The transition between orogenic and anorogenic magmatism in the western Mediterranean area: the Middle Miocene volcanic rocks of Isola del Toro (SW Sardinia, Italy)

The Isola del Toro is a small island (1 km2) about 5 nautical miles SW of Sardinia entirely made up of mildly alkaline sodic evolved rocks (benmoreites and trachytes) related to each other by fractionation processes. 40Ar/39Ar dating of an anorthoclase mineral separate from a trachyte gave an age of 11.83 ± 0.10 Ma. This age estimate falls within an interval previously considered a-magmatic in Sardinia. The results of this study shift the beginning of the Middle Miocene-Quaternary anorogenic volcanism of Sardinia back to the Late Serravalian. The transition between orogenic and anorogenic activity in Sardinia is markedly different from that recorded in the rest of the circum-Mediterranean area, where this transition is much more gradual, both in time and in terms of magma composition. On the other hand, the transition between orogenic and anorogenic magmatism in Sardinia is rather sharp and lacks hybrid compositions. The Isola del Toro volcanic rocks represent the first anorogenic products produced during the opening of the western Mediterranean Sea.

The Isola del Toro is a small island (1 km2) about 5 nautical miles SW of Sardinia. The island is entirely made up of mildly alkaline sodic evolved rocks (benmoreites and trachytes) related to each other by fractionation processes.In Sardinia there were two distinct magmatic episodes during the Cenozoic: the first,of Oligo-Miocene age (32-15 Ma), is considered to be related to subduction of a NW-dipping oceanic lithosphere; the second is upper Miocene-Quaternary (7-0.1 Ma), and shows anorogenic geochemical characteristics. 40Ar/39Ar dating of an anorthoclase mineral separate from a trachyte gave an age of 11.83 ± 0.10 Ma. This age estimate fall within an interval previously considered amagmatic in Sardinia. Initial 87Sr/86Sr ranges from 0.70481 to 0.70544, without correlation with SiO2, whereas 143Nd/144Nd ranges from 0.512783 to 0.512750, showing a good correlation with SiO2. The samples with the highest 87Sr/86Sr are likely to reflect contamination by sea water. Major and trace element and Sr-Nd isotopic compositions of the Isola del Toro volcanic rocks share more similarities with the oldest products of the upper Miocene-Quaternary magmatic phase (Capo Ferrato, Guspini, Rio Girone) than with those of the Oligo-Miocene Sulcis magmatism. The results of this study shift the beginning of the upper Miocene-Quaternary volcanism of Sardinia back to the late Serravalian, thus reducing the time gap of magmatic activity between the two magmatic phases to less than 4 Myr.