Metamorphic P-T evolution of garnet-staurolite-biotite pelitic schist and amphibolite from Keffi, north-central Nigeria: Geothermobarometry, mineral equilibrium modeling and P-T path

- The peak metamorphic conditions attained by the rocks under study, did not exceed middle-amphibolite facies.
- The study area presents evidence for variations of metamorphic response to tectonism.
- A magmatic arc tectonic setting is implied for the area under study by the result of this work.
- The P-T path for the rocks of the study area is inferred to be clockwise.

We report the first detailed petrological and pressure-temperature data from the pelitic schists and amphibolites of the Keffi area, north-central Nigeria, which is located on the eastern flank of the western Nigerian based, Great Nigerian Schist Belt. In one fresh exposure of the schist, staurolite-bearing and staurolite-absent, garnet-rich assemblages occur. All pelitic samples contain garnet, quartz, biotite, plagioclase, chlorite and ilmenite, but the staurolite-bearing assemblage contains euhedral to subhedral staurolites and very subordinate retrograde chlorites in addition. Mineral compositions applied to calculate metamorphic P-T conditions using different approaches reveal a temperature range of 570-630 °C for the garnet-biotite geothermometry. P-T pseudosection analyses calculated using THERMOCALC software for the suitable rock types, constrain garnet/staurolite equilibration within the range of 6.4-7.7 kbar and 610-630 °C. Empirical calculations and pseudo-section approaches indicate a clockwise P-T path for the rocks of the study area. The result of geothermobarometry (peak conditions) from this study is consistent with previous P-T estimations for the Pan-African episode on several areas within the Trans-Saharan Belt. All evidences point towards a magmatic arc tectonic setting for this area of study.

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