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The Timing and Duration of the Karoo Igneous Event, Southern Gondwana

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The Timing and Duration of the Karoo Igneous Event, Southern Gondwana JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. B8, PAGES 18,127-18,138,AUGUST 10, 1997 The timing and duration of the Karoo igneousevent, southern Gondwana R.A. Duncan,• P.R. Hooper,: J. Rehacek,2J.S. Marsh, 3 andA.R. Duncan4 Abstract.A volcanicevent of immensescale occurred within a relativelyshort period in early Jurassictime overlarge regions of the contiguousGondwana supercontinent. In southernAfrica, associated remnants of thick volcanic successionsof lava flows and extensive dike and sill complexesof similarcomposition have been grouped together as the Karoo Igneous Province. Correlativevolcanic and plutonic rocks occur in Antarcticaand Australia as the FerrarProvince. Thirty-twonew •OAr-•Ar incrementalt•eatmg experiments on feldsparsand whole rocks from Namibia,South Africa andEast Antarctica produce highly resolved ages with a vastmajority at 183+_ 1 Ma anda totalrange of 184 to 179 Ma. Theseare indistinguishablefrom recent,high- resolution40Ar-39Ar and U-Pb agedeterminations reported from the Antarctic portion of the province.Initial Karoo volcanism(Lesotho-type compositions) occurred across the entireSouth African craton.The ubiquitousdistribution of a plexusof generallynonoriented feeder dikes and sillsintruding Precambrian crystalline rocks and Phanerozoic sediments indicates that these magmaspenetrated the cratonover a broadregion. Lithosphere thinning of the continentfollowed the main pulseof igneousactivity, with volcanismfocused in the Lebombo-Nuanetsiregion, near the eventualsplit between Africa andAntarctica. Seafloor spreading and dispersion of eastand west Gondwanafollowed some 10-20 m.y. afterward.The volumeof the combinedKaroo-Ferrar province(•2.5 x 106km3) makes it oneof thelargest continental flood basalt events. The timing of thisevent correlates with a moderatemass extinction (Toarcian-Aalenian), affecting largely marine invertebrates. This extinction event was not as severe as those recorded at the Permian- Triassicor Cretaceous-Tertiaryboundaries associated with the Siberianand Deccan flood basalts events,respectively. The differencemay be dueto the high southerlylatitude and somewhat lower eruptionrates of the Karoo event. Introduction and Campbell, 1990; Duncan and Richards, 1991]. These rare, catastrophicevents may be spawnedby gravitational Plate tectonics is central to our current picture of Earth instabilitiesin a thermalboundary layer deepin the mantle dynamics. Ocean basins open and close, and continents (e.g., the core-mantleboundary), and evidenceis mounting disperseand coalescein a perpetual dance paced by upper that their occurrencecorrelates with profoundchanges in the mantle convection.The two major classesof volcanic activity surface environment, including mass biotic extinctions are understandable in terms of steady state processesof [Larson, 1991;Renne et al., 1995]. Additionalsignificance is crustal accretion at spreadingridges and plate recycling at attachedto the coincidenceof many continentalflood basalt subductionzones. However, a third class of volcanic activity, provinceswith continentalrifting [Morgan, 1981; White and that associatedwith mantle plumes, is unrelated to plate McKenzie, 1989;Storey, 1995]. Critical informationneeded tectonicsbut spansplate boundariesand interiorsof all ages. This latter type is volumetrically minor but is significant in to assessthese proposed correlations is the timing and sampling portions of the deeper mantle not routinely durationof igneousactivity relative to othergeologic events, such as tectonism and environmental crises. participatingin plate tectonics. Recently, much attention has focused on examining the The KarooIgneous Province (KIP) (Figure1) is oneof the proposalthat mantle plumesbegin with extremelyrapid, large largestand bestexposed of the classicMesozoic Gondwana volume eruptions of basaltic magmas that form the continentalflood basalt provinces [Erlank, 1984; Cox, 1988]. continental flood basalt provinces on land and oceanic It comprisesthick sequences of volcanicrocks preserved in plateausin the ocean basins [Richards et al., 1989; Griffiths erosionalremnants and a well-developedsubvolcanic plexus of dikes and sills scatteredthroughout southern Africa. Its formationis broadly correlatedwith the breakupof XCollegeofOceanic and Atmospheric Sciences, Oregon State University, Gondwana,although details of the temporalrelationship Corvallis. remainunclear. Karoo igneous rocks have also been emplaced 2Departmentof Geology, Washington State University, Pullman. in differenttectonic settings across the province, and there are 3DepartmentofGeology. Rhodes University, Grahamstown, Republic of South Africa. considerabledifferences in the lithostratigraphicsequences nDepartmentof GeologicalSciences, University of CapeTown, in thesedifferent settings. For example,the thick volcanic Rondesbosch,Republic of SouthAfrica. remnantsof Lesotho,south and centralBotswana, and central Namibia, including the huge volume of intrusives in the cratonicinterior of southernAfrica (the centralarea of Marsh Copyright1997 by the AmericanGeophysical Union. and Eales [1984]), are remarkablyhomogeneous tholeiitic Papernumber 97JB00972. basalts.In contrast,a diversesuite of nephelinites,picrites, 0148-0227/97/97JB-00972509.00 low- and high-Ti basalts, and rhyolites have built the 18,127 18,128 DUNCAN ET AL.: TIMING AND DURATION OF KAROO IGNEOUS EVENT Zimbabwe. 20 ø Nu.anetsi Serowe ............?i,-:::............ .... 25 ø trdap ""- • Lebombo . 30 ø [---] (• AntarcticaEast "':":':•pre-Karoorocks 35 ø + + + 500 km 15 ø 20 ø 25 ø 30 ø 35 ø E Figure 1. Schematicgeological map of South Africa, showingthe distributionof erosionalremnants of the Karoo Igneous Province [after Eates et at., 1984]. Major lava sectionsare named, and five major sampling localities are indicatedby circled numbers.The East Antarctica (Kirwan Mountains) location is shown in its predrift position. The dashedline indicatesthe southernand western limit of dolerites and sills, intrusive into Permo-Triassicage Karoo sedimentsand Precambriancratonic rocks. Dikes are both oriented,as in the WNW-ESE trendingswarm from Zimbabweto Namibia,and nonoriented. Lebombo-Mwenezi-Save monoclinal remnant along the We reporthigh-resolution age determinationsfrom 40Ar- northeasternmargin of the province,where the magmatism 39Ar incrementalheating experiments on whole basalts, was associatedwith lithosphericthinning and continental nephelinites, picrites, and rhyolites and feldspar separates rifting. from basalts and diabases from the complete stratigraphic Such differences have hampered lithological correlation range at widely separatedlocations in the KIP and from the between the widely spaced erosional remnantsof Karoo Kirwan Mountains, East Antarctica. The experimental magmatismand so limitedour understandingof the temporal procedureis able to identify excess-Ar and Ar-loss problems and spatialevolution of the provinceas a whole.Solutions to that have plagued previousradiometric studiesbased on K-Ar theseproblems have been frustratedby the wide rangeof methods. The overwhelming majority of the igneous activity reported radiometric ages for Karoo igneousrocks. For occurred within a narrow time frame, at 183 + 1 Ma, which example, the temporal correlationbetween the important matchesthe most recent results of datingof theAntarctic Lesotho and Lebombo remnants still remains unresolved. The Ferrar Province, indicatingthat this igneousprovince was total durationof Karoo magmatismis unknown,and a 85 m.y. rapidly eruptedover a vast, asymmetricregion of Gondwana, span of periodic activity suggestedby Fitch and Miller prior to continentalrifting. [1984] standsin contrastto the short duration of activity reportedfrom moderndating of otherflood basaltprovinces Sample Locations and Descriptions [Duncan and Pyte, 1988; Courtittotet at., 1988;Baksi and Farrar, 1991; Renne and Basu, 1991; Renne et at., 1991, Samples were collected during two field seasonsfrom 1992; Sinton et at., 1997]. Of wider importance are outcropsof lava flow sequencesand laterally extensivesills indications from a limited amount of new age data that the and dikes acrossa large area of southernAfrica, from southern Karoo provincemay be temporallyrelated to the largeFerrar and central Namibia to Lesotho and eastern South Africa Province in Antarctica-Australia[Encarnacion et at., 1996], (Figure 1). An additional set of samplesfrom basalt flows data which supportcontinental reconstructions that place from the Kirwan Mountains,East Antarctica, was providedby Africa andAntarctica adjacent in the earlyJurassic prior to the C. J. Harris. Samplescan be convenientlydivided into five breakupof Gondwana. coherentgroups according to location (Figure 1): (1) basaltic DUNCAN ET AL.: lIMING AND DURATION OF KAROO IGNEOUS EVENT 18,129 flows and sills of southern and central Namibia, (2) basaltic the flows is built of a sequenceof internal flow units, which. flowsof northeasternCape Province and Lesotho, (3) basaltic, grade from compact at the base to increasingly vesicular picritic, nephelinitic, and rhyolitic flows of the Lebombo basalt at the top. The base of each flow containspipe vesicles monocline, (4) doleritic dikes and sills of Transvaal and curved in the direction of flow. Such structures elsewhere Natal, and (5) basaltic flows of the Kirwan Mountains, have been proposedto indicate relatively low effusion rates Dronning Maud Land, western Antarctica. Stratigraphic from a nearbyeruptive center [Walker, 1970]. position within each group of lavas is clear
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