EPISODI VULCANICI IN SUCCESSIONI CARBONATICHE MEDIO-TRIASSICHE PREPIEMONTESI (COGOLETO, LIGURIA)

Abstract

Tuffaceous layers of Middle-Triassic age, weakly metamorphosed, are interbedded within a carbonatic succession outcropping near Cogoleto (Voltri Group, Pennidic Zone of the Alps). Volcanic glass, K-feldspar, plagioclase and biotite subordinate, constitute the primary mineralogical composition. From a palaeogeographic point of view the studied sequences belong to the Pre-piemontese or to the Outer Piemontese Zone, that is placed near the border of the lnner Piemontese Zone,that in Jurassic time opened in an oceanic basin. The microfacies and the chemistry of the carbonatic part of the sequence suggest that the lower part belongs to a shelf environment, while the upper part to a deeper environment.
In spite of the difficults of interpretation that metamorphic pyroclastic rocks present we arranged them in a ryolitic-ryodacitic rock-type and in a calc-alkaline chemical trend. Nevertheless we suggest that the pyroclastic layers fit in the early rifting af the Ligure-Piemontese oceanic context.