Archaean Erathem
Basement Complex
According to the first report on the geology of the Kola peninsula, related to the oldest formations were the supracrustal rocks of the Kola Group, i.e. garnet and micaceous gneisses, which were intruded by Archaean pyroxene diorites and oligoclase granites [51]. At the same time to the oldest formations of the Belomorian Zone related were the supracrustal formations of the Belomorian Group among which the predominant role was attributed to garnet and biotite gneisses [61]. Data on the diorites and granites collected during the after war period confirmed the conclusion that these rocks, united into a single tonalite-trondhjemite-granodiorite association, are intrusive into the Kola Group supracrustals [9]. Nevertheless, later an idea was conceived and widely spread that the rocks of this association including effusive formations compose the basement of the Early Precambrian rocks of the Kola peninsula and are identical to the TTG rocks whose age is more than 3.5 Ga [13]. A further development of this idea resulted in distinguishing huge masses of the rocks of the given association, which became known as the Early Archaean association of the primordial granitoids [42], in each zone of the Peninsula.
In the 1950s-1960s the research works and geological surveying resulted in a subdivision of the Kola Group into two units. The lower one includes biotite gneisses (hornblende-bearing ones as a rule), with amphibolites being sharply subordinated to them. The upper one includes various gneisses, amphibolites and banded iron formations, with garnet and sillimanite gneisses being predominant. An idea was also stated that the lower formations lie on an ultrametamorphic basement which is composed of different felsic gneisses (sometimes with relics of supracrustal rocks) and pyroxene diorites [18]. The analogous works carried out in the Belomorian Zone confirmed the lowest position of the Belomorian Group rocks. It is correlated well with our geological observations and geochronological data on the lowest position of the Kola and Belomorian groups supracrustals [1, 17, 32, 65; 67]. Of most probability is the same situation in the Keivy and Terskaya zones where the rocks of the lowest levels of the Belomorian Group are related to the lowest Archaean formations, as well. At the same time, the status of the oldest rocks should be preserved for some rocks which were distinguished earlier in the Early Archaean association of the primordial TTG rocks.
Model Rb-Sr and Sm-Nd ages of garnet and biotite gneisses of the Kola and Belomorian Groups including orthogneisses are within a rather short age interval and do not exceed 2.95 Ga when zircon ages are about 2.85-2.93 Ga [1, 17, 32, 65; 67, 70, 75]. Hence, the deposition of thick sedimentary strata that were transformed into the garnet gneisses, and the magmatic activity having resulted in the emplacement of huge masses of rocks that were original for the biotite and hornblende-biotite gneisses, took place practically, simultaneously. At the same time, it is obvious that a basement complex exposed on the surface is necessary for accumulating the thick garnet gneiss units extended in different zones up to 100 km. Judging from a sedimentary protolith of some garnet gneisses, and by their model Rb-Sr and Sm-Nd ages (not more than 2.94 Ga [1, 75]), as well as by absolute predominance of tonalites, trondhjemites, granodiorites, and/or their effusive analogues in the oldest conglomerate pebble, this basement is close in composition both to the primordial granitoid association and to the units of biotite and biotite-hornblende gneisses. So, it is logically to put these TTG units in the lowermost part of the Early Precambrian column section of the Kola peninsula, though at present we have no direct observations and isotopic data whether they are older than the garnet gneisses.
To summarize, the presumed basement in the Kola-Norwegian Zone is -composed of the formations of the lower unit of the Kola Group, i.e. the biotite and more rare hornblende-bearing gneisses with sharply subordinated amphibolites. In the Belomorian Zone, it is composed of various felsic gneisses of the Belomorian Group with the exception of the Chupa Formation gneisses as well as of amphibolite units distinguished from the Belomorian Group as younger formations [6]. In the Terskaya and Keivy zones as well as the Murmanskiy massif, the basement consists of a part of orthogneisses related earlier to the TTG association of the Saamian basement.
Thus, like in other Precambrian shields, the predominant part of the Kola Precambrian basement is composed of the so called grey gneisses of tonalite-trondhjemite-granodiorite composition. They contain a very small admixture of hornblende gneisses, thin layers of garnet-biotite gneisses, sometimes with kyanite or sillimanite, very rare observed were metagravelstones or other metasedimentary rocks. Thin amphibolite bodies are more often met, but their volume is also insignificant. In the Kola-Norwegian Zone, the layers of biotite-and magnetite-bearing amphibolites are characteristic of these gneissic formations, with their share being less than 1 per cent of the whole volume of the formations. Everything said above allows to adopt the view point mostly completely proved by the data obtained in study of the Kola Superdeep Borehole - the KS-3 [4] that these gneisses originated from thick units of andesite-dacite and dacite effusives, which contained thin layers of sedimentary rocks through all levels of their section. It is these felsic metavolcanites that have the oldest age of 2930+/-52 Ma yielded by zircon [17]. No doubt, these units should also contain plutonic rocks coeval to the effusives. These TTG gneissic units are mostly spread in the Belomorian Zone and less - in the Kola-Norwegian Zone.
In the Terskaya and Keivy zones as well as in the Murmanskiy massif these units are rather homogeneous and practically do not contain rocks that could be confidently considered as metasediments. The presence of hornblende-bearing gneisses in limited quantity as well as of amphibolites of uncertain genesis is typical for them. At the same time, in the Terskaya zone, these units are characterized by a banding due to an alternation of leucocratic and mesocratic varieties of biotite, epidote-biotite and hornblende-biotite gneisses with bands of up to 50 cm thick and of significant length.
Sometimes, the homogeneous units compose large structural forms, similar to domes or brachyforms whose contours were changed, to some extent, by later deformations. Such a situation is characteristic of the Terskaya Zone and Notozero Lake area. In the marginal parts of these structural forms, observed is an increased content of thin layers and lenses of amphibolites. So, these units should be considered as non-subdivided volcanic and plutonic formations among which intrusive rocks apparently predominate over effusive rocks and sedimentary rocks are practically completely absent [14].
In the Kola-Norwegian zone, the basement complex rocks were metamorphosed in both amphibolite and granulite facies, with granulite assemblages predominating; in all the rest territory mineral asseblages of amphibolite facies are observed.
In conclusion, it should be emphasized that inspite of the fact that the basement, distinguished by us, is similar lithologically to the Saamian basement of the Kola Precambrian distinguished earlier [13], it differs from the latter in principal. Firstly, the major part of the basement complex in question consists of other gneissic units, which as it was mentioned above are intruded by so called "Saamian" granitoids. Secondly, the formation of this complex is not considered to be an independent significant event, but it is related to the initial stages of the Late Archaean geological cycle during which the accumulation of the Kola and Belomorian Groups supracrustals and the emplacement of huge synkinematic intrusions of the granitoids and diorites of the Central Kola and Vezha Tundra complexes and coeval other plutonic rocks have occurred, and the metamorphism of all the rocks under granulite and amphibolite facies conditions took place. An analogical point of view on the early stages of the development of the Kola-Norwegian zone, in particular, was stated earlier [1].
Non-Stratified Terraines
Kola-Belomorian Complex
The oldest supracrustal rocks of the Kola peninsula and those of the adjacent part of Karelia were originally distinguished as the formations of the Archaean Kola and Belomorian Groups which were later united into the Kola-Belomorian Complex [13]. As to the origin of these rocks whose substantial part is represented in both Groups by garnet gneisses, the conclusions made by A.A. Polkanov [51] and V.M. Timofeyev [61] are also valid at present, who considered them to be metamorphosed sedimentary rocks. The mostly forcible argument is a distinct graded-bedding texture often observed in these gneisses as well as their specific chemical composition. So, we consider them as before to be the Archaean metasediments. The rocks associated with them and having lost their original structures are supposed to be of the same origin.
Many studies have shown that the Belomorian Group rocks have undergone an intensive and repeated metamorphic and deformational reworking that resulted in many cases not only in disappearance of their primary features, but it has become impossible to reconstruct an original stratigraphic section even in a small domain not speaking about where the top and bottom of the section are. The intensity of this reworking makes it impossible to determine the original co-relations between the geological bodies distinguished. This is just the main reason why all the stratigraphic schemes of the Kola-Karelian region are contradictive and sometimes even completely exclude each other. At its workshop held in the early 1993 in St. Petersburg, the Joint Regional Stratigraphic Committee on the North-Western Russia admitted the fruitlessness of the attempts undertaken stratigraphically to subdivide such rocks as Kola-Belomorian supracrustals, which we unite into a non-stratified complex and call it as before the Kola-Belomorian one. When its lithology is described, the term "unit" is used, with the position of distinguished units in the column section of the Legend having no meaning.
The conclusion has been made above that the Kola-Belomorian rocks lie on the basement complex and their isotopic ages are the oldest for the supracrustal formations of the Kola Archaean. Hence, they should be situated at the bottom of the stratigraphic sequence of the Early Precambrian of the Kola peninsula. The primary interrelations between rock units of the Kola-Belomorian complex that form real geological bodies and are shown in the Map, have been undetermined.
The unit of micaceous and garnet-micaceous gneisses. It was established as far back as twenties, that garnet, sillimanite and kyanite gneisses are widely spread in the Kola and Belomorian zones. The analysis of geological mapping data obtained later has shown that these gneisses sharply dominate through all the areas of the Kola Group rocks extension and are an important component of the Belomorian Group. They are the only rocks that often compose large bodies having distinctly reproduced boundaries. As a result, in both zones as well as in the Terskaya one, distinguished are terrigenous units that are mainly composed of garnet-micaceous (garnet) and micaceous (biotite and two-micaceous) gneisses. We decided it to be sensible to give these units their own names in any particular case bearing in mind that they form a single lateral set shown in the Legend.
In the Kola-Norwegian zone, this unit is composed of garnet-biotite gneisses containing sillimanite and cordierite, sometimes kyanite and more rare staurolite. In many places, in a sharply subordinated quantity observed are two-micaceous, biotite, hornblende and biotite-hornblende gneisses as well as amphibolites (including garnet ones), hornblende-pyroxene, hornblende-magnetite and pyroxene-magnetite granulites and banded ironstones. In some areas, these subordinated rocks were segregated into independent units, but even in maps of a 1:200 000 scale they form small isometric fields within the garnet gneiss unit, their location being irregular. So, all the rocks mentioned above are united into a single unit which is practically identical to the garnet gneisses unit by A.A. Polkanov and to the upper unit of the Kola Group according to later stratigraphic schemes. It is correlated well with the garnet gneiss unit that underlies the Proterozoic rocks in the KS-3 section in Pechenga. It is called as the Volshpakhk for it is close lithologically to one of the Kola Group formations having the same name [18]. Metamorphism of the Volshpakhk unit is zonal and its grade ranges from amphibolite to granulite facies.
In the near Imandra region observed is the absolute predominance of garnet-biotite gneisses in the Volshpakhk unit, sometimes with muscovite, kyanite, sillimanite, and cordierite. Banded ironstones, amphibolites, and amphibole gneisses are observed there rather rare.
Several units of the same lithology are distinguished in the KS-3 section. The thickness of the unit, underlying the Pechenga Lower Proterozoic rocks, is close to the total thickness of all other units [4]. The problem has not been solved yet, whether all these units are parts of one unit whose alternation with biotite gneisses is conditioned by folding, by doubling the section as a result of thrusting, etc., or whether they belong to the part of the Kola Group section that is not exposed.
In the Pechenga-Imandra-Varzuga zone to the south of the Pechenga area, biotite, garnet-biotite and kyanite-garnet-biotite gneisses (the latter are very often observed together with sillimanite or cordierite) compose rather a homogeneous unit similar lithologically to the Volshpakhk unit in the area of Kutskol'-Verkhneye Volch'ye Lakes. Rare interlayers of biotite-hornblende and hornblende gneisses are also observed. A fine graded bedding-like banding of a flyshoid type is characteristic of many types of the gneisses. Earlier, all these gneisses were united into the Vyrnim Group [60], that is why, the former name, the Vyrnim, is given to this unit.
In the Belomorian Zone, garnet-biotite and kyanite-garnet-biotite gneisses compose rather homogeneous bodies of different dimensions and morphology. They are usually of a linear form, sometimes are folded by structures of different orders, the synforms predominating among them. Sometimes in these gneisses one can observe cordierite, staurolite and sillimanite; two-micaceous and hornblende-biotite gneisses are contained in subordinate quantities, and sheet-like bodies of garnet and feldspar amphibolites are observed very seldom. Among the amphibolites, presented, probably, are rocks both those paragenetically related to Al-rich gneisses and the younger ones, tectonically or by any another way, introduced into the section of the non-stratified complex.
Some bodies have different lithology. For instance, to the south of the Upper Tuloma reservoir predominate two-micaceous gneisses with rare intercalations of biotite and biotite-hornblende gneisses with the exception of the N-S trending body on the Three Brothers Mountain, which is mainly composed of garnet-kyanite-biotite porphyroblastic gneisses. In the Pirenga Lakes area spread are fine-grained homogeneous biotite gneisses, sometimes with muscovite, garnet, epidote, and amphibole.
The Al-rich gneiss bodies shown in the Map were included into different formations of the Belomorian Group - the Loukhi, Chupa, Kaita Tundra, Yona, Rikolatva, and some other formations [12, 29]. While mapping, they were related to the upper unit of the Group. The Al-rich gneisses dominate in the Chupa,Yona, and Rikolatva formations, so the most extended body, which is practically the only one in the Kandalaksha and Loukhi areas, is distinguished as the Chupa unit; and a set of large bodies in the Yona area is united into the Rikolatva unit.
The unit of ortho- and paragneisses, amphibolites and calciphyres. In the Terskaya Zone, in the middle and lower stream of the Serga river, there is a unit that is characterized, first of all, by a significant variety of its lithology, i.e. metaterrigenous and metapyroclastic rocks as well as specific calc-silicate and sulphide-bearing deposits are widely spread side by side with basic and felsic metavolcanites. Hornblende-bearing gneisses (hornblende-biotite, garnet-hornblende-biotite, epidote-hornblende-biotite, sometimes with garnet) and amphibolites dominate in the supposed lower part of the section of this unit called as the Sergozero. Micaceous and garnet-micaceous (often with sillimanite) gneisses are often subordinate, sometimes observed are garnet-kyanite-micaceous and sulphide-bearing biotite gneisses. In the supposed upper part of this unit, biotite gneisses are of importance with thin layers and amphibolite lenses, the quantity of the latter reaching sometimes first dozens of per cents of the unit volume. In generall, the amphibolites are uniformly spread through the whole section. Also observed are thin layers of calc-silicate rocks, i.e. those of "calciphyres" composed of clinopyroxene, plagioclase, clinozoisite, scapolite and carbonate with the high pyrrhotite and pyrite content. From petrochemical reconstructions, a wide spectrum of volcanic rocks is determined (from basalts to dacites [42]), whereas sediments are represented only by greywackes.
In the Terskaya zone, distinguished is one more unit just of terrigenous lithology that is composed only of micaceous, two-micaceous and garnet-micaceous gneisses, which sometimes contain kyanite and more rare staurolite. The gneisses chemically correspond to slightly differentiated sediments (greywackes and subgreywackes). This unit was known before as the Chapoma Formation that is why this name is preserved. It builds up the central part of a large synform structure whose limbs are composed of the Sergozero unit rocks. It is supposed that the Chapoma unit is concordant with the Sergozero one and its relationship with younger Archaean rocks is tectonic.
The Vyrnim, Chupa, Rikolatva and Sergozero units have been metamorphosed under amphibolite facies conditions.
The unit of ortho- and paragneisses, amphibolites and banded ironstones. In the most south-eastern part of the Kola-Norwegian zone, several relatively thin bodies to 20 km in length composed of various gneisses and amphibolites (or pyroxene- and hornblende-bearing granulites) with intercalations of banded ironstones and (or) magnetite-bearing rocks were mapped among exposures of diorites, granodiorites and trondhjemites. As garnet gneisses do not dominate here, these bodies were united into an independent unit similar in composition to the Chudz-Yavr Formation of the Kola Group. It is sensible to give it the same name because it is in the Chudz-Yavr Lake area that the main part of its exposure is concentrated. This unit composes a tiny part of the Kola-Norwegian Zone; it is metamorphosed under amphibolite-to-granulite facies conditions.
The Annama Unit rocks crop out southwards from the Pechenga area. It is analogous to the Chudz-Yavr unit in composition and was known earlier as the Annama Group [60]. It has a highly layered structure changeable both in vertical and horizontal directions and is composed of hornblende-biotite, hornblende, biotite, two-micaceous, garnet-biotite (sometimes with sillimanite, kyanite, and staurolite as well as graphite) gneisses with lenses of banded ironstones and amphibolites. The banded ironstones form lenses of 60x300 m and are spread only in the eastern part of the expansion of this unit. Metamorphism of the Annama unit has occurred under amphibolite facies conditions.
The unit of ortho- and paragneisses, amphibolites and intermediate metavolcanites with banded ironstone deposits. In the near Imandra region distinguished is a unit whose lithology is close to that of the Chudz-Yavr unit. It differs mainly in dominating hornblende and biotite-hornblende gneisses and metavolcanites whose composition vary from basalt to rhyodacite, and in the presence of economic deposits of banded ironstones. As a whole, its section is various and is presented by the alternation of layers of hornblende, biotite-hornblende and biotite gneisses with thin layers of banded ironstones and garnet-biotite gneisses, sometimes with staurolite or sillimanite. It is highly identical to the Olen'ya Gora unit distiguished earlier, that is why the same name is preserved. It is metamorphosed under high-T amphibolite facies conditions.
In all zones distinguished are small bodies whose lithology is similar to that of the units of the Kola-Belomorian complex. They are shown in the Map by the same colours; but the proper names are not given to them.
Uncertain Age Complex
This complex unites rocks that like the Kola-Belomorian formations cannot be properly stratigraphically subdivided, but they are in the stratigraphic column section between the Archaean and Proterozoic supracrustals. The main part of the complex is composed of the Lapland Granulite Belt rocks, which are considered by most geologists of this country to be Archaean in age, while isotopic age data by foreign researchers suggest them to be Early Proterozoic. The age of the basic granulites, which are intruded by gabbro-anorthosites of the Pyrshin in the eastern part of the belt and of the Kolvitsa massif in the south-eastern branch, is limited by the ages of these massifs, i.e. 2452 +/-7 and 2450+/-10 Ma, respectively [45]. In addition, the Laplandian granulites are related to non-stratified formations for it is impossible to subdivide them in some areas into supracrustal and plutonic rocks, which is also shown in the Legend.
The unit of basic and intermediate granulites and enderbites. These rocks are characterized by an extremely high degree of shearing under granulite facies conditions up to the presence of ultramylonites, their structural zonation being distinctly observed, i.e. the degree of shearing decreases northwards while the intensity of granitization, charnockitization and enderbitization increases. The formation of ultramylonites in the south bottom of the granulite section observed is related to their thrusting southwards onto the rocks of the Belomorian part of the Kola-Belomorian complex.
In the southern marginal part of the belt predominantly spread are basic and intermediate granulites presented here by pyroxene and two-pyroxene varieties, usually garnet-bearing, which sometimes contain layers of felsic granulites, the so called eclogite-like rocks, and very seldom calciphyres. To the north all these rocks are replaced by dioritic gneisses that are amphibolized and biotitized as a rule, and orthopyroxene and high-Mg garnet are sporadically observed in them. There exist two mutually incompatible view points as to the nature of these rocks. They are considered to be mantle and (or) lower crust rocks tectonically intruded into the upper crust (i) or ordinary supracrustal rocks (mainly metavolcanites [36]) undergone high strain in high-P and high-T granulite facies (ii). In the last case the basic granulites and the rocks associated with them were united into the Ploskaya Tundra unit, and the intermediate ones - into the Por'ya Guba one [60]. We unite these rocks into the Laplandian complex including both supracrustal and plutonic formations.
The unit of felsic granulites. The granulites of the Laplandian Complex are replaced northwards by felsic granulites represented by sillimanite-garnet-biotite and garnet-biotite gneisses with subordinate biotite and cordierite-bearing gneisses as well as by garnet-quartz and hypersthene-bearing felsic rocks. They are usually mylonitized along their southern boundary. Fine graded banding found in them long ago (especially in the Finnish part of the belt) and the peculiar chemical composition as well as new data on the content of REE and oxygen isotopes [16] testify that the protolith of these granulites has been of the sedimentary origin*. They are known as metasediments of the Jaurijoki and Lotta units [60]. The names are preserved also in the Legend. The felsic granulites correspond chemically to psammites and more rare to pelites, sometimes admixtures of volcanic stuff are distinguished [36].
The unit of micaceous, chlorite-amphibole and carbonaceous schists, amphibolites and quartzites. It is located along the tectonic boundary between the Proterozoic rocks of the Imandra-Varzuga Zone and the Archaean gneisses of the Terskaya Zone. Earlier it was known as the Pikamskaya Formation of the Lower Proterozoic [60] and is mainly composed of mylonites and cataclasites. We believe that the presence of the Panarechka and Peschanoozerskaya Formations of the Lower Proterozoic and Upper Archaean, respectively, is possible here. Thus, this unit is a tectonic mixture of rocks of different ages, and poor exposures do not allow it to be stratigraphically subdivided, but at the same time it is a real geological body, which permits us to map it with the previous name as the Pikamskaya unit. Of most predominance in its lithology are chlorite-micaceous, epidote-chlorite-micaceous and two-micaceous schists, sometimes with garnet, limitedly spread are amphibole-bearing varieties as well as quartzites,
carbonaceous schists and amphibolites with epidote.
The unit of two-micaceous and quartz-feldspar schists. These rocks are exposed along the south-western boundary of the Pechenga Complex. They are weakly deformed and metamorphosed and are characterized by an obvious graded-bedding texture being indicative of the terrigenous origin of the protolith. The unit composed of these rocks preserves the same name as the Tal'ya unit. It consists of micaceous, garnet-micaceous, hornblende-biotite, muscovite, chlorite and carbonaceous schists, quartzites and seldom amphibolite. It may be Sumian-Sariolian or Lopian in age (Kozlova, Balagansky, in press).
Stratified Complexes
The Kola Archaean supracrustal units, besides the Kola-Belomorian formations, are
_____________________
* - see footnote on Page 52.
composed in many areas of gneisses, schists and amphibolites, whose original peculiar features are well preserved, which allows not only to determine the sedimentary-volcanic origin of the protoliths, but to reconstruct their stratigraphic sections. It is due to such easily recognized metamorphosed sedimentary and volcanic rocks widespread in these units as conglomerates, conglomerate-breccias, gravelstones, graded-bedded pelites, komatiites with spinifex texture, volcanic breccias, pillow lavas, and so on, and so forth, whose textures allow to determine the top and bottom of a section. The metamorphic complexes consisting of such rocks can be stratigraphically subdivided on the basis of lithostratigraphic data, which may be indirectly proved by similarity of the column section reconstructed for a certain complex by various researchers. Thus, the section of the Keivy supracrustal rocks metamor-phosed in amphibolite facies was reconstructed as far back as during the after war years and then it was only worked out in detail without any change in its main features [63].
The sections of these complexes are similar to each other, which makes it possible to correlate them and to make a single common section. As this is a deeper interpretation of original data, which are often not sufficient, then stratigraphic schemes of various authors differ in details and even in principal points from each other but they, nevertheless, are similar to each other as a whole. Thus, we may agree with the fact that the supracrustals have been deposited during one sedimentary-volcanic cycle and have formed a single stratigraphic section [13]. At the same time, a number of problems concerning the structure of some complexes, the common section and even their being possessed to this stratigraphic level remain open to discussion and need to be further studied.
The Upper Archaean age of these complexes spread in the Keivy, Kolmozero-Voron'ya and Terskaya zones, the north-western Belomorian province and in other areas is determined by both geological and single isotopic data available now. Many complexes unconformably lie on the oldest TTG rocks whose pebble is usual in conglomerates of the units discussed [13]. In the Voche-Lambina area found is pebble of the TTG rocks which had been metamorphosed and deformed before the supracrustals were accumulated [23], which testifies a regional unconformity between them and the older Kola-Belomorian formations having undergone their first structural-metamorphic reworking simultaneously with the TTG rocks.
The sections of particular stratified supracrustal complexes in a generalized way are shown in Fig.1*. The most complete and simply interpreted section is that of the Keivy Zone. It is based on a section compiled by L.Ya. Kharitonov [63] who took into account all the data obtained by the mid 1960s. In the common stratigraphic scheme of the Kola Precambrian created in the Geological Institute [13], it is this section that has been taken as a base to create the common section for these complexes. The Keivy section is subdivided into four levels which are distinguished as the main ones in the common section as well. Such a view point was proved as a whole by subsequent researchers [31] and is followed to in the Note.
The basement in the Keivy Zone is unknown. The first level (from top to bottom) is ter
_____________________________
* - see Fig.1 on Pages 18-19.
rigenous and is composed of deposits of the Kolovay and Kinemur Formations (Fig. 1). Granitoid pebbles in conglomerates of the Kolovay Formation indicate that it seems to be basal. These formations consist mainly of hornblende-biotite, biotite (sometimes with staurolite) and two-micaceous (often with garnet) gneisses after sandstones and arkoses with conglomerate lenses. Felsic volcanic rocks are mentioned. The rocks of the second level are represented by basic metavolcanic rocks with subordinate intermediate and felsic metavolcanics as well as by terrigenous sediments of the Patcherva Formation, which are supposed transgressively to overlie the Kinemur Formation rocks. These are amphibolites, sometimes metakomatiites, hornblende, epidote-hornblende, sericite and other gneisses and schists. Among metasedimentary rocks known are psammites (garnet-biotite gneisses), gravelstones, conglomerates and conglomerate-breccias, banded iron formations and iron-bearing schists, carbonatic sediments. The third level is represented by felsic metavolcanics of the Lebyazhka unit. Here widespread are hastingsite and microcline-bearing gneisses that have been considered to be resulted from alkali metasomatic reworking of the metavolcanics [26]. The section top is built up by terrigenous high differentiated deposits of the Chervurt, Vykhchurt and Pestsovaya Tundra Formations represented by conglomerates, sandstones, including quartzose ones, pelites, psammites, gravelstones, aleurolites, marls and dolomites. They form the fourth level of the section separated from the third one by the unconformity and weathering crust, and for a long time they have been considered to be the rocks of the sedimentary cover of an Archaean platform.
The supracrustal sequence of the Kolmozero-Voron'ya Zone*, its basement being unknown, has also well been studied, nevertheless, there exist some dozens of its versions. We have taken the version in which all the terrigenous sediments are related to the top of the section [30, 44]. It starts with the Polmos Formation of basic metavolcanics of the komatiite-tholeiite suite (Fig. 1), but we do not exclude the presence of some quantity of terrigenous basal deposits related to the Lyavozero Group. These are amphibolites with relics of pillow lavas and amygdales, sometimes they are ultrabasic rocks with spinifex textures. The Polmos Formation corresponds to the second level of the common section. Above it bedded are metavolcanics of the basalt-andesite-dacite suite united into the Voron'ya Tundra Formation; they correspond to the third level. High differentiated metasedimentary rocks of the Chervurt Formation (the fourth level) complete the section. As a whole, this terrane is close to those of the Late Archaean greenstone belts [24]. Among age determinations the most real ones are values of about 2.76 Ga (U-Pb ages of zircons from gneisses and schists) and of 2.65+/-0.06 Ga (Rb-Sr data on the same rocks) [48].
In the western part of the junction zone between the Murmansk Massif and Kola-Norwegian Zone, in a kind of a narrow bend, exposed are basic metavolcanics with metakomatiite lenses with spinifex textures [19], intermediate and felsic metavolcanics, terrigenous rocks are presented as well. They are united into the Ura Guba Formation, are well correlated with rocks of the western part of the Polmos Formation and correspond to the second level of the _____________________________
* - it is also known as the Polmos-Poros zone (V.V.B.).
common section. This formation is underlain by conglomerate-bearing terrigenous rocks that have been attributed to the first level (Fig. 1).
The Upper Archaean stratified supracrustals spread to the south of the Pechenga area are
distinguished as rocks of the Kaskama Formation. Its contacts are tectonized; interrelations with other rocks are uncertain. The formation volume is shortened against that of the Kaskama Formation distinguished earlier [60]. In its lithology of most predominance are basic and felsic metavolcanics, terrigenous rocks are almost absent. As a whole, the Kaskama section should correspond to the second level of the common section, and its uppermost part is probably related to the third level (Fig. 1). The correlation of the given formation with the terranes in question is disputable.
The rocks of the Korva Formation spread along the south boundary of the Lapland Granulite Belt are related to the same level on the base of a similar lithology and the section structure. They are mainly represented by felsic metavolcanics and various metasediments whose quantity increases to the top of the section with changing psammites into flysh-like rocks. The formation comprises also garnet-diopside amphibolites spatially connected with it. Since garnet amphibolites that occur along the south boundary of the belt and a part of amphibolites surrounding the Korva rocks have been related to younger units, the volume of the formation has been reduced against its former one [60]. In the common section, it corresponds to the third level (Fig. 1).
Metamorphosed basalts, andesite-basalt, andesite, felsic volcanics, carbonatic rocks, sandstones and quartzites , with andesite-basalts and andesites being predominant, are extensive in the area framing the Imandra-Varzuga Zone from the west. The felsic volcanics and the rocks associated with them were united into the Arvarench Formation before, so it is expedient to preserve this name. More basic rock underlying this formation were united into the Vite Guba Formation, but as this name has not been widely practised, we, like the geologists of the "Sevzapgeologiya", consider them to be the rocks related to the Pyalochnaya Formation which are expended here from the Terskaya Zone along the southern boundary of the Imandra-Varzuga Lower Proterozoic rocks. The Pyalochnaya Formation is regarded to the second level of the common section, the Arvarench Formation - to the third one (Fig. 1), the latter being unconformably overlain by Lower Proterozoic rocks of the Kuksha Formation [49]. Nevertheless, the Archaean age of the Arvarench Formation is disputable.
The Voche-Lambina Formation whose rocks are exposed in the same area is composed of four units [23]. The first, lowest, consists of intermediate and felsic metavolcanics (hornblende and biotite gneisses) and contains thin amphibolite bodies most of which are dykes. The second unit is composed of basal conglomerates and thin amphibolite bodies (metabasalts and metaandesite-basalts), biotite-hornblende and biotite gneisses (intermediate and felsic metavolcanics), as well as various biotite and hornblende gneisses with relics of graded-bedded and gravelstone structures. The third unit comprises only biotite and hornblende-biotite gneisses (metarhyodacites) with solitary conglomerate lenses. The fourth unit includes terrigenous (mainly conglomerates) and volcanic (amphibolites with relics of amygdales and pyroclastic textures, rhyodacitic gneisses) rocks. Many conglomerates are tuff-conglomerates, a pyroclastic admixture is also usual for other terrigenous rocks. The section of the formation corresponds to the third level of the common section (Fig.1). Model Sm-Nd and Rb-Sr ages of metarhyodacites do not exceed 2.76 Ga, whereas their Rb-Sr systems have been closed after the first metamorphic event 2.53+/-0.03 Ga ago [65, 69].
The Yona Complex is characterized by alternation of fine- and coarse-banded amphibolites (tholeiites and their pyroclastic analogues) with biotite-hornblende and hornblende gneisses (andesite-dacites). In the amphibolites presented are sheet- and lens-like bodies of ultrabasites , i.e. pyroxenitic and peridotitic metakomatiites. Terrigenous rocks are extremely rare here. Almost the whole Yona section corresponds to the second level of the common section; felsic metavolcanics completing the section are regarded to the third level. A felsic metavolcanic rock sample has been dated and a concordant age of zircon is 2778+/-4 Ma (Pozhilenko et al., unpublished). Since this age is very close to a model Sm-Nd age of the same sample [70], it is considered to be the age of volcanism. The Yona formations are separated from the basement and the Kola-Belomorian rocks by an unconformity.
The Irinogora Formation section is similar to the Yona Complex one (Fig. 1), its lower part corresponds to the second level of the common section, and its upper part is related to the third one. This sequence is completed with terrigenous rocks, small quantity of which being observed through the whole section. The Irinogora Formation rocks had been accumulated on the deformed and metamorphosed basement and after their first metamorphism and folding they were disconformably overlaid by the Lower Proterozoic basal formations [5].
A rather full section of supracrustals under consideration is reconstructed in the Terskaya Zone (Fig.1). In its bottom are terrigenous sediments, i.e. gneisses, mainly homogeneous or with a poor banding, sometimes with garnet and muscovite, seldom with kyanite and staurolite, with rare layers of muscovite-quartz schists and micaceous quartzites, hornblende-biotite schists and amphibolites with a lens of conglomerate-like rocks observed by L.I.Ivanova in the Munozero Lake area. Graded-bedded two-micaceous and garnet-micaceous schists predominate in the upper part of this section. These rocks are related to the Peschanoozerskaya Formation corresponding to the first level of the common section. The basic metavolcanics of the Pyalochnaya Formation with interlayers and lenses of intermediate and sometimes terrigenous rocks occur higher; they fill narrow synforms between domes and brachyforms in the basement complex. Metasedimentary rocks of the Vysokaya Zemlya Formation presumably occupy the uppermost position in this sequence. These two formations correlate well with the second level of the common section. Model Rb-Sr ages of felsic metavolcanics and metaandesite-basalts are 2.77 Ga, whereas their Rb-Sr systems have been closed 2.55+/-0.09 Ga ago, with this closure having been connected with the termination of regional metamorphism of these rocks [65].
In conclusion, let us consider the structure of the common section. The rocks of the first level are spread locally; they are established only in the Keivy and Terskaya zones and represented only by terrigenous rocks including conglomerates with TTG rocks pebbles. They are supposed to lie just on the basement. To this level also related is the Kislaya Guba Formation due to its basal position in the section of the Munozero dome. The second level is the marking one - only here the most part of the section is built up by basic volcanics and komatiites with which intermediate volcanites are associated; this association being present in all the zones and areas. The third level is also the marking one due to sharp predominance of felsic volcanics over all rocks and its rocks are expensive everywhere with the exception of the Ura Guba area. The volume of volcanics of the second and third levels build up the predominant part of the common section. The fourth, uppermost level is composed of terrigenous rocks whose original material is of high degree of differentiation. On this level observed is the redeposited deep chemical weathering crust as well as an unconformity between these rocks, on the one hand, and the third level formations and the basement rocks, on the other hand. The rocks of the fourth level are spread rather locally and established for certain only in the Keivy and Kolmozero-Voron'ya zones where they are deformed strongly enough but in general they lie gently. They occur also in the eastern part of the Imandra-Varzuga Zone. It is quite possible that the terrigenous sediments that complete the Terskaya Zone section, the Korva deposits and others thinner units having no proper names may be regarded to this level. Also it can not be excluded that the Al-rich rocks of the Yaurijoki and Lotta units of the Lapland Granulite Belt having the uncertain position between the Upper Archaean and Lower Proterozoic in the Legend might belong to the fourth level, too. In conclusion, the basal parts of the stratified complexes are lithologically close to paragneissic parts of the Kola-Belomorian Complex, so it becomes difficult to relate any particular unit to this or that complex, and a decision adopted is conditional as it is in the case with the Kislaya Guba Formation. Is is clearly shown from Fig.1 and the description of the sections that they change also laterally. We suppose it to be due to a zonation of the conditions under which sedimentation and volcanism took place and to the existence of several types of the sections of these complexes [13, 31].
Thus, the complexes considered are obviously volcanic with the homodromic evolution of volcanic processes. Locally and uncompletely occurring, terrigenous rocks of the lower part of their sections indicate the initial stage of the subsidences that rapidly started to develop as volcanic ones. These complexes are compatible with those of the greenstone belts, those extensive magmatic activity corresponds to the time interval of 2.8-2.7 Ga ago both in the Baltic and in other Early Precambrian shields. The uppermost, terrigenous part of the sections disconformably overlying the volcanic one seems to be an independent stage, compatible with the protoplatform one. Geochronological studies seem to amend somehow the above scheme of the structure and correlation of stratified supracrustal units of the Upper Archaean, but we hope that they will not throw it into the melting pot.
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