The anorogenic granitoids of Nigeria are classic exampl& of ring complexes associated with migrating mid-plate magmatisin. Most complexes represent eroded calderas of once active volcanoes, and range from 1 to 30 km in diameter. Petrology varies from peraluminous biotite granite, through hypersthene and fayalite granites to peralkaline aegirine. riebeckite-arfvedsonite granites rich in perthitic alkali feldspar. The complexes have intruded crystalline bascmcnt of Pan-African age. and dating indicates a progressive younging to the south.
The majority of the Nigerian anorogenic ring-complexes represent the roots of volcanoes with dominantly syenitic to granitic compositions. They are A-type granites of long-time duration. petrologically similar to. hut chronologically older than. their counterparts to thc north in Niger. In a fcw northern centres the volcanic rocks are preserved indicating an alkaline trend through hawaiite. mugearite, trachyte to rhyolite. Some mixing of contrasting magma compositions has occurred to produce trachyandesitic suites with deisequilibrium mineral reactions. Mixing can be identified from the major and trace chemical variations and petrological instability reactions.
Extensive mineralization by columbite. cassiterite, sphalerite and wolframitc suggest fluid interaction on a major scale which has affected much of the original mineral asscmhlages. In cases of extreme fluid activity, granites habe bccn converted into microclinites, albitites and greisens. Chemical data for major and trace elements have heen obtained for a comprehensive suitc of Mesozoic volcanic and plutonic rocks from Nigeria. Analyses of the crystalline basement rocks arc presented for comparative purposes. Pctrogenetic trends have been highlighted using selected hivariate diagrams in an attempt to reach a consensus view of the chemical evolution of the province, with due rcgard given to the effects of post-intrusion subsolidus Huid interactions. The series Rb. Th. Nb. La. Cc, Zr, Hf. and Y have been selected to monitor crystal-fluid partitioning. with the more compatible elcments showing greater partitioning to the fluid phase.