The Calatrava paradox to decipher the origin of carbonatites. A petrological insight on Finca La Nava, Calatrava Province (central Spain)

The paper presents a detailed petrographic and geochemical study of the pyroclastites outcropping at La Nava maar (Calatrava Volcanic Province-CVP). Here, extrusive carbonatites mix with melilite nephelinite lapilli and bombs, crustal and mantle debris. Nephelinite and carbonatite are not characterised by a HFSE distribution typical of immiscible pairs. ΣREE and La/Lu ratios reflect primitive mantle carbonates and kimberlites. Igneous carbonates have δ13C and δ18O isotopic ratios in the range of extrusive carbonatites and differ from local sedi- mentary carbonates. Carbonatites have 87Sr/86Sr and 144Nd/143Nd isotopic ratios similar to Calatrava leucitites but different to melilite nephelinite. The difference indicates that melilite nephelinites and carbonatites are co- eruptive but not comagmatic. The mingled magmatic convoy, underwent decarbonation processes, releasing Ca and Mg that reacted with parental silicate melt. Consequently, the nephelinitic melt evolved towards a melilititic composition producing Ca-rich overgrowths on mafic phenocrysts and reacted to produce high Mg#, low Cr + Ni forsterite. Mass balance calculations indicats that the nephelinite magma assimilated up to 30% of the carbo- natite magma. Mixing modelling based on the Sr and Nd isotopic composition of mantle xenoliths and carbo- natites indicate that La Nava carbonatite may represent the mantle metasomatic agent.