Kola Peninsula

Nitkina E.A.

Geological Institute KSC RAS, Apatity, Russia

[e-mail]

The Archaean basement complexes on the regional geological maps have called tonalite-trondemit-gneisses (TTG) complexes [8]. The processes of applying ultrametamorphism and melting in the basement complexes led to a change in the composition of rocks and minerals [8], including changes in isotopic zircon systems, that is the "rejuvenation" of age-dating. Different sizes of rocks and minerals, including zircon, which has the most stable structure, can be those relicts in the complexes.

More than 3.0 billion years dating of detrital zircons on the Kola Peninsula are widely known [5,7], which, according to Acad. F.P.Mitrofanov, shows a small transfer of material, i.e. massifs, of which this is brought zircon, are probably not far from the places of the zircons findings. In addition Archaean rocks are metamorphosed in the granulite facies metamorphism and there are small amounts of the terranes where the basement complex was metamorphosed in the amphibolite facies, including Ingozersky massiv.

Ingozersky block located in the Tersky Terrane of the Kola Peninsula is composed of Archean gneisses and granitoids [1, 6, Kharitonov, 1966]. In the previous studies [1,3,4,6,9] within Ingozersky block the following types of rocks were established: biotite, biotite-amphibole, amphibole-biotite gneisses, granites, granodiorites and pegmatites [2].

Preliminary U-Pb isotopic dating of samples held for biotite gneisses (H-10-01), amphibole-biotite gneisses (H-10-07) and biotite-amphibole gneisses (H-10-08).

A 25 kg sample of biotite gneisses (H-10-01) was taken in the region where the gneisses are situated, south-western coast of the Niznee Kapustnoe lake.

Fig. Isotope U-Pb diagrams: a – biotite gneisses Н-10-01 (single grains); b - biotite gneisses Н-10-01; c – amphibole-biotite gneisses Н-10-0; d – biotite-amphibole gneisses Н-10-08.

Zircons from this sample were analyzed using single grain U-Pb dating. The zircons with the oscillatory zoning were extracted: 1 – transparent prismatic dark brown crystals with facets of the prism {110} and dipyramid {111}; 2 – dark brown transparent prismatic crystals with facets of the prism {100} and dipyramid {111}; 3 – dark brown transparent prismatic crystals with facets of the prism {100} and dipyramids {111} and {331}. The U-Pb age, which was obtained from the three zircon populations, is 3149±46 Ma (fig. a).

Four zircon populations from the gneisses (sample Н-10-01) were picked for the isotope U-Pb dating by TIMS: № 1 – all zircons type in the fraction of the size of -0,075 mm; № 2 – long-prismatic to short-prismatic dark brown transparent crystals from fraction of the size of +0,15 mm; № 3 long-prismatic to prismatic dark brown transparent crystals from fraction of the size of -0,15 to + 0,1 mm; № 4 all zircons type in the fraction of the size of -0,1 to +0,075 мм. The coordinates of four points describe a discordia, which intersects the concordia at the point of 2697±9 Ma (fig. b).

Zircon concentrate was picked out of a 27 kg sample of the amphibol-biotite gneisses (H-10-07) – r. Umba – for the analysis as by single grain U-Pb dating as by TIMS. Five zircon populations were described as points on the U-Pb diagram: 1 – the second stage of the two-stage solution of dark brown transparent prismatic crystals; the second – dark brown transparent prismatic crystals with facets of the prism {100} and dipyramid {111}; the third – pale brown transparent prismatic to short-prismatic crystals with facets of the prisms {100}, {110} and dipyramid {111}; the forth – dark brown transparent prismatic crystals with facets of the prism {110} and dipyramid {111}; the fifth – the second stage of the two-stage solution of dark brown prismatic to short-prismatic crystals. The first point give the concordant U-Pb age of 2667±7 Ma (fig. c); and the rest zircon populations yielded the U-Pb age of 2725±2 Ma (fig. c).

A 24 kg sample of biotite-amphibol gneisses (H-10-08) contained 156 mg zircon concentrate was taken on the south-eastern coast of the Ingozero lake. Six zircon population was hand picked: (№1) – the first stage of the two-stage solution of dark brown transparent prismatic crystals; (№2) – pink transparent prismatic to short-prismatic crystals; (№3) – dark brown transparent prismatic crystals with facets of the prisms {100}, {110} and dipyramid {111}; (№4) – brown transparent crystals with facets of the prism {100} and dipyramids {111} and {331}; (№5) - brown transparent crystals with facets of the prism {100} and dipyramid {111}; (№6) – pale-pink transparent prismatic crystals with facets of the prisms {100}, {110} and dipyramid {111}. Six zircon populations yielded the U-Pb age of 2727±5 Ma (fig. d).

Thus, some U-Pb ages of the metamorphism processes in the TTG complex are obtained: 2697±9 Ma – for the biotite gneiss, 2725±2 and 2667±7 Ma – for the amphibole-biotite gneisses, and 2727±5 Ma for the biotite-amphibole gneisses. The age defined for the biotite gneisses by using single zircon dating to be about 3149±46 Ma corresponds to the time of the gneisses protolith formation.

Author are grateful to Akad. Mitrofanov F.P. and Bayanova T.B. for the consultations.

The work is supported by RFBR 11-05-00817.

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