Micas from the Khaluta carbonatite deposit, western Transbaikal region

Abstract

The Khaluta carbonatite deposit located in the western Transbaikal region was formed during the Late Mesozoic rifting in the southern framework of the Siberian Craton. Carbonatite is associated with shonkinite and syenite and is accompanied by fenitization. The composition of mica in more than 160 samples of country rocks, carbonatites, silicate rocks, and fenites was studied. The Fe3+ and Fe2+ contents, as well as oxygen isotopic composition, were determined. The Mg and Fe contents increase, whereas the Ti and Al contents decrease in micas when passing from silicate rocks and fenites to carbonatites. Micas from carbonatites are depleted in Al, enriched in Fe3+, and distinguished by high Si and F contents. According to our calculations, in some cases Al replaces Si in the tetrahedral site instead of replacement of Fe3+ as is characteristic of tetraferriphlogopite. Formally, the mica from carbonatites falls within the tetraferriphlogopite field, but typical inverse pleochroism is not always observable. The δ18O values of micas from carbonatite, shonkinite, syenite, and fenite are similar to those of mantle-derived silicate minerals. The δ18O values in the minerals coexisting with phlogopite testify to their isotopic equilibrium and make it possible to calculate the crystallization temperature of carbonatite.