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Article in Press + Model ARTICLE IN PRESS + MODEL Earth and Planetary Science Letters xx (2006) xxx–xxx 1 www.elsevier.com/locate/epsl 2 Cenozoic intraplate volcanism on New Zealand: 3 Upwelling induced by lithospheric removal ⁎ 4 K. Hoernle a, , J.D.L. White b, P. van den Bogaard c, F. Hauff c, D.S. Coombs b, 5 R. Werner d, C. Timm c, D. Garbe-Schönberg e, A. Reay b, A.F. Cooper b 6 a IFM-GEOMAR Leibniz Institute for Marine Sciences at the Christian Albrechts University of Kiel, Wischhofstr. 1-3, D-24148 Kiel, Germany 7 b Geology Department, University of Otago, PO Box 56, Dunedin 9015, New Zealand 8 c IFM-GEOMAR Leibniz Institute for Marine Sciences, Wischhofstr. 1-3, D-24148 Kiel, Germany 9 d Tethys Geoconsulting GmbH, Wischhofstraβe 1-3, 24148 Kiel, Germany 10 e Institut für Geowissenschaften, Christian Albrechts University of Kiel, Ludewig-Meyn-Strasse 10, 24118 Kiel, Germany 11 Received 21 December 2005; received in revised form 31 May 2006; accepted 1 June 2006 12 PROOF 13 Editor: R.W. Carlson 14 Abstract 15 Diffuse intraplate volcanism spanning the Cenozoic on the North, South,ED Chatham, Auckland, Campbell and Antipodes Islands of 16 New Zealand has produced quartz tholeiitic to basanitic/nephelinitic (including their differentiates) monogenetic volcanic fields and 17 large shield volcanoes. New 40Ar/39Ar ages, combined with published age data, show no correlations among age, location or 18 composition of the volcanoes. Continuous volcanism in restricted areas over long time periods, and a lack of volcanic age progressions in 19 the direction and at the rate of plate motion, are inconsistent with a plume origin for the intraplate volcanism. Although localized 20 extension took place during some episodes of volcanic activity, the degree of extension does not correlate with erupted volumes or 21 compositions. Major and trace element data suggest that the silica-poor volcanic rocks (primarily basanites) were derived through low 22 degrees of partial melting at deeper depths than the more silica-rich volcanic rocks (alkali basalts and tholeiites) and that all melts were 23 produced from ocean island basalt (OIB)-type sources, containing garnet pyroxenite or eclogite. The Sr–Nd–Pb isotope data indicate that 24 the silica-poor rocks were derived from high time-integrated U/Pb (HIMU)-type sources and the silica-rich rocks from more enriched 25 mantle (EM)-type sources, reflecting greater interaction with lithosphere modified by subduction beneath Gondwana. The first-order 26 cause of melting is inferred to be decompression melting in the garnet stability field of upwelling asthenosphere, triggered by removal 27 (detachment) of different parts of the subcontinentalRRECT lithospheric keel throughout the Cenozoic. In some cases, large thicknesses of keel 28 were removed and magmatism extended over many millions of years. Decompression melting beneath a thick craton generates melts that 29 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 30 reach the surface in their pure form due to the swamping effect of larger-degree melts formed at shallower depths. Different volcanic 31 styles in part reflect the mode of removal, and size and shape of detached parts of the lithospheric keel. Removal of continental 32 lithospheric mantle could be an important process for explaining the origin of diffuse igneous provinces on continental lithosphere. 33 © 2006 Published by Elsevier B.V. 34 35 Keywords: intraplate volcanism; continental diffuse igneous province; New Zealand; 40Ar/39Ar ages; geochemistry; lithospheric removal/ 36 detachment UNCO 37 ⁎ Corresponding author. Tel.: +49 431 6002642; fax: +49 431 6002924. E-mail address: [email protected] (K. Hoernle). 0012-821X/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.epsl.2006.06.001 EPSL-08225; No of Pages 18 ARTICLE IN PRESS 2 K. Hoernle et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx 38 1. Introduction continental areas is decompression melting of upwelling 49 shallow mantle that results from tectonic thinning of the 50 39 Since the plate tectonic model achieved wide accep- lithosphere (e.g. [2]). Neither of these models, however, 51 40 tance, intraplate volcanism has been primarily attributed can adequately explain diffuse volcanism known from 52 41 to mantle plumes [1], which now are generally pre- many continental areas globally. 53 42 sumed to represent cylindrical regions of mantle up- New Zealand's South Island has been the site of nume- 54 43 welling (∼100–300 km in diameter) from a thermal rous temporally and spatially dispersed episodes of in- 55 44 boundary layer such as the core/mantle boundary. The traplate volcanism throughout the Cenozoic (Fig. 1). This 56 45 mantle plume model is, however, being increasingly intraplate volcanism has produced: 1) scattered, low- 57 46 questioned, leading to the global “Great Plume Debate” volume alkalic dikes (e.g. Alpine Dikes) and monogenetic 58 47 (e.g. http://www.mantleplumes.org). The major alterna- volcanic fields (e.g. Waipiata volcanics), 2) several cubic 59 48 tive to the plume model for intraplate volcanism in kilometers of tholeiitic volcanic rocks erupted from 60 ROOF P CTED UNCORRE Fig. 1. Overview map of the Zealandia micro-continent, summarizing the published age data. Bathymetry is based on satellite altimetry and ship depth soundings [71]. Shading (see key in upper right hand corner) refers to water depth in meters. Inset: Blow-up map of Otago and southern Canterbury, South Island, showing the 40Ar/39Ar age data for intraplate volcanism from this study. Numbers beside the named localities give ages in Ma. Those without parentheses are Ar/Ar ages from this study (Table 1); those within parentheses are K/Ar and other dates from publications referred to in the text. Plate motion vector is from [43]. ARTICLE IN PRESS K. Hoernle et al. / Earth and Planetary Science Letters xx (2006) xxx–xxx 3 Table 1 (continued) t1:35 Sample Dated Dating Age±2 MSWDa % 39Ar t1:36 t1:1 Table 1 no. material method σ (Ma) in plateau t1:2 40Ar/39Ar step-heating and single-crystal/particle ages a 39 Canterbury: Timaru t1:3 Sample Dated Dating Age±2 MSWD % Ar b OU33298 Glass Single 2.6±0.4 1.14 (N =35) t1:38 no. material method σ (Ma) in plateau fusions Otago: Waiareka–Deborah t1:39 t1:5 LSI7 Matrix Step 39.5±1.8 0.83 69 Campbell Plateau: Auckland Island heating OU19560 Matrix Step 16.7±1.2 0.65 52 t1:41 t1:6 OU47055 Glass Step 34.2±0.4 0.84 100 heating heating OU19577 Matrix Step 16.7±0.1 0.53 99 t1:42 t1:7 Glass Single 34.3±0.9 3.31 (N b =13) heating fusions OU19606 Matrix Step 15.2±0.3 1.30 46 t1:43 t1:8 OU54929 Matrix Step 34.0±0.6 0.86 55 heating heating t1:44 t1:9 KK1 Hornblende Step 33.7±0.3 1.40 87 Campbell Plateau: Campbell Island heating OU36173 Matrix Step 7.3±0.3 1.30 70 t1:46 t1:10 Hornblende Single 34.1±0.1 0.87 (N b =14) heating fusions Feldspar Step 6.6±0.6 1.50 100 t1:47 t1:11 OU55008 Matrix Single 33.6±1.8 – (N b =2) heating fusions OU36174 Matrix Step 6.94±0.05 1.40 79 t1:48 t1:12 MSI99A Matrix Step 34.3±0.5 1.10 47 heating heating OU36176 Feldspar Single 7.5±0.3 0.67 (N b =15) t1:49 t1:13 PROOFfusion Otago: Alpine Dike Outlier a t1:50 b MSWD = Mean Squared Weighted Deviates. t1:15 OU42071 Matrix Single 20.7±0.4 1.09 (N =12) b N=Number of Single Fusion Analyses. t1:51 fusions t1:16 Otago: Waipiata t1:18 LSI4 Matrix Step 24.8±0.6 0.89 85 spatiallyED and temporally restricted centers (e.g. Timaru and 61 heating 62 t1:19 Geraldine lava flows), 3) closely related tholeiitic and OU54935 Matrix Step 22.0±0.3 1.15 67 63 heating alkalic volcanism (e.g. Banks Peninsula [3]), 4) large t1:20 LSI1 Matrix Step 21.4±0.4 0.92 87 composite shield volcanoes (e.g. Dunedin and Campbell 64 heating Island volcanoes) having edifice volumes of up to 65 t1:21 OU66573 Matrix Step 17.1±0.5 0.57 67 1200 km3, which can occur as clusters (e.g. Auckland 66 heating 67 t1:22 Islands and Banks Peninsula shield volcanoes). The cause Matrix Step 17.0±0.5 0.93 84 68 duplicate heating of this Cenozoic melting, which produced magmas t1:23 LSI3 Matrix Step 14.4±0.2 0.53 64 ranging from highly SiO2-undersaturated (e.g. basanitic 69 heating and nephelinitic) to quartz tholeiitic and their more 70 t1:24 Matrix Step 14.2±0.2 0.55 81 evolved differentiates, is poorly understood and contro- 71 duplicate heating 72 t1:25 RRECTversial. In order to explain the widely dispersed Cenozoic OU55018 Feldspar Step 14.1±0.6 1.50 61 73 heating intraplate volcanism on the (mostly submerged) New t1:26 MSI76 Matrix Step 11.1±0.6 1.60 72 Zealand micro-continent (Fig. 1), here referred to as 74 heating Zealandia, with the plume model, many dozens of small 75 t1:27 plumes (plume swarm) or a diffuse megaplume are 76 Otago: Dunedin Volcano 77 t1:29 required. There is, however, no geophysical (e.g. seismic LSI23 Matrix Step 16.0±0.4 0.66 63 78 heating tomographic) evidence for plume-like structures beneath 3 4 t1:30 LSI22 Matrix Step 14.6±0.4 0.53 78 New Zealand (e.g.
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