Trace-element geochemistry of Archaean and Proterozoic rocks from eastern Karelia, U.S.S.R.

Lithos, 21 (1988) 183-194 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

183

Trace-element geochemistry of Archaean and Proterozoic rocks from eastern Karelia, U.S.S.R. I.D. RYABCHIKOV ~, P. SUDDABY-', A.V. GIRNIS 1, V.S. KULIKOV ~, V.V. KULIKOVA~ and O.A. BOGATIKOV ~ 1.G. E. 31. Starometnoy 35, Moscow (U.S.S.R.) "-Imperial Co/h'~e, London (Great Britain) ~lnsti:zae ~/ Geolo,~y, Pelrozavodsk (U.S.S.R.)

LITHOS

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Ryabchikov, I.D., Suddaby, P., Girnis, A.V., Kulikov, V.S., Kulikova, V.V. and Bogatikov, O.A., 1988. Trace-element geochemistry of Archaean and Proterozoic rocks from eastern Karelia, U.S.S.R. Lithos, 21:183-194. Several komatiite and komatiitic basalt occurrences have been found in the greenslone successions of the eastern part of the Baltic shield (Soviet Karelia), with both Archaean and Proterozoic ages being represented. Some flows cxhibit internal layering with well-developed spinifex textures similar to those described from the classic komatiite provinces of Canada and Australia. Marked differences have been found in the trace-element geochemistry of these rocks. Some Archaean volcanics exhibit fiat REE distributions with approximately chondrilic ratios of Ca, Ti, A1, Zr, V and Y whereas others show depletion in LREE and Zr. Proterozoic komatiitic basalts differ from the Archaean ones by their relative LREE. Ba and Sr enrichment. These geochemical features are ascribed to an established heterogeneity in the Archaean mantle source and its further development by the Proterozoic. Major- and trace-element variations suggest relative immobility, during metamorphism, of all the analyzed elements except volatiles and alkalis. These trends being adequately explained by olivine with minor chromite fractionation. (Received May 12, 1986: accepted June 5, 1987)

Introduction

Geological setting and petrographic descriptions

K o m a t i i t i c m a g m a s m a y be p r o d u c e d by high degrees o f p a r t i a l melting o f m a n t l e material, a n d therefore they p r o v i d e i m p o r t a n t i n f o r m a t i o n a b o u t the g e o c h e m i c a l features o f their source. P r e c a m b r i a n k o m a t i i t e s a n d k o m a t i i t i c basalts have been r e p o r t e d f r o m all c o n t i n e n t s with the e x c e p t i o n o f A n t a r c t i c a . D e t a i l e d g e o c h e m i c a l i n f o r m a t i o n is available for the u l t r a b a s i c volcanics from the g r e e n s t o n e belts o f F i n l a n d ( J a h n et al., 1980). S i m i l a r greenstone belts have also been d e s c r i b e d in the eastern part o f the Baltic shield ( S o v i e t Karelia), where k o m a t i i t i c rocks o f b o t h A r c h a e a n a n d P r o t e r o z o i c age are f o u n d ( N a l i v k i n a , 1982). T h e present p a p e r reports on s o m e o f the geochemical features o f these rocks.

T h r e e m a j o r structural units are clearly distinguished in the s o u t h e a s t e r n part o f the Baltic shield (Fig. 1 ): T h e B e l o m o r s k y geoblock, a n d the Vodlozersky block, with the Vetreny P o y a s synclinorium zone between t h e m ( K r a t z , 1983). T h e B e l o m o r s k y geoblock is f o r m e d o f biotite, e p i d o t e - b i o t i t e a n d g a r n e t - a m p h i b o l e gneisses together with a m p h i b o l i t e s o f the B e l o m o r series (Sokolov et al., 1984). T h e rocks o f this series have u n d e r g o n e at least three e p i s o d e s o f m e t a m o r p h i s m and deformation. T h e f o r m a t i o n o f the Vodlozersky block p r o b a b l y c o m m e n c e d in the early A r c h a e a n . It is m a i n l y c o m p o s e d o f m e t a m o r p h o s e d m a g m a t i c rocks c h e m i c a l l y related to tonalites, plagiogranites, diorties a n d g a b b r o - d i o r i t e s . Rare relics o f a m p h i b o lites a n d u l t r a m a f i c s have also been found in this

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Fig. 1. Simplified geologicalmap of the Vetreny Poyas synclinorium zone, Lower Proterozoic ( Karelian ): 1= Vetreny Poyas suite komatiites and komatiitic basalts: 2= Kozhozero suite -- basalts, quartzites, carbonates, Vileng suite -- argillites, alleurolites, tuffites: 3= Kirich suite-- quartzites, basaltic andesites, basalts and komatiites, Kalgachsuite -- conglomerates. Upper Archaean (Lopian): 4= Vozhma series -- basalts, komatiites, rhyolites and metasediments: 5 =granites. Lower Archaean: 6=gneisses, amphibolites, thonalites: 7= faults; 8 = sample location. -

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sequence. These rocks were also subjected to at least three stages of deformation. The Vetreny Poyas synclinorium zone is characterized by a complex structure and evolution. Two structural levels are clearly distinguished in this zone, the lower being characterized by rocks of Archaean (Lopian) age and the upper by those of Proterozoic (Karelian) age. Lopian rocks form relics of local greenstone structures which are included in the Sumozero-Kenozero and Yuzhno-Vygozero greenstone belts (Rybakov et al., 1981 ). The best preserved are the Kamenoozero and Toksha structures which are dominated by metaeffusives and metasediments of the Vozhma series having a total thickness of 5-6 km (Fig. 2), This sequence includes numerous flows ofkomatiites and komatiitic basalts. These Lopian rocks are intruded by ultramafics, gabbros and the later plagiogranites which are accompanied by rhyolite and rhyodacite veins as well as small granitoid intrusives the zircon ages of which are about 2.8 Ga (Tugarinov and Bibikova, 1980). The Proterozoic level is represented by rocks forming a lower and middle Karelian sequence. The former contains terrigenous rocks, basalts and basaltic andesites with rare komatiitic flows (Kirich suite), having a total thickness of more than 1000 m (Fig. 3) and more than 260 m of conglomerates belonging to the Kalgach suite. The middle Kare-

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lian group includes quartzites, marbles, basalts (Kozhozero suite - - more than 400 m), aleurolites, argillites, conglomerates and tuffites (Vileng suite about 1.5 km) as well as more than 2 km of komatiites and komatiitic basalts of the Vetreny Poyas suite. Komatiitic rocks are thus seen to occur at three stratigraphic levels, Lopian, early Karelian and middle Karelian. Before describing these rocks in more detail it is necessary to clarify the somewhat contentious matter of komatiite terminology. The scheme used in the present paper is essentially the two-fold division of komatiites recommended by Arndt and Nisbet (1982). However, we also distinguish high- and low-magnesia varieties of komatiitic basalts with the boundary at 14 wt.% MgO. Lopian komatiites with well-preserved spinifex textures have been found in the region of the Kumbuksa river (Kulikova and Kulikov, 1981). Layered lava flows with well-developed cumulate, spinifex and flow-top breccia zones are exposed in this region. Greenschist-facies metamorphism has resulted in complete replacement of the magmatic minerals; however, the texture is well preserved and readily enables the primary mineralogy to be reconstructed. The lower zone which is several metres thick, is composed ofa serpentinized peridotite with relics of cumulative texture. The gradual upward -

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U~,o Fig. 2. Geological cross-sections o f Vozhma series (Lopian). / = f e l s i c volcanics: 2=basalls. mainly pillow-structured: 3 = massive basalts; 4 = greenschists; 5 = tuffites: 6 = carbonaceous schists: 7=quartzites: 8 = k o m a t i i l e s : 9=granites: 10= sample number and location.

transition to the spinifex zone is marked by the appearance of blade-like olivine aggregates up to 1.3 mm in length alongside the euhedral and pointed crystals of clearly cumulative olivine. The cores of these olivine blades were often filled with glass, which is now represented by fine aggregates of talc and tremolite. The spinifex-textured zone could be subdivided into four layers, each about 25 cm thick. The first and third layers (from base to top) have a lenticular appearance. These lenses which are up to

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Fig. 3. Schematic cross-sections o f Kirich suite and "Kalja" layered flow. / = s c h i s t o s e basaltic andesites: 2=amygdaloidal basalts and basaltic andesites; 3 = p i l l o w basalts; 4=luffs; 5 =brecciated lavas; 6 = k o m a t i i t i c basalts with spinifex texture: 7=melabasalts: 8 = m a f i c metabasalts: 9 = c a r b o n a t e chlorite-tremolite rocks: 10= chlorite-tremolite rocks; 11 = komatiites: 1 2 = arkoses, quartzites, quartz-sericitic and other schists: 13 = sample number and locality•

50 cm in length and 12-15 cm thick contained blades of olivine oriented approximately normal to the layering. Inside the blades magnetite crystals occur along fractures. These lenses are bordered by olivine blades up to 50 cm in length. The second layer contains extremely large (up to 1 m long) feather-like blades of olivine with a thickness of not more than 1.5 ram. Usually these blades form bundles situated subparallel to the layer boundaries. Dendritic crystals of magnetite and chromite sometimes form lattices on the olivine faces. The uppermost layer in the spinifex-textured zone contains

186 angular fragments 1.5-25 cm in size. These fragments consist of euhedral or less commonly elongated olivine crystals surrounded by a completely altered glassy groundmass. The material between the fragments consists of spinifex-textured komatiite, with packets of olivine crystals 1-3 cm length and 0.5-1 mm in width. Magnetite crystals form chains within the olivine plates. Between the plates, small ( 1 × 0.1 mm) needles of clinopyroxene replaced by actinolite occur at an acute angle to the olivine crystals. The top of the flows consist of breccias with rounded clasts 1-12 cm in size, only slightly displaced with respect to each other. Within the clasts rounded grains of olivine up to 1 mm size are observed. This breccia zone is about 10-25 cm thick. In addition to the layered flows with a well-developed spinifex texture, others with only a vague zoning are also present. The Kalja flow described below is taken as an example of the lower Karelian komatiites which have not all been studied in detail. This flow, which is named after its exposure located near the Kalja river in the Archangelsk district, is up to 100 m thick and has been chosen for its resemblance to the layered komatiitic flows of Canada in particular the "Fred Flow" o f A r n d t et al. (1977). As with the previous group, the primary rock textures have survived the metamorphism which has destroyed all the primary minerals with the exception of a few clinopyroxene relics in the komatiitic basalts. The following list shows from top to base the subdivisions of the flow, indicating where possible the relationships between the metamorphic rock and its igneous precursor: (I) (2 ) (3t (4) (5) (6) ( 7) (8 ) (9)

Brecciated komatiitic basalt Komatiitic basalt with spinifex texture relics Metabasalt Mafic metabasalt Metabasalt Carbonate-chlorite-tremolite rock Chlorite-tremolite rock, komatiite Komatiite with porphyritic texture Chlorite-tremolite rock, komaliite

5.0 50.0 10.0 6.0 6.0 3.5 7.0 8.0 0.5

m m m m m m m m m

The most abundant lavas in the region are the komatiitic basalts of middle Karelian age which are exposed over 5000 kin2; the total thickness in some places exceeding 2 km. Unlike those described above these rocks are quite fresh in places and contain abundant relics of magmatic minerals. Lava, tuff and vent facies are distinguished (Slusarev and Ku-

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Fig. 4. Cross-section o f Vetreny Poyas suite at Mount Shapochka. / = s c h i s t o s e and mylonitized komatiitic basalts with pillow structure; 2 = agglomerates and lava breccias: 3 = pillow komatiitic basalts (green): 4 = p i l l o w komatiitic basalts ( black): 5 = slag crust o f flows: 6 = rocks with olivine spinirex; 7=variolithic komatiitic basalts: 8 = d o l e r i t i c basalts: 9=komatiites; 1 0 = m a s s i v e komatiitic basalts: 1 / = s a m p l e number and location.

likov, 1973). The lava facies is composed of about 95% by volume of the Vetreny Poyas suite and includes numerous non-layered and sparsely layered flows ( Fig. 4 ). The non-layered flows are composed of komatiitic basalts including high-magnesia ones with massive or pillow structures 1.5-80 m thick. The layered flows usually consist of the following units (from top to base):

187 ( 1) (2) (3) (4) ( 5)

Brecciasand pumices Komatiitic basalts with olivine and clinopyroxene spinifex Komatiitic basalts, olivine absent Cumulative layer High-magnesiakomatiitic basalt

0.1-1.5 m 5-65 m 3-40 m 0-15m 0.1-0.2 m

Several varieties of komatiitic basalts are distinguished which differ in their mineral and textural features with gradual transitions between them. Low-magnesia komatiitic basalts are divided into plagioclase-free and plagioclase-bearing varieties. The major mineral in the rocks of the first group is clinopyroxene (up to 40 vol.%) often displaying chemical zoning with a high-magnesia pigeonite as the core. The remaining part of the rock consists of the devitrified glassy groundmass with accessory Fechromite and sometimes sulphides. Minor amounts of olivine (0-5%) are occasionally present. The rocks have massive, banded, variolitic and rarely amygdaloidal structures. Spinifex-like textures are in this case formed by clinopyroxene crystals [of plate random or rarely porphyritic types of Nesbitt (1971)]. The plagioclase-containing komatiitic basalts do not significantly differ from typical basalts in respect of their mineral composition and textures. Major minerals are clinopyroxene and plagioclase (51-65 mole% An), rarely olivine. Fe-chromite~ pyrrhotite, pyrite and chalcopyrite are found as accessories. The rocks are characterized by their massive structure and porphyritic, tholeiitic or pyroxene spinifex textures. High-magnesia komatiitic basalts consist of olivine (10-25 vol.%), acicular clinopyroxene (0-20 vol.%) and glassy groundmass (up to 80 vol.%). Fechromite is the most common accessory. These rocks have massive, pillow or amygdaloidal structures and olivine spinifex textures. Olivine phenocrysts are often characterized by chemical zoning from Fos5 in the core to Fo72in the rims.

Analytical procedures Trace elements were analyzed by inductively coupled plasma source spectrometry at Imperial College and King's College, London. The analytical procedures were essentially the same as described by Walsh et al. ( 1981 ) except that we have found it more convenient to use a lithium metaborate

(LiBO2) fusion followed by dissolution in dilute hydrochloric acid instead of digestion in HC104-HF solution. Major-element analyses were performed by wet-chemical methods at the Central Chemical Laboratory of the Institute of Geology of Ore Deposits (Moscow) (analysts - - G. Esikova and S. Vronskaya).

Results

Major elements The samples investigated embrace a wide compositional range from ultrabasic rocks with more than 40 wt.% MgO (127.20) to basalts with 8-10 wt.% MgO. The most magnesian melts in this range are represented by samples with olivine spinifex texture ( 127.8, 127.7) which contain about 30 wt.% MgO; rocks with higher MgO values being regarded as cumulates. These rocks are chemically similar to the typical high-magnesia komatiites of Canada (Arndt et al., 1977), Australia (Naldrett and Turner, 1977) and the Finnish part of the Baltic shield (Jahn et al., 1980). SiO2, TiO2, A1203 and CaO have a negative correlation with MgO and the compositional variations observed correspond closely with those that would be predicted when olivine fractionates from a melt of ca. 30 wt.% MgO. The observed variations suggest that these elements have not been significantly disturbed by the low-temperature alteration processes and that they have retained their primary magmatic abundances (Arndt, 1983). Some minor Ca redistribution has probably taken place as is indicated by the very low CaO values in the cumulative rock (127.20). In the case of the alkalis there is a very scattered distribution and a poor correlation with MgO content; this is undoubtedly a consequence of their high mobility under low-grade metamorphic conditions. Similar results are also seen for volatiles and for the Fe2+/Fe 3+ ratio, and we therefore present the data in Table 1 on a volatilefree basis with all iron expressed as FeO. In terms of their major-element chemistry the Archaean and Proterozoic rocks are indistinguishable and they have incompatible-element (Ca, A1, Ti ) ratios close to chondritic values.

43.4 0.20 4.49 8.02 0.06 43.8 n.d. 0.01 0.01

I t27-20

46.8 0.28 6.72 9.93 0.21 29.5 6.42 0.10 0.04

2 127-4

48.2 0.24 5.24 10.85 0.22 27.4 7.83 0.01 0.01

3 127-8 50.0 0.70 12.59 9.34 0.15 15.2 10.14 1.74 0.14

4 919 49.3 0.50 10.49 11.97 0.19 14.03 I 1.01 2.26 0.25

5 95/265 51.4 0.45 13.93 9.01 0.17 12.67 11.30 1.00 0.07

6 52 49.7 0.86 14.17 10.61 0.17 10.57 11.80 2.06 0.07

7 8039 46.5 0.48 17.0 11.34 0.23 10.41 11.90 1.62 0.52

8 49 49.7 0.73 13.92 10.01 0.17 9.33 14.92 0.71 0.51

9 5-1 49.2 0.77 15.7 11.64 0.17 8.15 12.46 1.64 0.27

10 7010-3 49.8 0.82 15.5 11.65 0.17 8.05 12.57 1.35 0.09

I1 7010 48.4 0.45 7.82 12.12 0.20 23.2 7.78 0.02 0.01

12 345-4 50.3 0.58 11.70 10.57 0.15 16.2 8.15 1.84 0.51

13 1774 49.5 0.59 I 1.40 10.86 0.17 16.3 8.81 1.98 0.39

14 1782 49.6 0.58 12.65 10.70 0.14 14.7 8.91 1.27 1.42

15 3516 52.9 0.56 12.25 8.28 0.16 11.82 10.56 2.03 1.46

16 918-2

1.00 2.10 0.29 0.99 0.27 0.08 0.15 0.53 0.09 0.34 0.40 0.06

0.50 1.80 0.47 1.30 0.46 0.14 0.39 0.61 0.12 0.38 0.43 0.05

1.10 3.00 0.56 1.93 0.67 0.26 0.70 0.83 0.16 0.44 0.43 0.06

35 n.d. 1920 35 1550 10 100 125 15 5.3

15 n,d. 1150 10 2000 I0 90 30 n.d. 3.0

25 0.1 2380 20 1400 10 115 190 15 4.0

3 127-8

I 2 127-20 127-4

n.d. = not detectable.

La Ce Pr Nd Sm Eu (id Dy Ho Er Yb Lu

Be Cr Cu Ni Sr V Zn Zr Y

na

Sample No.

3.30 8.70 1.77 7.05 1.89 0,52 1.90 2.18 0.41 1.27 1.28 0.19

40 0.2 2430 40 680 70 200 80 n.d. 12.6

4 919

1.10 2.60 0.72 2.64 0.83 0.35 1.07 1.60 0.31 1.01 1.06 0.16

35 0.1 1220 40 250 75 200 75 20 9.8

5 95/265

0.70 2.80 0.59 2.28 0.86 /).32 1.27 2.05 0.42 1.37 1.47 0.23

25 n.d. 670 70 390 45 200 90 20 13.7

6 52

1.50 5.00 0.95 2.86 1.32 0.50 1,72 2.32 0.46 1.30 1.45 0.24

15 0.1 420 20 185 90 200 90 40 15.5

7 8039

Trace and rare-earth element data ( in ppm ) for East Karelia komatiitic rocks

TABLE 2

0.60 2.20 0.77 2.38 0.90 0,38 1.31 2.12 0.46 1.46 1.57 0.25

185 0.2 470 25 250 120 240 105 25 14.6

8 49

3.40 7.10 1.59 5.89 1.81 0.59 2.30 2.92 0.59 1.80 1.85 0.29

75 0.5 520 95 190 125 240 95 50 18.9

9 5-1

1.80 3.90 1.00 4.11 1.58 0.62 2.16 2.97 0.59 1.98 1.86 0.30

30 0.3 440 95 180 115 250 95 40 19.0

10 7010-3

5.00 7.30 1.40 5.31 1.72 0.69 2.31 3.07 0.61 1.89 1.94 0.31

25 0.1 380 85 170 90 260 85 45 20.0

I1 7010

2.10 5.50 0.97 2.50 0.60 0.06 0.42 0.71 0.13 0.47 0.65 0.10

10 n.d. 2480 40 620 25 170 70 20 4.3

12 345-4

7.40 14.40 2.36 8.94 1.97 0.64 1.89 2.06 0.39 1.21 1.17 0.17

9.37 2.06 0.66 1.96 2.02 0.37 1.15 1.15 0.17

175 0.3 1840 85 580 170 180 85 45 12.1

14 1782

7.90 15.90 2.43

225 0.1 1830 95 540 200 170 80 45 12.3

13 1774

9.60 1.99 0.73 2.00 2.15 0.41 1.27 1.22 0.19

9.30 17.90 2.60

450 0.4 1330 35 490 130 180 85 45 13.4

15 3516

11.04 2.29 0.73 2.01 2.10 0.39 1.21 1.19 0.19

11.20 19.30 2.89

670 0.5 1220 170 330 450 I80 180 60 12.6

16 918-2

Samples Nos. I - I 1 are of Archaean age, Nos. 12-17, Prolerozoic. All analyses are given in volatile-free 100% basis, all iron-in FeO form. n.d. = not detectable.

SiO, l-iOAI:O~ FeO MnO MgO CaO Na,O K20

Sample No.

Major clement compositions i in vd.%l of komatiitic rocks of East Karelia

I~BI E I

11.87 2.77 0.90 2.90 2.93 0.58 1.67 1.71 0.27

11.40 23.30 3.22

235 0.3 980 50 185 245 220 95 50 19.0

17 345-18

52.8 0.71 13.63 10.39 0.16 8.73 10.25 3.25 0.08

17 345-18

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correlation with MgO up to about 18% MgO, but above this value shows rather a large scatter and may even show a reversal of the trend similar to that shown by Beswick (1982). Ni and Cr abundances are very similar in Archaean and Proterozoic rocks of the same MgO content. There is a strong negative correlation of Y, Zr and V with MgO content (Fig. 5c-e). Sr, Ba and Zn (Fig. 5 f a n d g) show similar relationships but there is much more scatter on the plots, probably due to these elements having a higher mobility during low-grade metamorphism. The Proterozoic rocks appear to be somewhat enriched in the more incompatible elements (Sr, Ba, Zr) relative to the Archaean samples.

Rare-earth elements (REE) 07010 •/

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. 52

+ 951265

Fig. 6. Chondrite-normalized REE plot for the Archaean komatihic rocks of eastern Karelia.

Trace elements Trace-element abundances (Table 2) vary systematically with the MgO contents (Fig. 5). The m a x i m u m Ni content (2000 p p m ) is found in the cumulative zone of the layered komatiitic flow (127.20). In the komatiitic basalts having 8-10 wt.% MgO, the Ni content falls as low as 150-200 p p m and the plot of Ni against MgO (Fig. 5a) exhibits an almost linear positive trend over the whole range of magnesia values. Cr also shows a good positive

1=

2 z::

I

t= ~-

o 345-18 e918-2 • 351"6 * 1774 I 7 8 2 e345~

c~ -- 0

Lc',

14o

~m E~

G'd

dy

E':

"~b

I~

Fig. 7. Chondrite-normalizcd REE plot for the Proterozoic komaliilic rocks of eastern Karelia.

The total REE content increases significantly as MgO decreases in both Archaean and Proterozoic samples. However, differences are apparent in the chondrite-normalized REE distribution patterns (Figs. 6 and 7) and at least two groups may be distinguished among the Archaean samples. Some samples have almost chondritic ratios of all REE and others are characterized by relative light rare-earth element (LREE) depletion: C e / S m = 0 . 6 - 0 . 8 (Fig. 6). Two samples (919, 127.8) have fairly flat patterns for both LREE and heavy rare-earth elements ( H R E E ) , but seem to be slightly enriched in the former: ( L R E E / H R E E ) n = I . 2 . However, in both these cases the degree of enrichment is comparable with the analytical uncertainty. For this reason and because samples (127.7) and (127.20) taken from the same flow as (127.8) show fiat REE patterns, we have attributed these two samples to the flat REE group. The Proterozoic samples differ markedly from the Archaean ones by showing a relatively strong LREE enrichment [ ( C e / S m ) , = 1.8-2.1] while maintaining a comparatively flat HREE distribution. This subdivision of the samples is also confirmed by other trace-element plots. The similarity in geochemical behaviour of Zr and the LREE on one hand and Y and HREE on the other results in a positive correlation between Ce/Sm and Zr/Y ratios (Fig. 8). kREE-depleted rocks appear to be relatively depleted in Zr and vice versa. The same relationship probably exists for Ba and Sr but here it is disguised by the high mobility of these elements during secondary alteration.

191 T~,BLE

S o m e Hacc- a n d m a j o r - e l e m e n t ratios in the k o m a t i i t i c rocks f r o m eastern Karelia Archaean ('at)/-M,()~ AI:{)/Ti()-

0.93 22.6

{ 0.7 - I . 4 9 ) * : ( 16.7-35.4) 116 ( 88 -150) 289 (197 -424) 2.4 ( 1.5 - 3 . 8 ) 0.94 ( 0 . 5 7 - 1 . 8 3 ) 0.86 ( 0 . 3 0 - 1 . 3 0 ) 0.34 ( 0 . 1 5 - 0 . 6 9 ) 0.101 ( 0 . 0 8 - 0 . 1 1 )

Ti'Zr

Ti'~ Zr,'Y {(c/Sm)N l {id/Yb),, {c,'h ~h/~

Proterozoic

P r i m i t i v e mantle* '

0.80 20.0 85 266 3.8 1.96 1.20 1.30 0.096

0.82 2() 110 275 2.5 1 I 0.32 0.091

( 0.7-1.0) ( 17.4-21.9) ( 56 135) (224 -295) ( 2.6 - 4 . 8 ) ( 1.81-2.15) ( 0.52-1.36) ( 1.19-1.53} ( 0.09-0.152)

* g a g o u t z el al. { 1979). *?Mean x alues v, ilh variation limits in brackets.

2 0

OA

0

8 0

0 0

0

[

0

2

3

4

5

Zr/¥

Fig. 8. Chondrile-normaIized Ce/Sm vs. Z r / Y plot for the Archaean ( open circles t and Proterozoic ( filled circles) komatiitic rocks. Solid triangle represents primitive lherzolite of Jagoulz ctal. (1979). In respect of all other analyzed elements there are no significant differences among these groups of rocks. The ratios of the moderately incompatible elements (V, Y, HREE) are very close to chondritic values (Table 3) and do not change significantly with the degree of differentiation.

Discussion Komatiitic melts with very high MgO contents (up to 30% and more) may have formed by high degrees of partial melting of a mantle peridotite. Although komatiitic major-element chemistry may be reproduced experimentally by lesser degrees of partial melting at very high pressures (Takahashi and Scarfe, 1985) high degrees of melting are required

to explain the trace-element content of komatiites (Arndt, 1986). Pure olivine, or olivine, orthopyroxene residua cause the concentration ratios of most elements to remain the same as in the original mantle. The similarity of refractory element ratios to chondritic values, noted by many investigators (e.g., Nesbitt and Sun, 1976; Nesbitt et al., 1979) led early workers on komatiites to believe that primitive mantle is a source for komatiites (e.g., Jahn and Sun, 1979). However, more recent detailed geochemical studies (e.g., Jahn et al., 1980; Cattell et al., 1984; Dupr6 et al., 1984) have shown that the source region ofkomatiitic melts could have been chemically heterogeneous; this also seems to be true for the east Karelian komatiites discussed in the present paper. The composition of primitive mantle now seems to be rather well constrained by detailed geochemical studies of mantle xenoliths (e.g., Jagoutz et al., 1979). Systematic differences between observed element ratios in komatiites and those in primitive mantle may be due to some of the following factors: (1) deviation of the source mantle composition from the model one; (2) preservation of phases other than olivine and orthopyroxene in the residuum; (3) differentiation of initial melts; (4) m a g m a mixing and contamination; and (5) element mobility during low-temperature alteration and metamorphism. Some of these factors have recently been discussed (Arndt, 1986). The last factor has apparently been responsible for the irregular variations of alkali elements in our samples and for this reason they will not be discussed further. The ratios of the moderately incompatible trace elements (Y, V, Zn, HREE) as well as Ca, Ti and AI are practically identical in all the aria-

192

lyzed samples and lie very close to primitive mantle values (Table 3). At the same time the variations in the absolute abundances of these elements could be attributed to olivine fractionation from komatiitic melts with ca. 30 wt.% MgO. The variations in Ni content are also consistent with olivine fractionation without any marked involvement of sulphide. The similar Ni abundances in the Archaean and Proterozoic rocks of similar MgO content supports the idea of a komatiitic composition for the initial Proterozoic magma even though non-cumulative effusives with more than 20 wt.% MgO have not been found in the Proterozoic sequences of east Karelia. The sharp decrease of Cr in the komatiitic basalts is probably due to a minor Cr-spinel fractionation. Saturation with respect to chromite is indicated by small amounts of chrome spinel in these rocks. Chromite fractionation may also be responsible for the slight decrease of V/Ti ratios from 0.063 in the komatiites (a strictly chondritic value) to around 0.053 in the komatiitic basalts. By considering the most incompatible trace elements we have distinguished three groups of samples (see previous section): (1) those with chondritic ratios of LREE/HREE and Zr/Y; (2) those depleted in LREE and Zr; and (3) those enriched in LREE, Zr, Sr and Ba. The negligible deviations of relatively mobile-element abundances
Under any realistic mantle conditions it is impossible to obtain LREE- and Zr-depleted liquids, as represented by the second group of samples, by a single-stage melting of a source having chondritic element ratios. This is because these elements are not incorporated into the possible residual minerals. For the same reason crystal fractionation could not have resulted in LREE and Zr depletion. The constancy of the refractory elements ratios as in the first group shows that olivine was the only phase responsible for the differentiation trends. Thus, the depletion of Archaean samples in LREE requires the presence of a depleted source mantle which is consistent with the isotopic data of Dupr6 et al. (1984). However, the existence of undepleted Archaean samples does not necessarily imply a differentiated mantle in the Archaean as these features could be attributed to processes of crustal contamination. The Proterozoic rocks in the third group differ significantly from the Archaean ones in that they show a strong enrichment in LREE. Zr, Ba and St. The lack of systematic variations in LREE/HREE and Zr/Y ratios indicates that again olivine was, apart from very minor chrome spinel, the only phase involved in the differentiation processes. Enrichments in highly incompatible elements, similar to those described in this paper, have previously been reported from other komatiitic rocks (Green, 1981 ; Sun, 1984) from Proterozoie mafic volcanics in southern Finland (Ehlers et al., 1986) and from some ultramafic dykes of Precambrian age (Weaver and Tarney, 1983). Arndt and Jenner (1986) have advocated contamination by sialic crustal material as the principal cause of such incompatible-element enrichment because of the high temperatures of komatiitic melts and the turbulent character of their flows inferred from fluid dynamic modelling (Huppert et al., 1984). The above results describe for the first time that systematic geochemical differences between komatiitic rocks of different ages have been reported within the same region. The features revealed in the Ce/Sm versus Zr/Y diagram (Fig. 8 ) are typical of mixing relations. These mixing relations may best be explained by the existence of enriched and depleted regions in the mantle source region of these komatiites. The involvement of depleted and enriched reservoirs with different degrees of mixing in the generation of the Proterozoic magmatism of the

193 Baltic shield is constrained by the Nd isotope data of Patchett and Kouvo (1986). The depleted reservoir probably corresponds to the convecting mantle. The enriched source on the other hand could be either continental crust or equally well enriched or metasomatised lithospheric mantle. In contrast with the situation described by Arndt and Jenner (1986), the Proterozoic volcanics of east Karelia exhibit great consistency in their REE patterns (Fig. 7). If we take into account that the Proterozoic rocks were sampled over a large area and from different stratigraphic units (Figs. 1-4) it is difficult to reconcile such regular patterns of trace and major elements with the rather fortuitous character of crustal contamination. We therefore favour the enriched lithospheric mantle as a source of the LREE-enriched volcanics of east Karelia. The enrichment of rocks in the incompatible elements may be a result either of the generation of melts in the lithospheric mantle or by the thermal erosion of previously enriched mantle material by initially LREE-depleted melts by a mechanism similar to that described by Huppert and Sparks (1985 ). Nalivkina (1982) described the different tectonic setting of late Archaean and Proterozoic greenstone belts on the Baltic shield and it is suggested that the more open rifting associated with the Archaean belts would make them less subject to the effects of continental lithospheric contamination than the latter. We feel that the involvement of different mantle reservoirs is the main cause of the geochemical variations seen in these east Karelia komatiitic rocks. Similar conclusions confirmed by isotopic data were reached by Weaver and Tarney (1983) when discussing some ultramafic Precambrian dykes from Scotland and Greenland.

Conclusions (1) During the greenschist-facies alteration of east Karelian komatiites, all elements except the volatiles, alkalis, Ba and Sr exhibit inert behaviour. (2) Komatiitic basalt liquids have been formed by the crystal fractionation of olivine and minor chromite from high-Mg komatiitic melts. (3) Differences in the REE distributions among Archaean samples (LREE-depleted and -undepleted rocks) could be a result of mantle heteroge-

neity, though contamination by crustal material cannot be ruled out. (4) The enrichment of Proterozoic rocks in less compatible elements is probably due to the involvement of enriched lithospheric mantle though again some crustal contamination may have taken place. The chemical distinctions between Archaean and Proterozoic komatiitic rocks are related to the different tectonic settings of the Archaean and Proterozoic greenstone belts.

Acknowledgements We would like to thank Dr. Nick Walsh and Mr. Peter Watkins for help with the trace-element analyses.

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