Page 138 - Caldera Volcanism Analysis, Modelling and Response

Case Study of the Abrigo Ignimbrite, Tenerife, Canary Islands 113

Table 2 (Continued )

Classi¢cation: clast type Description

FV4: Massive green phonolite Massive, sugary to glassy appearance; green crypto- to
medium crystalline groundmass; Minor
Afs+Agt+Mag+Sdl/Hyn+Tte phenocrysts; intergranular
to weakly trachytic groundmass (Afs+sodic Cpx7sodic
Am+Mag), which may contain minor curved Fsp
microlites or spherulites; or very fine to very
coarsely crystalline equigranular Fsp mosaic
groundmass with minor Agt, Tte and FeOx
FV5: Aphyric to porphyritic Light grey-blue and/or white micro- to medium crystalline
spherulitic phonolite groundmass; non- to weakly trachytic; spherical,
bow-tie and fan spherulites; straight and curved Fsp
microlites; minor intersertal glass or cryptocrystalline
texture
FV6: Porphyritic brown Porphyritic (mostly Fsp); caramel-brown; very fine to finely
phonolite crystalline groundmass; Afs+Pl+Bi+Mag+Tte
phenocrysts, AfsWPl; trachytic groundmass, straight,
curved and branching Afs laths and interstitial brown
glass
GV1: Dense, glassy volcanic Aphyric to porphyritic; glassy groundmass; felsic
clasts phenocrysts – Afs+Agt+Bi+Mag+Tte+Sdl/
Hyn+Ap; mafic/intermediate phenocrysts –
Pl+Afs+Cpx+Krs+Bi+FeOx (Mag)+Tte+Sdl/
Hyn+Ap; amorphous to cryptocrystalline
groundmass; some crystallites, spherulites; o30%
microvesicles
Note: Essential visually identifiable field classification criteria are highlighted in italics (NB: groundmass grainsize
was constrained from thin sections of representative samples); additional characteristics observed from
representative thin sections are listed in regular font. See Table 1 for definition of mineral abbreviations and
percentages.

noted that some mildly altered syenite clasts are chemically similar to the fresh
cognate Abrigo syenite suite, and their relationship to either the Abrigo or older
plutons has not been established. The finer-grained granular feldspar mosaic
groundmass observed in some corroded syenite clasts is likely to be the result of
thermal metamorphism of the wall rock (Bryan et al., 2000). Gabbroids and syeno-
gabbroids are interpreted to be juvenile and/or accessory mafic cumulates
(Neumann et al., 2000; Nichols, 2001).

3.3.2. Mafic/intermediate (MV) and felsic (FV) crystalline volcanic clasts
Grey, fine-grained basaltic to intermediate crystalline volcanic clasts are common
and range from dense (MV1–4, Table 2) to vesicular (MV5, Table 2), and from