Lower-Crustal Granulites and Eclogites from Lesotho, Southern Africa
- 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
Griffin, W.L., Carswell, D.A. and Nixon, P.H. (1979) Lower-Crustal Granulites and Eclogites from Lesotho, Southern Africa, 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/SP016p0059
- Published Online: 19 MAR 2013
- Published Print: 1 JAN 1979
Print ISBN: 9780875902135
Online ISBN: 9781118664858
- Granulite-suite nodules;
- Lower-crustal granulites;
- Mineral chemistry;
- Whole-rock analyses
Nodules of garnet granulites and “crustal” eclogites occur in many kimberlites around the edge of the Kaapvaal craton, but are apparently absent from pipes within the craton. In most Lesotho pipes this suite is dominated by basic cpx+plag+gnt±opx granulites, with smaller numbers of eclogites, garnet websterites and intermediate/acid granulites. Two-pyroxene granulites are common at Monastery Mine.
The basic granulites are essentially olivine basaltic, with minor normative Hy or Ne. The absence of plagioclase in eclogites and garnet websterites is controlled mainly by bulk chemistry; only rocks with >45% normative An+Ab contain modal feldspar. Microprobe data for 50 rocks demonstrate the consanguinity of the garnet granulite-eclogite-websterite suite. Clinopyroxenes from all garnetiferous samples are eclogitic (Jd/Ts > ½). Jd/Jd+Ts is proportional to Ab/Ab+An in the coexisting plagioclase (An 5–77%); Jd ranges from <5 to 30%. Garnets in all rock types resemble those from eclogites in gneiss and blueschist. High-sulfur (3.5–6.8% SO3) scapolite occurs in six samples.
Most samples of the garnet granulite suite yield T-P estimates between 550–700°C and 5–13 Kb, implying an origin in the lower crust. Seismic data on adjacent areas, measured densities, and data on nodule abundances suggest that the lower crust beneath Lesotho contains 50–707% basic granulite+eclogite+websterite, and 50–30% intermediate to acid granulite. Fe-rich garnet websterites give P estimates in the range 13–19 Kb, and may represent an important rock type in the uppermost mantle.
Our crustal P-T estimates lie closer to predicted oceanic geotherms than to the “Shield” geotherm, suggesting that the latter is only valid for the deeper parts of the upper mantle. The high T at lower-crustal depths during mid-Cretaceous time presumably was a consequence of the Karroo volcanic activity.