A complex quaternary ignimbrite-forming phonolitic eruption:: the poris member of the Diego Hernandez Formation (Tenerife, Canary Islands)

The Poris Member (0.28 Ma) of the Diego Hernandez Formation (Tenerife, Canary Islands) is a widespread succession of plinian fall, surge and non-welded ignimbrite deposits. It was erupted from the northeastern sector of the multicyclic Las Canadas Caldera, the summit caldera complex of the Las Canadas Edifice. We present new stratigraphic data which allow the evolution of the eruption to be divided into six main stages: (1) an initial surge-producing phreatomagmatic phase. (2) Generation of a buoyant plinian column culminating in vent-wall collapse and temporary vent blockage. (3) A resumption of phreatomagmatic activity, producing accretionary lapilli-bearing ash surges and a phreatomagmatic ignimbrite. (4) Sustained column collapse, resulting in the progressive aggradation of a complex, compositionally layered ignimbrite sequence. (5) A late plinian phase marked by sharp chemical zonation. (6) Final vent destruction and the generation of multiple surges and pyroclastic flows. Volume calculations indicate a total erupted volume in the order of 13-14 km(3) (3-4 km(3) dense rock equivalent), which together with facies and lithic analysis implies that a small-scale caldera. collapse occurred, contributing to the incremental enlargement of the Las Canadas Caldera. The geochemistry and mingling inter-relationships of four magmatic endmembers, ranging from alkaline mafic to evolved phonolitic liquids, indicate that two magma chambers fed stages 4 and 5 of the eruption sequence. The eruption was triggered when mafic magma intruded the two-chamber system. (C) 2002 Elsevier Science B.V. All rights reserved.