The structure, petrology and geochemistry of the Tibchi younger granite ring-complex, Nigeria

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 post- magmatic 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. Joint-controlled greisen vein development and primary mineralisation are among the most extensive in the Nigerian Younger Granites. Chemical analyses of fayalites, Ca-rich pyroxenes, amphiboles and a selection of biotites and chlorites show that these minerals approach very closely the iron end-members of their solid solution series. Two distinct but related pyroxene crystallisation trends are found in the porphyries. The 'granite porphyry pyroxene trend' involved a significant role of the acmite component in a way that suggests a modification of the 'quartz porphyry pyroxene trend' by increased oxygen fugacity. The presence of sufficient volatiles in the quartz porphyry magma was probably enough to account for the observed difference between the two trends. Pyroxene data on the whole indicate the Tibchi Complex to be of tholeiitic affinity, but with a mild alkalinity. Amphiboles are mostly postmagmatic in origin and define two divergent reaction series: a ferro- actinolite - ferroedenite series, and a ferroactinolite - ferrorichterite - alkali amphibole series. The compositions of the amphiboles are predestined by the compositions of the pyroxenes from which they formed. Despite the origin of the Tibchi, rocks from basaltic liquids, the presence of large volumes of granitic rocks suggests .extensive crustal anatexis as possible source of additional acid magma.