The Lapland charnockitic complex: REE geochemistry and petrogenesis

Lithos, 19 (1986) 95-111 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

95

The Lapland charnockitic complex: REE geochemistry and petrogenesis p. BARBEYI,2, j. BERNARD_GRIFFITHS2 and J. CONVERT2 1Laboratoire de P~trologie, Universit~ de Nancy L BP 239 54506 Vandoeuvre-les-Nancy C~dex (France) 2Centre Armoricain d'Etude Structurale des Socles, Universit~ de Rennes, 35042 Rennes C~dex (France)

LITHOS

Barbey, P., Bernard-Griffiths, J. and Convert, J., 1986. The Lapland charnockitic complex: REE geochemistry and pet~ogenesis. Lithos, 19: 95-111. The Lapland charnockitic complex is mainly composed of basic and intermediate granulites, with more restricted types including ultramafic rocks and one high-Mg enderbite occurrence. All these rocks are interbedded with or intrude the metasedimentary granulites (khondalites). The charnockites can be subdivided on the basis of their Y and REE contents into three groups. The first group is characterized by an increase in Y content with progressive differentiation and by fractionated LREE-enriched patterns quite similar to those of some Archean granulites. The REE distribution patterns of these rocks suggests an origin by partial melting of a LREE-enriched mantle source (metasomatism). The other groups and the high-Mg enderbite are depleted in Y and display strongly fractionated REE patterns either with U-shaped HREE and positive Eu anomalies similar to Archean TTG rocks despite more basic major element composition (group II and the high-Mg enderbite) or with strong HREE depletion (group III). The high-Mg enderbite approximates closely high-Mg andesites with respect to major and some trace elements and its REE pattern is identical to those of early Archean group III komatiites of the Onverwacht Group, South Africa. Parental magmas for group II rocks and the high-Mg enderbite are assumed to derive by partial melting of a garnet-enriched mantle source metasomatized by a LREE-enriched fluid, whereas those of group III could be produced by melting of quartzeclogites or garnet-amphibolites with fractionated REE patterns similar to those of group I. The overall petrographical and geochemical features of the Lapland charnockitic complex are consistent with an evolution of the Belomorian fold belt according to a collision model. (Received Devember 7, 1984; accepted after revision November 1, 1985)

Introduction Following Pichamuthu (1969), we use the term "charnockitic rocks" and "charnockites" to distinguish a series of basic to intermediate orthopyroxenebearing meta-igneous rocks (as opposed to khondalites of metasedimentary origin) which have crystallized or recrystaliized under water-deficient conditions. Basic and intermediate compositions are termed norites and enderbites (Streckeisen, 1974), or basic and intermediate granulites. The Lapland charnockitic rocks have been inter0024-4937/86/$03.50

© 1986 Elsevier Science Publishers B.V.

preted as calc-alkaline plutonics (Wright et al., 1978) or volcanics (Raith et al., 1982), but further investigations have shown the great complexity of this unit probably to be composed of both metavolcanic and plutonic rocks (Barbey, 1982). Recent geochronological data suggest that the granulite facies metamorphism (estimated at 1 . 9 - 2 . 0 Ga) was approximately coeval with the magmatism of the Granulite Belt (Bernard-Griffiths et al., 1984). The aim of this study is to re-examine the geochemistry of charnockitic rocks from the Lapland Granulite Belt. Their major, trace and rare-earth

96 element compositions are presented together with additional field and mineralogical data, in order to decipher the nature of their protoliths and to discuss their petrogenesis and their bearing on the evolution of the Granulite Belt. Field relations

The granulites of Lapland together with the Tana amphibolites (Fig. 1) constitute the axial zone of the Belomorides, an early Proterozoic fold belt (2.4-1.9 ~ Ga). The Granulite Belt consists predomin~tly of: (1) a major metasedimentary unit (khondalite suite) interpreted as a flysch-like graywacke-shale sequence; and (2) a unit of pyroxenebearing meta-igneous rocks (charnockitic complex) ranging in composition from ultramafic to intermediate (Barbey et al., 1984). All these rocks are characterized by granulite facies mineral assemblages giving evidence for a polyphase metamorphism (Raith et al., 1982; Barbey et al., 1984).

The charnockitic rocks although of widespread distribution within the Granulite Belt occur mainly in the vicinity of Ivalo and along the western side of the belt. A close examination of their relations with the regional structures (in particular with the main foliation) reveals a two-fold occurrence: (1) small foliae to several tens of meters thick intercalations within metasediments, conformable with the foliation and affected by the main deformation phases; and (2) crosscutting bodies (dikes, intrusions of variable size), less gneissose than the former and displaying relict igneous structures (viz. magmatic layering and breccia, porphyric structures, etc.).

Petrography The rock types have been described by Eskola (1952) which distinguished ultramafic, noritic and quartz-noritic granulites. Norites and enderbites constitute the dominant rock group, whereas restrict-

97

259

56

5

Utsjoki

J64e PJ73 ~ e L 234

VI 4

J.~38

I" 5

~lJ"

N

]

l

J9 •

J31 ~ J30e

L1683 L41;

Ivalo

•

5 KM

I

Fig. 1. Schematic map (previous page) of the Lapland Granulite Belt and adjacent areas. The Vaskojoki anorthosite in black. Boxes indicate sampled areas shown by the maps of sampling localities on this page, based on R o a d Map o f Ft'nland (1977).

ed or quite particular types include the ultramafics and one high-Mg enderbite occurrence. Only the most striking features and new data will be given below. Sample location is given in Fig. 1. Ultramafic rocks are mainly pyroxenites. They are constituted either of orthopyroxene-olivinespinel or orthopyroxene-clinopyroxene-hornblendeplagioclase or orthopyroxene-garnet assemblages. Some ultramafics occur as a crosscutting dike (sample L984) or brecciated fragments in a noritic dike

(sample L967) and correspond to cumulate of orthopyroxene with minor interstitial quartz and/or sulphides. Scarce occurrences of cortlandite have been reported in the southern part of the belt by Eskola (1952). Norites and enderbites occur generally as regular beds conformable with the structures of the khondalites, but locally as crosscutting dikes (samples J15 and L1669) or bodies (sample L259). Granoblastic, polygonal or flaser textures prevail in these rock

98 tions and sometimes occur as streaks enclosed in pyroxenes suggesting possible early crystallization (samples L234 and L149a). A quite uncommon sample (high-Mg enderbite J61) occurring as a dike, has been collected along the Tana fiver. The nature of its mineral phases makes it distinctive: bytownite (40%), orthopyroxene with a XFe of 0.32 (35%), phlogopite (10%), quartz (15%) and accessory minerals for less than 1% (voluminous rounded zircons, apatite, futile and opaque oxides).

types. The foliation is generally well defined by a compositional layering often accentuated by elongated streaks of pyroxene and ilmenite, or by the preferred orientation of quartz ribbons and biotite flakes. Nevertheless, we have identified clear magmatic textures (samples J12, J28, J36, L256, L259, L413, L1008 and L1048), exceptionally porphyritic (sample L256) or with lath-plagioclases (sample J63). Norites have variable colour index (25 < M < 60) and contain frequently significant amounts of clinopyroxene in addition to orthopyroxene and plagioclase. Garnet + clinopyroxene associations have only been found in samples characterized by whole rock XFe [= Fe/(Fe + Mg)] higher than 0.55 (Fig. 2). Enderbites are dominantly plagioclaseorthopyroxene rocks (10 < M < 35). Plagioclases are frequently antiperthitic and perthitic K-feldspar appears as single crystals only in the most differentiated members. In norites and enderbites, hornblende occurs generally as secondary crystals. However, small amounts of primary amphibole are seemingly present in a few cases (sample L34). Biotite can be found in stable contact with pyroxenes (sample L19) or as symplectitic intergrowths with quartz replacing earlier pyroxene or surrounding opaque oxides. Accessory minerals like zircon and apatite are frequently present in significant propor-

Metamorphism Chemistry of mafic mineral assemblages and related thermobarometry provide an insight into the nature of mineral phases which have crystallized from magmas emplaced during the high-grade metamorphic event, thus giving constraint for models of manna differentiation.

Amphibole fractionation The presence of CO2-rich metamorphic fluid implied by paragenetic constraints as well as by studies of cordierite (Armbruster et al., 1982) and

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0.6

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o

oo L166g o

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0

0

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0.4

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30

40

I

10

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I

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20

Fig. 2. Plot of whole-rock XFe [= Fe/(Fe + Mg)] against normative silica saturation defining the "garnet-in" index (Coolen, 1980). The transition between Gt-Cpx (filled circles) and Opx-P1 (open circles) assemblages corresponds to an index of 0.55 (the bulk XFe for exactly saturated composition), except for sample L1669. Note that the lower the index, the higher is the required pressure at which garnet is stable with clinopyroxene, at constant T.

99 fluid inclusions (Klatt, 1980; Barbey, 1982)makes extensive amphibole fractionation from synmetamorphic magmas unlikely. Only small amounts might have crystallized from water either retained in the magma or inherited from the surroundings.

Garnet fractionation Peak metamorphic conditions, estimated at 800 + 50°C and 8 -+ 1.5 kbar, are located within the Gt-Cpx-P1-Q stability field (Raith et al., 1982; Barbey et al., 1984). Besides, the simultaneous existence of Cpx-Gt and Opx-P1 associations in norites and enderbites is dependent on whole-rock compositions (Fig. 2). This suggests that garnet can have been on the liquidus of synmetamorphic igneous rocks, provided that magmas were sufficiently iron rich. Some basic dikes (e.g., sample L1669) emplaced after the main deformation episode and related peak metamorphic conditions, show evidence for garnet crystallization in magmas emplaced in the khondalites during the granulite facies metamorphic event. Sample L1669 is a mafic cumulate made up of a coarse-grained granoblastic groundmass of clinopyroxene, orthopyroxene and plagioclase (plus minor amount of hornblende, biotite and quartz) containing garnet remnants. Relict garnet crystals are surrounded or even completely resorbed by symplectitic intergrowths of plagioclase and elongate orthopyroxene droplets. The chemical composition of the orthopyroxene droplets arising from the garnet breakdown is identical to that of the groundmass forming orthopyroxenes. It is worth considering that this sample contained at a given time Gt + Cpx assemblages although having a XFe of 0.45 (Fig. 2). This suggests that garnet crystallized at pressures higher than those prevailing during the peak metamorphic conditions. Moreover, the pressure at which garnet was destabilized and equilibrated with its orthopyroxene-plagioclase reaction rim is estimated according to the calibration of Newton and Perkins (1982) at 6 -+ 1.5 kbar. Therefore, it appears that: (1) this mafic dike intruded the khondalites after the peak conditions of metamorphism (that is outside the Gt + Cpx stability field); and (2) that garnet, crystallized at higher pressures from a magma and carried as phenocryst, was subsequently destabilized in lower-pressure conditions during magma emplacement.

Analytical procedures Forty-nine representative samples collected all over the Granulite Belt (see location in Fig. 1) have been selected for whole-rock analyses. These were carried out at the University of Rennes. Major element concentrations have been determined by XRF spectrometry except for Mg and Na which were determined by atomic absorption. The analytical precisions are: 1% (MnO); 2% (SiO2, FeO, CaO, K20, TiO2); 3% (A12Oa, MgO, Na20); and 10% (P2Os). Trace element concentrations (except REE) were determined by XRF and the analytical precisions are ~ 5% when contents are > 20 ppm and 10% when contents are < 20 ppm. REE concentrations were determined by the isotope dilution method. The detailed analytical procedure used at Rennes can be found in Jahn et al. (1980). Analytical precisions are currently better than 2% except for La, Lu and Ce for which they are better than 5%.

Geochemistry The major and trace element results are presented in Tables 1-3. Basic and intermediate compositions have been distinguished following Gill (1981) using silica content on an anhydrous basis. Ultramafic rocks are not considered here.

Y and REE compositions Irrespective of their mode of occurrence, the Lapland charnockites can be subdivided on the basis of their Y and REE distribution patterns into three distinct groups plus the high-Mg enderbite. Group I shows an increase in Y concentrations (23-52 ppm) with progressive differentiation (Fig. 3), similar to the standard trend of Lambert and Holland (1974), and moderately fractionated REE patterns. The remaining samples comprising the two other groups and the high-Mg enderbite are characterized either by an impoverishment in Y as differentiation proceeds or by very low Y contents (Fig. 3) and display strongly fractionated REE patterns.

Group I. All REE patterns (Table i; Fig. 4a) are fractionated [(La/Yb)N = 3.4-12.8] for light REE (LREE) [(La/Sm)N = 1.3-3.2] as well as for heavy

K/Rb (La/Yb)N Yb N

La ( p p m ) Ce Nd Sm Eu Gd Dy Er Yb Lu Rb Sr Ba V Nb Zr Y Ni Cr Co

SiOa ( w t . % ) A120 a FeO MnO MgO CaO Na20 K20 TiO 2 P2Os

Sample No.

234 4.2

257 2 25 15 60 627 74

21.15 11.29 2.54 0.92 2.51 2.39 1.37 1.28 0.20 16 105

49.74 8.21 11.83 0.18 15.73 12.57 0.70 0.45 0.47 0.12

L34

275

75 589 2199 249 8 162 27 172 882 56

49.86 12.28 11.19 0.18 11.47 8.68 2.55 2.48 0.92 0.39

L19

Basic g r a n u l i t e s

158 12.75 8.51

160 27 55 30 60 82 36

34.17 85.25 46.85 11.26 1.61 10.50 7.33 2.93 1.77 0.23 159 328

50.32 19.44 9.85 0.09 6.1 6.09 3.30 3.03 1.12 0.66

L1008

311 4.45 10.14

14.21 37.18 20.50 4.61 1.03 4.12 3.58 2.17 2.11 0.34 32 154 734 300 5 39 22 147 1748 63

50.99 15.32 10.02 0.18 11.32 9.19 1.10 1.20 0.54 0.16

JS1

329 8.76 12.74

35.16 91.01 58.82 11.95 2.76 9.40 6.36 2.99 2.65 0.37 30 1337 932 197 8 49 31 143 292 61

51.00 15.83 8.93 0.18 9.66 9.02 2.67 1.19 0.96 0.61

J53

269

8 132 30

50 239 680

51.24 15.74 7.61 0.15 7.76 12.69 2.06 1.62 0.76 0.38

J108

526

15 148 18

9 542 255

51.32 18.47 8.82 0.18 6.36 8.60 3.01 0.57 2.07 0.67

L105

Major a n d t r a c e e l e m e n t c o m p o s i t i o n s o f L a p l a n d c h a r n o c k i t i c r o c k s : g r o u p I

TABLE 1

410

3 379 32

16 869 531

51.74 15.90 10.27 0.15 8.41 8.21 2.85 0.79 0.92 0.79

J54

136

28

78 2869 2697 197 9 232 24

51.99 18.48 8.18 0.10 6.57 8.95 2.15 1.28 0.67 1.63

J99

278 3.36 8.03

175 2 104 23 28 41 78

8.51 22.42 16.55 4.07 1.30 3.97 3.17 1.79 1.67 0.25 23 56O

52.13 21.85 8.08 0.11 5.02 9.65 1.48 0.77 0.75 0.16

L1040

K/Rb (La/Yb)N Yb N

La(ppm) Ce Nd Sm Eu Gd Dy Er Yb Lu Rb Sr Ba V Nb Zr Y Ni Cr Co

SiO2(wt.% ) Al20 a FeO MnO MgO CaO Na20 K20 TiO 2 P205

Sample No.

350

21

28 820 1074 184 7 259 26

53.14 19.96 9.24 0.11 4.2 7.32 3.11 1.18 1.21 0.53

672 7.03 8.70

19.28 48.04 26.28 4.97 1.33 4.10 3.51 1.95 1.81 0.29 10 561 327 136 6 71 19 45 325 45

53.21 19.18 8.03 0.13 5.76 8.95 2.95 0.81 0.67 0.32

J70

1439

2 89 33

3 388 32

374

1.5 534 71

55.21 15.85 13.3 0.20 4.05 7.53 2.13 0.26 1.20 0.26

J67

14 867 183

54.41 14.57 9.37 0.14 9.69 9.32 1.06 0.63 0.65 0.16

J56

Intermediategranulites

J13

297

11 33 26 39 98

50 427

56.66 18.30 7.79 0.12 4.26 6.48 3.13 1.79 0.94 0.53

L56

171

5 95 24

18 531 202

57.79 i7.36 10.14 0.17 3.61 7.40 1.73 0.64 0.91 0.25

J103

171

9 90 38

72 337 390

57.82 16.55 11.93 0.19 4.24 3.61 2.72 1.48 1.28 0.18

J101 58.32 17.58 9.64 0.13 3.23 6.72 1.33 1.15 1.65 0.25

180

200 10 164 26 33 41 60

53 446

L7

260

6 98 28

22 535 142

58.95 16.71 10.24 0.17 3.60 6.89 1.56 0.69 0.93 0.25

J102

782 6.95 11.25

132 18 259 31 33 54 44

24.62 51.33 26.73 5.32 2.21 4.94 4.24 2.57 2.34 0.36 12 362

58.97 16.83 9.55 0.11 3.20 5.51 2.91 1.13 1.45 0.35

L259

276

150 10 143 30 211 113 31

34 206

62.19 17.06 8.64 0.10 1.87 4.24 3.03 1.16 0.71

L1460

477

18 297 52

34 322 1268

62.78 16.45 7.34 0.17 2.30 4.67 2.84 1.93 1.20 0.30

J64

276

119 12 163 39 50 94 24

83 208

64.85 15.76 7.27 0.08 2.81 2.73 2.78 1.95 0.95

L1683

102 TABLE 2 Major a n d t r a c e e l e m e n t c o m p o s i t i o n s o f L a p l a n d c h a r n o c k i t i c r o c k s : g r o u p II a n d t h e high-Mg e n d e r b i t e Sample No.

Basic g r a n u l i t e s J63

SiO 2 ( w t . % ) AI203 FeO MnO MgO CaO Na20 K20 TiO 2 P205 La ( p p m ) Ce Nd Sm Eu Gd Dy Er Yb Lu Rb Sr Ba V Nb Zr Y Ni Cr Co K/Rb (La/Yb)N Yb N

51.16 18.86 8.42 0.13 9.25 9.48 1.58 0.40 0.46 0.25

3 420 127 147 2 39 13 97 3910 72 1107

L234 51.24 18.61 9.85 0.14 5.76 8.34 4.08 0.39 1.25 0.35

Intermediate granulites L1018 52.5 19.69 9.69 0.15 5.38 7.88 2.68 0.76 1.00 0.27

15.59 33.59 19.27 4.02 1.57 3.65 2.79 1.53 1.29 0.19 2.5 692

10 738

222 3 105 17 56 106 47

217 3 82 13 36 77 48

1295 7.98 6.20

631

J74 53.38 18.29 9.60 0.12 5.30 7.98 3.40 0.50 1.09 0.34

1.5 505 371 7 64 16

2767

REE (HREE) [(Gd/Yb)N = 1.6-4.8] and resemble those of the metavolcanic rocks from the Tana belt (H/3rmann et al., 1980). The REE concentrations do not systematically increase with increasing SiO2 and MgO contents, therefore suggesting that these rocks do not arise from the differentiation of a single parental magma. Sample L1008 shows together with a pronounced negative Eu anomaly (Eu/Eu* = 0.45), a quite high total REE content (EREE = 211). These features, the high P205 content (0.66%) and the presence in thin section of abundant clusters of voluminous apatite crystals indicate that this could be due to apatite accumulation. Sample L259 has a positive Eu anomaly (Eu/Eu* = 1.3) and although its REE pattern resembles those of group II

J62

High-Mg enderbite J149a

J50

J149b

J61

54.61 17.85 10.04 0.12 6.15 6.72 2.24 0.96 1.13 0.19

57.41 18.45 8.10 0.08 3.89 6.23 3.61 0.87 1.16 0.19

59.35 17.93 7.15 0.10 3.44 6.14 3.44 0.91 0.93 0.25

59.19 17.44 8.01 0.09 4.14 5.99 3.00 0.74 1.17 0.23

58.13 14.70 7.33 0.11 10.05 7.05 0.93 0.96 0.58 0.15

18.27 35.70 16.30 2.64 1.29 1.88 1.33 1.69 0.34 18 445 446 241 6 146 11 64 174 61

15.15 39.39 23.54 4.55 0.85 3.40 2.25 1.25 1.18 0.18 8 636 496 166 5 41 6 57 61 99

20.00 36.90 16.49 2.75 1.62 2.17 1.55 0.90 0.94 0.16 6 170 360 170 6 170 8 42 99 53

18.77 33.60 15.13 2.54 1.54 1.96 1.20 0.59 0.49 0.08 6 604 453 149 6 117 7 47 83 75

13.06 28.35 13.53 2.66 0.98 2.23 2.05 1.32 1.52 0.25 49 505 277 161 4 106 1,5 102

443 7.13 8.13

903 8.48 5.67

1259 14.11 4.50

1024 25.14 2.37

163 5.67 7.31

33

rocks, it differs by a higher HREE level for a same MgO content (YbN about 2.5 times). The LREE-enriched patterns of these rocks are similar to the patterns of many continental basalts (Basaltic Volcanism, 1981) despite lower TiO2 and higher Sr contents in Lapland rocks. Some Archean basic granulites from Madras, India (Weaver, 1980) and from Qianxi, China (Jahn and Zhang, 1984) show identical REE characteristics with similarly fractionated LREE-enriched patterns [(La/Yb)N = 1.6-5.3 in Madras granulites, 3.5-8.7 in Qianxi granulites].

Group 11. This group for which five samples are available, is rather homogeneous (Table 2; Fig. 4b).

103 The patterns are strongly fractionated [(La/Yb)N = 7.1-25.0], LREE enriched [(La/Sm)N = 1.820.3] but moderately HREE fractionated [(Gd/ Yb)N = 1.8-3.2] with typical concave-upwards shapes. They exhibit a noticeable positive Eu anomaly (Eu/Eu* = 1.27-2.05) except sample L149a (Eu/Eu* = 0.64). The (La/Yb)N and Eu/Eu* ratios increase with increasing silica content, but the former ratio is closely dependent on the HREE contents. Whereas YbN varies from 2 to 8 times chondritic values, Ce N remains fairly constant (40-50). The overall decrease in total REE level is closely conditioned by a rapid decrease in HREE abundances, suggesting garnet fractionation. These rocks have REE characteristics comparable to some Archean granulites with the same positive Eu anomaly (Weaver, 1980; Jahn and Zhang, 1984), but they differ by a more basic character with respect to major element compositions.

Group 111. Few data are available for this group (Table 3; Fig. 4c). The REE pattern of sample J52 is clearly fractionated [(La/Yb)N = 13.5] with LREE enrichment, and exhibits a slight positive Eu anomaly (Eu/Eu* = 1.06). The REE pattern of sample J36 is highly fractionated [(La/Yb)N = 232] with a strong HREE depletion. This pattern is quite similar to that given by Raith et al. (1982), quoted for comparison. Although they resemble TTG rocks (tonalite-trondhjemite-granodiorite) occurring in many Archean terrains (Martin et al., 1983; Jahn and Zhang, 1984), they differ by significantly lower silica (< 60% in group III rocks, > 68% in TTG rocks) and higher FeO, MgO and CaO contents. The high-Mg enderbite. Its REE pattern (Fig. 4b) is fractionated with a (La/Yb)N ratio (5.7) higher than those of LREE-enriched boninites [(La/Yb)N ,< 5.4, according to Cameron et al. (1983)]. The pronounced symmetrical U-shaped HREE pattern is similar to those of group II rocks and owing to the fact that they crop out in the same restricted area (see Fig. 1), this suggests genetic relationships between these rocks. Sample J61 resembles the highmagnesian rock from Papua given by Sun and Nesbitt (1978), but the REE concentration level is considerably higher in the Lapland high-Mg enderbite (about 5 times). The similarity in REE patterns with the group III komatiitic rocks from Onverwacht, South

Africa (Jahn et al., 1983) is much more striking: identical REE concentration level, tilted HREE, LREE enrichment, CaO/A1203< 1 and A1203/TiO2 > 20, but higher (Gd/Yb)N ratio (> 1). Finally, sample L1669 with depleted LREE [(La/Yb)N = 0.70] and weakly fractionated HREE [(Gd/Yb)N = 1.17] cannot be related to any other rock types. Its low REE concentration level is compatible with its cumulate nature.

Major and other trace element compositions Group I. The Mg number in norites are low (< 65) and suggest that the corresponding liquids have not been in equilibrium with mantle peridotite$. Moreover, their irregular Ni (28-172 ppm) and Cr ( 4 1 1748 ppm) contents, positively correlated with MgO imply either fractionation of olivine and pyroxene (sample L1040) or accumulation of pyroxene (samples L19 and J51). These features together with the fact that samples of intermediate composition are much more abundant than basic ones suggest that magmas have undergone igneous differentiation prior to their emplacement. P205 (0.161.63%) and Zr (39-232 ppm) contents are quite variable, but except for samples L1008, J51 and J53, their values normalized to N-type MORB (Pearce, 1982) show a relative enrichment. The Nb and Ti contents show significant depletion with respect to MORB, suggesting more affinities with volcanic arc rather than with continental basalts. Rb contents range between 9 and 159 ppm (average 63 ppm) and K/Rb ratios (136-526) are lower than those of both modern calc-alkaline and tholeiitic basalts, but quite similar to those of surrounding khondalites (200-550), suggesting chemical re-equilibration (Barbey and Cuney, 1982). Furthermore, orthopyroxene crystals are frequently replaced by biotite and the higher the content of metamorphic biotite, the lower the K/Rb ratios. Note that in such a process of retrogression, only K and Rb are added (through an aqueous fluid) to the original rock composition, so that the other elements are not significantly modified. A model calculation assessing the modal content of biotite is given in Fig. 5a. The samples define a trend distinct from the Shaw's Main Trend but close to a mixing line between metamorphic biotite and basalt average composition. More, the modal biotite content of samples corresponds closely to that estimated by modelling.

104

TABLE 3 M a j o r a n d t r a c e e l e m e n t c o m p o s i t i o n s o f L a p l a n d c h a r n o c k i t i c r o c k s : g r o u p 1II a n d u n d i f f e r e n t i a t e d s a m p l e s Sample No.

L256

SiO2(wt.% ) A1203 FeO MnO MgO CaO Na20 K20 TiO 2 P205

52.53 18.96 8.08 0.12 6.82 9.13 2.82 0.73 0.60 0.20

La(ppm) Ce Nd Sm Eu Gd Dy Er Yb Lu Rb Sr Ba V Nb Zr Y Ni Cr Co

7 567

7 71 17 61 310

K/Rb (La/Yb)N YbN

866

J15

J12

53.28 20.85 7.32 0.10 6.12 9.17 2.04 0.49 0.49 0.14

55.88 20.01 7.63 0.09 3.85 6.34 3.41 1.51 0.99 0.29

12 873 184 190 10 7 36 98 51

44 760 754 164 8 146 16 26 53 43

340

285

J52 57.21 17.92 7.25 0.10 5.05 6.60 3.55 1.07 0.86 0.38 28.16 60.28 29.30 5.38 1.86 4.21 2.86 1.44 1.83 0.21 11 852 479 138 7 128 15 73 133 55 808 13.48 6.63

L413 59.92 17.16 5.79 0.10 3.43 6.72 4.68 1.06 0.80 0.34

10 765 80 4 185 21 50 56 36 880

J60 60.71 16.49 7.10 0.09 3.95 5.48 2.87 2.24 0.84 0.24

52 360 754 7 133 21

358

J9 61.34 18.39 5.57 0.07 3.58 5.73 3.21 1.37 0.55 0.18

J10 61.53 18.79 5.46 0.07 2.78 5.70 3.64 1.21 0.60 0.21

37 482 465 105 4 97 111 28 76 51

29 491 447 116 5 98 11

307

346

J73 61.93 16.31 6.75 0.09 3.21 5.26 2.79 2.58 0.86 0.22

55 384 860 9 168 16

15

J31 62.05 18.12 5.23 0.07 3.33 5.64 3.30 1.58 0.53 0.16

36 457 563 105 4 97 11

19 389

364

The A1203 content of enderbites ranges between 16.5% and 20% and decreases slightly with increasing silica content as in tholeiitic series. Their trace element contents correspond approximately to those of medium-K andesites compiled by Gill (1981), except for K and Rb which do not appear to be primary.

50

y 3o

Group 11. It comprises norites and enderbites characterized by high A12Oa and rather high and constant TiO2 and P2Os contents. Their Ti/Zr (33-102)

i

o

o Q"•

0 0

0

0 <:~

10 o o

~0

D,'"

8-~

.m o .;o a ~721

CaO

Q" / ,' ,'r~

0 Q

o

Fig. 3. C a O ( w t . % ) v e r s u s Y ( p p m ) v a r i a t i o n d i a g r a m . S o l i d line: s t a n d a r d c a l c - a l k a l i n e t r e n d ( L a m b e r t a n d H o l l a n d , 1 9 7 4 ) . G r o u p I ( s o l i d circles), g r o u p II ( o p e n s q u a r e s ) , g r o u p III (stars), h i g h - M g e n d e r b i t e ( o p e n d i a m o n d ) , s a m ple L 1 6 6 9 ( s o l i d d i a m o n d ) , u n d i f f e r e n t i a t e d s a m p l e s ( o p e n circles) a n d s a m p l e 7 2 I f r o m R a i t h e t al. ( 1 9 8 2 ) .

105

J32 62.53 17.71 5.50 0.08 2.93 5.45 3.12 1.82 0.66 0.19

J79 63.45 17.99 4.90 0.07 2.41 5.00 3.58 1.88 0.54 0.18

49 433 576 105 5 115 14

48 510 532 89 5 95 13

15

13

308

325

J36 58.71 19.82 6.16 0.06 3.61 6.35 3.36 1.24 0.50 0.19

91.07 39.52 5.74 1.16 2.45 0.64 0.19 0.12 0.02 41 540 241 95 -228 3

18 251 233 0.58

J33 58.96 19.21 6.42 0.09 3.82 6.81 2.90 0.90 0.66 0.22

J18 59.08 19.01 6.30 0.07 3.55 5.96 3.42 1.51 0.77 0.34

J17 61.15 18.92 5.02 0.06 2.99 5.81 3.36 1.59 0.76 0.34

J30 61.88 19.09 5.16 0.07 2.62 5.89 3.70 1.81 0.62 0.16

J28 64.36 17.87 4.74 0.05 2.92 4.38 3.48 1.57 0.55 0.08

J71 65.13 15.79 4.96 0.06 3.39 5.03 3.32 1.45 0.67 0.19

L1669 49.41 14.21 14.17 0.22 9.63 9.45 1.10 0.44 1.37

3.58 11.18 9.71 3.12 1.01 3.77 4.04 2.61 2.58 13 559 830

13 189

-173 7

9 81 31

16

56 488 609 83 4 143 2 20 56 58

485

233

926

20 585 376 135 4 107 6

55 534 518 134 3 101 5

65 528 619 133 2 105 4

31 500 457 112 5 90 8

20

19

18

374

228

203

and Zr/Y ( 4 - 2 1 ) ratios, excepted samples J63 and J149a, are consistent with their fractionated REE patterns. Unlike the other group, their K and Rb contents (respectively < 1% and < 18 ppm) are low and their K/Rb ratios very high ( ~ 1000), suggesting depletion (Fig. 5b).

Group 111. Samples J52, J36 as well as the other undifferentiated samples show calc-alkaline affinities. The Ti/V ratios (29---40, excepted sample L413) remain constant whereas Ti and V contents fall with increasing differentiation, therefore corroborating the early fractionation of Fe-Ti oxides. Although the distribution of K and Rb in groups II and III is partly obscured by metamorphic overprint, the K/Rb ratios ( ~ 475) are not significantly different from those of calc-alkaline basalts and follow the Main Trend (Fig. 5b).

275

The high-Mg enderbite It is characterized by high silica and MgO, moderate TiO2 and low alkali contents and corresponds well to high-Mg andesites (Tatsumi and Ishizaka, 1982). Its CaO/TiO2 ratio (12) is lower whereas its Al203/TiO2 ratio (25) is higher than the pyrolite value (17 and 20, respectively; Sun and Nesbitt, 1978). This sample displays also a primitive Mg/(Mg + Fe) ratio (0.71) but a low Ni content (102 ppm) with respect to modern high-Mg andesites [200 ppm according to Tatsumi and Ishizaka (1982)]. On the whole, the incompatible element contents are somewhat higher than in the LREE-enriched boninites (Hickey and Frey, 1982). However, the high Ti, Zr and Nb contents are consistent with the enrichment trend given by Cameron et al. (1983) for boninites. Whereas the K content is intermediate

106

100

E Q

5C

Z

0 "r (J 0

~

~,,,,~

Group1 samples

Group3 samples J52

f_ L 1008 72 I

1c

5

a

L 1040--"j 70 J I

I

La Ce

N~

I

I

I

I

Sm Eu Gd

Dy

,

Er

1

I

Yb 2u

0.5

_~

Group2 samples

", I

l

C

L=I Ce

10

"

i

Nd

I

|

I

$m EuGd

I

0y

|

Er

l

I

Yb Lu

50

__._

L1669

o=

t~

d.c',,

1(3

.~

s'~',,G'~

o'y

E'r

I

,~b ,.,,

5

b

La Ce

Nd

Sm Eu Gd

Dy

Er

Yb Lu

Fig. 4. REE distribution patterns of Lapland charnockitic rocks. The chondritic normalization factors are taken from Jahn et al. (1980).

between those of sanukitoids and boninites, the Rb content is extremely high (49 ppm) and is very unlikely to be primary. Sr and Ba contents (505 and 277 ppm, respectively) are anomalously high with respect to boninites (Hickey and Frey, 1982).

Discussion

Unlike the well-documented changes of LIL elements, available data on granulites (Hamilton et al., 1979; Jahn and Zhang, 1984) as well as on

107

3000

1000

Basalts

b

L

rr" 500

,#...

0 i ill3t ~

."'" 1.~. ~.~..,,

...... . - - ' " "'"'... ',-... 0 ~ "".,.

~,;-,,~.o ~

.

.....

................. U?"~°:,C-;. ........ ii

SiZ,,e ~p

20Y.

100 I

a

........

,

i

10

1

I

t,

100

I

1000

Rb

3000 Q

1000

{3 t3

0

l

I:E 500

"

,,~1

°

-

8;

100

b

I

i

I

I

1o Rb

I

I

lOO

,

,

I

I

1ooo

Fig. 5. K/Rb ratios versus Rb (ppm). Shaw's Main Trend (arrow); New Britain island-arc reference suite (boxes) from Basaltic Volcanism (1981). a. Group I basic (filled circles) and intermediate (open circles) granulites; field of metasedimentary granulites (dotted line); biotite average composition calculated from K and Rb contents of biotites from Lapland charnockitic rocks (H6rmann et al., 1980; Convert, 1981; P. Barbey and M. Cuney, unpublished data, 1982) and from Heier and Billings (1970); mixing line calculated assuming that the original K and Rb contents of primary magmas are identical to that of the New Britain island-arc average basalt. b. Other samples; symbols same as in Fig. 3.

eclogites (Bernard-Griffiths et al., 1985) argue in favour to a relative immobility of REE during granulite facies metamorphism. Therefore, it has been assumed that fractionation of the REE due to metamorphism is of limited extent and that these elements can be used to model igneous processes.

Petrogenesis Group 1. Group I basic granulites display low Mg numbers, variable Ni, Cr and P2Os contents and both negative and positive Eu anomalies, suggesting that most samples have undergone some crystal

108

fractionation or accumulation and do not represent primary liquids. As indicated by isotopic data (Bernard-Griffiths et al., 1984), these rocks could originate in a same source region which could have been either the mantle or basalts separated from CHUR shortly before 1.9 Ga. However, subducted or underplated basalts cannot be a suitable source because genesis of basaltic liquids from that source would require quite improbable high degrees of melting. The mantle appears to be a more likely source for these rocks characterized by low ISr and by 61. Nd9" values close to CHUR. REE modelling shows that the LREE-enriched patterns of these rocks are not accounted for by fractional crystallization of LREE chondritic parental liquids because: (1) olivine, orthopyroxene, clinopyroxene and plagioclase have REE partition coefficients too low to induce important REE fractionation (see also Jahn and Zhang, 1984); and (2) garnet and amphibole cannot be liquidus phases due to the increase in Y concentrations with increasing differentiation. We therefore believe that the fractionated REE patterns are charac-

teristics inherited from the parental magmas. Model calculations show that these magmas could have been produced by various degrees of melting ( 1 5 20%) of a spinel-lherzolite (Fig. 6) metasomatized by a LREE-enriched fluid, just before melting as implied by Sm/Nd data (Bernard-Griffiths et al., 1984). Note that low degrees of partial melting (3-8%) of a garnet-lherzolite (OlssOpx2sCpxlsGts), with garnet in the residue, give crossing HREE patterns incompatible with the observed data (Fig. 4a).

Group 11. Group II rocks and the high-Mg enderbite display similar features with LREE-enriched patterns as well as depleted and tilted HREE. The decrease in HREE with increasing silica content from sample L234 to sample L149b cannot be related to garnet crystal fractionation only. These rocks are also characterized by positive Eu anomalies (except sample L149a) which increase with increasing differentiation. These anomalies similar to those described in other granulite facies rocks

30-407. PM

Group 2

High.Mg. enderbite I00 t

A - Gt:A rrlohibolite E - Eclog/te

51-

5o

i

1~.Cje La

La

j

15-20"/PM j LFtE enrichment 5OI2 ": 3Opx 0 : 1:5Cpx:: Sp 3 t 2

Yb

20"/.

/

Yb

PM

30"/. PM / \ LRE\ enrichment /

garnet in residue

5

no garnet in residue

01: Opx:Cpx:Gt I 50:20:

2

La

I' Yb

i La

Yb

Fig. 6. Diagram summarizing the petrogenetic models for the Lapland charnockitic rocks. Partition coefficients from Jahn et al. Gt = 40 (Arth and Hanson, (1980) for basaltic liquids, and from Gill (1981) for andesitic liquids, except DGet = 0.028 and Dyb 1975). Partial melting of garnet-bearing mantle source assumed to be governed by the melting equation of Mysen (1977): 0.63Gt + 0.41Cpx + 0.090px = 0.1301 + Liq The mantle metasomatic enrichment is assumed to correspond to a (La/Yb)N ratio of 5. Modal compositions of garnet-amphibolite and eclogite are: Hb14oPI43Gt,Q13and CpxssGt3oQls, respectively.PM = partial melting.

109 (Jahn and Zhang, 1984), cannot be accounted neither by clinopyroxene fractionation nor by separation of accessories like apatite (characterized by significant negative Eu anomaly) owing to the relatively high P205 contents of the samples (0.15-0.35%). As the extent of these anomalies is dependent on the silica content, they could be explained by the model of Moeller and Muecke (1984) suggesting that Eu can be stabilized in felspathic and highly polymerized silicate melts. The high-Mg enderbite has LREE concentrations higher than in LREEenriched boninitic rocks (Cameron et al., 1983), but quite identical to early Archean group III komatiites (Jahn et al., 1983) suggesting LREE enrichment. If this appears to be true, the source must have had (Gd/Yb)N ratios lower than unity to account for the tilted HREE patterns. This characteristic together with the high Al203/TiO2 (25)and low CaO/A1203 (0.5) ratios of the high-Mg enderbite with respect to pyrolite, suggest accumulation of garnet in the source. REE modelling shows that the parental magmas of group II rocks and of the high-Mg enderbite can be derived by different degrees of partial melting (20% and 30%, respectively) of the same garnet-enriched source, accompanied with LREE metasomatism (Fig. 6).

Group III. Group III intermediate samples J36 and No. 72I of Raith et al. (1982) are LREE-enriched and strongly HREE-depleted (Yb N < 1), suggesting that they could be derived by 30% of partial melting of basalts recrystallized into quartzeclogite (Fig. 6), characterized by fractionated REE patterns [(La/Yb)N = 5] similar to those of the group I, and separated from CHUR a short time before melting. Sample J52, less depleted in HREE could be produced by 40% of melting of the same basalts recrystallized into garnet-amphibolite (Fig.

6). The simultaneous emplacement of rocks belonging to groups I and II (syngranulitic plutonic rocks) and REE modelling suggest that the Lapland mafic and intermediate granulites could be produced by metasomatism and partial melting of an heterogeneous mantle locally enriched in garnet [similar to that proposed for the early Archean group III komatiites by Jahn et al. (1983)]. That garnet-enriched zone, characterized by nearly flat LREE and fractionated HREE [(Gd/Yb)N < 1], is compatible with the chemical features of the Lapland charnock-

ites. This model also accounts for the high A12OJ TiO2 ratios because accumulation of garnet leads to a concomitant enrichment in Al and HREE, the other elements (excepted Mg) remaining approximatively constant. Rocks belonging to group I are assumed to arise from melting of a LREE metasomatized mantle, whereas rocks of group II and the highMg enderbite could arise from a garnet-enriched segment of that mantle. Group III rocks are supposed to arise from partial melting of eclogites and garnet-amphibolites (subducted or underplated basalts with (La/Yb)N ratios of 5).

Geodynamic implications The geochemical and petrogenetic characteristics of the Lapland charnockitic complex prompt the following speculations: (1) The trace element features show that the Lapland charnockites comprise several rock groups characterized by LREE-enriched patterns quite similar to Archean and Proterozoic granulites (Coolen, 1980; Weaver, 1980; Jahn and Zhang, 1984). Model calculations based on REE suggest that parental magmas could have been produced by melting of an heterogeneous mantle metasomatized by a LREE-enriched fluid. The association of such a mantle metasomatism and carbonic metamorphism is probably not fortuitous since the CO2rich vapor phase, enriched in LREE and other LIL elements (Wendlandt and Harrison, 1979), occurring as a precursor of magma genesis, could also be responsible for the development of granulite facies metamorphic conditions. (2) The evolution of the Belomorides, discussed elsewhere (Barbey et al., 1984), together with field and petrographical data show that the interpretation of the Lapland charnockitic complex must not be restricted to that of an effusive suite and suggest the persistence of a magmatic activity throughout the whole evolution of the fold belt, comprising: volcanics and early intrusives emplaced during the geosynclinal phase and later recrystallized in highgrade metamorphic conditions, and syngranulitic plutonics emplaced directly in granulite facies conditions during the orogenic phase (subduction and continental collision). All groups comprise syngranulitic plutonics. However, the close similarity regarding REE between metavolcanics from the Tana belt (H/Srmann et al., 1980) and rocks of

110 group I suggests that this latter group could also comprise metavolcanics. (3) The plutonic rocks emplaced during the granulite facies metamorphism constitute a rock association reminiscent of active margin with calcalkaline rocks and more particularly with high-Mg andesite. Moreover, as the metasedimentary sequence (khondalites) is in many respect (rock association, geochemical characteristics, etc.) similar to Atlantic-type sediments, we believe that the development of the active margin assumed by some authors (Raith et al., 1982) occurred during the burial o f the sedimentary formations and their recrystallization in granulite facies conditions. A possible succession o f events could be proposed: - a phase of rifting led to the formation o f the fault-bounded troughs in Karelia, Finland, and in the Kola Peninsula, U.S.S.R.; -the geosynclinal period begun with an important episode of volcanism (Tana belt) and then culminated with the accumulation o f large thicknesses o f flysch-type sediments and with the emplacement of basic and intermediate volcanic rocks (Granulite Belt); - b u r i a l of these volcano-sedimentary formations (assumed by subduction) whilst an active margin develops; subducted or underplated basalts recrystallize into eclogites or garnet-amphibolites; - development o f granulite facies conditions and emplacement o f synorogenic plutonic rocks at the root o f the active margin; this activity continues later during the continental collision. The significant role played by garnet in magma differentiation (either through crystal fractionation or as a residual phase during partial melting) just before and during continental collision could be related to tectonic processes involving burial and large-scale thrusting achieving high-pressure conditions in the lower crust and in the upper mantle. Acknowledgments This work was financed under a cooperative research agreement between the Academy o f Finland and the CNRS (France). We also acknowledge financial assistance from the ATP "Geodynamique". Helpful discussions with B. Auvray, G. Gruau and B.M. Jahn, Rennes, are gratefully appreciated. We would like to thank J.C. Duchesne and an anonymous reviewer for their critical reviews o f this paper.

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111

teristics of boninite series volcanics: implications for their source. Geochim. Cosmochim. Acta, 46: 2 0 9 9 2115. HSrmann, P.K., Raith, M., Raase, P., Ackermand, D. and Seifert, F., 1980. The granulite complex of Finnish Lapland: petrology and metamorphic conditions in the lvalojoki-Inarijffrvi area. Bull. Geol. Surv. Finland, No. 308. Jahn, B.M., Auvray, B., Blais, S., Capdevila, R., Cornichet, J., Vidal, P. and Hameurt, J., 1980. Trace element geochemistry and petrogenesis of Finnish greenstone belts. J. Petrol., 21: 201-244. Jahn, B.M. and Zhang, Z., 1984. Archean granulite gneisses from eastern Hebei Province, China: rare-earth geochemistry and tectonic implications. Contrib. Mineral. Petrol., 85: 324-343. Jahn, B.M., Gruau, G. and Glikson, A.Y., 1983. Komatiites of the Onverwacht Group, S. Africa: REE geochemistry, Sm/Nd age and mantle evolution. Contrib. Mineral. Petrol., 80: 2 5 - 4 0 . Klatt, E., 1980. Seriengliederung, Mineralfazies und Zusammensetzung der Fliissigkeiteinschliisse in der pr~kambrischen Gesteinserien, Nordlapplande. University of Kiel, Kiel, 125 pp. Lambert, R.St.J. and Holland, J.G., 1974. Yttrium geochemistry applied to petrogenesis utilizing calcium-yttrium relationships in minerals and rocks. Geochim. Cosmochim. Acta, 38: 1393-1414. Martin, H., Chauvel, C. and Jahn, B.M., 1983. Major and trace element geochemistry and crustal evolution of Archean granodioritic rocks from eastern Finland. Precambrian Res., 21: 159-180. Moeller, T. and Muecke, G.K., 1984. Significance of Eu anomalies and silicate-melt and crystal-melt equilibria. A reevaluation. Contrib. Mineral. Petrol., 87: 2 4 2 250. Mysen, B.O., 1977. Experimental determination of crystalvapor partition coefficients for rare-earth elements to 30 kb pressure. Annu. Rep. Geophys. Lab., 1 9 7 7 1978, pp. 6 8 9 - 6 9 5 .

Newton, R.C., Perkins, III, D., 1982. Thermodynamic calibration of geobarometers based on the assemblages garnet-plagioclase-orthopyroxene-(clinopyroxene)-quartz. Am. Mineral., 67: 203-222. Pearce, J.A., 1982. Trace element characteristics of lavas from destructive plate boundaries. In: R.S. Thorpe (Editor), Orogenic Andesites. Wiley, New York, N.Y., pp. 5 2 5 - 5 4 8 . Pichamuthu, C.S., 1969. Nomenclature of charnockites. Indian Mineral., 10: 2 3 - 3 5 . Raith, M., Raase, P. and HSrmann, P.K., 1982. The Precambrian of Finnish Lapland: evolution and regime of metamorphism. Geol. Rundsch., 71: 230-244. Road Map of Finland, 1977. Road Map of Finland. Maanmittaushallituksen Karttapaino. Helsinki, scale 1:200,000 Sheets 17 and 19. Streickeisen, A., 1974. How should charnockitic rocks be named? In: J. Belli~re and J.C. Duchesne (Editors), G6ologie des domaines cristallins. Cent. Soc. G6ol. Belg., Liege, pp. 349-360. Sun, S.S. and Nesbitt, R.W., 1978. Geochemical regularities and genetic significance of ophiolitic basalts. Geology, 6: 6 8 9 - 6 9 3 . Tatsumi, Y. and Ishizaka, K., 1982. Origin of high-magnesian andesites in the Setouchi volcanic belt, southwest Japan, I. Petrographical and chemical characteristics. Earth Planet. Sci. Lett., 60: 293-304. Weaver, B.L., 1980. Rare-earth element geochemistry of Madras granulites. Contrib. Mineral. Petrol., 71: 2 7 1 279. Wendlandt, R.F. and Harrison, W.J., 1979. Rare earth partitioning between immiscible carbonate and silicate liquids and CO s vapor: results and implications for the formation of light rare earth-enriched rocks. Contrib. Mineral. Petrol., 69: 4 0 9 - 4 1 9 . Wright, A.E., E1 Hiyari, M.A., Jackson, A.J. and Fediukova, E., 1978. The structural history and geochemistry of the Lapland granulites, Finland. In: A.V. Sidorenko (Editor), Correlations of the Precambrian. Nauka, Moscow, pp. 223- 235.