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

[Marco E. Ciriotti]

Referenza:
▪ Vichi, G., Perna, M.G., Ambrosio, F.A., Rosatelli, G., Daniele, C., Broom-Fendley, S., Valdykin, N.V., Zaccaria, D., Stoppa, F. (2023): La Queglia carbonatitic melnöite: a notable example of an ultra-alkaline rock variant in Italy. Mineralogy and Petrology, 117, (in press).

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

[TOSATO FABIO]

Interessante per una regione, Abruzzo, non molto ricca di minerali.

[Alessio73]

Beh il problema è che alcune zone interessanti sono in zona parco. Anche questa ovviamente.
Tanto per dire ci sono due zone con delle anomalie radioattive, una delle quali ha l' aggravante... è in una grotta carsica.

[Marco E. Ciriotti]

Pubblicazione effettuata.

Referenza:
▪ Vichi, G., Perna, M.G., Ambrosio, F.A., Rosatelli, G., Daniele, C., Broom-Fendley, S., Valdykin, N.V., Zaccaria, D., Stoppa, F. (2023): La Queglia carbonatitic melnöite: a notable example of an ultra-alkaline rock variant in Italy. Mineralogy and Petrology, 117, 505–528.