The origin of the mafic and intermediate granulites in the Bakhuis Granulite Belt, West Suriname

The origin of the mafic and intermediate granulites in the Bakhuis Granulite Belt, West Suriname

Vos, H.

Molengraaff Fonds

2015

The Bakhuis Granulite Belt consists mainly of intermediate and mafic granulites that show banding on decimetre scale. Intercalations of Al-silicate-rich granulite and of quartzite are of a clear sedimentary origin and may indicate that the granulites are also of sedimentary origin. However, the granulites might also originate from volcanic deposits. To determine the protolith the chemical composition of the mafic and intermediate granulites and the nature and age of zircon in these granulites were examined. For zircon dating a SHRIMP was used. The distinction methods used to determine the protolith of the Bakhuis granulites are based on relatively old studies, for example Leake (1964), Shaw et al. (1965) and Winchester (1984) but are still used in current research. Most of these methods were designed for mafic amphibolites and may therefore be less suitable for intermediate rocks. Based on their chemistry, all analysed mafic bands have igneous protoliths. Intermediate granulites were found to have either an igneous or a sedimentary origin. However, most distinction methods are not reliable for intermediate samples. An intermediate granulite that compositionally suggests a sedimentary origin was found to be of volcanic origin in view of its euhedral zircon with magmatic zoning. Another intermediate granulite, of supposedly igneous origin, shows rounded zircon cores, as well as two clear zircon age groups implying a sedimentary protolith. Zircon dating of two intermediate granulite samples provided clear evidence for the age of the volcanic protolith and for the source rocks of the sediment. Zircon in the intermediate orthogranulite had an age of 2156 ± 6 Ma, which is similar to the age of volcanism in the Greenstone Belt. There was scarce evidence (based on one zircon) for younger and older volcanism in the Bakhuis Belt, with ages of 2125 ± 26 Ma and 2182 ± 16 Ma, respectively. The intermediate paragranulite contained zircon with two clear age groups. The younger group of 2178 ± 7 Ma is probably derived from older TTG intrusions in the Greenstone Belt. The older group has an age of 2241 ± 32 Ma, which is older than the Greenstone Belt, and probably originates from a nearby craton. Chemical analysis of calcsilicate granulites showed that they originate from a mixture of pelites/shales with limestone and/or dolomite. This suggests that most CO2 was removed from these marl-like rocks during metamorphism.

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Vrije Universiteit Amsterdam

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