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Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Mafic rocks of West Transbaikalia:

geochemical and geochronological data

Antsiferova T.

Geological Institute of SB RAS, Ulan-Ude, Russia,



At present, there are number of different models of large granitoid batholith formation. Mantle magmas as an additional source of heat and fluids play a key role there.  Direct evidence of mantle magma participation are combined dykes, melanocratic inclusions in granitoids and  synplutonic mafic rocks (Litvinovsky et al, 1993; Yarmolyuk et al, 1997; Gordienko et al, 2003).

The Late Paleozoic intrusive rocks (general trend from high-K calc-alkaline to alkali and peralkaline granitoids) totally occupy no less than 80 % of the territory of Transbaikalia (200,000 km2) (Litvinovsky et al, 1993; Kovalenko et al., 2004). Five intrusive formations were emplaced throughout the region. The largest from them is Barguzin complex of high-K calc-alkaline granites making up the Angara-Vitim batholiths (AVB) 150,000 km2 in area.

U-Pb and Rb-Sr isotopic zircon dating shows that the Late Paleozoic magmatic cycle lasted almost continuously from ~330 to ~275 Ma and included two stages of granitoid magmatism: early – 330-310 Ma (Barguzin complex is typomorphic for AVB) and at late stage formation of geochemically distinct intrusive suites occurred with total or partial overlap in time - 310-275 Ma (monzonite, quartz monzonite, quartz syenite) (Tsygankov et al, 2010, Litvinovsky et.al., 2011).

Mafic rocks are found in all intrusive complexes except for the earliest Barguzin suite. Gabbros, according to some estimates, come up to 10%, but their temporal relationship with associating granitoids in most cases is unknown. We assume that some part of gabbros could relate to granitoid magmatism. The evidence of such relationship is geochronological data and geochemical similarities of gabbros with synplutonic mafic rocks of dated granitoid plutons. Our studies were concentrated in the central part of the West Transbaikalia, in the lower course of Kurba River and neighboring areas of the Ulan-Burgasy Range, at 150-160 km north-east of Ulan-Ude. We have studied three mafic bodies confined to tree different granitoid suites that were formed in the interval 305-278 Ma (Tsygankov et al, 2010): Orefiev pluton (Chivyrkui intrusive complex), Khasurta pluton (Lower Selenga suite) and Unegetey pluton (Zaza suite).

In the TAS (total alkalis vs. SiO2) classification of Middlemost (1997) almost all the samples plot in the monzogabbro, monzodiorite, alkaline and normal gabbro fields. The Magnesium Mg#=100x (MgO / (MgO + FeO*) in rocks vary from 50 to 39%, TiO2 (0.8 – 1.5 wt. %, in mafic rocks of Khasurta pluton till 2.4 %). All gabbros are highly-aluminous (15 – 20 wt. % Al2O3). Mafic rocks from Orefiev and Khasurta pluton are potassium enrichment (0.5 – 2.7 and more wt % К2О) and can be referred to as high-K calc-alkaline to shoshonitic.  Unegetey rocks are differed by minimal potassium alkalinity that does not exceed 0.5 wt. % К2О. High-K calc-alkaline mafic rocks have higher contents of Ba and Sr (about 1000 ppm, in average), Rb (to 100 ppm) at comparatively not high Zr (no more than 200 ppm). The content of Ba and Sr in the rocks of Unegetey pluton is on average 600-800 ppm respectively, and Rb and Zr - 13-32 ppm respectively.

The REE patterns of the mac rocks from all three plutons are quite uniform. They are LREE-enriched (La/Yb)N

=8–23, and the Eu anomaly is generally weak to absent (Eu/Eu* 0.67 – 1.11). These characteristics indicate that these rocks are similar to continental basalts commonly found in interiors of cratons. Overall, all the mac rocks under consideration are distinguished from OIB by clear Nb (less Ti, Zr, Hf) troughs, and Pb, Sr peaks. The negative Nb anomaly reflects the geochemical characteristics of mantle source, and Pb, Sr peaks - the contribution of crustal contamination. More latest mafic rocks of West Transbaikalia have same geochemical features.

In a few samples δ18O was measured in plagioclase from Orefiev gabbro which showed mantle-like values (8.1–8.2‰). However, all mafic rocks are characterized by negative εNd(Т) values ranging from -7 to -4. The data set also indicates that mac rocks and associated granitoids show roughly similar isotopic compositions (Litvinovsky et al, 2011). It should be noted, that all Late Paleozoic mafic rocks of Transbaikalia have negative εNd(Т) values. It is likely that lithospheric mantle was enriched in the ancient acid crust material subducted during accretion and collision of the two continental plates. This inference is supported by mildly enhanced I(Sr) ratios, 0,7050 0,7061.

U-Pb isotopic zircon (from Ol-gabbro of Orefiev pluton) dating shows age of gabbros 290 ± 5 Ma (MSWD=0.44). In view of the fact that gabbros of Orefiev and Khasurta plutons have geochemical similarities, this age (290 Ma) can be extrapolated to the latter.  It agrees with geological and geochronological data according to which Q-syenite-monzonite of Lower Selenga suite (U-Pb and Rb-Sr age 283.7 ± 5.3) intrude mafic rocks of Khasurta pluton.

Thus, the geochemical similarity of mafic rocks indicates that the formation of mafic magmas was related to the same region of the mantle magma generation. U-Pb isotopic data confirm the synchronism of high-alkaline mafic and granitoid magmatism of the West Transbaikalia in the Late Paleozoic. According to isotopic age of Orefiev gabbros (290 Ma), these mafic rocks relate to high-K quartz-syenite and monzonite. The problem of mafic rocks associated with granites of Barguzin suite still open. 



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Kovalenko V.I., Yarmolyuk V.V., Kovach V.P., Kotov A.B., Kozlovsky A.M., Salnikova E.B., Larin A.M. Isotope provinces, mechanisms of generation and sources of the continental crust in the Central Asian Mobile Belt: geological and isotopic evidence // Journal of Asian Earth Sciences. 2004. Vol. 23. Р. 605-627.

Litvinovsky B.A., Tsygankov A.A., Jahn B.M., Be’eri-ShlevinY. Origin and evolution of overlapping calc-alkaline and alksline magmas: The Late Paleozoic post-collisionaligneos province Transbaikalia //Lithos. 2011. Vol. 125. P. 845-874.

Litvinovsky B.A., Zanvilevich A.N., Alakshin A.M. et al. The Angara-Vitim batholith is the largest granitoid pluton. Novosibirsk. UIGGM SB RAS. 1993. P. 141.

Tsygankov A.A., Litvinivsky B.A., Jahn B.M., Reichow M., Liu D.Y., Larionov A.N., Presnyakov S.L., Lepekhina Ye.N., Sergeev S.A. Sequence of magmatic events in the Late Paleozoic of Transbaikalia, Russia (U-Pb isotope data) // Geology and Geophysics. 2010. V.51. N 9. P.1249-1276.