ORIGINAL PAPER Petrology and phase equilibrium modeling of granulites from Obudu in the Benin-Nigerian Shield, Southeastern Nigeria: implications for clockwise P-T evolution in a collisional orogen Emmanuel Nwachukwu Ugwuonah 1,2 & Toshiaki Tsunogae 2,3 & Barth Nwoye Ekwueme 4 Received: 7 September 2017 /Accepted: 11 January 2019 # Springer-Verlag GmbH Austria, part of Springer Nature 2019 Abstract The Eastern Nigerian terrane forms the eastern flank of the Trans-Saharan Orogenic Belt within the Benin-Nigerian Shield. We report new petrological and P–T data of metamorphic rocks (migmatitic gneisses with subordinate granite gneiss, meta-quartz diorite, and meta-ultramafic rock) from Obudu in the southeastern flank of the Benin-Nigerian Shield for unraveling the tectono- metamorphic signatures of the eastern margin of the Trans-Saharan Orogenic Belt. The migmatized pelitic gneiss contains a peak mineral assemblage of plagioclase + rutile + garnet + biotite + K-feldspar + quartz ± liquid, based on which the peak metamorphic conditions are constrained as 850–890 °C and 9–10 kbar using phase equilibria modeling, optimal thermobarometry, and garnet- biotite geothermometry. Retrograde conditions were also estimated for a plagioclase + quartz + garnet + biotite + cordierite + rutile + sillimanite assemblage as 3.8 kbar/590 °C to 6.8 kbar/780 °C. The peak condition under granulite facies confirmed a marked increase of metamorphic grade from the greenschist facies in the western Benin-Nigerian Shield through the middle-upper am- phibolite facies in the north-central Nigerian terranes to the granulite facies in the eastern part of the Benin-Nigerian Block. The results presented in this study are the first evidence of granulite-facies metamorphism from the easternmost parts of the Benin- Nigerian Shield. The clockwise path obtained in this study might suggest a continent-continent collisional setting for the evolution of this region related to the closure of the Goiás-Pharusian Ocean and the formation of the Trans-Saharan Orogenic Belt. Keywords Granulite-facies metamorphism . Mineral equilibrium modeling . Geothermobarometry . Collisional orogen . Clockwise P-T path Introduction The Trans-Saharan Orogenic Belt (TSOB), which is a north- south-trending and 3000-km-long Pan-African orogen located between the West African Craton, the Congo Craton, and the East Saharan Block, is dominantly composed of various greenschist- to granulite-facies gneisses and migmatites (e.g., Caby 2003; Ananaba and Ajakaiye 1987). It has been regarded as a major suture zone formed by the closure of the Goiás-Pharusian Ocean and complex subduction/collision processes during the Neoproterozoic, possibly related to the Rodinia breakup and the Gondwana amalgamation (e.g., Caby 2003; Krӧner and Stern 2004; Cordani et al. 2013). The belt is mainly composed of the Hoggar Shield to the north and the Benin-Nigerian Shield to the south (Fig. 1a), which might represent isolated accretionary complexes and/or micro-continents trapped within the suture zone during the ocean closure (Cordani et al. 2013; Black 1978). Landsat im- agery and aerial photography clearly show the occurrence of north-south-running lineaments through the length of the TSOB (Ananaba and Ajakaiye 1987), which confirms the belt as a major collisional suture in northwest Africa. Previous petrological studies have shown variations in metamorphic Editorial handling: S. W. Faryad * Emmanuel Nwachukwu Ugwuonah emma.ugwuonah@unn.edu.ng 1 Department of Geology, Chukwuemeka Odumegwu Ojukwu University, Uli, P.M.B 02, Uli, Anambra State 431124, Nigeria 2 Division of Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, the University of Tsukuba, Tennoudai 1-1-1, Tsukuba-City, Ibaraki 305-8572, Japan 3 Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa 4 Department of Geology, University of Calabar, Etagbor, PMB 1115, Calabar, Cross-River State 540271, Nigeria Mineralogy and Petrology https://doi.org/10.1007/s00710-019-00652-4