Page 139 - Caldera Volcanism Analysis, Modelling and Response

114 A. Pittari et al.

Table 3 Description and classification of clasts of welded or lava-like breccia, and clastic
breccia.
Classi¢cation: clast type Description

W1: Green heterolithologic Angular fine-grained green and brown clasts in a partially
welded pyroclastic breccia altered flow banded, green cryptocrystalline groundmass;
clasts include (a) altered light green, microcrystalline
phonolite, (b) dark green, slightly coarser crystalline
phonolite, (c) brown, microcrystalline phonolite, (d)
strongly banded glassy to microcrystalline-textured
lithology and (e) fiamme, with spherulites
W2: Heterolithologic glassy Angular clasts of various altered and fresh feldspathic
volcanic breccia lithologies in a partially altered glassy to microcrystalline,
groundmass; lithic types include (a) altered lithology;
(b) coarsely crystalline phonolite/microsyenite; (c)
microcrystalline feldspathic lithology; (d) medium to
coarsely crystalline, trachytic, aphyric feldspathic
lithology and (e) coarsely crystalline equigranular
mosaic groundmass-textured lithology; also contains
broken crystals
W3: Porphyritic Crystals (Pl+minor Afs+Bi+Cpx+FeOx+Sdl/Hyn)
heterolithologic glassy and subordinate angular clasts of (a) altered lithology, (b)
volcanic breccia trachytic-textured felsic fine-grained, coherent lithology
and (c) fiamme; in a glassy to cryptocrystalline partially
altered groundmass with microlites
W4: Irregular laminated Broken Afs crystals in a partially altered, irregular laminated
volcanic rock with broken crypto- to microcrystalline groundmass
crystals
W5: Deformed obsidian–rich Poorly sorted; angular clasts of (a) deformed, attenuated
pyroclastic breccia obsidian (abundant) wrapped around or squeezed between
other fragments, (b) various phonolite types and (c) rare
vesicular ultramafic rock; in a green, altered groundmass
C1: Grey, lithic-rich breccia Poorly sorted, clast-supported; angular fragments of (a)
massive to vesicular basalt and (b) light grey and white
altered rock, in a grey mud matrix
C2: Pink ignimbrite Subangular basalt to phonolite, and minor red altered lithic
clasts (B20%); subrounded altered pumice (B20%);
crystals (B5%); in a pink fine ash matrix
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.

aphyric (MV1, Table 2) to porphyritic (MV2–4, Table 2). Alkali basalt and basanite
(MV1, Table 2, Figure 5a), which cannot be distinguished in the field, have
previously been observed on Tenerife (Fu ´ster et al., 1968; Scott, 1969; Aran ˜a, 1971;
Ridley, 1970; Borley, 1974; Ablay et al., 1998; Wolff et al., 2000; Bryan et al., 2002).