ABSTRACT
The 1 :50,000 geological map of the Tibchi Complex presented is characterised by a closed elliptical ring-dyke framework hitherto unpredicted by pioneer workers. Major rock units comprise a basalt - potassic granite suite. The structural history of the Complex involved a central shield volcano, succeeded by a caldera and fluidised intra-caldera volcanism. The model proposed is on the principle of positive magma pressure during both caldera formation and ring-dyke intrusion. With ash-fall tuff and rhyolite lavas resting directly on the subsided basement block within the ring-fracture, the Tibchi Complex shows the first clear exposure of the Jurassic landscape to be recorded in Nigeria. An olivine tholeiite - tholeiite - andesine basalt - trachyte sequence, and a microferrodiorite - microsyenite - granitic porphyries sequence are separately established but together appear to form a unified petrogenetic succession. The scarcity of modal plagioclase, and the low levels of calcium and magnesium, together with the extreme iron-enrichment,in the rocks,are accounted for. The petrology is complicated by various postmagmatic hydrothermal and metasomatic processes from which certain distinctive rock-types formed. These processes are described in full and explanations offered. Contact metamorphism previously unassociated with the Nigerian Younger Granites is recorded in a bifacial metamorphic aureole related to the biotite granite intrusion.