Page 140 - Caldera Volcanism Analysis, Modelling and Response

Case Study of the Abrigo Ignimbrite, Tenerife, Canary Islands 115

Table 4 Description and classification of pervasively altered lithic types.

Classi¢cation: clast type Description
A1: Amorphous Fe-rich clay Distinctive red, purple, pink and/or orange appearance due
and FeOx altered rock to Fe-rich clay and FeOx, sericitisation of Fsp,
cancrinitisation of Foids, and Cal precipitation;
alteration textures include reddish blebs,
disseminated FeOx, pseudomorphs of phenocrysts,
colloform banding, banding defined by variations in
the concentration of disseminated Fe-rich clay,
irregular hairline vein networks, open and in-filled
vughy porosity; relict primary textures variably
preserved
A2: Carbonatised alteration Breccia, often with mosaic or jigsaw fit texture of relict
breccia with relict clasts volcanic clasts within a pervasive anastomosing network of
microcrystalline Cal; disseminated FeOx; relict
microphenocrysts
A3: Non-descript clay and Pale grey, brown, yellow, yellowish-brown, cream appearance;
carbonate altered rock massive or with relict phenocrysts; pervasive
alteration to phyllosilicates and Cal; minor
amorphous Fe-rich clay, disseminated or streaks of
FeOx and zeolites; Cal in fractures and vughs and as
groundmass clots/blebs; relict primary textures
variably preserved
A4: Silky altered phonolite Massive, greyish-white and greenish-brown, silky lustre;
strong alteration to phyllosilicates and abundant
FeOx often replacing phenocrysts; minor colloform
banding within cavities; relict large tabular Afs and
trachytic textures
Note: Essential visually identifiable field classification criteria are highlighted in italics; additional characteristics
observed from thin sections of representative samples are listed in regular font. See Table 1 for definition of mineral
abbreviations.

Ankaramite (MV2, Table 2) is a highly porphyritic variety of alkali basalt or basanite
(W30% total olivine and pyroxene phenocrysts, Borley, 1974). Grey intermediate
volcanics (MV3–5, Table 2, Figure 5b)oftheLas Can ˜adas edifice follow the
plagioclase basanite–phonotephrite–tephriphonolite and trachybasalt–trachyandesite
trends (Fu ´ster et al., 1968; Scott, 1969; Ridley, 1970; Aran ˜a, 1971; Borley, 1974;
Ablay et al., 1998; Wolff et al., 2000; Bryan et al., 2002). Porphyritic intermediate
clasts (MV3–4, Table 2) in the Abrigo ignimbrite are distinguished from basaltic
clasts in the field by their visible phenocryst assemblage and occasional lighter
colour. Aphyric intermediate clasts are difficult to distinguish from mafic clasts in the
field and are included with MV1.
Felsic volcanic clasts (Figure 5c–h) are generally phonolitic in composition and
are commonly identified in the field by their distinct green groundmass (FV1–5,
Table 2), although some are also brown (FV6) ones. The groundmass consists