The lithology of the 1.305 Ga Kiglapait intrusion is dominated by a Lower Zone of troctolite, succeeded by an Upper Zone of olivine gabbro, ferrodiorite, and syenite with olivine composition of pure fayalite. The feldspar composition of the intrusion varies from An68 to An9 over a thickness of 8.4 km from the base to a sandwich horizon under an Upper Border Zone. The anhydrous nature of the Kiglapait syenites is shown by their high temperature, by the loss of minor biotite up-stratigraphy in the intrusion, and the absence of amphibole. The end-stage feldspar of the Kiglapait syenites is that of a solidus embedded in a solvus in a 3 kbar eutectic at 1000 °C. The end-member assemblage at temperature and pressure is invariant. The final bulk composition is relatively An-rich—An ~11%—with a composition of XOr = 1/3 when projected to the Ab-Or sideline. The experimental feldspar solvus when corrected for the effects of An and Ba and referred to 3 kbar penetrates the solidus and fits the experimental tie lines. These conditions precede a stage of local coarsening under subsolidus conditions that is found in colloform symplectites invading mesoperthite. The oligoclase-orthoclase symplectites are iso-compositional with their host mesoperthites. The coarsening is assumed to be related to a plausibly F-rich vapor phase that is locally consumed with time. The observed phase compositions indicate the end of exsolution at ~800 °C at 3 kbar on the binodal solvus.