Page 143 - Caldera Volcanism Analysis, Modelling and Response

118 A. Pittari et al.

isolated, anomalously large basalt clasts within massive ignimbrite, are most logically
accidental and derived from nearby basalt lavas.
Intermediate and felsic volcanic rocks are largely confined to the central Las
Can ˜adas edifice (Lower Group and Ucanca formation, Upper Group, Martı ´ et al.,
1994) and most are exposed within the caldera wall and upper edifice slopes, hence,
constraining an accessory or near-to-vent accidental origin for most intermediate-
felsic volcanic clasts. Limited outcrop of the Lower Group on the mid-southern
flanks and along the north coast, as well as younger small felsic centres (Caldera del
Rey and Montan ˜a de Guaza) near the southwest coast (see Fu ´ster et al., 1968),
could have contributed a minor fraction of the intermediate-felsic volcanic lithic
population locally.

3.3.3. Dense, glassy volcanic clasts (GV)
Dense, glassy volcanic clasts (GV1, Table 2) are aphyric to porphyritic, dense and
poorly vesicular (o30% microvesicles) and have an amorphous or cryptocrystalline
groundmass, sometimes with acicular microlites. Felsic/phonolitic (green) to mafic
(black) and banded compositional types occur. Most dense, glassy volcanic clasts are
interpreted to be juvenile clasts as they geochemically duplicate Abrigo pumice
(Nichols, 2001) are unaltered, and dense glassy bands are often co-mingled within
juvenile pumice clasts. However, care must be taken in interpreting all of these
lithic clasts as juvenile because older obsidian lavas have also been observed within
the caldera wall (e.g. Zafrilla, 2001).

3.3.4. Welded or lava-like volcanic breccias (W) and clastic breccias (C)
Clasts of welded or lava-like volcanic breccias (Table 3) consist of angular
fragmental components within a groundmass that displays cooling features
suggestive of a fluidal or plastic rheology, such as a glassy to devitrified crypto-
to microcrystalline texture or flow banding. Some clasts contain fiamme (e.g. W1,
W3, Table 3), deformed obsidian fragments (W5, Table 3), broken crystals (W4)
and/or a heterolithologic suite of lithic fragments (W1–3, W5, Table 3), all of
which indicate a pyroclastic origin.
Non-welded, matrix- or clast-supported lithic-rich clastic breccias, either
epiclastic or pyroclastic in origin, also occur. Two distinctive clastic breccia clast
types have been identified in the Abrigo ignimbrite (Table 3): grey, lithic-rich
breccia (C1) and pink ignimbrite (C2), although this list is probably not exhaustive.
Welded and rheomorphic pyroclastic deposits are common around the caldera
wall and on the upper slopes of the Las Can ˜adas edifice (Martı ´ et al., 1994; Zafrilla,
2001; Soriano et al., 2002, 2006). There is also limited exposure of welded
ignimbrites, which are lithologically distinct from proximal welded deposits, on the
western and southern coastal plains (i.e. Adeje Ignimbrite, Fu ´ster et al., 1994; Arico
Ignimbrite, Alonso et al., 1988), although clasts derived from these deposits have
not been identified within the Abrigo ignimbrite. Hence, most welded lithic clasts
are either accessory or near vent-derived accidental clasts.