U?Pb zircon geochronology and petrology of granitoids from Mitwaba (Katanga, Congo): implications for the evolution of the Mesoproterozoic Kibaran belt

Abstract

In Katanga (Congo), Mesoproterozoic Kibaran granitoids were previously divided into: (1) type A or G1 granitoids (1370±25 Ma); (2) type B or G2 granitoids (1308±25 Ma); (3) type C or G3 granitoids (1094±50 Ma); and (4) type D or G4 tin granites (ca. 977±18 Ma). Our field investigations indicate that igneous bodies previously described as G1 (A), G2 (B) and G3 (C) granitoids in the Mitwaba region were emplaced during the same early Kibaran compressional orogenic event (D1). The heterogeneous distribution of strain during D1 produced orthogneissic rocks along high strain zones versus slightly to unfoliated granitoids in low strain zones. New SHRIMP U–Pb zircon analyses yield the following igneous crystallisation ages for Mitwaba G1–G3 granitoids: Kisele monzogranite gneiss 1386.3±8.1 Ma, Kabonvia granodioritic augen gneiss 1385.5±7.4 Ma, Nyangwa monzogranite 1383.3±4.8 Ma, Kungwe-Kalumengongo monzogranite 1377±10 Ma and Fwifwi foliated leucomonzogranite 1372±9.6 Ma. All these granitoids were emplaced over a short period of time and could be coeval at 1381±8 Ma. This age dates the early Kibaran orogenic event (D1) in the Mitwaba region. Metamorphic zircons from the strongly deformed Kisele monzogranite gneiss yield an age of 1079±14 Ma, which possibly dates the D2 orogenic event in the Mitwaba area. The tin granites are younger and were emplaced at ∼1.0 Ga. The Mitwaba granitoids are two-micas and ilmenite-bearing strongly peraluminous S-type granitoids. The rare-earth elements (REE) patterns show a strong Eu negative anomaly, an enrichment and fractionation of light REE (LREE) and slightly fractionated to almost flat Heavy REE (HREE) patterns. Primordial-mantle normalized spidergrams are characterized by negative anomalies of Ti, Sr, Nb and subsidiary Ba. The emplacement of arc mafic magmas into the crust triggered the dehydration melting, at high temperatures, of a highly fertile and heterogenous metasedimentary source material producing a felsic melt which was mixed with the arc mafic magmas in variable proportions to produce these granitoids. The petrogenesis of these rocks and similar coeval occurrences defining a ca. 3000 km granitoid belt stretching from SE Congo to NE Congo, Burundi, Rwanda and Uganda indicates similar processes throughout the Kibaran orogenic system in central and eastern Africa.