Petrogenesis and Physical Volcanology of a compositionally zoned ignimbrite on Tenerife, Canary Islands, Spain

A Quaternary phonolitic Plinian volcanic eruption from Las Cañadas volcano on Tenerife emplaced a chemically zoned ignimbrite that shows gradational vertical variations in chemical composition, welding characteristics and lithofacies. In contrast with the majority of younger deposits from Las Cañadas volcano, it contains an abundant and varied population of phenocrysts and shows welding at some localities. This project aims to understand the chemical and mineralogical variations within the ignimbrite in terms of magma chamber processes and to determine the emplacement mechanism of the ignimbrite. The geochemical study of ignimbrite samples involved whole rock and mineral analyses using XRF spectrometry, EMP analysis and ICPMS techniques. The geochemical variations detected in the Arico deposits emplaced during both Plinian fall and pyroclastic flow phases of the eruption can be explained hypothesizing a petrogenetic model involving fractional crystallization in a single closed system magma chamber. Mineral compositions imply equilibrium conditions with the host liquid and therefore strongly support the fractional crystallization hypothesis. Although magma mixing is considered insignificant, involvement of AFC processes cannot entirely be ruled out. Whole-rock and mineral chemistry data confirm the existence of chemical vertical zoning in the Arico ignimbrite. The vertical double zonation in Magua locality records an eruptive waxing phase (leading to normal zonation) followed by a waning phase (leading to reverse zonation). The reverse zoning in Poris by the Sea locality can be explained by erosion of the normally zoned deposit before the reversely zoned section was deposited, or by lack of deposition of the normally zoned section. The complex record of zoning reversals found at the localities of Charco del Pino and Saltadero could be due to repeated erosion phases and/or bypass phases and may involve deposition of multiple lobes in different locations and times.