Is Oshurkovo gabbro comagmatic rock to carbonatite?

Doroshkevich A.G., Ripp G.S.

Geological institute SD RAS, Ulan-Ude, Russia.

There are some veins of carbonatites at northern part of Oshurkovo apatite-bearing basic massive. Veins are 0.6-1 meters wide and over 100-200 meters long. Carbonatites are composed of calcite, magnetite, phlogopite, apatite and barite-celestine. Monazite, allanite and zircon are accessory. The contacts of carbonatites with country rocks are sharp and marked by titanite-phlogopite-magnetite-potash feldspar contact zones. Carbon and oxygen isotopic values for calcite from carbonatites plot in the field of primary igneous carbonatites and oxygen isotopic values for silicate and oxide minerals are similar to mantle-derived minerals (table). Isotopic thermometers were used to calculate the temperature of carbonatite formation. Magnetite-phlogopite isotopic thermometer (Chacko et. al., 2001) gave temperatures of 932-946⁰С. Calcite-magnetite formula of Chiba et al. 1989 yields temperatures of formation up to 625⁰С. U/Pb analysis (SRIMP II, zircon) of the carbonatite yielded age of 126.6 Ma and its age is similar to that of the West Transbaikalia carbonatites (Khaluta, Yuzhnoe, Arshan).

Table.

Isotopic composition of  d ¹³C ‰, d¹⁸O ‰  and initial ⁸⁷ Sr/  ⁸⁶Sr ratios of minerals from

Oshurkovo carbonatites and pyroxene rocks.

In brackets – number of analyses.

Association of the carbonatites with silicate basic rocks of Oshurkovo massive has been favoured by many authors (e.g. Nikiforov et al., 2002), but the hypothesis is not supported by reliable data.  Dating by the Rb–Sr and К-Ar methods have given early Cretaceous ages for the basic rocks of Oshurkovo massive (Kuznetzov, 1980; Litvinovsky et al., 2002) and, thus, age determinations imply that the Oshurkovo basic rocks and carbonatites are broadly coeval. Our U/Pb analysis (SRIMP II, zircon) of the basic rocks (gabbroes) yielded age of 280,6±2,7 Ma and showed the time differences between formation of carbonatite and gabbro. In addition, Ca-bearing minerals (plagioclase, diopside-salite, amphibole and apatite) that formed at early crystallization stage are the main minerals in basic rocks of Oshurkovo massive. These facts are not consistent with hypothesis that Oshurkovo gabbroes and carbonatites are comagmatic rocks. 

In this connection, the most interesting as comagmatic rocks to carbonatites are pyroxene rocks that have been discovered recently in Oshurkovo massive area. Pyroxene rocks form dykes with taxitic texture. They are characterized by the presence of coarse grained schlieren. The rocks consist of diopside (60-80%) and potash feldspar. Calcite and apatite are filling interstices. Amount of calcite is up to 10-15%. Titanite, allanite and magnetite are minor minerals. Diopside has 10-25% aegerine minal and contains the melt inclusions that were homogenized at 708-720⁰С. Potash feldspar is characteristically enriched in barium (up to 7 wt. %) and contains 0,9- 1,9 wt. % of Na₂O. Magnetite has up to 8-10 wt. % of TiO₂. Composition of apatite from pyroxene rocks is similar to that from carbonatites (fig. 1). They both have higher REE, SO₃ and SrO (at 2-3 time) than mineral from Oshurkovo gabbroes.

Pyroxene rocks contain 42-45 wt. % of SiO₂, 1,2 wt. % of К₂О and 2,2 wt. % of Na₂О. They are rather different in composition, with higher Ca and lower Al₂O₃ than Oshurkovo gabbroes. The REE patterns of pyroxene rocks are similar to those for the carbonatites and different than basic rocks of Oshurkovo massive (fig. 2).

Thus, isotopic data can imply that the pyroxene rocks and carbonatites are probably coeval and have a common magma source (see table).

Fig. 1.  Amount of SrO, REE and SO₃ in apatites from gabbro (1), carbonatite (2) and pyroxene rock (3).

Fig.2. Chondrite-normalized plot of the REE in the rocks of Oshurkovo massive. Chondrite values are from McDonough & Sun (1995). 

This study was financially supported by RFBR 08-05-98028.

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