Page 158 - Caldera Volcanism Analysis, Modelling and Response

Case Study of the Abrigo Ignimbrite, Tenerife, Canary Islands 133

3.7.2. Conduit-vent evolution
Large lithic volumes and lateral lithic clast variations within the Abrigo ignimbrite
suggest that multiple vents along a ring fissure existed during the eruption (c.f.
Eichelberger and Koch, 1979; Heiken and McCoy, 1984; Hildreth and Mahood,
1986; Suzuki-Kamata et al., 1993; Rosi et al., 1996; Cole et al., 1998; Legros et al.,
2000). Deposits on the Bandas del Sur contain more syenite and felsic crystalline
volcanic clasts, fewer mafic crystalline volcanic clasts and significant variations in the
type of altered clasts compared to those on the north coast, indicating that northward-
and southward-directed pyroclastic flows were fed from different vents. This is further
supported by variations in juvenile clast geochemical populations on the northern
(North group) and southern to eastern coasts (South A group), reflecting different
vents tapping geochemically different parts of the magma chamber.
Subtle lateral variations in lithic componentry are also evident within both the
Sur-A and -C units across the Bandas del Sur (e.g. increase in felsic crystalline
volcanic and gabbroic clasts from west to east) suggesting that each southerly
directed pyroclastic flow was fed from multiple, simultaneously active vents
sourcing different country rock lithologies (e.g. Druitt, 1985). However, slight
lateral lithic clast variations may also be an artefact of point counting sites being at
slightly different stratigraphic levels within each unit, which is difficult to constrain
in vertically continuous sequences of massive ignimbrite. Thus, the variations could
also be explained by changes in wall rock excavation levels within a single vent.
Nevertheless, the west to east increase in the proportion of mafic juvenile clasts
across Tenerife, within laterally continuous Abrigo depositional units, reflects
similar west to east compositional variations within the Abrigo magma chamber
system and is consistent with simultaneous tapping of different parts of the magma
chamber from multiple vents.
Furthermore, localised concentrations of low-Nb syenite at Los Abrigos and of
only evolved juvenile clast compositions at San Miguel de Tajao, Poris de Abona
west and Barranco de la Cera (South Group B, Figure 13a) could have been caused
by imperfect mixing between the products from different vents. Mixing from
different vent-sourced pyroclastic flow pulses could occur either within the caldera
depression before spilling out into barrancos through passes in the caldera wall, or
by partial overlapping of pyroclastic flow lobes as they travel down the upper slopes.
A higher degree of mixing from multiple vents may be represented in the Abrigo
deposit at Poris de Abona (east), which has the most diverse suite of juvenile clast
and syenite compositions (Figure 13a).
The unique lithic clast population and juvenile clast geochemical group at Playa
de San Juan (west coast) is consistent with a separate pyroclastic flow lobe sourced
from a geochemically distinct part of the magma chamber and thus a different vent
from those that fed other Abrigo pyroclastic flow lobes.
The vent-derived lithic concentration zone in the upper Sur-A unit is restricted
to the southeastern flanks of the Las Can ˜adas edifice. This could be attributed
to a significant incremental (piecemeal) collapse event of the southeastern sector of
the caldera.
The Sur-C unit contains a higher abundance of deep lithic clasts (plutonic and
altered clasts) than the Sur-A unit and suggests that the focus of wall rock