The geochemistry and petrogenesis of basalts from the Taupo Volcanic Zone and Kermadec Island Arc, S.W. Pacific

Abstract

Basalts from the Taupo Volcanic Zone (TVZ), New Zealand, the Kermadec Island Arc (KA) and its back-arc basin, the Havre Trough show systematic variations in trace-element and isotope geochemistry which are attributed to differences in tectonic setting and source heterogeneity along a more or less continuous plate boundary. Basalts from the Kermadec Arc are characterised by low abundances of high field strength elements (HFSE) such as Ti, Zr, Nb, Ta and Hf and have high ratios of Ti/Zr and low ratios of Ti/Sc and Ti/V relative to typical MORB. Basalts from TVZ also show low abundances of the HFS elements relative to MORB but show lower Ti/Zr, higher Ti/V and Ti/Sc ratios and generally higher Zr abundances than KA most basalts. The Havre Trough basalt is mildly alkaline (< 1% normative nepheline) like many back-arc basin basalts from the Pacific rim, contrasting with the hypersthene normative TVZ and KA rocks. It has higher Zr than most TVZ basalts and all KA basalts. Ratios such as Ti/V, Ti/Sc and Ti/Zr are within the range of TVZ and MORB basalts but distinct from KA basalts. The depleted (relative to MORB) HFSE characteristics of the KA and TVZ basalts are complemented by high abundances of large ion lithophile elements (LIL), such as Ba, Rb and K, when compared to MORB, yielding the distinctive LIL-enriched pattern of subduction related rocks on a normalised multi-element plot. In contrast, the Havre Trough basalt is MORB-like. Chondrite-normalised Rare Earth Element (REE) patterns for the TVZ basalts show a field overlapping with that defined by the southern KA (Rumble Sea Mounts), with light REE enriched patterns (Ce/Yb n = ∼ 1.8−3) and flat heavy REE (Tb-Lu). Basalts from the northern KA are typically light REE depleted (Ce/Yb n = 0.5) or slightly enriched (Ce/Yb n = 1.5). The REE pattern of the Havre Trough basalt is distinctive from both the KA and TVZ fields, being richer in the heavy REE, yet similar to many basalts from back-arc basins. Sr and Nd isotopes show considerable variation from a depleted MORB-like ratio in the Havre Trough basalt ( TVZ-19, 87Sr/ 86Sr = 0.702556, ϵ Nd = +9.3 ) to radiogenic ratios in the Tarawera basalt (TVZ-4, 87Sr/ 86Sr = 0.7052, ϵ Nd = +2.2) from TVZ suggestive of crustal contamination. Between these extreme values the data from the KA and TVZ define distinctive arrays trending towards the evolved andesites, rhyolites and supracrustal basement rocks of central New Zealand. This curvilinear Nd-Sr array of the TVZ data is best explained as a result of crustal contamination. At the unradiogenic-Sr end of the array, the Sr and Nd isotopic ratios of primitive TVZ basalts overlap with those from southern KA suggesting that primitive TVZ basalts were derived from sources that are isotopically similar to those from which the southern KA basalts derived. Basalts from northern KA (Herald, Raoul and Macauley Islands) are appreciably less radiogenic than those from the central part (Curtis and L'Esperance) and the southern (Rumble Seamounts) part of the arc. Using non-slab-derived HFSE ratios and abundances as a guide to relative source depletion, it is inferred that the KA source varies from strongly depleted in the north to less depleted in the TVZ in the south. The KA source has generally experienced more extensive extraction of mafic magmas and hence is impoverished in incompatible HFSE. This leads to high Ti/Zr and low (absolute) Zr, Hf, Ta and Nb abundances. The Havre Trough source is relatively fertile with regard