Graphitization of organic matter and fluid-deposited graphite in Palaeoproterozoic (Birimian) black shales of the Kaya-Goren greenstone belt (Burkina Faso, West Africa)

摘要

Palaeoproterozoic black shales form an essential part of the Birimian volcanosedimentary belt in Burkina Faso, West Africa. The mean R-max values and the atomic H/C values of the bulk carbonaceous matter (BCM), together with rock structures and mineral assemblages, indicate that these carbon-rich rocks were metamorphosed to sub-greenschist and low-grade greenschist facies. X-ray diffraction reveals that the (002) 'graphite' peak width in half maximum (FWHM) ranges from 0.43 to 0.71 degrees 2 theta in sub-greenschist facies and from 0.27 to 0.41 degrees 2 theta in greenschist facies rocks, but the d(002) values in both groups of rocks are approximately the same (similar to 3.35 angstrom). The BCM of individual samples is composed of particles with very variable shape, reflectance and Raman spectra. Type I particles that predominate in sub-greenschist facies are fine-grained, irregular or elongate bodies 1 to 3 mu m in size. Their maximum reflectance (R-max) ranges between 2.5% and 8.2%, and Raman parameters R1 and R2 range from 0.5 to 1.4 and 0.5 to 0.8, respectively. Type II particles are lath-shaped, up to 40 mu m large bodies, commonly arranged parallel to white mica flakes. The number of these particles increases from sub-greenschist to greenschist facies. Maximum reflectance varies between 6% and 11.2% and R1 and R2 Raman parameters range from 0.05 to 0.7 and from 0.1 to 0.5, respectively. Type III particles occur in hydrothermally altered and sheared rocks; these are nodular aggregates composed of grains up to 10 mu m in size. This type of particles has very high reflectance (R-max = 11-15%) and its Raman spectra indicate a very high degree of structural ordering corresponding to well-ordered graphite. Type I particles represent original organic matter in the metasediments. Type II particles are believed to have been formed either in situ by solid-state transformation of Type I particles or by crystallization from metamorphic fluids. Gradual graphitization of the Type I organic particles and the growth of lath-shaped Type II particles from a fluid phase is assumed to have taken place under the peak metamorphic conditions associated with the burial of Birimian sediments during thrust tectonism, progressive tectonic accretion and crustal thickening during the D1 event of the Eburnean orogeny. The growth of equant, high-reflectance postkinematic nodular aggregates of Type III particles is ascribed to the reduction of CO2-rich fluids during a hydrothermal event associated with Late Eburnean D2 exhumation and strike-slip movements. Type I carbonaceous particles were only slightly affected by high-temperature, low-pressure contact metamorphism during intrusion of Late Eburnean magmatic bodies, whereas formation of Type II or III particles was not recorded in contact-metamorphosed rocks at all.