Explosive Phonolitic Magmatism: Mixing and Crystallization in the Fasnia Member, Diego Hernandez Formation, Tenerife, Spain

The Fasnia Member (309 ka) of the Diego Hernandez Formation (570 - 200 ka) represents one of the most significant voluminous explosive phonolitic eruptions in the history of Tenerife. The Fasnia's complex stratigraphy is comprised of multiple widespread plinian deposits, some lithic-rich, interbedded with pyroclastic flow and surge deposits. The Fasnia is divided into a Lower and Upper sequence based on minor localized reworking and gullying at a distinct stratigraphic interval, representing a short (days to weeks) eruption hiatus. The hiatus is important because it is a transition from early complex pulsatory activity to prolonged explosive plinian phases, reflected in the deposits. Petrologic data from Fasnia plinian pumice componentry (i.e. glass, minerals) require magma mixing between at least two distinct phonolitic magmas and a small volume of mafic- intermediate magma. Compositional variations within "high Zr" phonolite (1700-2950 ppm Zr) are consistent with a genesis through melting of preexisting syenitic plutons. The "low Zr" phonolite (460-800 ppm Zr) is an immature product of fractional crystallization from tephriphonolite. LA-ICP-MS trace element variations within juvenile clasts from the Lower Sequence show the evolved compositional range for the high Zr phonolite contrasting sharply with the low Zr phonolite. Coarseness of the mixture is on cm3 scale, both texturally and chemically. Upper Sequence juvenile clasts have chemically restricted intermediate trace element concentrations (1100-1800 ppm Zr) indicating further homogenization (to the mm3 scale) of the two-phonolite mixture, punctuated by a late appearance of relatively pure low Zr magma. Few hauyne crystals and only endmember titanite and clinopyroxene populations occur in the Lower Sequence, yet in the Upper Sequence an abundance of hauyne and an intermediate titanite is observed. We hypothesize that the during the Fasnia eruption hiatus, vigorous convection in the magma chamber partially homogenized the magma mixture which crystallized hauyne and intermediate titanite, but not clinopyroxene. The mixing event created conditions for sustained plinian activity.