Compositional Zonations of the Arico Ignimbrite, Tenerife, Spain: Implications for Depositional Processes

We present preliminary results of detailed sampling and EMP analyses of glassy matrix within populations of essential pyroclasts for the classic phonolitic Arico Ignimbrite (Schmincke and Swanson 1967). Purpose of this work is to define compositional zonations within the deposit in the Bandas del Sur region of southern Tenerife, and to correlate those zonation patterns between spatially separated areas. We test the possible use of compositional horizons as depochrons for the deposit. Data indicate that the ignimbrite is weakly normally zoned. At the medial Magua sampling location the lower 4 m of the deposit has MgO and CaO contents of 0.35 and 0.55% respectively (type-1 phonolite). The upper 7 m of the deposit (type-2 phonolite) appears to be continuously zoned from 0.40 and 0.55% at the lower end, to 0.6 and 0.95% (MgO and CaO) at the top. Distal (coastal) facies were observed to contain a heterogeneous mixture of types 1&2 phonolite + minor rhyolite. One interpretation of these data is that deposition from the original density current began in more distal areas prior to more proximal areas.