Volcanologic concerns of the siliceous metasedimentary xenoliths included in historic lava-flows of Lanzarote (Canary Islands)

Abstract

A large number of siliceous metasedimentary xenoliths appears in some historic lava flows of Lanzarote (Canary Islands). In most cases those xenoliths are constitute of α -cristobalite, α-trydimite , calc-silicates (wollastonite 2M and diopside) or paragenesis of calc-silicates and α -cristobalite. The presence of microfossils ghosts, detrital grains and sedimentary structures reveal that all these xenoliths are formed by thermal metamorphism of sedimentary rocks, many of them rich in radiolarians. The variable amount of radiolarians and the other components (calcareous microfossils, micrite, and mudstones) determine the formation of silica minerals or calc-silicates during the thermal transformations. The thermal transformation would have taken place in the contact of the sedimentary rock with an intrusive body or shallow magmatic chamber corresponding to an endogenous stage of the 173O-1736 eruption. The high temperatures reached in this process, together with the mineralogical composition of the siliceous sedimentary rock, rich in opal-CT, determine the formation of tridymite and cristobalite instead of quartz. The information obtained from the study of the transformed sedimentary xenoliths also allows us the quantification of depth and temperature of the magmatic chamber, which coincide with those obtained by other methods.