Primary origin of some trachytoid magmas: Inferences from naturally quenched glasses in hydrothermally metasomatized gabbroic xenoliths (Hyblean area, Sicily)

Hydrothermally-modified gabbroic xenoliths from the Hyblean tuff-breccia deposits (Sicily) consist of albitized plagioclase, Fe–Mg-rich clays, aegirine–augite, ±zeolites, titanite, apatite, magnetite, and hydrothermal zircon. Pockets of silicate glass with perlitic cracking occur in some samples forming 15–20% (by volume) of the rock modal assemblage. Electron microprobe analyses show the trachytic composition of the glass, with generally peralkaline sodic affinity [molar Al2O3/(Na2O+K2O)~0.8 (average value); molar Al2O3/(Na2O+K2O+CaO)~0.7 (average value); Na2O/K2O (wt.%)=1.7–2.3]. The glass trace element abundances, obtained by secondary ion mass spectrometry (SIMS) analyses are consistent with those of world-wide trachytes (e.g. Zr/Ti=0.15–018; Nb/Y=0.73–1). Relatively high abundances of Cl (700–1600 ppm) and F (>500 ppm) were also detected in the glass.