Page 123 - Caldera Volcanism Analysis, Modelling and Response

98 A. Pittari et al.

4. Conclusions 135
Acknowledgements 136
References 136

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 Can ˜adas 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 Can ˜adas 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
Can ˜adas caldera complex.

1. Introduction

Pyroclastic ejecta from explosive volcanic eruptions are key tracers for
subsurface volcanic processes. In particular, lithic clasts, or lapilli and block-sized
dense rock fragments, are often unique samples of the interior geology of volcanic
edifices (e.g. Eichelberger and Koch, 1979; Suzuki-Kamata et al., 1993; Cole et al.,
1998; Krippner et al., 1998). Whilst juvenile volcanic components (pumice,
crystals, vitric ash) are useful in understanding plutonic, conduit flow and magma
fragmentation processes, lithic clasts provide insight into destructive/erosional
conduit-vent dynamics (e.g. Heiken and McCoy, 1984; Hildreth and Mahood,
1986; Suzuki-Kamata et al., 1993; Rosi et al., 1996), which may be complex
during caldera-forming eruptions. Hence, lithic-rich pyroclastic deposits provide
sound geological evidence for the conditions leading to and dynamic processes
occurring during caldera-collapse events.
Debate over the dynamic processes related to the formation of the Las Can ˜adas
caldera, Tenerife, has centred on arguments in favour of a vertical collapse origin