Page 222 - Caldera Volcanism Analysis, Modelling and Response

Characterisation of Archean Subaqueous Calderas in Canada 197

unit with T a T a b beds contains pumice, lithic fragments and vitric grains and is
the result of subaqueous eruptive plume collapse. Local scours indicate turbulence
during bedload transport from high-density turbidity currents. The 1–5 m-thick
reworked pyroclastic and autoclastic facies consists of fine- to coarse-grained
bedded tuff and lapilli tuff beds that were remobilised down-slope via density
currents. The 5–20 cm-thick tuff contains graded T a b beds, and 10–50 cm-thick
coarse-grained tuff with 10–20% lapilli features low-angle shallow scours,
suggestive of unstable density currents with pyroclasts transported under bedload
conditions (Lowe, 1982; White and Busby-Spera, 1987).
The iron-formation lithofacies with magnetite, magnetite–jasper, and jasper
beds (2–100 cm thick) marks the top of the tuff horizons of the tuff–lapilli tuff units
(Figure 4C). This iron-formation facies developed via hydrothermal activity during
periods of volcanic quiescence. Large rip-ups of jasper were observed in pyroclastic
beds. Chown et al. (2002) suggested that iron-oxide formations in volcanic settings
may be due to cold water seeping, but chemical precipitation is an attractive and
commonly advocated alternative (Lascelles, 2007).

4.1.3. Felsic dyke swarm
A 5–7 km-thick, N-trending calc-alkaline rhyolite dyke swarm (Mueller and
Donaldson, 1992b; Dostal and Mueller, 1996; Table 1) documents the complex
evolution of the HMC. The western part of the dyke swarm can be traced 2.5 km
up-section and 2.8 km along strike. Outcrop zones display a dyke density of 80%
with small well-preserved remnant screens of carapace breccia. A dyke evolution
from aphanitic to porphyritic phases suggests phenocryst enrichment in a high-level
magma chamber. The various dyke phases based on cross-cutting relationships are:
(1) D-1a aphanitic and D-1b feldspar-phyric dykes, (2) D-2, quartz–feldspar-phyric
(o5% qtz), (3) D-3, quartz–feldspar-phyric (10–25% qtz), (4) D-4, dacitic feldspar-
phyric and (5) D-5a, b mafic dykes. Columnar jointing is the outstanding felsic
dyke feature, whereby multiple rows of columnar joints are arranged within
composite dykes (up to 25 m-thick). The flow direction of the swarm is from north
(base) to south (top) indicated by inverted V-shaped columnar jointed contacts
(Mueller and Donaldson, 1992b). The felsic dyke swarm is similar to the
distribution of mafic dykes in the rift zone of Iceland (e.g. Gudmundsson, 1983,
1984). Multiple rows of columnar joints indicate selective magma pulses (e.g.
Gudmundsson, 1984). Chilled, flow-banded and hyaloclastite dyke margins are
locally vesicular, have orb-like or microgranophyric textures, and contain quench
spherulites. High-temperature spherulites, located in the central portions of the
dyke, formed during slow cooling and nucleated preferentially around quartz
phenocrysts.

4.2. Middle formational stage (intrusive event)
The 2,731.8+2.2/ 2.0 Ma, up to 1 km-thick, E-trending Roquemaure sill, a
gabbro-quartz diorite (Figure 2B; Eakins, 1972), defines the middle formational
stage (Table 1; Mueller and Mortensen, 2002). Sill emplacement, like dykes,