Mafic and Ultramafic Xenoliths from the Kao Kimberlite Pipe
- F.R. Boyd and
- Henry O.A. Meyer
Published Online: 19 MAR 2013
Copyright © 1979 by the American Geophysical Union.
The Mantle Sample: Inclusion in Kimberlites and Other Volcanics
How to Cite
MacGregor, I. D. (1979) Mafic and Ultramafic Xenoliths from the Kao Kimberlite Pipe, in The Mantle Sample: Inclusion in Kimberlites and Other Volcanics (eds F.R. Boyd and H. O.A. Meyer), American Geophysical Union, Washington, D.C.. doi: 10.1029/SP016p0156
- Published Online: 19 MAR 2013
- Published Print: 1 JAN 1979
Print ISBN: 9780875902135
Online ISBN: 9781118664858
- Kao kimberlite pipe;
- Mineral chemistry;
- Phase assemblages;
- Ultramafic xenoliths
Ultramafic xenoliths from the Kao kimberlite pipe may be subdivided into oxide (± garnet)-bearing and garnet-bearing harzburgites and lherzo-lites. Oxide peridotites are divided into ilmenite and chromite spinel varieties. Both have coarse granular textures; the ilmenite bearing samples show cumulate features while the chromite spinel xenoliths are the product of metamorphic recrystallizationo The garnet peridotites occur in three textural types - coarse granular, porphyroclastic and mosaic. Coarse granular and porphyroclastic textured samples are a mixture of harzburgites and lherzolites while the mosaic textured rocks are solely lherzolites. Discrete coarse crystals of the ultramafic minerals also occur.
The coarse grained oxide peridotites occur. at the lowest, with the coarse grained, porphyroclastic and mosaic textured garnet peridotites occurring at progressively higher temperatures and pressures. Chromite spinel and coarse grained and porphyroclastic garnet peridotites have compositions indicating that they are refractory residues of earlier partial melting events. Mosaic textured garnet lherzolites have compositions that are more ‘primitive'. Ilmenite peridotites are crystal cumulates of mafic magmas in mantle magma chambers. The different rock suites seem to be from texturally and chemically distinct populations and are similar to other previously reported Northern Lesotho xenolith suites.
Eclogites have equilibrated at similar temperatures and pressure to the oxide peridotites and show changes from coarse granular to tabular textures with increasing temperatures and pressures. The textured distinctions also relate to differences in the CaO, Cr2O3 and Mg/Mg + Fe of garnet and Cr2O3 and Mg/Mg + Fe of clinopyroxene.
The estimated temperatures and pressures of equilibrium define a curvilinear path in which coarse grained samples are sub-parallel to estimated geothermal gradients for a static shield, while the gradients for the mosaic samples are slightly steeper and support the interpretation that dynamic processes are active at the time of kimberlite formation.