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
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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 amphibolite 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.
KeywordsGranulite-facies metamorphism Mineral equilibrium modeling Geothermobarometry Collisional orogen Clockwise P-T path
Constructive comments by two anonymous reviewers and journal editor Shah Wali Faryad are gratefully acknowledged. Chemical analyses of all the minerals by EPMA were carried out at the Chemical Analysis Division of the Research Facility Center for Science and Technology, the University of Tsukuba. This research work was partly supported by the Tertiary Education Trust Fund of Nigeria, which provided the initial funds for E.N.U.’s studies at the University of Tsukuba, Japan. Partial funding was provided by a Grant-in-Aid for Scientific Research (B) from the Japanese Society for the Promotion of Science (JSPS) (No. 26302009 and No. 18H01300) to T.T.
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