Metasomatic mantle origin for Mbuji-Mayi and Kundelungu garnet and clinopyroxene megacrysts (Democratic Republic of Congo)

Abstract

Mbuji-Mayi (east Kasai province) and Kundelungu (Shaba province) are the two kimberlite fields known for a long time in the Democratic Republic of Congo (DRC). Mbuji-Mayi intrudes the Archean basement (Congo–Kasai Craton) and is diamond-rich, whereas Kundelungu cuts across Paleoproterozoic basement (Bangweulu block) and is diamond-poor. The megacryst suites (or discrete nodule suites) of both fields include garnet and clinopyroxene megacrysts. The pyrope-rich megacrysts can be subdivided in three groups on the basis of their Cr contents: low-Cr (0.00–1.79 wt.%Cr 2O 3; Mg #: 72.8–84.0); medium-Cr (1.93–5.16 wt.%Cr 2O 3; Mg #: 76.2–86.3) and high-Cr (5.42–7.10 wt.%Cr 2O 3; Mg #: 79.2–84.6). There are no significant geochemical differences between the garnets from Mbuji-Mayi and from Kundelungu. Polymineral inclusions composed of K-rich hydrated phases (phlogopite and amphibole), fresh glass and Cr-spinels are identified in garnets from all three groups, in both localities, which suggest a common origin. Two groups of diopside megacrysts from Mbuji-Mayi are distinguished on the basis of their Ca content: low-Ca (Ca #: 39.5–42.1; 0.61–0.92 wt.%Cr 2O 3) and medium-Ca (Ca #: 44.1–48.5; 0.41–1.09 wt.%Cr 2O 3); they differ from a third group of high-Cr diopsides (Ca #: 47.1–49.4; 1.31–2.77 wt.%Cr 2O 3). The major element compositions of DRC megacrysts are distinct from those of many other megacryst suites worldwide: the clinopyroxenes are lower in Fe and Ti and higher in Mg and the garnets contain more Cr and significantly less Ti, Fe and Al. These DRC megacryst compositions are intermediate between those of peridotite minerals and those of kimberlite megacrysts from other localities. Most garnets have “normal” REE profiles ((La/Yb) N = 0.003–0.027), whereas clinopyroxenes display relative LREE enrichment ((La/Yb) N = 5.1–43.2). The REE patterns of garnet and clinopyroxene megacrysts are similar to those from metasomatized South African mantle lherzolites. The differences in composition between DRC megacrysts and those from other kimberlites might reflect different modes of formation. Some megacryst suites are related by fractional crystallization processes, the DRC garnet and clinopyroxene megacrysts display geochemical similarities with peridotites and may originate by metasomatic transformation and recrystallization of mantle peridotites.