THE ORIGINS OF HOLOCENE ERUPTIONS FROM FANTHAMS PEAK/PANITAHI, NORTH ISLAND, NZ: IMPLICATIONS OF U-SERIES DISEQUILIBRIA GENERATED DURING MAGMAGENESIS OF MAFIC ROCKS IN A BACK-ARC SETTING

Melt generation in the mantle beneath Fanthams Peak/Panitahi, a satellite vent on the southern flank of Mt. Taranaki, North Island, NZ is thought to result from back-arc related, fluid-flux melting associated with subduction of the Pacific plate at the Hikurangi trench, the southernmost expression of the Tonga-Kermadec arc. Some controversy surrounds this scenario because the back-arc position of Fanthams Peak (180+ km above the subducting slab) occurs where the slab has surpassed typical depths (~115 – 160 km) associated with front- and back-arc magmatism where dehydration and fluid-induced melting dominates. Because Fanthams Peak is in the back-arc, extensional processes associated with slab steepening may be partly responsible for melt production, despite eruptions preserving slab-dehydration characteristics. Four Holocene tephras, including two basaltic trachy-andesites, a trachy-basalt, and a rhyolite, were collected from the flanks of Fanthams Peak. Major and trace elements, radiogenic isotopes, and U-series isotopes (U, Th, and Ra) of Fanthams Peak tephras and crystals are variable but consistent with eruptions originating by melting subduction-fluid modified mantle. Groundmass major elements (MgO = ~4 wt %) reflect extensive differentiation and trace element characteristics (enrichments of Sr, K, Rb, Ba) are typical of arc magmas. Ba/Nb ratios are between 79.4 – 180.3, Rb/Nb ratios are between 9.9 – 11.9, Nb/U ratios are between 3.0 – 3.8, and Ce/Pb ratios are between 2.6 – 3.6, all of which are distinctive for back-arc basalts. Groundmass separates have uniform Sr, Nd, and Pb isotope ratios consistent with magmas originating by melting of a similar source. Eruption-age-corrected (²²⁶Ra)/(²³⁰Th) activity ratios of Fanthams Peak melts reflect the highest ²²⁶Ra-excesses recorded for continental arc rocks. However, groundmass separates are only slightly Th-enriched or approach secular equilibrium, while mineral phases (i.e., cpx and amphibole) are highly Th-enriched likely due to apatite inclusions. If ²²⁶Ra enrichments are imparted at the melting source, these melts record rapid magmatic differentiation and ascent (<1600 a). Alternatively, ²²⁶Ra enrichments may partly result from apatite fractionation, as apatite may influence Ra/Th characteristics of a magma.