Large layered anorthosite inclusion (Precambrian) in Beaver River Diabase (Beaver Bay Complex, Mesoproterozoic, 1096 Ma) (Split Rock Lighthouse, Minnesota, USA) 4

            Anorthosite in the Precambrian of Minnesota, USA.

The rock outcrop shown above occurs at the base of the Split House Lighthouse.  This 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.

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Taken on: June 8, 2015

Tags:

layered

anorthosite

megainclusion

Beaver

River

Diabase

Bay

Complex

Midcontinent

Rift

Intrusive

Supersuite

Precambrian

Proterozoic

Mesoproterozoic

plagioclase

feldspar

igneous

rock

Split

Lighthouse

Stony

Point

Minnesota « less

Sets appears in: • Minnesota Rocks & Minerals • Mesoproterozoic Rocks, Minerals, Fossils more » • Anorthosite & Metanorthosite • Beaver Bay Complex (Mesoproterozoic, ~1096 Ma; Split Rock Lighthouse, Minnesota, USA) « less

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