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THE ARSHAN REE CARBONATITES, SOUTHWESTERN TRANSBAIKALIA, RUSSIA: MINERALOGY, PARAGENESIS AND EVOLUTION

The Arshan carbonatites in the Transbaikalia region of Siberia consist of sovite with high concentrations of the REE, Sr, Ba, and SO 3 , and low concentrations of Nb and P 2 O 5 . There was a high fugacity of oxygen and fluid activity at the time of formation of these carbonatites, and they were later altered by hydrothermal solutions. Bastnasite formed as a magmatic mineral in the carbonatites, on the basis of petrographic evidence and fluid-inclusion data. The recrystallization of calcite, the replacement of bastnasite and phlogopite, and the formation of secondary sulfates, strontianite, calcite and fluorite resulted from a hydrothermal process. The 87 Sr/ 86 Sr values of minerals in the Arshan carbonatites are higher than in many other carbonatites, but are similar to those in the continental basalts of Transbaikalia. The δ 34 S CDT values resemble those of other Transbaikalia carbonatites and alkaline rocks. The δ 13 C V–PDB isotopic values in the carbonatite plot within the field defined for primary igneous carbonatites. On the other hand, δ 18 O V–SMOW values for the Arshan bastnasite is only partially within the usual range of mantle values. The altered carbonatites are characterized by lighter δ 18 O values, probably because of the influence of meteoric water. The carbonatites were formed from an enriched source in the mantle and crystallized rapidly at shallow depth.