PETROLOGY AND GEOCHEMISTRY OF THE ACQUA FORMATION, PANTELLERIA (ITALY): INSIGHTS INTO THE CONTRASTING ORIGINS OF COMENDITE AND PANTELLERITE IN A PERALKALINE MAGMATIC SYSTEM

The island of Pantelleria (Strait of Sicily Rift Zone, Italy) is the type locality for pantellerite, an iron-rich, peralkaline, silica-oversaturated volcanic rock that occurs most frequently as the felsic end member of bimodal suites in intraplate settings. Comendite is a similar, peralkaline silica-oversaturated volcanic rock that occurs in the same settings but lacks the iron enrichment. Although the Green Tuff, the most recent (~46 ka) ignimbrite erupted on Pantelleria, is comprised primarily of compositionally zoned pantellerite, seven of the eight ignimbrites known to have erupted earlier (189-85 ka) consist instead of compositionally zoned metaluminous trachyte, comenditic trachyte, and comendite. Although less peralkaline and lacking iron enrichment, these older units have a similar range of SiO₂ and nearly identical incompatible trace element ratios and rare earth element patterns as each other and the Green Tuff, strongly suggesting that they originated from similar parental magma despite their divergent paths. In this study, we present the petrography, geochemistry, and microanalyses of the 107 ka Acqua Formation (formerly “Units F and Q”), a potentially caldera-forming, compositionally zoned (comenditic trachyte to comendite) ignimbrite similar in estimated eruptive volume as the Green Tuff. The comenditic rocks are comprised of anorthoclase (Or₃₄), clinopyroxene, olivine, magnetite, and ilmenite, which contrasts with the pantellerites that tend to lack magnetite and include aenigmatite in the assemblage. From preliminary modelling we conclude that the petrogenetic differences are due to differences in oxygen fugacity, which may be the result of differences in pressure (depth), volatile content, or both.