A. V. Chistyakov, E. V. Sharkov, T. L. Grokhovskaya, O. A. Bogatikov, and G. N. Muravitskaya
Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry (IGEM)
N. G. Grinevich
Karelian Geological Survey, Petrozavodsk, Russia
The Europe-largest Burakovka Early Paleoproterozoic layered pluton of basic and mafic rocks (Figure 1), also known in literature as the Aganozero-Burakovka and Burakovka-Aganozero pluton, is located east of Lake Onega in an area almost wholly covered by Quaternary sediments. It was discovered in the early 1950s by drilling in the area of a high magnetic anomaly. Since 1964 extensive geological exploration and geophysical measurements have been made in the area under the guidance of V. A. Ganin, N. G. Grinevich, and V. N. Loginov (Karelian Geological Survey) for the purpose of the thorough study of the massif and the assessment of its ore potential. Along with these field operations, extensive research was done by geologists, petrologists, and geochemists in the Leningrad (St. Petersburg) and Moscow universities, in the Leningrad Mining Institute, and in many research institutes of the Russian Academy of Sciences, located in Moscow, St. Petersburg, and Petrozavodsk.
However, in spite of this extensive field work and laboratory research the Burakovka Pluton remains to be not enough thoroughly known. The papers available [Berkovskii et al., 2000; Chistyakov et al., 2000; Nikolaev et al., 1995; Sharkov et al., 1995a, 1995b] give but a general idea of the structure and composition of the Burakovka Pluton. This especially concerns its cumulate stratigraphy and the resultant vagueness of its structure and relationships among its different portions. The new data obtained in this study help to revise many views on the petrology of the massif, which is the subject of this paper.
The Burakovka layered pluton is localized in the southeast of the Baltic Shield in the Vodlozero block of the Karelian granite-greenstone province. This block is composed mainly of Archean tonalite gneissic granite ("grey gneiss") with an irregular network of greenstone belts. Some relict areas of the block include the oldest rocks of the Baltic Shield, namely tonalite gneiss with a model Sm-Nd age of ca. 3.54 Ga [Sergeev et al., 1990] and fragments of greenstone belts with an age of 3.45 Ga [Pukhtel et al., 1993].
The Burakovka Pluton has an irregular oval form, curved in map view, and is elongated in the NE direction. It is 50 km long, 13-17 km wide, and is as great as 630 km 2 in area. According to geophysical data, its thickness ranges between 5-7 and 10 km [Sobolev, 1993]. Earlier [Garbar et al., 1977], the massif was believed to be one intrusive body, and its present-day structure was interpreted as the result of the rejuvenation of tectonic activity in Late Proterozoic time, when the central and eastern parts of the intrusive massif had been raised stepwise along combined normal and strike-slip faults mainly of a roughly N-S strike. As a result of this faulting, the massif was broken into three main tectonic blocks: Burakovka, Shalozero, and Aganozero. The inferred magnitude of displacement between the latter two blocks was 1-2 km. In the previous reconstructions of the internal structure of the pluton and its generalized cross-section, the Aganozero "block" was interpreted as the lower part of the once united whole massif, and the Shalozero and Burakovka blocks, as its middle-upper parts.
At the same time, the geophysical sections (Figure 1) revealed that the Aganozero "block" was an independent funnel-shaped body. The Shalozero and Burakovka "blocks" are divided by a system of small faults with insignificant vertical displacements and constitute one body of a lopolith form. Each of these bodies has its own internal structure and orientation in space: the Aganozero block is elongated in a roughly S-N direction, whereas the Shalozero-Burakovka block strikes NE. This evidence suggests that the pluton consists of two independent intrusive massifs, Aganozero and Shalozero-Burakovka, which contact each other in the region of their upper parts and occur as one massif at the level of the pre-Quaternary erosion [Berkovskii et al., 2000; Chistyakov et al., 2000].
These new findings called for a more comprehensive investigation of the massif's structure. As has been mentioned above, the massif is almost wholly covered by Quaternary sediments, this hindering the study of its internal structure. The main evidence of the latter was provided by the results of drilling comparatively shallow holes (usually drilled to depths of 200-300 m, occasionally, to a depth of 500 m, very few being drilled to depths of 1200-1500 m). Until recently, the sections across different parts of the pluton were correlated using mainly geochemical data [Nikolaev et al., 1995; Sharkov et al., 1995a, 1995b]. This approach failed to provide accurate results, because it is based on the methods of mathematical statistics and fails to account for thin layering and for the real structure of the rocks (in particular, for relations between cumulus and intercumulus phases). Accordingly, where the compositionally close intrusive massifs are correlated, this approach can provide a very general pattern, similar not only for the pluton discussed but also for the other Early Proterozoic layered intrusive massifs of the Baltic Shield [Sharkov and Smolkin, 1998]. For this reason, we undertook a special study of the cumulate stratigraphy of the Aganozero and Shalozero-Burakovka intrusive bodies, which revealed significant differences between them. The representative analyses of their main cumulates are given in Tables 1 and 2.
The best known now is the Aganozero Body (AB) which is composed mainly of ultramafic rocks. This is a funnel-shaped body with its western side cut off by N-striking faults. The central part of the body is composed of gabbroids and has a trough-shaped form elongated in the northern direction. According to the geophysical data, the magma feeder of this intrusive body was located in its SW part.
The Shalozero-Burakovka Body (SBB) has a U-shaped form with the steeper southern sides and consists mainly of basic rocks in the pre-Quaternary section, although the drilling data suggest its rocks to be generally similar to those of the Aganozero Body. Judging by the geophysical and scanty drilling data, the basic rocks are underlain there by a thick unit (more than half of the body's total thickness) of ultrabasic rocks. The location of a magma feeder for this body remains to be uncertain.
Each of the bodies has its central (layered) series and a marginal zone composed of the rocks of the marginal border group. The rocks of the upper marginal zone seem to be demolished by erosion, and those of the lower zone have not been reached by drill holes. As follows from the drilling data, the contacts between the lateral marginal rocks and the Archean basement dip toward the centers of the bodies and are usually steep. The contact surfaces dip at angles of 60-70o in the southern surroundings of the Shalozero-Burakovka Body, at 35-40o in its southern surroundings, and at 50-60o in the Aganozero Body, the layering in the marginal border groups being conformable with the contact surface. As follows from the low-angle layering and trachytoid texture of the rocks from the central series ( 30o in the peripheral parts of the intrusions to 10o and less in their central parts), both bodies have internal structures independent of their contacts.
Ultrabasic zone (UZ) compose the lower units of the layered series in both bodies. As indicated by geophysical data, the total thickness of this zone in the Aganozero Body is about 6000 m, whereas in the Shalozero-Burakovka Body this zone is as thick as ca. 2 km. At the pre-Quaternary erosion level, this zone is most complete in the Aganozero Body, especially in its southern part. The holes drilled in the SBB penetrated merely the top of this zone. The ultrabasic zones have fairly distinct upper contacts which in most holes drilled through both bodies coincide with the major chromite horizons (see below). As follows from the drilling data, all rocks of this zone in the AB are serpentinized to different extents, at least to a depth of 700-900 m below the modern erosion level.
The internal structure of the ultrabasic zones is distinguished by a relatively invariable mineral composition of the rocks consisting mainly of olivine (Ol)-chromespinel (Chr) cumulates. Based on the ratio between the cumulus and intercumulus phase minerals, two subzones were distinguished in the Aganozero Body: the lower dunite subzone (2500-3000 m thick) consisting of dunite and olivinite, which are often devoid of intercumulus, and the upper peridotite subzone (200-400 m thick) composed mainly of poikilitic peridotite with intercumulus pyroxene and plagioclase amounting to 30-40 vol.%. The dunite subzone includes a few indistinct rhythms ranging between a few tens and a few hundreds of meters in thickness. The lower parts of the rhythms (75% of their thicknesses) are composed of dunite consisting of cumulate olivine (95-98 vol.%) and a small amount of cumulate chromite (1-2 vol.%), whereas the upper parts of the rhythms consist of interstitial pyroxene and some plagioclase and phlogopite (10-15 vol.%). In the peridotite subzone, the amount of poikilitic peridotite increases upward with the total growth of interstitial pyroxene (mainly clinopyroxene) up to 20-45 vol.% and plagioclase up to 10 vol.% with a decline of cumulate olivine to 50 vol.%. The two-member implicit rhythmic pattern is characteristic of this zone, too, except that its dunite interlayers have a minimum thickness here.
The upper sequence of this zone includes several chrome-spinel ore units ranging between 0.5 and 4.5-7 m in thickness, where the chrome-spinel content is as high as 50-75 vol.%. The thickest of them (Major Chromitite Horizon, MCH) was traced in the very top of the zone and often coincides with the contact of the overlying pyroxenite zone. This unit varies greatly in thickness and is lacking in some holes. It has a maximum thickness (5-6 m) in the central segment of the Aganozero intrusive body, where it was recorded in all holes drilled through this segment of the body, and also traced in the pre-Quaternary erosion surface in the form of occasional outcrops.
The internal structure of the main chromite unit is characterized by a frequent intercalation of peridotite, chromitite, and occasional dunite interlayers. The compositions of its dark-color minerals vary toward the growing Mg content. The contents of chrome-spinel in some layers vary from 10-15 to 60-70%. Grains ranging between the first fractions of mm and 1-2 mm in size occur as numerous accumulations among intercumulate pyroxenes and plagioclase, and in some rare cases as inclusions in olivine. In the chromite ore proper, chrome-spinels occur as veinlets, commonly restricted to contacts between the grains of interstitial pyroxenes or to cracks in the latter.
A layer combining several closely spaced thin chromitite interlayers, known as the Yakozero Unit, was found in the eastern segment of the Aganozero intrusive body in the peridotite subzone below the Main Chromite Unit (MCU).
Most of the holes showed a chromite ore layer, 5-7 m thick, in the top of the Ultrabasic zone (analog of the MCH of the Aganozero intrusive body).
To conclude, the main distinctive feature of the Ultrabasic zone in the SBB is the predominant development of poikilitic peridotite with the high contents of interstitial minerals, dominated by orthopyroxene and plagioclase, with the ubiquitous presence of phlogopite.
Pyroxenite zone (PZ) with a thickness of 190-200 m is exposed in the pre-Quaternary erosion surface of the Aganozero intrusive body and occurs merely as fragments in the Shalozero-Burakovka intrusive body, where its thickness is as small as 20-80 m. The upper contacts of these zones are marked by the ubiquitous occurrence of cumulate plagioclase.
The middle section of the pyroxenite zone includes a peridotite marker horizon, whose thickness grows from a few meters in the margins of the intrusion to a few tens of meters in its middle (see Figure 9). The marker usually consists of Ol+Chr cumulates with 70-75 vol.% of olivine in its middle and of Ol+Chr+Opx cumulates with 50-55 vol.% of olivine in the top and base.
Gabbronorite zone (GNZ) is best developed in the central part of the Aganozero intrusive body, where it has a maximum thickness (500 m), and was also recorded at the periphery of the Shalozero-Burakovka Body. The lower contact of this zone is defined by the stable development of cumulate plagioclase, and the upper, by the absence of orthopyroxene and the appearance of inverted cumulate pigeonite in the overlying rocks.
Aganozero intrusive body. In the central part of this body the gabbronorite zone has been subdivided into two subzones: the lower or banded subzone, 200-250 m thick, and the upper subzone having a roughly similar thickness.
The lower (layered) subzone is distinguished by the interlayering of websterite (Cpx+OpxOl Chr cumulate), orthopyroxenite (OpxOl Chr), norite (Pl+Opx), gabbronorite (Pl+Opx+Cpx), gabbro (Pl+Cpx), and gabbro-anorthosite (Pl). Based on the ratios of the cumulate minerals in the sequence, we distinguished several rhythms ranging from 20 to 185 m in thickness and having indistinct contacts between them. The base of each rhythm is dominated by Cpx. The olivine and chrome spinel that are also present there vanish toward the top of the subzone, where the dominant mineral in the gabbronorite is orthopyroxene. The tops of the rhythms were found to be enriched in sulfides and platinum-group minerals (PGM).
The lower part of this subzone contains a marker horizon, composed of interbedded poikilitic peridotite and olivine pyroxenite, varying within 3-5 m in thickness (see Figure 1). Another thinner marker horizon was found in the top of the sequence. It consists of poikilitic peridotite, bronzitite, and websterite containing up to 10-15 vol.% of chrome spinel in the top. These rock units are distinguished by the most magnesian compositions of their pyroxenes compared to the other rocks of the gabbronorite zone. The olivine from the rocks of the lower marker horizon occurs in isometric grains < 1.5 mm in size, whereas that of the upper marker is represented by very small (commonly a few fractions of mm), almost round grains. In some sites the marker-horizons rocks were found to contain Cu-Ni sulfide.
The upper subzone is composed mainly of gabbronorite including gabbronorite-anorthosite interlayers. A coarse layering was observed expressed in the alternation of meso- and leucocratic gabbronorite varieties and occasional anorthosite layers. The cumulate minerals are plagioclase (65-78 vol.%), augite (7-14 vol.%), and orthopyroxene (5-20 vol.%).
In the Shalozero-Burakovka intrusive body, the Gabbronorite zone has a thickness of < 100 m and is composed mainly of gabbronorite with interlayers of anorthosite, norite, and gabbro and resembles the upper gabbronorite subzone from the Aganozero Body. Orthopyroxene is seen to dominate in the total content of pyroxene in the rocks upward. The amount of cumulate plagioclase varies within 55-70 vol.%. The interstitial minerals are quartz, magnetite, and apatite, as well as some poor pentlandite-pyrite-chalcopyrite sulfide mineralization.
To sum up, the distinctive features of the Gabbronorite zone in the SBB are (1) its small thickness, (2) the absence of a different cumulate layered unit in the lower part of the zone (that is, the absence of any analog of this subzone of the Aganozero gabbronorite zone), (3) the absence of any distinct rhythmic zoning, (4) the absence of peridotite markers, and (5) a relative enrichment in sulfides.
Pigeonite Gabbronorite Zone (PGZ) was observed mainly in the Shalozero-Burakovka intrusive body and is poorly developed in the central part of the Aganozero body.
This zone is composed of gabbronorite with reversed pigeonite (Pig) and pigeonite-augite (plagioclase-two pyroxene and less common plagioclase cumulates). The northeastern segment of the Shalozero-Burakovka intrusive body showed a poorly expressed layering produced by the alternation of the layers of meso- and leucocratic gabbronorite varieties ranging between 1.5 and 10 m in thickness. Almost all of the studied samples were found to contain interstitial quartz, biotite, and K-feldspar in the amounts of < 5 vol.%, and also magnetite and apatite. The pyroxenes are often amphybolized. The gabbronorite is often seen to be cut by plagiomicrocline granite bodies ranging between 0.5 and 50 m in thickness (see below).
Several boreholes revealed a plagioclase harzburgite marker horizon (Ol+Opx+Chr cumulus) varying from 0.6 to 2.2 m in thickness in the gabbronorite of this zone's top.
As to the central segment of the Aganozero Body, only the tops of the borehole sections were found to contain gabbronorite with the cumulate grains of inverted pigeonite, which could be attributed to the pigeonite gabbronorite zone.
Magnetite Gabbronorite-Diorite Zone (MGDZ) is the uppermost zone in the sequence of the layered-series rocks in the Shalozero-Burakovka intrusive body and is absent in the Aganozero body. The cumulus consists of inverted pigeonite-augite (10-29 vol.%) and pigeonite (9-22 vol.%), titanomagnetite (3-12 vol.%), and plagioclase (oligoclase-andesine) (42-75 vol.%); interstitial K-feldspar and quartz, as well as apatite, were found occasionally in the top of the zone.
The rocks of this zone show a vague rhythmic pattern produced by the alternation of melanocratic and mesocratic rocks. The former were found to contain as much as 10-12% cumulate magnetite and as much as 51% pyroxene. The content of magnetite declines to 1-5% and that of pyroxene, to 20-35%, in the latter. Anorthosite interlayers were recorded in the tops of the rhythms.
Concluding the description of the cumulate stratigraphy of the Burakovka Pluton, it should be mentioned that both intrusive bodies show the following succession of the main typomorphic cumulates: Ol Chr (Ultrabasic zone) Opx Cpx Ol Chr (Pyroxene zone) Opx + Cpx + Pl Ol (Gabbronorite zone) Pig + Pig-Aug + Pl (Pigeonite gabbronorite zone) Pig + Pig-Aug + Pl + Mag (Magnetite gabbronorite zone which is absent in the Aganozero body, where its seems to have been eroded). In this respect the Burakovka Pluton does not show any differences from most of the Early Paleoproterozoic layered intrusions of the Baltic Shield, and also from the classical massifs, such as Bushveld (South Africa) and Stillwater (North America).
The Marginal Border Group is represented in both intrusive bodies by a group of contact rocks which usually occur as a band < 200-400 m wide along the periphery of the massif. Three zones can be distinguished in the sequence of the marginal rocks: a zone of direct contact, an outer (banded) zone, and an inner zone.
In the southern surroundings of the Shalozero-Burakovka intrusive body, the zone of direct contact is composed of microgabbronorite enriched in accessory sulfides of a pentlandite-pyrite-chalcopyrite assemblage and in titanomagnetite (totaling 5-10%) and ranging between 0.4 and 0.9 m in thickness. In the Aganozero Body, the contact gabbroids are represented by amphibolized quartz-bearing norite and gabbronorite, roughly 120 m thick (Hole 196), sometimes porphyritic, containing large (up to 2 mm long) plagioclase (An 44-63 ) crystals, often normally zoned, in a fine- and medium-grained plagioclase-pyroxene groundmass with small ( < 1 mm) veinlets and fine-grained patches of irregular form. The rock includes single sulfide and magnetite grains everywhere. There are occasional pyroxene and anorthosite interlayers and also fragments of a quartz-feldspar rock, which can be interpreted as the xenoliths of the host tonalite granite gneiss. A similar sequence was observed in the northeastern surroundings of the Shalozero-Burakovka intrusive body.
The outer (banded) zone, 10 to 210 m thick, occurs in the surroundings of the Shalozero-Burakovka intrusive body. It is composed mainly of fine- to medium-grained usually leucocratic gabbronorite (clinopyroxene Wo47En38Fs15, orthopyroxene Wo5En65Fs30, plagioclase An47 ) with interlayers of plagioclase websterite where ortho- and clinopyroxene grains are enclosed in plagioclase oikocrysts. The outer zone is mostly absent in the Aganozero body, where the near-contact gabbroids are replaced abruptly by the peridotites of the inner zone. The exception is Hole 177 drilled in the south of this body, where the rocks of the outer zone occur as a 20-meter layer of medium-grained plagioclase websterite which is changed by contact gabbroid closer to the contact.
The inner zone, 20 to 170 m thick, is present in both bodies and consists of chrome-spinel-bearing (1-5%) poikilitic serpentinized peridotite with occasional interlayers of plagioclase websterite in the Aganozero intrusive body and with those of websterite and less common peridotite in the Shalozero-Burakovka Body. The data available suggest that this difference in the structure of the internal zones depends on the type of the layered-series rocks, with which they are in contact. For instance, in the AB, the inner zone contacts the ultramafic rocks, whereas in the SBB, it contacts the mafic rocks in the west and the ultramafic rocks of the layered series in the east. The olivine from the peridotite samples collected in Hole 28a showed a magnesian number which was found to be the lowest for the rocks of the pluton (Fo52 ), the ortho- and clinopyroxenes yielded En78.5 and En46.6, respectively, and the interstitial plagioclase, An34-38. In the inner zone, the contents of SiO2, TiO2, Al2O3, FeO, Na2O, and K2O grow toward the contact with a decreasing MgO content caused by the enrichment of the marginal peridotite in interstitial plagioclase.
Therefore, the differences recorded in the structure of the marginal series can be associated with the composition of the adjacent rocks of the layered series. The most complete sequence of the rocks was observed in the Shalozero-Burakovka Body where we recorded a transition from the near-contact fine-granular gabbronorite via fine- to coarse-granular gabbronorite to plagioclase websterite and further, via its olivine variety, to the rocks of the layered series.
The compositions of minerals were determined throughout the sequences of the layered series of the Aganozero and Shalozero-Burakovka bodies. Measurements were made at IGEM RAN using an MS-46 Cameca microprobe with the experimental conditions of 15 kV and 50 mA. The time of measuring each element was 70 s.
Olivine is the main cumulate mineral in the ultrabasic zones and in the overlying peridotite marker horizons. In a 200-meter section examined in the top of the ultrabasic zone in the Shalozero-Burakovka Body, the Mg content of olivine declines up the section from Fo88 to Fo84 and increases again to 86-89% Fo in the Main Chromite Horizon, with the NiO content in the olivine varying between 0.45 and 0.55% and tending to decline up the zone. In the peridotite marker horizon of the Pyroxenite zone, olivine is more ferrous (Fo79-80 ) with the NiO content varying between 0.37 and 0.43%. Therefore the olivine composition varies from Fo88 to Fo79.
In the Ultrabasic zone of the Aganozero Body, the Mg content of olivine declines upward from 90 to 86% Fo, increasing again to Fo88 in the MCH. The relict olivine grains in the pigeonite-augite clinopyroxenite from the Pyroxenite zone showed a comparatively constant composition (Fo82-83 ). At the base of the Gabbronorite zone the olivine gabbronorite contains olivine with 80% Fo, its composition being similar to that of olivine in the lower peridotite marker horizon (Fo80-81 ). In the upper marker, olivine is more ferroan (Fo76 ). Therefore the Mg content of olivine varies from Fo90 to Fo76 up the section of the Aganozero Body.
Almost all of the olivine grains analyzed in the Burakovka massif are devoid of silica and Ca oxide, the olivine always containing isomorphous NiO (0.3 to 0.56 wt.%).
In the Ultrabasic zone of the Shalozero-Burakovka Body, the Mg content of the orthopyroxenes declines upward along a 200-meter section from En86 to En83, increasing again to 85-86% En in the main chromite unit. In the Pyroxenite zone, orthopyroxene has a Wo4En78Fs18 composition in the websterite below the marker and is more ferroan (Wo5-6En73-75Fs21-22 ) above it. In the gabbronorite interlayer of the Lower Pyroxenite horizon orthopyroxen has a Wo3En71Fs26. In the peridotite marker horizon, Opx contains 4-5% Wo with 77-78% En. Orthopyroxene has a composition of Wo4En66Fs30 in the Gabbronorite zone.
The pyroxene from the Pigeonite gabbronorite zone is represented by pigeonite (Pig) and pigeonite-augite (Pig-Aug). Orthopyroxene (Wo1.5-5En51-54Fs43-44 ) occurs as lamellas in the inverted Pig-Aug and as a matrix in the inverted Pig with a normal zoning: Wo1-3En54-58Fs41-44 in the cores and Wo2En46-54Fs44-52 in the margins. Therefore the lamellas in Pig-Aug and the matrix in the inverted pigeonite have almost the same composition. In the upper marker horizon, the orthopyroxene composition (Wo4En79Fs17 ) is close to that of the marker in the Pyroxenite zone. The composition of the orthopyroxene matrix of the inverted pigeonite from the magnetite gabbronorite zone varies upward from Wo2-5En54-56Fs40-44 to Wo4En38Fs59. Therefore the total variation in the Opx composition, including the exsolution texture compositions of the inverted Pig and Pig-Aug upward the layered series in the Shalozero-Burakovka Body, is Wo4En86Fs10 to Wo4En38Fs59.
In the Aganozero Body, Opx appears later than Cpx in the form of single interstitial grains in the rocks of the Ultrabasic zone. Orthopyroxene occurs as a cumulate mineral above this zone. In the Pyroxenite zone the quantity of this mineral is also subordinate to that of Cpx which occurs as scarce cumulate crystals, often highly corroded by the adjacent pigeonite-augite grains. The orthopyroxene has a Wo3En76Fs21 composition. In the lower intervals of the Pyroxenite zone, the pyroxenite contains, along with inverted metasomatic pigeonite-augite, inverted pigeonite showing exsolution textures in the form of coarse lamellas parallel to (001) and small augite grains of irregular form. The composition of the orthopyroxene matrix is Wo2En80Fs18, and that of the augite lamellas, Wo45-47En45-47Fs8.
The cumulate Opx has a composition of Wo3En76Fs21 in the lower part of the Gabbronorite zone, and Wo4En62Fs34 in the upper. There are orthopyroxenite interbeds where orthopyroxene has a composition of Wo3En65Fs32. The interstitial Opx in the peridotite marker horizons is more magnesian: W4En80Fs16 in the lower unit, and Wo3.5En75Fs21.5 in the upper.
Some holes drilled in the central part of the Aganozero Body intersected gabbronorite containing inverted pigeonite with an orthopyroxene matrix having a composition of Wo2En63Fs35, this pigeonite being the most ferrous one in the AB.
To conclude, the widest Opx variation range in the layered series of the Aganozero Body is Wo3En80Fs17 to Wo2En63Fs35.
High-Ca pyroxenes are represented by augite and pigeonite-augite, the latter being usually inverted. Some samples were found to contain uninverted pyroxene grains. The clinopyroxene compositions are presented in Tables 7 and 8 and displayed in Figure 13. In the Shalozero-Burakovka Body Cpx is present throughout the section of study. In the Ultrabasic zone, augite is among the main intercumulus phases, and its En content declines upward from En51 to En44 with some small variations throughout the section. In the main chromite unit, augite has a more magnesian composition, similar to bronzite. Single Pig-Aug grains (Wo38En51-55Fs7-11 ) were found in the ultrabasic zone. In the Pyroxenite zone, augite is substantially more ferroan, Wo46-48En43-45Fs9, in the pyroxenite below the peridotite marker horizon - Wo47En40Fs12 and in the gabbronorite interlayer - Wo47En40Fs12; the composition of intercumulus augite is Wo42-46En46-49Fs9-10 in the peridotite, and Wo47-48En41-42Fs10-12 in the upper pyroxenite layer. Augite has a composition of Wo46En40Fs14 in the Gabbronorite zone.
In the zone of Pigeonite gabbronorite, the composition of the augite matrix in the inverted pigeonite-augite varies from Wo45-46En34-36Fs18-19 in the cores of the grains to Wo43-45En35-36Fs20-21 in their rims, the composition of the inverted pigeonite lamellas being Wo44-47En40Fs16-19. In the Magnetite gabbrodiorite zone, the iron content of the augite matrix in the inverted Pig-Aug grows steadily up the section from Wo46-47En35-36Fs17-18 to Wo16En28Fs26. Some samples (e.g., 47/83.9) were found to contain pigeonite-augite with a composition of Wo33En30Fs38. Thus, the total variation of the augite composition across the layered series was found to be Wo43En51Fs6 in the ultrabasic zone to Wo46En28Fs26 in the zone of Magnetite gabbrodiorite.
In the Aganozero Body, Cpx is also among the main rock-forming minerals. It occurs as angular interstitial grains in the dunite of the ultrabasic zone, and as large oiko-crystals in the peridotite from the top of the zone. The Cpx composition in the ultrabasic zone corresponds to augite with a low iron content, minimal for the marginal rocks, which generally grows up the section from 4.5 to 7.8% Fs.
In the Cr-bearing peridotite of the Main Chromitite Horizon, clinopyroxene is slightly more magnesian(Wo43-46En47-50Fs6-7 ) and shows an elevated Cr2O3 content (up to 1.0-1.2 wt.%).
Clinopyroxene is more ferroan in the Pyroxenite zone. The clinopyroxene matrix in the inverted Pig-Aug from the metasomatic clinopyroxene and websterite has a Wo45-47En44-47Fs8-9 composition. The primary composition of pigeonite-augite, obtained with a defocused beam, was found to be Wo36.2En52.4Fs11.4, which is generally close to the Wo33En53Fs14 composition found for the undecomposed portion of one of the grains. The cumulate augite was found to be Wo47En44Fs9. In the wehrlites (Sample 16/159) augite is more magnesian - Wo45-47En44-46Fs8-9. Analyses of individual pyroxene grains did nor reveal any zoning, but showed some heterogeneity expressed in the compositional variations (mainly in terms of a Fe/Mg ratio) in some regions of the grains. All clinopyroxenes from the ultramafic part of the sequence were found to contain 0.12-0.8% Cr2O3 and < 0.08% NiO.
Above the Pyroxenite zone, augite occurs mainly as a cumulate mineral in a plagioclase-two pyroxene assemblage. In the Gabbronorite zone, Cpx is more ferroan (Wo45-46En41-44Fs10-13 ). Its Cr2O3 content is not higher than 0.1% and that of NiO, 0.09%, the most common values being 0-0.06% and 0.05%, respectively. In the peridotite marker horizons, the interstitial clinopyroxene has a composition of Wo44En47Fs10, this composition being most magnesian for the Gabbronorite zone. In the Pig-bearing gabbronorite from the top of the layered series, clinopyroxene has the highest iron content and a Wo45-46En40Fs14-15 composition.
To conclude, viewed along the section of the layered series in the Aganozero Body, the clinopyroxene composition varies from Wo47En48.5Fs4.5 at the bottom to Wo45-46En40Fs14-15 in the upper pigeonite-bearing gabbronorite.
In the Ultrabasic zone of the Shalozero-Burakovka Body, plagioclase is highly chloritized. As a result, we were able to examine merely two of the single grains: one of labradorite Or3Ab51An46, the other of oligoclase-andesine Or3Ab66An30. In the MCH, the interstitial plagioclase is represented by even more silicic oligoclase Or1Ab84An15. Intercumulate plagioclase has a composition of Or2-3Ab42-43An55 in the Pyroxenite zone below the marker horizon and is represented by labradorite Or2Ab44An53-54 and scarce oligoclase (Or2-4Ab71An25-27 ) above it. Plagioclase has a labradorite composition (Or4Ab45An51 ) in the rocks of the marker horizon.
Plagioclase is a cumulate mineral in the rocks above the Pyroxenite zone. In the Gabbronorite zone it is almost wholly unzoned and has a composition of Or2-4Ab46-48 An50-52. Plagioclase is slightly normally zoned in the Pigeonite gabbronorite zone, where its composition varies rather widely (Or2-3Ab46-58An40-51 ). However, because this zone was poorly studied, we did not find any systematics in these variations. Some plagioclase grains showed inverse zoning, from An42 in the core to An59 in the margin. We believe this pattern to have been caused by the roiling of the settling crystals by fresh magma portions [Higgins et al., 1997]. In the Magnetite gabbrodiorite zone the composition of plagioclase varies from andesine Or3-4Ab55-57An39-42 to Or2Ab66An32. The total variation of the cumulate plagioclase across the layered series was found to be An50 to An32 (without taking into account the interstitial phases of the Ultrabasic and Pyroxenite zones).
In the Ultrabasic zone of the Aganozero Body, the composition of interstitial plagioclase varies from Or1Ab47An52 to Or2Ab54An44. In the Pyroxenite zone, the composition of the interstitial plagioclase is variable too (Or1-3Ab38-48An50-60 ). It is only in the Gabbronorite zone that plagioclase occurs as the main cumulate phase, growing more silicic up the sequence, from Or1Ab42An57 to Or3Ab35An42. Some grains are normally zoned with the An component declining by 3-4% from the cores to the margins of the grains. Reversed zoning is less common. The plagioclase composition is Or0.5-5.5Ab50-63An39-49 in the peridotite of the markers. To sum up, our study of the cryptic layering in the Aganozero and Shalozero-Burakovka bodies of the Burakovka Pluton revealed the following variations in the mineral compositions up the sequences of the layered rocks (see Figures 11 and 12): in the latter, Fo88 declines to Fo79 in olivine, En86 to En38 in low-Ca pyroxene, En51 to En28 in augite, and An50 to An32 in plagioclase; in the former, Fo88.5 declines to Fo76 in olivine, En80 to En63 in low-Ca pyroxene, En48.5 to En40 in clinopyroxene, and An60 to An39 in plagioclase. Comparing these data, one can see that the olivine and pyroxenes have similar compositions in the ultramafic rocks of the layered sequences in both bodies. In the Shalozero-Burakovka Body, the clinopyroxene is more ferroan toward the top of the sequence, this possibly being accounted for by the more complete sequence of a gabbroid portion in its layered series.
Chrome-spinels are typomorphic accessory minerals in the rocks of the ultrabasic and pyroxenite zones and in the peridotites of the peridotite marker horizons. Their compositions are presented in Tables 11 and 12.
Chrome-spinel is represented by subferrialuminochromite also in the chromite ore units of both bodies, ore varieties with higher Mg and Cr contents (subferrichromite) occurring also in the Aganozero Body. The chrome-spinels found in the Yakozero Unit are distinguished by Cr2O3 as high as 56.81 wt.% (the maximum value for the Burakovka Pluton).
A special study of chromites in the Main Chromite Horizon in the Aganozero Body revealed the presence of numerous fine Ru-Os sulfide crystals ranked with a laurite-erlichmanite group [Smirnova and Dmitrienko, 1994]. Neither accessory chromite nor chromitites from the Shalozero-Burakovka Body have been studied in this respect.
Chrome-spinel grains were found to account for 3-5 to 20-30% in the peridotite marker horizons of the Gabbronorite zone of the AB and in the Pyroxenite zone of the SBB. The chrome-spinel was identified as subalumoferrichromate or subferrialumochromite. Magnetite was found in rims around some grains and is also developed in the cracks of the latter. In some places ilmenite lamellas were found. The chrome-spinel was found to contain less Cr and more Fe.
Summarizing the results of studying cryptic layering in the Burakovka Pluton, we can state that the character and variation range of its main typomorphic minerals is similar to those in the other large layered plutons, such as Bushveld (South Africa), the Great Dike in Zimbabwe, and others. For instance, in the Great Dike, the Mg content of olivine varies up the sequence from Fo94 to Fo85, and the compositions of cumulate clino- and orthopyroxene vary from En51 to En38 and from En92 to En58, respectively [Layered Intrusions, 1996]. In the west of the Bushveld Massif the composition of olivine varies from Fo89 to Fo83, and of orthopyroxene from En89 to En30 [Layered Intrusions, 1996]. In the Stillwater massif the total changes of the composition of olivine vary through the section from Fo91 to Fo81, that of clinopyroxene, from En52 to En42, and that of orthopyroxene, from En83 to En58 [Wager and Brown, 1970]. However, in contrast to other intrusive massifs, the Burakovka Pluton is distinguished by its more silicic plagioclase, the composition of which varies up the layered rock sequence from An60 in the Aganozero Body to An32 in the Shalozero-Burakovka Body.
Vein rocks are especially widely developed in the Shalozero -Burakovka Body, where they occur as compositionally various veins ranging from a few meters to a few dozens of meters in thickness, their dominating rocks being microgabbronorite, gabbronorite-pegmatite, granophyre, and potassic granite. Vein rocks are scarce in the Aganozero intrusion.
The vein-shaped bodies of microgabbronorite seem to have been feeders that channeled fresh magma portions into the crystallizing intrusive chambers, while the gabbronorite-pegmatite bodies were local accumulations of residual magma in the cumulate rocks.
Of particular interest are vein granites. These rocks are widely developed in the northeastern part of the Shalozero-Burakovka Body, where they occur mainly in the Pigeonite Gabbronorite Zone. The granites are gray or light gray, massive, mainly fine- and medium-grained rocks which occur as cross-cutting veins ranging from 0.5 cm to a few dozens of meters in thickness. They vary in composition from biotite leucogranodiorite to K-granite. The granite veins usually have distinct, rectilinear or angular contacts with the host gabbronorite and occasionally include small fragments of the latter. Neither chill zones nor any low-temperature alterations of the host rocks were observed. The isotopic and geochemical characteristics of the granites [Bogina et al., 2000] preclude their formation by way of the basement rock melting and suggest the same source of origin, common for the granites and their host mafic rocks. The geological, petrologic, and isotopic-geochemical data available suggest that the granites originated synchronously with the gabbroids in situ within the zone of Pigeonite gabbronorite. It is believed that these granites were derived from the residual interstitial silicic melt that was retained in the almost solidified cumulates of this zone. Where the process of contraction was operating, this magma must have been sucked into shrinkage cracks under the resulting vacuum effect.
The Shalozero-Burakovka Body also includes widely developed late gabbroid dikes which are believed to be the late Paleoproterozoic formations of the Pudozhgora submeridional dike belt.
To study the compositions of the rocks typical of the Burakovka Pluton samples were collected from the zones identified in the Aganozero and Shalozero-Burakovka bodies, which had been least subject to secondary alteration, and also a few samples from the rocks of the Marginal Border Group.
Methods of Geochemical Study. The bulk compositions of the rocks were determined using a conventional chemical analysis in the Central Chemical Laboratory of the Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry (IGEM), Russian Academy of Sciences. The contents of trace elements were determined by X-ray fluorescence analysis. Several methods were used to determine REE contents. Some REE were determined by neutron activation analysis, with the radiochemical separation of "noise" elements, in the Geological Institute, Russian Academy of Sciences, using a method proposed by Lyapunov et al. . The convergence of the analytical results was estimated using a BCR-1 standard. Most of the samples were analyzed by ICP-MS in the IGEM Central Laboratory. Analyses were made using a quadrupole Plasma Quad PQ2+Turbo mass spectrometer, a VG Instruments product. The correct use of the technique was checked using certified standard solutions and an AGV international standard sample.
In spite of some insignificant differences, all rocks of the pluton showed a generally similar REE distribution trend. The total REE concentrations grow normally from the mafic cumulates at the bottoms of both bodies to the gabbronorite in their tops (Table 13). In the case of the Shalozero-Burakovka body the REE content grows from 10 ppm in poikilitic peridotite to 72.8 ppm in magnetite gabbronorite, this variation range being 2.5 to 20.6 ppm in the Aganozero Body. The rocks also show a successive enrichment in LREE: the Ce/Yb ratio grows from 13.7 to 49.2 ppm in the SBB and from 1.5 to 4.5 in the AB.
The REE distribution patterns in both bodies of the pluton are similar to the patterns observed in similar layered intrusions in the Baltic and other shields, and also in the high-Mg basalts of the same age from the Vetreny Belt. All rocks studied in the Burakovka Pluton yielded similar REE distribution patterns, characterized by the growth of REE concentrations up the rock sequence with the growing LREE contribution suggesting the accumulation of the latter in the residual melt during the directional solidification of the intrusions. The higher REE enrichments of the SBB rocks relative to the same rocks of the Aganozero Body suggest the relative enrichment of the primary magma (or magmas) parental for the former.
The isotopic studies of our rock samples were carried out in the Institute of Precambrian Geology and Geochemistry, Russian Academy of Sciences, St-Petersburg. For this purpose we collected samples along the sections across the layered series of both bodies (see Figure 2), which contain all major rock types, least subject to secondary alteration. The methods of Sm-Nd and Rb-Sr studies are described in [Amelin and Semenov, 1996], and the results are summarized in Table 14.
The intrusion of each body was apparently accompanied by the injections of new magma portions into the solidifying chambers, the evidence of this being the emplacement of peridotite marker horizons (see below). These new magma injections are characterized by lower 87Sr/86Sr(T) ratios (0.7019 for Shalozero and 0.7032 for Aganozero) and by higher e Nd (T) values ( - 1.32 for Shalozero and - 2.35 for Aganozero). The pigeonite-augite clinopyroxenite from the Aganozero body (Sample 273/5.4) showed an abnormally high 87Sr/86Sr(T) value (0.7060) and a low eNd (T) value ( - 3.58). On this basis we interpret it as a product of subsolidus metasomatism. This isotopy seems to suggest that crustal fluids were involved in the process of its formation.
Our study revealed that along with the material and structural characteristics common to the other large Early Paleoproterozoic layered intrusions of the Baltic Shield, the Burakovka Pluton has its own particular features which call for a special discussion. The most important among them are differences between the rock sequences, compositions, and ages of its individual bodies, as well as the origin of the marker horizons and the structure of the southeastern segment of the Shalozero-Burakovka Body (Hole 67).
Origin of the marker horizons. As has been mentioned above, the specific feature of the pyroxenite, gabbronorite, pigeonite gabbronorite, and magnetite gabbronorite zones in both bodies is the occurrence of thin-bedded ultrabasic-rock marker horizons. The emplacement of these rock units among the cumulates of much lower temperatures suggests the injections of fresh magma portions into the solidifying magma chamber. This new magma had a higher density and spread over the chamber floor displacing the old, more evolved magma upward. As a result, the new magma was immediately involved in crystallization and later - in convection, as it lost its density, the composition of magma in the intrusive chamber was equalized, and the crystallization trend returned almost to its original state.
The fact that fresh magma was periodically injected into the solidifying magma chambers is proved by the findings of cumulate plagioclase crystals with a reversed zoning, the fact emphasized above. This kind of zoning could hardly be produced during magma crystallization in a stagnant zone, but was quite feasible where the settling crystals were stirred up by a fresh, hotter magma.
Above, we called the attention of the reader to the unusually great thickness of the ultramafic zone in the Aganozero body (as high as six kilometers). It is absolutely impossible to envision a magma that might have produced such a great thickness of olivine-chromite cumulates in the case of the single-act filling of an intrusive chamber. The presence of a poorly expressed rhythmic layering in this zone can be interpreted as the indication that its formation might have been associated with the multiple additions of fresh ultramafic melt into the crystallizing magma chamber. Inasmuch as the newly formed cumulates are almost indistinguishable from the pre-existing cumulates here, in contrast to the gabbroid portion of the intrusion, they built up the ultramafic rock sequence producing an impression of the single-act formation of the zone. In this case the old evolved magma, enriched in the products of the magma crystallizing differentiation, must have been displaced continuously upward to form the upper, basic portion of the body, as the supply of fresh magma declined in connection with the attenuation of magma generation in the mantle.
So far, we cannot offer an unambiguous interpretation of the rocks crossed by Hole 67. There are three possible versions: (1) the lower megarhythm consists of the rocks of the Marginal Border Group, (2) the megarhythms are characteristic of the entire Shalozero intrusive body, but are not expressed as obviously in the other parts of the body, and (3) this structure is characteristic only of this segment of the body.
Version (1) does not agree with the higher-temperature rocks of the lower megarhythm where cumulate olivine is much more magnesian than that in the rocks of the Marginal Border Group (see above). Version (2) is also improbable, because the composition of the dark-colored silicates from the ultramafic rocks of the lower megarhythm is similar to that of the ultramafic rocks from the Ultrabasic Zone, and that from the upper megarhythm is more ferrous, similar to the composition of minerals from the peridotite marker horizon. Apparently, the more suitable version is (3) which suggests that this macrorhythmic pattern is a specific feature of the SE segment of the Shalozero body.
This macrorhythmic pattern is not a unique feature of layered intrusions. One of the examples is the Zlatogorsky layered intrusion in Northern Kazakhstan, where there are two macrorhythms with thick dunite layers at the base, which are replaced, via the zones of rhythmic ultrabasic and basic cumulate interbedding, by norite and gabbronorite layers in the top [Sharkov, 1980].
Similarities and differences in the cumulate stratigraphy and composition of the Aganozero and Shalozero-Burakovka rocks. The results of our study revealed the following specific features in the structure and composition of these two intrusive bodies. Both bodies are characterized by the high degree of layering, and their sections reflect the same main sequence of cumulate parageneses, this providing a basis for distinguishing similar zones in them. However, the more detailed study of the structure and composition of each zone revealed substantial differences between them. One example is that the Ultrabasic zone of the Shalozero-Burakovka Body contains only poikilitic peridotite, whose intercumulate phases are dominated by plagioclase with ubiquitous phlogopite. Conversely, the upper peridotite subzone of the ultrabasic zone in the Aganozero body includes dunite layers, with almost wholly absent plagioclase and scarce phlogopite.
Differences are even more substantial in the pyroxenite zones. In the Shalozero-Burakovka Body this zone is as thick as 80 m and consists mainly of websterite, orthopyroxenite, and their olivine varieties. In addition, its middle portion includes a peridotite layer interpreted as a marker horizon for the Shalozero Body. The Pyroxenite zone in the Aganozero Body is ca. 200 m thick and is dominated by clinopyroxenite with a noncumulus texture. The main mineral composing up to 90% of the rock volume is inverted pigeonite-augite whose grains are seen to intensively corrode the single grains of cumulate augite and orthopyroxene. In the lower part of the zone, a clinopyroxene aggregate replaces the peridotite minerals with merely single relict areas remaining intact. All rocks of the Pyroxenite zone in the Aganozero Body contain numerous small inclusions mainly of a quartz-carbonate material.
The Gabbronorite zone of the Shalozero-Burakovka Body also has a reduced thickness and does not include a banded subzone represented by interlayered rocks of different compositions and accounting for up to half of the thickness of the Gabbronorite zone. We failed to correlate the structures of the overlying zones. In the Aganozero body, the Pigeonite gabbronorite zone is very thin, and the rocks of the Magnetite gabbrodiorite zone are absent. Moreover, in contrast to the Aganozero body, the pyroxene in the SBB is dominated by orthopyroxene, and there are intersticial quartz and orthoclase grains (2-3 vol.%) over the whole sequence of the Pyroxenite and overlying zones.
The comparison of the compositions of cumulate silicate minerals revealed their substantial similarity in the similar portions of the Aganozero and Shalozero-Burakovka bodies, this reflecting a similarity between the physicochemical conditions of magma crystallization during the emplacement of these intrusions.
To sum up, our analysis of the structure and composition of the Aganozero and Shalozero bodies shows that with the general sequence of cumulate mineral assemblages, common for both bodies, they show a number of significant differences. Our data suggest that the Aganozero and Shalozero-Burakovka intrusive bodies were derived from similar parental magmas, which, nevertheless, were slightly different in composition.
Age relations between the Aganozero and Shalozero-Burakovka intrusions. These two intrusive bodies are separated by a large fault, and there are two different views as to their age relations: (1) both bodies were intruded at the same time, and differences in their structure were caused by the tectonic activity that occurred during the emplacement of the pluton [Tevelev and Grokhovskaya, 1999]; (2) the two bodies of the Burakovka Pluton have different ages, the SBB being younger than the AB [Berkovskii et al., 2000]. However, as a result of our study we found that the Aganozero Body was intruded 50 million years later than the Shalozero-Burakovka intrusion (see above).
To sum up, our study proved the Early Paleoproterozoic Burakovka Pluton to consist of two independent intrusive bodies, AB and SBB, which were emplaced with an interval of ca. 50 million years, the pluton itself being a long-lived magmatic center. The intrusions were derived from similar-type but slightly different parental magmas of a silicic high-Mg series with similar isotopic and geochemical characteristics, this being responsible for the absence of principal differences in the physico-chemical trends of their crystallization. Both bodies were emplaced in the course of the regular replenishment of the solidifying intrusions by new portions of fresh magma from a mantle magma-formation source and also from intermediate crustal magma chambers rising through the crust by the mechanism of zone melting (see above).
This situation does not seem to be unique for the Baltic Shield. According to recent evidence [Sharkov et al., 2001; Smolkin et al., 2001], the Monchegorsk Complex in the Kola Peninsula, ranking second in size in the region concerned, also consists of two intrusive bodies: the Monchegorsk Ni-bearing pluton of ultrabasic and basic rocks and the Monche-Chuna-Volchie-Tundra gabbroid massif, which were emplaced with a period of 30-50 million years between them [Sharkov et al., 2001; Smolkin et al., 2001].
With a very similar assemblage of cumulates, almost identical for most of the intrusive bodies, the latter differ in the development of their particular rocks. Usually, the massifs are dominated by basic cumulates, such as gabbronorite or gabbronorite-anorthosite, although there are some exceptions. For instance, it has been shown above that in the Aganozero Body ultrabasics dominate over basic rocks, the olivine-spinel cumulates having an unprecedented thickness of 6 km. This thickness is not higher than 2 km in the SBB, being a few hundred meters in most of the similar massifs. The same is true of the other cumulates; some bodies are dominated by pyroxene (usually orthopyroxene) cumulates, others, by plagioclase-pyroxene cumulates, still others, by plagioclase cumulates, and so on. Commonly found are various violations of the cumulate stratigraphy, associated with the repeated injections of new portions of fresh magma into the crystallizing intrusive chambers [Sharkov and Smolkin, 1998].
Apparently, differences in the composition of the layered intrusions were associated with the particular functioning mechanisms of individual magma systems, which were controlled by the flow rate of magma ascending from its source and intermediate chambers, by particular geomechanic conditions in the surrounding region, and also by the composition of the Archean crust in a given area, different under the granite-greenstone regions and under the granulite belts.
A good illustration for this is the Burakovka Pluton. The differences between its Aganozero and Shalozero bodies, revealed in this study, seem to have stemmed from the different contributions of the components which, as follows from [Amelin and Semenov, 1996], participated in the generation of this pluton's magma. This is in good agreement with our data on the long life of the Burakovka magma source. Differences in the rock compositions of the bodies can be explained by the crustal contamination of magma during its generation, by the different lithologies of these rocks, by differences in the source depths of the parental magma derived from the same mantle superplume, as has been demonstrated by Abaouchami and Hofmann  using the Pb isotope ratios of the Hawaiian magmas. It appears that all of these factors were valid in our case.
As has been shown above, during the Early Proterozoic, roughly 2.5 to 2.3 billion years ago, the eastern part of the Baltic Shield was a large igneous rock province of a siliceous high-Mg series, similar in its geological position to Phanerozoic trap basalt regions, but greatly different from them by the composition of its magmas, typical of subduction environments [Sharkov et al., 1997, 2000].
The distinctive features of the rocks of this series are the high contents of SiO2, MgO, and Cr, the medium contents of Ni, Co, Cu, and V, and the low contents of Ti, alkalies, and Nb. The predominant rock is basalt, the associated basaltic andesites and andesites being often characterized by an elevated Mg content. As far as the recent igneous rocks are concerned, these features are characteristic only of island-arc rocks, especially of the boninite series. These rocks resemble boninites and modern island-arc tholeiites in terms of their contents of major, trace, and rare-earth elements with their typical Nb and Ti minima and also in terms of their high Al2O3/TiO2 ratios. Appreciable differences between them were found only in the isotopic characteristics of the SHMS rocks: the e Nd (T) value varies from - 1.0 in the ultramafics to - 3.1 in the gabbroids. A similar pattern was found in the other similar layered intrusions: in Bushveld in South Africa [Schiffries and Rye, 1989] and in Stillwater in North America [Lambert et al., 1994].
The unusual geologic position of the origin of these magmas and the specific features of their geochemistry and isotopy suggest a principally different origin of the SHMS magma, as compared to the similar Phanerozoic magmas. We believe that the origin of the magmas discussed was associated with the extensive contamination of the parental high-T mantle magma with the Archean crustal material as it rose toward the Earth's surface. The leading mechanism seems to have been the "floating" of the high-T magma through the crustal rock sequence by melting the top by way of zone refinement [Sharkov et al., 1997]. The layered intrusions seem to have acted as intermediate collectors where these melts accumulated (Figure 26).
In the Kola and Karelian cratons, igneous activity manifested itself in the formation of large layered intrusions, gabbronorite dike swarms, and volcanic sedimentary belts restricted to the linear graben-shaped structures. Here, the SHMS volcanics are represented by a great variety of rocks ranging from low-Ti picrite and basalt to andesite, dacite, and rhyolite, the dominating rocks being basalts. These volcanogenic-sedimentary belts had been continental rifts that developed mainly in subaerial environments.
The intermediate mobile belts of the Belomorian type were distinguished by a dispersed intrusive magmatism which resulted in the emplacement of numerous small synkinematic intrusions of basic and ultrabasic rocks of the same types found in the layered intrusions, except that here individual intrusions are composed of one rock variety, namely, peridotite, gabbronorite, or anorthosite (e. g., Belomorian drusitic complex). Highly subordinate are enderbite-charnockite massifs. The granulite belt does not contain any SHMS rocks, the main igneous rock types being crustal enderbite and charnockite.
To sum up, in contrast to the Phanerozoic foldbelts, the distinctive feature of the Early Paleoproterozoic tectono-magmatic activity in the Baltic Shield was the coexistence of three main types of structural domains:
(1) large regions of rising and extension, accompanied by intensive mantle magmatism and active erosion (Kola and Karelian cratons); magmatism was of within-plate type; there is no evidence of any processes that operated at active plate boundaries;
(2) a region of compression and subsidence between them marked by abundant crustal enderbite-charnockite magmatism (Laplandia-Umba granulite belt), where excess of crustal (including sedimentary) material accumulated at the expense of erosion and denudation in adjacent areas;
(3) peculiar belts of the low-angle tectonic flowage of crustal material from regions of extension toward granulite belt, such as the Belomorian belt, which originated in the extension environment and had a mixed character of magmatism.
Of interest is the point of the initial size of the Early Paleoproterozoic province of boninite-like magmatism, the Baltic province, discussed above, being its part. Similar rocks are traceable in the basement of the Russian Platform, in Scotland, Greenland, in the Canadian Shield (Matatchewan and Herst dike swarms and volcanic rocks at the base of the Huron basalt plateau) and in the Wyoming Craton [Heaman, 1997]. According to Heaman's paleomagnetic and stratigraphic reconstructions, these cratons were initially parts of the Laurentia-Baltia supercontinent and separated in the Late Proterozoic. The initial size of this igneous province was supposed to be 2500 km long and at least 1500 km wide, the size comparable with the largest Phanerozoic trap provinces.
As mentioned above, this type of igneous activity developed widely in the east of the Baltic Shield throughout the Early Paleoproterozoic, this suggesting the long existence (ca. 200 million years) of a superplume [Sharkov et al., 1997]. A distinctive feature of geologic processes at that time was the localization of the centers of tectono-magmatic activity in the same geologic structures (Pechenga, Imandra-Varzuga, Vetrennyi Belt, etc.) with closely spaced periods (spaced 10-50 million years apart) of volcanic and intrusive activities (see Figure 27). Our evidence on the long-lasting activity of the Burakovka igneous center are in good agreement with this statement. Although there are some geochemical and isotopic differences between the individual bodies, the physico-chemical trends of their crystallization did not have any principal differences, as would be expected. Differences in the lithology can be explained either by differences in the degree of crustal rock contamination in the course of magma generation, or by differences between the depths of the parental magma in the same superplume.
Judging by geochemical data and the mineral composition of the highest-temperature cumulates including high-Mg olivines and pyroxenes, as well as chromites, the primary mantle magma was produced by the melting of a highly depleted ultramafic material. According to the isotopic and geochemical data available, as the newly produced magma ascended toward the Earth's surface, it assimilated the material of the ancient lower crust, getting enriched in Si, Al, and Ca, this explaining the predominance of basic rocks in the intrusions. The above evidence suggests that the composition of the early Paleoproterozoic mantle plumes was drastically different from that of the Phanerozoic plumes depleted in ultrabasic material. The primary melts were very hot (ca. 1600o C [Girnis and Ryabchikov, 1988]), this promoting the assimilation of crustal material, the process uncharacteristic of the Phanerozoic intraplate magmatism.
The emergence of local plumes responsible for the origin of long-lived magmatic centers, marked by large layered intrusions, was obviously associated with local rises on the superplume surface, where the mantle material was melted at the expense of decompression, and where the magmas of the siliceous high-Mg series were produced.
1. The Burakovka layered pluton of mafic and ultramafic rocks is one of the largest layered massifs in the Baltic igneous province of siliceous high-Mg rocks. It consists of two independent bodies (Aganozero and Shalozero-Burakovka), each having its own internal structure, which contact each other in their tops.
2. The two bodies of the pluton have a similar rock sequence consisting of five differentiated zones: ultramafics, pyroxenite, gabbronorite, pigeonite gabbronorite, and magnetite gabbronorite (the latter was found only in the Shalozero-Burakovka Body). However, with this general structural similarity, these bodies differ markedly in the character of their cumulate stratigraphies and, to a lesser extent, in their rock compositions.
3. A distinctive feature of the pluton is the presence of markers: single interlayers of high-temperature ultrabasic cumulates in the sequence of lower-temperature formations. Their origin is believed to be associated with the replenishment of fresh magma portions into the crystallizing magma chambers. The same mechanism explains the macrorhythmical pattern recorded in the SE segment of the Shalozero-Burakovka body.
4. As follows from the geochemical, mineralogic, and isotopic-chronological data, the two bodies were produced by the compositionally siliceous high-Mg magmas, except that the Aganozero Body was intruded 50 million years later than the Shalozero-Burakovka Body, the Sm-Nd isochron age of the former being 2372 22 Ma ( e Nd = -3.220.13 ) and that of the latter, 2433 28 Ma ( e Nd = -3.140.14 ).
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