Page 154 - Caldera Volcanism Analysis, Modelling and Response

Case Study of the Abrigo Ignimbrite, Tenerife, Canary Islands 129

3.6. Geological configuration of the Las Can ˜adas edifice prior to the
Abrigo eruption
The large variety of lithic types and the high lithic clast volume indicate that the
Abrigo ignimbrite sampled a large number of lithological units both in the
subsurface of the Las Can ˜adas caldera and from the ground surface on the flanks of
the edifice. A large fraction of the lithic population within the Abrigo ignimbrite is
likely to be accessory, although the exact proportions of accidental to accessory
clasts are unknown. Deep-level accessory lithic clasts (i.e. plutonic and most of the
hydrothermally altered clasts) constitute approximately 35% of the lithic population.
The variety of fresh volcanic accessory lithic clasts embedded in juvenile pumice
clasts also suggest that a substantial proportion of the volcanic lithic population was
derived from shallow subsurface unaltered zones.

3.6.1. Plutonic/metamorphic core and the Abrigo magma chamber
Shallow syenitic plutons were emplaced at depths of 4–7 km below the Las Can ˜adas
summit, multiple times over the depositional history of the Upper Group (1.6Ma)
(Wolff, 1987; Wolffetal.,2000; Ablay et al., 1995, 1998; Bryan et al., 2000) and a
solidified plutonic complex is likely to form much of the core of Tenerife. The Diego
Herna ´ndez formation consists of four distinct chemostratigraphic sequences,
characterised by Nb/Zr ratios from phonolitic juvenile clasts, and are named from
oldest to youngest: DHFbs (high Nb/Zr), DHF I (low Nb/Zr), DHF II (very low
Nb/Zr) and DHF III (high Nb/Zr, also represented by cognate syenites in the
Abrigo ignimbrite; Wolff, 1987; Nichols, 2001; Wolff et al., 2000; Edgar et al., 2007).
Each sequence represents a unique cycle of volcanic eruptions with a common
magma chemistry (e.g. DHF III includes the Abrigo ignimbrite and pre-Abrigo
plinian eruptions). At least three earlier geochemically distinct pre-DHF plutons have
been identified: (a) one of low Nb/Zr type, represented by accessory syenites found
in the Caleta and Fasnia Members (Diego Herna ´ndez formation, Edgar et al., 2007;
cf. Wolff et al., 2000); (b) another that conforms to a high Nb/Zr trend, represented
by syenite clasts in units of the Guajara and Ucanca formations; and (c) an additional
augite syenite to alkali gabbro pluton beneath the southern coastal plain.
Most hydrothermally altered syenite clasts from the Abrigo ignimbrite are
derived from pre-DHF I plutons. Hence, the Abrigo magma chamber is likely to
have formed within or adjacent to significantly older pre-Diego Hernandez
formation pluton(s) and erupted fragments of these crystallised plutonic lithologies
(Figure 14).
The pre-eruption configuration of the Abrigo magma chamber probably
involved: (a) an upward zoned body of less evolved (relatively low Zr content) to
more evolved (higher Zr) crystal mush, capped by a crystal-poor supernatant liquid
layer; (b) crystallised syenite chamber walls, from which fresh syenite lithic clasts
were derived; and (c) a mafic magma body which had intruded from below
(Nichols, 2001). Rare microsyenite and recrystallised syenitic lithic clast types were
possibly sourced from finer-grained outer parts of the crystallised chamber wall,
related sills and dykes and/or an associated metamorphic aureole.