MULTIPLE LIQUID LINES OF DESCENT FROM ALKALI BASALT TO PERALKALINE RHYOLITE: EXAMPLES FROM PANTELLERIA, ITALY

Silica-oversaturated, peralkaline (mol Na+K/Al > 1) trachyte and rhyolite can be further classified as either “pantelleritic” or “comenditic,” with the former more enriched in iron relative to aluminum (1.33*FeO^T > Al₂O₃ - 4.4) than the latter. Both types of rock frequently comprise the felsic end-member in bimodal (basalt-rhyolite) suites commonly described in intraplate volcanic settings. Although in most cases the felsic rock is pantellerite or comendite, there are several instances where both types of rock occur. One such place is on the island of Pantelleria, situated in the Strait of Sicily Rift Zone in the central Mediterranean Sea and the type locality for pantellerite, where eruptions of ignimbrite over the past 190 ka have been pantelleritic (such as the most recent, 45.7 ka eruption of the Green Tuff) or comenditic. A salient feature of these ignimbrites is that despite their differences in major element geochemistry and mineral assemblages, they have nearly identical trace element patterns and incompatible trace element ratios, all similar to the mafic (alkali basalt) end-member. This strongly suggests that the differences between the petrogenesis of comendite and pantellerite do not arise from differences in their parental magmas but likely reflect differences in the storage conditions of the magma reservoirs, which affect the intrinsic parameters (temperature, pressure, volatile solubility, and oxygen fugacity) that control the fractionating mineral assemblages and liquid line of descents from the basalts. In this study, we use quantitative modelling methods to compare the describe and compare the mineralogy and geochemical evolution of pantelleritic and comenditic suites at Pantelleria to determine the processes that drive the petrogenesis of alkali basalt to one end-member or the other.