The Petrology, Geochemistry and Geochronology of Back-Arc Stratovolcanoes in the Southern Kermadec Arc-Havre Trough, SW Pacific
Authors:   , Zohrab Alexander 
Published: November 2021

<p>The Kermadec Arc-Havre Trough (KAHT) is widely regarded as a classical example of an intra-oceanic arc-back-arc system, where subduction-driven arc magmatism is focused at the Kermadec volcanic arc-front, and magmatism within the Havre Trough back-arc system results from decompression-related melting. In detail, however, the Havre Trough has not been well-studied, and data for very few lavas have been reported.  Recent mapping undertaken in the southern Havre Trough has resulted in the discovery of several prominent submarine stratovolcanoes, Gill Seamount, Rapuhia Seamount and the related Rapuhia Ridge, Yokosuka Seamount, and Giljanes Seamount, situated in the middle of deep rifts and on elevated crustal plateaux. The origin and evolution of these stratovolcanoes is unknown. The first detailed dataset of whole rock major and trace element geochemistry, mineral chemistry, and ⁴⁰Ar/³⁹Ar isotope data, for lavas erupted from these volcanoes is presented here, and used to investigate the processes that drive volcanism in the Havre Trough back-arc.  ⁴⁰Ar/³⁹Ar ages obtained from back-arc stratovolcanoes range from ca. 1167 - 953 ka for Gill Seamount, and ca. 107 - 50 ka for Rapuhia Ridge. These ages overlap with known ages for arc-front lavas, indicating that both back-arc and arc-front volcanism are coeval. These ages are all significantly younger than the inferred initation of Havre Trough rifting ca. 5 - 6 Ma.  Lavas analysed from Gill Seamount and Rapuhia Ridge are basaltic to basaltic-andesitic in whole rock composition and contain a phenocryst assemblage of olivine ± orthopyroxene + clinopyroxene ± plagioclase. Lavas from Rapuhia Seamount, Yokosuka Seamount and Giljanes Seamount range from andesitic to dacitic in composition, and have a phenocryst assemblage consisting primarily of plagioclase ± clinopyroxene ± amphibole ± Fe-Ti oxides ± apatite. Variations in mineral assemblages and whole rock compositions of the lavas are consistent with crystal fractionation of their respective phenocryst phases. The more evolved compositions of Rapuhia Seamount, Yokosuka Seamount and Giljanes Seamount, all sited on an elevated crustal plateau, are inferred to result from prolonged assimilation + fractional crystallisation (AFC) in the mid- to upper- crust.  Mineral compositions provide additional evidence for fractional crystallisation, and most crystals are inferred to have crystallised in equilibrium with their host melt. However, compositions of some olivine phenocrysts in Gill Seamount and Rapuhia Ridge indicate multiple populations of olivine, suggesting their magmatic systems were open to contributions from secondary processes. Variations in Or content in plagioclase crystals for a given lava suite suggests the sample suites crystallised from melts with different starting K₂O compositions.  Elevated ratios of Nb/Yb in the mafic Gill Seamount and Rapuhia Ridge lavas indicate the back-arc volcanoes and ridges originated from a less depleted mantle than that present underneath the Kermadec volcanic arc-front, likely a consequence of trenchward advection of mantle within a suprasubduction wedge and/or partial melting of a fusible enriched mantle component.   All whole rock samples from these back-arc volcanoes have trace element characteristics that resemble those of typical volcanic arc magmas, indicating that they are variably modified by subducting plate-derived components despite their rear-arc setting. However, the extent of fluid enrichment is less than that at the Kermadec volcanic arc-front. Elevated REE patterns and (La/Sm)N ratios suggest the subduction-component modifying back-arc volcano magmas is dominated by subducting sediment. This sediment component is not consistent with aqueous fluid transfer or bulk mixing, but by the addition of a sediment-derived partial melt with residual accessory phases monazite + zircon + rutile. HFSE/REE fractionated trace element patterns overlap for unmodified basalts from Gill Seamount and Rapuhia Ridge, and Rumble V Ridge back-arc constructional volcanism to the south. This suggests that a similar mechanism triggers constructional back-arc volcanism at both locations in the southern Havre Trough, likely a consequence of thermal anomalies inferred to be present in the mantle wedge (Todd et al. (2011)).</p>