Geochemistry of carbonatite-silicate pairs in nature: A case history from Central Italy

The IUP carbonatites and kamafugitic foidites definitely form intra-outcrop conjugate pairs. All these co-eruptive rocks have parallel trace element patterns, namely REE, which implies a dilution-concentration relationship among them but always with higher contents in primary calcite. Based on current textural and compositional criteria, as well as comparable experimental data, we attribute this feature to liquid immiscibility dominant over crystal fractionation at crustal pressure. This relatively late immiscibility phenomenon is superimposed on co-magmatic features shown by inter-outcrop conjugate rock couples. In fact if we consider San Venanzo kamafugite and Polino Ca-carbonatite, or Grotta del Gervo kamafugite and Oricola Ca-carbonatite, we note couple by couple that they are chemical heteromorphs erupted in isolation in different place. The REE distribution is another distinctive feature of these couples and shows a marked crossover at MREE level. A fact we interpret as produced by near mantle-solidus immiscibility. After this early phenomenon the two members of the couple can erupt in a near primary state carrying mantle xenoliths or undergo some evolution including settling out mantle xenoliths and crystals and fractionation and finally exsolve a carbonatitic residuum by immiscibility.