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Igneous Rock Associations 10. Komatiites Jaroslav Dostal

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Igneous Rock Associations 10. Komatiites Jaroslav Dostal Document generated on 10/02/2021 8:06 a.m. Geoscience Canada Igneous Rock Associations 10. Komatiites Jaroslav Dostal Volume 35, Number 1, March 2008 Article abstract Komatiites are ultramafic volcanic rocks that occur mainly in Archean and URI: https://id.erudit.org/iderudit/geocan35_1ser02 Paleoproterozoic greenstone belts. These olivine-rich rocks are assumed to have crystallized from magmas that have about 28–30 wt% MgO. They are See table of contents characterized by spinifex texture - platy or skeletal crystals of olivine set in a glassy matrix. Chemically, komatiites resemble peridotites and have high MgO but low SiO2, TiO2 (<1 wt%), K2O (<0.5 wt%) and incompatible trace element Publisher(s) contents. Most of their compositional variations can be accounted for by olivine fractionation. Komatiites are conventionally considered to be derived The Geological Association of Canada from high temperature melts that have eruption temperatures of about 1600EC and are produced by high degrees of anhydrous melting of mantle plumes. The ISSN abundance of Archean komatiites, their decrease through the Proterozoic and extreme rarity in the Phanerozoic have been taken as evidence for secular 0315-0941 (print) cooling of the mantle. However, the plume model has recently been challenged. 1911-4850 (digital) In marked contrast, it has been proposed that komatiites originate via hydrous melting at a shallow depth in subduction environments at significantly lower Explore this journal melting temperatures than those invoked by the plume hypothesis. This new model thus challenges traditional views of the early evolution of the Earth. Nevertheless, it appears that many komatiites are plume-related. In addition to Cite this document the information they provide about the tectonics and the thermal evolution of Archean Earth, komatiites are economically important because they host Dostal, J. (2008). Igneous Rock Associations 10. Komatiites. Geoscience Canada, locally significant magmatic Ni-sulfide (Ni-Cu-PGE) mineralization. 35(1), 21–31. All rights reserved © The Geological Association of Canada, 2008 This document is protected by copyright law. Use of the services of Érudit (including reproduction) is subject to its terms and conditions, which can be viewed online. https://apropos.erudit.org/en/users/policy-on-use/ This article is disseminated and preserved by Érudit. Érudit is a non-profit inter-university consortium of the Université de Montréal, Université Laval, and the Université du Québec à Montréal. Its mission is to promote and disseminate research. https://www.erudit.org/en/ GEOSCIENCE CANADA Volume 35 Number 1 March 2008 21 SERIES extreme rarity in the Phanerozoic have 1600°C, et qui proviennent d0un haut been taken as evidence for secular degré De fusion de panaches man- cooling of the mantle. However, the téliques anhydres. L’abondance des plume model has recently been chal- komatiites archéennes, leur moindre lenged. In marked contrast, it has been abondance au Paléozoïque et leur proposed that komatiites originate via extrême rareté au Phanérozoïque ont hydrous melting at a shallow depth in été interprétés comme étant l’effet d’un subduction environments at signifi- refroidissement séculaire du manteau. cantly lower melting temperatures than Par-centre, récemment, le modèle des those invoked by the plume hypothesis. panaches mantéliques a été remis en This new model thus challenges tradi- question. On a proposé qui au con- tional views of the early evolution of traire les komatiites proviendraient de Igneous Rock Associations the Earth. Nevertheless, it appears that fusions hydratées de faibles pro- 10. many komatiites are plume-related. In fondeurs en milieux de subduction, et Komatiites addition to the information they pro- à des températures de fusion significa- vide about the tectonics and the ther- tivement plus basses que celles sup- mal evolution of Archean Earth, posées par l’hypothèse des panaches. Jaroslav Dostal komatiites are economically important Ce nouveau modèle remet donc en Department of Geology because they host locally significant question la perspective traditionnelle Saint Mary’s University magmatic Ni-sulfide (Ni-Cu-PGE) sur les premiers stades de l’évolution Halifax, NS, Canada, B3H 3C3 mineralization. de la Terre. Ceci dit, il semble que E-mail: [email protected] nombre de komatiites soit relié à des SOMMAIRE panaches. Au-delà des informations SUMMARY Les komatiites sont des roches vol- qu’elles fournissent sur la tectonique et Komatiites are ultramafic volcanic caniques ultramafiques que l’on retrou- l’évolution thermique de la Terre rocks that occur mainly in Archean and ve principalement dans des bandes de archéenne, les komatiites sont impor- Paleoproterozoic greenstone belts. roches vertes archéennes et paléopro- tantes économiquement, étant les These olivine-rich rocks are assumed térozoïques. On suppose que ces roches hôtes de minéralisations mag- to have crystallized from magmas that roches à fort contenu en olivine ont matiques de sulfures de nickel (Ni-Cu- have about 28–30 wt% MgO. They are cristallisé à partir de magmas ayant des EGP) en certains endroits. characterized by spinifex texture - platy teneurs de 28 à 30% en poids de MgO. or skeletal crystals of olivine set in a Typiquement, elles présentent une tex- INTRODUCTION glassy matrix. Chemically, komatiites ture spinifex; c’est-à-dire en cristaux Komatiites are rare ultramafic volcanic resemble peridotites and have high lamellaires ou squelettiques d’olivine and subvolcanic rocks that occur, pre- MgO but low SiO2, TiO2 (<1 wt%), dans une matrice vitreuse. Chimique- dominantly, in Archean and Paleopro- K2O (<0.5 wt%) and incompatible ment, les komatiites ressemblent aux terozoic greenstone belts. These rocks trace element contents. Most of their péridotites et présentent des teneurs contain more than 18 wt% MgO (Le compositional variations can be élevées en MgO mais basses en SiO2, Bas 2000) and are distinguished from accounted for by olivine fractionation. TiO2 (<1% en poids), K2O (<0.5% en other magnesium-rich rocks, such as Komatiites are conventionally consid- poids) ainsi qu’en éléments traces picrites and meimechites, by having ered to be derived from high tempera- incompatibles. La plupart de leurs vari- spinifex texture (characterized by spec- ture melts that have eruption tempera- ations compositionnelles peuvent s’ex- tacular arrays of subparallel or ran- tures of about 1600EC and are pro- pliquer par le fractionnement de l’o- domly-oriented skeletal, platy and blad- duced by high degrees of anhydrous livine. Par convention, on considère ed olivine crystals set in a glassy melting of mantle plumes. The abun- que les komatiites proviennent de mag- groundmass; Fig. 1). Because all dance of Archean komatiites, their mas de hautes températures qui ont komatiites do not display spinifex tex- decrease through the Proterozoic and des températures d’éruption d’environ ture, Arndt and Fowler (2004) defined 22 Figure 1. a) Komatiite flow showing a a zone of spinifex-textured olivine blades overlying a cumulate zone (Pyke Hill, Munro Township, Ontario, Abitibi greenstone belt; photo courtesy of A. Fowler); b) Boulder of olivine spinifex-textured flow top komatiite. Top of flow is toward hammer handle. Olivine textures become rapidly coars- er away from upper chilled margin of the flow and evolve from < 1 mm equant crystals (not visible) to radiate spinifex needles (2-10 mm long) to coarse bladed spinifex crystals (10-30 cm long; plate spinifex A3) in flow interior. Olivine textures are preserved by metamorphic mineral assemblage of serpentine + chlorite + tremolite (Spinifex stream area, Barberton Mountainland, Mpumulunga, South Africa; Dann and Grove 2007; photo courtesy of T. Grove). Hammer han- dle is ~ 0.5 m in length; c) Spinifex texture in thin section (Pyke Hill). b Note skeletal spinifex-textured olivine crystals largely altered to serpentine with clinopyroxene spherulites (photo courtesy of A. Fowler). komatiites as ultramafic rocks that either contain spinifex texture or are related to rocks that do (see Kerr and Arndt 2001). Komatiitic lavas are assumed to crystallize from magmas that have ultramafic compositions. Komatiites were first identi- fied by Viljoen and Viljoen (1969) in the Archean greenstone belt in the c Barberton Mountainland of South Africa as a distinctive and “new” class of magnesium-rich (20–30 wt% MgO) volcanic rocks. Up until then, genuine ultramafic lavas were not known. Viljoen and Viljoen (1969) recognized that many high-Mg rocks in the Bar- berton greenstone belt were lava flows of significant lateral extent and thick- ness, with chilled or brecciated tops, amygdules, pillows and the distinctive quench textures that were subsequently named “spinifex” (after an Australian spiky grass - Triodia spinifex). The Viljoen brothers inferred that the rocks had characteristics of erupted liquids with distinctive chemical composition and named them after the Komati GEOSCIENCE CANADA Volume 35 Number 1 March 2008 23 Figure 2. Proportions of komatiites in volcanic packages of greenstone belts as a function of age (modified after de Wit and Ashwal 1997, and Condie 2001). River flowing through the type locality. aqueous volcanic successions of Pre- Figure 3. Variations in texture across These 3.5 Ga flows are among the old- cambrian greenstone belts. De Wit and a several metre-thick komatiite flow est known ultramafic rocks. Ashwal (1997) estimated that komati- showing well developed layering. The Similar ultramafic
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