Cenozoic intraplate volcanism on New Zealand: Upwelling induced by lithospheric removal

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Abstract

Diffuse intraplate volcanism spanning the Cenozoic on the North, South, Chatham, Auckland, Campbell and Antipodes Islands of New Zealand has produced quartz tholeiitic to basanitic/nephelinitic (including their differentiates) monogenetic volcanic fields and large shield volcanoes. New 40Ar/39Ar ages, combined with published age data, show no correlations among age, location or composition of the volcanoes. Continuous volcanism in restricted areas over long time periods, and a lack of volcanic age progressions in the direction and at the rate of plate motion, are inconsistent with a plume origin for the intraplate volcanism. Although localized extension took place during some episodes of volcanic activity, the degree of extension does not correlate with erupted volumes or compositions. Major and trace element data suggest that the silica-poor volcanic rocks (primarily basanites) were derived through low degrees of partial melting at deeper depths than the more silica-rich volcanic rocks (alkali basalts and tholeiites) and that all melts were produced from ocean island basalt (OIB)-type sources, containing garnet pyroxenite or eclogite. The Sr–Nd–Pb isotope data indicate that the silica-poor rocks were derived from high time-integrated U/Pb (HIMU)-type sources and the silica-rich rocks from more enriched mantle (EM)-type sources, reflecting greater interaction with lithosphere modified by subduction beneath Gondwana. The first-order cause of melting is inferred to be decompression melting in the garnet stability field of upwelling asthenosphere, triggered by removal (detachment) of different parts of the subcontinental lithospheric keel throughout the Cenozoic. In some cases, large thicknesses of keel were removed and magmatism extended over many millions of years. Decompression melting beneath a thick craton generates melts that are likely to be similar to those from the base of the mid-ocean-ridge melting column. At mid-ocean ridges, however, these melts never reach the surface in their pure form due to the swamping effect of larger-degree melts formed at shallower depths. Different volcanic styles in part reflect the mode of removal, and size and shape of detached parts of the lithospheric keel. Removal of continental lithospheric mantle could be an important process for explaining the origin of diffuse igneous provinces on continental lithosphere.

Introduction

Since the plate tectonic model achieved wide acceptance, intraplate volcanism has been primarily attributed to mantle plumes [1], which now are generally presumed to represent cylindrical regions of mantle upwelling (∼ 100–300 km in diameter) from a thermal boundary layer such as the core/mantle boundary. The mantle plume model is, however, being increasingly questioned, leading to the global “Great Plume Debate” (e.g. http://www.mantleplumes.org). The major alternative to the plume model for intraplate volcanism in continental areas is decompression melting of upwelling shallow mantle that results from tectonic thinning of the lithosphere (e.g. [2]). Neither of these models, however, can adequately explain diffuse volcanism known from many continental areas globally.

New Zealand's South Island has been the site of numerous temporally and spatially dispersed episodes of intraplate volcanism throughout the Cenozoic (Fig. 1). This intraplate volcanism has produced: 1) scattered, low-volume alkalic dikes (e.g. Alpine Dikes) and monogenetic volcanic fields (e.g. Waipiata volcanics), 2) several cubic kilometers of tholeiitic volcanic rocks erupted from spatially and temporally restricted centers (e.g. Timaru and Geraldine lava flows), 3) closely related tholeiitic and alkalic volcanism (e.g. Banks Peninsula [3]), and 4) large composite shield volcanoes (e.g. Dunedin and Campbell Island volcanoes) having edifice volumes of or greater than 1200 km3, which can occur as clusters (e.g. Auckland Islands and Banks Peninsula shield volcanoes). The cause of this Cenozoic melting, which produced magmas ranging from highly SiO2-undersaturated (e.g. basanitic and nephelinitic) to quartz tholeiitic and their more evolved differentiates, is poorly understood and controversial. In order to explain the widely dispersed Cenozoic intraplate volcanism on the (mostly submerged) New Zealand micro-continent (Fig. 1), here referred to as Zealandia, with the plume model, many dozens of small plumes (plume swarm) or a diffuse megaplume are required. There is, however, no geophysical (e.g. seismic tomographic) evidence for plume-like structures beneath New Zealand (e.g. [4]). In addition, 3He/4He isotope ratios of fluid inclusions in mantle xenocrysts and basalt phenocrysts from the South Island appear to support derivation of the intraplate magmas from degassed upper mid-ocean-ridge basalt (MORB)-type mantle beneath New Zealand [5].

Many often contradictory models have been proposed to explain the Cenozoic intraplate volcanism on Zealandia. Major continental rifting associated with the separation of New Zealand from West Antarctica ceased in the mid-Cretaceous, but Weaver and Smith [2] proposed that Cenozoic intraplate volcanism was at least in part related to shallow upwellings related to local rifting events. In contrast, based on an apparent crude WNW-ESE age progression of volcanism, Adams [6] and Farrar and Dixon [7] proposed that intraplate volcanism resulted from Zealandia over-riding a former spreading center, manifested as a NNE-trending line of asthenospheric upwelling. Coombs et al. [8] noted the similarity in Sr and Nd isotopic composition of Cenozoic basalts on the South Island of New Zealand to basalts from Australia and Marie Byrd Land, Antarctica. These three continental blocks were contiguous prior to 100 Ma but rifted apart at c. 84 Ma, suggesting that the Cenozoic volcanism resulted from melting of common, and therefore lithospheric, source materials (e.g. [8], [9], [10]). Lithospheric sources have been proposed for the nephelinitic to tholeiitic volcanism in the South Auckland volcanic field [11] and on the Chatham, Campbell and Antipodes Islands [12]. In contrast, Finn et al. [4] included the Cenozoic intraplate volcanism on New Zealand within a “Diffuse Alkaline Magmatic Province”, encompassing the easternmost part of the Indo-Australian Plate, West Antarctica and the southwest portion of the Pacific Plate, and attributed this magmatism to interaction between the uppermost asthenospheric mantle and subduction-modified subcontinental lithosphere.

In order to better understand the origin of Cenozoic intraplate volcanism on New Zealand, we have acquired new age (40Ar/39Ar) and geochemical (major element, trace element and Sr–Nd–Pb isotopic) data to assess possible age progressions in volcanism and the sources of magma for volcanic rocks from Otago on the South Island of New Zealand and from the Auckland and Campbell Islands on the Campbell Plateau.

Section snippets

Age determinations

Twenty-six intraplate volcanic samples from the South Island of New Zealand and the Auckland and Campbell Islands on the Campbell Plateau (see Supplementary File 1 for sample descriptions and locations) have been dated with the 40Ar/39Ar method (age data are summarized in Table 1; analytical methods, detailed age data and age diagrams are presented in Supplementary File 2).

Six samples from the Waiareka–Deborah Formation [8] near Oamaru northeast of Dunedin (Fig. 1 inset) have ages ranging from

Temporal and spatial variations of Cenozoic intraplate volcanism

Late Cretaceous and Cenozoic tholeiitic to nephelinitic intraplate volcanism has been commonplace and widespread in New Zealand and on the Chatham Rise and the Campbell Plateau [2]. The Late Cretaceous intraplate volcanism (∼ 100–65 Ma) forming the Mandamus Igneous Complex (e.g. [23]), the Tapuaenuku Igneous Complex [24], the Hohonu Range [16] and the Southern Volcanics of the Chatham Islands [12], [25] is probably associated with the final phase of Gondwana breakup in which the micro-continent

Acknowledgements

We thank D. Rau, J. Sticklus, S. Hauff for analytical assistance, C. Adams for sharing unpublished K/Ar age data on Otago volcanism, M. Portnyagin for comments on this manuscript, and the “Great Plume Debate” Chapman Conference in Fort William (2005) for talks and discussions that helped further develop ideas in this paper. We are especially grateful to Lindy Elkins-Tanton and Kurt Panter for constructive reviews which helped to improve this manuscript significantly. We acknowledge Otago

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