LITHOLOGIC, PETROLOGIC, AND GEOCHEMICAL INVESTIGATION OF THE LOWER LAKE VERMILION FORMATION

The Lake Vermilion Formation is a Late Archean metavolcanic and metasedimentary formation located in northeastern Minnesota. The outcrops investigated are located on highway 169 approximately seven miles southwest of Tower, MN, and include lithologies interpreted to be the oldest within the sequence. The lithologies investigated lie on the northern side of the Wahlsten Fault, a strike slip fault with an east-west orientation. Lithologies include: (1) Felsic meta-tuff/volcaniclastic unit composed of fine to medium (0.5-2.0 mm) tephra, which form strata ranging from lamina to medium beds (0.1-3.0 cm thick). Massive beds as much as 5m thick are also present. (2) Intermediate-mafic schist, fine to medium grained, strongly lineated, with porphyroblasts of amphibole. (3) Mafic metabasalts interbedded within meta-tuff and schist. (4) Mafic hypabyssal, high-level sills. While primary depositional features are present, all of these lithologies form lens–like bodies, which have been disrupted and sheared.

All these lithologies have been metamorphosed to greenschist-amphibole facies. Geochemical data (whole-rock geochemical analyses) support field interpretations and appear to indicate a series of juxtaposed volcanic-derived units that include both continental arc-derived felsic volcanic material, as well as basaltic material of oceanic arc origin. The geochemical data and petrologic observations also indicate several intrusive events that exhibit different chemical and mineral compositions. Based on field evidence from bedding within the felsic volcanics, the younging direction is to the south. Basaltic flows are most abundant to the north, and transition to felsic volcaniclastic rocks, which are more abundant in the south. These observations suggest that the lithologies mapped in this outcrop record tectonic juxtapositioning of packages of volcanic rocks that range from basaltic through to felsic, possibly capturing a snapshot of magma evolution over time.