32 17.04 13.48 -
2.21 13.78 5.45 7.30 6.28 8.29 2.82 1.47
49.98
1
2.83
2.04 23.62 29.98 19.55 14.62 3.79 3.58 -
1.46 15.63 2.42 8.11 7.89 10.67 2.74 0.34 -
49.30
2
Oceanic basalis
3.72
2.34 20.53 28.66 22.23 13.39 4.85 4.27
48.35 1.93 14.70 2.90 8.55 8.14 11.14 2.39 0.39 -
3
5.33 3.92 2.87
4.51 2.04 21.82 29.57 19.95 9.09 -
15.22 7.48 2.97 7.61 10.96 2.54 0.34
49.89 1.49
4
4.79
3.22 20.53 26.86 22.81 16.00 2.31 3.48 -
48.58 2.49 14.27 2.37 10.86 6.49 10.88 2.40 0.54 -
5
1.98
6.64 -
0.39 7.93 24.62 29.86 15.15 13.42 -
49.88 1.02 16.77 4.47 5.55 6.17 9.60 2.84 1.31
1
2.44
2.84 32.01 27.68 14.09 8.65 8.52 3.76 -
49.14 1.23 16.16 2.48 6.83 7.07 8.70 3.62 0.46 -
2
Island-arc basalts
1.86
16.98 4.88 -
27.03 17.35 30.22 1.08 -
48.21 0.95 17.99 3.26 6.40 6.28 6.16 2.11 4.40 -
3
1.39 30.17 26.83 17.04 12.55 0.41 1.64 7.39 2.61
49.06 1.34 15.54 8.30 1.70 8.23 9.57 3.47 0.23 -
4
4.46
6.78 -
0.48 5.99 29.51 22.21 16.75 13.81
48.68 2.25 14.34 4.48 7.68 5.74 8.29 3.34 0.97
5
Precambrian basalts: type I-subtypes 2, 2, 3, 4; abyssal basalts: 1 and 2 (Grachev, 1977); 3 and 4 (present paper); 5 = Iceland (Grachev, 1977); island arcs: 1,2,3,4,5 = main types of basalts (present paper).
Or Ab An Ix HY 01 Mt Hm Ilm
Q
K2O
FeG MgQ CaO NaaO
Fe203
A1203
SiGz TiOz
1
I
Early Precambrian basalts
Averages and CIPW norms of main types of Early Precambrian and oceanic basahs (cluster analysis)
TABLE II
206
r,g
4.--.-
AM
Fig. 6. CaO-MgO-A1203 diagram for Early Precambrian basalts. Symbols are the same as in Fig. 4; o, b, c = basalt trends of eastern Finland (Blais et al., 1977), Botswana, South Africa (Key et al., 1976) and Yakabindie, Western Australia (Naldrett and Turner, 1977).
The Early Precambrian felsic volcanism is a peculiar problem because acid lavas and pyroclastics have great thicknesses of about 3-4 km. However, very often pyroclastics are well developed in one place but in another they are almost absent. A fine example is the Shebandowan area in Canada (Smith, 1978). A similar feature is common for belts of the Kola peninsula, West Australia, South Africa. The common feature of pyroclastics is an exclusively broad development of volcano-elastic rocks (lapilly and psammite tuffs, breccias, agglomerates, lahar deposits). Usually they are rhythmically bedded when mafic lavas in the lower part grade upward into pyroclastic accompanied by thin acid lava layers.
Gee et al. (1976) suggest that felsic volcanic sequences may be formed from a number of separate central volcanoes or volcanic centres. Such a conclusion allows us to understand the disappearance of pyroclastics in the individual parts of the same greenstone belt. Many authors suggest that activity of such central volcanoes in the Early Precambrian were in conditions similar to recent island arc systems. It has been based upon the occurrence of andesites and dacite-rhyolites in upper volcanic sequences. However, the comparative analysis of frequency distribution and chemistry of these rocks has shown that they have nothing to do with island-arc volcanics. It is possible to note the following main differences: (1) Intermediate volcanic rocks (strictly andesites) are absent from most greenstone belts with the apparent exception of Canadian ones. In some cases, for example in the Sturgeon Lake volcanic belt in Canada, andesites are referred to as rocks with SiOz range between 51-53% (Franklin, 1978) in other regions (Barberton, Bulawayo in South Africa) the rocks described as andesites (Harrison, 1970) in fact are dacites (SiOz > 63%) or basaltic andesites (Si02 < 57%). The true andesites are scarce, but in island arcs they constitute more than 60% of the entire rock-volume (Erlich, 1966). It is necessary to take into account the paucity of inte~ediate rocks in Cenozoic rift zones despite their occurrence in Iceland, Ethiopia and Chara rifts. (2) The felsic rocks (rhyolites and rhyo-dacites) together with basalts from the bimodal contrasting basalt-rhyolite association. The latter is an important feature of all the greenstone belts where acid volcanism took place. So Goldich and Peterman (1978) pointed out that “this bimodal distribution is a problem common to the Archaean as well as to younger rock suites” (p. 231). The equivalent of the Early Precambrian basalt-rhyolite association within young complexes is the basalt-trachyte (rhyolite) formation of recent oceanic islands with well developed Daly gap, falling to andesites. With all the reasons we may suppose that in the Early Precambrian the basaltrhyolite association was also caused by the activity of the central type volcanoes located on proto-oceanic crust. The pattern of the differentiation trend on an AFM diagram differs from the field of Kuril-Kamchatka zone, the typical Cal-alkaline series (Fig. 7). One cannot but admit that the pattern of three volcanic belts is considered as an example of typical island-arc systems formed due to different mechanism and under different conditions. (3) The sign~ic~t difference between the chemists of greenstone belts and island arcs basalts (Table II) is shown by A1203, CaO, Na,O and K,O contents. Gottini-Rittmann’s diagram (Fig. 8) confirms this conclusion. Furthermore, Hawkesworth and O’Nions (1977) have shown that Archean greenstone belt volcanics and those of modem island arcs are distinguished by REE and trace elements. (4) And finally, the difference lies in the absence of lateral zonality in greenstone belts volcanics. As is well known, the latter is an important character of modem island arc volcanism. As a whole, all above data suggest
208
”
\i
i.
”
v
”
\,
'M
”
Fig. 7. Fields and trends of volcanic series of present island arcs, rifts, and Precambrian greenstone belts; 1 = Kuril-Kamchatka (Erlich, 1966); 2 and 3 = basalt-trachyte association; (2) Udokan Range, Baikal Rift (present paper), and (3) the Azores (White et al., 1979); 4 = Rainy Lake, Canada, and 5 = Vermillion, Canada (Goldlich and Peterman, 1978).
1.5 b 49 1.0
0.5 -0.5 01
02
0.
0.5
l3
4 (’ 0
&l&o 5 [-‘\,6 k
\7
209
that greenstone belt volcanism in the Early Precambrian is very similar to that of the tholeiites series of modern oceanic and continental rift zones. In belts of the first and second stages it has the oceanic trend and in the third stage belts, the features of transitional rift volcanism take shape (Grachev, 1977). TECTONIC STYLE OF GREENSTONE
BELT DEVELOPMENT
It is evident that any attempt to find modern structure equivalents of greenstone belts must include a comparative analysis of both Archaean and recent volcanism. Tectonically one may only speak about two main types of volcanics. The first one includes volcanics of extension areas (continental and oceanic rifts), the second one is referred to as volcanics of early erogenic (island arc) and late erogenic stages of fold belts. As mentioned above, the greenstone belt volcanism has nothing to do with that of island arcs as well as with fold belts (young and mature). Moreover, a comparative analysis of basalts of all tectonic regimes throughout the earth’s history suggests that greenstone belt volcanics formed in areas of extension of earlier protocrust, i.e. in rifting environments. That is why it is possible to determine the greenstone belts as rift structures. Because of the absence of the island arcs in Early Precambrian, there is no sense in using the marginal basin model for greenstone belts origin. Noting the similarity between the greenstone belts and modern rifts one takes into account the apparent features of distinctions. In the Early Precambrian greenstone belts the olivine alkaline (Ne-normative) basalts and other alkaline rocks are unknown, at the same time they are widespread in modern continental rifts. The chemistry of mafic volcanics and sedimentary sequences of greenstone belts led to the conclusion about the existence of all three stages of rift development: continental, transitional and oceanic (Grachev, 1977). Although all the presently known greenstone belts formed on pre-existing sialic crust, the latter differed greatly from the Phanerozoic continental crust in thickness. A primary crust being very thin, only tholeiitic lavas would be generated under low pressure conditions. During the course of granite-greenstone terrain development the early crust may have thickened bringing into effect the mechanism of the formation of the mature continental crust. The main part of the present continental crust has already been created as a result of Early Precambrian tectogenesis about 1.8-2.0 b.y. ago. There are two cycles in the evolution of greenstone belts. The first cycle is characterized by graben formation and syntectonic volcanic pile accumulaFig. 8. Early Precambrian basalts on Rittmann-Gottini diagram. 1 = airerage compositions of Archaean basalts (cf. Table I), 2 = averages of abyssal tholeiites and abyssal volcanic glasses, 3 = averages of main types of island arc basalts (for numbers cf. Table II), 4,5, 6 = fields of average basalt compositions of Archaean, mid-oceanic ridges and island arcs, 7 = tholeiite = alkaline basalt - hawaiite trend (Rittmann, 1973).
210
tion. Sedimentation took place within the graben. The next cycle was marked by the growth of granitic diapir uplift and erosion resulting in the formation of mountain relief and appearance of coarse-grained sediments containing granite material. Similar stages are known in Cenozoic continental rift development (Grachev, 1977). The lower sequence consists of fineg-rained rocks, and upper portion is referred to as a molasse. In oceanic rifts at the initial stage of spreading the second stage is marked by the deposition of turbidites. Such a similarity of sections of upper portions of greenstone belts sequences and Cenozoic rift filling was associated with mountain-relief formation due to erosion of uplifted rift shoulders. In greenstone belts the reason for the growth of granite diapirs is an isostatic rise (Ramberg, 1967) in the Cenozoic continental rifts; the uplift of shoulders reflects the gravity instability of the anomalous mantle. CONCLUSIONS
The comparison of structure, volcano-sedimentary filling and magmatism of both greenstone belts and modern rifts shows that all of them have many features in common. Therefore it is possible to conclude that rifting is a principle process of the oceanic crust formation in the geological history of the earth. Besides, the morphology and structure of sequences of greenstone belts indicates that in the Early Precambrian rifting no basins comparable with modern oceans were formed. However the numbers of greenstone belts established in the Precambrian shields is so large that their total width is commensurable with, for example, the Atlantic Ocean. In the Early Precambrian subduction did not take place since no significant volumes of andesites have been found. This suggests that plate tectonics did not operate during most of Precambrian time. ACKNOWLEDGMENTS
The authors are grateful to Drs. M.S. Markov and A.A. Saveljev for discussions. We also thank Mr. V.I. Mishin for developing the computer program and Mr. N.B. Phillipov for technical assistance. REFERENCES Anhaeusser, C.R., Mason, R., Viljoen, M.J., and Viljoen, R.P., 1969. A reappraisal of some aspects of Precambrian shield geology. Geol. Sot. Am. Bull., 80: 2175-2200. Blais, S., Auvray, B., Capdevila, R and Hameurt, I., 19’77. Les series komattques et tholeiitiques des uintures archeennes de roches vertes de Finlande orientale. Bull. Sot. Geol. Fr., XIX: 965-970. Dimroth, E., 1972. The Labrador geosyncline revisited. Am. J. Sci., 272: 487-506. Dimroth, E., Baragar, W.R.A., Bergeron, R. and Jackson, G.D., 1970. The filling of the Circum-Ungava geosyncline. In: A.J. Boer (Editor), Symposium on Basins and Geosynclines of the Canadian Shield. Geol. Surv. Can. Pap., 70-40: 45-142.
211 Erlich, E.N., 1966. The petrochemistry of Cenozoic Kuril-Kamchatka volcanic province Nauka, Moscow, 280 pp. (in Russian). Fedorovsky, V.S. and Leytes, A.M., 1968. On the Early Proterozoic geosynclinal troughs in the Olekma-Vitim Mountain Land. Geotectonica, 4: 46-61. (in Russian). Franklin, J.M., 1978. Petrochemistry of the South Sturgeon Lake volcanic belt. Proc. 1978 Archean Geochemistry Conf. Univ. Toronto Press, Toronto, Ont., pp. 161-180. Gee, D.R., Groves, D.J. and Fletcher C.J., 1976. Archaean geology and mineral deposits of the Eastern Goldfields. Guideb. Excursion 42A, 25th Int. Geol. Congr., Sydney. Goldich, S.S. and Peterman, Z.E., 1978. Geology and geochemistry of the Rainy Lake area. Proc. 1978 Archaean Geochemistry conference, Toronto Univ. Press, Toronto, Ont., pp. 209-234. Grachev, A.F., 1977. The rift zones of the Earth. Nauka, Leningrad, 248 pp. (in Russian). Grachev, A.F. and Fedorovsky, V.S., 1970. On unified nature of rifts, aulacogenes and geosynclinal troughs. Soviet Geology: 12, 121-122 (in Russian). Also in: Int. Geol. Rev., 2,197l. Harrison, N.M., 1970. The geology of the country around Que Que. Bull. Geol Sure. Rhodesia, 67: 125. Hawkesworth, C.I. and O’Nions, R.K., 1977. The petrogenesis of some Archaean volcanic rocks from Southern Africa. J. Petrol., 18: 487-520. 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. Key, R.M., Litherland M. and Hepworth I.V., 1976. The evolution of the Archaean crust of northeast Botswana. Precambrian Res., 3: 375-413. Konkin, V.D., Ruchkin, G.V. and Fedorovsky, V.S., 1975. The comparative analysis of the Precambrian suture structures of Karelia and BaikaI area. Geotectonika, 3 : 15-26 (in Russian). Kratz, K.O. (Editor), 1978. The geology and petrology of the Archaean granite greenstone terrain in Central Karelia. Nauka, Leningrad, 264 pp. (in Russian). Kuenen, Ph.H., 1963. Turbidites in South Africa. Trans. Geol. Sot. S. Afr., 66: 191-195. Melson, W.G., Byerly, G.R., Nelen, J.A., O’Hearn, T., Wright Th.1. and Vallier, T., 1977. A catalog of the major element chemistry of abyssal volcanic glasses. In: B. Mason (Editor), Mineral Sciences Investigations 1974-1975, Smithson. Contrib. Earth Sci., 19: 31-60. Naldrett, A.I. and Turner, A.R., 1977. The geology and petrogenesis of a greenstone belt and related nickel sulfide mineralization at Yakabindie, Western Australia. Precambrian Res., 5: 43-103. Naqvi, S.M., 1972. The petrochemistry and significance of Jogimsrdi traps, Chitaldrug schist belt, Mysore. BuII. Volcanol., XXXY,: 1069-1093. Polyakov, A.I., 1978. The chemical composition of magmatic rocks. In: V.V. Beloussov, V.I. Gerasimovsky (Editors), Iceland and Mid-Oceanic Ridge. Geochemistry, 1978: 7-54. Ramberg, H., 1967. Gravity, Deformation and the Earth’s Crust as Studied by Centrifuged Models. Academic Press, London, 214 pp. Rittmann, A., 1973. Stable Mineral Assemblages of Igneous Rocks. A Method of CaIculation. Springer-Verlag, Berlin, 353 pp. Robonen, V.J., Rybakov, S.J., Ruchkin, G.V., Konkin, V.D., Svetova, A.J. and Sergeeva, N.E., 1978. Iron Pyrites of Karelia. Nauka, Leningrad, 192 pp. (in Russian). Smith, I.E.M., 1978. Volcanic and Plutonic Rocks of the Lake She Bandowan Area. Proc. 1978 Archaean Geochemistry Conference. Toronto Univ. Press, Toronto, Ont., pp. 185-191. Srinivasan, R. and Sreenivss, B.L., 1972. Flood basal& from Dharwars of Mysore, India. Bull. Volcanol., XXXY: 824-840. Viljoen, M.I. and Viljoen, R.P., 1969. Introduction to the geology of the Barberton granite-greenstone terrain. Geol. Sot. S. Afr. Spec. Publ., 2: 9-28.
212 White, W.M., Tapia, M.P.M. and Shilling, J.G., 1979. The petrology and geochemistry of the Azores islands. Co&rib. Miner. Petrol., 69: 201-203. Williams, D.A.C., 1972. Archean ultramafic, mafic and associated rocks, Mt. Monger, Western Australia, J. Geol. Sot. Aust., 19: 163-188. Wilson, I.F., Bickle, MI., Hawkesworth, C.I., Nisbet, E.G. and Orpen, I.L., 1978. Granitegreenstone terrain of the Rhodesian Archaean craton. Nature, 271: 23-27. Zagorodny, V.G., Mirskaya, D.D. and Suslova, S.N., 1964. Geological structure of the Pechenga volcano-sedimentary sequence. Nedra, Leningrad, 84 pp. (in Russian).