The Mordor Complex, a series of highly differentiated potassic rocks (phlogopite lherzolite, phlogopite wehrlite to syenite) which intrudes Precambrian gneiss and amphibolite of the Arunta Block, central Australia, is a rare example of highly potassic magmatism of pre‐Phanerozoic age. Sm‐Nd and U‐Pb whole‐rock isochrons confirm a previously published Rb‐Sr emplacement age of ∼ 1150 Ma. The magmas of the complex possessed a range of initial isotopic compositions, with initial87Sr/86Sr ˜ 0.71 (from Langworthy & Black 1978),ϵNd from ‐9.5 to ‐11.6, initial 206Pb/204Pb from 16.33 to 16.85 and initial207Pb/204Pb from 15.46 to 15.56, and were derived from isotopically evolved, ‘enriched mantle’ sources, in common with many Phanerozoic examples of potassic magmatism. Evolution of the highly negative initialϵNd values requires that Sm/Nd fractionation occurred in the magma sources at least ∼ 830 Ma prior to emplacement of the complex, suggesting that the Mordor sources were generated either prior to or shortly after formation of Arunta Block crust at ∼ 2000 Ma. The processes generating enriched mantle sources, such as those from which many Phanerozoic examples of potassic magmatism are derived, have therefore operated since at least the mid‐Proterozoic.