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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 H 2 O 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.

The Grader layered intrusion is part of the Havre-Saint-Pierre anorthosite in the Grenville Province (Quebec, Canada). This intrusion has a basin-like morphology and contains significant resources of Fe–Ti–P in ilmenite and apatite. Outcropping lithologies are massive oxide alternating with anorthosite layers, banded ilmenite–apatite–plagioclase rocks and layered oxide apatite (gabbro-)norites. Drill cores provide evidence for stratigraphic variations of mineral and whole rock compositions controlled by fractional crystallization with the successive appearance of liquidus phases: plagioclase and ilmenite followed by apatite, then orthopyroxene together with magnetite, and finally clinopyroxene. This atypical sequence of crystallization resulted in the formation of plagioclase–ilmenite–apatite cumulates or “nelsonites” in plagioclase-free layers. Fine-grained ferrodiorites that cross-cut the cumulates are shown to be in equilibrium with the noritic rocks. The high TiO 2 and P 2 O 5 contents of these assumed liquids explains the early saturation of ilmenite and apatite before Fe–Mg silicates, thus the nelsonites represent cumulates rather than crystallized Fe–Ti–P-rich immiscible melts. The location of the most evolved mineral and whole rock compositions several tens of meters below the top of the intrusion, forming a sandwich horizon, is consistent with crystallization both from the base and top of the intrusion. The concentrations of V and Cr in ilmenite display a single fractionation path for the different cumulus assemblages and define the cotectic proportion of ilmenite to 21 wt.%. This corresponds to bulk cotectic cumulates with ca. 8 wt.% TiO 2 , which is significantly lower than what is commonly observed in the explored portion of the Grader intrusion. The proposed mechanism of ilmenite-enrichment is the lateral removal of plagioclase due to its relative buoyancy in the dense ferrodiorite melt. This plagioclase has probably accumulated in other portions of the intrusion or has not been distinguished from the host anorthosite.

The plutonic rocks in the Kwandonkaya complex, located within the NYG province of Nigeria, have some hypersolvus granites composed mainly of orthoclase microperthite and interstitial annite. These are inferred to have formed from a relatively F-poor, and relatively dry felsic melt. During cooling, Al–Si order was not completely achieved when the inversion of sanidiness to orthoclase and exsolution occurred. A majority of the granites contain intermediate to low microcline with annite to siderophyllite. The samples were incipiently modified in the subsolidus at very low fluid–rock ratios. Drusy granites result from resurgent boiling and volatile loss, which produced orthoclase-dominant feldspar and zoned zinnwaldite, with microcline lining cavities, whereas late loss of volatiles resulted in low microcline and zinnwaldite and metasomatism associated with cassiterite-topaz mineralization. Mica composition in both types of drusy granite is similar and seems to have been fluid-buffered. Albitization was rock-buffered and resulted in variable degree of Al–Si order in K-feldspar and mild modification of mica composition. Key factors affecting both the degree of Al–Si order of K-feldspar and mica compositions at Kwandonkaya seem to be the degree of volatile build-up and loss, and extent of fluid–rock interactions.

In the Kwandonkaya Complex, an A-type metaluminous to peraluminous granite complex in northern Nigeria, the presence of Fe2+-rich ferromagnesian phases (fayalite, hedenbergite and amphibole) at the initial and annite at final stages of crystallisation indicates relatively reduced melts throughout (