Browsing by Author "Pope, Neil Mark"

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    Petrology and Structure of the Late Precambrian Mafic Sills East of Silver Creek, Lake County, Minnesota
    (1976-05) Pope, Neil Mark
    The 200 foot (60 meters) thick Silver Creek Cliff sill and the 600 foot (183 meters) thick Lafayette Bluff sill intrude the Keweenawan lava flows of the North Shore Volcanic Group, about 4 - 6 miles northeast of Two Harbors, Lake County, Minnesota. The sills are mainly ophitic olivine diabase, consisting of plagioclase, olivine, pyroxenes, opaques, apatite, and secondary interstitial material. The Lafayette Bluff sill also contains plagioclase phenocrysts and amygdaloidal minerals. The Silver Creek Cliff sill contains minor amounts of olivine-free diabase and pegmatitic diabase in discontinuous lenses and layers. Geologic relations and whole-rock analyses and microprobe analyses of individual minerals indicate the olivine-free diabase crystallized from a residual liquid of the olivine diabase and. the pegmatitic diabase crystallized from a still later residual liquid. The Lafayette Bluff sill contains xenoliths of anorthosite along the shore of Lake Superior, northeast of Crow Creek, and a granodiorite unit at the base of the sill at Lafayette Bluff. No evidence was found to indicate a definite genetic relation between the granociorite or anorthosite and the olivine diabase. The regional attitude of the flows is a northeasterly strike and a gentle dip of about 8 - 10° to the southeast. The Silver Creek Cliff sill appears to be nearly flat-lying and conformable with the lava flows except in the vicinity of Encampment River where it cross-cuts the flows and is here interpreted as the feeder zone. The Lafayette Bluff sill forms a syncline - anticline structure. The lava flows in the trough of the syncline strike NW to NE and dip up to 50°. Faulting at the contacts of the diabase with the lava flows suggest the sill has moved downward relative to the flows. Chemically the sills are similar to one another. The minor amount of differentiation in both sills suggests slight Fe-enrichment toward the center of the sill from the top and base. Very little alkali-enrichment is evident. The sills are chemically compared to several other Late Precambrian intrusives and extrusives of northeastern Minnesota, and the Keweenaw Peninsula, and to the Tertiary Skaergaard intrusion in Greenland. Of the two general mafic magma types in northeastern Minnesota, both sills appear to be similar to the high-alumina olivine diabase type rather than the Logan type.