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The Pan-African orogeny along the boundaries of the West African craton |
1 Geosciences, Université de Montpellier 2, Place E. Bataillon, F-34095 Montpellier-cedex, France (e-mail: caby{at}gm.univ-montp2.fr)
2 GEOTER, rue Jean Monnet, Clapiers, France
3 Direction Nationale de la Géologie et des Mines, BP 223, Bamako, République du Mali
4 Hansa Geomin Consult Gmbh Africa, BP 24257 Dakar, Senegal
The Neoproterozoic Gourma fold and thrust belt exposed in eastern Mali includes in its inner part high-pressure, low-temperature metasediments and scarce metabasites. This high-pressure metamorphic unit is characterized in the Ansongo region by garnet–glaucophane–paragonite assemblages and eclogites of basaltic derivation, whereas phengite–garnet–rutile mineral assemblages characterize the metapelites. Thermobarometric estimates on the metabasites suggest peak pressure around 13–15 kbar and temperature of 500±50 °C for the Seyna Bela garnet glaucophanite and glaucophane-bearing eclogite, and 16 kbar at 600±50 °C for the Tin Hama phengite eclogite, values indicative of palaeogeothermal gradients of about 10 °C km–1 typical of subduction settings. The high-pressure unit may represents a giant allochthon emplaced on top of very low-grade metasediments. It represents the southern extension of the ultrahigh-pressure rocks. Garnet pyriclasites from the Amalaoulaou massif, which represents the roots of a c. 800–730 Ma Neoproterozoic island arc, underwent a medium-temperature metamorphic overprint characterized by barroisite–paragonite assemblages; that is, of same grade as the decompression that affected the eclogites and garnet-blueschists. The Gourma high-pressure metamorphic belt formed as a consequence of the east-dipping subduction of the Neoproterozoic passive palaeo-margin of the West African craton. The presented P–T estimates suggest that subduction-related palaeogeothermal gradients during the late Neoproterozoic period along the main Pan-African suture were similar to those reconstructed for Tertiary Alpine-type belts.
