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ABSTRACT

The 7–9-km-thick North Shore Volcanic Group (NSVG) constitutes the volcanic products of the 1.1 Ga Midcontinent Rift System in northeastern Minnesota. With close physical, chemical, and volcanological analogies to Tertiary-to-modern Iceland, these flows accumulated in a gradually subsiding basin over a mantle plume centered beneath modern Lake Superior between 1108 and 1094 Ma. They are essentially undeformed, except for local faulting and disruption associated with hypabyssal intrusions.

Geochemically the NSVG is bimodal, dominated by basalts and rhyolites, but includes a complete tholeiitic Fe-enrichment suite that ranges from primitive olivine tholeiite through transitional basalt, basaltic andesite, andesite, and icelandite to rhyolite. The mafic magmas were partial melts of the plume and lithospheric mantle, variably modified by crystal fractionation in crustal chambers and by crustal interaction. Many, but not all, of the rhyolites were derived largely from partial melting of Archean crust.

The volcanic rocks were erupted subaerially, primarily from fissures, though there is some evidence for central volcanoes. Some of the rhyolites are very large and widespread, and were emplaced as high-temperature lavas and rheoignimbrites that crystallized primary tridymite.

During their accumulation and subsidence, these plateau volcanics were subjected to burial/hydrothermal metamorphism, resulting in secondary mineral associations that range from greenschist (epidote-chlorite-albite±actinolite) to zeolite (thomsonite-scolecite-smectite) facies.

This field trip will allow participants to examine outcrops throughout the stratigraphic section of the NSVG, including structural relations, volcanology, geochemical diversity, burial metamorphism, and associated hypabyssal intrusions.

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