Alkaline mafic rocks from basic-ultrabasic massifs of the Aldan Shield: silicate-carbonate-salt immiscibility during their crystallization

Rokosova E. Yu.^1,2, Panina L.I.²

¹Novosibirsk State University, Novosibirsk, Russia

²V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

 

rokosovae@gmail.com

 

The origin of carbonatites is one of the most complicated and debated questions of recent petrology. The study of melt inclusions in minerals showed the presence of silicate-carbonate-salt immiscibility in rocks of basic-ultrabasic alkaline massifs of the Aldan Shield - Inagli, Ryabinovyi and Malyi Murun.

Primary completely crystallized silicate-carbonate-salt inclusions were studied in diopside of the shonkinites from Inagli massif. It was found that, diopside crystallized from homogeneous carbonate-salt silicate melt at 1170-1190°C. Homogeneous carbonate-salt silicate melt was separated into silicate and carbonate-salt fractions at 1150-1160ºC. The composition of silicate fraction evolves from alkali-basalt to alkali-trachyte. Carbonate-salt fraction has an alkali-carbonatite composition and were enriched in SO₃, Cl.

At the crystallization stage of diopside (1120-1190ºC) of the micaceous shonkinites from Ryabinovyi massif the melt was heterogeneous and consisted of immiscible silicate, carbonate-salt, and carbonate fractions. The composition of silicate fractions evolves from alkali-basalt to melaphonolite-alkali trachyte. The carbonate-salt and carbonate melts separated from silicate magma were enriched in Ca, alkalies, CO₂, S, Cl.

At the crystallization stage of monticellite and diopside-augite of melilite-monticellite-olivine rocks from Malyi Murun massif the initial melt was separated into silicate and salt (sulfate-carbonate) fractions at 1260-1280 ºC (Panina, Usol’tseva, 2000). The melt of the sulfate-carbonate inclusions was separated into immiscible alkali-sulfate and Ca-carbonate liquids at 1170-1200 ºC. It was concluded that alkaline sulfate-carbonate liquid immiscibility was responsible for the appearance of calcium carbonatite melts, and their crystallization at 800-900 ºC resulted in the formation of calcite carbonatites of the Malyi Murun massif.

Conclusion: the carbonatite melts are enriched alkalies, sulfur, chlorine and occurred as a result of silicate-carbonate-salt liquid immiscibility of initial melts at a high-temperature (1120-1280 ºC) stage of their evolution in considered alkaline massifs of the Aldan Shield.

 

Panina L.I., Usol’tseva L.M. The role of liquid immiscibility of calcitic carbonatites from the Malyi Murun massif (Aldan) \\ Geology and Geophysics , 2000, v.41, №5, p. 655-670 (in Russ.)