Major, Trace Elements and Sr-Nd Isotopic Characteristics of High-Pressure and Associated Metabasites from the Pan-African Suture Zone of Southern Togo, West Africa

Eclogites and garnet-bearing or garnet-free amphibolites from Lato, Toutouto and Ahito-Meliendo hills in southern Togo, were analyzed for their bulk rock geochemistry and Sr and Nd isotopic compositions. Major and trace element compositions for these rocks indicate that precursor magmas belong to tholeiitic to subalkaline series which may have evolved by fractional crystallisation. However, some intense sheared eclogites from southern Lato hills display secondary LREE-loss patterns not typical of magmatic protoliths. Measured ratios and epsilon values have been recalculated at 800 Ma. 87Sr/86Sr initial values for the eclogites and amphibolites range from 0.70215 to 0.70460 and from 0.70126 to 0.70307, respectively. 147Sm/144Nd values for both eclogites and amphibolites range from 0.14510 to 0.183320 and are lower than chondritic values of 0.1966. Overall 143Nd/144Nd values range from 0.512804 to 0.512862, corresponding to initial ƐNd(T) from +4.9 to +8.8 that yielded TDM model ages from 1.6 to 0.72 Ga. One eclogite from south Lato hill displays 143Nd/144Nd values of 0.512862, corresponding to initial ƐNd(T) values of +8.83 and TDM model age of 0.72 Ga, that constrains its minimum age. However, eclogites from northern Lato and garnet amphibolites yielded older model ages of 1.65 and 1.43 Ga respectively, suggesting crustal contamination of the surrounding metasedimentary pile. Key words: Eclogites, Sr-Nd isotopes, Pan-African suture zone, Togo

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

The Dahomeyide orogen, in Togo and adjoining parts of southeast Ghana and Benin, represents the suture of West Africa Craton (WAC) into northwest Gondwana. The suture zone corresponds to a narrow and lithologically diverse area with high pressure granulite complexes. 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 better understand the geodynamic evolution of the Dahomeyide belt in Togo. The methodology implemented is based on a synthesis of previous works, a petrographic study of 20 thin sections, and a geochemical study through discrimination diagrams of 15 rock samples. Results show 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 the amphibolite facies. The Djabatoure massif rocks also display tholeiitic affinity, enriched LREE, and negative anomalies in Nb, Zr and Ti; all these characteristics indicate subduction zone magmtism. These features are consistent with protoliths of tholeiites, N-MORB, and volcanic arc basalts affinities. The Djabatoure massif rocks were emplaced in an oceanic environment and likely originated  from a metasomatized mantle

Magmatic flare-up causes crustal thickening at the transition from subduction to continental collision

Above subduction zones, magma production rate and crustal generation can increase by an order of magnitude during narrow time intervals known as magmatic flare-ups. However, the consequences of these events in the deep arc environment remain poorly understood. Here we use petrological and in-situ zircon dating techniques to investigate the root of a continental arc within the collisional West Gondwana Orogen that is now exposed in the Kabyé Massif, Togo. We show that gabbros intruded 670 million years ago at 20–25 km depth were transformed to eclogites by 620 million years ago at 65–70 km depth. This was coeval with extensive magmatism at 20–40 km depth, indicative of a flare-up event which peaked just prior to the subduction of the continental margin. We propose that increased H2O flux from subduction of serpentinized mantle in the hyper-extended margin of the approaching continent was responsible for the increased magma productivity and crustal thickening

Transition from subduction to collision recorded in the Pan-African arc complexes (Mali to Ghana)

International audienceThe 1000 km-long suture zone of the Dahomeyide belt, exposed from Southeast Ghana to South Mali, corresponds to a narrow and lithologically diverse area with symptomatic coronitic HP granulitic massifs. Based on a review of published petrological, geochemical and geochronological data along the Dahomeyide belt we propose a global scenario for the closure of the Pharusian ocean between the West African craton (WAC) and the Benino-Nigerian shield during the end of the Neoproterozoic. The onset of a long-lived oceanic subduction by 800–780 Ma is recorded by early magmatism in the Amalaoulaou intra-oceanic arc in Mali, contemporary to the Gourma and Adrar des Iforas to the North, and in Brazil to the South. The first occurrence of tonalitic plutons dated at 720 Ma, and the development of a forearc system around 650 Ma mark the onset of active margin subduction beneath the Benino-Nigerain shield and its northward prolongation in Mali. Oceanic subduction beneath the active margin ended between 640 and 630 Ma with the onset of subduction of the WAC continental margin while subduction related magmatism continues till ca. 600 Ma on the upper plate. During a short period between 620 and 610 Ma, the forearc system and the tip of the active continental margin were buried synchronously. The positive Bouguer anomaly observed to the East of the suture zone, in Benin, and not beneath the suture zone itself supports the occurrence of a massive mafic body at the base of the crust that could correspond to the underplated buried forearc. Ultimately, the exhumation and partial amphibolitization of the suture zone and a shift from a calc-alkaline magmatism to anatectic magmatism along with the onset of strike-slip faulting in the upper plate marks the transition from continental subduction to continental collision between 610 and 580 Ma. This work highlights the importance of the Pharusian suture zone s.l. To our knowledge, it represents a unique example in the world in where the forearc system is buried and partly exhumed at the transition from subduction to collision