The kamafugite-carbonatite associations of Central Italy: silicate-carbonate immiscibility
Nikolaeva A.T. 1,2, Panina L.I.1
1 V.S. Sobolev Institute of Geology and Mineralogy SB RAS
2 Novosibirsk State University
The silicate-carbonate immiscibility investigation in experimental systems and in natural melt inclusions suggests that this process plays a major role in the origin of some natural carbonatites (Hamilton, Kjarsgaard, 1993; Lee, Wyllie, 1997; Panina, Motorina, 2008).
It is interesting to study the silicate-carbonate immiscibility in the kamafugite complexes of Central Italy because within these complexes carbonatites are spartially associated with alkaline high-Ca rocks. The main district of kamafugite-carbonatite locality distribution in Central Italy is Intramontane Ultra-alkaline Province (IUP).
We obtained the data for Cupaello volcano, that is represented by carbonatite tuff and kalsilite-bearing melilitite lava flow about 750 m long (Stoppa, Cundari, 1995). Primary silicate-carbonate inclusions were studied in the clinopyroxene phenocrysts. During the heating experiments of inclusions at 1080-1150°C silicate-carbonate immiscibility occures in the melilititic melt conserved in inclusions. Composition of the separated carbonate melt is characterized (wt.%) 1.7-4.81 SiO2, 0.1-0.2 TiO2, 0.5-1.1 FeO, 0.6-1.3 MgO, 23.1-31.2 CaO, 1.4-4.5 Na2O, 8.9-13.2 K2O, 3-4.4 BaO, 2.4-3.1 SrO, 0.3-1.95 P2O5, 0.06-0.44 Cl, 0.2-0.4 SO3. It is enriched with alkalies, predominantly K, Ba and Sr.
The silicate-carbonate immiscibility (Panina et al., 2003; Sharygin, 2001) was also found during the formation of olivine melilitite and melilitolite of Pian di Celle volcano (IUP). Silicate-carbonate immiscibility occured at 1240 °Ñ and persisted to 670 °Ñ in conserved melt in olivine and melilite of these rocks. The carbonate globule consists of (wt.%): up to 19-32 CaO, 18-24 SiO2, 2.4-4 Al2O3, 3-5.9 FeO, 5-11 MgO, 0.3-1 Na2O, 1.5-2 K2O, up to 0.1 BaO, 0.6-0.8 SrO, 0.1-1 P2O5, 0.1-0.2 Cl, 0.1-1 SO3. This composition differs from that of Cupaello with high amount of Si, Al, Fe, Mg and less – alkalies, Sr, and Ba, and is close to composition of Fort Portal carbonatite (Uganda).
Melt inclusion study in minerals of melilitites from IUP volcanoes indicates that carbonatites spartially associated with melilititic rock of this province were formed as a resulted of silicate-carbonate immiscibility in initial magmas.
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