Plutonic rocks

Plutonic rocks crystallize from molten magma that is intruded deep below the earth’s surface. Exposed at the surface by erosion, they occur mainly in mountain belts and ancient “shield” areas of continents. Many of the world’s principal ore deposits are associated with plutonic rock bodies; the rocks themselves may be also exploited economically, as in the production of granite for building stone.

Plutonic Rock Occurrence

Plutonic rocks crystallize from molten silicate magmas that intrude deep into the earth’s crust. This same magma (molten silicate liquid) may eventually flow out onto the surface of the earth as lava that crystallizes to produce volcanic rocks. Volcanic rocks can be distinguished from plutonic rocks by the relative size of their mineral grains. Rapidly cooled volcanic rocks have nearly invisible crystals (or glass), while the slowly cooled plutonic rocks have larger crystals clearly visible to the naked eye.

Because they form deep underground, plutonic rocks require special circumstances to become exposed at the surface. Uplift of the crust in a mountain range or high plateau accompanied by nearly constant erosion by streams or glaciers may eventually uncover once-buried plutonic rocks. The best places to see excellent exposures of these rocks are in mountain ranges such as the Rockies, Sierra Nevada, and Appalachian ranges of North America, the Alps of Europe, and the Himalayas of Asia, among many others. Another major plutonic rock terrain exists in “shield” areas found on all the world’s major continents. These areas comprise the ancient cores of the continents and consist of rocks that are billions of years old, most of the rocks having once existed in ancient mountain ranges now eroded down to relatively flat plains. In North America, this area is called the “Canadian Shield” and covers most of Canada and the northern portions of the states of Minnesota, Wisconsin, Michigan, and New York; buried extensions of the shield underlie much of the rest of the eastern and central United States. In certain places, these rocks are exposed on the surface, as in the center of the Ozark Plateau of southeastern Missouri.

Plutonic Rock Varieties

Plutonic rocks come in many varieties depending upon the chemistry of the parent magma and their mode of emplacement in the crust. Igneous magmas vary chemically between two major extremes: “felsic” magma, in which the concentration of dissolved silica (silicon dioxide) is high and the concentrations of iron and magnesium are relatively low, and “mafic” magma, in which the concentration of silica is low and the concentrations of iron and magnesium are relatively high. Granite and related rocks (generally light-colored) are produced by crystallization of felsic magmas. Their light color and the presence of the mineral quartz (silicon dioxide) distinguish granitic rocks from the dark plutonic rock gabbro, which crystallizes from mafic magmas. Other rocks, such as diorite, crystallize from magma that is intermediate in composition between felsic and mafic extremes. Diorite and its relatives are generally gray-colored and are commonly mistaken for granite. For example, much of the Sierra Nevada range in eastern California is composed of granodiorite, although it is popularly known as a “granite” mountain range.

Granitic or Diorite Bodies

Plutonic rocks are emplaced in the crust in a variety of geometric forms, collectively called “plutons.” By far the largest plutons are batholiths, huge masses of granite, diorite, or both with surface exposures exceeding 100 square kilometers. In western North America, some batholiths are exposed over a considerable portion of whole states, such as the Boulder and Idaho batholiths of Montana and Idaho, and the Sierra Nevada and Southern California batholiths of California. Batholiths also occur in the Appalachian Mountains, particularly in the White Mountains of New Hampshire and in parts of Maine. Most batholiths attain their large size by the successive addition of smaller plutons called “stocks.” Stocks are generally exposed over tens or hundreds of square kilometers.

Minor plutonic bodies include dikes and sills, tabular intrusions that commonly represent magma that has filled in fractures that either cut across layers in country rock (dikes) or that intruded parallel to rock layers (sills). Some sills fill up with so much magma that they expand and force the overlying layers of rock to bow upward. Such plutons are called “laccoliths,” and some, like the Henry Mountains of southeastern Utah, have attained the scale of small mountains.

Gabbroic Bodies

Batholiths, stocks, and laccoliths are predominantly granitic or diorite bodies. Mafic (gabbroic) intrusions occur in their own particular geometric forms, mostly as dikes and sills. Some of these bodies reach enormous size and are exposed over areas that rival large stocks or even granitic batholiths. Many of these bodies show evidence of having concentrated layers of crystals that gravitationally settled after crystallization. Known variously as “gravity-stratified complexes” or “layered mafic-ultramafic complexes,” these bodies are commonly rich sources of economically important metallic ores, particularly those of chromium and platinum. The best examples of these bodies are in South Africa (the Bushveld and the Great Dyke), Greenland (the Skaergaard Complex), and North America (the Stillwater complex in Montana, the Muskox complex and Kiglapait complex in Canada, and the Duluth Gabbro north of Lake Superior).

Other mafic-ultramafic complexes may produce massive layers of metallic sulfides rich in copper, nickel, and variable amounts of gold and silver, among other metals. In North America, the Sudbury “nickel irruptive” in Ontario, Canada, is a well-known example of a rich sulfide ore body associated with gabbroic magma. Melting to produce this magma has been attributed to the ancient impact of a large meteoroid. Gravitational segregation of metallic oxides is also known from gabbroic magmas. The rich iron deposits in Kiruna, Sweden, are believed to form from the settling of large blobs of liquid iron oxide that later crystallized to the mineral magnetite. Titanium deposits in anorthosites (feldspar-rich gabbro) at Allard Lake, Quebec, may have formed by gravitational concentration of iron-titanium-rich fluids within the gabbroic magmas that also produced the associated anorthosite rock.

Pluton-Producing Magmas

The origin of the magmas that create plutons depends in large part on their chemical compositions. Most granite-composition magma probably arises by partial melting of siliceous metamorphic rocks in the deeper parts of the continental crust. Rocks in these regions have compositions that are already close to granitic, so melting them leads inevitably to the production of granitic liquids. These liquids (magma), being less dense than the surrounding, cooler rocks, rise through the crust and join with other bodies to make stocks and, possibly, batholiths.

Mafic magmas of the kind that crystallize gabbro originate in the upper mantle, where they arise by the partial melting of mantle peridotite. Peridotite is the major constituent of the upper mantle and consists of the mineral olivine (iron-magnesium silicate) with minor pyroxene (calcium iron-magnesium silicate) and other minerals. Laboratory experiments have shown that partially melting peridotite at pressures like those in the mantle produces mafic liquids capable of crystallizing gabbro (or basalt at the surface).

A special kind of gabbro, anorthosite, is produced by the separation and concentration of plagioclase feldspar from the gabbroic magma subsequent to its production in the mantle. Because plagioclase commonly is less dense than the surrounding iron-rich gabbroic liquid from which it crystallized, plagioclase crystals may literally float to the top of the magma chamber to form a concentrated mass of feldspar. Anorthosite makes up most of the light-colored regions of the moon (the lunar highlands) but also occurs as large intrusions on Earth, as in the Duluth Gabbro complex of northern Minnesota; in the Adirondack Mountains of New York State; the Laramie Anorthosite of Wyoming; and the Kiglapait, Nain, and Allard Lake complexes in Labrador. Mysteriously, most of Earth’s anorthosite bodies were generated during only one restricted period in geologic time, between about 1 and 1.5 billion years ago.

The origin of diorite magma is complicated by the fact that it can arise in a variety of ways. Granodiorite plutons, like those in the Sierra Nevada range, represent magma reservoirs under now-vanished volcanoes. The magma that produced these intrusions was generated by melting along a subduction zone, where the Pacific Ocean lithospheric plate (crust plus uppermost mantle) was sinking under the North American plate in a zone roughly parallel to the West Coast. A similar process is currently producing the active volcanoes in the Cascade Range of the Pacific Northwest. Any number of parent materials being pulled down to great depth (and, thus, greater temperature) in a subduction zone may mix with subducted ocean water and subsequently melt to produce diorite-type magma. These materials include hydrated (water-saturated) oceanic basalt lava, and even some hydrated mantle materials (serpentine). Ample evidence suggests that some diorite is also produced by the physical mixing of granitic magma produced by the melting of continental crustal materials, and gabbroic magma produced in the mantle. These contrasting liquids intermingle on their way upward, finally intruding as intermediate-composition diorite magmas.

Alkaline Plutons

One other group of plutonic rock is worthy of mention, even though surface exposures are relatively rare. These are the “alkaline” intrusives, so named because they tend to be enriched in the alkali elements potassium and sodium and relatively depleted in silica. Alkaline magmas originate by partial melting of mantle peridotite at extreme depths and high pressures (20 kilobars or greater). Because high pressure acts to discourage melting (even at high temperatures), special conditions are required (commonly tectonic rifting) to generate alkaline magmas, making them among the rarest of igneous rocks.

A fairly familiar variety of alkaline-type volcano-intrusive body is the kimberlite “pipe” that in some areas contains diamond. Diamond-bearing kimberlites are especially common in South Africa and other African countries, India, and Russia. North American localities include Murfreesboro, Arkansas, and the Arctic of Canada. Kimberlite rock itself is a complex mixture of mantle and crustal fragments, carbonate minerals, and silicate minerals crystallized from the alkaline magma. Diamonds, if present, were swept up by the magma from high-pressure areas of the mantle and propelled by expanding carbon-dioxide gas to the surface at speeds estimated, in some cases, to exceed supersonic velocities. Surface deposits (diatremes) of kimberlite consist of volcanic ash ejected during powerful volcanic explosions.

Other important alkaline plutons include the nepheline syenites, light-colored, coarse-grained rocks consisting mostly of the mineral nepheline (sodium aluminum silicate). Important North American localities include Magnet Cove, Arkansas, and Bancroft, Ontario. Depending on the particular locality, these rocks are potential repositories of rare elements, and thus unusual minerals. Commercial exploitation of nepheline syenites has produced the metals beryllium, cesium, thorium, uranium, niobium, tantalum, and zirconium, to name just a few. These rock bodies may also be a fertile source of apatite (hydrated calcium phosphate), an ore of phosphorus, and the dark-blue mineral sodalite (sodium aluminum silicate chloride), used as a semiprecious gemstone and for sculpted carvings. Another alkaline plutonic rock rich in rare elements and minerals is carbonatite, a magmatic rock composed mostly of calcium, magnesium, and sodium carbonates. Like nepheline syenites, carbonatites are highly prized for their mineral treasures, although they are relatively rare, and their individual surface exposures are limited in size. The best examples are in the East African rift and in South Africa.

Study Methods

Plutonic rocks are igneous rocks; thus, their study entails the same methods that might also apply to volcanic rocks. Field studies of plutons include the construction of geologic maps showing spatial distribution and structure of the various rock types in the pluton. Most plutons contain more than one type of igneous rock or at least show some chemical and mineralogical variation from one locality to the next. During mapping or other field surveys, samples are normally collected to be chemically analyzed by various techniques, including atomic absorption analysis, X-ray fluorescence, or neutron activation analysis. Individual minerals in the rocks may be chemically analyzed using an electron microprobe, a machine that gives a full chemical analysis of a 1-micron spot on a single mineral in a matter of minutes. More sophisticated analyses include isotopic abundance ratios and radiometric ages determined from mass spectrometry. Collected chemical data are normally plotted on diagrams that help show how and why the plutons may have changed over time, and may lead to an understanding of the ultimate origin of their parent magmas.

The identities, textures, and spatial orientations of minerals in rocks are assessed by preparing microscopic slides of thin slices of the rocks called “thin sections.” A skilled igneous petrologist (geologist who specializes in igneous rocks) can determine much about the history of a plutonic rock merely by studying a thin section under a microscope. The identification of constituent minerals under the microscope serves to classify the rock, and the relative volume of individual minerals provides hints about the rock’s chemistry. For example, a rock with a high volume of dark, mafic (iron and magnesium-rich) minerals would suggest a high concentration of iron and magnesium in that rock compared to one with fewer dark minerals.

Economic Value

Plutonic rocks are the source of many of the raw materials that are used in industrial society. Granite and diorite are used as construction stones in buildings and monuments and as crushed stone for roadways and concrete. Gabbro, however, is generally shunned for decorative purposes because its high iron content causes it to oxidize (rust) over time. Conversely, the special kind of light-colored gabbro, called “anorthosite” (mostly plagioclase feldspar), is prized as a polished building facing stone; it is also used to grace floors or countertops in banks and office buildings.

Additionally, many of the richest metallic ore deposits originate in or adjacent to plutonic bodies. The extensive list of metals from granitic deposits includes copper, gold, silver, lead, zinc, molybdenum, tin, boron, beryllium, lithium, and uranium. Gabbroic and related deposits contribute nickel, iron, titanium, chromium, platinum, copper, gold, and silver.

Plutons are also the source of some of the most precious and semiprecious gemstones. For example, the precious gems topaz and emerald occur in ultra-coarse grained granitic deposits called “pegmatites.” Pegmatites also provide the semiprecious gems aquamarine (a blue-green form of emerald), tourmaline (elbaite), rose quartz, citrine (yellow quartz), amethyst, amazonite (aqua-colored microcline feldspar), and zircon (zirconium silicate). Also, large mica crystals from pegmatites are used as electrical and thermal insulators, and feldspar minerals are powdered to make porcelain products and potassium-rich fertilizer. The principal source of the element lithium, used in lubricants and psychoactive drugs, is granitic pegmatite. Lithium is obtained from spodumene (a lithium pyroxene) and lepidolite (a lithium mica), minerals that occur exclusively in pegmatites.

In addition to providing a highly desirable construction stone, anorthosite plutons may also be the source of the semiprecious gemstone labradorite. Labradorite is a high-calcium form of plagioclase feldspar (the major mineral in anorthosites) that displays a green, blue, and violet iridescence similar to the play of colors in the tail of a male peacock. Gem-quality labradorite crystals may be made into jewelry, and polished slabs of labradorite anorthosite are used as countertops, desktops, and building facings.

Geologic Value

In a larger sense, plutonic rocks (particularly granitic and diorite plutons) form the bulk of the world’s continents. During the early evolution of the earth, low-density granitic plutons rising from the early, primitive mantle coalesced to form the cores of the first continents. In later eras, continents have continued to grow, as plutons and overlying volcanic rocks have added new material to the margins. This process is well illustrated by the plutonic terrain of the Sierra Nevada range, the andesite (the volcanic equivalent of diorite) volcanoes of the Cascades of the Pacific Northwest, and the Andes Mountains of South America. Were it not for the formation and expansion of continents by the continuing addition of plutons over geologic time, life on Earth would be considerably different. Without continents and the dry land they provide, life would be confined to the oceans, with obvious implications for the evolution of human beings.

Finally, plutonic rocks in natural settings enhance the beauty and general aesthetic value of the landscape. Deeply eroded plutons have produced some of the most striking landscapes in North America, Europe, and Asia, many of which have been set aside as parks and recreation areas. In North America, especially magnificent landscapes in eroded plutons occur at Yosemite National Park in the Sierra Nevada range of California, the Boulder batholith area of Montana, the high peaks area of the Adirondack Dome of New York State, and the White Mountains of New Hampshire.

Principal Terms

anorthosite: a light-colored, coarse-grained plutonic rock composed mostly of plagioclase feldspar

batholith: the largest type of granite/diorite pluton, with an exposure area in excess of 100 square kilometers

feldspar: an essential aluminum-rich mineral in most igneous rocks; two types are plagioclase feldspar and alkali feldspar

gabbro: a coarse-grained, dark-colored plutonic igneous rock composed of plagioclase feldspar and pyroxene

granite: a coarse-grained, commonly light-colored plutonic igneous rock composed primarily of two feldspars (plagioclase and orthoclase) and quartz, with variable amounts of dark minerals

magma: a molten silicate liquid that upon cooling crystallizes to make igneous rocks

pluton: generic term for “intrusion”

stock: a granite or diorite intrusion, smaller than a batholith, with an exposure area between 10 and 100 square kilometers

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