Date of Award
Master of Science (MS)
D. Perkins, III
Alteration in the Lake Vermilion Formation took place at low water/rock ratios. Calculations using the method of Grescens (1967) indicate that the amount of open-system elemental exchange was slight. During secondary alteration, Ca, Na, K, Rb, Sr, and Ba were very mobile; Mg and Fe were less mobile; Si, Al, Ti, Zr, Y, and Ni were slightly mobile. Chemically the rocks resemble modern spilites and keratophyres.
The dacites and basalts are depleted in Ti, Zr, and Y relative to Tertiary igneous rocks of similar composition. The Lake Vermilion Formation basalts display a volcanic arc basalt trend of constant Ti02 with increasing Zr, similar to low-K tholeiites of the island arc tholeiitic series. The few Lower Ely basalt .samples are similar. The Upper Ely samples display a trend of increasing Ti~ with increasing Zr, similar to ocean-floor basalts.
The Vermilion igneous rocks' Ti-Zr-Y systematics are of the convergent plate-margin "Andean" type of Pearce and Norry ( 1979). Partial melting of mantle peridotite could produce the Ti-Zr-Y covariations observed in the basalts. Partial melting of amphibolite or eclogite with biotite in the residuum could produce the Ti-Zr-Y trend observed in the dacites.
The graywackes of the Lake Vermilion Formation have compositions resembling the dacites and are derived from them. The chloritic facies graywackes may be equivalents of the biotitic graywackes which have been subjected to retrograde metamorphism. The slates and many graywackes are higher in Fe203T, MgO, Ni, and Cr than the dacites, indicating the presence of mafic components or derivation from intermediate (andesitic) volcanics. No evidence for plutonic components in the LVF graywackes was found.
The Vermilion igneous and sedimentary rocks closely resemble rocks formed in young ensimatic island arcs in their chemistry, mineralogy, and abundance of rock types. Formation of the district by an accretionary-tectonic mechanism could explain its structural and geochemical characteristics.
Robinson, Scott E., "Geochemistry and petrology of the Lake Vermilion Formation, Ely Greenstone Belt, northeastern Minnesota" (1985). Theses and Dissertations. 247.