Rhyolite petrogenesis and meteoric–hydrothermal alteration at the Maghnia volcanic massif, Northwest Algeria

Abstract

Volcanic rocks from the Maghnia region of northwest Algeria consist of fresh to altered rhyolitic units that were emplaced during late Miocene time. Petrographic observations and geochemical data indicate that rhyolitic rocks were derived from a hybridized peraluminous magma. Petrogenetic modeling, REE contents and oxygen isotope data of quartz and feldspar and spessartine-rich garnet allows the reconstruction of the fractional crystallization between 680 and 780°C and pressure of less than 0.5GPa. Rhyolitic units are partially weathered and form an economic deposit of montmorillonite. Spectroscopic data and stable isotopes (O, H) of glass and montmorillonite indicate that the residual glass was partially hydrated by meteoric water at temperatures between 31 and 54°C. The chemical mass balance indicates that rhyolitic rocks were altered to montmorillonite by meteoric water enriched in Ca, Mg and Sr. The combination of the chemical mass balance and the 87Sr/86Sr initial ratios of rhyolite and montmorillonite indicate that continental water/rock interaction was related to the surrounding Miocene lacustrine limestone.