The role of liquid immiscibility of calcitic carbonatites from the Malyi Murun Massif (Aldan)

Silicate salt liquid immiscibility has been established at 1280-1260 degree s C in the inclusions from pyroxene and monticellite from melilite-monticel lite-pyroxene-olivine rocks of the Malyi Murun Massif. Melilite and spinel of the rocks contain alkaline high-calcium sulfate-carbonate inclusions, wh ich at a microlevel might have reflected the process of spatial separation of salt melts from silicate magma. In this study, similar salt inclusions w ere also found in pyroxene and calcite of the above rocks and in the minera ls of pyroxenites and lamproites. They contain (wt. %): CaO - to 20-40, alk alies - to 27 (with strong predominance of Na over K), SO3 - to 28-38, and CO2 - to 9-30 (calculated). Minor amounts (few percent and less) of SiO2, F eO, MgO, SrO, BaO, F, and Cl were also revealed. In general, the compositio n of alkaline high-calcium sulfate-carbonate inclusions appeared to be clos e (except the high concentrations of SO3) to the composition of sodium carb onatitic lavas of Oldoinyo Lengai (Tanzania). At the same time, these inclusions displayed alkaline sulfate-carbonate imm iscibility at about 1170-1200 degrees C. The fact of spatial separation of the alkaline high-calcium sulfate-carbonate melt was confirmed by the prese nce of inclusions of both alkali-sulfate and Ca-carbonate compositions. Alk alies (to 40 wt.% K2O and 9 wt.% Na2O) and SO3 (to 40-42 wt.%) played a lea ding role in the chemical composition of alkali-sulfate inclusions, whereas CaO (49-55 wt.%) and CO3 (to 40 wt.%) are most important in carbonate incl usions. Minor amounts of Si, Mg, P, Fe, Ba, Sr, F, and Cl are also present. The composition of carbonate inclusions shows a good correlation with the composition of calcitic carbonatites of the Malyi Murun Massif. A conclusion is made that three-phase alkaline sulfate-carbonate-silicate l iquid immiscibility was the reason for the appearance of calcium carbonatit e melts as a result of separation of alkaline high-calcium sulfate-carbonat e melts from silicate magma and further (at a lower-temperature stage) sepa ration and removal of alkalies from them in the form of alkaline-sulfate as sociates. Crystallization of calcium carbonate melts at 750-950 degrees C w as responsible for the formation of calcitic carbonatites, and the wide dev elopment of fenitization and formation of sulfate minerals was related to t he effect of alkaline sulfate melts on surrounding rocks.