A Synthesis of the Granitic Rocks of the Karagwe‐Ankole Belt in Burundi, Eastern and Central Africa

ABSTRACT We synthesise the published geochemical and geochronological data on the granitic rocks that have been reported from the Karagwe‐Ankole belt (KAB) in Burundi, eastern and central Africa. On the basis of petrographic characteristics and geochemical compositions, these granites are classified as S‐ and A‐types. The S‐type granites consist of widespread granites in the KAB named Gr1, Gr2, Gr3 and are calc‐alkaline and peraluminous, two‐mica types. These granites were emplaced at ~1375 Ma during bimodal magmatism ascribed to extensional setting. The Gr1‐3 granites are characterised by high initial 87 Sr/ 86 Sr (0.707–0735) suggesting crustal contamination or to a lesser extent the composition of their source region. The A‐type granites consist of (i) Kabanga‐Musongati (KM) and Gitega‐Makebuko‐Bukirasazi (GMB) granitoid rocks clubbed into Gr4 group. These Gr4 granites were emplaced locally during ~1205 Ma intracratonic magmatism, representing minor additions to the crust in the Western Domain of the KAB. The Gr4 granites of GMB massifs are characterised by negative εNd (−1.3 to −1.4). This εNd, coupled with low initial 87 Sr/ 86 Sr values (initial 87 Sr/ 86 Sr average = 0.702) for the coexisting mafic rocks, suggests an ocean island basalts (OIB)‐type asthenosphere/lower continental lithospheric mantle origin, with only slight contamination by the lower crust. The granites (A‐type) from the KM belt of mafic and ultra‐mafic complexes are slightly different from those of GMB. These are characterised by the initial isotopic ratios (SrIR average = 0.708; εNd average = −7.133) which indicate an old continental lithospheric mantle origin. (ii) Buraniro A‐type granite (dated at 748 ± 2 Ma) occurring within the Upper Ruvubu Alkaline Plutonic Complex (URAPC) in association with other intrusions such as gabbro‐diorite rocks, alkaline rocks such as feldspathoidal syenite, monzonite, ijolite and carbonatite. The URAPC which hosts Buraniro granite was interpreted to have emplaced under an extensional regime associated with Rodinia breakup. They exhibit initial 87 Sr/ 86 Sr ratios and initial ε Nd values varying from 0.7088 to 0.7214 and from +2.2 to −4.4, respectively, whereas their ε Hf (i) values vary from 0.7 to −10.2. These isotopic data of URAPC granites are indicative of a contamination of the mantle‐derived magmas with crustal material. In addition to the S‐ and A‐type granites, there is an igneous activity that occurred at ca. 976 Ma in the KAB and gave rise to tin‐granites. This late igneous activity is less documented in Burundi and can be investigated in the Ta‐Nb‐Sn ore deposits hosted in leucogranite and pegmatite lenses in the northern part of Burundi.