Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (3) Citing Articles via Google Scholar Google Scholar Articles by Doyle, P. M. Articles by le Roex, A.P. Search for Related Content GeoRef GeoRef Citation

Fine-grained pyroxenites from the Gansfontein kimberlite, South Africa: Evidence for megacryst magma – mantle interaction.

Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa, P.O. Box 31316, Tokai 7966, South Africa, e-mail: geo-odyssey{at}ananzi.co.za.

D. R. Bell

Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa, Department of Geological Sciences and Department of Chemistry and Biochemistry, Arizona State University, Tempe AZ 85287–1604, USA., e-mail: David.R.Bell{at}asu.edu

A.P. le Roex

Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa, e-mail: aleroex{at}geology.uct.ac.za

The Gansfontein kimberlite contains a suite of fine-grained xenoliths dominated by orthopyroxene, and containing ilmenite, phlogopite, and occasional garnet, with minor quantities of olivine and sulphide. Lamellar intergrowths of orthopyroxene and ilmenite were observed in one sample. The fine grained orthopyroxenite assemblages were observed as discrete xenoliths, as a vein in lherzolite, and as a zoned margin surrounding a megacrystic dunite. The minerals are characterized by intra- and inter-grain chemical heterogeneity, but are on the whole compositionally similar to those in the abundant, highly evolved Cr-poor megacryst suite at Gansfontein. However, they differ to varying degrees from megacrysts in the concentration of minor elements such as Cr, Al and Ti. Mineral compositions in a pyroxenite vein in lherzolite are higher Cr and Mg#, and lower in Fe³⁺ than in the discrete fine-grained pyroxenites, indicating chemical interaction with peridotite. A single zircon-bearing mica-clinopyroxenite has mineral compositions similar to MARID xenoliths.

Fine-grained orthopyroxenites, recognized previously from the Weltevreden and Mzongwana kimberlites and interpreted as rapidly crystallized magmas, are here suggested to result from a reaction between megacryst magma and solid mantle peridotite. Mica-clinopyroxenite may represent the liquid end-product of this reaction. Chemical and modal differences of orthopyroxenites from megacrysts result from reaction with peridotitic components, lack of buffering by typical megacryst mineral assemblages, and possibly shallower origins. Textures and fine-scale chemical disequilibrium indicate that reaction postdates some episodes of megacryst formation and was probably underway when the xenoliths were sampled by ascending kimberlite. Orthopyroxene-garnet thermobarometry indicates an origin of one Gansfontein pyroxenite at ~1215°C and ~3.3 GPa, similar to the locus of megacryst crystallization under East Griqualand.

The Gansfontein pyroxenites indicate interaction of magmas at shallow levels within the subcontinental lithospheric mantle and provide evidence for melt-peridotite reaction that includes a possible reaction relationship between Ti-rich melt and olivine. Similar phase relations and processes may also play a role in megacryst petrogenesis, for which these pyroxenite xenoliths could represent a small-scale, initial stage analogue. The liquid end-products of such reaction may be important components of kimberlites and mantle metasomatic fluids.

This article has been cited by other articles:

				C. Bonadiman, M. Coltorti, S. Duggen, L. Paludetti, F. Siena, M. F. Thirlwall, and B. G. J. Upton Palaeozoic subduction-related and kimberlite or carbonatite metasomatism in the Scottish lithospheric mantle Geological Society, London, Special Publications, January 1, 2008; 293(1): 303 - 333. [Abstract] [Full Text] [PDF]

				M. de Wit The Kalahari Epeirogeny and climate change: differentiating cause and effect from core to space South African Journal of Geology, September 1, 2007; 110(2-3): 367 - 392. [Abstract] [Full Text] [PDF]

				D. R. Bell and R. O. Moore Deep chemical structure of the southern African mantle from kimberlite megacrysts South African Journal of Geology, June 1, 2004; 107(1-2): 59 - 80. [Abstract] [Full Text] [PDF]