Page 153 - Caldera Volcanism Analysis, Modelling and Response

128 A. Pittari et al.

Figure 13 (a) Major geochemical groups of juvenile pumice and dense glassy clasts ( JP; North
group, South groups A and B, San Juan group; see text for de¢nition of groups) and variations
in cognate and accessory syenite clasts (Syt) within the Abrigo ignimbrite around the Las
Can ì adas edi¢ce. Geochemical sample localities or groups of localities (ellipses) are indicated
and the number of analysed samples are shown in parentheses. (b) Geochemical variations in
Mn, Zn and Nb/Zr within only the phonolitic population of juvenile clasts between the San
Juan, North, South A and South B geochemical groups as depicted in (a).

(c) Localised San Juan group, at Playa de San Juan on the west coast, is unique with the
phonolitic juvenile clasts having distinctly high Nb/Zr, Zn and Mn (Figure 13b).

Geochemical variations in cognate and accessory syenite lithic clasts also occur
within the Abrigo ignimbrite (Figure 13a). Cognate Abrigo syenite is generally fresh
and classified as high-Nb/Zr type, although some partly altered varieties could also be
cognate. Older accessory syenites are generally altered and may be of low- and high-
Nb/Zr type (Wolffetal.,2000). High-Nb/Zr syenites dominate the fresh and
altered Abrigo syenite population, with the following exceptions (Figure 13a): (a)
both high- and low-Nb/Zr syenites occur locally near Poris de Abona; and (b) only
altered low-Nb/Zr syenites have been recognised geochemically around Los Abrigos.