Geochemical Characteristics of the Nigerian anorogenic province

Abstract

The anorogenic granitoids of Nigeria are classic examples of ring complexes associated with migrating mid-plate magmatism. 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 basement 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, but chronologically older than, their counterparts to the north in Niger. In a few 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 wolframite suggest fluid interaction on a major scale which has affected much of the original mineral assemblages. In cases of extreme fluid activity, granites have been converted into microclinites. albitites and greisens. Chemical data for major and trace elements have been obtained for a comprehensive suite of Mesozoic volcanic and plutonic rocks from Nigeria. Analyses of the crystalline basement rocks are presented for comparative purposes. Petrogenetic trends have been highlighted using selected bivariate diagrams in an attempt to reach a consensus view of the chemical evolution of the province, with due regard given to the effects of post-intrusion subsolidus fluid interactions. The series Rb, Th, Nb, La, Ce, Zr, Hf, and Y have been selected to monitor crystal-fluid partitioning, with the more compatible elements showing greater partitioning to the fluid phase.