THE NORTH SHORE VOLCANIC GROUP: MESOPROTEROZOIC VOLCANIC ROCKS OF THE MIDCONTINENT RIFT SYSTEM: A REVIEW

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 in two pulses, ~1108-1107 and 1100-1094 Ma. They are essentially undeformed, except for local faulting and disruption associated with hypabyssal intrusions.

The lavas formed in two separate sub-basins located northeast and southwest of a prominent bisecting gravity low. This feature, which corresponds to a block of Archean basement that remained standing as a crustal ridge during regional subsidence, separates volcanic successions that formed independently. However, the uppermost volcanic unit in the NSVG straddles the boundary between the two sub-basins. In addition to the volcanic rocks, occasional interflow redbed sandstones, up to 75 m thick, are also present, particularly within the northeastern basin.

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 greatly predominant mafic magmas were partial melts of the plume and lithospheric mantle, and were modified to varying degrees by crystal fractionation in crustal chambers. Many but not all of the rhyolites were derived largely from partial melting of Archean and possibly Paleoproterozoic crust.

The volcanic rocks were erupted subaerially, primarily from fissures. Some of the rhyolites are very large (100’s of km³) and widespread, and were emplaced as high-temperature lavas and rheoignimbrites that crystallized primary tridymite.

All of the NSVG has been affected to some degree by hydrothermal/burial metamorphism, with 27 secondary minerals recognized so far. The more permeable flow tops and bases have undergone more intensive mineralogical changes than the massive interiors, and display mineral assemblages which range from lower greenschist facies (epidote-chlorite-albite+/-actinolite) at the base of the NSVG to zeolite facies (thomsonite-scolecite-smectite) at the top.