Megacrysts and ultramafic xenoliths from Kundelungu kimberlites (Shaba, Zaire)

Some twenty kimberlite pipes outcrop along the eastern and western borders of the Kundelungu plateau. Shaba Province, Zaire. They are arranged roughly along two north-south trending alignments. The pipes probably intruded the Bangweulu Block, which stabilized around 1800 Ma. The exceptionally fresh kimberlites contain mantle-derived nodules (peridotites and eclogites), as well as megacrysts which may reach up to several cm in diameter. The most important megacrysts are garnets, ilmenites, clinopyroxenes, orthopyroxenes and olivines. Micas and diamonds are rarely observed. The clinopyroxenes can be subdivided in two groups: (1) a Ca-rich, low-T type, similar to the Cr-rich diopsides found in 'depleted' (granular) peridotites; and (2) subcalcic clinopyroxene comparable to the megacrysts and to the clinopyroxenes of 'fertile' (sheared) peridotites. The orthopyroxenes are less frequent and are Ca-poor enstatites (0.07-0.42 wt.% CaO) and Ti-bronzites (CaO <1.3 wt.%). All the analysed garnets are Ca-rich (>4.5 wt.% CaO) and all fall in the Iherzolite field defined by Sobolev et al., 1973. The low-Ca garnets which appear in many diamond-bearing kimberlites have never been observed in Zaire, neither in the diamond-poor Kundelungu pipes nor in the diamond-rich Mbuji-Mayi pipes. The ilmenites define a trend close to the 'magmatic Mg-enrichment trend'. The olivine macrocrysts have Fo contents comparable to those of peridotites (Fo90-93). The ultramafic nodules comprise Iherzolites, harzburgites, pyroxenites, wehrlites and dunites. The granular textures and P-T equilibrium conditions (770-1380 degrees C and 28-61 kbar) deduced from their mineral compositions, show clearly that they were derived from a mantle zone on the continental geotherm (90-190 km depth). The eclogite nodules, which are less frequent, contain only two mineral phases (pyrope-almandine-grossular and omphacite), and the texture and the mineral compositions are similar to those of Roberts Victor eclogites. Our findings support the conclusion of Nixon and Condliffe (1989) that low-T peridotites, eclogites and pyroxenites derived from 'depleted' lithosphere, while Cr-poor garnet, subcalcic diopside and bronzite megacrysts cristallized from fertile asthenosphere.