Some data on the comendite type area of S. Pietro and S. Antioco islands, Sardinia

Abstract

A summary of the available data on the peralkaline rocks of S. Pietro and S. Antioco islands, together with, new chemical analyses and some preliminary K-Ar ages are reported.

Peralkaline rocks occur as ignimbrites, lava flows and domes usually deeply affected by hydrothermal alteration. Pantelleritic varieties are found within the dominantly comenditic association, which display K₂O contents higher than Na₂O ones. K-Ar data indicate that these peralkaline rocks have a middle Miocene age (≅ 15 m.y.). They occur in close field association with coheval andesitic and subalkaline acid volcanics belonging to the final products of the Tertiary calc-alkaline volcanic cicle of Sardinia.