Spatial relations and bulk chemical compositions of three bodies of multiphase leucogranites and two categories of granitic pegmatites in the Archean Red Cross Lake greenstone belt, Sachigo Subprovince of the Superior Province in the Canadian Shield, indicate a comagmatic origin and an enormous range of fractionation. The leucogranite + pegmatite assemblages, probably emplaced at similar to 2.65 Ma, represent an upward fingering-out system presumably rooted in a deeper-seated granitic parent, which was derived by anatexis of felsic lithologies of the greenstone belt, with possible contribution from granitoid components of the flanking plutonic terranes. The leucogranitic melts so derived underwent extensive fractionation, culminating in the lepidolite pegmatites that solidified within the relatively low-P -high-T stability field of petalite. The most primitive fine-grained NGR leucogranite shows a K/Rb of 303. In contrast, the most evolved LPG lepidolite-type pegmatites [with albite >> K-feldspar, K-, Rb-and Cs-dominant lepidolite, spodumene + quartz after petalite, elbaite, beryl, pollucite, amblygonite, and (Ta, Sn)oxide minerals] average K/Rb 1.8, and Cs 4199, Be 236, Ga 161, Nb 14, and Ta 349 ppm. Crystal-liquid and gravity-assisted liquid-liquid fractionation and differential solubilities of HFSE in the melts could have cooperated with selective complexing of elements and mass-controlled diffusion rates to attain the extreme enrichment and fractionation observed at the scale of pegmatite dikes down to local crystal-matrix levels. The bulk chemical composition of the dikes was not affected by mylonitization. The oxygen-isotope signature of the pegmatites also suggests relatively dry conditions during shearing, with very limited exchange with the host rocks. In contrast, the Rb-Sr isotopes are extensively disturbed in both the leucogranites and LPG dikes.