THE STRUCTURE, PETROLOGY AND GEOCHEMISTRY OF THE TIBCHI YOUNGER GRANITE RING-COMPLEX, NIGERIA
THE STRUCTURE, PETROLOGY AND GEOCHEMISTRY OF THE TIBCHI YOUNGER GRANITE RING-COMPLEX, NIGERIA
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Date
1979-09
Authors
IKE, ECHEFU CYRIACUS
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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.
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THE STRUCTURE, PETROLOGY AND GEOCHEMISTRY OF THE TIBCHI YOUNGER GRANITE RING-COMPLEX, NIGERIA, THE STRUCTURE, PETROLOGY AND GEOCHEMISTRY OF THE TIBCHI YOUNGER GRANITE RING-COMPLEX, NIGERIA