Abstract
The Rb-Sr isochron age of igneous ankerite-calcite and siderite carbonatites in central Tuva is esti- mated at 118 ± 9 Ma. The following ranges of initial values of O, C, Sr, and sulfide and sulfate S isotopic com- positions were established: δ 18 e carb = +(8.8-14.7)‰, δ 13 e carb = -(3.6-4.9)‰, δ 18 e quartz = +(11.6-13.7)‰, δ 34 S pyrite = +(0.3-1.1)‰, and ( 87 Sr/ 86 Sr) i = 0.7042-0.7048 for ankerite-calcite carbonatite and δ 18 e sid = +(9.2- 12.4)‰, δ 13 e sid = -(3.9-5.9)‰, δ 18 e quartz = +(11.2-11.4)‰, δ 34 S pyrite = -(4.4-1.8)‰, δ 34 S sulfate = +(8.6- 14.5)‰, and ( 87 Sr/ 86 Sr) i = 0.7042-0.7045 for siderite carbonatite. The obtained isotopic characteristics indicate that both varieties of carbonatites are cognate and their mantle source is comparable with the sources of Late Mesozoic carbonatites in the western Transbaikal region and Mongolia. The revealed heterogeneity of isotopic compositions of carbonatites is caused by their contamination with country rocks, replacement with hydrother- mal celestine, and supergene alteration.
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