Anorthosite
Photographed by Michael P. Klimetz
SOUTH AFRICA
Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar (90-100%) and a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the most common mafic mineral associates. Terrestrial anorthosite is divided into two types: Proterozoic anorthosite (also known as massif or massif-type anorthosite) and Archean anorthosite. These two types of anorthosite have different modes of occurrence, appear to be restricted to different periods in Earth's history, and are thought to have had different origins. Lunar anorthosites on the other hand, constitute the light-colored areas of the Moon's surface and have been the subject of much research. Since they are primarily composed of plagioclase feldspar, most of Proterozoic anorthosites appear, in outcrop, to be grey or bluish. Individual plagioclase crystals may be black, white, blue, or grey, and may exhibit an iridescence known as chatoyancy on fresh surfaces. The feldspar variety labradorite is commonly present in anorthosites. The mafic mineral in Proterozoic anorthosite may be clinopyroxene, orthopyroxene, olivine, or, more rarely, amphibole. Oxides, such as magnetite or ilmenite, are also common. Most anorthosite plutons are very coarse grained, that is, the individual plagioclase crystals and the accompanying mafic mineral are more than a few centimeters long. Less commonly, plagioclase crystals are megacrystic, or larger than one meter long. However, most Proterozoic anorthosites are deformed, and such large plagioclase crystals have recrystallized to form smaller crystals, leaving only the outline of the larger crystals behind.
While many Proterozoic anorthosite plutons appear to have no large-scale relict igneous structures (having instead post-emplacement deformational structures), some do have igneous layering, which may be defined by crystal size, mafic content, or chemical characteristics. Such layering clearly has origins with a rheologically liquid-state magma. Smaller amounts of anorthosite were emplaced during the Archaean (ca 3,800-2,400 Ma), although most have been dated between 3,200 and 2,800 Ma. They are distinct texturally and mineralogically from Proterozoic anorthosite bodies. Their most characteristic feature is the presence of equant megacrysts of plagioclase surrounded by a fine-grained mafic groundmass.
Photographed by Michael P. Klimetz
Beaver Bay Complex
MINNESOTA
Photographed by Michael P. Klimetz
Whiteface Mountain
NEW YORK
Photographed by Michael P. Klimetz
Whiteface Mountain
NEW YORK
Photographed by Michael P. Klimetz
Stillwater Complex
MONTANA
Photographed by Michael P. Klimetz
Nye
Stillwater County
MONTANA
Photographed by Michael P. Klimetz
[Garnetiferous]
Whiteface Mountain
Adirondacks
NEW YORK
Photographed by Michael P. Klimetz
[Garnetiferous]
Whiteface Mountain
Adirondacks
NEW YORK
Photographed by Michael P. Klimetz
[Garnetiferous]
Whiteface Mountain
Adirondacks
NEW YORK
Photographed by Michael P. Klimetz
Saranac Lake
Essex County
NEW YORK
Photographed by Michael P. Klimetz
Saranac Lake
Essex County
NEW YORK
Photographed by Michael P. Klimetz
Saranac Lake
Essex County
NEW YORK
Photographed by Michael P. Klimetz
Saranac Lake
Essex County
NEW YORK
Photographed by Michael P. Klimetz
Saranac Lake
Essex County
NEW YORK
Photographed by Michael P. Klimetz
Saranac Lake
Essex County
NEW YORK
Photographed by Michael P. Klimetz
Saranac Lake
Essex County
NEW YORK
Photographed by Michael P. Klimetz
Finland
Duluth Complex
MINNESOTA
Photographed by Michael P. Klimetz
Finland
Duluth Complex
MINNESOTA
Photographed by Michael P. Klimetz
Finland
Duluth Complex
MINNESOTA
Photographed by Michael P. Klimetz
Finland
Duluth Complex
MINNESOTA
Photographed by Michael P. Klimetz
Northshore
Duluth Complex
MINNESOTA
Photographed by Michael P. Klimetz
Northshore
Duluth Complex
MINNESOTA
Photographed by Michael P. Klimetz
Northshore
Duluth Complex
MINNESOTA
Photographed by Michael P. Klimetz
Laramie Anorthosite-Norite Complex
Laramie Range
Bosler
Albany County
WYOMING
Photographed by Michael P. Klimetz
Laramie Anorthosite-Norite Complex
Laramie Range
Bosler
Albany County
WYOMING
Photographed by Michael P. Klimetz
Laramie Anorthosite-Norite Complex
Laramie Range
Bosler
Albany County
WYOMING
Photographed by Michael P. Klimetz
Laramie Anorthosite-Norite Complex
Laramie Range
Bosler
Albany County
WYOMING
Photographed by Michael P. Klimetz
Laramie Anorthosite-Norite Complex
Laramie Range
Bosler
Albany County
WYOMING
Photographed by Michael P. Klimetz
Laramie Anorthosite-Norite Complex
Laramie Range
Bosler
Albany County
WYOMING
Photographed by Michael P. Klimetz
Laramie Anorthosite-Norite Complex
Laramie Range
Bosler
Albany County
WYOMING
Photographed by Michael P. Klimetz
Laramie Anorthosite-Norite Complex
Laramie Range
Bosler
Albany County
WYOMING
Photographed by Michael P. Klimetz
Laramie Anorthosite-Norite Complex
Laramie Range
Bosler
Albany County
WYOMING
