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Vol. 10 (2007 year), No. 1
Chupin V.P., Sorokina G.A., Vetrin V.R.,
On the genesis of zircon from Archaean plagiogneiss of the Kola Superdeep Borehole (thermobarogeochemical data)
The data on geochemical and structural-mineralogical peculiarities of rock-forming minerals from the homologous sedimentary rocks of the Luchlompolo sedimentary Formation were the result of investigation of SG-3 deep rocks, their surface and near-surface homologues. Each of these groups has the same stratigraphic position of samples but a different depth level. These minerals are microclines, carbonates and quartzes. The surface and near-surface samples of microclines, carbonates and quartzes have identical composition and spectroscopy data. But in the context of crystal structure defects they have noticeable distinctions as compared with abyssal homologues. The revealed structural distinctions for microclines and dolomites are manifested in the degree of crystal structure defects: it decreases with depth for microclines and increases for dolomites. This is caused by a different metamorphism grade. The distribution of admixture Mn-ions in the possible structural sites of dolomite may be caused by the combined influence of metamorphism and a "depth factor". Displacement of isomorphism of Al admixture centres in quartz in the direction of alkaline varieties, with increase of the depth level, is connected with an analogous process. The tendencies of decreasing the natural part of paramagnetic Al-centres and oxygen vacancy concentrations in quartz with depth can be explained by the "depth factor".
(in Russian, стр.8, fig. 4, tables. 2, ref 16, Adobe PDF, Adobe PDF 0 Kb)
Vol. 10 (2007 year), No. 1
The results of new isotopic investigations of the Archaean rocks from the Kola Superdeep Borehole (SG-3) – gneisses by the U-Pb method on zircon and amphibolites by the Sm-Nd model method on the whole rock have been presented. Besides, some rocks of the Proterozoic Pechenga structure and its Archaean framing have been dated by the U-Pb method on zircon, baddeleyite and other accessory minerals. Tonalite gneisses from SG-3 unit II yielded the oldest age of 2814±17 Ma. Zircon from the rest gneiss units in the section yielded an age of 2.78-2.55 Ga. This age fixes metamorphic events of the Archaean. Model ages of protolith from amphibole rocks are divided into three groups: 2.50-2.25, 2.79-2.65 and 3.15-2.89 Ga. Amphibolites from the first group are comagmatic to basic-ultrabasic dykes of the northern framing and to volcanites from the lower formations of the Pechenga structure. Amphibolites from the second group correspond to the basic rocks of the Late Archaean greenstone belts that are very common in the NE Baltic shield. Amphibolites with the oldest protolith are likely to be relics of the Early Archaean greenstone belt on the enzymatic base. All the ages obtained on the rocks from SG-3 do not correlate with depth which is likely to reflect lamellar-block structure of the section. New geochronological data on the Pechenga structure are evidence of a few previously unknown magmatic impulses in its formation. The dating results for granitoids from the Archaean framing fix their Proterozoic age and metamorphic changes at the final stage of the Svecofennian orogeny.
(in Russian, стр.14, fig. 4, tables. 2, ref 47, Adobe PDF, Adobe PDF 0 Kb)
Vol. 10 (2007 year), No. 2
Turkina O.M., Vetrin V.R.
Isotopic-geochemical systematization and geodynamic setting of formation of the Late Archaean tonalite-trondhjemite complexes in the Kola Superdeep Borehole section and Pechenga structure framing
The Proterozoic Pechenga complex penetrated by the Kola superdeep borehole SG-3 (0-6840 m) represents a rhythmic alternation of volcano-sedimentary formations with co-magmatic bodies of gabbro-wehrlites and intrusions of gabbro-diabases and dacite-andesite porphyries. We have studied bodies of metapicrite rocks from the Nikelska group – the Matert and Zhdanov Formations. Metapicrites are medium-grained rocks with variable alteration, some of them metamorphosed at the greenschists facies. In the studied rocks olivines were totally altered. Clinopyroxenes correspond to augite, rarely to hedenbergite. Amphiboles occur in several generations, the first kaersutites co-exist with clinopyroxenes. Amphiboles around clinopyroxenes correspond to edenite and the amphiboles which originated during the metamorphosis under the greenschist facies PT conditions correspond to actinolites, ferroactinolites and tremolites. We compared metapicrites from SG-3 and ferropicrites on the surface. They are geochemically very similar. The studied Proterozoic metapicrites from the Matert and Zdhanov Formations have a range of SiO2 between 38.69 and 48.2 wt%. The MgO content varies from 10.53 to 33.94 wt%, the Al2O3 content varies from 2.64 to 11.28 wt%, the TiO2 content varies from 1.25 to 4.18 wt% and CaO content varies from 1.14 to 13.99 wt%. The Fe2O3 content varies from 16.06 to 20.73 wt%, Mg# from 50.16 to 77.29 wt% and LOI from 3.90 to 10.30 wt%. High concentrations of compatible elements Ni, Co, Cr in metapicrites from the Matert and Zhdanov Proterozoic Formations indicate a mantle source. REE-chondrite normalized patterns of these rocks are different from the MORB basalts pattern, they are enriched in LREE. The concentrations of most HFSE and REE are considered to reflect the primary magmatic distribution and correspond to E-type MORB and tholeiitic WPB.
(in Russian, стр.11, fig. 7, tables. 3, ref 44, Adobe PDF, Adobe PDF 0 Kb)
Vol. 15 (2012 year), No. 2
Vetrin V.R., Yakovlev Yu.N., Chupin V.P.
Sources of the terrigenous material in the formation of metasedimentary rocks of the Archaean basement of the Paleoproterozoic Pechenga structure
The basement of the Paleoproterozoic Pechenga structure was cored by the Kola Superdeep Borehole SD-3 at the depths of 6842-12262 m. It consists of alternating strata of metavolcanic dacite, plagiorhytodacite rocks and high-alumina gneisses; the protoliths of these rocks corresponded mainly to graywackes. Resulting from the examination of zircons from metaterrigenous rocks of the 1st, 3rd, and 9th strata of the SD-3, the detrital, anatectic, metamorphogenic, and contact-metasomatic genetic types have been identified. Detrital zircons include several age groups. The most homogeneous, i.e., comparable to zircons from tonalite gneisses (bottoms of the SD-3 section) and from surrounding rocks, zircons have appeared to be those from gneisses of the deepest 9th stratum. The data on the age of these zircons, along with a poor rounding of the grains, signify formation of the host gneisses' protoliths owing to washing out and redeposition of material. Widening of alimentation areas, which supplied terrigenous material into sedimentation basins, took place during formation of alumina gneisses of the 3rd and especially 1st strata of the section.
(in English, стр.6, fig. 3, tables. 1, ref 14, Adobe PDF, Adobe PDF 0 Kb)