Origin of late Archean granite: geochemical evidence from the Vermilion Granitic Complex of northern Minnesota

Abstract

The 2,700-Ma Vermilion Granitic Complex of northern Minnesota is a granite-migmatite terrane composed of supracrustal metasedimentary rocks, mafic rocks, tonalitic and granodioritic plutonic rocks, and granite. The metasedimentary rocks are predominantly graywacke, which has been regionally metamorphosed to garnet-sillimanite-muscovite-bearing biotite schist, and has locally undergone anatexis. The mafic rocks form early phases within the complex and are of two types: (1) basaltic amphibolite, and (2) monzodiorite and essexite rich in large ion lithophile elements (LILE). The members of the early plutonic suite form small bodies that intrude the metasedimentary rocks and mafic rocks, producing an early migmatite. The granite is of two distinct varieties: (1) white garnet-muscovite-biotite leucogranite (S-type; Chappell and White 1974) and (2) grayish-pink biotite-magnetite Lac La Croix Granite (I-type). The leucogranite occurs in the early migmatite and in paragneissic portions of the complex, whereas the Lac La Croix Granite is a late-stage intrusive phase that invades the early migmatite and metasediment (producing a late migmatite) and forms a batholith. This study focuses specifically on the origin of granite in the Vermilion Granitic Complex. Chemical mass-balance calculations suggest that the S-type two-mica leucogranite had a metagraywacke source, and that the I-type Lac La Croix Granite formed via partial fusion of calc-alkaline tonalitic material, which may have been similar to rocks of the early plutonic suite. This model is satisfactory for petrogenesis of similar Late Archean post-kinematic granites throughout the Canadian Shield.