| The Kwandonkaya complex is one of the Mesozoic Nigerian Younger Granite complexes, which belong to the within-plate A-type granite spectrum. The complex, comprised of three main intrusive centers, contains subvolcanic rocks, plutonic rocks, and localized occurrences of altered granite. In this study, the field, petrographic and mineral chemical characteristics of the complex are investigated, a model of evolution of the subvolcanic rocks is formulated, and the mode of genesis of the parental melt are proposed.; The Fe-rich mafic minerals documented in the subvolcanic rocks suggest a comagmatic suite, and low f(O{dollar}sb2){dollar} at the time of crystallization (ca. QFM). The subvolcanic rocks are inferred, from mica chemistry, to have equilibrated with a fluid phase characterized by low average {dollar}fsb{lcub}rm(HF){rcub}/fsb{lcub}rm(HCl){rcub}{dollar} values ({dollar}10sp{lcub}-1.74{rcub}{dollar} at 773 K); values of the same ratio are higher in plutonic rocks (10{dollar}sp{lcub}-0.67{rcub}{dollar} at 673 K), and they are even higher values in the drusy and mineralized variants of the biotite granite. The occurrence of orthoclase, orthoclase + microcline, and microcline reflects decreases in the degree of undercooling, related to size of intrusive unit, and in the An content of sanidine{dollar}rmsb{lcub}ss{rcub},{dollar} which precluded complete ordering. The successful conversion of a monoclinic precursor to microcline indicates an increase in degree of volatile build-up and retention, and in extent of rock-fluid interactions in the subsolidus range of temperatures. The temperature of closure was between 450 and 300{dollar}spcirc{dollar}C for orthoclase-bearing rocks, and between 450 and 300{dollar}spcirc{dollar}C for those with microcline {dollar}pm{dollar} orthoclase.; This granite also contains dark enclaves, formed by mingling of a hybrid magma with the evolved, but not on a scale that noticeably affected its geochemical features. Petrologic modelling of the felsic subvolcanic rocks show that the rock suite could have evolved by 29% Rayleigh fractionation of liquidus phases (cumulate: sanidine{dollar}rmsb{lcub}ss{rcub}{dollar}: 88%, fayalite: 6%, hedenbergite: 5%, ilmenite: 1%). Trace element evolution requires 50% fractionation of the same phases. This disagreement, between the major and trace element models, results from the role of zircon, chevkinite, and allanite, not required in the major-element model. Selective trace-element enrichment followed by nonmodal melting of a lower crustal rocks, of granulitic composition, could produce the required parental melt by 20 to 60% melting. |
