Page 223 - Caldera Volcanism Analysis, Modelling and Response

198 W.U. Mueller et al.

requires a local or regional extension (Wharton et al., 1994) and extensional
processes are readily accommodated in an arc setting. As the sill represents an
intracaldera intrusion, it may be considered a caldera resurgence phase. The sill has
been employed as a marker unit to separate lower and upper evolutionary stages
(see Mueller and Mortensen, 2002 for details) and separates caldera evolution into
two distinct events. Grain size differences, characterising this massive, dark green
intrusive body, suggest a polyphase history. The fine- to coarse-grained gabbro
contains pods with intergrowths of cm-scale hornblende, pyroxene, plagioclase and
interstitial quartz. The pods yielded the age of the sill. A subophitic (7ophitic)
to hypidiomorphic granular texture is prevalent: pyroxene and plagioclase are
commonly uralitised and saussuritised. The sill is a tholeiitic ferrogabbro (17–21%
FeO; Eakins, 1972; Mueller and Dostal, unpublished data), and felsic dykes cross-
cut the sill while it was still unconsolidated, as suggested by magma mixing textures.

4.3. Upper formational stage (second caldera event)
A gradual up-section change from felsic- to mafic-dominated volcanism is recorded
where tholeiitic, normal to Mg-rich basalts are interstratified with calc-alkaline
rhyolites (Figure 2B). The lithological diversity of the second caldera event is
striking. Three major lithofacies, which occur roughly in equal proportions,
characterise the upper formational stage: (1) felsic and mafic volcanic lithofacies,
(2) volcaniclastic and iron-formation lithofacies and (3) mafic dykes and sills
(Table 1).

4.3.1. Felsic volcanic lithofacies and mafic volcanic lithofacies
The up to 500 m-thick felsic volcanic lithofacies displays the features of effusive lava
flows with autoclastic fragmentation (e.g. Yamagishi and Dimroth, 1985; Kano
et al., 1991). The coherent to brecciated felsic flows, 2–30 m-thick, have lobate
terminations and flow banded margins. Brecciated upper portions of flows contain
massive to stratified lapilli tuffs related to limited reworking and density current
deposition (De Rosen-Spence et al., 1980; Kano et al., 1991). The 50–80 m-thick
mafic volcanic lithofacies with tholeiitic and Mg–basalt (10–11% MgO) contains
2–10 m-thick massive-columnar jointed, pillowed and brecciated flow units
inherent to submarine flows (Wells et al., 1979; Dimroth et al., 1985). This
lithofacies includes a 30–40 m-thick chaotic breccia facies of randomly oriented
rafts of magnetite–jasper iron-oxide formation (Figure 3B), segments of felsic flows,
mafic flows and dismembered volcaniclastic deposits that have undergone extensive
hydrothermal alteration. A collapse breccia is inferred to have formed at the caldera
margin and is similar to the megabreccia at the Dorobu caldera (Miura and Tamai,
1998). Heterolithic breccias, 2–5 m-thick, are dominated by basalt clasts and minor
chert, BIF fragments and felsic clasts. The massive heterolithic breccia facies with
entrained pillows are debris flow deposits that formed adjacent to the caldera
margin and have undergone hydrothermal silicification.