Petrography, geochemistry and U-Pb zircon age of the Matongo carbonatite Massif (Burundi): Implication for the Neoproterozoic geodynamic evolution of Central Africa

The Matongo carbonatite intrusion belongs to the Neoproterozoic Upper Ruvubu alkaline plutonic complex (URAPC), that is located in Burundi along the western branch of the East African Rift. Beside the Matongo carbonatite, the URAPC alkaline complex comprises feldspathoidal syenites, diorites, quartz-bearing syenites and granites. Three main fades have been recognized in the Matongo carbonatite: (1) Sovites represent the dominant fades. Two varieties have been recognized. A scarce coarse-grained sovite (sovite I), which is altered and poorly enriched in REE (4 < Sigma REE < 8 ppm), is encountered in highly fractured zones. A fine-grained sovite (sovite II), which is made of saccharoidal calcite, commonly associated with apatite, aegirine and amphibole, is abundant in the intrusion. Sovite II is enriched in LREE (442 < Sigma REE < 1550 ppm, 49 < La-N/Yb-N < 175). (2) Ferrocarbonatites, that form decimeter-wide veins crosscutting the sovites, are characterized by a LREE enriched patterns (225 < Sigma REE < 1048 ppm, 17 < La-N/Yb-N < 64). (3) K-feldspar and biotite-rich fenite fades (silicocarbonatites) have been recognized at the contact between the carbonatites and the country rock. They are likewise LREE-enriched (134 < Sigma REE < 681 ppm, 25 < La-N/Yb-N < 46). Additionaly, late hydrothermal MREE-rich carbonatite veinlets can be found in sovite I. They are characterized by moderate enrichment in REE (Sigma REE = 397 ppm), with a MREE-humped pattern (La-N/Yb-N = 3.7). The different fades represent the typical magmatic evolution of a carbonatite, while the silicocarbonatites are interpreted as resulting from the fenitisation of the country host-rocks. In addition, the most REE-depleted and fractionated facies, i.e. the coarse-grained sovite facies and the late calcite veinlets testify for hydrothermal processes that occurred after carbonatite emplacement and result from REE mobilization and redistribution. Large idiomorphic zircon crystals (megacrysts), found in the vicinity of the carbonatite can directly be related to the carbonatite evolution. They have been dated at 705.5 +/- 4.5 Ma (U-Pb concordant age, LA-ICP-MS). Similar zircon megacrysts of the Lueshe carbonatite (DRCongo) have been dated and give a concordant age at 798.5 +/- 4.9 Ma (U-Pb, LA-ICP-MS). Considering that an extensional tectonic regime occured at that time in Central Africa - what remains debated - both ages could relate to different stages of Rodinia breakup, with uprise of mantle-derived magmas along Palaeoproterozoic lithospheric zones of weakness. (C) 2014 Elsevier Ltd. All rights reserved.