Page 233 - Caldera Volcanism Analysis, Modelling and Response

208 W.U. Mueller et al.

collapse and redistributed by sediment gravity flow processes is favoured (Mueller
et al., 1994). Phase 2c (0.8–1.5 km thick) contains andesite, dacite and rhyolite
flows, dykes and endogenous domes. A 90–580 m-thick massive and pillowed
andesite with pillow breccia and lapilli tuffs is well exposed in the eastern part of the
central segment (Figure 7C, D). Dacite flows are massive with local, angular breccia-
size fragments. The 150–920m rhyolite units are flows, dykes and endogenous domes,
and are associated with tuff, lapilli tuff and lapilli tuff breccia. Columnar joints, 5–20cm
in diameter, characterise flows (Figure 9A, B) and high-level endogenous domes.
Lateral flow morphology over 1–2km includes (Figure 7A–D) changes from massive
to 1–20 m-thick, west-closing lobes with massive centres and a marginal metre-thick
flow-banding lapilli tuff breccia (Figure 9C) and laminated-stratified tuff. These
features compare favourably to subaqueous facies models of Yamagishi and Dimroth
(1985) and de Rosen-Spence et al. (1980). Large massive bodies, characterised by
uniform phenocryst content, massive facies and local intrusive contacts are interpreted
as endogenous domes (Goto and McPhie, 1998). Massive to lobate rhyolitic flows are
products of viscous flow (Kano et al., 1991; Manley, 1992, 1996).

5.2. Normetal caldera phases 3 and 4
The 100–600 m-thick, aphanitic rhyodacite–rhyolite phase 3 volcanism in western
and central segments (Figures 7a, d, 8a, b) contains 75–275 m-thick flow units
traceable for 10 km along strike. Flow units display a change over 2–3 km, from
massive to 3–30 m-thick massive or flow-banded lobes to lapilli tuff breccia, which
grade up-section and laterally into massive, 1–10 m-thick lapilli tuff (2–5 cm-size)
and 1–10 m-thick laminated tuff composed of 2–10 cm-thick beds. Well developed
8–15 cm-wide columnar joints are predominant in the basal parts of flows. Sills and
dykes crosscut the previous phase 2 flows and feature local lobes with flow banded
chilled margins. Similarly, massive to flow-banded units and lapilli tuff breccias
alternate in the central segment. Metre-scale flow banding with mm to cm thick
sericite-rich bands mark the contact between massive flows and lapilli tuff breccia.
Collectively, these features are hallmarks of lateral volcanic facies changes in
subaqueous felsic flows (de Rosen-Spence et al., 1980; Yamagishi and Dimroth,
1985; Kano et al., 1991) and domal structures. Water depths may range between
200 and 1,000 m (Kano et al., 1991). The lapilli tuffs and laminated tuffs formed
from high- and low-concentration turbidity flows (Lowe, 1982), representing either
reworked autoclastic debris or local explosive hydroclastic products.
A 475 m-thick rhyodacite–rhyolite unit of phase 4 (Figure 9D) in the central
segment is composed of a massive unit and dykes that contain 3–10 mm large quartz
and feldspar phenocrysts (Qfp3; Figure 7C, D). The 15–20 m-thick Qfp3 dykes cut
phase 2 rocks. The dykes are massive with 30 cm-thick flow banded margins and
have 10–15 cm-thick chilled margins at the contact with phase 2c Qp1 lapilli tuffs.
The geometry, large-scale change in flow band orientation from NW–SE to
NE–SW and intrusive nature of the unit favour the interpretation of a high-level
endogenous dome (Burt and Sheridan, 1987; Goto and McPhie, 1998) and the
dykes of similar phenocryst composition are considered feeders to the dome.