Page 220 - Caldera Volcanism Analysis, Modelling and Response

Characterisation of Archean Subaqueous Calderas in Canada 195

4.1.1. Coherent and brecciated felsic lithofacies
The coherent and brecciated felsic lithofacies contains lava flow units, and
exogenous and endogenous domes. The 5–150 m thick, massive, lobate, flow-
banded to brecciated felsic lava flows are prominent indicate different volcanic facies
of the flow (De Rosen-Spence et al., 1980; Yamagishi and Dimroth, 1985).
Observed flow bands at the margins of felsic units are typical of endogenous domes
(Goto and McPhie, 1998) and extrusive flows/lobes (Yamagishi and Dimroth,
1985). Hyaloclastite (lapilli tuff) with lobate structures are well preserved
(Figure 4A) in remnant screens within the feeder dyke swarm, and represent
carapace breccias. The aphyric to quartz–feldspar-phyric flows have structureless
centres, which grade into in-situ breccia and flow-banded hyaloclastite margins
(Figure 4B), consistent with a subaqueous setting (e.g. Yamagishi and Dimroth,
1985). Flow terminations display contorted flow banding, and in situ to disrupted
autoclastic breccia. Dykes locally billow into domal-lobate structures (Mueller and
Mortensen, 2002), which caused inflation of the edifice. The large m-scale lobes
(Figure 4B) commonly display radial-oriented columnar joints; these joint patterns
are complex due to ingress of water along fractures that result in new cooling fronts.
The vesicularity in flows and clasts varies significantly (5–70%) and reflects
changing volatile conditions. Phenocryst rich flow units are consistent with the
formation of high-viscosity thick stubby felsic flows (Yamagishi and Dimroth,
1985), whereas aphanitic flows have a lower viscosity and hence are more extensive.
Although pumice is commonly associated with violent eruptions, it is also an
integral component of lava flows (Fink and Manley, 1987), as observed in the HMC
carapace breccias.

4.1.2. Volcaniclastic lithofacies and iron-formation lithofacies
The volcaniclastic lithofacies, which includes a pyroclastic facies, and a reworked
pyroclastic and autoclastic facies, is interstratified with the iron-formation
lithofacies (Figure 4C). The 9–71 m-thick pyroclastic facies is a product of low
viscosity, deep-water fountaining eruptions (e.g. White, 2000, 2004) and is divided
into (i) a basal 7–20 m-thick, massive lapilli tuff breccia with C-shaped pumice and
segregation pipes (Figure 4D, E, F), (ii) a middle up to 51 m-thick, stratified lapilli
tuff (Figure 4G), and (iii), an up to 2 m-thick turbiditic tuff–lapilli tuff (Figure 4C).
The massive lapilli tuff breccia contains clusters of isolated and compressed irregular
vesicle-rich amoeboid- to C-shaped clasts with chilled margins, fluidal textures and
vesicle trains (Figure 4F). These deposits originated from hot magmatic fountains
insulated from the ambient medium, water, by a steam carapace. Segregation pipes
with Fe-lining at pipe margins (Figure 4E) developed locally and support hot
emplacement consistent with vapour phase streaming (Fisher, 1979). The erosive
power of the hot lapilli tuff breccias is substantiated by entrained m-scale BIF rip-up
clasts (Figure 4D). The stratified lapilli tuff unit (Figure 4G) reflects the change from
magmatic to phreatomagmatic processes caused by the ingestion of water into the
magma column (Mueller and White, 1992). Stratified units in 20–50 cm-thick
couplets are composed of vesicle-rich blocky lapilli, and individual layers locally
pinch out laterally and display low-angle discordances. The turbidite tuff–lapilli tuff