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  • Parana Magmatic Province^Tristan da Cunha plume system:xed versus mobile plume, petrogenetic considerations and

    alternative heat sources

    M. Ernesto a;, L.S. Marques a, E.M. Piccirillo b, E.C. Molina a, N. Ussami a,P. Comin-Chiaramonti c, G. Bellieni d

    a Department of Geophysics, University of Sao Paulo, Rua do Matao 1226, 05508-900 Sao Paulo, SP, Brazilb Dipartimento di Scienze della Terra, University of Trieste, Trieste, Italy

    c Dipartimento di Ingegneria Chimica, dellAmbiente e delle Materie Prime, University of Trieste, Trieste, Italyd Dipartimento di Mineralogia e Petrologia, University of Padova, Padua, Italy

    Received 30 November 2000; accepted 25 November 2001

    Abstract

    Paleomagnetic reconstructions demonstrate that the Tristan da Cunha (TC) plume, which is usually related to thegenesis of the high- and low-Ti flood tholeiites of the Parana Magmatic Province (PMP), was located V1000 kmsouth of the Parana Province at the time of the magma eruptions. Assuming plume mobility, and considering the low-velocity zone identified in the northern portion of the PMP as the TC fossil plume (V20 from the present TCposition), the plume migrated southward from 133^132 (main volcanic phase) to 80 Ma at a rate of about 40 mm/yr.From 80 Ma to Present the plume remained virtually fixed, leaving a track (Walvis Ridge) compatible with theAfrican plate movement. However, geochemical and Sr^Nd^Pb isotopic data do not support that the tholeiites fromWalvis Ridge, Rio Grande Rise and Parana can result from mixing dominated by the TC plume and mid-ocean ridgebasalt components. The similarity among the high-Ti basalts from Rio Grande Rise, part of Walvis Ridge (525A) andthe Parana Province suggests that delaminated subcontinental lithospheric mantle must be considered in their genesis.Regional thermal anomalies in deep mantle mapped by geoid and seismic tomography data offer an alternative non-plume-related heat source for the generation of intracontinental magmatic provinces.? 2002 Elsevier Science B.V. All rights reserved.

    Keywords: Parana Magmatic Province; Tristan da Cunha; mantle plumes; paleomagnetic reconstructions; geoid anomalies

    1. Introduction

    The origin of the Early Cretaceous ood tholei-ites of the Parana Magmatic Province (PMP), in-cluding the equivalents in Etendeka (Namibia)and Angola (Fig. 1), is commonly related to theTristan da Cunha (TC) plume (e.g. Richards etal., 1989; White and McKenzie, 1989; Milner

    0377-0273 / 02 / $ ^ see front matter ? 2002 Elsevier Science B.V. All rights reserved.PII: S 0 3 7 7 - 0 2 7 3 ( 0 2 ) 0 0 2 4 8 - 2

    * Corresponding author. Tel. : +55-11-3091-4755;Fax: +55-11-3091-5034.

    E-mail addresses: marcia@iag.usp.br (M. Ernesto),leila@iag.usp.br (L.S. Marques), picciril@univ.trieste.it (E.M.Piccirillo), eder@iag.usp.br (E.C. Molina), naomi@iag.usp.br(N. Ussami), comin@univ.trieste.it (P. Comin-Chiaramonti),giuliano@dmp.unipd.it (G. Bellieni).

    VOLGEO 2484 11-10-02 Cyaan Magenta Geel Zwart

    Journal of Volcanology and Geothermal Research 118 (2002) 15^36

    www.elsevier.com/locate/jvolgeores

  • and Le Roex, 1996; Gibson et al., 1995; Courtil-lot et al., 1999), although geochemical evidenceled Peate (1997), Comin-Chiaramonti et al.(1997) and Marques et al. (1999) to discard therole of the TC mantle source in the generation ofthe Parana melts except as a source of heat. Theconcept of mantle plumes since the initial propo-sition by Wilson (1963) became so popular thatvirtually all large igneous provinces are dogmati-cally assumed to have been associated with amantle plume, although the weaknesses of thismodel have already been evidenced by Anderson(1996, 1998), Smith and Lewis (1999) and Sheth(1999), among others. In the case of the ParanaProvince, geological, geochemical and petrologicaldata (e.g. Piccirillo and Mel, 1988; Peate andHawkesworth, 1996; Comin-Chiaramonti et al.,1997; Peate, 1997) demonstrated that the sourcemantle signatures of TC volcanics are not appre-ciable in the Parana Large Igneous Province(LIP). This indicates that the plume head^hot-spot system, formerly proposed by Wilson(1963), cannot easily be assumed a geodynamicmodel in order to explain the genesis of the Para-na magmatism, particularly if the genesis of theParana ood tholeiites is considered compatiblewith models involving edge-driven convectionprocesses of upper mantle (e.g. Anderson,1994a,b; King and Anderson, 1998; Sheth,1999; King and Ritsema, 2000; Tackley, 2000),as recently documented for the Central AtlanticMagmatic Province (e.g. McHone, 2000; De Minet al., 2002).The Parana ood tholeiites have often been

    used in the literature as one of the best examplesof a continental LIP genetically related to theplume-head, mainly because the oldest part (113Ma) of the Walvis Ridge volcanic chain is locatedclose to the Etendeka plateau (e.g. OConnor andDuncan, 1990), which represents the easternmostextent of the PMP in a pre-Atlantic conguration.Therefore, there is a strong tendency to interpretthe Walvis Ridge as a trace left by the TC hot-spot, in spite of the fact that it is very dicult tointerpret the expected western symmetrical mag-matic chain (i.e. Rio Grande Rise) in terms of asimple mantle plume^hotspot model (see below).However, the location of the TC plume relative

    to the South American plate by the time of Para-na volcanism is still unconstrained as the plumehead emplacement under the lithosphere predatesthe South Atlantic opening. The oldest oceanicmagnetic anomaly (M4), at Parana latitudes datesto 126.5 Ma (Nurnberg and Muller, 1991), and nohotspot trace was recognized onshore. Geochro-nological data, based on the 40Ar/39Ar method(Renne et al., 1992; Turner et al., 1994, Renneet al., 1996a; Ernesto et al., 1999), show thatthe main volcanic activity in the PMP took placeat 133^132 Ma, before the South America andAfrica separation (Piccirillo et al., 1988). There-fore, between 113 Ma (Walvis Ridge) and 133^132 Ma (main Parana volcanism), there is atime interval of about 20 Ma of unknown historyregarding the location of the plume relative to thecontinents.Various attempts to locate the TC plume have

    been made, and there is a consensus that it mustbe placed beneath the PMP since this provincewould represent the major surface expression ofthe plume. In doing so, most of the authors admitTC as a xed point in relation to the mantle (e.g.OConnor and Duncan, 1990; Turner et al., 1996;Courtillot et al., 1999), and the Western Gondwa-na supercontinent (South American and Africanplates) is reconstructed by forcing the PMP overthe TC plume. Paleolongitudes and paleolatitudesof the supercontinent are therefore set arbitrarilyin terms of the TC^PMP relationship.The mantle plume geodynamic model was orig-

    inally conceived as a xed thermal mantle anom-aly generated at lower mantle^outer core depthsover which the lithospheric plate moved, leaving atime-related volcanic track. However, this modelhas shown inconsistency in the case of some LIPsbelieved to be related to mantle plumes and,therefore, a mobile plume with velocities exceed-ing 100 mm/yr (e.g. Molnar and Stock, 1987;Vandamme and Courtillot, 1990; Duncan, 1990)must be considered.Local scale and high resolution tomography

    from VanDecar et al. (1995) has mapped a low-velocity zone in the mantle, extending from 100 to600 km, which has been interpreted as the fossilTC plume in the northeastern Parana Province.However, no geoid anomaly or surface expression

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    M. Ernesto et al. / Journal of Volcanology and Geothermal Research 118 (2002) 15^3616

  • of the TC thermal anomaly is recognized in thisregion (Molina and Ussami, 1999), except for theIpora and Alto Parana|ba Late Cretaceous alka-line provinces (Comin-Chiaramonti et al., 1997,and references therein) further to the north.In addition, according to the plume model, it

    would be expected that geochemical and isotopicsignatures would be compatible with a dominantcontribution of asthenospheric mantle materialsin the basalt genesis (Campbell and Griths,1990; Arndt and Christensen, 1992). For thePMP, in spite of the large amount of petrological,geochemical and isotopic data available, there isno consensus about the mantle sources involvedin the basalt genesis. An origin in the lithosphericmantle, without signicant geochemical and iso-topic plume signatures, is proposed to explain the

    geochemical and isotopic characteristics of thelow- and high-TiO2 PMP tholeiites, their spatialdistribution in the province and the associationwith coeval alkaline and alkaline-carbonatiticmagmatism (e.g. Piccirillo and Mel, 1988; Peateand Hawkesworth, 1996; Comin-Chiaramonti etal., 1997; Marques et al., 1999). On the otherhand, according to other interpretations (Gibsonet al., 1995, 1999; Milner and Le Roex, 1996),some basalt compositions could reect the geo-chemical imprint of the TC mantle plume.The purpose of this paper is to (1) summarize

    the geochemical and isotopic data of the Paranabasalts and their comparison with those of RioGrande Rise, Walvis Ridge, South Atlantic Mid-Ocean Ridge and TC volcanic rocks, particularlyfor their mantle source characteristics ; (2) test the

    Fig. 1. Location of the PMP in South America, and the corresponding Angola and Etendeka provinces in Africa. The TC hot-spot is shown along with other Atlantic hotspots. Bathymetric data and ages for Walvis Ridge and Rio Grande Rise are accord-ing to Muller et al. (1993).

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    M. Ernesto et al. / Journal of Volcanology and Geothermal Research 118 (2002) 15^36 17

  • hypotheses of both the xed and the mobile TCplume, and set constraints on the location of theTC plume using an updated paleomagnetic data-base for the Early Cretaceous from South Amer-ica and Africa; (3) discuss