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Transactions (Doklady) of the Russian Academy of Sciences / Earth Science Sections, Vol. 347A, No. 3, 1996, pp. 513-515. Translated from Doklady Akademii Nauk, Vol. 347, No. 6, 1996, pp. 799-801. Original Russian Text Copyright ї 1996 by Nikolaev, Koptev-Dvornikov, Ganin, Grinevich, Kireev, Pchelintseva, Vislova. English Translation Copyright ї 1996 by еДад зЫН / Interperiodica Publishing (Russia).

GEOCHEMISTRY

Vertical Structure of the Aganozero-Burakovsky Layered Massif and Distribution of Major Elements in Its Section.
G. S. Nikolaev*, E. V. Koptev-Dvornikov**, V. A. Ganin***, N. G. Grinevich***, B. S. Kireev**, N. F. Pchelintseva**, and T. Yu. Vislova****
Presented by Academician V.A. Zharikov November 3, 1994 Received November 4, 1994

The Middle Proterozoic Aganozero-Burakovsky Massif is the largest layered intrusion in Europe. It contains chromite and Ni-silicate mineralization and is a potential source of platinum [1]. The intrusion (630 km2) is located in the eastern Onega region (40-50 km to the north of the Pudozh settlement) within Archean metabasites of the Vodlozero block of the Baltic Shield. It is a lopolith with bottom depth varying from 6.5 to 2 km. A thick cover of Quaternary deposits overlay it. Previous results of investigations [2, 3] were based on the geophysical data and study of scarce outcrops and several shallow boreholes. As a result, the massif outlines, its layered structure, and its thickness were determined. Deep drilling and geological mapping carried out by the Karelian Geological Expedition over the last 10 years created new possibilities for study of the massif. This paper is a first attempt to reconstruct the complete intrusion section and quantitatively describe the major element distribution from its bottom to the top. Our petrological pattern is based on the data on 80 boreholes with depths of 100-400 m (rarely > 400 m). With depth, each borehole shows a trend of increase in the iron mole fraction f = Fe/(Fe + Mg) in the rocks and a trend of decrease in normative plagioclase number An = (Al - Na - K)/(Al + Na + K). This allows us to demonstrate the vertical standard section (thickness about 6600 m) using data on the 24 most representative boreholes. The section is based on 539 samples related to vertical coordinates of the section.
* Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, 117975 Russia. ** Moscow State University, Vorob'evy gory, Moscow, 119899 Russia *** Karelian Geological Expedition, pr. Uritskogo 65, Petrozavodsk, 185030 Russia **** Department of Geology and Geophysics, University of Minnesota, 108 Pillsbury Hall, 310 Pillsbury Drive S.E., Minneapolis, MN 55455, USA

Based on petrographic study and distribution patterns of major elements, the standard section was divided into upper and lower contact zones, and layered series (Ucz, Lcz, LS). From bottom to top, LS is subdivided into five zones, whose rocks have different mineral assemblages of the first structural group (cumulus parageneses). Zone boundaries are defined by the abundant appearance of typomorphic cumulus parageneses. Variations in proportions of cumulus phases result in rock diversity in each zone, while the same taxon may correspond to different cumulates because of the variable porosity of cumulus. The ultrabasic zone (Uz) (3000-3500 m thick) contains chromite-olivine cumulates and is composed of dunites, harzburgites, lherzolites, and werhlites. An increased intercumulus mineral levels (to 40% of the total volume) in the upper 400-600 m resulted in the zone being subdivided into lower and upper subzones. Chromite content usually does not exceed several percent, with the exception of the upper zone, where it increases to 30-80% in separate beds with thickness from 1-2 cm to 1.5 m. Some beds with cumulus clinopyroxene, whose thickness increases upward, occur in the upper 150 m of the Uz. The pyroxenite zone (Pz) (100-200 m thick) contains chromite-olivine-clinopyroxene, ortho-clinopyroxene, and clinopyroxene cumulates and is composed of wehrlites, websterites, orthopyroxenites, melanocratic gabbro-norites, and clinopyroxenites. The gabbro-norite zone (GNz) (1100 m thick) mainly contains ortho-clinopyroxene cumulates and is composed of gabbro-norites, norites, and gabbros. The fine contrast alternation of gabbroids, pyroxenites, and peridotites observed in the lower 450 m was used to subdivide this zone into lower (banded) and upper subzones. The upper subzone shows a more uniform distribution of the rock-forming minerals and consists of alternating gabbro-norites and subordinate norites. The gabbro-norite zone with inverted pigeonite (GNPz) (1150 m thick) contains clinopyroxene-pigeonite-plagioclase cumulate and is composed of gabbro- norites with orthopyroxene presented by pigeonite.

513


514 Relative height 1.0 0.8 0.6 0.4

NIKOLAEV et al. Absolute height, m 6000

4000

2000 0.2 0 38 58 0 20 3 13 SiO2, wt % Al2O3, wt % FeO, wt % 1.0 6000 0.8 0.6 0.4 2000 0.2 0 0 1.0 40 1 Na2O, wt % K2O, wt % 0 0.5 P2O5, wt % 0 2 TiO2, wt % 0 Cr, ppm An f 6000 0 5000 4000 0 50 0 10 MgO, wt % CaO, wt% 0

0.8 0.6 0.4 2000 0.2 0 0 Ol, vol % 1 2 100 0 50 0 50 OPx, vol % CPx, vol % 3 4 5 0 50 Pl, vol % 6 0 f, An 7 8 1 0 4000

Distribution patterns of major elements and normative minerals in the vertical section of the Aganozero-Burakovsky intrusion. (1) UPz, LPz; (2, 3) Uz: (2) lower and (3) upper subzone; (4) Pz; (5, 6) GNz: (5) lower (banded) and (6) upper subzones; (7) PGNz; (8) MGNDz.
TRANSACTIONS (DOKLADY) OF THE RUSSIAN ACADEMY OF SCIENCES / EARTH SCIENCE SECTIONS Vol. 347A No. 3 1996


VERTICAL STRUCTURE OF THE AGANOZERO-BURAKOVSKY LAYERED MASSIF

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Indistinct layering caused by alternation of leucocratic and meso-varieties is observed in the zone section. The magnetite gabbro-norite-diorite zone (MGNz) (760 m thick) contains magnetite-pigeonite- clinopyroxene-plagioclase cumulates and is composed of alternating leucocratic and mesocratic gabbro- norite-diorites with gradual transition. The content of cumulus magnetite varies from 30% (at the bottom of the mesocratic layers) to 1-2% (in the leucocratic layers). An content reaches minimum values in the upper part of zone, where it marks the most fractionated rocks of the intrusion and the boundary between the upper part of this zone and the LS. The presence of LPz and UPz (sampled thickness is 330 and 90 m, respectively) indicates that the complete section of layered series (6100-6700 m in total thickness) is available for investigation. The cumulus composition are graded upward from high- to low-temperature parageneses: Ol-Chr (Uz), Cpx-Opx-Ol-Chr (Pz), Pl-Cpx-Opx (GNz), Pl-Cpx-Pig (PGNz), Pl-Cpx-Pig-Mt (MGNDz). This sequence is disrupted in two cases: (1) at the boundary between the LPz and LS; and (2) at the boundary between the LS and UPz. Boundaries between the UPz, LPz, and LS was defined by inflection points in the plots of major element distribution. The distribution patterns of major elements and normative minerals though intrusive section conveniently illustrate the proposed scheme of the massif 's structure. To elucidate the main patterns of the element distribution, which is complicated by small and medium-scale layering, initial distribution plots (except for P2O5) were processed with tenfold smoothing by a three-point sliding window (figure). When passing from one zone to another (from one to another cumulate paragenesis), chemical composition changes and, as a result, normative content of that mineral, which becomes the cumulus component in the overlaying zone, increases. For example, the appearance of cumulus clinopyroxene is accompanied by an increase in Ca content, while the appearance of cumulus magnetite is accompanied by an increase in Fe and Ti concentrations. The maximum concentration of P2O5 at the boundary between the LS

and UPz marks the most fractionated rocks, as it is an independent criterion of the reliability of this boundary. The average weighted composition (in wt %) of the intrusion is as follows:
SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 47.05 0.38 7.76 10.24 0.14 27.46 5.13 1.51 0.29 0.03

(similar to Karelian komatiites [4]). This may be taken for the parental magma composition, if the intrusion either is of the monophase type, or if the probable additional injections did not influence the magmatic evolution inside the chamber. The umulate sequence shows the order of crystallization of the parental magma: Ol + Chr OPx + CPx - Ol - Chr OPx + CPx + Pl - Ol Pig + CPx + Pl - OPx Pig + CPx + Pl + Mt (minus denotes the incongruently melting minerals). This order does not contradict the mechanism of crystallization of komatiitic melts. ACKNOWLEDGMENTS This study was supported by the Russian Foundation for Basic Research, project nos. 93-05-10 8282 (E.V. Koptev-Dvornikov and B.S. Kireev) and 94-0516 098a) (G.S. Nikolaev). REFERENCES
1. Ganin, V.A., Grinevich, N.G., and Pchelintseva, N.F., Abstracts of Papers, VII Mezhdunarodnyi Platinovyi Simpozium (VII International Platinum Symposium), Moscow: 1994, p. 27. 2. Lobanova, A.B., Polikarpov, V.K., and Shinkarev, N.F., Sov. Geol., 1975, no. 9, pp. 132-137. 3. Garbar, D.I., Sakhnovskaya, T.P., and Cherchel', E.K., Izv. Akad. Nauk SSSR, Ser. Geol., 1977, no. 8, pp. 100-112. 4. Komatiity i vysokomagnezial'nye vulkanity rannego dokembriya Baltiiskogo shchita (The Early Precambrian Komatiites and High-Magnesium Volcanites of the Baltic Shield), Bogatikov, O.A., Ed., Moscow: Nauka, 1988.

TRANSACTIONS (DOKLADY) OF THE RUSSIAN ACADEMY OF SCIENCES / EARTH SCIENCE SECTIONS Vol. 347A No. 3 1996