DescriptionLarge anorthosite xenolith in ophitic ilmenite gabbro (anorthosite series, Duluth Complex, Mesoproterozoic, 1099 Ma; Keene Creek East Skyline Parkway roadcut, Duluth, Minnesota, USA) 1 (22039818790).jpg
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)
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