Evidence of Early Oligocene submarine volcanism in the Caltanissetta Basin (Central-Southern Sicily)

Petrological and geochemical data of volcanic rocks from several sites of the Caltanissetta Basin (central-southern Sicily) are discussed to provide information about volcanism in the northern portion of the african plate. Volcanics occur as large isolated blocks, mainly as pillows and subordinate lava flows, enclosing sedimentary levels of marly limestones. They are packed in several paleogene and neogene clayey lithologies, which are correlated to the sedimentation into the Miocenic Foredeep. The age of volcanism is referable to early Oligocene (Rupelian), as revealed relative dating of the interpillow calcareous sediments. The studied samples are transitional and poorly evolved alkali basalts. Petrographic study highlights a discrete uniformity for the most samples, with porphyritic texture characterized by olivine, clinopyroxene and plagioclase phenocrysts in a microcrystalline groundmass, composed of the same phases plus opaque minerals. Major and trace element data are poorly variable and the trends show low degree of fractional crystallization with some contribution of mineral accumulation. Abundance and ratios of incompatible elements resemble OIB-type volcanics from intraplate environment. A slightly different garnet lherzolite sources that underwent a low partial melting degree can be hypothesized for the studied rocks. The oligocene volcanism of the Caltanissetta Basin results as different magmatic pulses intruded along lithospheric fractures originated in response to flexure and uplift of the african paleomargin, in the pre-subduction stage and then incorporated in the future Sicilian accretionary prism.