TETRAD-EFFECT IN RARE EARTH ELEMENT PATTERNS OF THE PALEOZOIC RARE METAL GRANITOIDS FROM THE OKA ZONE, EAST SAYAN

Yasnygina T.A.*, Rasskazov S.V.*, Osipova T.A.**, Ershov K.V.*

*Institute of the Earth’s Сrust SB RAS, Irkutsk, Russia, ty@crust.irk.ru

**Institute of Geology and Geochemistry UB RAS, Ekaterinburg, Russia

Tetrad-effect is a periodical changing of the rare earth element (REE) properties caused by formation of aqueous complexes. The stability constants of the complexes versus atomic number of the REE vary not smoothly but periodically (Fidelis, Siekierski, 1966; Byrne, Li, 1995). The like changes of REE properties are revealed at 2-5 kbar and 200-500 ºC by experiments with the aqueous complexes (Haas et al., 1995). In the natural objects the tetrad-effect appears as anomaly of the smooth form of the chondrite-normalized REE pattern plotted as a split of the spectra into four curved segments (tetrads): La-Nd, Sm-Gd, Gd-Ho и Er-Lu. The tetrads are convex or concave correspond to M- and W-shaped patterns respectively.

The split of the REE pattern into tetrads was observed experimentally at the conditions of the immiscibility between aluminosilicate and aluminofluoride melts. In the aluminofluoride melt appeared the M-shaped pattern and in the aluminosilicate melt – complementary W-shaped. Apart from the melts fluoride containing minerals produced by one of the runs were analyzed. Cryolite (Na₃AlF₆) showed the tetrad-effect and fluorite (CaF₂) was not revealed that (Veksler et al., 2007).

The tetrad-effect (ТЕ_i) is estimated through the deviation of a measured concentration of the middle elements of a given tetrad i from interpolated values calculated using the first and the last elements (Irber, 1999). The M-shaped pattern shows ТЕ_i>1.1 and the W-shaped - TE_i<0.9. The using of first, third and forth tetrads (ТЕ_1-4) gives a rather correct estimation.

The M-shaped pattern of the tetrad-effect was revealed in granitoids (Hetcht et al., 1999; Irber, 1999; Monecke et al., 2007 и др.). It was found in the palaeozoic leucogranites and rare metal granitoids of the Shagaite-Gol structure in the Samsal massif (ТЕ_1-4 1.06-1.33), of the Sahir-Shuluta massif (ТЕ_1-4 1.07- 1.25) and Honchen vein complex (ТЕ_1-4 0.95 - 1.18) in the Oka zone of eastern Sayan.

The REE and others trace element concentrations were determined by ICP-MS using the different methods of sample preparation. The samples were prepared by acid digestion with HF and HNO₃ in the tephlon closed vessels (analyst M.E. Markova, IEC SB RAS, Irkutsk) and by fusion with Li-tetraborate and dissolution of aliquots with HNO₃ (analyst N.N. Pahomova, IGC SB RAS, Irkutsk). The ICP-MS measurements were performed using a VG PlasmaQuad PQ 2+ and Agilent 7500 quadrupole ICP mass spectrometers at the Baikal Equipment Center of Joint Use.

The tetrad-effect appears in the REE-spectra of evolved granitic melts. The REE patterns of all analyzed samples of granitoids have a negatively Eu-anomaly. The rock forming minerals have the curved REE pattern with tetrad-effect too, and the patterns of mica have higher TE_1-4 then feldspars. Feldspar represents by amazonite with minor albite and microcline. Albitization and muskovitization can well be observed in the thin section. Biotite (zinnwaldite) of the Shagaite-Gol granites is characterized by relatively high concentrations of Li (0.34-0.56 %), Rb (0.33-0.36 %), Zn (<0.4%), Nb (577-630 ppm), Ta (120-176 ppm), Ga (<120 ppm) and Th (34-77 ppm).

The tetrad-effect is more significant in the albitized granites and greisens at Zinnwald (East Germany) then in the granites affected by greisenization and argillization. It is supposed that the tetrad-effect in the granites could appear due to interaction between fluid and granitic melt before or during albitization (Monecke et al., 2007). As the content of K-feldspar in rock decreases and it is displaced by albite, Rb and Rb/Sr decrease too. After (Boulvais et al., 2007), the albitization is characterized by decreasing TE_1-4 and Zr/Hf. It is observed in general negative correlation between TE_1-4 and Zr/Hf in the East Sayan granites with high Rb/Sr therefore tetrad-effect is not the result of albitization.

Concerning ТЕ_1-4 and the other geochemical characteristics, the rare metal plumasite and alkali granites of the Oka zone are likely to the granites of the Bogemian massif and the Baerzhe and Quanlinshan granites respectively (Yasnygina, Rasskazov, 2007).

Using correlation coefficients between ТЕ_1-4 and the other geochemical characteristics for the data access including 15 samples of rare metal granites of the Oka zone the increasing of ТЕ_1-4 is related by enhanced SiO₂, Rb/Sr, Nb/Zr and Y/Ho. The ТЕ_1-4 has negatively correlation with TiO₂, Fe₂O₃, FeO, Sr, Ba, La/Sm, Sm/Yb, Zr/Hf, Nb/Ta and La/Ta. The positive correlation indexes were calculated for CaO, Al₂O₃, Sr and Nb/Zr and the negative – for agpaitic coefficient, K₂O, K₂O/Na₂O, Zr/Hf, La/Sm and La/Ta for more representative data sampling including 36 samples of palaeozoic granites of the Oka zone.

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