Late mesozoic carbonatites of western Transbaikalia: Mineralogical, chemical, and isotopic (O, C, S, and Sr) characteristics and relationships to alkaline magmatism

Late Mesozoic carbonatites occur in the western part of the western Transba ikalia volcanic (rift) province. They are known in six localities: Khalyuta , Oshurkovo, Arshan, Yuzhnoe, Torei, and Borgoi. Carbonatites mainly occur as veins and volcanic facies were found only in the Khalyuta Complex. The m ain mineral of the carbonatites is calcite. Peculiar reticulate barite, som etimes also fluorite and bastnaesite are typomorphic minerals of the carbon atites. The carbonatites are characterized by high contents of Pa, Sr, and REE (LREE-enriched pattern) and low contents of Ta, Nb, and Zr. The O, C, a nd S isotopic composition of unaltered carbonatites delta(18)O ranges from 6.0 to 10.7 parts per thousand and delta(13)C ranges from -4.1 to -6.5 part s per thousand) is similar to those from other world regions. The oxygen an d carbon isotopic composition indicates that the carbonatites were derived from a mantle source and then experienced secondary alteration. The Sr isot ope composition and other geochemical characteristics of the carbonatites a re similar to those of associated alkaline silicate rocks (alkali gabbros a nd syenites, teschenites, shonkinites, and nepheline syenites). The Late Me sozoic alkaline magmatic rocks (silicate and carbonate) are restricted to t he horst framing of the Gusinoe Ozero graben. They were originated owing to the activity of the Nizhnyaya Selenga magmatic center above a mantle hot s pot.