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