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 withthose 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 (N500 ppm) were also detected in the glass. Careful macroscopic and microscopic observations exclude the possibility that external silicate melt infiltrated the xenolith. The occurrence of glass pockets between the mafic clay assemblages and the feldspar grains, along with comparisons between chemical compositions of the glass and the surrounding minerals,suggest that the glass is due to the melting of a eutectoid system consisting of Na-rich alkali feldspar, Fe–Mg rich clays and aegirine–augite. Halogens had probably played an important role in the partial melting process by decreasing the melting temperature of modal minerals, especially feldspar. The occurrence of these trachytic glasses lends support to petrologic models suggesting that partial melting of a hydrothermally altered, brine-rich oceanic crust induced by shallow-seated basic intrusions can produce primary trachytoid melts. This may explain the “Daly-gap” characterizing some oceanic within-plate volcanoes.