Объяснительная записка к геологической карте северо восточной части

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* - see Fig.2 on Pages 24-27.

The Kola stratotype of the Sumian is situated in the eastern Imandra-Varzuga Zone and is represented by the Purnach, Kuksha and Seidorechka formations whose rocks form a suc­cession of certain geological bodies with the established stratigraphic interrelations [33, 34]. The exception is the basal horizon of terrigenous rocks of the Purnach Formation. The au­thors have placed it at the bottom of the Purnach Formation in both the Legend and Map. Data available make it impossible to determine the position of this horizon properly because it is situated northward of the Purnach marginal fault separating the Sumian succession from the sequence of the Malye Keivy area where there are no reliable marking layers for these two sequences to be correlated. Since the position of the conglomerate horizon in the Malye Keivy area is the key for correlating the successions in the Pechenga-Varzuga and Keivy Zones, the data available and the authors' point of view should be considered in a detail.

In the southern margin of the Purnach Block in a 15 kilometer's distance east from Ro­manov Lake (the Ambarny Stream), quartzose metasediments of the Pestsovaya Tundra Formation conformably lie on the weathering crust of the basement gneisses. The metasedi­ments dipping southwards are disconformably overlaid by a conglomerate member, its section being regressive. The member is overlaid by andesitic-basaltic metavolcanics, which are petrochemically identical to the Purnach Formation volcanics. These two units, i.e. the Pestsovaya Tundra Formation and the conglomerate member, are continuously extended westwards in the Malye Keivy Ridge where they are overturned and dip northwards. The overturned layering is indicated, on the one hand, by the whole succession, which is reverse against the stratotype succession in the Keivy area as well as in the Purnach Block margin in question, and on the other hand, by graded-bedded textures of the conglomerates. Besides, the layering of the Pestsovaya Tundra metasediments on basic volcanics is completely un­known. So, to consider the succession of the Malye Keivy Ridge to be in normal (non-over­turned) layering is not right because it contradicts many facts. It is believed that the optimum interpretation of the Malye Keivy succession may be a version that the Purnach Formation has the basal conglomerate layer that occurs on the Pestsovaya Tundra rocks. In this case, we may explain the presence of the regressive succession (such a succession is seldom enough), which has been quickly overlaid by volcanics of a slightly differentiated andesite-basalt asso­ciation at the incremental sinking in a marginal zone of the volcanic subsidence.

The correlation of the Sumian formations of the Pechenga-Varzuga Zone and the schist part of the Bol'shye Keivy succession as coeval rocks deposited under different sedimentation settings [47] is not satisfactory substantiated. This model takes the overturned section of the Malye Keivy Ridge to be normal and ignores the complete absence of any traces of volcanic stuff in high differentiated sedimentary formations of the Keivy Group at the boundary of these two sequences, i.e. two palaeozones - volcanic and protoplatform ones.

A part of the Panajarvi Formation have usually been related to the Sumian. It has been made rather traditionally on the lithological basis, and the specific association of rocks com­posing this section has not been taken into account, whereas it is the feature that is of the decisive importance for the Sariolian level. The formation of the Sariolian rocks was estab­lished to have occurred under conditions of reactivisation of faults separating crustal blocks, which resulted in the volcanic-molassic type of the Sariolian succession. It consists of con­trasting coarse-grained terrigenous rocks and felsic and basic tuffs and lavas. The succession contain also unconformities, disconformities and weathering crusts. Such sections are mostly typical of structures that have started their development at the Sariolian time. They are char­acterized by more stable features in inherited subsidences of the Pechenga-Varzuga Zone.

The Kandalaksha, Kareka, Keulik-Kinerim and Korva Tundra amphibolite units are conventionally related to the Purnach Formation level for their sections and regional positions are similar both to the Purnach ones and to each other. This conclusion is disputable and needs evidences.

The Karelides sections shown in Fig.2 probably need a short commentary on their re­gional regular pattern, bases of correlation and problems concerning the subdivision of the upper department of the Karelides.

A general natural feature of the Karelian Complex is the presence in it of three subcom­plexes separated by interruptions and unconformities and characterized by different relation­ships between the stratigraphic sedimentary and volcanic elements. The lower subcomplex unites formations consisting of a pair of subformations - sedimentary and volcanic ones hav­ing sharp boundaries without any traces of intercalations. The upper subcomplex is character­ized by sedimentary, volcanic and mixed rocks interbedded with each other and composing layers, members and units. The boundaries between these units are sometimes conditional, and they are distinguished by a certain rock association dominating in a unit section. The middle subcomplex is characterized by stratigraphic elements of both upper and lower sub-complexes, with a two-member structure of each formation being preserved on a large scale.

The structure of the upper subcomplex of the Karelides in the Pechenga-Varzuga Zone is complicated by the tectonic zonation: There are the dome-block domains the first folding phase of which resulted from the vertical movements, and the second folding phase reflected the horizontal compression which in turn resulted in thrusting as well. Stratigraphic interre­lations were greatly complicated by faults that have disturbed the succession. The initial inter­relations between only two lower units may be considered to be established. The upper basal unit has survived in the Imandra-Varzuga structure and is located in synclinal folds cores. A basalt unit of the South Pechenga domain related earlier to this stratigraphic level is correlated now with the volcanics of the Pil'gujarvi Formation by the presence in it of the marking felsic tuff horizon with typical carbonate concretions. The reliable stratigraphic in­terrelations between the third from the bottom unit of greenstone schists and picritic por­phyrite tuffs, and the lower units in the South Pechenga domain, are not established yet. As to the structure type, it is similar to that of the pyroclastic-terrigenous part of the Pil'gujarvi subformation in the south-eastern part of the North Pechenga domain (Lammas Lake).

The peculiar features of the Karelian Complex composition make it possible to recon­struct climatic conditions on land by the lithology of terrigenous associations and those of sedimentation basins by the composition of chemogenic rocks [43]. As stratigraphic indica­tors, used were other signs that allow to suppose a regional development and change of pecu­liar conditions of lithogenesis. They are alga horizons, the salt content in water of palaeobasins, widespread crusts of chemical weathering, tillites and regional unconformities. Of correlation importance are the following temporal boundaries. A distinct change of humid environment for arid has occurred on the boundary between the Sariolian and Upper Jatulian times. In sections, the action of climatic factors has resulted in that the Jatulian rocks are often red-coloured and occur on two levels in most sections. The lower level contains gray- and pink-coloured terrigenous and carbonate-terrigenous formations, and the upper one - distinct motley- and red-coloured rocks. A climatic change has been distinctly determined on the boundary between the Jatulian sections with aridity signs and the Zaonezh'e (Ludikovian) formations possessing humidity signs. An indicator of this climatic change is the development of carbonaceous-black shale complexes [28].

The complex of the regional correlation elements has permited to create a lateral set of synchronous formations shown in the common stratigraphic column sections (Fig.2).

The correlation of these isolated sections is complicated by lack of reliable geochronolog­ical data. At present, the K-Ar age determinations of glauconite and rocks are from 1050 Ma to 670 Ma and correspond mainly to the Upper Riphean [11]. Besides, some researchers say about the resetting of the K-Ar isotope systems, which has resulted in wrong ages, and these ages may not coincide with true ones. The conclusions made on the base of biostratigraphic data are more definite. By microphytological analyses data [41], the sediments of the Tur'inskaya and Terskaya Formations are quite definitely related by the complex of micro­phytofossils to the Middle Riphean, besides, the Tur'inskaya Formation is distinguished as the older one similar to the Yurmatinskaya Group of the South Urals, and the Terskaya Formation is similar to the lower part of the Serdobskaya Group.

Late Riphean deposits in the Sredny peninsula and sedimentary complexes situated in the Kil'din peninsula and are correlated with them, are characterized by both microfossils set typical to the Upper Riphean and alga stromatolites Gymnosolen ramsayi Steinm. Lately, by microphytofossils set [41], the Upper Riphean age is determined for the upper part of the Sredny peninsula section (the Kuyakanskaya and Pumanskaya Formations), which was earlier compared with the Vendean formations of the Tanafjord Group in the Varanger peninsula.

A more detailed comparison between the sections of the Sredny peninsula and the Kil'din Island is made on the base of lithological correlations. In the section of the Sredny peninsula distinguished are deposits of several transgressive-regressive cycles, they being separated by

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• 
  - see Fig.3 on Page 30.
  

stratigraphic and, perhaps, angular unconformities [37, 46]. Of most distinction are two lower cycles characterized by the sedimentation, at first, of current formations, and then deltaic and near-sea-coast formations up to the remote shelf ones during transgressive stages and of the reverse sequence completed by the deposition of carbonatic formations of drying up lagoons during regressive stages. Such a conformity is not observed in the Kil'din Island, but the first cycle regressive stage and the transgressive stage of the second cycle are supposed to be present in that section. More detailed correlations are, probably, uncorrect at present.

The position of the turbidite sediments complex in the Rybachy peninsula is highly dis­putable at present. The propositions stated by A.A. Polkanov [50] about the older age and a thrust tectonic boundary between the rocks of the Rybachy complex and the sediments of the Sredny peninsula, have been revised basing on the geological and petrographic studies [46, 57]. These researchers, after Ye.M. Lyutkevich and L.Ya. Kharitonov, supposed the normal stratigraphic interrelation to occur. At present the most substantiated notions are those of the Middle Riphean age of the deposits [37]. They are based on the similarity between the de­posits observed in the Rybachy peninsula and the Kongefjord Formation of the Varanger peninsula. Noted practically are a full likeness between the turbidity flow deposits composing the Perevalnaya, Zubovskaya and Tsypnavolok Formations and the absence of high dense current deposits being the analogues of the Motovskaya and Lonskaya Formations. In addi­tion, the Middle Riphean age of the Kongsfjord Formation deposit being sufficiently cor­rectly substantiated, a rather close location of the sections (not far than 100 km), correlation with the Timan Middle Riphean turbidite formations [27], its being located in all three re­gions northwards from the established thrust zone make it possible to relate the units of the Rybachy peninsula to the Middle Riphean.

A correlation scheme of all the upper Proterozoic formations of the Varanger and Kola peninsulas, and the Urals is, probably, far from being completed, but its principal outlines may be now suggested on a base of ideas in [37, 50] and data in [38, 41, 73] (Table).

Table. Correlation of Upper Proterozoic deposits in the Kola peninsula and northern Norway.

Note: age boundary – 10⁶ years; groups: Ryb – Rybachy, Kil - Kil'din, Vol – Volokovaya, Bar. Sea – Barents Sea, Lok – Lokvikfjellet; formation – italics.

INTRUSIVE COMPLEXES

The scale of this Map made it impossible to exhibit volcano-plutonic associations, which will be, nevertheless, noted while characterizing concrete magmatic complexes. All said above concerns the dykes united into four regional groups of different age. Anatexitic granites widespread all over Late Archaean complexes are not considered here by the authors. Their development took place during a very long period of time and resulted from metamorphism of supracrustal rocks under amphibolite and granulite facies conditions.

The characteristics given below is mainly based on the data compiled in the works men­tioned above.
Archaean Erathem
The oldest plutonic rocks are tonalites, trondhjemites, granodiorites and enderbites re­lated earlier to the ancient Saamian basement. We have excluded them from the basement taking into account data about their geological setting and isotopic age (see the section "The basement complex"). The given rocks are regarded to the "tonalite-trondhjemite-granodiorite" association [9]. They are rather distinctly established in those cases when they are of intrusive origin, which differs them from chemically similar rocks of the basement complex. These rocks are widely spread in the Murmansk block and in the Kola-Norwegian zone.

The emplacement of the earliest intrusive basites and harzburgites is considered to be

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* - Russian geologists use the term "magmatic formation" (literal translation), which means a natural comagmatic igneous rocks association that naturally appears in a certain geological setting during the development of Earth's crust geotectonic elements of the same kind, i.e. they differ from each other only in age (A Dictionary of Geology, V.2. 1978, Moscow, p.381)(V.V.B.).

linked with the formation of a wide spectrum of volcanic rocks related to the Lopian strati­fied complexes. The initial ultrabasic and basic magmatism is not characteristic of the older Kola-Belomorian paragneissic complex. The consideration of the Annama unit with the basic volcanic rock horizons as a member of this complex makes no difference because the Al­larechka Complex harzburgites being spatially connected with the Annama unit rocks have no genetic relationship with the latter for there are no any their volcanic analogues in the unit's section.

The rocks of the Central Kola and Murmansk tonalite, trondhjemite and granodiorite complexes are spread through vast areas in the Murmansk block and the Kola-Norwegian zone. Tonalite and trondhjemite are dominant. Besides, granodiorite is widely spread in the massifs of the Keivy zone and in the Tersky Complex and diorite - in the Ingozero and Tu­loma Complexes in the Kolvitsa-Laplandian zone. In the Northern Karelia Zone related to this group are diorites and TTG rocks occurring in the Pyaozero Lake area, including nu­merous bodies of gabbroic rocks. As a whole, mostly spread are TTG rocks. The contacts between the granitoids and the country rocks are clear, conformal or cross-cutting; some­times they have contacts with breccia and migmatite zones. Along with the independent massifs, TTG rocks form widely spread migmatite fields. The latter are mapped southwards the Nota river, to the north-west of the Salny Tundra Mountains, in the upper flow of the Strel'na River and eastwards from the Kuolajarvi area.

Peculiar features of all the granitoids considered are their location in the cores of domes and the homogeneous composition, they are also migmatized and microclinized, and contain pyroxenite and gabbro inclusions. By their age and composition, they are close to the or­thogneisses of the Kola-Belomorian Complex, which allows to suppose their being possessed to a single volcano-plutonic association.

Most the massifs are differentiated. Leucogabbros and gabbro-anorthosites predominate in the Tsaga lopolith and sheet-like bodies of the Acha Complex. Typical to them are peri­dotites and pyroxenites with Fe-Ti ores. Olivine gabbroids are widely spread in sheet-like bodies of the Bear Lake and Poger-Yavr Complexes. These two complexes are characterized by dykes d₁, which are composed of gabbro-anorthosite, leucogabbro, gabbro-diabase, olivine gabbro and hypersthene gabbro are also spread in the Murmansk Block. Porphyroclast-like plagioclase is characteristic of all these rocks. Specific chemical features for most the rocks under consideration are the high Fe and Ti content, the higher alkali content, K playing a considerable role. The lowest Fe content is characteristic of gabbroids of the Upper Ponoy and Murmansk Blocks.

The rocks have been affected by subalkali and alkali granites. The age of zircon from the Bear Lake Complex gabbro is 2610+/-12 Ma, but it could have been captured from the country rocks [45].

The bodies of quartz monzonites, monzodiorites and syenites show a spatial link with the gabbro-anorthosites mentioned above. These plate-like bodies are situated above a large gab­bro-anorthosite body of the Bear Lake Complex and are changed southeastwards to dykes in the basement gneisses. Monzonite, monzodiorite and syenite veins are widespread in the marginal part of the Tsaga massif. The rocks in question are characterised by chemical and mineral features of the gabbros associated with them.

The Jokan'ga and Verkhneponoysky (Upper Ponoy) complexes of subalkali granites com­plete the late Archaean magmatism. Their spatial link with the gabbro-anorthosites is estab­lished. The granites are represented by the largest Jokan'ga Massif and by some smaller mas­sifs in the Murmansk Block. In the Upper Ponoy Block the granitoid massifs are spread southwards of the Bear Lake Complex gabbro-anorthosites. The contacts between the gran­ites under consideration and the basement gneisses, older TTG plutonic rocks and gabbro-anorthosites are cross-cutting. The massifs are represented by dykes and sheet-like bodies. The most widespread rocks are represented by lepidomelan-ferrogastingsite, while monzonites and granodiorites occur to a lower extent. Chemically, the rocks are similar to rapakivi gran­ites. The age of 2760+/-80 Ma obtained for granitoids [53] needs to be revised for it contra­dicts the observed succession of the emplacement of the Upper Archaean plutonic rocks.