Article

Structure and petrology of newly discovered volcanic centers in the northern Kermadec–southern Tofua arc, South Pacific Ocean

Journal of Geophysical Research Atmospheres (Impact Factor: 3.44). 01/2008; 113. DOI: 10.1029/2007JB005453

ABSTRACT The NZAPLUME III expedition of September-October 2004 to the northern
Kermadec-southern Tofua (NKST) arc, between 28°52'S and 25°07'S,
resulted in the discovery of at least seven new submarine volcanic
centers and a substantial caldera complex adjacent to the previously
known Monowai Seamount. The volcanic centers form a sublinear chain that
coincides with the Kermadec Ridge crest in the south (Hinetapeka) and
diverges ˜45 km westward of the ridge crest in the north ("V")
just to the south of where the Louisville Ridge intersects with the arc.
All of the centers contain calderas or caldera-like structures, as well
as multiple cones, domes, fissure ridges, and vent fields. All show
signs of recent eruptive and current hydrothermal activity. There are
strong structural controls on edifice location, with cones and fissure
ridges typically associated with faulting parallel to the regional
˜12° strike of the arc front. Several of the calderas are
ellipsoidal, orientated northwest-southeast in the general direction of
least compressive stress. Sampled volcanic rocks, representing the most
recently erupted lavas, are all low-K tholeiites. Two of the centers,
Gamble and Rakahore, yielded only high-silica dacite to rhyolite (69-74
wt% silica), whereas two others, Monowai and "V," yielded only basalt to
andesite (48-63 wt% silica). Mineral assemblages are
plagioclase-pyroxene dominated, with accessory Fe-Ti oxides, apatite,
olivine, and quartz/tridymite/cristobalite, typical of dry volcanic arc
systems. Hornblende occurs only in a felsitic rhyolite from Hinepuia
volcanic center, and zircon is absent. Glass contents range to 57% in
basalts-andesites (mean 20%), and 97% in andesites-rhyolites (mean 59%)
and other quench textures, including swallow-tailed, plumose, or
dendritic crystal forms and crystallites, are common. Most lavas are
highly vesicular (≤63%; mean 28%) and have low volatile contents
(mostly <2 wt%) which, together with the occurrence of tridymite or
cristobalite, indicates explosive eruption and rapid cooling. Exceptions
are rocks from "U" volcanic center, which have low vesicularity and low
glass contents across a wide compositional range, indicating effusive
eruption. Disequilibrium mineral textures, the frequent occurrence of
xenoliths and xenocrysts, and macroscopic evidence for magma mingling
indicate that many of the lavas are hybrids, having resided only a short
time in upper crustal reservoirs prior to eruption. Silicic magmas are
major components of NKST arc volcanism and caldera formation is the
dominant eruptive style. The scale of silicic magmatism is in marked
contrast to the dominant basaltic-andesitic magmatism in the southern
Kermadec arc. With evidence from other arcs, silicic magmatism is now
recognized as a major feature of intraoceanic arcs globally.

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