Petrology and geochemistry of rocks from the basement of the Pechenga paleorift

Geological Institute, Kola Science Centre, Russian Academy of Sciences, Apatity, Murmanskaya oblast, Russia

O. M. Turkina

United Institute of Geology, Geophysics, and Mineralogy, Siberian Branch of Russian Academy of Sciences, Novosibirsk, Russia

J. Ludden

Centre for Petrographic and Geochemical Researches, Nancy (CRPG-CNRS), France

Contents

Abstract

Introduction

The early history of the Earth is one of the most interesting problems of modern geology. The pivotal point of this problem is the nature of the primary crust in various areas, its composition, age, and genesis. An area convenient for studying these problems is the northeastern part of the Baltic Shield and, within this area, the Pechenga mining district, which is composed of Proterozoic sedimentary-volcanic rocks and Archean amphibolite-gneiss complexes. These were penetrated by the Kola Superdeep Borehole to a depth of 12,261 m. The geoblock hosts large Cu-Ni sulfide deposits, is exposed well enough at the surface, and was thoroughly examined by a variety of geological and geophysical techniques.

To a depth of 6842 m, the Kola Superdeep Borehole (KSDB) intersected the Early Proterozoic Pechenga sedimentary-volcanic complex and farther, up to the hole bottom, was drilled in the Archean metamorphic rocks of the basement of the Pechenga riftogenic structure. The geology, origin succession, age, and composition of Precambrian rocks from the KSDB section itself and from its closest surroundings were described in numerous papers and monographs [Archean Complex..., 1991; Kola Superdeep Borehole, 1984, 1998; Kremenetsky and Ovchinnikov, 1986; Magmatic..., 1986; and others]. The Pechenga Complex was studied particularly extensively, which made it possible to conduct a layer-by-layer correlation between rocks from the KSDB section and their surface analogues. The correlation and petrology of the Archean rocks are not as obvious because of their extensive folding, polycyclic metamorphism, and migmatization. There is still no consensus as to which rocks are analogues of the Archean complex of KSDB, whether these are the high-grade gneisses of the Kola Group exposed north and northeast of the Pechenga structure or the complexes of the Late Archean granite-greenstone terrane east and northwest of it. Only scanty information was obtained on the transformation processes of the Archean basement rocks in the course of the Early Proterozoic rifting, a process that was responsible for the intense reheating of the Archean rocks and their intrusion by vast masses of mantle material.

We carried out a geological and petrological-geochemical study of the principal types of Archean rocks from KSDB and from the framing of the Pechenga structure. The precise geochemical data obtained allowed us to reconstruct the composition of the protoliths. Much attention was paid thereby to the geochemistry of REE and some major, minor, and trace elements (Al, Mg, Ti, V, Th, Zr, Hf, Ta, Nb, Y, and others), which are the least mobile during regional metamorphism and metasomatism. We revealed the importance of the REE composition of the tonalite-trondhjemite gneisses for the correlation of Precambrian rock sequences. Rocks of the KSDB Archean complex were demonstrated to be similar to rocks of the Archean granite-greenstone terrane, and the Archean basement rocks were determined to have been significantly affected by processes related to the origin of the Early Proterozoic Pechenga paleorift.

Geological Overview

Figure 1

The Kola Superdeep Borehole, which reached a record depths of 12,261 m, was drilled in the northern portion of the Pechenga structure in the northwestern part of the Murmansk district area, which is situated in the Kola subprovince of the Baltic Shield and includes the Kola Peninsula, part of northern Karelia, Finnish Lapland, and the Finnmark area in northern Norway. The subprovince consists of large crustal blocks (Murmansk, Kola-Norwegian, Inari, Notozero, Keivy, Belomorian, and Tersk), separated by Archean and Early Proterozoic mobile belts (Northern Kola, [Smolkin et al., 2000]; Tersk-Allarechka, Pechenga-Imandra-Varzuga, and Lapland-Kolvitsa; Figure 1).

The 2.3- to 1.8-Ga Pechenga structure is part of an Early Proterozoic continental rift zone, which crosses the Kola Peninsula and consists of two major parts (Imandra-Varzuga and Pechenga) and a number of smaller local structures (Polmak, Pasvik, Keulik-Kenirim, and Ust' Ponoi). The origin and further evolution of the zone were controlled by the extension of the Archean basement and the development of its dome-shaped uplift, subsequent folding, subsidence, and a final folding episode during the orogenic stage [Kola Superdeep Borehole, 1998; Smolkin et al., 1995; Zagorodnyi et al., 1982]. It is believed that, during the development of the rift zone, its western and eastern flanks were partly juxtaposed with the Late Archean Tersk-Allarechka greenstone belt (Olenegorsk belt; [Dobrzhinetskaya et al., 1995]), and, thus, this structure is "embedded".

The Poritash fault divides the Pechenga rift structure into the Northern and the Southern tectono-stratigraphic subzones, which differ in the compositions of their rocks and the styles of tectonic deformations. The southern margin of the Southern Pechenga subzone nearly everywhere occurs in contact with the Tersk-Allarechka greenstone belt, and only its western part is bounded by the granite-greenstone rock assemblages of the Inari block. The rocks of the Northern Pechenga subzone, where the Kola Superdeep Borehole was drilled, were combined into two series: Nikel and underlying Luostari, each of which is further subdivided into two volcanogenic suites, alternating with initially sedimentary lithologies. The volcanic evolution is reflected in the systematic compositional variations of the rocks from basaltic andesites and trachyandesitic basalts (Luostari suite) to tholeiitic basalts and ferropicrite-basalts (Nikel suite). The intrusions are sheet-shaped gabbro-diabase bodies, massifs of mafic and ultramafic rocks, and hypabyssal intrusions of andesite porphyry. The metamorphic grades of the rocks vary from the prehnite-pumpellyite facies to the lower and middle grades of the amphibolite facies, with the metamorphic grade increasing from upper to lower stratigraphic units [Kola Superdeep Borehole, 1984]. The Northern Pechenga subzone is hosted by gneiss-amphibolitic complexes of the Kola-Norwegian block (an Early Proterozoic composite terrane [Balaganskii et al., 1998]), whose northwestern termination is made up of the Svanvik-Neiden and Titovsky Archean segments.

The southwestern boundary between the Svanvik-Neiden terrane and the Inari block is drawn along a system of faults, which controlled the position of the Early Proterozoic Polmak-Pasvik-Pechenga structures. In the east, it is bounded by a northeast-trending deep fault, which is marked by granite porphyry intrusions of the Litza-Araguba Complex [Vetrin et al., 1975]. The northern boundary with the Titovsky segment is also tectonic; it served as the conduit for the Kirkenes tonalite-trondhjemite pluton (dated at 2.8 Ga; [Levchenkov et al., 1995]) and is marked by the development of a diversity of anatectic granites and migmatized rocks, dated at 2.8-2.15 Ga [Ts'on' et al., 1988; Vinogradov, 1978]. The predominant supracrustal associations of this segment are Archean tonalite-trondhjemite complexes (~80-90%) with scattered relics of greenstone structures, the largest of which is the Bjrnevatn greenstone structure, consisting of micaceous gneisses, schists, amphibolites, and BIF [Siedleska et al., 1985]. The U-Pb zircon age of the tonalite-trondhjemite gneisses was estimated at 2.8-2.84 Ga, with the metamorphic age of 2.7 Ga [Levchenkov et al., 1995]. The gneisses are intruded by granite porphyry and granodiorite plutons (dated at ~2.5 Ga) and granite and pegmatite dikes (2.7-2.5 Ga). The metamorphic grades of the rocks do not exceed the middle part of the amphibolite facies. According to deep seismic sounding results, rocks in the northwestern surroundings of the Pechenga structure can be traced beneath this structure and account for a significant part in its basement [A seismological..., 1997]. Analogous rock associations make up the eastern part of the Inari block [Kesola, 1991]. The Inari block and the Svanvik-Neiden segment of the Kola-Norwegian block seem to have composed a single granite-greenstone terrane in Late Archean time and were separated in the Early Proterozoic during the development of the Palmak-Pasvik-Pechenga-Imandra-Varzuga structure.

The Titovsky segment of the Central Kola block near Pechenga Bay of the Barents Sea is dominated by aluminous gneisses of the Kola Group, which trend to the northwest and extend to Norway, where they are referred to as the Jarfiord Gneiss. It is believed that the aluminous gneisses rested on a basement of banded enderbites and two-pyroxene crystalline schists (Hompen Gneiss), dated at 2902 9 Ma [Levchenkov et al., 1995]. The aluminous gneisses, whose protoliths consisted of hydromica clays, sands, and clayey graywackes, were determined to have lateral metamorphic zoning with the metamorphic grade decreasing from northwest to southeast from the granulite to amphibolite facies [Facies..., 1977; Vinogradov and Vinogradova, 1993]. The age of the early metamorphism of the aluminous gneisses was constrained by a variety of techniques to the interval of 2.88-2.83 Ga [Avakyan, 1992; Balashov et al., 1992], with the latest metamorphic episode dated at 2.7 Ga. The gneisses were determined to bear evidence of seven to eight folding events in the regime of subhorizontal or tangential motions [Dobrzhinetskaya, 1978]. The aluminous gneisses are intruded by rocks of the gabbro-tonalite (2.83 Ga), granodiorite (2.76-2.73 Ga), and monzonite-syenite (2.73 Ga) composition, which compose, respectively, pre-, syn-, and postmetamorphic intrusions [Nordgulen et al., 1995; Vetrin et al., 1995].

It follows that the composition and age of supracrustal rocks, their metamorphic grades, and the ages of intrusions are remarkably different in the Titovsky and Svanvik-Neiden segments. According to Condie [1981], they can be attributed to, respectively, the terranes of high-grade metamorphic (granulite gneissic) and granite-greenstone rocks. The former were produced at a contraction coupled with granulite metamorphism and emplacement of charnockite-enderbite intrusions, and the latter developed under crustal extension, in relation to the ascent of mantle material and reomorphism of the host rocks during the development of granite-greenstone terranes.

Geologic Criteria for Correlations Between Rocks of the KSDB Archean Complex and Rocks in the Framing of the Pechenga Structure

Rock Complexes

Figure 2

In the sequence penetrated by the Kola Superdeep Borehole, the Paleoproterozoic sedimentary-volcanic rocks of the Pechenga structure are underlain (at depths of 6842-12,261 m) by Archean rocks, which can be subdivided into five cyclic units. Each of them is, in turn, composed of two members, which are dominated by migmatized biotite-plagioclase gneisses of tonalite-trondhjemite composition (~45%) and gneisses with high-Al minerals (gneisses with HAM, ~20%), which originally correspond to volcanic and sedimentary rocks, respectively [Archean..., 1991; Kola Superdeep Borehole, 1984; Kremenetsky and Ovchinnikov, 1986]. About 30% of the sequence falls to amphibolites and iron quartzites (BIF), and ~5% is accounted for by granitoid veins. In the lower part of the KSDB section, below a depth of 11,708 m, an amphibolite-tonalite-trondhjemite complex (ATTC, Figure 2) was recognized. The Archean complex of KSDB was metamorphosed under amphibolite-facies (pre-Pechenga) conditions and, later, under low-temperature amphibolite, epidote amphibolite, and, more rarely, greenschist facies (Pechenga) conditions [Duk et al., 1989; Kola Superdeep Borehole, 1984]. Some researchers believe that the Archean rocks of KSDB contain the most ancient granulite-facies relics [Magmatic..., 1986], but this hypothesis was not confirmed by later mineralogical [Yakovleva, 1991] and isotopic geochemical [Chen et al., 1998] studies.

Metamorphism to the amphibolite- and epidote amphibolite facies of the late (Pechenga) episode was associated with migmatization of rocks of the Archean complex expressed in development of patchy, conformable, and, mostly, nebular migmatites. Their amounts are as high as 15-20% in the gneisses with aluminous minerals and increase upsection from 20-30% in the biotite-plagioclase gneisses of unit 10 to 50-60% in the gneisses of unit 2.

The age of the protoliths of the gneisses was constrained by isotopic geochemical techniques to 2950-2850 Ma, and the ATTC granitoids were dated at 2835-2832 Ma. Equal dates were determined for the tonalitic gneisses of units 1, 2, and 8. The gneisses with HAM are some 30-150 m.y. younger than the underlying tonalites-trondhjemites. The migmatites developed at approximately 2740 Ma, during the closing stages of the Late Archean metamorphic event. The Proterozoic migmatites were dated at 2225-2150 Ma, and the postkinematic veins of porphyritic and equally grained granites were intruded at 1766 Ma [Bibikova et al., 1993; Chen et al., 1998; Timmerman and Daly, 1995; Yakovlev et al., 2000].

The data cited above come in conflict with the recognition of Early Archean rocks in the lower part of the succession penetrated by KSDB [Vinogradova et al., 2000]; neither was confirmed the correlation between the upper part of the succession and the rocks of the greenstone tectono-stratigraphic complex of the Vyrnimskaya and Tal'inskaya formations in the southern framing of the Pechenga structure [Vinogradova et al., 2000], because the T¹⁴³Nd/¹⁴⁴Nd(DM) model ages of the latter formations are no older than 2.3-2.4 Ga [Pozhilenko et al., 2000].

It is expedient to search for associations analogous to the rocks of the KSDB Archean complex bearing in mind that the probable analogues should be coeval, have similar mineralogical and chemical compositions, be characterized by quantitatively similar rock proportions in the rock complexes, and have protoliths of similar composition. Obviously, none of the aforementioned features taken alone (for example, similarities in chemistry) cannot be put forth as a proof of an analogy between the rocks compared. It is equally inaccurate to compare rock complexes metamorphosed to distinct metamorphic facies, because a comparison of such complexes, whose rocks differ mineralogically, would require evidence of the preservation of their original chemistry under different P-T conditions. In particular, this pertains to migmatized and granitized rock complexes metamorphosed to the amphibolite and granulite facies.

Another aspect that makes it difficult to compare the rocks exposed at the surface and those in the KSDB Archean complex is the transformation of the latter under the effect of the Early Proterozoic mantle-crust interaction, related to the development of the Pechenga riftogenic structure. Because of this, correlation between the KSDB Archean complex and complexes exposed at the surface remain ambiguous. The KSDB Archean complex is commonly paralleled with the gneisses of the Kola Group, which consists of two units: lower, composed of biotite-plagioclase gneisses with subordinate amounts of aluminous gneisses, amphibolites, magnetite-garnet-pyroxene schists, and iron quartzites; and upper, dominated by garnet-biotite-plagioclase gneisses with sillimanite, staurolite, and andalusite [Kola Superdeep Borehole, 1984; Magmatic..., 1986]. The aluminous gneisses of the upper unit are restricted mostly to the Titovsky segment of the Central Kola block, which consists of magmatic and metamorphic rocks of specific textures and compositions that have never been reliably identified in the KSDB rocks. Because of this, it seems to be useless to search for analogues of the KSDB Archean complex among the high-grade gneisses on the Barents seashore [Guberman et al., 1994; Kozlov et al., 2001], where there are virtually no tonalite-trondhjemite gneisses analogous to those dominating in the Archean complex of the borehole.

According to geological-geophysical data, certain similarities with the KSDB Archean complex are displayed by tonalite-trondhjemite gneisses with layers of aluminous gneisses and amphibolites that compose the Svanvik-Neiden segment of the Central Kola block. These rocks occur mostly in the northwestern, northeastern, and eastern framing of the Pechenga structure. In earlier schemes, the tonalite-trondhjemite gneisses and related amphibolites were considered to be the lower gneiss unit of the Kola Group [Kola Superdeep Borehole, 1984; Magmatic..., 1986] or the basement complex [Radchenko et al., 1994]. We examined these rocks in much detail in the northwestern surroundings of the Pechenga structure, mainly in northern Norway.

In accordance with their geologic setting, textural and structural features, quantitative rock proportions, and deformation styles, the gneisses of the Svanvik-Neiden segment are subdivided into a series of complexes (Varanger, Svanvik, Garsjo , and others; [Dobrzhinetskaya et al., 1995; Siedleska et al., 1985]). The most diverse rock associations are typical of the Garsjo gneisses in the northwestern part of the area (Figure 1). This complex is dominated by biotite and more rare amphibole-biotite gneisses of tonalite-trondhjemitic composition (50-60%) with strata of garnet-biotite and sillimanite-garnet-biotite gneisses (10-20%) and amphibolites (10-20%) and iron quartzites layers. The amounts of migmatites and granite or pegmatite veins are, respectively, 10-20 and 5-10%. The U-Pb metamorphic age of the Garsjo gneisses was estimated at ~2.7 Ga, and two fractions of magmatic zircon were dated at 2840 Ma [Levchenkov et al., 1995]. The age and the rock assemblage of the Garsjo gneisses are most close to those of rocks in the KSDB Archean complex, differing from the latter by less intense migmatization and smaller volumes of mafic rocks [Vetrin et al., 1999].

The Svanvik gneisses are exposed at the western contact with the Pechenga structure and compose an intrusive complex consisting of at least two phases. The predominant rock types are banded tonalitic gneisses with layers and lens-shaped bodies of feldspar amphibolites. The rocks of the second phase are equally grained massive or unclearly gneissose tonalites. The U-Pb age of their magmatic crystallization is 2825 34 Ma [Levchenkov et al., 1995].

The Varanger gneisses are separated from the Garsjo gneisses by a tectonized zone of northeastern trend. The dominant rocks of this complex are tonalite-trondhjemite gneisses with massive or fine- to coarse-banded structures. Their U-Pb isochron age was determined to be 2803-2813 Ma [Levchenkov et al., 1995]. The tonalites contain enclaves of feldspathic and garnet-bearing amphibolites with iron quartzites (jaspilite) lenses, bodies of garnet-biotite, two-mica, and garnet-biotite-sillimanite gneisses. In places, the aluminous gneisses and associated amphibolite and iron quartzites (jaspilite) bodies compose greenstone structures (Valen, Bjornvatn, and others) with conformable tectonized contacts with respect to the host tonalite-trondhjemite gneisses.

Correlations

Balashov and Vetrin [1991] suggested that the KSDB Archean complex is an intensely migmatized and granitized fragment of an Archean granite-greenstone terrane. Its principal rock types are gneisses of tonalite-trondhjemitic composition in association with amphibolites, gneisses with HAM, and iron quartzites (jaspilites). The latter three rock types are, along with tonalite-trondhjemite, the predominant rock associations of Precambrian greenstone belts [Condie, 1981]. In the lower part of the sequence, the rocks recognized as the amphibolite-tonalite-trondhjemite complex (ATTC) have blastogranitic textures, are devoid of layering, and bear no layers of metasediments, that indicate plutonic precursors of tonalite-trondhjemite gneisses.

Analogous rock assemblages compose the Svanvik-Neiden segment of the Kola-Norwegian block. The tonalite-trondhjemite gneisses that are dominant in the Svanvik complex comprise a number of phases and in places retain relict porphyroblastic textures. The Garsjo and Varanger complexes are characterized by a clearly pronounced rock banding, defined by intercalating of tonalite-trondhjemitic gneisses with various proportions of mafic minerals, amphibolites, gneisses with HAM, and iron quartzites (jaspilites). The aforementioned structural and textural features of these rocks and their composition testify to the predominantly sedimentary-volcanogenic genesis of the main rock associations of the complexes and their crystallization under disequilibrium near surface conditions. The Garsjo and Varanger tonalite-trondhjemite gneisses include numerous relict bodies of amphibolites, gneisses with HAM, and iron quartzites (jaspilites). Their rock associations including BIF and composition are close to those of rocks in the Bjornevatn greenstone structure, and they seem to be relics of greenstone structures (possibly, of different ages), which were intruded by tonalite-trondhjemites.

Petrography and Geochemistry

Presented below are the results of petrographic and geochemical examinations of the tonalite-trondhjemite gneisses, gneisses with HAM, and amphibolites, which are the principal rock types from the KSDB Archean complex and the framing of the Pechenga structure.

Samples and Analytical Techniques

Samples of the KSDB core and rocks from the northwestern framing of the Pechenga structure were analyzed for major and trace elements by ICP-MS, ICP, INAA, XRF, and mass spectrometry at the Centre for Petrographic and Geochemical Researches (CRPG-CNRS) in Nancy, France (32 samples); United Institute of Geology, Geophysics, and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia (19 samples); Geological Institute, Kola Science Center, Russian Academy of Sciences, Apatity, Russia, and the Institute of the Mineralogy, Geochemistry, and Crystal Chemistry of Rare Elements, Moscow (accordingly, 4 and 7 samples). The Sm-Nd isotopic analyses of the rocks were conducted at the Laboratory of Geochronology at the Geological Institute, Kola Science Center. The results are summarized in Tables 1, 2, 3, 4, 5, 6, 7, and 8.

Petrography

Kola Superdeep Borehole.

The KSDB Archean complex is dominated by tonalite-trondhjemite gneisses, which are usually variably migmatized. To reproduce the primary characteristics of the rocks, we examined the least altered of their samples, mainly from the lower part of the sequence penetrated by the borehole (units 8 and 10). The plagiogneisses in question were subdivided into two types. One of them comprises mesocratic and more rare leucocratic varieties with granoblastic and blastogranitic textures. The mesocratic gneisses (Table 1, Samples 46a, 44895, 45001-3, 36305, 40302-2) consist of plagioclase (50-55%, 31-40% An), quartz (25-30%), and biotite (10-15%); Sample 45001-3 contains both biotite and hornblende. The leucocratic gneisses (Samples 39866-6, 39288-6 and Sample 36009 of microclinized gneiss) contain 5-10% biotite and 60-65% plagioclase (24-27% An). The accessory minerals of all rocks are magnetite, ilmenite, apatite, zircon, sphene, and allanite. The gneisses of the second type (Samples 30085/1a and 34944) are higher in biotite (up to 20%) or muscovite and epidote, which replace primary mineral assemblages.

The gneisses with HAM are characterized by the absence of potassic feldspar and the strong dominance of micas over aluminous minerals. Our samples represented two-mica gneisses with sillimanite (Table 2, Samples 22627 and 37926) and garnet-biotite gneisses (Sample 41426-3).

In accordance with their geologic setting and composition, the orthoamphibolites were subdivided into rocks of the dike facies (predominant type; Table 3), metapyroxenite (Sample 9608), garnet-clinopyroxene metagabbro (Sample 43745), and cummingtonite-hornblende metagabbro (Table 5, Sample 27023). The dike rocks are feldspathic amphibolites and, more rarely, garnet-feldspar amphibolites (Sample 42167). The most common textures of the rocks are nematoblastic and granonematoblastic; samples 42167 and 44369-2 show relict ophitic, poikilitic, and allotriomorphic-granular textures.

Complexes Exposed at the Surface.

The plagiogneisses generally have granoblastic or, more rarely, porphyroblastic and blastogranitic textures. In contrast to the analogous rocks from KSDB, these plagiogneisses are characterized by relict porphyritic textures with plagioclase and quartz phenocrysts. The Svanvik gneisses, which occur in contact with the Pechenga structure, are locally microclinized, muscovitized, and chloritized. The main minerals of the plagiogneisses are plagioclase (23-40% An), quartz, and biotite, with hornblende encountered only in the most melanocratic varieties (Table 6, Samples 56-1, 65-15, 91-170, and 90-30). The accessories are epidote, allanite, apatite, zircon, magnetite, ilmenite, and, occasionally, sphene.

The gneisses with HAM have granoblastic, porphyrogranoblastic, and nematoblastic textures and typically exhibit broad variations in the contents of rock-forming minerals, which determines transitions from melanocratic amphibole-biotite gneisses with muscovite and sillimanite (Table 2, Samples 70-3, 68-5) to biotite and two-mica gneisses with sillimanite (Samples 46/99 and 68/1), garnet (Sample 16/99), and sillimanite plus garnet plus biotite (Sample 80-1).

The feldspathic amphibolites generally have a less variable composition than that of rocks from the KSDB Archean complex and consist of plagioclase (40-50%), amphibole, and, locally, biotite (  10%). The accessory minerals are sphene, magnetite, apatite, and occasional allanite. The textures of the rocks are nematoblastic and granonematiblastic. The amphibolites hosted by the Svanvik gneisses (Samples 109-2 and 107/99) are biotitized, epidotized, and carbonatized.

Geochemistry

Kola Superdeep Borehole

Plagiogneisses.

Figure 3

The KSDB biotite plagiogneisses correspond to tonalite and, more rarely, trondhjemite in major element and normative composition (Table 1). Most of the rocks contain > 15% Al₂O₃ at 70% SiO₂, have low K₂O/Na₂O ratios, and high CaO concentrations, which are typical of Archean tonalite-trondhjemite-granodiorite series (TTG) [Barker, 1979]. According to their mineralogical, major-, and trace-element composition, the rocks can be classed into two types. The predominant gneisses of type A correspond to high-Al TTG. They are characterized by a strongly fractionated REE patterns, (La/Yb)_n = 18-83, with a moderate enrichment in LREE, La_n = 33-116, and a depletion in HREE, Yb_n = 1.4-2.3, with unclear Eu anomalies (or without them, Figure 3a), which is typical of Archean TTG [Martin, 1994]. A microlcinized gneiss (Sample 36009) has REE and Zr, Hf, Ta, Nb, Y (i.e., HFSE - high field strength element) concentrations similar to those in the gneisses but differs from them by elevated contents of Ba. There are two noteworthy exceptions. Sample 39288-6 is low in LREE and shows a weakly fractionated REE pattern with a pronounced Eu maximum (Figure 3b). Its high SiO₂ concentration is associated with depletion in mafic components and HFSE, which was due to the low concentrations of mafic and accessory minerals. These compositional features seem to have been resulted from the fractional crystallization of the parental melt. The other exception, Sample 40302-2, represents more melanocratic rocks with a higher plagioclase content and low concentrations of accessory minerals. These compositional features can, perhaps, be interpreted as reflecting of the origin of the protolith as a plagioclase-enriched migmatite leucosome.

Figure 4

Gneisses of the other type (type B) were encountered in unit 2, which was thoroughly sampled by A. A. Kremenetsky, who provided us with the analyses of these gneisses cited in this paper. Gneisses of type A dominate in this unit [Kremenetsky and Ovchinnikov, 1986]. Compared with the gneisses of type A, which have the same SiO ₂ concentrations, these rocks are enriched in Ti, Fe, Mg, REE (La _n = 142-210, Yb _n = 4.1-5.7), and HFSE (Figures 3c, 4c), which are concentrated in mafic and accessory minerals. The rocks are characterized by a weakly fractionated REE patterns, a feature generally uncommon for the gneisses of type A and typical Archean high-Al TTG. The multi-element patterns of these gneisses normalized to the primitive mantle have gentler slopes for the gneisses of type B. The rocks have pronounced Sr minima, which are also atypical of Archean grey gneisses (Figure 4c).

Gneisses with High-Al Minerals.

Figure 5

The gneisses with HAM are characterized by broad variations in silica contents (Table 2, 51-75%), a component negatively correlated with most other elements. The rocks show fractionated REE patterns and a decrease in their concentrations with increasing SiO₂ contents. Another distinctive feature of the gneisses is their enrichment in LREE and HFSE, as well as relatively high concentrations of Cr, Ni, Co, and V. These data are generally consistent with the sedimentary nature of the protoliths of these rocks, which could consist of rocks of argillite-aleurolite composition. Compared to typical Archean shales [Taylor and McLennan, 1985], the gneisses with HAM are depleted in HREE and are characterized by higher (La/Yb) _n = 19-33 ratios (Figure 5), which is most probably explained by the significant contribution of grey gneiss source to the origin of these sedimentary rocks.

Amphibolites

In terms of normative composition, the amphibolites of the dike facies correspond to olivine tholeiite (four samples) or quartz tholeiite (two samples; Table 3). According to A. A. Kremenetsky, rocks of this type account for 70-80% of amphibolites in the KSDB Archean complex, belong to Fe-amphibolites, and form a part of the Late Archean volcano-plutonic tholeiite association. Some dike rocks of analogous composition were suggested to generated synchronously with the Pechenga volcanism [Kremenetsky and Ovchinnikov, 1986]. The occurrence of Proterozoic amphibolites among gneisses of the KSDB Archean complex is confirmed by zircon age (1836 161 Ma) of an amphibolite from a depth of 11,795 m [Bibikova et al., 1993].

Figure 6

Figure 7

Figure 8

As follows from Table 3, the dike amphibolites are classed into two distinct types based on their compositional features. The amphibolites of the first type are characterized by a moderately fractionated REE patterns at elevated concentrations of LREE ((La/Yb)_n = 4.6-6.9, La_n = 70-90; Figure 6). The amphibolites of the second type are depleted in LREE (La_n = 12-29), have flat REE patterns ((La/Yb)_n = 1.0-1.1; Figure 7a), and are poorer in Fe, Ti, Th, Nb, and Zr and richer in Mg, Ni, and Pb than the first-type amphibolites.

Compared to Archean amphibolites from the surroundings of the borehole, the first-type amphibolites are enriched in REE and HFSE, exhibit more fractionated REE patterns (Figures 6, 8). They differ from REE-enriched TH2 basalts of Archean greenstone belts [Condie, 1981] by even higher concentrations of REE, Fe₂O₃, TiO₂, and V but lower concentrations of MgO and Cr, which could be explained by the derivation of basalt protoliths by the fractionation of a more primitive, Mg-rich melt. Among the Pechenga metavolcanics, rocks most closely approaching the amphibolites in terms of concentrations of REE and other trace elements, particularly HREE and HFSE, are the melanocratic basalts of the Zapolyarnyi Formation and the trachybasalts of the Pirttijarvi Formation (Figure 6, Table 3). At the same time, the latter have elevated concentrations of Al₂O₃ and Na₂O, relatively low contents of CaO and MgO at SiO₂ similar to those of the amphibolites, a feature incompatible with the concept that the amphibolites and Pirttijarvi basalts were produced by the differentiation of the same melt. A composition most similar to that of the KSDB amphibolites was identified in the basalts of the Zapolyarny Formation, which were derived from a relatively shallow source with an age of approximately 2.1 Ga [Kola Superdeep Borehole, 1998].

Figure 9

The revealed compositional similarities between the metavolcanic rocks of the Early Proterozoic Pechenga structure and amphibolites from the KSDB Archean complex are confirmed by the results obtained on the Sm-Nd systematics. The maximum model age of the amphibolites calculated under the assumption of the depleted composition of the regional mantle, T_Nd(DM), is 2.16-2.33 Ga (Table 4), that determines the lower age limit for the origin of the protoliths. Given the compositional similarities between the amphibolites and the Zapolyarnyi metavolcanics in the Pechenga structure, which were dated at 2114 52 Ma [Smolkin et al., 1995], we assumed the same age for the amphibolites analyzed. For this age, the e _Nd value of the amphibolites is 0.77-2.69, i.e., close or lower than the e _Nd3.5 for the depleted mantle with an age of 2.1 Ga [DePaolo, 1981] (Figure 9). Lower e _Nd values are usually interpreted based on the assumption of either the enriched nature of the source or the contamination of the melts with upper crustal material [Faure, 1986]. The latter interpretation is in conflict with relatively low concentrations of Rb, Ba, and Pb in the Zapolyarnyi basalts. Hence, the e _Nd values calculated for the first-group amphibolites most probably reflect the derivation of the parental melts from a moderately depleted mantle source, insignificantly enriched in incompatible elements.

For the Pirttijarvi trachybasalts, which were dated at 2214 54 Ma [Smolkin et al., 1995], the model age T _Nd (DM) = 2460 Ma and e _Nd = 0.93, which lies within the range of e _Nd values determined for the amphibolites from the KSDB Archean complex.

In contrast to the first-type amphibolites, the second-type amphibolites have major- and trace-element compositions obviously similar to those of amphibolites from the surroundings of the borehole and the TH1 primitive tholeiites of Archean greenstone belts [Condie, 1981]. These similarities are further supported by the near flat REE patterns, relatively low concentrations of Ti, P, LREE (La _n = 11.6-11.9), Zr, and Nb at elevated concentrations of Mg, Cr, and Ni. The amphibolites differ from the basalts of the Majarvi Formation of the Pechenga structure (which have a similar major-element composition) by lower concentrations of LREE, Ti, Ba, Sr, Zr, Cr, and Ni (Table 3, Figure 7a).

All amphibolites representing intrusive rocks in the KSDB Archean complex have REE concentrations and patterns notably different from those of the Archean amphibolites in the surroundings of KSDB (Tables 5, 8). Among these rocks, the highest LREE concentrations and the most fractionated REE patterns, with (La/Yb) _n = 12.2, are typical of metapyroxenites. As follows from Figure 7b, there is practically full coincidence between the REE patterns of these rocks and the pyroxenites of the Nyasyukka Complex [Smolkin et al., 1995] in the northern framing of the Pechenga structure. The complex was dated at 1956 20 Ma by the Sm-Nd isochron technique [Huhma et al., 1996]. The maximum estimates of the model age, T_Nd(DM), of the studied metapyroxenite sample is 2200 Ma, which is consistent with its assignment to the Early Proterozoic.

The metagabbro (Samples 43745 and 27023) have major- and most trace-element concentrations analogous to those of the metadolerites and gabbronorites from dikes in the northern framing of the Pechenga structure (Table 5), which belong, according to Zh. A. Fedotov [Smolkin et al., 1995], to picritodolerite and gabbronorite complexes dated at, respectively, 2200 and 2555 Ma. This correlation is confirmed by the Proterozoic model age of Sample 43745 (2222 Ma). It is also worth noting the elevated MgO contents of rocks in the samples examined (6.4-8.0% MgO) at SiO₂ contents of 53-51% and U-shaped REE patterns with a minimum at MREE and maxima over both LREE and HREE. These compositional features are inherent to the rocks of the marianite-boninite association typical of young island arcs [Ewart et al., 1977]. In the Baltic Shield, ancient boninites are restricted mostly to the Sumian and Sariolian age levels [Sharkov et al., 1997] but were also encountered among younger rocks of the Pechenga-Varzuga belt and its surroundings. Figure 7b presents the REE patterns of the studied metagabbro and paleoboninite samples from the Baltic Shield [Sharkov et al., 1997], which show mostly similar configurations.

Complexes Exposed at the Surface

Biotite Plagiogneisses.

The chemistry of the Garsjo gneisses corresponds to that of tonalite and trondhjemite, and they range from quantitatively dominant aluminous ( < 15% Al₂O₃ at SiO₂ = 70%) to less abundant low-Al ( < 15% Al₂O₃ ) varieties with variable concentrations of REE. Based on REE pattrens, at least three types of these rocks can be recognized (Figure 10):

type A comprises rocks with strongly fractionated REE patterns, (La/Yb)_n = 12-88, extremely depleted in HREE (Yb_n = 1-3) and less strongly depleted in LREE (La_n = 28-76);

type B is characterized by a moderate REE fractionation, (La/Yb)_n = 20-32, with a deletion in HREE (Yb_n = 3-5) and elevated concentrations of LREE (La_n = 90-126);

type C has weakly fractionated REE patterns, (La/Yb)_n = 7-28, is enriched in LREE (La_n = 98-203) and HREE (Yb_n = 7-13).

Figure 10

The first type includes meso- and leucocratic rock varieties with Al₂O₃ and CaO concentrations decreasing with increasing SiO₂ and without correlations of the Fe, Mg, and Ti with SiO₂. The irregular variations in the contents of Fe, Mg, Ti, HFSE, and HREE are controlled by variations in the amounts of mafic and accessory minerals, most probably, because of the primary heterogeneity in the textures and structures of the protolith. The rocks are characterized by varying concentrations of both LREE and HREE at clearly pronounced Eu maxima. Their distinctive features are high Sr concentrations and very low contents of HREE and Y, which are only comparable with or lower than those of typical Archean TTG. The elementary patterns of these rocks have steep slopes with an enrichment in LILE and LREE relative to HFSE at well pronounced negative anomalies of

Figure 11

Nb, Ti, and P (Figures 10a, 11a), i.e., features typical of Archean grey gneisses [Martin, 1994].

Having similar SiO ₂ contents, the plagiogneisses of type B are higher in Ti, Fe, HFSE, and HREE than the type-A gneisses (Figures 10b, 11b), that were resulted from higher concentrations of mafic and accessory minerals. The compositional differences between the gneiss types could not be due to the fractional crystallization of a single parental magma or distinct partial melting degrees of the source, because, at similar SiO₂, Al₂O₃, and CaO concentrations, the gneisses differ in the concentrations of REE and HFSE. This led us to suggest that the rocks were generated under different P-T conditions and/or were derived from distinct sources.

The low-Al gneisses of type C contain 66-70% SiO₂ and elevated concentrations of TiO₂, Fe₂O₃, REE, and HFSE but relatively low concentrations of Sr. The multi-element patterns of the type-C gneisses are characterized by gentler slopes than those of the gneisses of types A and B (Figures 10c, 11c). These compositional features of the rocks are generally atypical of Archean grey gneisses, which usually correspond to types A and B. At the same time, rocks of tonalite-trondhjemitic composition with weakly fractionated REE were identified among the TTG of the Superior Province in Canada [Feng and Kerrich, 1992], Amitsoq gneisses in Greenland [Shimizu et al., 1998], in the Ancient Gneissic Complex of Swaziland [Hunter et al., 1984], and in other shields.

Figure 12

Figure 13

The plagiogneisses of the Varanger Complex exhibit no clearly pronounced correlations between SiO₂ (68.1-71.4%) and other major components but have a clear positive correlation between the LREE and Th concentrations (because of the simultaneous occurrence of these elements in accessory minerals). The gneisses are characterized by a fractionated REE patterns with (La/Yb)_n = 11.6-40.1 at low concentrations of HREE (Yb _n = 1.5-1.9, Figure 12a). The progressive depletion of the gneisses in LREE and MREE is coupled with an increase in the Eu maximum that relates with decrease in REE-bearing mineral contents, such as allanite, epidote, and apatite. It could reflect the fractionation of these minerals during the parental melt evolution. The most conspicuous features of these rocks are their high LREE, Rb, and Ba concentrations at low concentrations of HREE and pronounced Ta, P, and Ti minima (Figure 13), i.e., typical features of Archean grey gneisses.

The biotite gneisses of the main phase of the Svanvik Complex are characterized by all distinctive features of the high-Al trondhjemites: a strongly fractionated REE patterns and high Sr contents up to 800 ppm (Table 6, Figure 12b). The second-phase trondhjemites are more leucocratic (70.4-73.2% SiO₂ ) and are depleted in Al, Ca, Fe, Ti, Mg, LREE, and Sr compared with the first-phase gneisses (Figure 12c). These features confirm genetic relations between the rocks of the two phases of this complex and can be accounted for by the fractionation of amphibole, plagioclase, and allanite during crystallization of the parental melt.

Gneisses with High-Al Minerals.

Figure 14

The SiO₂ concentrations of biotite and amphibole-biotite gneisses with muscovite, sillimanite, and garnet range from 52.3 to 70.5% (Table 2) and are negatively correlated with CaO, Fe₂O₃, TiO₂, and MgO at the absence of pronounced correlation with alkalis, Rb, Ba, Sr, and REE. The gneisses have moderately fractionated REE patterns (La/Yb)_n = 4.7-16.2, Figure 14, which is similar to this parameter of Archean shales; [Taylor and McLennan, 1985]. The distinctive compositional features of the gneisses with HAM, such as regular variations in the least mobile major elements coupled with an uneven distribution of trace elements in rocks of variable silicity, suggest that the protoliths of the rocks could consist of weakly differentiated sediments ranged from argillite to sandstone. The various degrees of HREE depletion may be indicative of variable contributions of mafic rocks and TTG to these sediments.

Amphibolites.

In terms of normative composition, the amphibolites correspond mostly to olivine tholeiite and, less often, to alkaline basalt (Samples 65-16 and 32/99) or quartz tholeiite (Table 8, Sample 107/99). The amphibolites are characterized by relatively low concentrations of LREE and gently sloping REE patterns (La/Yb)_n = 0.7-2.0. Except for most mobile elements (such as Rb, Ba, and Pb), the multi-element patterns of the rocks are quite flat, with small negative Ti anomalies (Figure 8). These features make the rocks sharply different from the Proterozoic amphibolites in the KSDB Archean complex but similar to TH1, the most widespread basalts of Archean greenstone belts [Condie, 1981], and to the modern subduction-related tholeiites [Pearce et al., 1995]. Exceptions are amphibolites of Samples 107/99 and 105-2/99 from the Svanvik Complex, which are enriched in REE, Rb, Ba, Pb, and Sr, perhaps, because of their extensive migmatization (3.2-5.4% K₂ O and up to 51.3% SiO₂ ). In terms of Sm-Nd isotopic composition the mantle source of amphibolites from the Garsjo Complex was a depleted or strongly depleted ( e _Nd = 2.15-3.3 for an age of 2.84 Ga (Table 4).

Reconstruction of the Protolith Composition

Reconstruction of the genesis and composition of the protoliths provides valuable information for correlation between deep-seated rocks and their analogues exposed at the surface. The composition of the protoliths was identified based on the assumption of the isochemical character of metamorphism with respect to the least mobile elements: REE, HFSE, and some major elements (Al, Mg, and Ti).

Gneisses with High-Al Minerals

Figure 15

The gneisses with HAM are pervasively enriched in Fe, Mg, Ti along with LREE and HFSE, perhaps, because of the concentration of these elements in the detrital and clay material of the initial sedimentary rocks. The negative correlation between LREE and SiO₂ suggest that the protoliths were produced at the active participation of weathering and sedimentary differentiation processes. The most informative elements of the gneisses with HAM are REE, HFSE and Th. The Garsjo HAM-bearing gneisses have narrow range of these elements and consequently correspond to moderately differentiated sediments (Al₂O₃/SiO₂ = 0.2-0.32), which are similar in this parameter to gneisses from the KSDB sequence (Al₂O₃/SiO₂ = 0.16-0.44). REE are least likely to be redistributed during sedimentary and subsequent diagenetic processes [McLennan, 1989], so we used these elements to interpret origin of the gneisses within the framework of the model for mixing of material from different sources (tholeiitic basalts and gneisses of tonalite-trondhjemitic composition) [Taylor and McLennan, 1985]. Our estimates point to a significant involvement of the TTG source at a smaller input of the material of high-Mg mafic rocks in the protoliths of the gneisses with HAM from the KSDB sequence as compared with their analogues exposed at the surface (Figure 15). This conclusion implies the significant contribution of local erosion sources to the composition of sediments in ancient granite-greenstone terranes. These areas contained rocks of various compositions in different quantitative proportions. If this is actually the case, the use of metasediments for the purposes of correlation seems to be hardly justified.

Amphibolites

In order to identify the distinctive features of the source of the Proterozoic dike amphibolites and metavolcanic rocks of the Pechenga structure, one can use the approach proposed by Kerrich and Wymen [1997]. In multi-element plots normalized to the primitive mantle (Figure 6b), a line passing through the points of the most conservative elements (HREE, Ti, Hf, and Nb) has a negative slope. It indicates the enriched mantle source of the amphibolites and metabasalts of the Pirttijarvi and Zapolyarnyi formations of the Pechenga structure. Sm-Nd isotopic data shown that most of the KSDB amphibolites were derived from a source of composition intermediate between those of the depleted mantle and CHUR, supports this conclusion. The application of this prosedure [Kerrich and Wymen, 1997] to the Archean amphibolites (Figures 7a, 8) led us to conclude that the Archean mantle was more depleted, that, in turn, is consistent with analogous conclusions made on the basis of the Sm-Nd systematics. Of course, the Proterozoic age of the bulk of the amphibolites precludes reliable correlation between mafic rocks in the lower part of the KSDB succession with Archean amphibolites in the surroundings of the borehole.

Tonalite-Trondhjemite Gneisses

Gneisses of tonalite-trondhjemitic composition dominated among the suites of deep-seated rocks and analogous rocks exposed at the surface differ in textures, structures, and composition. The magmatic genesis of the gneisses follow from the similarities between their compositions with those of modern volcanic rocks (adakites) and are generally consistent with experimental data on the origin of the parental melts from mafic sources. The textural and structural features of the tonalite-trondhjemite gneisses allow for their subdivision into volcanic and coeval (or younger) plutonic facies. The gneisses are characterized by broad variations in the concentrations of trace elements, which depended on the crystallization and melting conditions. In HREE contents, the rocks vary from strongly depleted to enriched types. The KSDB plagiogneisses can be subdivided into two geochemically contrasting types. One of them (type A) is strongly depleted in HREE and dominates in the succession penetrated by the borehole, while the rocks of the other type (type B) are enriched in these elements and were encountered only in the upper portion of the succession. The Garsjo tonalite-trondhjemite gneisses exposed at the surface were classified into three geochemical types, which mostly include depleted and moderately enriched varieties. The low-Al gneisses with elevated HREE concentrations are less abundant. The presence of distinct geochemical rock types among the tonalite-trondhjemite gneisses of KSDB and its surroundings makes them suitable for reconstructions of the protolith composition and for correlation purposes.

The origin conditions of the parental melts for gneisses belonging to discrete geochemical types may be constrained based on their Sm and Nd isotopic compositions. In an e _Nd vs. T diagram, the points of the gneisses define two discrete fields with different e _Nd values for the gneisses of types A and B (Figure 9, Table 4). The lower field comprises the points of gneisses with relatively low e _Nd (0.69-1.05 at an average of 0.80) with relatively high model ages (2911-2930 at an average of 2917 Ma), most of which fall into type A. The upper field includes composition points with e _Nd = 1.32-2.68 (average 2.02) and model ages of 2778-2877 (average 2825) Ma, most of which belong to type B. The Sm-Nd systematics of the rocks suggest the origin of the type-A gneisses by the melting of more ancient and less depleted source than those of the type-B gneisses, which had a relatively shot crustal residenbe age. The concept of different protoliths of the type-A and B gneisses is also confirmed by the distinct Sm/Nd ratios of the rocks (0.172 and 0.161, respectively), whose values are though to be preserved in granitoids produced by the melting of the same source material [Faure, 1986]. The ¹⁴³Nd/¹⁴⁴Nd ratios of the amphibolites (0.512920-0.512922) exceed this value of CHUR (0.512636) and provide evidence of the essentially restitic nature of the mantle source of these rocks.

The concentrations of trace elements in a melt are known to be controlled by the respective concentrations in the source, its melting degree, and the bulk partition coefficients of these trace elements between the melt and residue, whose values depend, first and foremost, on the mineralogical composition of the residue. Hence, it can be suggested that the differences in the REE compositions of the type-A and B gneisses were resulted from both the compositional differences between their sources and a lower melting degree of the source of the type-B gneisses. A less probable scenario is the enrichment of the protolith of the type-A gneisses in incompatible elements immediately before its melting, because in this case the melts derived became enriched in REE but retained isotopic signatures of the depleted source.

Figure 16

According to experimental studies [Beard and Logfren, 1991; Rapp and Watson, 1995; Rapp et al., 1991; and others], the melts of tonalite-trondhjemitic composition most probable form by the partial melting of metabasic sources. An increase in the garnet and clinopyroxene in the residues in equilibrium with the partial melts and a decrease in the hornblende and plagioclase with increasing temperature and pressure result in progressive depletion of the melts in HREE and enrichment in Sr. To evaluate the P-T conditions under which the melts were generated and the composition of the most probable residues, it is expedient to use an Yb-Eu diagram [Turkina, 2000], which shows variations in the boundary concentrations of REE in the model geochemical types of melts (Figure 16). As follows from this diagram, most points of the tonalite-trondhjemite rocks plot near the fields of initial or evolved melts, which are shifted toward lower Yb and Eu concentrations in response to fractionation of amphibole and plagioclase during a melt crystallization. The KSDB tonalite-trondhjemite gneisses are localized within the fields of the two recognized geochemical types of the rocks. The protoliths of the gneisses depleted in HREE are close to the composition of melts generated in equilibrium with garnet-enriched amphibolite or eclogite residues at P15-16 kbar. Conversely, the protoliths of the gneisses enriched in HREE may have been derived under lower P-T, with the separation of garnet amphibolite residue. Similar melting conditions were also typical of the protoliths of the Garsjo tonalite-trondhjemite gneisses of types A and B. For type C most strongly enriched in HREE the partial melts could be derived only in equilibrium with garnet-free amphibolite residues corresponded to pressures of no more than 8 kbar. The tonalite-trondhjemite gneisses of the Varanger and Svanvik complexes are moderately depleted in HREE and generally belong to geochemical type A, whose protoliths were most probably formed in equilibrium with a garnet amphibolite residue, as also were the protoliths of ATTC in the lower part of the KSDB sequence. Hence, the analogy between the tonalite-trondhjemite gneisses penetrated by the borehole and those exposed at the surface is based not only on similarities in their compositions but also on the identity of their residual associations and the compositions of the metabasite sources.

Processes of Proterozoic Magmatism and Metasomatism in the Basement Rocks of the Pechenga Structure

The most intense Proterozoic processes that affected the basement rocks of the Pechenga structure were the intrusion of numerous mafic-ultramafic bodies, retrograde metamorphism to medium- and low-temperature amphibolite and epidote amphibolite facies, intensive migmatization, and intrusions of 1.76-Ga postkinematic granites.

Rocks of Mafic-Ultramafic Composition

As was demonstrated above, the inspected amphibolites from the lower part of the KSDB succession include rocks of Archean and Proterozoic ages, with the obvious predominance of the latter rocks (no less than 80% of the total amphibolite amount). The metabasalts of the dike facies seem to have served as magma conduits for the Pechenga volcanic rocks, while the amphibolites developing after the intrusive rocks are similar to the Proterozoic pyroxenites and paleoboninites from the framing of this structure. The mafic intrusive rocks in the Pechenga basement can be correlated with Proterozoic complexes of gabbroids, picrodolerites, Ti- and Fe-rich metadolerites, and metapicrites, which compose dike swarms in the framing rocks and have ages of, respectively, ~2555, 2200, and 1950 Ma. As follows from studies of paleorifts, the development of dike swarms of different ages is a characteristic feature of magmatism in these structures during different stages of their evolution [Balagansky et al., 2001; Bridgewater et al., 1995; Kaz'min and Byakov, 1997]. Similarly to the Pechenga metabasalts, it was proposed (based on Sm-Nd isotopic data) that the Proterozoic amphibolites were derived from a depleted mantle source, which was variably enriched in incompatible elements.

Metamorphism and Migmatization

Figure 17

The Early Proterozoic retrograde metamorphism proceeded under low-temperature amphibolite, epidote amphibolite, and, seldom, greenschist facies conditions. G. G. Duk identified metamorphic zoning of the andalusite-sillimanite type produced during two metamorphic episodes: its high-temperature stages (of the amphibolite and epidote amphibolite facies) are coeval with the Luostari unit of the Pechenga Complex, and the higher temperature rocks correspond in age to the Nikel unit [Duk et al., 1989]. The metamorphic processes of the amphibolite and epidote amphibolite facies were accompanied by migmatization of the rocks of the KSDB Archean complex. This process gave rise to a diversity of morphological types of migmatites. At the surface, analogous migmatites and granitic rocks compose a zone more than 20 km long and 2-6 km wide in the eastern framing of the Pechenga structure, which trends farther northwestward to the territory of Norway (Figure 1). The U-Pb zircon age of migmatized rocks in the lower part of the KSDB succession is 2225 Ma. The migmatization occurred locally, only beneath the Pechenga structure and its nearest surroundings, and was an apparently unequilibrated and allochemical process with respect to the host metamorphic rocks. It led to the depletion of the rocks in Al₂O₃, CaO, and Na₂O and their enrichment in SiO₂, TiO₂, FeO, K₂O, and some trace elements (Rb, Ba, Nb, Zr, REE, Pb, Cu, Cr, Ni, Co V (Figure 17), F, P, and CO₂, which are typical of alkali-enriched rocks. In studying the isotopic composition of common Pb, it is commonly assumed that migmatization is triggered and maintained by fluids that segregated from a chamber of melts parental for the subalkaline volcanics of the Kuetsjarvi unit of the Pechenga Complex [Vetrin et al., 2002a]. Fluid flows, directed from the crystallizing magma to the chamber roof, brought about migmatization of the upper crustal rocks through the precipitation of some major and trace elements from the fluids due to a decrease in solubilities of these elements in the fluids in response to a decrease in T and P. The same factors seem to have diminished the trace element concentrations in the rocks at the upper crustal levels as compared with he migmatized rocks in the lower parts of the succession and in the volcanics of the Kuetsjarvi unit, while the principal tendencies in the distribution of lithophile and siderophile elements remain similar (Figure 17). Data on the age of the KSDB migmatized rocks and "red granites" from the northeastern margin of the area led us to estimate the duration of the migmatization process at 50-70 m.y. [Vetrin et al., 2000, 2002a].

Postkinematic Granites

Figure 18

At depths of 9100-11,200 m, the Archean succession penetrated by KSDB was determined to contain numerous dikes of fine-grained, sometimes weakly porphyritic granites, which are analogous to the phase-4 granites of the Litza-Araguba Complex (LAC). The complex is exposed in the form of a chain of massifs in the eastern framing of the Pechenga structure. The concordant U-Pb zircon age of granite from depths of 9100-9700 m is 1765 2 Ma [Kola Superdeep Borehole, 1998]. A quite similar crystallization age value, 1762 9 Ma, was yielded by zircon from the LAC vein granite exposed at the surface [Vetrin et al., 2000]. The initial ⁴ He/ ³ He ratio of He entrapped by the crystallizing granite was estimated at approximately (3-5) 10 ⁵ (Figure 18). The genetic interpretation of the initial ⁴He/³He ratio was carried out within the framework of the mixing model of He from the continental crust and mantle. Based on studies of mantle xenoliths, the mantle source was assumed to be the enriched mantle of the area. Taking the values of the ⁴He/³He ratio in the enriched mantle and continental crust to be equal to, respectively, 6.7 10⁴ and 1 10⁸, we concluded that the values of ⁴He/³He = (3-5) 10⁵ of the granite could have been resulted from mixing of the mantle and crustal components in the proportion of approximately 1 to 4-7. These values indicate the essentially crustal nature of the entrapped fluid, which contained as little as ~13-22% mantle constituent [Vetrin et al., 2002b]. The mantle-crustal model for the fluid genesis in granitoids is well compatible with the petrologic model for the genesis of these rocks mostly by the anatectic melting of the crust under the effect of mantle melts [Vetrin et al., 1975].

Conclusions

The basement of the Pechenga riftogenic structure, which was penetrated by the Kola Superdeep Borehole over a depth interval of 6842-12,261 m, is dominated by tonalite-trondhjemite gneisses, amphibolites, iron quartzites (jaspilites), and gneisses with aluminous minerals. Analogous rock associations are exposed at the surface as the Svanvik-Neiden segment in the northwestern part of the Kola-Norwegian block. The rocks of both the KSDB Archean complex and the Svanvik-Neiden segment have identical age and correspond to volcano-sedimentary and plutonic rocks typical of granite-greenstone terranes.

The geochemical features of the gneisses with HAM, such as high concentrations of LREE, HFSE, and mafic elements concentrated in the detritus and clay material along with the REE decreasing with increasing SiO ₂, testify that the genesis of the protoliths involved the processes of weathering and sedimentary differentiation. The gneisses with HAM recovered by KSDB and sampled at the surface have similar, but not the same, sources. It was determined that local erosion sources contributed to the composition of the gneisses, and much material from high-Mg mafic rocks was involved in the gneisses with HAM from KSDB as compared with the analogous rocks exposed at the surface.

The great majority (  80%) of amphibolites in the KSDB Archean complex are Early Proterozoic rocks of the dike facies, which seem to have formed magma feeders for the Pechenga volcanics. They were derived from a variably depleted mantle source with an insignificant input of material from an enriched mantle reservoir. The mafic intrusive rocks in the basement of the Pechenga structure can be correlated with the Proterozoic gabbronorite, picrodolerite, and Ti- and Fe-rich metadolerite and metapicrite complexes composing dike swarms in the framing of the structure and having ages of, respectively, ~2555, 2200, and 1950 Ma. The Late Archean amphibolites penetrated by KSDB and exposed at the surface have concentrations of major and trace elements close to those in TH1 basalts of Archean greenstone belts, fundamentally differ from Proterozoic amphibolites, and were derived from a depleted or strongly depleted mantle source.

Tonalite-trondhjemite gneisses dominate among the deep- and shallow-seated rocks of the granite-greenstone terrane and can be subdivided into plutonic and stratified volcanic facies, which include various genetic types. The variable composition of tonalite-trondhjemite gneisses make them convenient for correlating of rocks at different depths. The data obtained on the Sm-Nd isotopic systematics indicate that the protoliths of the REE-enriched gneisses (type B) have been derived from the depleted mantle-related metabasites ( e _Nd = 1.05-2.68), whereas the source of the REE-depleted gneisses (type A) seem to have been less depleted in incompatible elements and radiogenic Nd ( e _Nd = 0.58-1.73). The gneisses depleted in HREE were formed in equilibrium with residues of garnet-rich amphibolitic or eclogitic composition at least 15-16 kbar. The differences between the trace-element compositions of the gneisses belong to geochemical types A and B could be resulted from different source compositions and a lower melting degree of the type-B source. Conversely, the gneisses enriched in HREE may have been generated under lower pressures, with residues of garnet-free amphibolites.

Contrary to the associations exposed at the surface, the rocks of the KSDB Archean complex were extensively altered by Proterozoic magmatic and metasomatic processes related to the development of the Pechenga structure. The most active Proterozoic processes that affected the basement of the structure were the intrusion of numerous mafic-ultramafic dikes, retrograde metamorphism to the medium-temperature amphibolite and epidote amphibolite facies, migmatization, and the emplacement of postkinematic granite intrusions at 1.76 Ga. Most Proterozoic processes show evidence of their relations with mantle sources. In general, the amounts of Proterozoic material introduced into the Archean rocks in the lower part of the borehole section plus the remobilized material of the Archean crust were evaluated at 30% (  12-15% amphibolites, ~3% vein granites, and ~15% migmatized rocks).

Acknowledgments

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