GENESIS OF CRYSTAL-RICH EPICLASTIC ROCKS FROM SUBAQUEOUS SILICIC LAVADOMES - ROLE OF THERMAL-SHOCK ON QUARTZ PHENOCRYSTS

Deposits in the Palaeozoic marine basin of the Sarrabus region (SE Sar dinia, western Mediterrariean, Italy) show the presence of rhyolitic m icropillow lithofacies in the uppermost, sector of extrusive subaqueou s acid domes. These particular volcanic rocks contain quartz phenocrys ts affected by curved fractures attributed to thermal shock produced i n the magma-water interface. The flanks of subaqueous domes contain pr oximal epiclastic sandstones and greywackes characterized by the prese nce of non-mature angular quartz clasts, very similar to the occurrenc es detected in the rhyolitic micropillows. Therefore, a syngenetic (re ferred to acid domes) and proximal (unreworked) character is inferred for these epiclastic deposits. A genetic model is proposed to explain the association of lithofacies (volcanic and epiclastic) in direct rel ationship with the subaqueous growth and erosion of the domes. This pr ocess of genesis of texturally unmature epiclastic rocks not related t o explosive acid volcanism seems-rather new; and the conclusions of th e paper may be in sharp contrast with some of the criteria previously described for distinguishing between pyroclastic and epiclastic crysta l-rich rocks.