Early Precambrian Bedrock Geology of the Northwest Angle, Lake of the Woods County, Minnesota

Abstract

The Northwest Angle is located in northwestern Minnesota, bounded by the Lake of the Woods, Manitoba, and Ontario. About 4 km2 of outcrop scattered over 250 km2 area was investigated, during ten weeks of field work. The Northwest Angle contains four major rock units: a supracrustal unit; a tonalitic unit; a granitic unit; and a mafic dike unit. The supracrustal rocks show evidence of amphibolite prograde metamorphism; they are well foliated, and lineated. The foliation is given by compositional banding while the lineation is caused by the alignment of hornblende prisms in the plane of foliation. Tonalitic rocks show evidence of amphibolite grade metamorphism, the degree of foliation is variable. The granitic and mafic dike rocks show evidence of deuteric alteration; most outcrop areas contain massive rock, but two of the granite outcrop areas, both in the northwest part of the peninsula are foliated. The supracrustal rocks appear to be composed of intermediate-mafic, calc-alkalic and tholeiitic volcanics mafic to ultra mafic hypabysal intrusions, and sediments which are mineralogically similar to the volcanics. The plutonic rocks of the area show varied metamorphic effects, but maintain an igneous texture overall. The structure of the area is complex. The predominate structural grain is northeast - southwest and is produced by foliations found in the supracrustal, and tonalitic units. The granite and supracrustal rocks which crop out in the northwest part of the peninsula have foliations trending northwest to southeast. It appears that northeast trending isoclinal folding of the supracrustal rocks is responsible for the northeast trending structural grain; northwest trending structural features are attributed to detachment, or rotation of large crustal blocks, during plutonic emplacement. The apparent order of geologic events is: (1) deposition of supracrustal rocks; (2) isoclinal folding of supracrustal rocks along a northeast trend; (3) emplacement of tonalitic intrusives; (4) emplacement of granitic intrusives; (5) emplacement of large mafic dikes. Isoclinal folding of the supracrustal rocks may be comtemporaneous with tonalitic emplacement.