Объяснительная записка к геологической карте северо восточной части

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].

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.

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.

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.

carbonaceous schists and amphibolites with epidote.

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* - 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

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* - 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).

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.