Precambrian Pyroclastic Deposit in the Basement Rocks of Igarra, Southwestern Nigeria – Implications for Identification of the Ancient Suture Zones of the Trans-Saharan Orogen

This is the first-time report of a metamorphosed pyroclastic deposit in the Precambrian Basement shield of Nigeria. Igarra area is in the eastern part of the Southwestern Nigeria Basement which prior to the separation of the Gondwanaland by the early Cretaceous triple-junction rifting was part of the supercontinent heartland continuous with the Eastern Nigerian Basement Massifs. Field evidence is all we need to describe a pyroclastic deposit of Ignimbrite character. The ratio of the average thickness to the spread of the deposit was used to classify it as a High Aspect Ratio Ignimbrite (HARIs). Petrography was also used to prove that the pyroclastic is a mixture of both pelitic and mafic components. The results show that the schistose rocks of Igarra area are grouped into two namely; the metapelitic group and the metapyroclastic group. The metapelitic group are well foliated and therefore syn-orogenic, while the metapyroclastic group are weakly foliated and therefore syn- to late-orogenic. The mixing of pelitic and igneous components in the metapyroclasitc schists suggests a flowing of the pyroclastic discharge into a valley in a marine environment (probably the subduction trench) before the final stages of the orogeny. This result adds another layer of understanding to the identification of ancient suture zones which demarcated paleo-environmental and paleo-tectonic domains of the Precambrian Rodinia to Gondwana transition system of the Trans-Saharan Orogen.