Pan-African subduction–collision event evidenced by high- P coronas in metanorites from the Agou massif (southern Togo)

Abstract

The ultrabasic–basic complexes from SE Ghana and Togo to N Bénin are generally considered as an unique lithotectonic unit which may decorate the suture zone of the Pan-African collision belt. In the southern region of Togo, the Agou massif is one of these massifs mainly consisting of granulite-facies rocks. Metagabbronorites and metagabbros outcrop in the innermost part of the Agou Igneous complex (AIC), whereas in the outermost part, relict boudins of metagabbronorites are surrounded by related retrogressed rocks such as layered Grt-bearing amphibolites and amphibolites. The latter are intruded by leucocratic to melanocratic veins, including dykes of microgabbros. Post-solidus cooling of the metagabbronorites leads to the production of HT-MP granulites (M1 granulite stage: Opx1 + Cpx1 + Pl1 + Ilm ± Sil; T > 850 °C, P = 8 kbar). Following this, an evolution toward high- P coronitic granulites is indicated by early Cpx and Grt coronite developed around Opx1 and Pl1: (granulite 2 stage: Cpx2 ± Grt2 + Pl2 + Qtz + Sil + Rt: 770 °C, 10 kbar). Three successive stages of retrogression are then observed from (i) hornblende-granulite facies (700–750 °C, 9 kbar), (ii) to amphibolite facies (700 °C, 6–7 kbar) and (iii) finally greenschist facies conditions (550–500 °C, 3–4 kbar). The inferred P– T path defines an unusual anti-clockwise loop characterized by a slight pressure increase. According to their calc-alkaline signature and their metamorphic evolution, the granulites of Agou are derived from deep-seated basic intrusion equilibrated in the lower continental crust under MP-HT conditions and never experienced an early eclogite stage. This contrasts with granulites derived from eclogites that are also described in SW Togo. In order to explain the existence of two types of metabasites in Togo, we propose that the Agou massif corresponds to basic magmas emplaced and cooled at the base of an active continental margin (western part of the Benino-Nigerian shield) whereas the former eclogites reflect subduction of the passive margin of the west African Carton. During subduction process or at the transition from subduction to collision at c. 610 Ma, burial of the edge of the active margin in the subduction zone led to the formation of coronitic granulites. During the Pan-African orogeny at c. 580 Ma the coronitic granulites together with the eclogites were exhumed under amphibolite facies conditions.