Page 135 - Caldera Volcanism Analysis, Modelling and Response

110 A. Pittari et al.

3.3. Lithic clast types and their provenance
Eight major groups of lithic clasts in the Abrigo ignimbrite have been recognised
(Tables 1–4): nepheline syenite (S); syeno-gabbroids and gabbroids (G); mafic and
intermediate crystalline volcanics (MV); felsic crystalline volcanics (FV); glassy
volcanics (GV); welded or lava-like volcanic breccias (W); pyroclastic and epiclastic
breccias (C) and altered clasts (A). The major subgroups (e.g. MV1) were identified
in the field, and then described in more detail through petrographic analysis of hand
samples and thin sections. Quantitative analysis of the lithic distributions both
vertically and laterally in the Abrigo ignimbrite was undertaken using this lithic
classification scheme.
Coarse-grained coherent lithologies were defined as having an average grainsize
of greater than 3 mm, medium-grained, between 1 and 3 mm, and fine-grained, less
than 1 mm. Fine-grained coherent lithologies have been subdivided further
according to McArthur et al. (1998): coarsely crystalline (500 mm–1 mm); medium
crystalline (250–500 mm); finely crystalline (125–250 mm); very finely crystalline
(63–125 mm); microcrystalline (4–63 mm) and cryptocrystalline (o4 mm).

3.3.1. Nepheline syenite (S), syeno-gabbroids and gabbroids (G)
Nepheline syenite clasts of the Abrigo ignimbrite have been described by Wolff
(1987), Wolff et al. (2000) and Nichols (2001) and consist of a coarse-grained
intergranular framework of alkali feldspar (B60%) and feldspathoids (nepheline and
sodalite; 25–30%, Figure 4a, b); lesser interstitial aegirine-augite, arfvedsonite and
ilmenite; minor biotite, titanite and magnetite; late-stage stellate aegirine clusters;
and exotic accessory minerals. Wolff (1987) and Wolff et al. (2000) identified both
fresh syenite (o10% of syenite clasts; S1, Table 1) and accessory hydrothermally
altered syenite (W90% of syenite clasts; S4, Table 1). Fresh clasts have strong
chemical affinities with Abrigo pumice and, being holocrystalline, form one end of
a textural continuum in glass content that includes glass-bearing syenites (W90%
crystals), quenched ‘‘crystal mush’’ (50–90% crystals) and porphyritic pumice
(o50% crystals); hence, they are considered to be essentially juvenile in origin
(Wolff, 1987; Wolff and Toney, 1993; Wolff et al., 2000; Nichols, 2001). The
primary crystalline texture within some clasts is partly corroded and replaced by a
micro- to medium-crystalline granular mosaic of subhedral to anhedral feldspar
(Figure 4c; S3, Table 1). Fresh microsyenite has also been observed (S2, Table 1).
Coarsely crystalline mafic clasts (i.e. gabbro, syeno-gabbroid/gabbroid,
pyroxenite, Figure 4d; G1–G6, Table 1) are a rare component of the lithic
assemblage and have only been found on the northeastern and southeastern flanks
of the Las Can ˜adas edifice. The mineral assemblages and textures are variable and
most have compositions intermediate between syenite and alkali gabbro.
Nepheline syenite, syeno-gabbroid and gabbroid clasts are derived from plutonic
and hypabyssal rocks, which are not exposed at the surface on Tenerife. Fresh
cognate syenite clasts (S1) are thought to represent crystallised margins of the Abrigo
magma chamber (Wolff, 1987; Nichols, 2001), extracted from depths of 4–7 km
(Wolff, 1987; Ablay et al., 1995, 1998; Bryan et al., 2000). Fresh microsyenite (S2)