Anorthosites

Anorthosites are coarse-grained, intrusive igneous rocks composed principally of plagioclase feldspar. They are useful for what they reveal about the early crustal evolution of the earth, and they are the source of several economic commodities.

Anorthosite Composition

Anorthosites are igneous rocks that are composed primarily of plagioclase feldspar (calcium-sodium-aluminum silicate). Minor minerals in these rocks may include pyroxene minerals (calcium-iron-magnesium silicates), iron-titanium oxides, and, in metamorphosed varieties, garnet. All anorthosites are coarse-grained, plutonic rocks (they crystallize at depth), and they may have plagioclase crystals 10 centimeters or more in length. Because the color of plagioclase changes with minor changes in chemical composition, anorthosites come in a variety of colors. Light gray is the most common, but dark gray, black, light blue, green, and brown varieties are known.

Anorthosite Types

In the earth, there are two types of anorthosite: massif-type anorthosites and layered intrusive anorthosites. The former occur as large lens- or dome-shaped intrusions that may be exposed by erosion over an area of several square kilometers. The latter variety is associated with intrusions of gabbroic (iron-rich, low-silica) magma that has segregated into mineralogically distinct layers, with anorthosite occurring in the uppermost layers. The lighter areas on the moon, known as the “lunar highlands,” are composed of anorthosite that originated in a similar manner to terrestrial, layered intrusive anorthosites.

Massif-type anorthosites are unusual among igneous rocks in their restricted distribution in time and space and their composition. They contain at least 85 percent plagioclase, with the other silicate minerals being either augite (high-calcium pyroxene), hypersthene (low-calcium pyroxene), or both. Ilmenite and apatite commonly occur as minor accessory minerals. Massif anorthosites are found almost exclusively in a wide belt from the southwestern United States, through Labrador and on the other side of the Atlantic through Sweden and Norway. Another belt extends from Brazil through Africa (Angola, Tanzania, Malagasy), Queen Maud Land in Antarctica, and across Bengal, India, to Australia (These regions were once adjacent before continental drift dispersed them). The best examples of massif anorthosites in the United States are in the Adirondack Mountains in northeastern New York State. Another large anorthosite body, the Laramie Anorthosite, is in Wyoming. In Canada, the Nain and Kiglapait intrusions in northern Labrador are notable for their excellent surface exposures of anorthosite and associated rocks.

Anorthosite Formation

Ages determined with radiometric dating techniques demonstrate that nearly all anorthosites, including the layered-intrusion varieties, are Precambrian; most are between about 1,700 million and 1,100 million years old. No lava flows of anorthositic composition are known; they are exclusively plutonic. Additionally, experimental evidence shows that liquids of anorthositic composition cannot be produced by any known process of melting at depth in the earth. Although specific mechanisms are not fully understood, it is generally accepted that anorthosites form from concentrated plagioclase crystals that have previously crystallized from gabbroic or other magmas. Massif anorthosites are commonly associated with gabbroic rocks called “norites” (gabbro in which the pyroxene mineral is hypersthene), and it can be demonstrated in most cases that anorthosite is produced by the norite magma becoming progressively richer in plagioclase.

Anorthosites in layered intrusives clearly result from the same processes that produce the other layered rocks associated with the anorthosite layers. Layered intrusives, commonly termed “layered complexes,” are generally tabular bodies that vary in thickness from a few hundred to thousands meters thick. They consist of extremely iron- and magnesium-rich rocks called peridotites in their lower reaches, but they grade upward into gabbros and, in some cases, anorthosites in their upper extents. This layering of different rock types is attributed to the gravitationally controlled settling of dense minerals to the bottom of a large body of originally gabbroic magma. Rock layers of different compositions are built up over time as different minerals crystallize and are deposited on the chamber floor. Plagioclase generally crystallizes relatively late in this process and, depending on its composition and that of the enclosing silicate liquid, may actually float to the top of the magma chamber instead of settling to the bottom. Anorthosite deposits are known to have been formed by both settled and floated plagioclase. In both cases, anorthosite does not form directly as magma, but instead results from the concentration of plagioclase by different densities as the magma evolves.

Anorthosite Distribution

Layered intrusive anorthosites occur in the early Precambrian Stillwater Complex of western Montana and the late Precambrian Duluth Complex that parallels the north shore of Lake Superior in northern Minnesota. The Stillwater contains three anorthosite units, each about 400 to 500 meters thick. In contrast, the Duluth Complex, about 40 kilometers thick, is mostly anorthosites and “gabbroic anorthosites” (somewhat richer in iron-magnesium silicates than anorthosites), with a mostly unexposed, relatively thin peridotite unit at its base. Norites and troctolites (olivine plus plagioclase) occur as minor associated rocks. Other layered anorthosites occur in the Bushveld complex in South Africa, the Fiskenaesset Complex in Greenland, and the Dore Lake and Bell River complexes in Ontario, Canada, among others. All of the layered intrusions in the United States are Precambrian.

Without question, the most obvious (but least accessible) exposures of anorthosite occur on the moon. Relatively rare on Earth, anorthosite is the dominant rock type on the moon, where it forms the bulk of the rocks in the lunar highlands. The lunar highlands are the moon’s ancient, highly cratered, light-colored areas. The dark areas are basalt (iron-rich silicate rock) lava flows that fill huge craters blasted in the highlands by large meteorite collisions. Ever since the first moon rocks were returned to Earth by the Apollo 11 astronauts in 1969, scientists have studied lunar anorthosites and associated rocks for clues to how the moon’s crust and interior originated and evolved. The generally accepted model postulates that early in its history, more than 4 billion years ago, the entire surface of the moon became molten to a depth of several kilometers. This “magma ocean” then cooled, and as it cooled, it crystallized various silicate and oxide minerals. The heavier, iron-rich minerals, being denser than the silicate magma, sank to form deep-seated layers, similar to the peridotite layers in the Stillwater and Bushveld complexes described earlier. Plagioclase, in contrast, was less dense than the iron-rich liquid, so it floated to the top, solidifying to form the early lunar highland anorthosites. Over time, the highlands became increasingly cratered by the meteorite impacts, until they acquired their present appearance about 3 billion years ago. Since that time, meteorite impacts have been sporadic. Interestingly, lunar highland anorthosite occurs with minor quantities of norite and troctolite, two rock types that also are commonly associated with terrestrial anorthosites. To lunar scientists, this peculiar group of associated rocks is the “ANT suite”; ANT is an acronym for anorthosite, norite, and troctolite.

Assessing Anorthosite Origin and Significance

Paradoxically, the origin of terrestrial anorthosites is not as clear-cut as the origin of lunar anorthosites, and many models and hypotheses have been offered to explain their unique composition and space-time relationships. No explanation is readily accepted as applying to all or most anorthosite occurrences, but enough is known about anorthosite bodies to allow some good, educated guesses. Scientific work on anorthosites has included numerous field-mapping projects to determine their spatial extent and structure. Laboratory work has included radiochemical dating studies to determine the absolute ages of anorthosites and associated rocks and experimental studies that have explored possible parent materials and melting environments of anorthosite magmas.

Several factors must be considered in assessing the origin and significance of anorthosite bodies—specifically the massif varieties. First, anorthosites are restricted, for the most part, to the very narrow time band between about 1.7 and 1.1 billion years before the present, as determined mostly by potassium-argon, uranium-lead, and rubidium-strontium dating. Second, these bodies occur in belts where orogeny (mountain building) may have occurred before or after their emplacement but where they intruded during anorogenic times or during rifting. They are unequivocally igneous as opposed to metamorphic; that is, they did not result from some other type of rock that changed in the solid state but rather from molten magmas. This fact is determined in the field by examining anorthosite contacts with older rocks to see whether these rocks show evidence of thermal heating caused by intrusion or hot magma. Anorthosites lack minerals that contain water in their structures, so water was not an important constituent of anorthosite parent magmas. Finally, many anorthosite complexes, such as those in New York’s Adirondack Mountains, show an association not only with norites and troctolites but also with pyroxene-bearing granitic rocks called charnockites.

Although experimental evidence shows that a liquid of plagioclase composition cannot be generated under high pressures from any known earth materials, magmas can form from rocks in the mantle or lower crust that, under anhydrous (water-free) conditions, are capable of generating the plagioclase “mush” (crystals plus a small amount of liquid) by plagioclase flotation. Laboratory melting experiments show that one type of magma, called “quartz diorite” (relatively siliceous magma of the type erupted in 1980 by Mount St. Helens), could produce a plagioclase mush at great pressures. Intrusion of this mush at higher levels in the crust could squeeze out some of the interstitial liquid, which would crystallize as charnockites. Norites and gabbroic anorthosites represent cases where the plagioclase mush has trapped greater or smaller amounts of more iron-rich silicate liquid.

Whatever the precise mechanism might have been, it produced large volumes of a very unusual suite of rocks over a geologically brief time interval. No doubt this extensive melting event required higher heat flow in the crust and mantle than is now present in areas of anorthosite’s occurrence, or in most other areas of the earth. The cause of this brief episode of high heat flow is still uncertain.

Decorative and Practical Applications

Anorthosites are not very common in the earth’s crust, and most people are not as familiar with this stone as they are with the more common building stones, granite and marble. Ironically, anyone who has ever seen the moon has seen huge spans of anorthosite; it is the principal rock type composing the lunar highlands, the dominant light-colored areas of the moon.

Anorthosite is used as a building stone or for decorative building facings. A beautiful variety composed mostly of the high-calcium plagioclase known as labradorite is in particular demand. Labradorite exhibits blue, violet, or green iridescence that varies in color with the angle of incidence of light. This rock, a dark gray rock that exhibits bluish flashes, is called larvikite, after Larvik, Norway. Anorthosite composed of labradorite plagioclase is sometimes polished and used in tabletops and floor or wall panels. It has other decorative and practical applications as well. Like other feldspar-rich rocks, anorthosite can be used to make ceramic products. Porcelain bath fixtures, insulators, and dining ware are made from finely pulverized feldspar that is heated to very high temperatures. The resistance to heat and electricity and the general durability of porcelain come from the same properties inherent in feldspar.

Anorthosites and associated rocks are also the sites of economically exploitable iron-titanium deposits in some localities. The ore occurs mostly as ilmenite (iron-titanium oxide) and magnetite (iron oxide), mostly concentrated in associated rocks such as norite. Some notable deposits occur in the following places: Lofoten, in northern Norway; Allard Lake, Quebec; Iron Mountain, in the Laramie Range of Wyoming; Duluth, Minnesota; and Sanford Lake (near Tahawus) in the Adirondacks of New York State.

Principal Terms

crust: the upper layer of the earth and other “rocky” planets; it is composed mostly of relatively low-density silicate rocks

gabbro: coarse-grained, iron-magnesium-rich, plutonic igneous rock; anorthosite is an unusual variety of gabbro

hypersthene: a low-calcium pyroxene mineral

magma: molten silicate liquid plus any crystals, rock inclusions, or gases trapped in that liquid

mantle: a layer in the earth extending from about 5 to 50 kilometers below the crust

massif: a French term used in geology to describe very large, usually igneous intrusive bodies

norite: gabbro in which hypersthene is the principal pyroxene; it is commonly associated with anorthosites

peridotite: the most common rock type in the upper mantle, where basalt magma is produced

plagioclase: a silicate mineral found in many rocks; it is a member of the feldspar group

plutonic: formed by solidification of magma deep within the earth and crystalline throughout

Precambrian: The span of geologic time extending from early planetary origins to about 540 million years ago

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