Kirsten N. Nicholson

Cretaceous‐oligocene multiphase magmatism on three kings islands, northern New Zealand

Abstract The Three Kings Islands, New Zealand, contain some of the few on‐land autochthonous examples of Cretaceous‐Oligocene magmatism in the Southwest Pacific region. Analyses of primarily mafic lavas reveal two different geochemical signatures, both predominantly medium‐K series lavas with Mg# <40 and characteristically flat, high field strength element and high large‐ion lithophile element contents normalised against mid ocean ridge basalts (MORB). Group 1 lavas have continental arc signatures and were generated by partial melting of an N(normal)‐MORB source indicating formation during the final stages of arc volcanism as crustal thinning began, during spreading along the eastern Gondwana margin. The younger Group 2 lavas were generated from an enriched mantle source with high degrees of partial melting and have minor continental signatures; we propose they represent Oligocene arc volcanism. These results support the hypothesis that between the mid cretaceous and early Tertiary, the New Zealand sector of eastern Gondwanaland underwent long periods of compressional tectonics resulting in subduction‐related volcanism and associated magmatism.

Alteration, age, and emplacement of the Tangihua Complex Ophiolite, New Zealand

Abstract The Tangihua Complex, New Zealand, represents an upper sequence of Late Cretaceous oceanic crustal material: massive basalt flows, pillow lavas, and dolerites. Three phases of alteration are preserved within the complex, each characterised by zeolite precipitation, which correlate to stratigraphic position. The mylonitised sole contains greenschist assemblages (c. 325°C) grading upwards into the initial phase of alteration (250–300°C), and is characterised by actinolite, epidote, albite, and Narich zeolites. This phase is cut by lower temperature veins of chlorite‐smectite and Carich zeolites. The final alteration phase (<50°C) precipitated K+and Ca2+‐rich minerals, including apophyllite and calcite. Disruption of Ar/Ar spectra around 50 Ma correlate with rifting in the Loyalty Basin and initiation of obduction along the Loyalty‐Three Kings Ridge system. We suggest that these events resulted in initial dismemberment, alteration, and movement of the ophiolite, whereas Ar/Ar plateaux at 25–35 Ma correspond to ophiolite emplacement and the last phases of alteration.

Characteristics of drift pumice from New Caledonia beaches

Siliceous drift pumice was collected from a total of 40 beaches around the main island of New Caledonia, Southwest Pacific, in order to determine its provenance. New Caledonia is enclosed by a barrier reef lagoon whose 2008 designation as a UNESCO World Heritage Site brought attention to the environmental degradation caused by a century of open cast nickel mining. The frequent, voluminous pumice eruptions in the Southwest Pacific provide ample source material that is somewhat durable, highly transportable in water, and easy to collect and analyze. Geochemical and mineralogical analyses were used to identify the source of the pumice in order to map the transport vector across the open ocean and into the lagoon. Drift pumice was sampled during 2008 and 2010. The mineral assemblage of the pumice was consistently calcic plagioclase, clinopyroxene, orthopyroxene, and opaque minerals. All of the pumice was of fairly uniform geochemistry: low in mafic elements, low in alkalis, with LILE enriched compared to HFSE, and negative Eu, Ti, and Zr anomalies. The pumice is predominately dacitic and tholeiitic. This geochemical signature was consistent with published data from the Tonga arc, which is further supported by the mineralogy. With the exception of two samples (which probably came from either the Kermadec arc or Vanuatu) all of the pumice comes from the Tonga arc. The samples from 2008 are consistent with pumice erupted from Metis Shoal in 2006, and the majority of 2010 samples are consistent with pumice erupted from an unnamed volcano (0403-091) that erupted in 2001.