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Morphology and Composition of Chromite in Komatiites

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Morphology and Composition of Chromite in Komatiites Canadian Mineralogist Vol. 30, pp. 303-317(1992) MORPHOLOGYAND COMPOSITIONOF CHROMITEIN KOMATIITES FROMTHE BELINGWEGREENSTONE BELT, ZIMBABWE MEI.FU ZHOU ANDROBERT KERRICH Department of Geological Sciences,IJniversity of Saskatchewan,Saskatoon, Saskatchewan S7N 0W0 ABSTRACT Chromite grains in nearly fresh Archean komatiites from the late Archean Belingwegreenstone belt (ca. 2.7 Ga), in Zimbabwe, have diversemorphologies: skeletal and dendritic chromite is restricted to spinifex zones, whereas euhedralgrains occur throughout the flows. Systematicdifferences in chromite compositionsate presentbetween the spinifex (iop) and cumulate (base)zones of the flows and betweenskeletal and euhedral chromite. Two stagesof crystallizationhave beenidentified: an early euhedralcore is mantled later by skeletalchromite. The core is enriched in Mg and Cr, but depleted in Fe, Ti and Mn relative to the skeletal rim, in a manner similar to variations in compositionsof euhedralchromite from the top to the base of the flows. Accordingly, the euhedralchromite and the core of skeletalchromite in the spinifex zonescould have formed at an early stage,in equilibrium with a large volume of magma, whereasthe skeletalmantle could have formed rapidly from a small volume of evolvedresidual liquid after eruption of the magma.Thermometry based on the pair chromite-olivine may provide an estimateof the ambient conditions of the komatiitic liquids, Chromite in komatiites has uniform Al contents and characteristic compositional trends, distinct from chromite in mafic-ultramafic rocks from other geodynamicsettings, thereby providing an independentmeans of discriminatingits origin. Keywords:chromite, morphology, chemicalcomposition, late Archean, komatiite, geodynamicsetting, Zimbabwe. Sountetne Dans les komatiites de la ceinture de rochesvertes de Belingwe,au Zimbabwe, presquefraiches malgr6 leur 6ge arch$entardif (environ 2.7 Ga), les grains de chromite possbdentdes morphologiesdiverses. Les grains skelettiques et dendritiquessont limit6s aux zonesir spinifex, tandis que les grainsidiomorphes sont r6pandusi traversles coul6es. Nous remarquonsdes diff6rencessyst6matiques dans la composition entre la chromite des zones ir spinifex et des cumulats, situ6evers le haut et i la basedes coul6es,respectivement, et entre les grains skelettiqueset idiomorphes. Deux 6tapesde cristallisationsont envisag6es:un coeut idiomorphe est recouvertpar des surcroissancesskelettiques. Le coeurmonlre un enrichissementen Mg et Cr, et un appauvrissementen Fe, Ti et Mn par rapport aux surcroissances skelettiques,de fagon paralldleaux variations compositionnellesddcel6es dans la chromite idiomorphe entre le haut et la basedes coul6es. Les grains idiomorpheset le coeur desgrains skelettiquesdes zones ir spinifex seseraient form6s ir un stadepr6coce, en 6quilibre avecun volume important de magma, tandis que les surcroissancesskelettiques ont cristallis6rapidement, d partir de volumesrestreints de liquides plus 6volu6s,suite ir l'6panchementdu magma. Une analysethermom6trique fond6e sur la chromite et l'olivine en €quilibre permetde caract6riserles conditionsambiantes des liquides komatiitiques. La chromite des komatiites possddeune teneur uniforme en Al et des compositions caractaristiquesqui la distinguentde la chromite desroches mafiques et ultramafiquesd'autres milieux 96odynamiques. Ceci permet une discrimination de son mode d'origine par une mdthodeinddpendante. (Traduit par la Rddaction) Mots-clAsichromile, morphologie,composition chimique, arch6en tardif, komatiite, milieu g6odynamique,Zimbabwe. INTRODUCTIoN documented(e.g., Fleet & MacRae 1975, Arndt et al.1977, Donaldson1982, Donaldson et ol.1986). Chromite is a sensitiveindicator of magmatic This may stem from the fact that komatiites environments(e.9., Irvine 7965, 1967, Dick & predominantly occur in Archean greenstonese- Bullen 1984). Studiesof chromite compositions quences, where they have experiencedvariable Alpine ultramafic rocks intensities of hydrothermal alteration and havebeen mainly limited to ' and layered intrusions. No systematicanalyses of metamorphisminto assemblagesof serpentine,talc chromite from komatiites have been conducted, and chlorite; fresh primary chromite rarely is although its occurrencein theserocks has been preserved. Komatiites formed from distinctive 303 304 THE CANADIAN MINERALOGIST high-Mg magmas that have had histories of well-preservedsuccession of Archean lavas and crystallization,cooling, alteration and metamor- sedimentaryrocks, which were depositedon an phism quite different from those of mafic and older tonalitic crust (Bickle et al. 1975.Nisbet er ultramafic magmasemplaced in plutonic or upper ql. 1977,Nisbet 1987).The volcanic-sedimentary mantle environments.Accordingly, fresh chromite sequencehas been divided into two groups: the in komatiites may provide critical information lower or MtshingweGroup (2.8-2.9Ga) and upper about their conditionsof crystallization. or NgezGroup (2.7 Ga) (Bickleer a/. 1975,Nisbet Samplesof relativelyunaltered komatiite, com- et al. 1977,Nisbet 1987).The lower group occurs posed almost totally of primary minerals and in the western, southwestern,and southeastern slightly devitrified glass,have been obtained from parts of the area, and is uncomformablyoverlain the Belingwegreenstone belt and are the freshest by the upper group, which consistsof the Manjeri Archeansamples yet found (Nisbetet al. 1987).In (thin sedimentaryrocks), Reliance(komatiites and the present investigation, a detailed petrographic komatiitic basalts), Zeederbergs (basalts) and and electron-microprobestudy of the different Cheshire(mainly shallow-water sedimentary rocks) texturalvarieties of chromitehas beenperformed. formations(Fig. lA). Komatiitesoccur in the Bend On the basisof their chemicalcomposition, these and Brooklandsformations in the lowergroup, and varieties can be compared with chromite from in the RelianceFormation in the uppergroup. The "normal" ultramafic rocks; differencesbetween RelianceFormation hasbeen studied in the greatest them are discussedin termsof the implicationsfor detail (Nisbet et al. 1977, Nisbet 1987). This the conditionsof their crystallization. formation, which is 0.5-1.0 km thick, consists principally of komatiitic basalts,komatiites and GEoLocrcnl SrtrrNc oF KoMATTTTES komatiitic basaltsfrom stratigraphicbase to top. Komatiitic flows in this formation are the freshest The Belingwe greenstonebelt, a late Archean that have been observedin an Archean lerrane, granite-greenstoneterrane with ca. 2.7 Ga isotopic particularlyas found along the SASKMAR I drill ages, is located in the southeasternpart of the hole. which intersectsseveral flows (Nisbetet a/. Zimbabwe Archean craton (Nisbet et al. 1977, 1987)(Fig. 1B); the samplesfor this study have Nisbet 1987).This supracrustalbelt consistsof a beenchosen from rhis section. 0m 50m 'toom Driilsir\ ?j Anderson's 661 Cheshire l-l ZeedeferOs Randomspinifox - Formation H Formation ffi]4 tFI Reliancg I Manjeri ffi Orienredspinifex Formailon Formation Ftc;.l. (A) Simplifiedgeological map of rheupper group, Belingwegreensrone belt, south-centralZimbabwe, illustraring the locationof the SASKMAR drill core, which penetratedthe RelianceFormarion. Modified after Nisbet et al. (1987). (B) Stratigraphicsection of the SASKMAR drill coreshowing rhar rhe sequence of komariiticflows intersectedin the core correspondswith surfaceoutcrop in the area [after Renneret ol. (1992)]. CHROMITE IN KOMATIITES. ZIMBABWE 305 142 I qE- nrF _aet E ErgrqEo S I '$5 el H+152 9L tt I ia I AE 2.) oo ?t ET162 I utt2t tt o lit 8 s 6 I 100M9/(Mg+Fe) 172 It t t- n 50 60 70 75 Edgeofzoned Chr ffi**^* FgHB-lsubzoneE in spinifexzone Euhedralu Core J,Tilspioir"*,-"[I Hmff"t tI Chrin spinifexzone Frc. 2. Stratigraphicsection and variationsof lO0Mg/(Mg+ Fe) in chromite(Chr) throughthe SASKMAR drill core from 142to 112m, includingthree flows, of which the uppermostflow is best developedand containsthe random olivine spinifexand platy olivinespinifex (A), hopperolivine (B*l) and equantolivine (82-4) subzonesfrom top to base. These sampleswere taken from the 142-172-m cumulatezone are mostly pristine,although some depth interval, and come from three flows, serpentineand magnetiteoccurs along fractures. including Onias'sand Cemuson's(Fig. lB), each The cumulatezone also containsmatrix together of whichcontains spinifex-A and cumulate-Bzones with minor skeletalolivine. (Fig. 2). Onias's flow has all subzonesnormally associatedwith komatiiticflows (Donaldson1982), OccURRENCEeNo MonpHolocY oF Cunonatrr and contains random spinifex and platy olivine spinifexsubzones in zoneA, and an uppermostB-l Chromiteis ubiquitousas an accessorymineral subzonein zoneB (Fig. 2). throughout the spinifex and cumulate zones of The groundmassof the rocks from the spinifex these flows. Distinct morphologicalvarieties of zonehas beendevitrified to a variableextent and chromitehave beenidentified in the spinifexand has been partly replaced by greenschist-faciescumulate zones (Fig. 3). Their distribution is assemblages,as have someolivine crystals,which systematicallyrelated to position in each of the show a euhedralor skeletalhabit. The matrix of three flows studied.Most of the chromitein the thespinifex zone also contains minor clinopyroxene cumulate zones and some in the spinifex zones and featherytremolite. Someof the hopper crystals occursas euhedral,octahedral or subhedralcrys- includepigeonite, and typicallyhave a lanternor, tals, l5 to 100pm in diameter,either disseminated rarely,
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