THE TIMING OF MARID METASOMATISM IN THE KAAPVAAL MANTLE - AN ION PROBE STUDY OF ZIRCONS FROM MARID XENOLITHS

The precise timing of metasomatic events in the Kaapvaal craton lithos pheric mantle is poorly constrained and therefore it has proven diffic ult to relate mantle metasomatism in the source to igneous activity at the surface. A suite of mica-amphibole-rutile-ilmenite-diopside-beari ng xenoliths (MARID) represents the product of such metasomatic fluids or melts. In this paper we report results of a SHRIMP and cathodolumi nescence study on zircons from four MARID samples from the Kimberley c luster of pipes. Textures combined with experimental evidence suggest that zircons are late crystallizing phases in MARIDs that form close t o the solidus where the major precipitating phases are phlogopite and K-richterite. Cathodoluminescence patterns reveal a complex growth his tory of the zircons involving strong deformation, repeated events of c rystallization, resorption and/or modification of the zircon compositi on, possibly over several millions of years. Many of the U-Pb ages are significantly older than the intrusion age of the group I kimberlites of the Kimberley cluster. Zircon ages in a MARID xenolith from the We sselton kimberlite vary in the range 113 +/- 3 Ma to 142 +/- 3 Ma, com pared with a pipe age of 84 +/- 3 Ma. In two samples from the De Beers , Bultfontein or Dutoitspan pipes, ages range from 85 +/- 5 to 119 +/- 2 Ma in one and 80 +/- 4 to 90 +/- 2 Ma in the other. The ages obtain ed for the latter are indistinguishable from the pipe age of kimberlit es belonging to the younger group I kimberlite event (ca. 80-95 Ma). Z ircons in this sample, as well as the younger ages in the other, may r eflect real ages but most likely have lost radiogenic Pb as a result o f deformation and fluid or melt interaction. The presence of pre-group I ages in two of the samples demonstrates unambiguously that the meta somatism that yielded the older crystallization ages cannot be related to the group I kimberlite event. Instead, minimum ages for the MARID zircons coincide with the period of group II kimberlite magmatism in t he central to southeastern Kaapvaal craton. This coincidence in ages w ould be consistent with the contention based on experimental evidence, that MARID-type rocks can be derived from group LI precursor magmas b y olivine fractionation and exsolution of a carbonatitic component, An alternative possibility, not excluded by the zircon minimum ages, is a genetic relation between MARID metasomatism and Karoo magmatism. (C) 1998 Elsevier Science B.V. All rights reserved.