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

摘要

The Isola del Toro is a small island (similar to 1 km(2)) 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. Ar-40/Ar-39 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.