English: Anorthosite in the Precambrian of Minnesota, USA.

This is Split Rock Lighthouse, built atop Stony Point along the northern shoreline of western Lake Superior. It was built in 1909 and 1910 in response to the wrecks of nine different lake vessels during a winter storm in late November 1905.

The rock outcrop at the base of the lighthouse complex is part of a large layered anorthosite inclusion that is hosted in a diabase intrusion (not visible here) of the Beaver River Diabase. Anorthosite is a coarsely-crystalline, intrusive igneous rock composed principally, or ~exclusively, of plagioclase feldspar (= calcium sodium aluminosilicate, NaAlSi3O8 to CaAl2Si2O8). The layering is caused by slight compositional and/or crystal size differences, originally produced by differential crystal settling in an ancient magma chamber.

Note: the anorthosite layering is the tilted banding from upper right to lower left.

Description of this rock unit from Miller et al. (2001): "Anorthosite inclusion - Gray to pale-green, coarse- to medium-grained anorthosite to gabbroic anorthosite (more than 90 percent plagioclase); forms large (less than 500 m) blocks in unchilled ophitic olivine diabase."

Geologic unit & age: large anorthosite inclusion (Precambrian) hosted in Beaver River Diabase (Beaver Bay Complex, Midcontinent Rift Intrusive Supersuite, late Mesoproterozoic, 1096 Ma)

Locality: rocky base of Split Rock Lighthouse, Stony Point, northern shore of western Lake Superior, northeastern Minnesota, USA (47° 12’ 00.36” North latitude, 91° 22’ 01.45” West longitude)

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Reference cited:

Miller et al. (2001) - Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Minnesota Geological Survey Miscellaneous Map Series Map M-119.