Summary
Migration of residual liquid can potentially affect the textures and mineral compositions in layered intrusions, but is difficult to conclusively demonstrate. In the Upper Zone of the Bushveld Complex a metabasaltic xenolith forms a locally impermeable horizon, which acted as a barrier to vertically migrating residua. Increased Ab content in plagioclase, and K2O and Zr in whole-rock analyses in the anorthosite directly below the xenolith, compared to the same horizon along strike of the xenolith, demonstrate trapping of residual liquid and/or fluid beneath the xenolith.
Comparison of Cu/Ni and Cu/S ratios of the mineralised anorthosite in the normal sequence and below the xenolith suggests that these are primary magmatic sulphides which crystallised within the anorthosite and are not derived by sinking of dense interstitial sulphide liquid originally associated with the overlying magnetite layer or introduced hydrothermally.
Zusammenfassung
Die Migration von Restschmelzen kann Texturen und Mineralzusammensetzungen in geschichteten Intrusionen beeinflussen, jedoch ist es schwierig, dies eindeutig nachzuweisen. In der Upper Zone des Bushveld-Komplexes bildet ein metabasaltischer Xenolith einen lokal undurchlässigen Horizont, der für vertikal migrierte Residuen als Barriere fungierte. Erhöhte Ab-Gehalte in Plagioklas, sowie erhöhte K2O und Zr-Werte in Gesamtgesteins-Analysen in Anorthosit direkt unterhalb des Xenolithen-verglichen mit der Zusammensetzung des gleichen Horizontes in Streichen des Xenolithen-weisen auf Konzentration von Restschmelzen und/oder Fluiden im Liegenden des Xenolithen hin. Der Vergleich von Cu/Ni und Cu/S Verhältnissen der mineralisierten Anorthosite in der normalen Abfolge mit denen unterhalb des Xenolithen zeigt, daß es sich hier um primäre magmatische Sulfide handelt, die innerhalb des Anorthosits kristallisierten. Diese Sulfide lassen sich nicht auf das Absinken von dichter Sulfidschmelze, die ursprünglich mit den überlagernden Magnetit-Bändern in Zusammenhang standen, und auch nicht auf hydrothermale Zufuhr zurückführen.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
References
Ballhaus CG, Stumpfl EF (1986) Sulfide and platinum mineralization in the Merensky Reef: evidence from hydrous silicate and fluid inclusions. Contrib Mineral Petrol 94: 193–204
Barnes SJ (1986) The effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusons. Contrib Mineral Petrol 93: 524–531
Bedard JH, Sparks RSJ, Renner R, Cheadle MJ, Hallworth MA (1988) Peridotitic sills and metasomatic gabbros in the Eastern Layered Series of the Rhum Complex. Quart J Geol Soc London 145: 207–224
Boudreau AE, Kruger FJ (1990) Variation in the composition of apatite through the Merensky Cyclic Unit in the western Bushveld Complex. Econ Geol 85: 737–745
Butcher AR (1985) Channelled metasomatism in Rhum layered cumulates-evidence from late-stage veins. Geol Mag 122: 503–518
Cawthorn RG, McCarthy TS (1980) Variations in Cr content of magnetite from the Upper Zone of the Bushveld Complex-evidence for heterogeneity and convection currents in magma chambers. Earth Planet Sci Lett 46: 335–343
Cawthorn RG, McCarthy TS (1981) Bottom crystallization and diffusion control in layered complexes: evidence from Cr distribution in magnetite from the Bushveld Complex. Trans Geol Soc S Afr 84: 41–50
Cawthorn RG, Poulton KL (1988) Evidence for fluid in the footwall beneath potholes in the Merensky Reef of the Bushveld Complex. In:Pritchard HM, Potts PJ, Bowles JFW, Cribb SJ (eds) Geo-Platinum 87. Elsevier, London, pp343–356
Cawthorn RG, Meyer PS, Kruger FJ (1991) Major addition of magma at the Pyroxenite Marker in the western Bushveld Complex, South Africa. J Petrol 32: 739–763
Czamanske GK, Scheidle DL (1985) Characteristics of the banded-series anorthosites. Montana Bureau Mines & Geology, Special Publication 92, pp 334–345
Harmer RE, von Gruenewaldt G (1991) A review of magmatism associated with the Transvaal basin-implications for its tectonic setting. S Afr J Geol 94: 104–122
Irvine TN (1980) Magmatic infiltration metasomatism, double diffusive fractional crystallization, and adcumulus growth in the Muskox intrusion and other layerd intrusions. In:Hargraves RB (ed) Physics of magmatic processes. Princeton University Press, New Jersey, pp326–383
Irvine TN (1982) Terminology in layered intrusions. J Petrol 23: 127–162
Klemm DD, von Gruenewaldt G, Henckel J, Dehm R (1985) The geochemistry of titanomagnetite in magnetite layers and their host rocks. Econ Geol 80: 1075–1088
Lindstrom DJ (1976) Experimental study of the partitioning of the transition metals between clinopyroxene and coexisting silicate liquids. Thesis, University of Oregon, 188pp
Maclean WH (1969) Liquidus phase relations in the FeS-FeO-Fe3O4-SiO2 system, and their application in geology. Econ Geol 64: 865–884
McCarthy TS, Cawthorn RG (1983) The geochemistry of vanadiferous magnetite in the Bushveld Complex: implications for crystallization mechanisms in layered complexes. Mineral Deposita 18: 505–518
McCarthy TS, Cawthorn RG, Wright, CJ, McIver JR (1985) Mineral layering in the Bushveld Complex: implications of Cr abundances in magnetite from closely spaced magnetitite and intervening silicate-rich rocks. Econ Geol 80: 1062–1074
Molyneux TG (1974) A geological investigation of the Bushveld Complex in Sekhukhuneland and part of the Steelpoort valley. Trans Geol Soc S Aft 77: 329–338
Morse SA (1986) Convection in aid of adcumulus growth. J Petrol 27: 1183–1214
Palacz ZA, Tait SR (1985) Isotopic and geochemical investigation of unit 10 from the eastern layered series of the Rhum Intrusion, northwest Scotland. Geol Mag 122: 485–490
Reynolds IM (1985) The nature and origin of titaniferous magnetite-rich layers in the Upper Zone of the Bushveld Complex: a review and synthesis. Econ Geol 80: 1089–1108
Tait SR, Huppert HE, Sparks RSJ (1984) The role of compositional convection in the formation of adcumulate rocks. Lithos 17: 139–146
Viljoen MJ, Scoon R (1985) The distribution and main geological features of iron-rich dunite and pegmatoidal bodies of the Bushveld Complex. Econ Geol 80: 1109–1128
Von Gruenewaldt G (1973) The Main and Upper Zones of the Bushveld Complex in the Roosenekal area, eastern Transvaal. Trans Geol Soc S Afr 76: 207–227
Von Gruenewaldt G (1976) Sulfides in the Upper Zone of the eastern Bushveld Complex. Econ Geol 71: 1324–1336
Von Gruenewaldt G, Klemm DD, Henckel J, Dehm RM (1985) Exsolution features in titanomagnetites from massive magnetite layers and their host rocks of the Upper Zone, eastern Bushveld Complex. Econ Geol 80: 1049–1061
Wager LR, Brown GM (1986) Layered igneous rocks. Oliver and Boyd, Edinburgh, 588pp
Wager LR, Brown GM, Wadsworth WJR (1960) Types of igneous cumulates. J Petrol 1: 73–85
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Cawthorn, R.G., Street, J. Vertical migration of residual magma in the upper zone of the Bushveld Complex. Mineralogy and Petrology 51, 345–354 (1994). https://doi.org/10.1007/BF01159736
Received:
Accepted:
Issue Date:
DOI: https://doi.org/10.1007/BF01159736