Petrology of high-pressure granulite facies metapelites and metabasites from Tcholliré and Banyo regions: Geodynamic implication for the Central African Fold Belt (CAFB) of north-central Cameroon

摘要

The Tcholliré and Banyo high-pressure granulites occur mainly as highly strained small lenses, bands or elongated bodies interbedded with gneisses and migmatites in the Central African Fold Belt (CAFB) in north-central Cameroon. They were previously attributed to Palaeoproterozoic but are now shown to be Pan-African. These granulites are made up of two occurrences of metapelites with garnet-kyanite-sillimanite-cordierite-biotite-quartz-plagioclase and metabasites containing garnet-clinopyroxene-orthopyroxene-hornblende-quartz-plagioclase.Eight samples were analysed in detail by electron microprobe for mineral chemistry.Parageneses in both metapelites and metabasites highlight three main stages witnessed by prograde, peak and retrograde mineral assemblages. The prograde stage is preserved as well-defined inclusion trails of kyanite, biotite, plagioclase, quartz, and rutile within porphyroblasts of garnet in metapelites; or with more or less clinopyroxene, hornblende, plagioclase, quartz, ilmenite, rutile and apatite, within porphyroblasts of garnet in metabasites. The peak stage, displaying heterogranular granoblastic texture is characterised by porphyroblastic garnet-kyanite-K-feldspar-biotite in metapelites and garnet-clinopyroxene-plagioclase-quartz in metabasites. This was followed by decompression and cooling during the retrograde stage marked by aggregate of sillimanite prisms presumably after kyanite and cordierite corona around garnet in metapelites and by symplectites or vermicular structures of orthopyroxene-plagioclase and orthopyroxene corona around garnet in metabasites. Geothermobarometric study shows that granulite facies metapelites and metabasites from both Tcholliré and Banyo regions recrystallised under peak pressure-temperature conditions of 13-14. kbar and 800-900 °C. They experienced similar clockwise P-T path with nearly isothermal decompression.Our results provide evidence for a substantial crustal thickening during the Pan-African continent-continent collision but show that there is no significant isothermal decompression as commonly observed in recent collisional orogens. The reconstructed P-T paths are in some ways reminiscent of the ones reported in Precambrian " mixed-hot orogens"