Ultramafic nodules from the Kimberley pipes, South Africa

Abstract

Garnet lherzolite and harzburgite xenoliths from the Kimberley pipes differ from suites collected in a number of other areas in southern Africa in two important respects: 1. (1) There are no sheared peridotites with high (> 1100°C) equilibration temperatures. 2. (2) The low-temperature (< 1100°C) peridotites from the Kimberley pipes have textures ranging from coarse to fluidal and disrupted mosaic-porphyroclastic. In most nodule suites from southern Africa and elsewhere in the world, these low-temperature lherzolites are little deformed. Petrographic and electron microprobe study of the Kimberley peridotites shows that they are depleted in basaltic components and have mineral compositions similar to those found for low-temperature, mainly coarse peridotite xenoliths from northern Lesotho and other areas. Abundant sheared dunites that occur with peridotite xenoliths in the Kamfersdam kimberlite are more Fe-rich than the peridotites and may have originated as cumulates. The Kimberley pipes contain a discrete nodule suite that shows many of the distinctive chemical and physical characteristics noted for these nodules in Lesotho and other areas. The more Fe-rich discrete nodules, however, which generally have lower equilibration temperatures and which include pyroxene-ilmenite lamellar intergrowths, have not been found in the Kimberley pipes. Estimation of temperatures and depths of origin for the garnet lherzolites and for the enstatite discrete nodules shows an inflected geotherm for Kimberley that is similar to those found for other localities within the boundaries of the Kaapvaal Craton.