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Journal of Petrology Volume 42 Number 5 Pages 855-876 2001
© Oxford University Press 2001
The Palaeoproterozoic Komatiite–Picrite Association of Finnish Lapland
1GEOLOGICAL SURVEY OF FINLAND, PO BOX 77, FIN-96101 ROVANIEMI, FINLAND
2GEOLOGICAL SURVEY OF FINLAND, PO BOX 96, FIN-02151 ESPOO, FINLAND
3DEPARTMENT OF GEOLOGY, UNIVERSITY OF TASMANIA, GPO BOX 252-79, HOBART, TAS. 7001, AUSTRALIA
Received January 25, 2000; Revised typescript accepted July 21, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL BACKGROUNDTHE KOMATIITE-PICRITE...THE JEESIOROVA AND PEURAMAA...GEOCHEMISTRYCLASSIFICATION OF HIGH-MgO...NEODYMIUM ISOTOPE DATA AND...DISCUSSIONREFERENCES |
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The large range of chemical variation within intimately associated highly magnesian volcanic rocks in the Palaeoproterozoic Central Lapland Greenstone Belt prompted the construction of a new classification scheme for MgO-rich volcanic rocks, based on an [Al2O3] vs [TiO2] diagram where the axes are the Al2O3 and TiO2 contents (in mole proportions) of the rocks projected from the olivine composition. This diagram places the Lapland rocks in the fields of Ti-enriched komatiites and picrites. Komatiitic rocks are depleted in both light and heavy rare earth elements (LREE and HREE) relative to middle REE (MREE) and possess relatively high TiO2 even in the most LREE-depleted varieties, whereas picritic rocks approach geochemically Hawaiian picrites. Seven clinopyroxene and whole-rock pairs analysed for Sm–Nd isotopes yield an average age of 2056 ± 25 Ma for the komatiites. Uncontaminated komatiites and picrites have similar positive
Nd values (+4) indicating generation from a mantle source with a long-term depletion in LREE relative to MREE. Geochemical characteristics of the komatiite–picrite association, including REE and Nb/Y–Zr/Y systematics, indicate chemical heterogeneities in the source region, which seem to have been created by complex depletion and enrichment processes shortly before or related to a dynamic melting process. The high MgO contents of the rocks coupled with chemical similarity between the Lapland and Hawaiian picrites supports a mantle plume model for their genesis. Nevertheless, the geotectonic evolution appears to have proceeded without significant regional uplift shortly before volcanism. KEY WORDS: Finland; komatiite; Nd isotopes; Palaeoproterozoic; picrite
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDTHE KOMATIITE-PICRITE...THE JEESIOROVA AND PEURAMAA...GEOCHEMISTRYCLASSIFICATION OF HIGH-MgO...NEODYMIUM ISOTOPE DATA AND...DISCUSSIONREFERENCES |
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Komatiitic lavas with high MgO contents are often considered to represent high degrees of mantle melting and consequently to approach their mantle sources in composition (e.g. Bickle et al., 1976
). However, as pointed out by Arndt (1986), they can be a dirty window into the Archaean mantle because of some complicating factors. These include the high temperature of high-MgO liquids, which makes them susceptible to crustal contamination during ascent or eruption (Huppert & Sparks, 1985). Another feature of komatiites—variable chemistry in terms of Al2O3/TiO2 or heavy rare earth element (HREE) ratios—could have arisen from garnet fractionation, and if this fractionation took place during partial melting (e.g. Ohtani, 1990), then the komatiite composition may not reflect that of the source mantle. Observations on ultra-depleted and enriched melt inclusions in early crystallized minerals from recent primitive oceanic basalts have shown that melts produced by partial melting of upwelling mantle undergoing decompressional melting can be much more varied in their trace element characteristics than the erupted lavas themselves (Gurenko & Chaussidon, 1995). This feature has been explained by mixing of liquids produced by a dynamic melting process from a homogeneous mantle source (Gurenko & Chaussidon, 1995). Dynamic melting has also been suggested to account for the chemical variability of komatiites and picrites in Gorgona Island, Colombia (Arndt et al., 1997).
Soon after the identification of the first komatiites in the late 1960s (Viljoen & Viljoen, 1969), it became apparent that there is a considerable heterogeneity within these rocks in terms of certain elements (e.g. Al/Ti, La/Sm, Gd/Yb; Nesbitt & Sun, 1976). Two principal chemical types were recognized: one having very high CaO/Al2O3 (
1·5) and occurring in the early Archaean Barberton Mountain Land, and the other possessing CaO/Al2O3 1·0 and occurring in late Archaean greenstone belts in various localities (Brooks & Hart, 1974). Later these two varieties were defined as Al-depleted and Al-undepleted komatiites by Nesbitt et al. (1979). As more geochemical and isotopic data have become available around the world, this simple picture of the distribution in time and space of these two komatiite types has turned out to be complex. Although the early Archaean komatiites are dominantly of the Al-depleted type, there are exceptions (e.g. Collerson et al., 1991; Puchtel et al., 1993a, 1993b; Lahaye et al., 1995). Also, late and middle Archaean greenstone belts contain numerous examples of komatiite and komatiitic basalt occurrences that display low Al2O3/TiO2 ratios deviating considerably from the chondritic value typical of Al-undepleted komatiites (e.g. Nesbitt et al., 1984; Cattell & Arndt, 1987; Schaefer & Morton, 1991; Xie et al., 1995; Fan & Kerrich, 1997; Tomlinson et al., 1999). Low Al2O3/TiO2 may be the consequence of depletion in Al, but not necessarily always exclusively so. In contrast, it seems that often it is due to an overabundance of TiO2 compared with normal Al-undepleted komatiites. Major element heterogeneity of Archaean primitive volcanic rocks cannot therefore be explained only in terms of depletion or non-depletion in aluminium, but there is another dimension of major element variation related to enrichment in TiO2, which is often coupled with enrichment in iron. Some of the Archaean rocks are sufficiently high in iron and titanium to allow them to be called ferropicrites (Stone et al., 1995; Francis, 1999) or iron-rich komatiites (Green & Schulz, 1977). Depletion in titanium in turn leads to rock compositions approaching recent boninites (Kerrich et al., 1998). The chemical heterogeneity within Archaean primitive volcanic rocks is even more pronounced when incompatible trace elements are concerned.
In this paper we present data for Palaeoproterozoic MgO-rich volcanic rocks from northern Finland, which indicate that chemical heterogeneity among primitive magmas was not less significant in the early Proterozoic than in the Archaean. As shown in Fig. 1, the Finnish occurrences are part of a longer belt extending to northern Norway. A characteristic feature of komatiites in this belt is enrichment in titanium and other high field strength elements (HFSE), which led Barnes & Often (1990) to call these rocks Ti-rich komatiites. The belt has also become well known because of the presence of large amounts of fragmental komatiites classified as pyroclastic or volcaniclastic (Saverikko, 1985; Barnes & Often, 1990; Räsänen, 1996), which are not so common in most Archaean komatiite successions.
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When considering the Lapland rocks together with other Palaeoproterozoic primitive volcanic rocks in the Fennoscandian Shield, such as the Pechenga ferropicrites and Onega picrites (Hanski & Smolkin, 1989; Hanski, 1992), there seems to be a more or less complete chemical variation in both major and trace elements. This observation raises the question of the classification of these primitive rocks and gave us an impetus to construct a new classification scheme for such volcanic rocks based on their Al2O3 and TiO2 contents. Another objective of this study was to provide new Sm–Nd isotope data for komatiitic and picritic rocks from Lapland to better constrain their age, which has been contentious until now. To this end, we have measured Nd isotopic compositions for whole rocks and primary clinopyroxenes from the Jeesiörova and Peuramaa areas at the southern edge of the Kittilä greenstone belt, where the rocks are relatively well preserved. We also provide new geochemical data and discuss the petrogenesis of these volcanic rocks with special reference to mantle plume models.
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GEOLOGICAL BACKGROUND |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDTHE KOMATIITE-PICRITE...THE JEESIOROVA AND PEURAMAA...GEOCHEMISTRYCLASSIFICATION OF HIGH-MgO...NEODYMIUM ISOTOPE DATA AND...DISCUSSIONREFERENCES |
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In contrast to most Precambrian terranes, most widespread komatiitic metavolcanic rocks in Finnish Lapland are early Proterozoic rather than Archaean in age. Archaean komatiites, found in eastern Lapland, have suffered from pervasive tectono-metamorphic modifications and have therefore attracted limited attention from petrologists. Instead, the Proterozoic komatiites have served as targets for several studies in northern Finland (e.g. Saverikko, 1985
; Räsänen et al., 1989; Lehtonen et al., 1998) and in northern Norway (Henriksen, 1983; Krill et al., 1985; Barnes & Often, 1990; Davidsen, 1994). These komatiites are part of the Central Lapland Greenstone Belt, which extends from the eastern border of Finland in the Salla area through the Sodankylä and Kittilä areas and to the northern border between Finland and Norway (Fig. 1). The most recent summary of the geology of the central part of the belt has been reported by Lehtonen et al. (1998).
The early Proterozoic komatiites principally occur at two stratigraphic horizons, referred to here as the lower and upper komatiites. They are separated by an assessed time span of 400 my (Lehtonen et al., 1998). These two occurrences clearly differ in their associated rock types, environment of eruption and geochemical characteristics. The lower komatiites, belonging to the Onkamo Group, have a stratigraphic position close to or at the bottom of the Proterozoic supracrustal sequence and have been most thoroughly studied in the type area at Möykkelmä, where they form part of a komatiite–tholeiite sequence of 250 m thickness extruded directly on an Archaean granitoid basement (Räsänen et al., 1989). The geochemical characteristics of these komatiitic rocks, including high light REE (LREE)/HREE and negative Ta anomalies, suggest a considerable amount of Archaean upper-crustal contamination. There exist no radiometric dates for the lower komatiites and associated rocks of the Onkamo Group. However, these rocks can be correlated with dated komatiitic rocks from the Vetreny Belt in Russian Karelia, on the basis of their similar stratigraphic position, lithology and geochemical characteristics. Recently, Puchtel et al. (1997) have obtained Sm–Nd isochron ages of 2449 ± 35 Ma and 2410 ± 34 Ma for Vetreny Belt komatiites.
The upper komatiites are associated with phyllites and black schists of the Savukoski Group deposited on a sequence of cratonic quartzites of the Sodankylä Group (Lehtonen et al., 1998). These komatiites occur together with picritic metavolcanic rocks and are therefore described below as the komatiite–picrite association. Komatiites and picrites display similar structures and textures and therefore we can distinguish them only on the basis of chemical composition, particularly the TiO2 content, which is clearly higher in picrites. Figure 1 depicts the areal distribution of this rock association in the Central Lapland Greenstone Belt and its continuation in northern Norway, totalling 400 km in length. Komatiitic varieties are abundant and well exposed in the Sattasvaara and Kummitsoiva areas, and have previously been described by Saverikko (1983, 1985) and Räsänen (1996), whereas picritic varieties are well developed in the Sotkaselkä area (Lehtonen et al., 1998) (see Fig. 1). Samples for this study were obtained from two localities, Jeesiörova and Peuramaa, situated in the most southwestern part of the komatiite belt (Figs 1 and 2).
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After recognition of their komatiitic affinity, the ultramafic volcanic rocks in Central Lapland were regarded by some workers as Archaean in age (Gaál et al., 1978; Saverikko, 1983, 1985). This opinion has been questioned over the last decade, on the basis of detailed field geological studies resulting in the consensus that most komatiites in Finnish Lapland belong to Palaeoproterozoic schist belts deposited unconformably on the Archaean basement (e.g. Lehtonen et al., 1998). However, owing to the lack of any direct age determination on the komatiitic rocks in Finnish Lapland, their precise age has remained contentious. The only available dating of komatiitic rocks is that performed by Krill et al. (1985) using the Sm–Nd method on rocks from the Karasjok greenstone belt in Finnmark, northern Norway. They obtained an isochron age of 2085 ± 85 Ma, but this result has to be regarded with caution as the real eruption time (Hanski & Smolkin, 1989; Puchtel et al., 1998), because it relies on a geochemically heterogeneous population of whole-rock samples collected from several locations along the greenstone belt. Two other options have been presented. According to the stratigraphic scheme by Silvennoinen (1985), komatiite-bearing formations are assigned to the Lapponian Supergroup, meaning that they are pre-Jatulian and hence must have an age of >2·2 Ga. Recently, Puchtel et al. (1998) showed that the Onega plateau basalts in Russian Karelia possess an age of >1·98 Ga, similar to that obtained for the Pechenga ferropicrites (Hanski et al., 1990), and put forward a model in which the primitive volcanic rocks in these two areas together with komatiitic rocks in Central Lapland are genetically related to magmatism of the same starting mantle plume upwelling beneath the Fennoscandian Shield.
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THE KOMATIITE–PICRITE ASSOCIATION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDTHE KOMATIITE-PICRITE...THE JEESIOROVA AND PEURAMAA...GEOCHEMISTRYCLASSIFICATION OF HIGH-MgO...NEODYMIUM ISOTOPE DATA AND...DISCUSSIONREFERENCES |
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As mentioned above, komatiites and picrites show similar volcanic structures in the field and cannot be distinguished from each other without geochemical analyses, and therefore some uncertainty exists about their areal distribution. Nevertheless, it is evident that komatiitic varieties are more abundant than picritic ones, although there are areas dominated by picritic rocks. As a whole, there seems to be no systematic distribution of different types within the komatiite belt. The most common eruption style of both komatiites and picrites as subaqueous volcaniclastic deposits and pillow lavas suggests that no large lateral magma movements through magma channels on the surface took place and hence both the magma types were extruded from vents located relatively close to each other. The age relationship between the komatiitic and picritic rocks is also unclear, although we know that in some areas the first volcanic rocks erupted on phyllites are komatiitic in composition and picritic dykes have been observed to cut komatiitic tuffs. Hence, at least some of the komatiitic rocks are older than the picritic ones.
The field characteristics of the Lapland primitive volcanic rocks have been described in the literature (Saverikko, 1983, 1985; Barnes & Often, 1990; Davidsen, 1994) and therefore need only brief mention here. Unsorted, agglomerate-like deposits are typical, with the size of rounded or angular fragments ranging from 1 to 10 cm. These deposits can be tens of metres thick and they are interlayered with fine-grained laminar tuffs. Massive lavas are common, and pillow lavas and associated pillow breccias are locally abundant. They alternate with volcaniclastic units reaching hundreds of metres in thickness and several kilometres in length. Differentiated lava flows with peridotitic and pyroxenitic cumulates in the lower part and gabbroic rocks in the upper part are not common but are found in the Sattasvaara area. Very rare olivine spinifex-textured rocks are found in these layered flows (Räsänen, 1996).
Even though the komatiitic and picritic metavolcanic rocks have often delicately preserved their original textures, the primary silicate mineralogy is commonly replaced by regional metamorphic minerals, which formed under greenschist to epidote amphibolite facies. The komatiitic and picritic lavas are usually tremolite–chlorite rocks with various amounts of serpentine, and basaltic komatiites contain actinolitic amphibole with small amount of oligoclase plagioclase. The magmatic olivine is always altered to serpentine or chlorite, and pyroxene usually to amphibole, but relics of magmatic pyroxene are occasionally still preserved.
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THE JEESIÖROVA AND PEURAMAA AREAS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDTHE KOMATIITE-PICRITE...THE JEESIOROVA AND PEURAMAA...GEOCHEMISTRYCLASSIFICATION OF HIGH-MgO...NEODYMIUM ISOTOPE DATA AND...DISCUSSIONREFERENCES |
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The Jeesiörova area is located 25 km east of the village of Kittilä. There exists a lens-like, NW-trending, 3 km x 12 km area of dominantly komatiitic rocks partly overlain by younger coarse-clastic quartzites and conglomerates of the Kumpu Group (Fig. 2). We have drilled through the lower contact of the komatiite-bearing formation. The drill-core shows that the black schists at the base are overlain by
10 m of dolomite and a similar interval of chert. The latter in turn is overlain by tholeiitic lavas 22 m thick. These are followed by 50 m of fragmental komatiites and >90 m of massive, amygdale-bearing komatiites. In general, the komatiitic formation is elsewhere poorly exposed, except in the central part of the area where several outcrops of komatiitic rocks occur in the form of amygdaloidal massive flows, pillow lavas and tuffs. Some of the pillows display variolitic structures. Ultramafic lavas are often porphyritic with abundant olivine phenocrysts. There also exist some outcrops of tholeiitic basalt lavas but they seem to make up only minor intercalations within the komatiitic sequence. Microscopic investigations have revealed that some of the outcrops are composed of ultramafic rock with a cumulate texture with poikilitic clinopyroxene, indicating that these represent lower parts of differentiated lava flows or sills. Also, gabbroic and doleritic rocks are encountered whose geochemistry (low LREE/HREE, for example) is consistent with derivation from a komatiitic parental magma. A few porphyritic lavas have been found that possess chemical compositions transitional between komatiite and picrite.
Although it has limited exposure, the Jeesiörova area was chosen for this study because of its mineralogical preservation, which is the best so far encountered in the komatiite belt in northern Finland, despite the closeness to a long fault zone, the ‘Sirkka line’, on the southern edge of the Kittilä greenstone complex (Fig. 2). In this area, olivine has been replaced by secondary minerals, but clinopyroxene occurs as a poikilitic intercumulus mineral in olivine cumulates, prismatic grains in gabbroic differentiates and as needle-like crystals in pillowed and massive lavas. In addition, ultramafic lavas contain euhedral chromite grains, which were liquidus phases together with olivine during magma emplacement. These chromites are often translucent and brown in colour, and contain small melt inclusions, demonstrating that they are very fresh.
The Peuramaa area is located 30 km NW of Jeesiörova. The lavas in this area cover a smaller area than those at Jeesiörova and are mostly basaltic (Fig. 2). These occur as massive lavas, tuffs and pillow lavas. Cogenetic mafic dykes also occur. Picritic rocks are interbedded with the mafic lavas and occur either as massive, porphyritic lavas or lapilli tuffs. As is shown below, geochemical data demonstrate that all the rocks in the Peuramaa area form a co-magmatic suite. The primary minerals are not so well preserved here as at Jeesiörova. All the primary silicates are destroyed, but fresh chromites have been observed in picritic rocks.
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GEOCHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDTHE KOMATIITE-PICRITE...THE JEESIOROVA AND PEURAMAA...GEOCHEMISTRYCLASSIFICATION OF HIGH-MgO...NEODYMIUM ISOTOPE DATA AND...DISCUSSIONREFERENCES |
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Whole-rock major and minor element abundances were determined by X-ray fluorescence analysis (XRF) using a Phillips PW 1480 spectrometer at the Geological Survey of Finland and a Phillips PW 1410 spectrometer at the University of Tasmania, Australia. Rare earth elements and other trace elements were mostly analysed by inductively coupled plasma mass spectrometry (ICP-MS) using a Perkin–Elmer Sciex Elan 5000 instrument at the Geological Survey of Finland and an HP 4500 instrument at the University of Tasmania. Some samples were also analysed previously for REE by instrumental neutron activation analysis (INAA) in connection with the Lapland Volcanite Project (Lehtonen et al., 1998
). Representative whole-rock major and trace element analyses are reported in Tables 1 and 2.
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Regardless of the relatively well-preserved nature of the rocks at Jeesiörova and Peuramaa, we concur with Barnes & Often (1990) that whole-rock analyses of rocks in the komatiite belt are not good indicators of the original contents of mobile elements such as Na, K, Rb, Ba and Sr, and we therefore do not discuss these elements further. Instead, the coherent behaviour of many more immobile elements, including HFSE and REE, makes them valuable in characterizing the rocks and the processes involved in their genesis.
The total range of the measured MgO contents of komatiitic rocks varies from 31% to <10%. Because of the porphyritic nature of many of these rocks, as well as the presence of clear cumulate textures in some samples, enrichment of intratelluric olivine and chromite has evidently taken place. Massive lavas contain normally 22–24 wt % MgO whereas in pillow lavas MgO can be as high as 27 wt % (calculated on a volatile-free basis) but is most commonly between 14 and 18 wt %. Therefore, strictly speaking, many of the Jeesiörova magnesian lavas are komatiitic basalts rather than komatiites. It seems that the high MgO attained by volcaniclastic (19·2–24·8%) and pillow lavas (24·7–26·1%) in the Karasjok greenstone belt are not usual in our study area. Abundant (intratelluric) chromite grains often produce high Cr concentrations of 4000–5000 ppm. The Al2O3/TiO2 ratio in komatiitic rocks is typically around 13, thus falling well below the chondritic value of 22. This has been shown to result from a relatively high TiO2 content (Barnes & Often, 1990; Lehtonen et al., 1998), which in our sample suite is 0·6 wt % at an MgO content of 24 wt %, and 0·8 wt % at MgO of 15 wt % (Fig. 3). CaO/Al2O3 is most often close to 1·0 although there is some variation, most probably as a result of mobility of CaO.
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Picritic tuffs at Peuramaa most often have MgO contents close to 17–18 wt %, which may be representative of the magma composition. Picritic lavas have a lower MgO content (11–15 wt %). Chromium and nickel behave coherently with MgO and do not distinguish komatiitic rocks from picritic. Compared with komatiitic rocks, the picrites have a much higher TiO2 at the same MgO content and consequently their Al2O3/TiO2 ratio is very low, in the range of 5·0–6·5. The Jeesiörova komatiites possess Ti/V close to the value observed in Archaean komatiites (16, Nesbitt & Sun, 1976), whereas the Peuramaa picrites have considerably higher values of 30. The difference is caused by the higher TiO2 in picrites, as the V content of these two magma types is comparable. The most striking difference between the komatiitic and picritic suites is the elevated HFSE concentrations in the latter; this feature is not restricted to titanium but occurs also for Zr and particularly Nb and Ta. Niobium contents in picritic rocks reach 10 ppm, whereas in komatiitic rocks they remain below 1 ppm. This is reflected in high Nb/Y ratios in picrites as shown in a Zr/Y vs Nb/Y plot in Fig. 4. Fitton et al. (1997) utilized this diagram to distinguish plume-related Icelandic basalts and picrites from N-MORB (normal mid-ocean ridge basalts). Picrites from our study area fall totally within the Icelandic field, whereas the komatiitic rocks plot at the lower edge of the N-MORB field.
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) rocks from Lapland plotted in a Nb/Y vs Zr/Y diagram showing the fields for Icelandic basalts and N-MORB after Fitton et al. (1997)
The difference between komatiites and picrites is also pronounced when considering their REE characteristics. In Fig. 5, chondrite-normalized REE profiles are shown for various rock types from the Jeesiörova area. Komatiitic lavas have hump-shaped patterns with very low (La/Sm)CN of 0·2–0·5 in pillow lavas and slightly higher value of 0·6 in massive lavas, whereas (Gd/Yb)CN varies between 1·3 and 1·8 (Fig. 5a and b). Arndt et al. (1989) have observed similar REE patterns in Brazilian komatiites in the Crixas greenstone belt and suggested that they are at least in part due to mobility of HREE. We have analysed a few primary clinopyroxenes for REE, and their HREE patterns closely resemble the corresponding whole-rock patterns (Fig. 5f) and demonstrate that the REE characteristics of the rocks are magmatic features. Some komatiitic samples display negative europium anomalies, most probably as a result of secondary alteration (see Sun & Nesbitt, 1978). Doleritic and gabbroic rocks follow the same middle REE (MREE)-enriched pattern, indicating that they are fractionation products of a similar parental magma to that of the magnesian volcanic rocks (Fig. 5d). Figure 5c displays analyses of a tholeiitic lava and two ultramafic lavas, which have transitional composition between komatiite and picrite. Mafic tuffs are present as two chemically distinct varieties, one with an LREE-depleted REE pattern and the other with enrichment in LREE (Fig. 5e). Because of the presence of chromite phenocrysts, the latter type is peculiar in having exceptionally high Cr of 1500–1750 ppm for its MgO content of 7–8 wt %.
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Chondrite-normalized REE patterns for rocks from the Peuramaa area are presented in Fig. 6. These include analyses of mafic pillow lavas, massive lavas, tuffs and dykes, and picritic massive lavas and tuffs. All display similar sloping, LREE-enriched patterns with a total range of (La/Sm)CN of 1·4–1·7 and (La/Yb)CN of 4·4–5·4. This feature coupled with similar ratios of other incompatible elements, such as Ti/V, Zr/Y and La/Nb, suggests that the rocks at Peuramaa form a co-magmatic suite produced by fractional crystallization from a similar parental magma.
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To emphasize the huge spread in the abundances of LREE, a combination of selected chondrite-normalized REE patterns of komatiitic to picritic rocks from the Jeesiörova and Peuramaa areas is displayed in Fig. 7. In the whole suite, La concentrations vary from <2 times chondritic to 40 times chondritic and all the rocks show fractionated HREE patterns. Along with increasing LREE/MREE, there is a concomitant increase in HFSE concentrations.
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CLASSIFICATION OF HIGH-MgO VOLCANIC ROCKS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDTHE KOMATIITE-PICRITE...THE JEESIOROVA AND PEURAMAA...GEOCHEMISTRYCLASSIFICATION OF HIGH-MgO...NEODYMIUM ISOTOPE DATA AND...DISCUSSIONREFERENCES |
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On the basis of the above discussion it is evident that there is a relatively large variation in the composition of magnesian volcanic rocks from both the Archaean and Proterozoic greenstone belts, which has led to them being described using terms such as Ti-rich komatiites, iron-rich komatiites, Al-depleted komatiites or Al-undepleted komatiites (Green & Schulz, 1977
; Nesbitt et al., 1979; Barnes & Often, 1990). On the other hand, younger magnesian volcanic rocks are commonly classified as picrites, with good examples being those from Hawaii, Iceland, Baffin Bay, the Deccan Trap and the Karoo. A closer chemical comparison reveals that there is an overlap in composition of rocks termed picrites and komatiitic basalts or komatiites. A characteristic feature of many Phanerozoic picritic magmas is an elevated TiO2 content compared with Archaean komatiites, but no well-established definitions exist in the literature that could be used to classify a magnesian volcanic rock either as picrite or komatiite. Textural criteria are not unequivocal, as spinifex textures have also been recognized in picritic rocks (Hanski & Smolkin, 1995).
The revised IUGS classification for high-Mg and picritic rocks recommends that volcanic rocks with MgO between 12 and 18 wt %, SiO2 between 30 and 52 wt % and (Na2O + K2O) < 3 wt % are called picrites (Le Bas, 2000). This scheme has some weaknesses when applied to our study area or to Precambrian greenstone belts in general: it is based on mobile elements (Si, Na, K) and totally eliminates the terms komatiitic basalt or basaltic komatiite. Furthermore, it cannot distinguish between fractionated products (MgO < 18 wt %) of magmas ranging from various types of komatiites to meimechite.
As the magnesian rock suites have often undergone olivine fractionation or olivine accumulation, a strict value of TiO2, for example, is not a good criterion. Nor is the Al2O3/TiO2 ratio, because both these components vary. We have developed a new [Al2O3] vs [TiO2] diagram (Fig. 8a) in which [Al2O3] and [TiO2] are contents of these components in mole proportions (normalized to unity) as projected from the olivine composition. They are calculated using the following equations: [Al2O3] = Al2O3/(2/3 -MgO -FeO), [TiO2] = TiO2/(2/3 -MgO -FeO). For simplicity, all iron is taken as ferrous. The rationale of the projection scheme has been graphically explained by Hanski (1992). All rock or liquid compositions that lie on an olivine control line have constant [Al2O3] and [TiO2] values and therefore a fixed position on the diagram. The diagram exhibits fields for boninites, Al-undepleted komatiites, Al-depleted komatiites, Ti-enriched komatiites, Ti-enriched and Al-depleted komatiites, picrites, Al-depleted picrites and meimechites. Shown in Fig. 8b are lines with constant Al2O3/TiO2 ratios as expressed in wt %. The boundaries between the separate fields in Fig. 8 could be drawn in many different ways. We have chosen the boundaries nonparallel to the axes of the diagram mainly for two reasons. First, komatiites and boninites are most effectively distinguished by a line projecting to the origin and therefore having a constant Al2O3/TiO2 ratio. The lines between the overlying fields separating komatiites, Ti-enriched komatiites and picrites were drawn parallel to the komatiite–boninite boundary. Second, the reason for an oblique instead of vertical boundary between the Al-depleted and Al-undepleted rock types is the observation that many natural compositional trends of picritic rocks exhibit a slight decrease in [Al2O3] with increasing [TiO2]. As a result of the method of construction of the diagram, there is some loss of information, as the diagram does not distinguish between komatiites and komatiitic basalts, for example. This has to be done on the basis of MgO content. Thus the fields of the diagram represent rock series. Moreover, additional attributes can be used for rocks with some additional peculiarities, such as a high iron content (ferropicrites, ferrokomatiites).
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In Fig. 8b, analyses of the Kittilä komatiite–picrite association are compared with literature data on komatiites from Barberton, Munro Township and Gorgona Island in an [Al2O3] vs [TiO2] diagram. The Al-undepleted Munro Township and Gorgona Island komatiites straddle the line with Al2O3/TiO2 of 20. The majority of the Barberton komatiites, typical examples of Al-depleted komatiites, possess variable but very low [Al2O3] values compared with the Munro Township or Gorgona komatiites. Several of the Barberton samples, however, overlap with the Al-undepleted komatiites in terms of the Al2O3/TiO2 value. Some of them are also relatively high in [TiO2]. Compared with the Munro Township komatiites, the Jeesiörova komatiites display similar [Al2O3] values but are in most cases distinctly higher in [TiO2] and plot within the field of Ti-enriched komatiites. The Peuramaa picrites in turn have still higher [TiO2] and accordingly plot within the picrite field.
In Fig. 9, the Lapland komatiite–picrite association is compared with picrites from the North Atlantic volcanic province and Hawaii. The trend formed by the data on the Baffin Bay and Greenland picrites demonstrates the non-vertical slope of many natural suites. Barnes & Often (1990) pointed out the similarity between the Karasjok komatiite and Baffin Bay picrites. Figure 9 shows, however, that these two magmas differ somewhat with respect to their Al2O3 contents. The Peuramaa volcanic rocks mimic Hawaiian picrites not only in their Al–Ti systematics but also in their overall chemical composition including trace elements and therefore it is more appropriate to call them picrites than komatiitic basalts or Ti-rich komatiitic basalts.
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NEODYMIUM ISOTOPE DATA AND THE AGE OF THE ROCKS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDTHE KOMATIITE-PICRITE...THE JEESIOROVA AND PEURAMAA...GEOCHEMISTRYCLASSIFICATION OF HIGH-MgO...NEODYMIUM ISOTOPE DATA AND...DISCUSSIONREFERENCES |
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For isotopic analyses, clinopyroxene separates were made from seven samples from Jeesiörova using standard separation methods following purification by hand-picking. Six of the samples represent komatiitic volcanic rocks and one of them is a doleritic dyke. In addition to these mineral separates and their whole-rock samples, whole-rock Nd isotopic analyses were carried out for two komatiitic lavas and one gabbroic sample. Because no primary silicates are preserved in the picritic suite at Peuramaa, we had to resort to whole-rock isotope analyses, which were carried out on two picrite samples. Nd isotopic compositions were analysed on a VG SECTOR 54 mass spectrometer at the Geological Survey of Finland. Measurements of 143Nd/144Nd for the La Jolla standard during 1995–1999 have yielded a ratio of 0·511851 ± 0·000006 (SD, n = 48). The analytical procedure has been described by Peltonen et al. (1996)
. The results are shown in Table 3.
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The pyroxene separates have relatively large range in 147Sm/144Nd (0·284–0·428) probably owing to their different modes of occurrence (poikilitic to prismatic) and likely different cooling conditions in which they crystallized. The 147Sm/144Nd values are always higher than those of the corresponding whole rocks (0·231–0·285). On the other hand, Sm and Nd concentrations in pyroxenes can be either lower or slightly higher than in the whole rocks. Picritic rocks display low 147Sm/144Nd (0·152, 0·156). As a whole, the dataset thus possesses a large spread in 147Sm/144Nd. When plotted in a 147Sm/144Nd vs 143Nd/144Nd diagram, the analyses form a linear trend (Fig. 10). There is, however, some scatter [mean square weighted deviation (MSWD) = 16] indicating heterogeneity in the initial ratios. Regression of the whole dataset of 25 analyses yields an age of 2081 ± 29 Ma, which is identical to that published by Krill et al. (1985) for the Karasjok komatiites (2085 ± 85 Ma).
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There are several examples in the literature of Sm–Nd studies on mafic to ultramafic rocks that have resulted in erroneous isochron ages based on whole-rock samples (Cattel et al., 1984; Hegner et al., 1984; Chauvel et al., 1985, 1993; Gruau et al., 1987, 1990, 1992; Claoue-Long et al., 1988; Jahn & Ernst, 1990; Tourpin et al., 1991). The reasons include heterogeneities in the source, crustal contamination or mobility of REE as a result of post-crystallization alteration, particularly carbonatization. One of the aims of our study was to assess whether the age obtained by Krill et al. (1985) is realistic. They used whole rocks having both LREE-depleted komatiitic and LREE-enriched picritic compositions. Regressed separately, their analytical data for komatiites and picrites yield very different ages: 2180 Ma for the former and <1700 Ma for the latter. The age of 2085 Ma obtained using the whole dataset is valid only if the komatiitic and picritic rocks had a similar initial Nd value. Given the large difference in REE chemistry between komatiites and picrites, the Nd isotopic heterogeneity in the corresponding source regions is a possibility that evidently increases the uncertainty of the obtained whole-rock isochron age for the Karasjok metavolcanic rocks. Simple calculations show that if the Nd values for the individual komatiite samples were between +2·8 and +4·2 and those for picrite samples between +4·3 and +4·9, both datasets would plot on separate but parallel isochrons with an age of 2200 Ma. Thus a relatively small heterogeneity in the source region would produce easily an age error of >100 my.
From the above discussion it follows that we regard it as more reliable to try to calculate the age using primary minerals and their whole rocks. The seven whole-rock–clinopyroxene pairs analysed from komatiites at Jeesiörova provide Sm–Nd ages ranging from 2019 ± 44 to 2118 ± 86 Ma with a weighted average of 2056 ± 25 Ma (1 SD). On the basis of this age, the initial Nd values range from +2 to +4 for individual samples, with the most LREE-depleted komatiites having the highest Nd. Using the same age, we obtained initial Nd value of +3·5 and + 3·7 for the two picritic samples from Peuramaa. The high Nd values of +4·0 and +3·7 obtained for the gabbroic and doleritic samples, respectively, corroborate the conclusion reached on the basis of geochemistry on the genetic relationship between these rocks and the komatiitic lavas.
These new isotopic results are compatible with the data reported by Krill et al. (1985) from Finnmark. The isotopically most depleted samples from both the picritic and komatiitic rocks display similar high Nd values of +4. As a whole, there is thus no correlation between the Nd isotopic composition and the major element composition of the volcanic rocks. However, when the whole-rock analyses of the komatiitic sub-population are considered, there seems to be a positive correlation between Nd and 147Sm/144Nd (Fig. 11). This means that Nd is the highest for the most LREE-depleted samples. The decrease in Nd with an increase in LREE/HREE may be a mantle source character or a consequence of crustal contamination.
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The Sm–Nd isotopes show that the age obtained by Krill et al. (1985) is close to the new results we obtained using whole rocks and mineral separates from a single area. The reason for this is that komatiites and picrites of the Kittilä–Karasjok belt were derived from an isotopically homogeneous mantle source at least in terms of neodymium isotopes. Both Ti-enriched komatiites and picrites in Lapland were produced from a mantle source with a long-term depletion in LREE relative to MREE.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDTHE KOMATIITE-PICRITE...THE JEESIOROVA AND PEURAMAA...GEOCHEMISTRYCLASSIFICATION OF HIGH-MgO...NEODYMIUM ISOTOPE DATA AND...DISCUSSIONREFERENCES |
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Neodymium isotopic evolution
One way to explain the observed variation of
Nd in the komatiitic rocks is crustal contamination. Two possible sources of crustal contamination can be envisaged. The komatiites may have digested pelitic sediments of the Savukoski Group on which the komatiite–picrite unit of the same group is lying or they may have interacted with continental rocks deeper in the crust during their ascent or in an intermediate magma chamber. We have previously analysed pelitic metasediments for their Nd isotopic and trace element compositions from 60 km east of Jeesiörova (Hanski et al., 1997) and assume that the pelitic rocks underlying the komatiite suite in the present study area possess similar elemental and isotopic compositions. Mica schists and black schists have high REE concentrations, are LREE enriched and possess low Nd(2060 Ma) values ranging between -3·7 and -6·7. Even minor amounts of contamination with such crustal material should be readily detectable in terms of isotopic and trace element variations in the komatiitic rocks. We use the isotopically and geochemically most depleted komatiite sample as the uncontaminated parental magma. Results of the mixing calculations demonstrate that the observed range in Nd and 147Sm/144Nd can be accounted for by assimilation of up to 4–6% of pelitic material.
In Fig. 12, calculated Nd isotopic evolutions of two samples, one komatiitic and the other picritic, are shown together with the corresponding REE profiles of these samples. It is seen that the evolution lines cross each other at a value that is very close to that of the proposed contemporaneous upper mantle as defined by Nägler & Kramers (1998). The contrast between the positive Nd values, indicating a long-term LREE depletion in the source, and the observed LREE-enriched character of the picritic rocks is similar to the situation with the Hawaiian tholeiitic picrites and alkali basalts and Pechenga ferropicrites, for example (Stille et al., 1986; Hanski et al., 1990). The possible explanations for these results are that such melts were derived by very low degrees of partial melting or by dynamic melting processes so that parent–daughter abundance ratios were changed markedly during partial melting or that the source experienced enrichment in incompatible elements by a ‘metasomatizing’ process, which must have occurred relatively shortly before partial melting to avoid the influence on the initial isotopic signature. Utilizing the most LREE-depleted komatiites and typical LREE-enriched picrites as representing the relative proportions of LREE in the mantle source before and after the enrichment event, it can be shown that every 50 my extension of the enrichment event back in time from the melting event at 2060 Ma would decrease the initial Nd(2060 Ma) value of the picritic rocks by one unit. The result would not be much different if allowance is made for some Sm–Nd fractionation during partial melting.
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Low LREE/MREE ratios in komatiites are in harmony with their positive Nd values. It is generally presumed that in the case of komatiites the Sm–Nd fractionation during partial melting is negligible. The most LREE-depleted komatiitic lava analysed for Sm–Nd has 147Sm/144Nd ratio of 0·275 (equivalent to f = 0·401 in Fig. 12). The mantle source with such isotopic characteristics would possess relatively low Nd values before 2·1 Ga, clearly off the depleted mantle source (DM). The Lapland komatiites have hump-like chondrite-normalized REE patterns with (Gd/Yb)CN varying between 1·3 and 1·8. Specifically, if the source had chondritic relative abundances of HREE before melting, then the heaviest REE were retained more preferably in the source compared with the lighter ones, most probably as a result of the influence of residual garnet. Model calculations using batch melting of garnet peridotite demonstrate that if the source was unfractionated with respect to HREE and these elements were fractionated during melting to the extent observed in the Lapland komatiites, and LREE could not escape from mutual fractionation during melting, then the mantle source would have had even higher Sm/Nd than that measured for the komatiites. In Fig. 12, a Nd isotopic evolution line is also shown for the source of the komatiite sample assuming that this source had originally chondritic Gd/Yb and produced the observed REE profile of the melt with (Gd/Yb)CN of 1·63. This corresponds to Sm–Nd fractionation from f Sm/Nd = 0·85 to f Sm/Nd = 0·40 upon melting. Provided that the depleted model mantle with positive Nd is the principal source for primitive magmas, the LREE depletion seen in the most depleted komatiites should have take place shortly before or during the melting event that produced the komatiites. A similar conclusion was reached above concerning a potential enrichment event related to the generation of the picrites, and hence there seems to have been a complicated fractionation process in the source mantle shortly before or at 2·06 Ga.
The relation between picritic and komatiitic magmas
The association of LREE-depleted and LREE-enriched Archaean komatiites has been explained by a combination of crystal fractionation and crustal contamination (Cattell, 1987). Barnes & Often (1990) noted the Ti- and LREE-enriched nature and heterogeneity of the Norwegian Karasjok komatiites and examined several potential explanations for their unusual geochemistry compared with typical Archaean komatiites. They concluded that alteration, upper-crustal contamination or low-pressure crystal fractionation cannot be invoked in this connection. It suffices to note here that the same conclusion applies to the relation between komatiitic and picritic rocks of this study, and the reader is referred to Barnes & Often (1990) for detailed discussion on the processes mentioned above.
We are confronted with a difficult question: How much of the observed heterogeneity is related to fractionation processes during partial melting and how much to compositional differences in the mantle sources? The implication of the Nd isotope studies is that if there was significant variation in the mutual ratios of REE in the mantle, this was created very soon before the generation of the komatiite–picrite association. This association has two particular features, the large variation in LREE/MREE within the rock suite and concomitant depletion in both LREE and HREE in the komatiitic rocks, which any melting models should be able to explain if these fractionations are attributed to partial melting effects.
Various melting models have been called upon to account for chemical variations in oceanic basalts, particularly as a consequence of findings of ultradepleted melt inclusions in near-liquidus minerals. For example, Gurenko & Chaussidon (1995) have successfully replicated the observed range of incompatible elements in depleted and enriched melt inclusions in olivine from Icelandic tholeiites by using a critical (dynamic) melting model. This is an intermediate process between pure fractional and batch melting and assumes that a critical amount of melt is retained in the mantle residue. The same model was utilized by Arndt et al. (1997) to explain moderate to extreme trace element depletion in the Gorgona Island komatiitic lavas. Fractional melting depletes the residue in incompatible elements efficiently, and consequently incremental melts may differ considerably in composition from each other; this difference is also reflected in the composition of the pooled melts. Dynamic melting thus has a potential for creating both LREE-enriched and LREE-depleted melts from the same mantle source and is therefore an attractive hypothesis in our case.
To simulate polybaric, near-fractional melting we have carried out model calculations using incremental, non-modal batch melting (Shaw, 1970) with small steps of 1 kbar pressure release, as this approach allows changing melting modes to be taken into account easily. We consider decompressional melting of upwelling mantle with two potential temperatures leading to different pressures of solidus intersection, 28 and 40 kbar, and accordingly initiation of melting within the spinel and garnet stability fields, respectively. We performed calculations using six starting mantle compositions with flat or variously LREE- and/or HREE-depleted REE patterns, as shown in Fig. 13. Other input parameters in the model are given in the caption to Fig. 13.
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Pooled melt compositions produced from various sources are compared with measured rock compositions in Fig. 13 in terms of primitive mantle normalized La/Nd and Nd/Yb. Melting in the spinel peridotite facies produces curved trends approximately parallel to the komatiite–picrite array (Fig. 13a), but only those mantle sources having high Nd/Yb already before melting (model compositions 4 and 6) are able to create melt compositions with HREE fractionation similar to that observed in the komatiites and picrites. It is immediately evident that the whole observed range of elemental ratios cannot be achieved by dynamic melting to any reasonable degree in these conditions. It is also questionable whether low degrees of melting can produce high-MgO liquids at the pressures of the spinel lherzolite facies.
As all the rocks in the study area display high Gd/Yb (i.e. a strong ‘garnet signature’), partial melting in the garnet peridotite facies could provide a relevant explanation. Irrespective to the source composition, the calculated decompression melting paths produced by low degrees of melting approximately parallel the komatiite–picrite array but are shifted to much higher (Nd/Yb)PM values (Fig. 13b). On the other hand, high-degree melts tend to form nearly horizontal trends with little variation in (La/Nd)PM crossing the komatiite–picrite array. The same problem occurs for melts produced by melting of garnet lherzolite and those from spinel lherzolite, namely the limited variation of La/Nd compared with the measured dataset. Computations demonstrate that mantle sources with low (La/Nd)PM (<1) obviously fail to recover REE characteristics of the picritic rocks. Instead, those characteristics are successfully reproduced by LREE-enriched model composition 2 in Fig. 13.
Figure 13a and b assumes continuous mixing of melt increments after their extraction from the source. Much larger compositional variation in pooled melts derived from a single mantle source can be attained if the early formed melts are isolated from subsequent melt increments in some stage of the melting process, either because of tapping of the magma chamber, in which melt aggregation takes place, or pooling of the later melt increments in a separate chamber. Figure 14 illustrates the effect of isolation of the first formed pooled melts on the subsequent compositional evolution of the melts. Two starting materials are used, an enriched source (EM, Fig. 14a) and a depleted source (DM, Fig. 14b), with the latter chosen to be in accord with a long-term LREE-depleted character of the source as indicated by the obtained Nd(2060 Ma) values. It is interesting to note that in this kind of melting process, both strongly LREE-enriched and LREE-depleted melts as well as melts with pronounced MREE humps can be generated from both the EM and DM sources. However, only the EM source allows melts to be generated that correspond in their REE characteristics to those of the Peuramaa picrites. The Jeesiörova komatiites can instead represent moderate- to high-degree, second-stage pooled melts from both sources.
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Methods have been developed for calculating an approximate degree of partial melting for melts without having to make assumptions about the mantle source composition. One of them is based on the concentration ratios of two incompatible elements in at least two, variously enriched or depleted melt compositions thought to have been generated from the same source (e.g. Hofmann & Feigenson, 1983). Zou & Zindler (1996) derived equations that allow application of this principle to melts produced by a dynamic melting process. Using this dynamic melting inversion method (DMI) and elemental ratios such as La/Nd in picrites and komatiites from our study area, it can be shown that extremely low degrees of partial melting (<<0·1%) would be required for the picrites if they were produced from the same source as the komatiites by a one-stage dynamic melting process.
The DMI modelling together with the forward modelling presented in Figs 13 and 14 strongly indicates that progressive dynamic melting of the same LREE-depleted source with the picrites representing a low degree of melting and the komatiites a subsequent higher degree of melting fails to account for the chemical variability of these rocks. The picrites require a source enrichment in incompatible elements, which may or may not be an integral part of the melting event that produced the komatiites, but in any case was not an ancient one as shown by the Nd isotopic results. It can also be concluded that even though the komatiites are products of high degree of melting, the geochemical compositions do not necessarily reflect accurately the incompatible element ratios of the original mantle source.
We have made some other observations that support the notion that the mantle source for the komatiite–picrite association was chemically heterogeneous. As shown in Fig. 4, picrites and Ti-enriched komatiites appear in totally different fields in a Nb/Y vs Zr/Y plot. Fitton et al. (1997) demonstrated that the discrimination power of the diagram is insensitive to low-pressure fractional crystallization, the effects of variable degrees of mantle melting, source depletion through earlier melt extraction or crustal contamination, and hence the sub-parallel arrays must reflect different source characteristics. Following Fitton et al. (1997), the positions of the komatiitic and picritic rocks on the diagram indicate fundamental chemical differences in their respective mantle source compositions.
High chondrite-normalized or primitive mantle normalized Gd/Yb ratios in komatiitic rocks at Jeesiörova are not accompanied with drastic depletion in Al2O3 as is evidenced by the [Al2O3] vs (Gd/Yb)PM plot in Fig. 15. The situation is hence very different from that of the Barberton komatiites, whose chemical characteristics are thought to have resulted from equilibration with majorite garnet deep in the mantle (e.g. Herzberg, 1995). High Gd/Yb in the Lapland rocks may thus be a consequence of another kind of process, such as preferential enrichment of the source in LREE relative to HREE. This hypothesis makes the history of the mantle source region even more complicated, because this potential enrichment event must have followed another depletion event as deduced from the LREE-depleted nature of the rocks.
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In Fig. 16, the Jeesiörova rocks are compared with the Munro Township komatiites in a [TiO2] vs (Ce/Sm)CN diagram. Further evidence for source heterogeneity is provided by the behaviour of Ti and LREE: even the most LREE-depleted komatiitic basalts from Jeesiörova have a relatively high TiO2 content compared with Munro komatiites and other MgO-rich magmas with similar La/Sm. Because enrichment in TiO2 is usually coupled with high LREE/HREE in magnesian magmas, a literature survey showed that those rocks having [TiO2] >0·015 and (Ce/Sm)CN <1·0 are extremely rare. The authors are not aware of any conditions during partial mantle melting that could change the behaviour of Ti and REE in such a way that the same source would produce Jeesiörova Ti-enriched komatiites and Munro-like komatiites with both having a similar degree of LREE depletion. Thus, the reason for the difference between these two rock types is most probably found in the mantle source composition, which was more Ti rich in the case of the Lapland komatiitic rocks. These rocks provide further evidence for the presence of heterogeneous mantle beneath the Fennoscandian Shield during early Proterozoic time. When considering all MgO-rich Palaeoproterozoic volcanic rocks in the Fennoscandian Shield, it can be seen that besides LREE, FeOtot has a positive correlation with TiO2. Compared with a trace element heterogeneity, large differences in the iron contents in MgO-rich volcanic rocks are not so easy to explain by a dynamic melting process. There seems to be a more or less gradual change from Ti-enriched komatiites and picrites found in the Kittilä–Karasjok belt through picrites of the Onega region to ferropicrites found in the Pechenga region, and we infer that their mantle sources probably had an analogous compositional variation. One explanation for the heterogeneity could be lherzolitic mantle veined with garnet pyroxenite or eclogite (see Cordery et al., 1997
; Francis et al., 1999). However, in our case, this is difficult to test without the use of isotopic systems other than Nd.
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A single plume source model
The highly magnesian nature of the komatiitic and picritic rocks, pointing to high potential temperatures of the mantle source, and the similarity between the Hawaiian and Lapland picrites, can be used as arguments for a mantle plume origin for these rocks. In their single plume model, Puchtel et al. (1998) related the generation of the Onega basalt plateau, the Pechenga ferropicrites and komatiites of the Kittilä–Karasjok belt to the ascent and partial melting of the same starting mantle plume. This interpretation was supported by similar (1·98 Ga) ages obtained for the primitive volcanic rocks at Pechenga (Hanski et al., 1990) and in the Onega region (Puchtel et al., 1998) as well as tholeiitic basalts from the Kittilä area (Hanski et al., 1998). These last rocks, however, are not spatially or genetically related to the magnesian metavolcanic rocks studied in this paper, but instead are part of the Kittilä Group, which is interpreted to represent an allochthonous, dominantly basaltic oceanic crustal block (Hanski, 1997; Hanski et al., 1998).
Two fundamental phenomena of the plume-related processes make it problematic to reconcile the genesis of the komatiites and picrites in Lapland with a single starting plume hypothesis similar to that proposed by Puchtel et al. (1998) or with the plume model in general. These are the duration of volcanism and the uplift of the continental crust before plume magmatism. Recent improved age determinations of volcanic rocks related to many flood basalt provinces thought to be linked to a plume head (e.g. the North Atlantic Tertiary Province, Karoo province, and Siberian and Deccan Traps) have shown that the volcanic rocks were extruded in a very short time interval of only a few million years, with the bulk of the total volume of volcanism erupted even within 1 my (e.g. White & McKenzie, 1995). The plume tail magmatism in turn may persist for as long as 200 my, but in a given locality, it will last only a few million years, because of movement of the overlying plate across the hotspot conduit. Although the geological evolution appears to have been very similar in Central Finnish Lapland, the Kola Peninsula and southern Karelia, the age data reported for the komatiitic and picritic rocks from Kittilä suggest an age difference of more than 50–80 my between these rocks and the picrites in the other two areas. Assuming that the ages are correct, the timing of the primitive volcanism in Central Lapland appears to be too offset from that in the Kola Peninsula and southern Karelia for them to be connected to the same plume head. On the other hand, the generation of these magmas could be attributed to plume conduit magmatism. The distance between the Central Lapland komatiite–picrite association at Kittilä and the Onega plateau is 700 km, which would translate to a plate movement of 1·4 cm/year with an age difference of 50 my, provided there has not been any large block movement in the Archaean basement since their formation. This value is not excessively unrealistic given potential large errors in this kind of calculation. However, accepting the similar age for the Pechenga and Onega rocks, they do not form, together with the Kittilä rocks, a coherent plume track with progressive age change along its strike.
The plume theory also predicts large and rapid regional uplift of the Earth’s surface as a result of the positive buoyancy of hot plume material, reaching as much as 1–2 km in height and covering laterally an area similar to that of the underlying flattened plume head (White & McKenzie, 1989). Uplift caused by plume activity beneath a continent will start 10–20 my before the onset of volcanism and lead to an increasing erosion rate, development of unconformities and deposition of continental sediments. This effect is recognized in the sedimentary record in the Karoo province and West and East Greenland, for instance (Dam et al., 1997; White, 1997).
All three areas (Onega, Pechenga, Central Lapland) linked to a mantle plume by Puchtel et al. (1998) have a similar general geological evolution. In the Onega region, ‘flood basalts’ belong to the Ludicovian rocks deposited on epicontinental quartzite- and dolomite-bearing Jatulian strata. In the Ludicovian sequence, volcanic rocks are underlain by abundant calcareous and argillaceous sediments including highly carbonaceous rocks called shungites (Galdobina & Melezhik, 1986; Buseck et al., 1997). At Pechenga, ferropicritic rocks occur in the upper part of the Pilgujärvi Sedimentary Formation (‘Productive Formation’) and intercalated with tholeiitic basalts in the overlying volcanic sequence. The Productive Formation ranges in total thickness between 600 and 1000 m and comprises a variety of sedimentary rock types, but is dominated by carbonaceous and sulphidic pelites (Melezhik et al., 1994).
As mentioned above, the komatiitic and picritic rocks in Central Lapland were deposited on phyllites and black schists of the Savukoski Group representing part of a thicker transgressive sedimentary succession having epicontinental quartzites and minor carbonate rocks of the Sodankylä Group at lower stratigraphic levels (Lehtonen et al., 1998). In summary, in all three areas under discussion, older cratonic sedimentary units at lower stratigraphic levels give way to carbon-rich pelitic rocks, indicating gradual tectonic evolution from shallow-water to deeper-water regimes. The sedimentary records hence do not reveal any substantial regional uplift shortly before the voluminous eruptions of komatiites, picrites and basalts began. In this sense, the situation is similar to that in the Noril’sk region, where no plume-related uplift has been documented and the volcanic sequence lies above thick, coal-bearing sedimentary strata (Fedorenko et al., 1996).
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ACKNOWLEDGEMENTS |
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We thank Nick Arndt, John Tarney and Majorie Wilson for their constructive comments on an earlier version of the manuscript.
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FOOTNOTES |
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*Corresponding author. Telephone: +358-0205504260. Fax +358-02055014. E-mail: eero.hanski{at}gsf.fi
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REFERENCES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDTHE KOMATIITE-PICRITE...THE JEESIOROVA AND PEURAMAA...GEOCHEMISTRYCLASSIFICATION OF HIGH-MgO...NEODYMIUM ISOTOPE DATA AND...DISCUSSIONREFERENCES |
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