ISOTOPE-GEOCHEMICAL FEATURES OF APOULTRABASIC METASOMATITES OF THE SAYAN-BAIKAL FOLDED AREA
M.V. Rampilova, G.S. Ripp, M.O. Rampilov, B.B. Damdinov, L.B. Damdinova, V.F. Posokhov
Geological Institute, Siberian Branch of the Russian Academy of Sciences, Ulan-Ude, Russia
Keywords: Serpentinite, nephrite, listvenite, geochemistry, isotope geochemistry, fluid inclusions
Abstract
The paper is concerned with a geochemical study of apoultrabasic metasomatites of the Ospa-Kitoi, Parama, and Ust’-Kelyana ophiolite massifs located in the southern folded framing of the Siberian craton. The isotope (O, C, H, Sr, and Rb) systems of dunites, serpentinites, nephrites, listvenites, and talc-carbonate rocks are studied. The isotopic composition of oxygen in olivines from dunites is characterized by δ18O = 4.6-5.5 ‰. The δ18O values of serpentinites (4.67-7.35 ‰) point to the mantle genesis of fluids and might have been inherited from ultrabasic rocks. Nephrites are slightly enriched in heavy oxygen isotope (δ18O = 6.13-9.54 ‰). This indicates that their fluid phase was transported from serpentinites and captured a small portion of the crustal component. The widest variations in δ18O values, from 8.12 to 17.46 ‰, are observed in minerals from listvenites. Carbonates from these rocks show a highly heterogeneous isotopic composition of oxygen (δ18O = 12.9-18.8 ‰) and carbon (δ13C = -2.8 to +2.8 ‰). These rocks formed with the contribution of metamorphogenic fluids. According to the isotopic composition of hydrogen, the examined serpentinites are divided into two groups: with δD values specific to “magmatic water” (δD = -73.50 to -85.00 ‰) and those typical of meteoric fluids (δD = -151.90 to -167.20 ‰). The listvenites are characterized by low Rb and high Sr contents. Their 87Sr/86Sr values (0.70702-0.70971) indicate the contribution of a crustal source. The study of fluid inclusions in minerals from listvenites has shown that the rocks formed under relatively low-temperature conditions. The homogenization temperatures of fluid inclusions in quartz and magnesite from listvenites of the Ospa-Kitoi massif are 184-290 ºC and 122-182 ºC, respectively. In the Parama massif, the homogenization temperature of fluid inclusions in quartz is 130-170 ºC. The solutions that formed listvenites of the Ospa-Kitoi massif were slightly saline (TDS = 2.9-8.4 wt. % NaCl eq.), with NaCl and Na2CO3 being the main salt components.
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