Chapter 3 The Use of Lithic Clast Distributions in Pyroclastic Deposits to Understand Pre- and Syn-Caldera Collapse Processes: A Case Study of the Abrigo Ignimbrite, Tenerife, Canary Islands

Abstract

Lithic-rich pyroclastic units and facies are often associated with caldera-forming eruptions. Petrographic and quantitative studies on the variety of lithic types, and the spatial and vertical variations in their proportions, provide a powerful tool for understanding (a) the subsurface and pre-caldera geology, and (b) conduit-vent processes during caldera eruptions. In particular, lithic assemblages may include unique samples of deep plutonic-basement features, hydrothermal systems and ancient volcanic landforms destroyed by caldera fragmentation. When interpreting caldera eruptions, studies of lithic clasts can constrain vent configurations, and the depth and style of conduit wall rock fragmentation. The 186 ka Abrigo ignimbrite, representing the last major caldera-forming eruption of the Las Cañadas volcanic edifice, Tenerife, contains a diverse lithic population including (a) syenite, and rare syenogabbroid and gabbroid fragments from a deep plutonic-contact metamorphic core, (b) abundant altered fragments, representing a relatively deep extensive zone of hydrothermal alteration, and (c) shallow- and surface-derived mafic to felsic, crystalline and glassy volcanic clasts, and welded to non-welded pyroclastic and epiclastic breccia clasts, all of which are consistent with being derived from a pre-Abrigo constructive phase of the Las Cañadas edifice. Significant lateral variations in the proportions of lithic clast types, within depositional units, is consistent with an eruption involving multiple vents around a caldera that underwent piecemeal collapse, and this is further supported by lateral geochemical variations in juvenile clast populations. Vertical variations in lithic clast proportions between depositional units suggest an increasing depth of conduit wall rock fragmentation during the eruption. This study highlights vertical caldera collapse as a major process in the evolution of the Las Cañadas caldera complex.