Charnockites and UHT metamorphism in the Bakhuis Granulite Belt, western Suriname: Evidence for two separate UHT events

The Bakhuis Granulite Belt in western Suriname is an ultrahigh-temperature (UHT) metamorphic terrain in the centre of the Paleoproterozoic (Transamazonian) Guiana Shield. Next to the UHT granulites, the belt contains a 30 by 30 km body of orthopyroxene-bearing granitoids: the Kabalebo charnockites. This setting offers an excellent opportunity to investigate the source and origin of charnockite magmatism and the common association of charnockites with (ultra)high-temperature metamorphic terrains. We present a detailed geochemical dataset and LA-ICPMS zircon U/Pb ages with the aim to investigate the geochemical and geochronological relationship between charnockite magmatism and UHT metamorphism in the Bakhuis Granulite Belt. The Kabalebo charnockites have a characteristic trace element signature with elevated K₂O, P₂O₅, Zr, REE and Ba coupled with mobile element depletion, which is a consequence of high-temperature melting of anhydrous but fertile granulitic crust. Field and geochemical evidence suggests that the intermediate granulites in the Bakhuis Granulite Belt are the source of the Kabalebo charnockites. The new U/Pb zircon ages indicate that charnockite magmatism (1993–1984 Ma) postdates UHT metamorphism (2.07–2.05 Ga) by at least 60 Myr. We argue that it is not possible to maintain a thermal anomaly >200 °C in excess of a normal geothermal gradient for such a prolonged period and hence conclude that the Bakhuis Granulite Belt has experienced two distinct periods in which temperatures >950 °C were reached in the lower crust.The presence of comagmatic metadolerite enclaves in the charnockites establishes that mafic magmatism occurred contemporaneously with, and was the likely heat source for, charnockite magmatism at 1.99–1.98 Ga. In contrast, the 2.07–2.05 Ga UHT metamorphic event is not associated with felsic or mafic magmatism in the Bakhuis Granulite Belt or nearby Guiana Shield and postdates the suturing of the juvenile North Guiana TTG-greenstone belt with the West African Shield by at least 10 Myr. We postulate that the UHT metamorphism at 2.07–2.05 Ga is the result of mantle upwelling in a slab tear in the subducted West African slab that formed as the result of crustal scale shearing and boudinage. Prior to the final stabilisation of the Amazonian–West African Shield at 1.90 Ga, northward subduction at 1.99–1.98 Ga caused the emplacement of voluminous hot, mafic magma, resulting in partial melting of the Bakhuis granulite suite to form the Kabalebo charnockites. Charnockite magmatism was roughly contemporaneous with the emplacement of a large belt of shallow granites and felsic volcanic rocks in the SW Guiana Shield. Despite their similar age, the inherited zircon populations suggest that the charnockites are derived from a distinct, juvenile source while the felsic volcanic rocks include an Archaean protolith.