Regional geologic relationships and geochemical data support the idea that late-stage biotite granite of the Lac La Croix type may have formed by partial melting of previously migmatized metasedimentary rocks in the Quetico subprovince of the Superior craton. The proposed parent material is an intimately layered rock composed of approximately half to two-thirds biotite schist and half to one-third granitoid leucosome; the bulk chemical attributes of such a material were estimated from detailed mapping and drilling of two large outcrops near Pearl Lake, Minnesota.
Previous work has demonstrated that the granitoid leucosome of the layered migmatite is the intrusively injected product of partial melting of a metasedimentary protolith. Therefore the proposed derivation of the younger Lac La Croix-type granite from layered migmatite in effect represents a second distillation of granitic liquid from meta-sedimentary rocks that had been enriched ingranitic components in the first cycle of partial melting and melt migration. It is postulated that the implied two-stage melting of metasedimentary rocks was related to thermal consequences within the Quetico accretionary complex as tectonism along the Quetico-Wabigoon boundary alternated between subduction and transform kinematics during Late Archean time.