Deep-Seated Processes Govern Contrasted Eruption Styles at Felsic Submarine Neovolcanic Centres, Eastern Manus Back-Arc Basin (EMB), Papua New Guinea

Abstract:

Two types of EMB volcanic edifice are characterised respectively by essentially aphyric and distinctly porphyritic lavas. The former build elongate extensional ridges by effusive eruption. The latter are explosively erupted volcanoes overlying older backarc sequences. Exemplars of each style, Pual Ridge and SuSu Knolls, are extensively studied by academic expeditions and commercial operations of Nautilus Minerals Inc.

NE-striking, 20km long, 600m high Pual Ridge (crest 1650mbsl) comprises a sequence of laterally extensive andesite tube flows and dacite-rhyodacite sheets with aa- and pahoehoe-like structure. Cm-thick quenched glass rinds and local collapse pits indicate unusually fluid lavas. Scarce phenocrysts are often resorbed. Rare xenoliths provide insight to deeper processes.

SuSu Knolls (base ~1700mbsf) comprising three conical volcanoes. North Su (crest 1150mbsl), older South Su (1320mbsl), and a degraded smaller edifice (Suzette, 1520mbsl) are constructed from debris blocks several metres across scattered among finer equivalents, mostly lacking glassy rinds. Abundant phenocrysts (Plag-Cpx-Opx-TiMag) lie within microlitic vesicular groundmass. Clasts of altered lava were also exhumed from the volcano interiors during explosive eruption. Flank collapse mass flows distributed large blocks several km from the knolls and formed distal gravelly volcaniclastic deposits. Hydrothermal explosions following fault-induced pressure release created a distal volcaniclastic sand-silt apron up to 10km from North Su. Clast fabrics, bulk geochemistry, and lack of hemipelagic sediment cover denote a recent North Su source.

Fractionation trends and mineral compositions of EMB effusive and explosive eruptives are closely comparable, and vesicularity suggests similar magma water contents. Neither these factors nor eruption depth satisfactorily explain effusive versus explosive behaviour. Contrasted aphyric and porphyritic characters, however, imply important deep-seated processes. Thicker basement beneath explosive eruption sites offers scope for slower or interrupted uprise leading to phenocryst precipitation with consequent lowered magma temperature and increased viscosity, favouring explosive eruption relative to faster uprise and higher T fluidity at effusive sites.