Abstract

The alkali feldspars in members of the epizonal Kwandonkaya complex, one of the Younger Granite anorogenic plutons in north-central Nigeria, became frozen in a state of arrested reaction as the complex cooled. High sanidine solid-solution, the liquidus phase expected in porphyritic rocks in early ring-dykes and in cone sheets, has given way to exsolution-induced assemblages involving more ordered species, owing to interaction with an aqueous fluid as a necessary catalyst. The rocks now contain grains of orthoclase perthite, possibly zoned structurally, and structurally intermediate microcline; well-ordered microcline is rather sparse. The three intrusive centers, Buri, Jubga–Ziem and Panshanu, show a progression in terms of the time available for equilibration and the activity of H2O in the melt. The feldspars of this metaluminous complex define a transition from a hypersolvus to a subsolvus texture. We document clear signs of magma degassing in miarolitic pockets in an apical region at the Jubga–Ziem intrusive center. The peraluminous compositions that result, leading to unusual clay-mineral structures surrounding the insoluble accessory phases, are an expression of loss of alkalis due to degassing, and not an attribute of the felsic magma in this association. The prevalence of orthoclase and intermediate microcline in perthite at Kwandonkaya, either separately or coexisting, is considered representative of other centers of Mesozoic A-type granite and syenite emplacement in Nigeria.

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