Caught in the act — The first few hours of xenolith assimilation preserved in lavas of the Rockeskyllerkopf volcano, West Eifel, Germany

Abstract

Lithic sedimentary xenoliths in magmatic pyroclastic deposits in the Rockeskyllerkopf Volcanic Complex of the Quaternary West Eifel volcanic field preserve evidence of the earliest stages of melting, reaction with and assimilation into their silica-undersaturated basanite host. The xenoliths have a 0.5–1.5 mm thick surface glaze of rhyolitic glass and internally have both silica-undersaturated and silica-saturated glasses that are the result of in situ melting of micas and clay minerals and reaction of these melts with the surrounding quartz matrix. Mixing between the rhyolite glaze and host basanite resulted in the formation of a clinopyroxene- and glass-rich hybrid zone. The glazed xenoliths are commonly found with either no coating of lava or with void space between them and the host. This unusual occurrence is due to vesiculation of the glaze during eruption which led to physical separation of the xenolith from its surrounding melt. Simple calculations indicate that assimilation occurred at ~ 1180 °C over a period of ~ 12 h and that all the textural features including deformation of the partially molten xenoliths, occurred during transport and eruption rather than after deposition.