The Petrology, Geochemistry and Petrogenesis of Volcanic Rocks from the Southern Kermadec Oceanic Island Arc, South West Pacific

Abstract

The Kermadec Island Arc marks the locus of oceanic island arc volcanism between 25°s and 37°s associated with convergence and subduction of the Pacific Ocean Plate beneath the Australian Plate. This thesis is concerned with the petrology, geochemistry and petrogenesis of volcanoes from the southern part of the arc, which is entirely submarine. The volcanoes studied are Rumble III, Rumble IV, Rumble V, Tangaroa and Clark, the latter representing the southernmost volcanic edifice of the Kermadec arc system. Samples were collected by dredging on two separate cruises: (1) by the RV Rapuhia in February 1992 and (2) by the RV Akademic Lavrenteyev in July 1994. Out of the total number of dredge samples, 51 rock samples were selected for detailed study. Rock samples are described petrographically and analysed by electron microprobe and X-Ray fluorescence spectrometry for major and trace elements. Representative samples have been analysed by Instrumental Neutron Activation Analysis for selected trace elements, including the Rare Earth Elements, Th and U. A further subset was analysed by Thermal Ionisation Mass Spectrometry for Sr, Nd and Pb isotopes. Several samples have been dated radiometrically by the 40Ar/39Ar method. Chemically analysed samples range between basalt and dacite, with some volcanoes displaying considerable diversity, and others very little. For example: Rumble III samples are basalt and basaltic andesite, Rumble IV samples are basalts, basaltic andesites, andesites and dacite, Rumble V basaltic andesites, Tangaroa basaltic andesites, andesites and dacites and Clark basalts, basaltic andesites and dacites, including an unusual K-rich suite. Fractional crystallisation, involving varying proportions of Olivine + Plagioclase + Clinopyroxene + Orthopyroxene + Magnetite, acting on independent magma batches is the favoured model for producing compositional variation in individual volcanoes. Differences in incompatible element abundance and ratios between volcanoes suggest source heterogeneity influenced by previous melt extraction events and variable flushing of the source by fluids (and sediment) from the down-going slab. These variations are most marked for Clark volcano, where the suite of K-rich lavas, coexisting with more "normal" Kermadec Arc magmas, show distinctive phase relations (Olivine + clinopyroxcne, no plagioclase) and radiogenic isotope compositions more typical of TVZ lavas. The K-rich suite are also enriched in Light Rare Earth Elements (LREE), with Ce/Ybn ~ 7-10 in comparison with the flat to LEE to LREE depleted nature of the "normal" magmas (Ce/Ybn ~ 2 - 0.7). This is interpreted to reflect a significant contribution of subducted, continent-derived, sediment to the slab - derived component.