Page 141 - Caldera Volcanism Analysis, Modelling and Response

116 A. Pittari et al.

a b

0.4 mm
cm
c d

Pl

FeOx Cpx

0.2 mm 0.2 mm
Figure 4 (a) Photograph of fresh syenite, and photomicrographs of (b) syenitic alkali feldspar
framework with interstitial aegirine and aegirine--augite (ppl), (c) corroded syenitic frame-
work minerals in a sub- to anhedral feldspathic mosaic groundmass (ppl) and (d) ¢ne-grained,
subhedral granular gabbro (ppl). Mineral abbreviations: Pl, plagioclase; Cpx, clinopyroxene;
FeOx, iron oxide.

predominantly of alkali feldspar and subordinate clinopyroxenes, amphiboles and
iron oxides. Felsic clasts vary from aphyric to porphyritic (Figure 5c), and occur in a
variety of textural types (Table 2). Massive clasts (FV4, Table 2) have groundmass
textures varying from intergranular to weakly trachytic with straight, curved or
branching feldspar laths, to an equigranular felsic mosaic (Figure 5e). Spherulitic
textures may also be present (Figure 5f). Mottled (FV2, Table 2, Figure 5d)or
foliated (FV3, Table 2) felsic volcanic clasts commonly contain felsic and dendritic
mafic domains (Figure 5g) and a strongly trachytic groundmass of fibrous, curved
and branching feldspar microlites (Figure 5h).
Mafic, intermediate and felsic crystalline volcanic rocks appear to be derived
mainly from lava flow units, although some could also be derived from shallow
dykes and sills. Clasts that are altered, or occur as lithic inclusions in pumice clasts,
are likely to be accessory in origin (see Section 2.3) and include some foliated,
massive and spherulitic felsic lithologies and minor aphyric and plagioclase-phyric
intermediate/mafic volcanic clasts.
Basaltic source rocks are ubiquitous throughout Tenerife, and it is difficult to
constrain whether fresh basaltic lithic clasts are accidental or accessory. However,
pebble- and cobble-sized basalt lithic concentration zones on the Bandas del Sur, or