Page 144 - Caldera Volcanism Analysis, Modelling and Response

Case Study of the Abrigo Ignimbrite, Tenerife, Canary Islands 119

Pyroclastic and/or epiclastic deposits occur intermittently around the Las
Can ˜adas edifice and within the caldera wall, and the provenance of individual lithic
clast types is uncertain. The abundance and variety of angular lithic fragments, poor
sorting and interclast mud/ash matrix suggests that these clasts were derived from
widely exposed ignimbrite deposits on Tenerife.

3.3.5. Pervasively altered lithic clasts (A)
The Abrigo ignimbrite is characterised by an abundance of pervasive hydro-
thermally altered clay-rich, red, orange, yellow, brown, white and/or grey lithic
clasts (A1–A4, Table 4; Figure 6a). Relict primary textures of mafic to phonolitic
lavas include phenocrysts, the remnant intergranular or trachytic feldspar
groundmass texture, and unaltered textural domains or pseudoclasts. Coarse-
grained samples are likely to be syenitic. Alteration includes conversion of
ferromagnesian minerals to clay and disseminated iron oxides, sericitisation of
feldspars, cancrinitisation of feldspathoids, calcite precipitation and zeolitisation.
Iron-rich clay minerals and phyllosilicates often occur as amorphous masses,
irregular blebs or in-filling cavities. Calcite occurs as clots, irregular anastomosing
veins or in-filling fractures. Some cavities show clear fibrous, botryoidal and
colloform textures with alternating layers of secondary minerals (Figure 6b).
Exposure of hydrothermally altered volcanic rocks around the Las Can ˜adas
edifice is limited to a few isolated patches associated with fractures within the
southern and western parts of the caldera (Aran ˜a, 1971). The abundance of altered
lithic clasts within the Abrigo ignimbrite cannot be accounted for by the sparse
surface exposure of hydrothermally altered rock. It is possible that surface alteration
was more extensive prior to the destructive Abrigo eruption; however, there is little
evidence for this as much of the older caldera wall lithologies are still quite fresh.
Hence, a large fraction of the altered clast population was likely to have been
derived from hydrothermally altered rocks occurring at depth.

cm

0.2 mm
a b
Figure 6 (a) Photograph of pervasively altered lithic clasts and (b) photomicrograph of an
altered clast showing botryoidal and colloform banded alteration textures (ppl).