Lithostratigraphic analysis and geochemistry of a vitric spatter-bearing ignimbrite: the Quaternary Adeje Formation, Cañadas volcano, Tenerife

Abstract

"The 1.5-Ma Adeje Formation in SW Tenerife contains an ignimbrite sheet with remarkable textural and chemical complexity. A basal Plinian pumice-fall layer is overlain by a partly welded compound ignimbrite in which phonolitic pumice lapilli and dense obsidian spatter rags with irregular, fluidal-shaped margins are supported in a poorly sorted tuff matrix. The lower ignimbrite flow-unit contains accretionary lapilli in its upper part, overlain by an ash-pellet-bearing fallout layer from a co-ignimbrite plume. The upper ignimbrite flow-unit comprises a locally welded massive lapilli-tuff that grades up into lithic breccia containing juvenile obsidian blocks and both cognate and vent-derived lithic blocks. Geochemically, the Adeje Formation shows two distinct juvenile populations that relate to crystal-poor and crystal-rich magma types. Crystal-rich juvenile clasts contain multiple compositions of ilmenite and magnetite, and crystal aggregates of bytownite (An79-86). The varied assemblage of juvenile clasts reflects an eruptive style that may have involved rapid changes in magma chamber pressure associated with caldera collapse, and possibly the disruption of a lava lake. The Adeje eruption started with a Plinian explosive phase that rained ash and pumice lapilli across SW Tenerife; followed by pyroclastic fountaining feeding density currents with explosive ejecta of juvenile glassy material producing the coarse, spatter-bearing ignimbrite facies. A short pause between pyroclastic density currents is recorded by the co-ignimbrite ash and pellet-fall bed. The climactic phase of the eruption probably involved caldera subsidence as recorded by a widespread massive heterolithic breccia."