Early Precambrian mafic rocks of the Fennoscandian shield as a reflection of plume magmatism: Geochemical types and formation stages

N. A. Arestova, S. B. Lobach-Zhuchenko, and V. P. Chekulaev

Institute of the Precambrian Geology and Geochronology of the Russian Academy of Sciences, St. Petersburg

E. G. Gus'kova

St. Petersburg Branch of the Institute of the Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (SPbF IZMIRAN), St. Petersburg

Contents

Abstract

Introduction

The last decade witnessed how the previously governing plate tectonic paradigm of the Earth's history gave way to the new theory of the global Earth Tectonics. From the standpoint of this theory, the Earth developed through the processes of core growth, plume tectonics, and plate tectonics, which first operated sequentially and then jointly [Devias, 1997; Kumazava and Maruyama, 1994; Maruyama et al., 1994; etc.]. In this succession of mechanisms, plume tectonics, whose refinement was contributed to by many studies of the last decade [Campbell and Griffiths, 1990, 1992; Condie, 2001; Dobretsov et al., 2001; Grachev, 1998, 2000; Maruyama, 1994; etc.], is thought to have played the leading role at early phases of the Earth's evolution. Studies by a number of workers in the Early Precambrian have shown that the plume-tectonic mechanism was dominant during the Early Precambrian stages of geologic history [Abbott, 2001; Campbell and Griffiths, 1990, 1992; Vrevsky, 2000; etc.].

The most promising approach in unraveling mechanisms that were likely to operate in the Early Precambrian is the study of compositions of basites and ultrabasites, derivatives of mantle melts, to elucidate their source composition, melting conditions, and subsequent melt evolution.

The focus of our work is on Early Precambrian (3.4-2.4 Ga) basites and ultrabasites of the eastern Fennoscandian shield. Our study draws on the recent results regarding the conditions and history of formation of Early Precambrian (Archean) crust in the eastern part of the shield [Lobach-Zhuchenko et al., 1998, 2000b, 2003]. Among these results is the conclusion that, alongside the previously established age heterogeneity of the Archean domains (Fenno-Karelian, Belomorian, and Kola) of the eastern Fennoscandian shield, there exists an age heterogeneity of the shield's largest ancient entity, the Fenno-Karelian granite-greenstone province. The oldest portion of the Fenno-Karelian granite-greenstone province is the Vodlozero domain, whose crust started forming at 3.2-3.4 Ga. Later on, the crust of the western Karelian (3.1-3.0 Ga), Kola, and Belomorian (3.0-2.9 Ga) domains began to form. The crust of the youngest, central Karelian domain is less than 2.85 Ga old. Another approach in scrutinizing mafic-ultramafic magmatism is centered on establishing the principal stages of formation and evolution of Early Precambrian crust of the shield [Lobach-Zhuchenko et al., 2001].

Detailed petrologic and geochemical studies of Early Precambrian komatiites, basalts, and mafic intrusives of the eastern Fennoscandian shield, carried out in recent years [Arestova and Glebovitsky, 2003; Chekulaev et al., 2002, 2003; Lobach-Zhuchenko et al., 1998, 2002a, 2003; Puchtel et al., 1997, 1998, 1999; Vrevsky, 2000], enabled the researchers to identify (by analogy with modern basites generated in a variety of geodynamic settings) the rocks whose generation was likely related to mantle plumes [Campbell and Griffiths, 1992; Kerr et al., 2000; etc.]. Such basites, derived from high-temperature melts, are the focus of this study.

Characteristics of Early Precambrian Basites of the Fennoscandian Shield

Over the past 15 years, a large number of reliable isotope age determinations, most of which are listed in Table 1, have been carried out on Fennoscandian shield basites. These basites fall into five age groups: (1)  > 3.1 Ga, (2) 2.99-2.91 Ga, (3) 2.88-2.80 Ga, (4) 2.72-2.66 Ga, and (5) 2.50-2.41 Ga. Given below is the analysis of how basites of various age groups are distributed over the area of the Fennoscandian shield and of the geochemical types of the basites constituting these age groups.

Basites With Ages Older Than 3.1 Ga

Figure 1

Mafic rocks with the oldest age determinations (3.4 Ga) are found in the southeastern Vodlozero domain, the oldest in the Fennoscandian shield (Figure 1) [Kulikova, 1993; Puchtel et al., 1991]. The basites are represented by the Volotskaya Sequence komatiite-basalt assemblage (Table 2, nos. 1, 2). The peridotites of the Sequence are high-Mg rocks with ca. 27% MgO (no. 1, Table 2). Currently, there is no doubt that peridotitic komatiites with > 24% MgO in spinifex-textured varieties are plume derived. Liquidus temperatures for melts initial to such komatiites, as calculated using the formula proposed by [Nisbet et al., 1993], range 1550-1600^o C, which corresponds to a mantle temperature of ca. 1800^o C and is far in excess of mantle temperatures in the Archean or Early Proterozoic, as calculated by [Richter, 1988]. Compositions of the Volotskaya Sequence peridotitic komatiites (27% MgO, 47% SiO₂, CaO/Al₂O₃ = 0.7-1.29, Al₂O₃/TiO₂ = 15-24, 1900-1300 ppm Ni, e _Nd (t) = +1.2) suggest that they are derived from high-temperature mantle and are undepleted or slightly depleted in silica. The basalts associated with these komatiites are high in Ni (760-150 ppm) and have Nb/Y and Zr/Y ratios ( > 0.1 and 2-3, respectively) that place them in the field of rocks generated in oceanic or continental-margin plateaus (volcanic rifted

Figure 2

margins; Figure 2) [Kerr et al., 2000; Marsoli et al., 2000]. Although, according to our own geological data, such an old age requires additional validation, it can be safely assumed that high-temperature melts (plume derivatives) first appeared as early as > 3 Ga ago.

Basites With 2.99-2.91 Ga Ages

The next (and longest) stage of mafic magmatism takes the time span between 2.99-2.91 Ga. The basites of this stage are widespread within the ancient Vodlozero domain and are represented by intrusions in the central part and by volcanics (komatiites and basalts) in the marginal parts of the domain (Figure 1). This stage, which lasted ca. 75-80 m.y., is divisible into three episodes.

Figure 3

The earliest episode is featured by intrusive magmatism, as exemplified by the Lairuchei layered intrusion (composed of gabbropyroxenite, gabbronorite, anorthosite, and diorite), situated in the central part of the domain and dated at 2.987 11 Ga. Geochemical features of the basites composing the intrusion (Table 2, nos. 3, 4) are: high mg# (0.79-0.68), 22-7% MgO, and high Cr (1000-350 ppm) and Ni (800-200 ppm). According to Campbell and Griffits's data, NiO > 600 ppm at 16% MgO may point to a plume provenace for the initial melt. Note, however, that characteristics of high-temperature melts, such as elevated (as compared to Archean komatiites) SiO₂ contents, (Nb/La) _<UNDEF> N = 0.5, Ti/Zr = 40, an evolved distribution of the rare earth elements ((La/Yb)_N = 5-10) (Figure 3), and e _Nd (t) = - 0.8 to - 2.5, suggest crustal contamination for the initial melt. Mass balance calculations using major- and trace elements and AFC model calculations based on the ( e _Nd -La/Sm) ratio suggest melt generation conditions that involved 15% assimilation of the Lairuchei tonalite by a plume melt with mantle REE abundances [Arestova, 1997].

Figure 4

Another episode is represented by mafic-ultramafic volcanics of the western and eastern margins of the Vodlozero domain; it is dated at 2.96-2.94 Ga (Figure 1). Peridotitic komatiites with 24.8-29.6% MgO contents in spinifex textured varieties and 44.4-47.2% SiO₂ are found in all the greenstone belts (Table 2, nos. 5, 7, 9, 11). They belong to the silica-undepleted type (CaO/Al₂O₃ = 0.5-0.9, Al₂O₃/TiO₂ = 15-25) and are high in Ni (950-1450 ppm) and Cr (2000-4000 ppm). The komatiites have unfractionated REE patterns ((La/Yb)_N = 1 0.1, (Gd/Yb)_N = 1 0.1) and REE abundances 1.5-4 times the chondritic at (Nb/La)_N of ca. 1 (Figure 4a). Less frequently, the komatiites are depleted in the light REE and have (La/Yb)_N = 0.6-0.7, (Gd/Yb)_N = 1, and (Nb/La)_N = 1-1.2. The e_Nd (t) value in komatiites from the western surroundings of the domain is +1.7, and from the eastern, +2.2. In the Hautavaara belt komatiites (the domain's western margin), Ni contents (650 ppm) are lower than in komatiites from the other belts. The Hautavaara komatiites are enriched in the light REE ((La/Yb)_N = 1.3 0.1 and (Gd/Yb)_N = 0.9 0.1) and have (Nb/La)_N ratios of 0.5-0.7 and negative e_Nd (t) values; this set of evidence suggests crustal contamination for the komatiitic melt (Figure 4a).

Basalts found in the Oster, Palaya Lamba, and Hautavaara belts along the western margin the Vodlozero domain and in the Kenozero belt at its eastern margin, have different geochemical characteristics. Thus, basalts associated with komatiites have mantle Ti/Zr ratios (100-110), unfractionated REE patterns ((La/Sm) _N = 1.0-0.9 and (La/Yb) _N = 1.1-1.2), and REE abundances 7-14 times the chondritic (Table 2, nos. 6, 8, 10, 12; Figure 4b). The high Ni contents ( > 100-150 ppm), Nb/Y ratios in excess of 0.1, Zr/Y ratios of 2-3, and Nb/La = 0.9-1.11 of this basaltic group, are similar to those of oceanic plateau rocks. The e_Nd (t) value in high-temperature uncontaminated basalts ranges from +0.5 to +3.2, suggesting source heterogeneity and/or mixing of melts from depleted and undepleted sources.

Geochemical features of the Hautavaara belt komatiites and basalts (reduced Ni contents, La/Yb > 1, Nb/La < 0.8 (Table 2, nos. 5, 6)) imply that these rocks make part of a plateau generated on continental crust. Komatiites occurring in association with basalts at Palaya Lamba are also likely to represent a fragment of a plateau generated on continental crust. This is evidenced by the surviving low-angle attitudes of volcanic flow units and by the superimposed deformations (high angle schistosity related to the subsequent accretionary phase and, at the same time, conformable to bedding planes, as should be expected in an oceanic plateau obducted onto a continental margin).

Figure 5

Detailed studies performed in the Oster and Semch greenstone belts have shown that alongside basalts similar to those just described, these belts contain basalts whose chemical features and spatial association with andesites suggest an analogy with modern volcanics generated in island-arc and backarc basin settings (Figure 5). Later on, basalts that were generated in various geodynamic settings underwent tectonic juxtaposition, to form a collage.

Figure 6

At the northern margin of the Vodlozero domain, mafic-ultramafic volcanics are dated at 2913-2916 Ma [Lobach-Zhuchenko et al., 1999; Puchtel et al., 1999; Sochevanov et al., 1991]. These volcanics occur in the Shilos and Kamennye Ozera bimodal greenstone belts. Komatiites are only encountered in the Kamennye Ozera belt, where they are represented by peridotitic varieties that, where spinifex textured, show high MgO, Cr, and Ni contents (Table 2, no. 16). The komatiites are depleted in the light REE ((La/Yb) _N = 0.6-0.7), their medium- and heavy REE contents corresponding to those of primitive mantle (Figure 6). The komatiitic (Nb/La) _N ratio is 0.9-1.0, suggesting lack of crustal contamination of the melts. Geochemical characteristics of the komatiites testify to their having originated from high-temperature plume melts in oceanic or rifted continental margin settings.

Basalts of the northern margin have a broad range of compositions. In the Shilos belt, two basalt groups are discerned (Table 2, nos. 14, 15, Figure 6b). Both basaltic groups are high temperature rocks, considerable distinctions between them occur in their Ti and Zr abundances and REE enrichment degrees. Group 1 basalts are light REE depleted ((La/Yb)_N = 0.5-0.7, (La/Sm)_N = 0.6) and slightly HREE depleted ((Tb/Yb)_N = 1.2). Their REE contents are 2.5-3.5 times the primitive mantle (Figure 4b). Group 2 basalts have (La/Yb)_N = 1.9 and (La/Sm)_N = 1 and REE contents 6-8 times the PM values. Both basaltic groups lack evidence of crustal contamination; their (Nb/La)_N ratio ranges of 0.8-1.5. The early stage tholeiites of the Kamennye Ozera greenstone belt (Table 2, nos. 17, 18) fall into groups with distinctive mg# (from 0.62 to 0.53) and high Cr and Ni abundances. These tholeiites are depleted in the light REE and show no crustal contamination, their (Nb/La)_N ratio ranges of 0.8-1.7.

Figure 7

Geochemical characteristics of both basaltic sub-groups of the Shilos belt and early stage basalts of the Kamennye Ozera belt are consistent with those of modern oceanic plateaus or continent margin plateau basalts (Figure 7). The discrepancies in the e_Nd (2916) values of the Shilos basite isochron (+1.6) [Sochevanov et al., 1991] and those of Kamennye Ozera (+2.7) [Puchtel et al., 1999] are due to different isotope compositions of their initial melts and imply melt derivation from various parts of a heterogeneous source, which is feasible if melting occurs in the plume head.

Basites With 2.88-2.80 Ga Ages

Figure 8

Figure 9

Figure 10

The third episode of mafic magmatism, dated to the 2.88-2.81 Ga time interval, is recorded in various domains of the Fennoscandian shield (Figure 8). During this stage, komatiitic and basaltic eruptions took place in the Kola Peninsula (Polmos-Poros belt, Uraguba, and Korvatundra, dated at 2.88 Ga [Vrevsky, 2000]), in northern Karelia (early volcanism of the north Karelian system of greenstone belts, dated at > 2.81 Ga [Kozhevnikov, 2000]), and in the western Karelian domain (Kostomuksha belt, dated at 2.84-2.81 Ga [Lobach-Zhuchenko et al., 2000a; Puchtel et al., 2000a]). High-temperature peridotitic komatiites with 22-31% MgO and 44.4-47.2% SiO ₂ (Table 3, nos. 1, 4-6) are present in all the belts of this particular stage. They belong to the undepleted or slightly silica-depleted type (CaO/Al₂O₃ = 0.6-0.9, Al₂O₃/TiO₂ = 15-25) and have high Ni (800-1600 ppm) and Cr (2000-4000 ppm) contents. Komatiites from most belts of this stage have unfractionated REE patterns ((La/Yb)_N = 1 0.1, (Gd/Yb)_N = 1 0.1), REE abundances 1.5-3 times the chondritic (Figure 9), and a (Nb/La)_N ratio of ca. 1. Our own detailed study of komatiites from the Kostomuksha belt in the western Karelian domain shows them to be depleted in the light REE ((La/Yb)_N = 0.4-0.6, (Gd/Yb)_N = 1.04-1.16, and (Nb/La)_N = 1-1.2). The e_Nd (t) value in the komatiites ranges from +2.7 to +2.9, pointing to generation of their initial melts from a depleted source and to lack of crustal contamination. Basalts of greenstone belts of this stage (Table 3, nos. 3, 7-9) have mantle Ti/Zr ratios (100-116), unfractionated REE patterns ((La/Yb)_N = 1.0-0.9), and REE abundances 7-14 times the chondritic (Figure 9). The high Ni contents (75-135 ppm), the ratios of Nb/Y = 0.1 and Zr/Y = 2-2.5 (Figure 10), and Nb/La > 1.0 are earmarks of rocks generated in oceanic or continental plateaus in the absence of crustal contamination. However, the basaltic sequence of the Kostomuksha greenstone belt, alongside uncontaminated basalts (Nb/La > 1, e_Nd (t) = 2.9), hosts basalts that probably suffered contamination (Table 3, nos. 10, 11), their e_Nd (t) value ranging from - 3.1 to +0.9 [Lobach-Zhuchenko et al., 2000a]. This implies that basalts of the Kostomuksha greenstone belt are plume-derived melts erupted in a volcanic rifted margin setting.

Figure 11

Intrusive mafic magmatism of the 2.85-2.84 Ga interval is recorded to have taken place on the western and northern margins of the Vodlozero domain, which had been formed through accretion. Mafic intrusives (the Semch, Palaya Lamba, and Shilos massifs; Table 3, nos. 12-16) have rather high SiO₂ contents at elevated (0.5-0.8) mg#, Ti/Zr ratios of 40-80, an evolved REE pattern ((La/Yb)_N = 5-12) (Figure 11), negative Nbanomaly ((Nb/La)_N = 0.8-0.5), and negative e_Nd values (from - 0.8 to - 1.5), features that are suggestive of a variable degree of crustal contamination for the initial melts.

Basites With 2.72-2.66 Ga Ages

Figure 12

Figure 13

The Late Archean stage of mafic magmatism was less vigorous as compared to the preceding ones. It spans a ca. 2.72-2.66 Ga time interval. This stage is distinguished by moderate- and low-pressure granulite metamorphism first affecting a number of regions in southwestern and western Karelia (Lake Tulos, Lake Kuito), south-central Finland in the vicinity of Iisalmi (all the structures of the western Karelian domain), and between Lake Kovdozero and Lake Notozero in the Belomorian domain. Basites of this stage are chiefly developed in northern Karelia and the Belomorian region. They are represented by: the late-stage komatiites of the north Karelian group of belts ( > 2705 Ma) [Shchipansky et al., 1999] and, apparently, of Finland [Gruau et al., 1990; Jahn et al., 1980]; the gabbronoritic and high-Ti, high-Fe gabbroic intrusions dated at 2.69 Ga in the regions of Lake Notozero and the Tupaya Guba Bay of Lake Kovdozero [Lobach-Zhuchenko et al., 1993, 1995]; and dikes developed throughout the shield (Figure 12; Table 4, nos. 1-5). Geochemical characteristics of both the volcanic and intrusive rocks alike (in particular, high Ni contents at high mg# of the rocks) testify to a plume related provenance for their initial melts [Campbell and Griffiths, 1992], whereas the elevated SiO₂ contents, evolved REE patterns, Ti/Zr ratios of 60-70, and negative Nb anomaly (Nb/La < 1) (Figure 13) point to a considerable crustal contamination of initial melts for both the intrusive and volcanic rocks. Evidence for a

Late Archean plume within the Karelian province includes (besides the formation of high-temperature basites) the emplacement (in the 2.70-2.68 Ga time interval) of postorogenic intracratonic granitoid intrusions featured by high contents of HFS elements such as Y, Zr, Ti, and Nb [Lobach-Zhuchenko, 2002].

It is open to discussion whether or not this particular age group includes the mafic-ultramafic volcanics from greenstone belts of eastern Finland. Not inconceivably, they belong to the preceding stage. Komatiites of these belts have the following compositional parameters: 22-27% MgO at 0.80-0.77 mg# and 9.5-19.8% MgO at 0.76-0.50 mg#; CaO/Al₂O₃ ratios of 0.72-0.87, Al₂O₃/TiO₂ = 16-17, and Ti/Zr = 110; and Ni contents of 800-1500 ppm and 300-650 ppm [Gruau et al., 1990; Jahn et al., 1980]. In this age group, komatiites make three sub-groups with dissimilar REE patterns: (1) light REE depleted ((La/Sm)_N = 0.3-0.6 and (Gd/Yb)_N of ca. 1.0); (2) with a flat REE pattern or a slight LREE enrichment ((La/Sm)_N of ca. 1 and (Gd/Yb)_N = 1.0-1.32); and (3) with REE abundances 1.5-2 times the mantle values and HREE depletion ((La/Sm)_N = 0.7-0.95 and (Gd/Yb)_N = 1.7-1.4). The basalts are also divisible into three groups: (1) uncontaminated, with Ti/Zr ratios of 100-110, (La/Sm) = 0.62, (Gd/Yb)_N of ca. 1.0, (La/Sm)_N of ca. 1.0-0.96, and (La/Yb)_N of ca. 1.0, and with REE abundances 7-20 times the chondritic; (2) contaminated, with Ti/Zr = 70, (La/Sm)_N of ca. 1.5-2, and (La/Yb)_N of ca. 2-4; (3) subalkaline basalts with a high mg# (0.60), relatively high total alkali abundances (up to 6%) and Rb, and high Zr, P₂O₅, and REE. These rocks also have fractionated REE patterns ((La/Sm)_N of ca. 4 and (Gd/Yb)_N of ca. 4). The existence within the same greenstone structure of such a broad compositional variety of high-temperature volcanics is in good agreement with their plume origin and is not inconsistent with eruptions in a rifted continental margin setting.

Basites With 2.50-2.41 Ga Ages

Figure 14

Figure 15

Figure 16

The next stage of mafic-ultramafic magmatism belongs to the Early Proterozoic period and spans a 2.5-2.43 Ga time interval. Basites of this stage are widespread throughout the Fennoscandian shield and occur as both volcanic and intrusive varieties (Figure 14). These basites are represented by the layered intrusions of the Kola Peninsula, northern Karelia and Finland, Burakovskaya intrusion in southeastern Karelia, numerous drusite intrusions (coronitic gabbronorites) of the Belomorian region, and volcanics of the Vetreny belt and a number of smaller Sumian structures in Karelia and the Kola Peninsula. In terms of geochemical and isotope characteristics, both the volcanic and intrusive basites of this age are closely similar (Table 4, nos. 6-16). All the basites of this group have elevated SiO ₂ contents at increased MgO and high Ni abundances. Among all the rocks of this group, our most detailed studies were centered on the Belomorian drusites. Drusite composition plotted on variations diagrams show silica enrichment (SiO₂ 2-4% higher than in Archean komatiites and basalts or in their later, Proterozoic counterparts; Figure 15), owing to which their initial melts are not infrequently referred to as being "boninite-like'' [Sharkov et al., 1994]. On most variations diagrams, the drusites fall into two suites: magnesian (mg# = 0.81-0.49) and high-Fe (mg# = 0.53-0.29), featured by contrasting differentiation trends. According to their MgO/TiO ₂ ratios, the magnesian drusites plot in the komatiitic field, and the high-Fe ones, in the tholeiitic field (Figure 15). Drusites of the magnesian suite are dominant. They are typically high in Zr and have low Ti/Zr (50-80) and a high Zr/Y (3-6) ratios, as compared to the mantle. These rocks are high in Cr (130-3652 ppm) and Ni (up to 1300 ppm). In the high-Fe drusite suite, the Ti/Zr ratio ranges 60-140, and the Zr/Y ratio is similar to that in the magnesian drusites. Drusites of both suites are enriched in the light REE ((La/Yb)_N = 5-10, (La/Sm)_N = 2.5-3.5, and (Gd/Yb)_N =1.2-2.0) (Figure 16), the total REE content in the high-Fe drusites being higher than in the magnesian ones. The drusites of both suites have negative Nb (Nb/La < 1) and phosphorus anomalies and positive Pb anomaly, just like the crustal tonalites and gneisses. We interpret the contrasting chemistries of the drusites of the two suites within the same massif as evidence of cryptic layering, which is traceable even in the marginal portions of the massifs, where the drusites are turned to garnet amphibolite. All the basites of this group (drusites, mafic rocks from layered intrusions, and volcanics) have e_Nd values between 0 and - 2.5 (Table 1).

Melts initial to the high-Mg rocks with elevated silica contents and low Ti/Zr ratios may have been generated in three ways: (1) melting of water saturated mantle wedge in subduction zones (boninite model), (2) assimilation of felsic crustal material by mantle melt, and (3) mixing of high-temperature plume melts with partial melts of depleted harzburgitic lithospheric mantle. The boninite model for drusite melt generation is at odds with the high TiO₂ contents and the low Al₂O₃/TiO₂ ratio. Besides, oxygen isotope composition data from igneous minerals in the drusites ( d¹⁸ O ranging 8-4) testify to their crystallization from dry melts [Salye et al., 1983]. The fact that the drusites are light REE enriched and have high Ni (600 ppm or more) with at least 15% MgO is rather suggestive of a plume nature for their parental melt [Campbell and Griffiths, 1992].

Figure 17

We have presented quantitative model calculations that involve contamination of picritic or komatiitic melts by crustal material (Belomorian granite or biotite-garnetgneisses) (Figure 17) and mixing of an EM1 type enriched melt with a high-magnesian, high-silica melt derived from residual harzburgite mantle [Lobach-Zhuchenko et al., 1998]. Mass balance calculations using both major and the rare earth elements allow for both processes. The low Nb/La ratio, assimilation and mixing calculations in the La/Sm - e_Nd reference frame, and the e_Nd variations and values in comparatively small massifs, all suggest that the most likely model is the one that invokes contamination of an undepleted plume melt by Late Archean crustal material.

Conclusions

Analyzing the timing, spatial position, and distribution of Early Precambrian basites throughout the Fennoscandian shield point to a number of regularities. Although it is impossible to establish the duration of the earliest radiometrically dated stage of mafic magmatism, it can be ascertained that each successive stage spans a time interval of 70-80 m.y. The early stages of high-temperature mafic magmatism ( > 3.1 and 2.99-2.91 Ga) are confined to the most ancient core of continental crust on the Fennoscandian shield; namely, the Vodlozero domain with crustal age of 3.2-3.4 Ga. The first long-lasting stage of mafic magmatism (2.99-2.91 Ga) took place following a pattern that is classical to a number of modern plumes, and in which the first occurrences of high-temperature plume magmatism emerge in the central part of the continent, and subsequent ones, in its marginal parts. In the case in point, such first occurrence is the Lairuchei intrusion (2.99 Ga, in the central part of the Vodlozero domain) with the subsequent komatiite and high-temperature basalt volcanism along the western and eastern margins of the domain (2.94-2.96 Ga) and, possibly, at its northern margin (2.91 Ga). In all likelihood, beneath the Vodlozero domain there existed a deep seated plume (super-plume, or a first order plume, to use the terminology of Dobretsov and co-workers) that was rising from the interface between the core and the lower mantle. This particular inference is favored by the fact that the Nd isotope characteristics of a number of komatiites and mafic intrusions are explicable assuming mixing of melts derived from undepleted and depleted mantle sources or contamination of an undepleted mantle melt by crustal material. The rising mantle plume generated rift structures near the boundary and at the margins of the Vodlozero domain. Brittle deformations that caused rifting related to the rising mantle plume did not result in breakup of the newly formed continental crust. The dominant values of model Nd ages (T _DM ) in the 2.9-3.0 Ga time interval, obtained from rocks derivative from basalts that make up the younger domains [Lobach-Zhuchenko et al., 2000a, 2000b], suggest that basalts of this age were developed over a significant area outside the Vodlozero domain.

The next stage of high-temperature mafic magmatism (2.88-2.80 Ga) is expressed within the Kola and western Karelian domains with crustal ages of 3.0 and 3.1 Ga and on the north of the younger, central Karelian domain. Generation of komatiites and high-temperature tholeiite lavas in these domains provides evidence of a super-plume that ascended within these domains. The ascent of this plume initiated rifts beneath the continental crust of the Kola and western Karelian domains and beneath the oceanic plateau in the northern part of what is now the central Karelian domain. According to [Vrevsky, 2000], the higher liquidus temperatures for the komatiites from greenstone belts of the Kola domain and the greater depth of generation of their initial melts, as compared to the liquidus temperatures for the older komatiites from the Vodlozero domain, should imply a higher ascent for the early plume.

As the deep mantle plume ascended beneath the northern to northwestern part of the shield, its south-southeastern part (namely, the northwest margin of the Vodlozero domain) provided the stage for extensive development of mafic magmatism and granitoids with ages of 2.85-2.80 Ga, whose generation is attributed to underplating [Lobach-Zhuchenko et al., 1999]. These facts suggest the existence, in parallel to the superplume, of another plume that may have been less deep seated, and that was initiated at the interface between the lower mantle and the upper mantle, inasmuch as this magmatism immediately postdated the termination of subduction processes at the western margin of the Vodlozero domain. The ascent of this mantle plume may have been triggered by mantle slab sinking to the interface between the lower mantle and the upper mantle.

The last of the Archean stages of high-temperature mafic magmatism with ages of 2.72-2.66 Ga occurs in the north Karelian belts, in the Karelian part of the Belomorian area (the regions of Lake Notozero and the Tupaya Guba Bay of Lake Kovdozero) and, possibly, in the western Karelian domain. This magmatism took place also immediately after the subduction processes at the boundary of the Karelian and Belomorian domains. Accordingly, the mantle plume that ensured generation of high-temperature mafic melts, rose from the interface between the lower mantle and the upper mantle immediately following the end of the subduction processes. The majority of high-temperature melts, of both volcanic and plutonic provenance alike, suffered crustal contamination, which points to the existence of thick continental crust.

Early Proterozoic high-temperature mafic magmatism at 2.50-2.41 Ga was the most extensive areally and the longest lasting on the Fennoscandian shield. Nearly all the researchers of high-temperature basites of this stage attribute this magmatism to the ascent of an extensive deep super-plume [Amelin and Semenov, 1996; Arestova and Lobach-Zhuchenko, 1996; Hanski et al., 2001; Lobach-Zhuchenko et al., 1998; Puchtel et al., 1997; etc.]. Circumstantial evidence for the existence, in the 2.5-2.41 Ga time interval, of a long lived heat source that occupied virtually the entire area of what is now the Fennoscandian shield may be provided by paleomagnetic data. Nearly all the Archean basites measured yielded an additional magnetic component, whose age is estimated at 2.5-2.45 Ga [Arestova et al., 1999, 2000]. A hallmark of the mafic rocks of this stage is that all the volcanic and plutonic varieties without exception show varying degrees of crustal contamination, which is a further evidence that by the beginning of the Early Proterozoic there had been formed a thick continental crust, probably continuous beneath the entire eastern (Archean) part of the Fennoscandian shield.

Our analysis of the spatial position, timing, distribution, and geochemical characteristics of the high-temperature Early Precambrian basites of the Baltic shield suggests the following conclusions:

1. In the Early Precambrian of the Baltic shield (3.4-2.4 Ga), established are five stages of high-temperature mafic-ultramafic magmatism, most of which is attributable to the action of the plume-tectonic mechanism that ensured the inflow of lower mantle material and heat to cause melting in the upper mantle and crust.

2. The plume-derived high-temperature komatiitic melts, showing no crustal contamination and originating from depleted sources, are heterogeneous in terms of their Nd isotope compositions; accordingly, they either are derivatives from second-order plumes or result from mixing of plume material and plume-entrained portions of depleted upper mantle.

3. The plume-derived melts intruded both the newly formed continental crust and surviving oceanic crust, giving rise to deep-seated intrusions in the rather thick continental crust and to volcanics in continental and oceanic plateau settings. As a rule, intraplate rifting occurred in marginal parts of the sialic domains.

4. The process of interaction between initial mafic melts and crustal material (plume-crustal interaction) is established starting from the second recorded stage of mafic-ultramafic magmatism.

5. With increasing thickness of the continental crust of the Baltic shield, the degree of crustal contamination of mafic melts became progressively higher, to reach a maximum at the end of the Archean and beginning of the Proterozoic, during the fourth and, especially, the fifth stage of magmatism.

Acknowledgments

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