Mafic and ultramafic enclaves in Ustica Island lavas: Inferences on composition of lower crust and deep magmatic processes

Abstract

Ustica Island, southern Tyrrhenian Sea, is constituted of Quaternary alkaline volcanics. A variety of enclaves representative of deep to supra-crustal settings were recently found in a hawaiitic lava flow. Enclaves consist of: (i) Ultramafic meta-cumulates, i.e. clinopyroxenites and wherlites characterized by variably deformed porphyroclastic to granoblastic textures. (ii) Mafic cumulates, i.e. gabbros (± amphibole) and troctolites, the first often characterized by frequent amphibole breakdown coronas (olivine + Ti-augite + plagioclase + magnetite + ilmenite + rhönite) in response to an H 2O decrease during the ascent, while the troctolites interpreted as meta-cumulates. (iii) Microsyenites, consist of anorthoclase and Fe-clinopyroxene organized in a granular sub-ipidiomorphic texture. Amphibole is absent in Ustica lavas and is found only in some old, now exposed, sub-intrusive volcanic bodies. This evidence suggests a late appearance of amphibole on the liquidus, at a high crystal content that inhibits further ascent of the magma. The importance of the amphibole as a medium pressure liquidus phase in Ustica mafic magmas is in the bearings on the geochemistry of lavas e.g. in buffering Na and Ti abundances, in trace elements partitioning, etc. Density measurements pointed out higher values for clinopyroxenites (3160 to 3300 kg/m 3) than for gabbros (ca. 2900 kg/m 3). Given the density contrast between enclaves and host lavas (2790 kg/m 3) and assuming appropriate rheological models, we calculated a minimum ascent rate of 0.01 m/s, corresponding to an ascent time in the range of 5–29 days for a depth of entrapment of 25 km.