Petrographic and Geochemical Characteristics of the Akaba Igneous Massif from the Pan-African Orogen in Togo, West Africa

The Dahomeyide orogen in Togo and adjoining parts of southeast-Ghana and Benin represents the West Africa Craton (WAC) suture into northwest Gondwana. The suture zone is a narrow and lithologically diverse region with high pressure granulites complexes. The Akaba massif located in the central part of Togo, belongs to this suture zone. Petrographical and whole rock geochemical analyses are used to evaluate the characteristics, petrogenesis and mode of emplacement of the Akaba massif. The new data suggest that the Akaba massif is built by layered amphibolites, garnet-free granulites and metapyroxenites. These rocks were equilibrated under granulite facies conditions and subsequently partially retrogressed to the amphibolite facies. Geochemical suggest that the granulites may have preserved the geochemical imprints of their igneous protoliths. They display tholeiitic affinity, slightly enriched LREE, no Eu anomaly and negative anomalies in Nb and Zr indicate subduction zone magmatism. These features are consistent with protoliths of N-MORB affinities. The Akaba massif rocks were emplaced up in an oceanic arc environment and likely originated from a metasomatized mantle. Keywords: Granulites, Petrography, Geochemistry, Akaba massif, Pan-African orogen, West Africa. DOI: 10.7176/JEES/13-7-03 Publication date:September 30th 202

Petrographic and Geochemical Characteristics of the Djabatoure Massif Metamagmatites from Pan-African Orogen in Togo, West Africa

The Dahomeyide orogen in Togo and adjoining parts of SE-Ghana and Benin records the suture of West Africa Craton (WAC) into NW Gondwana. The suture zone corresponds to a narrow and lithologically diverse area with HP granulites complexes. In Togo, the Djabatoure massif, located in the central part of Togo, belongs to the suture zone. The aim of this paper is to present the petrographic and geochemical characteristics of the Djabatoure massif in order to understand its implication in the geodynamic evolution of the Dahomeyide belt in Togo. The methodology implemented is based on a synthesis of previous works, a petrographic study of twenty thin sections and a geochemical study through discrimination diagrams of fifteen samples. The result shows that the Djabatoure massif is composed of granulites, pyroxenites, amphibolites, talcschists and gneisses. These rocks were equilibrated under granulite facies conditions and subsequently partially retrogressed to amphibolite facies. They display tholeiitic affinity, enriched LREE and negative anomalies in Nb, Zr and Ti indicating subduction zone magma. These features are consistent with protoliths of tholeiites, N-MORB and volcanic arc basalts affinities. The Djabatoure massif rocks were set up in oceanic environment and are thought to have been derivated from a metasomatized mantle

Geochemical Characteristics of Granitoids (Ho Gneiss) from the Pan – African Dahomeyide Belt, Southeastern, Ghana: Implications for Petrogenesis and Tectonic Setting

The Pan-African Dahomeyide belt exposed in Southeastern, Ghana, consist of granitoid gneisses locally known as Ho gneiss. These rocks are thought to be part of the West African Craton (WAC) which was reworked during the Pan-African Orogeny, and may be time equivalent with the Kara gneisses. Petrographical and whole rock geochemical analyses have been used to evaluate the characteristics, petrogenesis and mode of emplacement of the granitoids. The new data reasonably suggest that the Ho gneiss consist mainly of biotite augen gneisses of both mafic and felsic rock suites. Geochemically, these rocks show tonalitic to quartz monzonite but mainly granodiorites affinities. They are metaluminous to weakly peraluminous, I-type, magnesian to ferroan and calcic to alkali calcic. With these varying features, the granitoids mimic mantle derived magma source which mixed or mingled with crustally derived melt. The rocks display varying REEs and trace elements patterns but, their LREEs and LILE enrichment with  the noticeable enrichment in Rb, Ba, K and especially Pb and negative Ti, Ta, Nb observed among majority of the samples are typical signature of ‘‘arc rocks’’ or continental crustal materials. Their incompatible trace element ratios, such as Th/U (1.07 – 13.87), K/Rb (272 – 574.47), Th/Yb (0.79 – 15.09), Ta/Yb (0.25 -0.64 ), Ce/Pb (mainly 1.62 – 7.88 ) and high Ba/Nb (19.55 – 314.17, with TA = 1565.00), are similar to those of the continental crust. The rocks are characterised by subduction related Sr/Y content (< 100), except sample MA8 that shows high concentration of Sr/Y (227.71). The magnesian affinity reflects relatively hydrous, oxidizing source which is consistent with origins that are broadly subduction related. Their high – K nature also points to an important petrogenetic role of remelting and differentiation of arc – accretionary complex crust. These geochemical signatures are likely to be related to metasomatism of the sub – continental lithospheric mantle via crustal recycling. In conclusion, the granitoids may have from melting of igneous source in a subduction related environment. Keywords: Pan-African belt, Dahomeyide, Southeastern Ghana, Ho gneisses, Geochemistry, petrogenesi

Petrographic and Geochemical Characteristics of the Djabatoure Massif Metamagmatites from Pan-African Orogen in Togo, West Africa

The Dahomeyide orogen in Togo and adjoining parts of SE-Ghana and Benin records the suture of West Africa Craton (WAC) into NW Gondwana. The suture zone corresponds to a narrow and lithologically diverse area with HP granulites complexes. In Togo, the Djabatoure massif, located in the central part of Togo, belongs to the suture zone. The aim of this paper is to present the petrographic and geochemical characteristics of the Djabatoure massif in order to understand its implication in the geodynamic evolution of the Dahomeyide belt in Togo. The methodology implemented is based on a synthesis of previous works, a petrographic study of twenty thin sections and a geochemical study through discrimination diagrams of fifteen samples. The result shows that the Djabatoure massif is composed of granulites, pyroxenites, amphibolites, talcschists and gneisses. These rocks were equilibrated under granulite facies conditions and subsequently partially retrogressed to amphibolite facies. They display tholeiitic affinity, enriched LREE and negative anomalies in Nb, Zr and Ti indicating subduction zone magma. These features are consistent with protoliths of tholeiites, N-MORB and volcanic arc basalts affinities. The Djabatoure massif rocks were set up in oceanic environment and are thought to have been derivated from a metasomatized mantle

Caracterisation petrostructurale du massif granulitique de djabatoure

L’intrusion de Djabatouré est l’un des plus importants massifs du complexe basiqueultrabasique de Djabatouré-Anié. Ce complexe représente un élément de la zone de suture de la chaîne panafricaine des Dahomeyides au Togo. Cette intrusion comprend trois unités lithostructurales que sont d’Ouest à l’Est : Aflanyon, Adiaé et Foukpa. Les unités occidentales sont essentiellement constituées de  granulites à grenat tandis que l’unité orientale réunit des granulites sans grenat, des pyroxénites et des amphibolites. Les paragenèses magmatiques sont à Pl1±Opx1+Cpx1 ayant subi une phase de granulitisation à Pl2+Opx2±Cpx2+Grt et une phase tardive  amphibolitique à Pl3+Hbl±Qtz. Ces paragenèses traduisent une évolution tectono-métamorphique allant d’une granulitisation à une rétromorphose dans le faciès amphibolite associée à la mise en place des nappes. Les microstructures relevées définissent trois phases de déformation : une phase Dn soulignée par la paragenèse de granulitisation (phase de collision), une phase Dn+1 correspondant à la foliation principale Sn+1 et associée à l’amphibolitisation (phase tangentielle), et une phase Dn+2 matérialisée par le plissement de la foliation principale. Enfin l’intrusion basique de Djabatouré présente des minéralisations à corindon observées dans l’unité de Foukpa. Mots-clés : Togo, Panafricain, Dahomeyides, Zone de suture, Pétrostructures,Granulites.   The Djabatoure intrusion is the most important massif of Djabatoure-Anie mafic to ultramafic complex. This complex represents an element of the Dahomeyide panafrican suture zone in Togo. This intrusion is composed of three lithostructurals units which are from west to east: Aflanyon, Adiae and Foukpa. The Western units are made of garnetbearing granulites while the Eastern ones joins are composed of massive garnet-free granulites, pyroxenites and amphibolites. The magmatic parageneses are with Pl1±Opx1+Cpx1 which underwent a granulitization phase with Pl2+Opx2±Cpx2+Grt and later amphibolitic phase with Pl3+Hbl±Qtz assemblages. These  parageneses show a tectono-metamorphic evolution through granulitization to a retrograde metamorphism in the amphibolite facies associated with nappes setting. The microstructures define three deformation phases: a Dn phase underlined by the paragenesis of granulitization (collision phase), a Dn+1 phase corresponding to the main Sn+1 foliation which is associated to the amphibolitization or tangential phase and a Dn+2 phase materialized by the folding of the main foliation. Finally the Djabatoure mafic intrusion contains thecorundum mineralizations observed in the Foukpa unit. Keywords : Togo, Panafrican, Dahomeyide, Suture zone, petrostructures, Granulites