Mafic and ultra-mafic enclaves from Ustica island lava: inferences on deep magmatic processes

Silvio Giuseppe Rotolo, Massimo Pompilio, Marina Alletti

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19 Citations (Scopus)

Abstract

Ustica Island, southern Tyrrhenian Sea, is constituted of Quaternary alkaline volcanics. Avariety of enclaves representative ofdeep 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 (F amphibole) and troctolites, the first often characterized by frequent amphibole breakdown coronas (olivine+Tiaugite+plagioclase+magnetite+ilmenite+rhfnite) in response to an H2O decrease during the ascent, while the troctolitesinterpreted as meta-cumulates. (iii) Microsyenites, consist of anorthoclase and Fe-clinopyroxene organized in a granular subipidiomorphictexture. Amphibole is absent in Ustica lavas and is found only in some old, now exposed, sub-intrusive volcanicbodies. This evidence suggests a late appearance of amphibole on the liquidus, at a high crystal content that inhibits further ascentof the magma. The importance of the amphibole as a medium pressure liquidus phase in Ustica mafic magmas is in the bearings onthe 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/m3) than for gabbros (ca. 2900 kg/m3). Given the density contrast between enclaves and host lavas (2790 kg/m3) and assuming appropriate rheological models, wecalculated a minimum ascent rate of 0.01 m/s, corresponding to an ascent time in the range of 5–29 days for a depth ofentrapment of 25 km.
Original languageEnglish
Pages (from-to)151-167
Number of pages17
JournalLithos
Volume84
Publication statusPublished - 2005

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All Science Journal Classification (ASJC) codes

  • Geology
  • Geochemistry and Petrology

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