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A Cenozoic Diffuse Alkaline Magmatic Province (DAMP) in the Southwest Pacific Without Rift Or Plume Origin

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A Cenozoic Diffuse Alkaline Magmatic Province (DAMP) in the Southwest Pacific Without Rift Or Plume Origin Article Geochemistry 3 Volume 6, Number 1 Geophysics 16 February 2005 Q02005, doi:10.1029/2004GC000723 GeosystemsG G ISSN: 1525-2027 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society A Cenozoic diffuse alkaline magmatic province (DAMP) in the southwest Pacific without rift or plume origin Carol A. Finn U.S. Geological Survey, Denver Federal Center, MS 945, Denver, Colorado 80226, USA R. Dietmar Mu¨ller School of Geosciences and University of Sydney Institute of Marine Science, University of Sydney, Edgeworth David Building F05, Sydney, New South Wales 2006, Australia Kurt S. Panter Department of Geology, Bowling Green State University, Bowling Green, Ohio 53503-0218, USA ([email protected]) [1] Common geological, geochemical, and geophysical characteristics of continental fragments of East Gondwana and adjacent oceanic lithosphere define a long-lived, low-volume, diffuse alkaline magmatic province (DAMP) encompassing the easternmost part of the Indo-Australian Plate, West Antarctica, and the southwest portion of the Pacific Plate. A key to generating the Cenozoic magmatism is the combination of metasomatized lithosphere underlain by mantle at only slightly elevated temperatures, in contrast to large igneous provinces where mantle temperatures are presumed to be high. The SW Pacific DAMP magmatism has been conjecturally linked to rifting, strike-slip faulting, mantle plumes, or hundreds of hot spots, but all of these associations have flaws. We suggest instead that sudden detachment and sinking of subducted slabs in the late Cretaceous induced Rayleigh-Taylor instabilities along the former Gondwana margin that in turn triggered lateral and vertical flow of warm Pacific mantle. The interaction of the warm mantle with metasomatized subcontinental lithosphere that characterizes much of the SW Pacific DAMP concentrates magmatism along zones of weakness. The model may also provide a mechanism for warming south Pacific mantle and resulting Cenozoic alkaline magmatism, where the oceanic areas are characterized primarily, but not exclusively, by short-lived hot spot tracks not readily explained by conventional mantle plume theory. This proposed south Pacific DAMP is much larger and longer-lived than previously considered. Components: 16,174 words, 10 figures, 1 table. Keywords: geochemistry; geophysics; tectonics; alkaline magmatism; metasomatism. Index Terms: 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 3099 Marine Geology and Geophysics: General or miscellaneous; 1099 Geochemistry: General or miscellaneous. Received 2 March 2004; Revised 5 November 2004; Accepted 22 November 2004; Published 16 February 2005. Finn, C. A., R. D. Mu¨ller, and K. S. Panter (2005), A Cenozoic diffuse alkaline magmatic province (DAMP) in the southwest Pacific without rift or plume origin, Geochem. Geophys. Geosyst., 6, Q02005, doi:10.1029/2004GC000723. 1. Introduction Australia, the Tasman Sea, New Zealand, and the Antarctic plate extending east of the Australia- [2] Cenozoic alkaline igneous rocks cover conti- Antarctic discordance, south of the Pacific- nental fragments of East Gondwana and adjacent Antarctic Ridge and west of the Antarctic Peninsula oceanic lithosphere in parts of Antarctica, eastern (Figure 1). This region is distinguished by dis- Copyright 2005 by the American Geophysical Union 1 of 26 Geochemistry 3 finn et al.: alkaline magmatic province Geophysics 10.1029/2004GC000723 Geosystems G Figure 1. Topographic and bathymetric map of south Pacific [Sandwell and Smith, 1997]. The SW Pacific DAMP study area is indicated by the thick black line. AP, Antarctic Peninsula; AT, Adare Trough; BI, Balleny Islands; BFZ, Balleny Fracture Zone; CP, Campbell Plateau; CR, Chatham Rise; CS, Coral Sea; LHR, Lord Howe Rise; LR, Louisville Ridge; LTK, Lau-Tonga-Kermadec trench; NFB, North Fiji Basin; NQ, Northern Queensland; MBL, Marie Byrd Land; MI, Macquarie Island; P-DG, Peter I and De Gerlache Seamounts; RS, Ross Sea; TAM, Transantarctic Mountains; TS, Tasman Sea; TZ, Tasmania; WA, West Antarctica; WV, Western Victoria. tinctly low velocity upper mantle with variably developing a general model for magmatism is to enriched geochemical signatures (Figure 2). The synthesize a variety of the modern data sets, igneous activity has been related to adiabatic whichhasnotbeenpreviouslydone.Inthis decompression melting due to rifting [Johnson, paper, we describe the geological, geochemical, 1989; Tessensohn, 1994; Wo¨rner, 1999] or strike- and geophysical characteristics of the crust and slip faulting [Rocchi et al., 2002a, 2003], large mantle that we use to define a diffuse Cenozoic mantle plumes [Behrendt, 1999; LeMasurier and alkaline magmatic province (DAMP). On the Landis, 1996], or numerous separate, small hot basis of our synthesis and analysis of the limi- spots [Gaina et al., 2000; Lanyon et al., 1993; tations of existing models, we identify the key Sutherland, 1991]. As we will argue, all of these combination of elements required to bring about models are flawed, whether the magmatism is the regional alkaline magmatism in the largely considered as separate or related events. continental fragments of East Gondwana and suggest an alternate model linked to late Creta- [3] Accumulating geological, geophysical and ceous slab detachments to explain these charac- geochemical data collected over the last several teristics. Finally, this model will be used to show years provide a foundation to revisit the issue of that the largely continental magmatism in the SW the origin of the volcanism. Our approach toward Pacific DAMP may be part of a much broader 2of26 Geochemistry 3 finn et al.: alkaline magmatic province Geophysics 10.1029/2004GC000723 Geosystems G Figure 2. Rayleigh wave 150s group velocity map (120 km depth) [Larson and Ekstro¨m, 2001]. Only long-lived hot spots with long traces [Clouard and Bonneville, 2001; Gaina et al., 2000; Ritsema and Allen, 2003] underlain by low-velocity perturbations in the upper mantle are shown. Abbreviations same as Figure 1 and AAD, Australian- Antarctic Discordance; LHR, Lord Howe Rise; RS, Ross Sea; TAM, Transantarctic Mountains, BH, Bellingshausen Sea. The SW Pacific DAMP study area is indicated by the thick black line. This region also includes the expected low velocities associated with the mid-ocean ridges. The white line locates the projection of the location of the postulated 130 Ma subducted plate in the mantle [Mu¨ller et al., 1993]. province encompassing much of the south igneous rocks extend 4400 km from offshore of Pacific. its northern coast south to Tasmania and Victoria (Figure 1). The 100–300 km wide belt contains 2. Characteristics of the Cenozoic scattered local volcanic centers whose estimated Alkaline Magmatism thickness derived from gravity modeling averages 70 m, yielding a total upper crustal (<2 km) volume of 0.02–0.07 Â 106 km3 (surface 2.1. Location of Alkaline Magmatism area times estimated thickness) [Wellman and [4] Cenozoic dominantly alkaline igneous rocks McDougall, 1974]. In New Zealand, Cenozoic cover large, but discontinuous portions of the intraplate volcanic rock is widely distributed along SW Pacific (Figure 1). The continental basement 1000 km of the coastlines in 3 distinct, 50– to the Cenozoic alkaline magmatism formed as a 100 km wide, mostly mafic provinces from the result of subduction processes and is composed of northeastern part of the North Island, to Auckland, Paleozoic-Mesozoic arc plutonic roots of magmatic and the southern portions of the South Island. arcs, and accreted sedimentary and oceanic crust, Compared to Australia, most of the predominantly covered in part by Jurassic igneous rocks [e.g., mafic volcanic centers are considerably smaller Dalziel, 1992]. In East Australia, the exposed, with estimated volumes an order of magnitude mainly mafic (alkaline and tholeiitic basalts), lower [Weaver et al., 1989]. The thinned continental 3of26 Geochemistry 3 finn et al.: alkaline magmatic province Geophysics 10.1029/2004GC000723 Geosystems G crust of the Campbell Plateau region and seafloor [Weaver et al., 1989]. Intermittent Cenozoic between Australia and Antarctica host scattered volcanism in the Campbell Plateau region began alkaline volcanic islands [Johnson, 1989] and small 40 Ma (Chatham Islands) and extends to 1Ma seamounts (Figure 1). (Antipodes Islands) [Weaver et al., 1989]. In Ant- arctica, magmatism started at least 48 Ma in [5] Cenozoic alkaline igneous rocks crop out at the the Transantarctic Mountains–Ross Sea region edge and offshore of West Antarctica (Marie Byrd [Tonarini et al., 1997], 37 Ma in Marie Byrd Land seamounts and Peter I Island) and parts of Land [Rocchi et al., 2002b] and continues today East Antarctica (e.g., Balleny Islands), on islands [LeMasurier and Thomson, 1990] (Figure 1). in the Ross Sea and in the Transantarctic Moun- Because much of the region is covered by ice, tains (Figure 1). In contrast to much of the rest of age data are sparse, but indicate pulses of activity in the region, 20 volcanoes have summit elevations the Upper Oligocene-lower Miocene in the western greater than 2500 m, but thousands of small cinder Ross Sea and Marie Byrd Land regions and the cones and flows are also observed [LeMasurier last 10 Myr, with 20 volcanoes manifesting and Thomson, 1990]. In the mostly ice-covered Holocene activity [LeMasurier and Thomson, regions of West Antarctica, only a few volcanic 1990; Rocchi et al., 2002a]. edifices have been imaged in sub-ice bedrock
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