page_title: Duluth Complex
section_title: Contemporary landforms
hierarchical_section_title: Duluth Complex / Contemporary landforms
caption_attribution_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 &quot;foreign rocks&quot; 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 &amp; 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)
context_page_description: The Duluth Complex, the related Beaver Bay Complex, and the associated North Shore Volcanic Group are rock formations which comprise much of the basement bedrock of the northeastern part of the U.S. state of Minnesota in central North America. The Duluth and Beaver Bay complexes are intrusive rocks formed about 1.1 billion years ago during the Midcontinent Rift; these adjoin and are interspersed with the extrusive rocks of the North Shore Volcanic Group produced during the same geologic event. These formations are part of the Superior Upland physiographic region of the United States, which is associated with the Laurentian Upland of the Canadian Shield, the core of the North American Craton.
context_section_description: The Precambrian bedrock of the Duluth Complex and the North Shore Volcanics are not buried beneath layers of later sedimentary rock, as is common further south; much of this bedrock is close to or at the surface.  Glaciers scoured away earlier soils, and as is typical of the Canadian Shield, the new topsoils are thin and poor, being derived from the rock beneath or nearby rather than from deep layers of glacial till, which is intermittent and relatively shallow over most of the region.  Consequently, much of the bedrock is exposed, except for the sediments and glacial till in the watershed of the Saint Louis and Cloquet Rivers inland on the west.
  Gabbro outcroppings anchor both ends of the complex.  They dominate the city which gave the Duluth Complex its name, and also form part of Pigeon Point, the easternmost point of Minnesota.  In between, Superior's shoreline from Duluth to the international border has been likened to one long volcanic outcrop, albeit interrupted by parts of the Beaver Bay Complex, such as the anorthosite cliffs at Split Rock Lighthouse adjacent to basalt flows. Prominent relics of volcanism include rhyolitic cliffs at Palisade Head, basaltic lava flows at Gooseberry Falls, and the Sawtooth Mountains further east.  Along the lakeshore can be found quartz-banded thomsonite and agate gemstones created by mineral infilling of gas cavities formed when the lava flows cooled.
  Interior highlands include Eagle Mountain and the Misquah Hills.  Most of the eastern part of the Superior National Forest and its Boundary Waters Canoe Area Wilderness (BWCA) is located on the Duluth Complex, and its exposed Late Precambrian bedrock formations are characteristic features of the region.  The inland lakes lie in hollows formed by differential erosion of the gabbro intrusions.  These depressions were given their final form by glacial scouring during recent ice ages, creating the irregularly shaped and rocky-shored lakes which are hallmarks of the wilderness.
caption: Large anorthosite xenolith in ophitic ilmenite gabbro in Duluth
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