THE ORIGIN OF TRACHYTE AND THE “DALY GAP” AT PANTELLERIA, ITALY

Volcanic rocks that comprise the island of Pantelleria, Italy, consist of basalt, trachyte, and pantellerite, with a “Daly gap” that spans ~49 to 62 wt% SiO₂. At Pantelleria and many other peralkaline volcanic centers, the relationship between the basaltic and trachytic magma is ambiguous: major- and trace-element models and isotopic data are often consistent with an origin for trachyte from either fractional crystallization or equilibrium batch melting, and the presence of the “Daly gap” is often cited as prima facie evidence for the latter. We have tested both hypotheses with mass-balance, trace-element, and thermodynamic (MELTS) modelling. From these results we conclude that metaluminous trachyte formed as a result of 70 to 75% low-pressure (0.1 GPa) fractional crystallization of an assemblage of plagioclase, clinopyroxene, olivine, magnetite, and apatite from a hydrous (1.0-1.5 wt% H₂O) transitional basalt magma at relative oxygen fugacities approximately one log unit below the fayalite-magnetite-quartz buffer (FMQ-1). Thermodynamic modelling suggests that the “Daly gap” is the result of rapid differentiation. Some metaluminous trachyte lavas have positive Eu anomalies, high K/Rb ratios, high concentrations of Ba, and low concentrations of incompatible trace elements; these are interpreted to be the result of up to 40% accumulation of alkali feldspar. Comenditic trachyte, pantelleritic trachyte, and pantellerite formed after an additional 20 to 80% fractional crystallization of an assemblage dominated by alkali feldspar from metaluminous trachyte magma.