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Journal of Petrology Advance Access published online on May 20, 2005
Journal of Petrology, doi:10.1093/petrology/egi047
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oupjournals.org
Received March 15, 2004
Accepted April 4, 2005

Article

Contrasting Group I and Group II Eclogite Xenolith Petrogenesis: Petrological, Trace Element and Isotopic Evidence from Eclogite, Garnet-Websterite and Alkremite Xenoliths in the Kaalvallei Kimberlite, South Africa

K. S. VILJOEN 1*, D. J. SCHULZE 2, and A. G. QUADLING 3

1 GEOSCIENCE CENTRE, DE BEERS CONSOLIDATED MINES LTD, P.O. BOX 82232, SOUTHDALE, 2135, SOUTH AFRICA; BERNARD PRICE INSTITUTE OF GEOPHYSICAL RESEARCH, HUGH ALLSOPP LABORATORY, UNIVERSITY OF THE WITWATERSRAND, PRIVATE BAG 3, WITS, JOHANNESBURG, 2050, SOUTH AFRICA
2 DEPARTMENT OF GEOLOGY, UNIVERSITY OF TORONTO, ERINDALE COLLEGE, MISSISSAUGA, ONTARIO, CANADA L5L 1C6
3 GEOSCIENCE CENTRE, DE BEERS CONSOLIDATED MINES LTD, P.O. BOX 82232, SOUTHDALE, 2135, SOUTH AFRICA

* To whom correspondence should be addressed.
K. S. VILJOEN, E-mail: fanus.viljoen{at}debeersgroup.com


  Abstract

Eclogite xenolith varieties at Kaalvallei include accessory-free bimineralic xenoliths, diamond-bearing eclogite, corundum-bearing eclogite, garnet-websterite, alkremite and spinel-bearing eclogite. The xenoliths can be accurately classified into previously defined Group I and Group II varieties on both petrographic and geochemical principles. Kaalvallei Group I eclogites (including diamond-bearing eclogite) are considered to derive from a heterogeneous protolith. Eclogite genesis might have been by residue formation associated with the dehydration and partial melting of a protolith consisting of variably mixed, subducted ocean floor basalt and sediment. Kaalvallei Group II eclogite xenoliths are likely to have formed through crystallization of small-volume melts within conduits in old, enriched subcontinental lithosphere. Kaalvallei websterite xenoliths might be petrogenetically related to Group II xenoliths. Isotopic data for Kaalvallei corundum-bearing eclogite and alkremite xenoliths do not provide constraints on petrogenesis. Spinel-bearing eclogite xenoliths are ultradepleted in virtually all trace elements, with very low light rare earth element contents, relatively high heavy rare earth element concentrations, extreme 87Sr/86Sr (~0·915) and extreme 143Nd/144Nd (0·517) isotopic compositions. These xenoliths are considered to be the residues of a partial melting event.

Keywords: eclogite xenoliths; trace elements; REE; isotopic composition; diamonds.
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