Multiple compositional megacryst groups from the Uintjiesberg and Witberg kimberlites, South Africa

Abstract

Mineral chemistry of megacrysts from the Uintjiesberg and Witberg kimberlites has been used to delineate a number of distinct populations at each locality. The broad and overall complex compositional trends of clinopyroxene megacrysts have been simplified into Low-Cr, High-Cr and High-Al populations. Most of these populations display related inter-element trends that can easily be interpreted to result from fractional crystallisation from single or similar magma batches. The Low-Cr population, at both localities, shows chemical characteristics that fall within, or close to, the compositional range for Cr-poor megacrysts from typical southern African Group I kimberlites such as the Monastery kimberlite, whereas the High-Cr populations are different, as they have compositional trends that initiate at elevated magnesium-number and Cr 2 O 3 contents. Compositions similar to the High-Al clinopyroxene megacrysts are unusual in kimberlites but are similar to megacrysts derived from alnoites and alkalic basalts. The origin of the different megacryst groups points towards the existence of multiple magmas prior to intrusion of the kimberlite. Such magmas could be generated through the differential interaction of an alkaline mafic magma with the sub-continental lithospheric mantle or originate from heterogeneous sources. Pressure and temperature calculations show that the depth of formation for megacrysts for the off-craton Uintjiesberg kimberlite is −1.0 GPa less than for the on-craton Witberg kimberlite.