ORIGIN OF NEOARCHEAN METASEDIMENTARY ROCKS NEAR PIKE LAKE, VERMILION DISTRICT, NORTHEAST MINNESOTA

The Vermilion district in northeast Minnesota is a Neoarchean greenstone belt containing clastic and chemical metasedimentary rocks intercalated with meta-volcanic rocks. Previous work identified abundant greywackes and turbidite deposits that preserve primary sedimentary textures. At Pike Lake Dam, these rocks display tabular plane beds and laminations at varying scales (mm – cm), alternate between clay, silt, and sand sized material, and preserve sedimentary structures like graded bedding, flame structures and other load structures, locally derived mud chips, and convolute bedding. We studied an enigmatic body of massive buff- to grey-colored rocks exposed near Pike Lake Dam on Route 77 that contains distinctive dark-colored, internally-laminated rock fragments ranging from centimeter to meter scale. The lithic fragments typically display sharp irregular boundaries and decrease systematically in size to the south end of the outcrop, away from a large middle section composed entirely of dark colored, fine-grained material. We sampled both the massive host rock and the dark-colored internally-laminated fragments for textural and mineral petrography, major element geochemistry using x-ray fluorescence, and mineral identification using x-ray diffraction. The host rock consists of sub-angular to sub-rounded mono-crystalline quartz, composite quartz, feldspar, and putative rock fragment framework grains that vary from sand to granule size, supported in a micaecous matrix. Dark-colored internally-laminated fragments consist of mud-sized quartz, biotite, feldspar and opaques, and preserve very fine, mm-scale laminae, commonly defined by minor grain size differences. We interpret the massive host unit as sedimentary in origin due to: (1) rounded framework grains, (2) a dominant two-component quartz and feldspar grain composition, (3) presence of fine-grained matrix support, and (4) the occurrence of putative rock fragments. The large rock fragments also have a sedimentary origin and were introduced to the host during deposition. Future work will focus on detailed textural and geochemical study to identify a potential depositional mechanism and to evaluate the degree of weathering prior to sediment deposition.