New isotope data of carbonatites and albitite from Gremyaha-Vyrmes massif
Groznova M.V., Zaitsev V.A., Sorokhtina N.V., Kogarko L.N.
Gremiakha-Vyrmes massif is located in north-east part of Kola Peninsula. Massif is extended in north-east direction; it is 19 km long and 4-6 km width. It crosses mica- and oligoclase- granite gneisses of late-Achaean Kola – Belomorean Sea complex.
Massif includes four spatial-connected rock complexes, which were formed one after another. They are: 1) peridotites- pyroxenites- gabbro- anorthosites - and akerite-pulaskites; 2) alkaline syenites- alkaline granites 3) nepheline syenites – juvites-ijolites-urtites (Kukharenko et al, 1971; Arzamastsev et al, 2006); 4) veins of alkaline metasomatites and carbonatites with the thickness up to tens cm, crossing the rocks of basite-ultrabasite and foidolite complexes (Osokin, 1987; Savatenkov et al, 1999). Alkali metasomatites and carbonatites are the latest phase in massif. Earlier the similar rocks, located to secondary tectonic disturbances, were considered as the late hydrothermal veins, connected with region-metamorphic processes (Polcanov et al, 1967).
Carbonatites belong to soevite type. Calcite-rich carbonatites are located in Aegirine Navolok section in the central zone of massif and are surrounded by the foidolitic rocks. We analyzed two parageneses - rare graphite-bearing calcite-rich carbonatites and graphite-free carbonatites. The first association, which is essentially alkaline, consists of calcite, aegirine, biotite, substantial amount of albite and orthoclase, graphite, fluorapatite, titanite, zircon, some unknown Ca and K silicates, rare ferriallanite-(Ce) and sulphides. The second association consists of ilmenite in place of titanite, minor amount of albite and orthoclase, pyrochlore. Chemical composition of calcite from both parageneses is identical and the mineral enriched in Sr up to 3 wt %. Graphite forms individual spherulites, sometime it contains single inclusions of calcite.
Punlished data of the age of Gremiakha-Vyrmes rocks show their paleoproterozoic age. Variation of ages is from 1600-1675 Ma (K-Ar method (Polcanov et al, 1967)) to 2070±110 Ma (Sm-Nd method by foidolite complex rocks (Kogarko, Karpenko, 1998)) and 2034±46 Ma (Sm-Nd method by calcite in carbonatites (Savatenkov et al., 1999)).
To establish more detailed age of carbonatites from this massif we studied Rb/Sr and Sm/Nd isotopic systems for two samples of carbonatites and one sample of albitite from drill core on Aegirine Navolok.
Carbonaites was divided to silicate and carbonate parts by the chemical method. 30-50 g of crushed rock was put in 2M HCl to dissolve carbonate fraction. Solution was decanted. Isotope study was carried out on mass-spectrometer «Tims Triton» (GEOKHI RAS).
For graphite-bearing carbonatite Sm-Nd method shows age 1922±26 Ma and initial isotopic ratio 43Nd/144Nd(T)=0.510261±18 , ENd =+2.2, MSWD=0.31 (fig. 1). Bulk-rock isotopic composition of graphite-free carbonatite and albitite, recalculated to this age shows similar initial ratio 143Nd/144Nd(T=1922Ma)= 0.511591 ENd =+1.93 and 0.510266 ENd =+1.91 correspondingly.
Rb-Sr method shows for graphite-bearing carbonatite the age estimation 1713±16 Ma, and initial isotopic ratio 87Sr/86Sri=0.702619±13, MSWD =1.11 (fig. 2), and for graphite-free carbonatite - 1730±16 Ma, initial isotopic ratio 87Sr/86Sri=0.702719±12, MSWD =0.0023 (fig. 3). For albitite measured 87Sr/86Sr ratio is 0.707421±26 with 87Rb/86Sr ratio of 0.50. After recalculation to the ages, estimated from carbonatites by Rb/Sr method, shows estimation 87Sr/86Sri=0,6949, that is notably lower than we can see for carbonatites.
Fig. 1. Isochrone for graphite-bearing carbonatite from Gremiakha-Vyrmes.
Fig. 2. Isochrones for graphite-free carbonatite (left) and for graphite-bearing carbonatite (right) from Gremiakha-Vyrmes.
We suggest that Sm/Nd age estimation shows the true age of rock crystallization. Rb/Sr age estimations can be explained by late metamorphic event (Pushkarev, 1990).
Referenses
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