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

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.