Early Precambrian tectonic-igneous evolution in the Vermilion district, northeastern Minnesota
The Vermilion district and adjacent areas are part of the greenstone-granite terrane of northern Minnesota and consist of a thick succession of subaqueous volcanic rocks, derivative sedimentary rocks, and intrusive granitic rocks, which formed during the interval 2,750 to 2,700 m.y. ago. The volcanic-sedimentary succession now constitutes an eastward-trending, upright, tight anticlinorium between flanking granitic batholiths. The folding was broadly contemporaneous with emplacement of the oldest recognized plutonic rocks, which range in composition from granite to tonalite, and is attributed to compression caused by the relative upwelling and convergence of the buoyant granitic bodies. Metamorphism of the supracrustal rocks to greenschist and, locally, middle-amphibolite facies accompanied the folding. Later, anorogenic monzonite–quartz monzonite and other syenitic rocks intruded the supracrustal rocks, apparently as diapirs. Regional strike-slip faulting followed emplacement of the anorogenic plutons. The principal fault, named the Vermilion fault system, transects several greenstone-granite complexes; within the district, stratigraphic offsets indicate a right-lateral strike-slip separation of 17 to 19 km along the fault. The strike-slip faulting marked the transition from an unstable crust dominated by vertical tectonic movements to a more stable crust capable of sustaining regional fractures.