Ree distribution in the late Hercynian dykes from Sardinia

The late Hercynian dyke magmatic rocks of Sardinia develops in two main stages, with different compositional and tectono-magmatic features, one preceding and the other following Permian volcanism. The first stage which is present all throughout the island – is represented by orogenic type products ranging in composition from basaltic andesites to ryolites and peraluminous rhyolites. These were erupted during two phases one Carboniferous, and the other Permian in age. In the successive late Permian stage basaltic products are dominant. These are mostly transitional, from mildly sodic to mildly potassic, and are associated with subalkaline tholeitic rocks, in the vicinity of shear zones in the northen part of the island. Scarce peraluminous rhyolites are also found in the late Permian stage. Sach a varied geographycal distribution of basaltic types seems to be linked to the different crustal thinning of the northern and central southern parts of the island. The REE patterns and the values of the initial Sr isotopic ratio make it possible to differentiate the contamination phenomena present in the magma source zones from those accompanying the ascent, and hence the evolution of the melts. In the calcalkaline orogenic suite two groups of rocks with different values of REE and initial Sr isotopic ratio have been identified, these can be traced back to the difference in composition of the source zone.