La Queglia carbonatitic melnöite: a notable example of an ultra-alkaline rock variant in Italy

Abstract Very primitive ultramafic igneous rocks occur at Mt. La Queglia (Abruzzo, Italy). They form a strongly deformed sill–dyke system now tilted vertically. These were initially classified as alnöite and, subsequently, have been suggested to be carbonatitic olivine melilitite. However, further investigation and interpretation of these is needed due the presence hand-specimen-scale textural variation suggesting complex petrogenesis. We study texture, mineral chemistry, whole-rock geochemistry define three main rock-types. (1) A brecciated rock with an ocellar texture composed calcite pseudomorphs after melilite, plus fresh diopside in groundmass mica, aegirine, garnet, calcite, apatite, perovskite, titanate chlorite. Zoned ocelli this show amoeboid shape, agglutination, menisci typical plastic state. (2) quenched showing spinifex containing long feathery phenocrysts cpx mica suspended nepheline, Ti–rich magnetite, abundant some K-feldspar zeolites. (3) coarse-grained intergranular glauconite, mixture spinel group accessory barite, pyrite, chabazite-K. The extreme SiO 2 -undersaturation (33.5–37.3 wt% ), high MgO contents TiO /Al O 3 ratios. Rock-type 1 has lower Mg number Mg# = 100 × [Mg/(Mg + Fe 2+ )], higher Ca Ca# [Ca/(Ca Mg)], Cr (up 720 ppm) Ni 379 ppm), rare earth elements (REE) well La/Lu ratio, compared rock-type 2. Perovskite chromite accumulation seems important agent during differentiation. shows REE cross-over light (L)REE concentration carbothermal residuum. are end-member other Upper Cretaceous Paleogene Italian lamprophyres, low degree melting HIMU (a colloquialism for “high-?”; referring mantle domains 238 U/ 204 Pb) garnet-bearing source.