Metamorphic evolution of Neoproterozoic eclogites from south Togo (West Africa)

Abstract

Isotropic, sheared and retrogressed eclogites with garnet amphibolites outcrop in the Lato and Toutouto Hills (southern Togo), intercalated within high-grade metasediments, mainly quartzites and subordinate metapelites. These metabasites belong to a series of ultramafic-mafic (UM-M) complexes scattered throughout the Pan-African reactivated Dahomeyides Belt along the ‘so-called’ suture zone. Estimation of the crystallisation conditions of various eclogitic assemblages allow two distinct units to be distinguished in the Lato Hills, separated by a west-vergent thrust, as an upper unit (19 ± 4 kbar; 700 ± 95°C) and a lower unit (13 ± 2 kbar; 650 ± 50°C). These two units display different decompression paths. The upper unit underwent coeval cooling and decompression as the lower unit was transformed to granulite/high-pressure amphibolite-facies (8 ± 2 kbar; 720 ± 60°C), then to amphibolite (5 ± 1 kbar; 560 ± 60°C) and finally greenschist-facies conditions (3 ± 1 kbar; 475 ± 25°C). The Toutouto assemblage preserved only retrograde assemblages under granulitic to amphibolitic facies conditions, suggesting that this unit corresponds to the southward prolongation of the lower Lato unit. Such P-T evolutions support a dynamic change from subduction to continental collision during the formation of the Dahomeyides Belt. It resulted in the amalgamation of the West African Craton to central Gondwana in the late Neoproterozoic times.