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Thermal history of the 3.5–3.2 Ga Onverwacht and Fig Tree Groups, Barberton greenstone belt, South Africa, inferred by Raman microspectroscopy of carbonaceous material

¹ Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA

Raman spectra of carbonaceous material were collected in situ from samples of cherts of the Onverwacht and Fig Tree Groups in the central Barberton greenstone belt. The spectra feature two dominant peaks characteristic of disordered carbon: the D peak at 1310 cm^–1 and the O peak at 1580–1600 cm^–1. D peak positions and relative peak intensities and areas indicate that all samples have been altered to lower greenschist facies or above. No correlation was observed between maximum temperature and stratigraphic position or degree of hydrothermal alteration, implying that metamorphism in the central Barberton greenstone belt was regional and unaccompanied by the flow of large quantities of hydrothermal fluids. Samples from the Marble Bar Chert of the Pilbara block, Western Australia, have been heated to the same extent as samples from Barberton. This study demonstrates the use of Raman spectra of carbonaceous material as a sensitive geothermometer for low-temperature metamorphic facies. This application could also be used to establish the antiquity of putative microfossils from metamorphic terranes.

Key Words: carbonaceous rocks • chert • metamorphism • Barberton greenstone belt • Raman spectra

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