Page 136 - Caldera Volcanism Analysis, Modelling and Response

Case Study of the Abrigo Ignimbrite, Tenerife, Canary Islands 111

Table 1 Description and classification of nepheline syenite, syenogabbroid and gabbroid
clasts.
Classi¢cation: clast type Description
S1: Fresh nepheline syenite Coarse-grained, crystalline; Afs+Ne
(7Sdl)+Agt+Aeg+Arf7Bi7Tte a
S2: Fresh nepheline As for S1, but medium-grained
microsyenite
S3: Recrystallised syenite and White to grey-blue, peppery appearance; micro- to medium-
microsyenite crystalline; granular mosaic Fsp groundmass; minor
glassy domains; minor FeOx and Fe–Mg silicate-rich
domains; strongly corroded relict syenitic textures;
occasional spherulitic texture
S4: Hydrothermally altered As for S1, but cloudy-white altered appearance; sericite,
nepheline syenite and calcite, cancrinite, FeOx and Fe-rich clay alteration
microsyenite phases
G1: Fine-grained subhedral Equigranular, fine-grained; Cpx (B45%)+Pl
granular gabbro (B40%)+Mag (B10–15%)7Ap (up to 5%)7glass
(2–3%), subhedral crystals
G2: Porphyritic syeno- B50% coarse-grained (up to 4.5 mm) Fsp phenocrysts in a
gabbroid fine-grained Fsp+mafic-rich groundmass; Fsp (Pl+An)
(B75%)+Am (B8%)+Cpx (B4%)+Bi
(B5%)+Foids (B2%)+FeOx (B5%)+Ap (B1%);
commonly partially resorbed Fsp; diffuse grain
boundaries across groundmass
G3: Vitric hornblende Coarse-grained; brown Am (50%)+Fsp (10%)+Mag
melagabbro (10%); 15% interstitial yellow-brown glass; 10% pore
space and vesicles
G4: Porphyritic microgabbro Medium to coarse-grained Cpx+Mag phenocrysts in an
equigranular fine-grained groundmass; CpxWPlWMag;
partially altered
G5: Pyroxenite Equigranular, coarse-grained, mostly pyroxene;
TiAug+minor FeOx
G6: Coarse-grained gabbroid Cpx (up to 30 mm)WPl (+ minor Bi) (Handsample of
C. Edgar)

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. Mineral abbreviations: Fsp, feldspar;
Afs, alkali feldspar; An, anorthoclase; Pl, plagioclase; Ol, olivine; Cpx, clinopyroxene; Aug, augite; TiAug,
titanaugite; Aeg, aegirine; Agt, aegirine-augite; Am, amphibole; Arf; arfvedsonite; Krs, kaersutite; Bi, biotite; Foid,
feldspathoid; Ne, nepheline; Sdl, sodalite; Hyn, hau ¨yne; Tte, titanite; Ap, apatite; FeOx, iron oxide; Mag,
magnetite; Cal, calcite. Percentages represent visual estimates of the whole rock or of a mineral group.
a
Mineralogy from Wolff (1987), Wolff et al. (2000) and Nichols (2001).
was probably derived from the outer extremities of the magma chamber and within
the proximal hypabyssal system. Heavily altered syenite clasts (S4), that are chemically
distinct from the cognate clasts, have been attributed by Wolff et al. (2000) to one
or more solidified pre-Abrigo plutons, constituting a portion of the wall rock
around the Abrigo magma chamber/conduit (cf. Bryan et al., 1998b, 2000). It is