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Description

Large anorthosite xenolith in ophitic ilmenite gabbro in the Precambrian of Minnesota, USA.

These rocks are part of the widespread Duluth Complex, a ~1.1 billion year old suite of igneous rocks in northeastern Minnesota that is associated with volcanism in the Lake Superior segment of the ancient Mid-Continent Rift System.

The dark-colored rocks are gabbro. The light-colored mass is a single large anorthosite xenolith within the gabbro. Xenoliths are "foreign rocks" that sometimes occur in igneous rocks - they form by detachment of magma chamber wall rocks. In this case, the magma chamber contained gabbroic magma and the walls were anorthosite.

Gabbro is a mafic, phaneritic (coarsely-crystalline), intrusive igneous rock dominated by the minerals plagioclase feldspar and pyroxene. Anorthosite is a mafic, phaneritic, intrusive igneous rock dominated by just plagioclase feldspar.

Geologic unit & age of gabbros: anorthosite series, Duluth Complex, late Mesoproterozoic, 1099 Ma

Age of anorthosite xenolith: unknown, but Precambrian, likely derived from the lower crust

Locality: Keene Creek East outcrop - roadcut along the northern side of Skyline Parkway, Duluth, northeastern Minnesota, USA (46° 45’ 08.52” North, 92° 10’ 29.93” West)
Date
Source Large anorthosite xenolith in ophitic ilmenite gabbro (anorthosite series, Duluth Complex, Mesoproterozoic, 1099 Ma; Keene Creek East Skyline Parkway roadcut, Duluth, Minnesota, USA) 1
Author James St. John

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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/22039818790 ( archive). It was reviewed on 9 October 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

9 October 2019

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current 06:17, 9 October 2019 Thumbnail for version as of 06:17, 9 October 20194,000 × 2,391 (4.3 MB)Ser Amantio di NicolaoTransferred from Flickr via #flickr2commons
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