Bimodal sill emplacement from an inhomogeneous source: a case study of olivine-gabbronorite and diabase with Ni-Cu sulphides in a sill within Permian sediments from Western Sicily, Italy.

A small outcrop of mafic rocks, located between the Permian-Triassic sediments of the Cenozoic tectonic unit of the Maghrebian chain in Western Sicily, is an olistolith made up of olivine gabbronorite within diabase. Textures, microprobe analyses, and major and trace element geochemistry confirm the co-genetic nature of the various rocks: the gabbronorite is the result of olivine and orthopyroxene accumulation, with interstitial clinopyroxene and plagioclase, in a framework of a gabbrodioritic diabase. At least two stages of magmatic crystallisation during ascent in the continental crust are described, on the basis of reconstructed conditions of T, P and XH2O. The primitive magma was reconstructed by means of mass balance calculations on major and trace elements, and crustal contamination was taken into account. The uprising post-Hercynian extensional and continental tholeiitic magma, enriched by an amphibole upper mantle source (high HFSE/LILE and LREE/LILE), crystallised chromite and troctolite cumulate in the upper crust chamber; an immiscible Cu-Ni sulphide magma formed during late increased crustal contamination. The residue (≈10 wt%), a hydrated, S-rich crystal mush and suspended differentiated magma, thus intruded into the subvolcanic sill (P≈100 MPa) diabase being intruded by gabbronorite crystal mush. Late hydrothermal alterations along progressive, extensional propagation of cracks forming columnar joints, together with late sepiolite-group crystallisation, are other features of the outcrop.