The mafic-ultramafic intrusions in the Bakhuis Granulite Belt, Western Suriname: heat source for UHT metamorphism?

Abstract

The Bakhuis Granulite Belt (BGB) is a PaleoProterozoic granulite facies domain in west Suriname, South America. Mineral assemblages such as sapphirine + quartz and orthopyroxene + sillimanite + quartz indicate that the belt experienced Ultrahigh-Temperature (UHT) metamorphism, which was dated at 2,07 – 2.05 Ga (De Roever et al., 2003a). In the SW part of the belt charnockites dominate over granulites. Earlier research (Klaver, 2011; Klaver et al., 2015a) indicated that charnockite magmatism is the result of a second, younger UHT-like event at 1,99-1,98 Ga. The heat source for UHT metamorphism and the younger UHT-like event has not yet been identified, but might be related to the mafic-ultramafic intrusions which are present throughout the BGB. Fieldwork, petrography, geochemical investigation and U-Pb dating indicated the presence of two generations of mafic magmatism in the BGB. The Moi-Moi Metagabbro is characterized by its association with ultramafic rocks and by the common presence of cm-sized poikilitic hornblende crystals in mafics and ultramafics. They contain a range of different cumulate and non-cumulate rocks, related to each other by fractionation and accumulation of olivine, pyroxene and plagioclase from a hydrous, tholeiitic parental magma. The rocks show a characteristic subduction signature in their trace elements, which, together with the hydrous nature of the magma, points to subduction-related magmatism. The Moi-Moi Metagabbro was dated at 1984 +/- 4 Ma by U-Pb zircon dating and clearly postdates the UHT metamorphism in the BGB. However, this age is identical to the age of the charnockite magmatism. Underplating of the (meta)gabbroic magma or slab detachment is suggested to have been the heat source for the charnockite magmatism. The younger Charlie Gabbro crystallized from a tholeiitic HAB melt. Variation within the series is explained by the fractionation and accumulation of plagioclase and olivine. U-Pb baddeleyite dating resulted in an age of 1970 +/-17 Ma. Just like the Moi-Moi Metagabbro, the Charlie Gabbro rocks show characteristic subduction related trace element patterns, but they lack hydrous magmatic minerals. The Charlie Gabbro magmatism is expected to be related to the waning stage of northward subduction during the final phase of the Trans-Amazonian orogeny. The Charlie Gabbro does not show metamorphism, whereas the Moi-Moi Metagabbro rocks show a metamorphic overprint in the high amphibolite facies to possibly granulite-facies. Its age, of 1970 +/- 17 Ma, indicates the end of Trans-Amazonian metamorphism in the BGB.