The Petrology and Geochemistry of Archean Volcanics, Western Vermilion District, Northeastern Minnesota

Abstract

The Archean Vermilion greenstone belt in northeastern Minnesota was sampled stratigraphically for petrologic and geochemical study. The oldest unit, the Ely Greenstone, is divided into three members: lower, Soudan Iron-formation and upper members. The lower member consists of calc-alkaline pillowed flows of basalt and andesite composition and mafic to felsic tuffs and breccias. The majority of flows and breccia fragments are highly amygdaloidal suggesting shallow deposition. Geochemically, the lower Ely volcanic rocks are similar to recent island-arc calc-alkaline rocks, but have lower Al2o3 and Y contents. A model involving partial melting of amphibolite or garnet amphibolite is proposed for these rocks. The upper Ely member consists largely (>90%) of pillowed to massive tholeiitic basalt. Two distinct chemical types are recognized in the upper Ely; a low TiO2 , low FeOT/MgO, and a high TiO2 high FeOT/MgO group. Very few intermediate compositions have been found. Mass balance calculations using both major and trace elements suggests that these two basalt groups can be related by low pressure fractional crystallization of olivine, plagioclase and pyroxene in the ratios 5:50:45. The apparent abundance of high TiO2 basalts with few intermediate compositions suggests that the tholeiitic magmas fractionated in shallow chambers isolated from the main magma reservoir and were only periodically tapped. The upper basalts are compositionally similar to other Archean basalts and have characteristics in common with ocean floor and island arc tholeiites. The Newton Lake Formation consists of a mafic member and a felsic member. The felsic member consists of calc-alkaline andesites and dacites mainly of fragmental nature which are chemically similar to the lower Ely member calc-alkaline rocks and probably had a similar origin. The mafic member consists of pillowed flows and layered to nonlayered mafic-ultramafic sills. The basalts are distinguished by a wide variety of crystal morphologies and textural types with many having skeletal pyroxene phenocrysts in a spherulitic matrix. A model involving supercooling and changing rates of coolings can account for the range in observed textures. Two major chemical basalt types have been identified. One is characterized by high MgO, varying Al2O3/TiO2 ratios and marked iron enrichment with decreasing Al2o3. These are similar both texturally and compositionally to basaltic komatiites from Australia and Canada. A model involving fractionation in the shallow layered sills is proposed to explain the range of flow compositions. The other basalt group is distinguished by having high NgO, FeOT, CaO and incompatible elements (except Y) with low but constant Al2o3/TiO2 ratios and marked iron enrichment with increasing Al2o3. These basalts share similarities to South African komatiites and to the so called high iron tholeiite suite in Munro Township, Canada. It is suggested, based on textural and chemical characteristics, that these basalts may represent a chemically distinct komatiite type. Petrologic modeling has been largely unsuccessfull in relating the two Newton Lake basalt types. Varying degrees of partial melting of distinct mantle sources seem necessary for these two types. Geologic and petrologic observations suggest that the Vermilion greenstone belt developed through the coalescence of petrologically distinct volcanic centers. Calk-alkaline volcanism appears to have been more or less continuous while basaltic volcanism was intermittent in nature. Mass balance considerations imposed by the geologic and petrologic conclusions require rapid recylcling and replenishment of source material to generate the calc-alkaline volcanic rocks which in turn provided sediment to form the source for the intrusive "granitic" batholiths. In terms of modern analogs, a marginal-basin-island-arc setting seems most compatible with the available data. However, for a better understanding of early crustal evolution, further attention should be directed at determining the unique interaction of tectonic-igneous processes during the Archean.