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: 3.43). 08/2008; 113(B8). 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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    • "This special issue on " Metallogenesis and Mineralization of Intraoceanic Arcs: Part I " is intended to complement and expand on an earlier issue on active magmatic, tectonic, and hydrothermal processes at intraoceanic arcs (Embley et al., 2008). Papers related to the Kermadec arc in that issue included topics such as, the morphology and petrology of eight volcanic centers that comprise the northernmost part of the Kermadec arc (Graham et al., 2008), the collapse and regrowth of the Monowai cone volcano (Wright et al., 2008), an ocean-bottom hydrophone experiment at Brothers volcano (Dziak et al., 2008), fossilized filamentous microbes contained in silica-rich chimneys found at Giggenbach volcano (Jones et al., 2008), and microbes collected from the active vents sites of Brothers volcano (Stott et al., 2008). In this issue, all of the papers are focused on the Kermadec arc with a common theme of understanding the physicochemical conditions for the formation of sea-floor hydrothermal ore deposits. "

    Full-text · Article · Dec 2012 · Economic Geology
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    • "(similar to the 11 km 3 calculated by Graham et al., 2008) (Fig. 6A). "
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    ABSTRACT: Morphometric analysis of multibeam bathymetry and backscatter data is applied to Monowai, a submarine volcano of the active Tofua-Kermadec Arc to map and document the structure and evolution of the volcanic centre. Low rates of erosion and sedimentation, and pervasive tectonic and magmatic processes, allow quantification through detailed structural analysis and measurement of deformation. The Slope, Aspect, Curvature, Rugosity, and Hydrology (flow) tools of ArcGIS provide a robust structural interpretation and the development of a model of Monowai evolution.A nested caldera structure with a volume of ~ 31 km3 and a stratovolcano of ~ 18 km3 dominate the magmatic constructs. The outer caldera is elongate along 125°, and the inner caldera along 135°. Numerous parasitic cones and fissure ridges are also observed, oriented at 039° and 041°, respectively. Northeast trending faults (with a regional average strike of 031°) are widespread within this part of the backarc, forming a nascent rift graben to the west of the Monowai caldera complex. The distribution of throw varies spatially, reaching a maximum total along-rift of 320 m and across rift of 120 m, with greater throw values measured in the west.Elongation directions of the two nested calderas are near-perpendicular to the trends of faults and fissure ridges. The inner caldera is more orthogonal to the magmatic constructs (fissure ridges and aligned vent cones) and the outer caldera is approximately orthogonal to the regional fault fabric, suggesting a strong interaction between magmatic and tectonic processes, and the directions of the horizontal principal stress. We present a detailed morphometric analysis of these relationships and the data are used to interpret the spatial and temporal evolution of the tectono-magmatic system at Monowai, and classify the type of rifting as transtensional. Similar analysis is possible elsewhere in the Kermadec backarc and within other regions of submarine volcanism.
    Full-text · Article · Sep 2012 · Journal of Volcanology and Geothermal Research
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    • "For a long time the dominant products were thought to be basaltic andesites and andesites. However, recent dredging by the Australian research vessel Southern Surveyor has shown that the majority of volcanoes are submerged caldera structures that are in fact often dominated by silicic (generally dacitic) rocks (Graham et al., 2008). A similar observation has been made in the Izu arc (Tamura et al., 2009) and so there is an emerging realization that the amount of silicic magma emplaced in primitive oceanic arcs (and thus their potential volcanic hazard) has been significantly underestimated. "
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    ABSTRACT: Fonualei is unusual amongst subaerial volcanoes in the Tonga arc because it has erupted dacitic vesicular lavas, tuffs and phreomagmatic deposits for the last 165 years. The total volume of dacite may approach 5 km(3) and overlies basal basaltic andesite and andesite lavas that are constrained to be less than a few millennia in age. All of the products are crystal-poor and formed from relatively low-viscosity magmas inferred to have had temperatures of 1100-1000 degrees C, 2-4 wt % H2O and oxygen fugacities 1-2 log units above the quartz-fayalite-magnetite buffer. Major and trace element data, along with Sr-Nd-Pb and U-Th-Ra isotope data, are used to assess competing models for the origin of the dacites. Positive correlations between Sc and Zr and Sr rule out evolution of the within-dacite compositional array by closed-system crystal fractionation of a single magma batch. An origin by partial melting of lower crustal amphibolites cannot reproduce these data trends or, arguably, any of the dacites either. Instead, we develop a model in which the dacites reflect mixing between two dacitic magmas, each the product of fractional crystallization of basaltic andesite magmas formed by different degrees of partial melting. Mixing was efficient because the two magmas had similar temperatures and viscosities. This is inferred to have occurred at shallow (2-6 km) depths beneath the volcano. U-Th-Ra disequilibria in the basaltic andesite and andesite indicate that the parental magmas had fluids added to their mantle source regions less than 8 kyr ago and that fractionation to the dacitic compositions took less than a few millennia. The 165 year eruption period for the dacites implies that mixing occurred on a similar timescale, possibly during ascent in conduits. The composition of the dacites renders them unsuitable candidates as contributors to average continental crust.
    Full-text · Article · May 2012 · Journal of Petrology
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