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Plutonic Geology Colin H. Donaldson University of St. Andrews

R. John Reavy Michael John O’Mahony University of Cork

I. Historical Development II. Depths of Pluton Emplacement III. Characteristics and Nomenclature of Plutonic Rocks IV. Nature and Origin of Plutons V. Plate Tectonic Occurrence of Plutons VI. The Origin and Evolution of Magma in the Earth—Key Plutonic Processes VII. Economic Uses of Plutonic Rocks

GLOSSARY Batholith A very large intrusion (>100 km2 in plan) which normally consists of from several to many individual plutons. Most batholiths are dominated by granitoid rocks but gabbroids are also possible. Dike A sheetlike intrusion that cross-cuts structures in intruded rocks. Gabbro A silica-poor plutonic rock consisting of approximately equal proportions of the two minerals pyroxene and plagioclase feldspar. Granite A silica-rich plutonic rock consisting of quartz, alkali feldspar, plagioclase feldspar, biotite mica ± amphibole. Intrusion (a) The action by which magma is emplaced into preexisting rock; (b) a body of rock formed by injection of magma. IUGS rock classification The International Union of

Geological Sciences system of classification based on the relative proportions of minerals in a rock. Laccolith A very large, saucer-shaped intrusion that is more or less conformable with the bedding of the intruded rocks. Layered pluton One that is heterogeneous because of parallel bands in the rock picked out by variation in relative mineral proportions, crystal size, or crystal shape. Most are gabbroid in composition. Migmatite A coarse-grained heterogeneous rock consisting of a mixture of high-grade (gneissose) metamorphic rocks and granitic igneous rock. Orogenic belt An elongate region characterized by severely deformed rocks, which represents the site of current or formerly active mountain building. Plutons are abundant in such sites. Partial melting The creation of magma by the incomplete melting of a rock within the earth.

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492 Pegmatite An especially coarse-grained igneous rock (crystals >1 cm) formed within large plutons and/or injected from a pluton into the country rock. Pluton Any massive body of igneous rock created by solidification of magma. Plutonic rock One formed underground by solidification of magma. Rock texture The appearance that a rock has depending on characteristics of the crystals that form it. In plutonic rocks textures are phaneritic, implying that crystals can be seen with the naked eye, i.e., crystals exceed ca. 1 mm in length. Sill A sheetlike intrusion that is conformable with the bedding of the intruded rocks. TAS rock classification A chemical classification of igneous rocks based on the total alkalis content and silica content. Transformation/granitization A hypothesis that granitoid rocks form by the conversion of crustal rocks in the solid state to a granite mineralogy by the action of metasomatic fluids without melting occurring. Xenolith A fragment of rock which has been incorporated into a magma during its emplacement and which has not been digested.

PLUTONIC GEOLOGY is the study of the processes concerned with the origin, movement, and emplacement of molten rock (magma) inside the earth. The scope of plutonic geology is displayed in the flow diagram in Fig. 1a and in the imaginary cross section through the crust in Fig. 1b which set out its processes, from the generation and segregation of magma within the earth, to its ascent through the crust, to the chemical processes that may alter its composition en route, to the formation of intrusions in which solidification occurs, with or without further chemical processing. This article explores the rocks produced and the processes portrayed in the chart, placing particular emphasis on plutonic rock types, on the nature and origin of intrusions, and on the plate tectonic associations of intrusions. (More information on the chemical processing of magmas can be found elsewhere in this Encyclopedia.) Due to their high temperature and semi-molten condition most nascent magmas have a density lower than the surrounding and overlying rocks and so ascend due to buoyancy, penetrating preexisting or newly opened cracks in the rocks. Heat loss to the enclosing rocks induces crystallization and so most magmas actually solidify underground rather than penetrating the crust and erupting as lava or ash. These newly formed underground rocks constitute the intrusive or plutonic association of igneous rocks. As Hutton realized in 1785, it is subsequent uplift

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(a) FIGURE 1 (a) Schematic representation of igneous processes occurring at progressively shallower levels in the earth. [Adapted, with permission, from Wyllie, P. J. (1971). “The Dynamic Earth,” Wiley, New York, Fig. 4.5.] (b) The “life” of a magma, from generation, through ascent, to emplacement within the earth or on the surface. [Adapted, with permission, from Basaltic Volcanism Study Project. (1981). In “Basaltic Volcanism on the Terrestrial Planets,” Pergamon Press, New York, Chapter 3, Fig. 3.1.1.]

and erosion that expose intrusive rocks at the surface for scientific investigation.

I. HISTORICAL DEVELOPMENT At the close of the 18th century Neptunism was the ruling theory of geology, maintaining that all rocks had formed as sediments from a globe-covering ocean. Part of the evidence used to discredit the theory was that granite can be found cutting across the bedding of other rocks and is therefore younger than them (see below, at the end of this section). Therefore granite must have been a mobile material that injected the bedded rocks. The succeeding theory, Plutonism, held that while some geological processes take place at the earth’s surface, others operate inside the earth (hence the term’s allusion to the Roman god of the underworld). The concepts of igneous rocks and of the underground existence of molten rock (magma) had been added to geological thinking. The significance of plutonism may be gauged from the fact that at least two-thirds of the earth’s crust is now recognized to be made of intruded rock and that magma intrusion has been the main crust-building process on all the terrestrial planets.

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the rocks is assisted by the use of several kinds of laboratory investigation. The crystallization of magmas at high temperature and pressure can be reproduced in experiments conducted in heating and squeezing apparatus. Mechanical aspects of how magmas move underground and how they create space when they intrude is advanced through the use of model fluid experiments and computer modeling of the processes involved. Possible genetic relations among plutonic rocks are tested by use of various mineralogical and geochemical analyses of rocks. Integration of the results of these methods has led to a qualitative understanding of the underground behavior of magmas. Neptunism and Plutonism—Werner and Hutton

(b) FIGURE 1 (continued )

Although Charles Lyell advocated in his “Principles of Geology” (1830) that all underground geological processes should be regarded as plutonic, thus including metamorphic rocks, this practice was never widely adopted. It is now customary to regard “plutonic” as encompassing only those underground processes that involve molten or partially molten rock. Plutonic geology is one branch of igneous geology; the other, volcanic geology, is concerned with the study of surface igneous processes. In that volcanoes are fed with magma from underground, there is inevitably some overlap between the two branches. The processes that create plutonic rocks can never be seen and so have to be investigated indirectly through observations of solidified intrusions, now exposed by lengthy erosion. Understanding the field relations and nature of

Until the late 18th century, western geological thinking was constrained to fit in with the teachings and beliefs of the Church. The earth was regarded as a few thousand years in age, and all rocks, even coarsely crystalline ones such as schist, granite, and gabbro, were regarded as sedimentary deposits precipitated from the water of a primeval, receding ocean. As the century closed, the greatest exponent of this Neptunian view of the earth was Abraham Werner (1749–1817), professor at the Mining Academy of Freiburg in Saxony, Germany, who considered that granite and schist were the earliest precipitates from the flood waters and so should always underlie all other rock types. This view was challenged by the Scot James Hutton (1726–1797), a trained doctor, retired farmer, and entrepreneur, who at the age of 55 moved to the city of Edinburgh and devoted himself to the study of philosophy sensu lato, including geology. It is often said that the best geologist is the one who has seen the most rocks. This was certainly true of Hutton, who had traveled widely in Scotland, England, Holland, Belgium, and France. He built a considerable personal experience of field geology that was the basis of his rejection of Neptunism in a paper delivered to the fledgling Royal Society of Edinburgh in 1785 and published in 1788, the grandly titled “Theory of the Earth.” Key evidence in Hutton’s claim that magma is involved in the creation of igneous rocks came from a field expedition to Glen Tilt in Perthshire. In the river bed he examined the contact between granite and schist and found veins of granite cutting through the bedding planes and micaceous foliations in the schist, unequivocal evidence that the granite had invaded and injected the schist. This was an observation that Hutton subsequently made on other granite plutons elsewhere in Scotland, again demonstrating that older rocks are intruded by granite. By the time of Werner’s death, geologists throughout Europe had verified Hutton’s observations and supported his

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494 inference about the existence of magma in the crust. Neptunism became a spent hypothesis.

II. DEPTHS OF PLUTON EMPLACEMENT In the late 19th century magmas were considered to solidify in three types of environment: r In a “plutonic” setting, deep in the crust r In a “hypabyssal” setting, at shallow levels in the crust r At the surface following a volcanic eruption

While the distinction between hypabyssal and volcanic setting was clear, that between plutonic and hypabyssal never was because an absolute value could not be assigned to the depth at which any exposed intrusion had been emplaced. Other criteria had to be employed to make the distinction: r The thickness of the intrusion r The grain size of the rock in the intrusion (grain size

increases with intrusion thickness because cooling rate, and hence time available for crystals to grow, is proportional to intrusion thickness) Thus thin, “minor” intrusions containing glassy or finegrained rock were viewed as quickly cooled in a nearsurface environment, whereas thick, “major” intrusions with coarser grains were regarded as being of deep origin. However, occurrences of minor intrusions in deeply eroded, high-grade metamorphic terrains and of major intrusions that geological relations showed to have formed within a few kilometers of the earth’s surface demonstrated the unreliable nature of the plutonic versus hypabyssal distinction. Most petrologists are now content to distinguish between plutonic and volcanic settings only. In 1959 the American geologist A. F. Buddington proposed a threefold division of granite intrusions depending on their internal and external relations. r Epizone plutons, formed down to depths of 6–9 km,

are normally structureless and have discordant relations to the intruded rocks, which show little or no metamorphism by an intrusion. r Mesozone plutons, formed at depths between 6 and 12 km, have contact relations that are in part concordant and in part discordant with the intruded rocks. There may be well-developed structures in the plutonic rock. The intruded rocks frequently consist of low-grade, regionally metamorphosed rocks, which next to the intrusion may be strongly deformed and thermally metamorphosed.

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r Katazone plutons, formed at depths below 12–15 km,

have concordant contacts, are often part of a migmatite zone and possess internal structures that are generally conformable with structures in the intruded rocks which are high-grade regional metamorphic types (schists and gneisses). Unfortunately, this scheme is no better than the plutonic–hypabyssal one at providing a clear-cut division between its zones. In any case many exceptions are known. For example, detailed investigations of granite plutons in western Ireland by Pitcher and Berger in the 1960s demonstrated that intrusions with characteristics of each of the three types had been emplaced at essentially the same depth. In short, plutons have no reliable field or textural characteristics with which to assess the depth at which any particular intrusion has been emplaced. On the other hand, in the last 30 years the depth of emplacement of some intrusions has been determined using the compositions of certain minerals. For example, the aluminum content of the amphibole hornblende is sensitive to the pressure at which it grows from rock melt. By conducting laboratory experiments in which hornblende is grown from molten rock at a variety of pressures, the relationship between pressure (equivalent to depth in the crust) and aluminum content of the mineral can be calibrated. Thus comparison of the composition of hornblende crystals in an intrusion with the experimental calibration enables the emplacement depth to be estimated. Uncertainties in such estimates are, however, considerable, ranging from ca. ±2.5 to ±5 km.

III. CHARACTERISTICS AND NOMENCLATURE OF PLUTONIC ROCKS A. Mineralogy Most igneous rocks are dominated by silicate minerals.1 However, of the roughly 22,000 minerals known to science, only a very few crystallize from magmas. These are conveniently subdivided into a mafic group, a term that designates those that contain magnesium, ferrous, and possibly ferric iron, and which are all dark-colored minerals (olivine, orthopyroxene, clinopyroxene, hornblende, biotite, garnet, and spinel), and a felsic group, from feldspar and silica, which are all light-colored minerals [quartz, alkali feldspar, plagioclase feldspar and feldspathoid minerals (nepheline, kalsilite, leucite, sodalite), muscovite, and melilite]. 1 The main exceptions are the very scarce rock types carbonatite, in which carbonate minerals predominate, and kimberlite, in which silicate and carbonate minerals occur in subequal proportions.

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Other silicate minerals can be present but usually in small amounts, ≤5%, for example, zircon and sphene. Nonsilicate minerals, e.g., apatite (calcium phosphate), magnetite (iron oxide), ilmenite (iron, titanium oxide), and pyrite (iron sulfide), may also be present, but they, too, are rarely abundant. All of these minerals crystallize from rock melt at high temperatures (≥750◦ C) and are designated the primary minerals in an igneous rock. Following crystallization it is common in cooling plutons for fluid to react with the primary minerals causing their replacement by secondary ones. These include numerous hydrated silicate minerals such as clay minerals, certain amphiboles, and epidote, as well as carbonates (e.g., calcite), and oxides (e.g., hematite and limonite). The fluid may be residual from solidification of the magma or it may be ground water from the rocks surrounding the pluton which penetrated cracks that opened in the cooling, shrinking pluton. A hydrothermal circulation system is driven by heat loss from the pluton. Yet further decomposition of the primary minerals can occur when the pluton is unroofed and exposed to weathering at the earth’s surface, causing additional hydration, oxidation, and carbonation reactions. B. Textures Texture is the overall appearance that a rock has because of the size, shape, arrangement, and crystallinity of its constituent crystals. Because they cool slowly, plutonic rocks rarely contain glass and are normally 100% crystalline. The crystals commonly partly or wholly enclose one another, making for an interlocking arrangement, indicating that during growth from melt they competed with one another for space. [Metamorphic rocks can also display interlocking of crystals (unlike most sedimentary rocks), but igneous rocks rarely possess the arrangement of minerals in parallel layers (“foliation”) that characterizes the majority of metamorphic rocks.] The term plutonic rock is traditionally taken to mean an igneous rock with a “phaneritic” texture, which means that the individual crystals can be distinguished with the naked eye. This term is imprecise because it is subjective; some rocks that a geologist had identified as plutonic in his/her youth would have to be reclassified as volcanic in middle age! Indeed, while some petrologists take the lower limit as 2 mm, others put it at 1 mm. The vast majority of rocks known to be of plutonic origin from their geological occurrence have a grain size of >1 mm but <1 cm. From laboratory crystallization experiments with rock melts it is known that minerals mostly grow at rates of ca. 10−10 –10−12 cm sec−1 . Hence growth of individual crystals in plutons may extend from tens to thousands of years, consistent with estimates of the

cooling rates of plutons from heat transfer theory. Rocks in a minority of plutons possess crystals that greatly exceed 1 cm (“supercoarse” rocks), reaching 1–2 m across, though even these are dwarfed by the giant crystals up to 15 m long found in a very few such plutons. Supercoarse rocks are referred to as pegmatites and most are of granite composition. Pegmatites have not had longer to crystallize than other plutonic rocks but rather had a growth accelerator in the form of a high concentration of H2 O and other gases dissolved in the melt. This reduces the viscosity of the melt and speeds ion transport from melt to crystals. Two arrangements of crystals dominate plutonic rocks. In one the crystals are of approximately the same size forming equigranular texture; the magma is inferred to have undergone crystallization only in the pluton. In the other, the crystal size is roughly bimodal, with some crystals (phenocrysts) clearly larger than others (the groundmass), forming a porphyritic texture. Whereas the groundmass is considered to have solidified in the pluton, the phenocrysts are usually assumed to have grown in the magma as it rose in the crust. From crystal inclusions in the phenocrysts, zoning patterns in them, and their composition it is sometimes possible to reconstruct information about the ascent and crystallization histories of the magma before it came to rest in the pluton (cf. Fig. 1a). C. Classification Rock names have no systematic derivation; place names, a physical characteristic, a dominant mineral, ancient Egyptian, Greek, and Latin words, and modern German, French, English, and Russian words have all been used in making names. Many were created by enthusiastic 19th century petrologists working throughout Europe and North America, who produced different names for the same, or essentially the same, rock. Rocks can be named and identified either from their physical characteristics or their chemical compositions. Until the start of the 20th century, the former approach was used exclusively and it remains the basis on which the field geologist identifies rocks. 1. Simple Mineralogical Classification The key physical features used for classification are: r Presence or absence of particular minerals r Relative abundances of the minerals present2 r Average grain size of the crystals 2 Which is also reflected in the “color index” of a rock, the relative proportions of light- and dark-colored minerals, and in its density, properties that may also be used in naming the rock.

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FIGURE 2 Mineralogical classification of the common plutonic rocks. [Adapted, with permission, from Duff, D. (ed.) (1993). “Holmes’ Principles of Physical Geology,” 4th ed., Chapman and Hall, London, Fig. 5.7.]

These features can be determined either from a hand specimen of rock, assuming that the crystals are large enough to identify individual minerals (ca. >1 mm) or by examining a thin slice of the rock with the aid of the petrological microscope. The procedure requires determination of the volume (or “modal”) percentages of each mineral. When working in the field this would usually be a visual estimate but when a thin slice of the rock is available, accurate determination is possible by means of point counting—laying out a grid of perhaps 1000 points and recording the mineral beneath each grid intersection. Figure 2 is a widely used classification that employs these properties. It shows, for example, that the rock type diorite is coarse grained, may or may not contain a small amount of quartz, is dominated by plagioclase feldspar3 (ca. 75%), contains up to 10% biotite mica along with 10–20% hornblende, and may have up to a few percent of pyroxene. As with all rock classification schemes, this one necessarily has discrete boxes for each rock type and the unwary will suppose that rocks can be precisely identified, as in a biological classification. However, igneous rocks differ 3 The figure indicates that the composition of the plagioclase feldspar varies with rock type. Plagioclase varies in composition from anorthite (CaAl2 Si2 O8 ) to albite (NaAlSi3 O8 ), with all compositions in between possible. Compositions are designated as a percentage of anorthite in the crystal, e.g., An60 for 60% anorthite and 40% albite. This property is readily measured with the petrological microscope and provides additional information for rock identification.

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FIGURE 3 Total alkalis versus silica (TAS) chemical classification diagram for the common and less common plutonic rocks. [Modified from MacKenzie, W S., Donaldson, C. H., and Guilford, C. (1982). “Atlas of Igneous Rocks and Their Textures,” Longman, Harlow, Fig. L.]

from plants and animals in that characteristics merge between rock types, reflecting the fact that in origin they may be directly related by various processes, e.g., partial melting or fractional crystallization or rock assimilation. Thus the boundaries to boxes are best viewed as indistinct or perforated, with one rock type merging seamlessly into another. 2. Chemical Classification Using Total Alkalis and Silica Contents Chemical analyses of igneous rocks show that ca. 40– 75 wt% of a rock consists of SiO2 ,4 and that this amount increases systematically from gabbro to diorite to granite (cf. Fig. 2). This offers another means of rock identification, by coupling crystal size with SiO2 content. Not all igneous rocks are represented in Fig. 2, but it does include the commonest ones, i.e., >90% by volume in the crust. A bivariate chemical diagram is employed to show the remainder, with SiO2 “content” of a rock plotted against the combined Na2 O and K2 O “contents” (Fig. 3), the TAS (total alkalis versus silica) diagram. The commonest igneous rocks are those with relatively low contents of alkali metals for a particular SiO2 “content” (cf. Figs. 3 and 4). Chemical classification of rocks is an essential starting point in petrological research; indeed many other aspects 4 Strictly, there is no SiO in a rock unless it contains quartz. Use of 2 the parameter SiO2 is merely a convention in reporting the composition in which the silicon in the rock is all assumed to be bonded to oxygen to make silica molecules. In practice it is combined with several elements in the various silicate minerals of an igneous rock.

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of the composition are useful in investigations of genetic relations among rocks. For more-routine work and for field investigation physical properties that are much cheaper to determine are those typically used for rock identification. 3. The IUGS Mineralogical Classification By the mid-20th century it became clear that the basis of naming rocks from their physical and chemical properties was not uniformly applied and that two centuries of uncontrolled development of nomenclature had resulted in many multiple names for rocks with only minor, or even no, differences in characteristics. In 1968 the International Union of Geological Sciences set up a Subcommission on the Systematics of Igneous Rocks to examine the problems of classifying igneous rocks and to present recommendations on how they should be named. Work began on the plutonic rocks and recommendations for their classification were approved by the IUGS in 1972. Recommendations about the nomenclature of volcanic rocks and exotic plutonic rocks were made and approved over the following 15 years and are collected in the Subcommission’s principal report (Le Maitre et al., 1989), which included recommendations for the removal of the majority of the 1500 rock names that then existed. The scheme adopted is another mineralogical one, again using the relative abundances of minerals and their grain size. It assumes that plutonic rocks contain crystals that are visible to the naked eye but did not specify an absolute lower crystal size for plutonic rocks. The IUGS scheme for plutonic rocks has three parts: r Rocks whose total content (M) of mafic group

minerals is ≥90 % (includes those on the extreme left-hand side of Fig. 3) are classified according to the relative proportions of those minerals. r Rocks with M ≤ 90% (includes the majority of the rocks in Fig. 3) are classified according to the relative proportions of felsic group minerals. r In the field, an accurate measure of the volume percent of each mineral present cannot be obtained and so a provisional name is identified, based on approximate amounts of the felsic minerals present. This can be refined later in the lab when an accurate knowledge of the mineral content is obtained. a. Rocks with M ≥ 90%—the “ultramafic” rocks. Two diagrams are employed, depending on whether hornblende is a significant constituent or not: the olivine– orthopyroxene–clinopyroxene triangle and the olivine– hornblende–total pyroxene (orthopyroxene + clinopyroxene) triangle (Fig. 4, top). A number of polygonal fields

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FIGURE 4 IUGS classification of plutonic ultramafic rocks (upper triangles) and detailed classification of gabbroic rocks (middle and lower triangles). [Redrawn, with permission, from Le Maitre, R. W., et al. (1989). “A Classification of Igneous Rocks and Glossary of Terms,” Blackwell, Oxford, Figs. B6 and B8.]

et al. (1989) provide further diagrams (their Fig. B.7.a) to define what is “normal.” The QAPF classification produces a general “root name” for a rock. The petrologist is then free to add qualifiers that may be a mineral present in small quantities (e.g., hornblende gabbro), a textural term (e.g., porphyritic granite), a chemical term (e.g., Sr-rich granite), a genetic term (e.g., contaminated diorite), a tectonic term (e.g., collision zone granite), or any other term that usefully describes special qualities that the rock possesses.

c. “Field classification.” If a rock is very darkcolored, it should be referred to as “ultramafic”; otherwise it is classified using a simplified QAPF diagram (Fig. 6). This field classification is also used in informal discussion of the origin of common plutonic rocks, when petrologists will refer to just two broad groups of rocks— granitoids and gabbroids. The former are usually assumed to include the granitoids and syenitoids of Fig. 6 and the latter the dioritoids and gabbroids, though some

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FIGURE 5 IUGS classification of plutonic rocks using the QAPF diagram. [Redrawn, with permission, from Le Maitre, R. W., et al. (1989). “A Classification of Igneous Rocks and Glossary of Terms,” Blackwell, Oxford, Fig. B4.]

petrologists consider that granitoids also encompasses dioritoids.

IV. NATURE AND ORIGIN OF PLUTONS A. General Features of a Pluton When magma intrudes the crust it will ordinarily have a higher temperature than the enclosing “country” rocks penetrated. Gabbroic magma is emplaced in the crust at ca. 1100–1250◦ C and granitic magma at 750–950◦ C, whereas the temperature range of intruded rocks is ca. 0–500◦ C; heat must flow from magma to rock. Magma in proximity to rock will cool especially quickly and crystallize rapidly, creating fine-grained or even glassy rock. This chilled margin varies in thickness from a few millimeters to several tens of centimeters and forms an insulating rind to the pluton, slowing further heat loss from the magma and causing coarser crystals to grow inside the pluton. The heat that passes to the country rock raises its temperature and brings about simple recrystallization of some minerals (e.g., quartz grains), decomposition by dehydration of others (e.g., clays, micas, and amphiboles), and reactions between minerals to produce new ones (e.g., of

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FIGURE 6 IUGS-recommended classification for classification of plutonic rocks in the field. NB: Rocks ending “-oid” may be replaced by “-ic rock,” depending on linguistic preference. [Redrawn, with permission, from Le Maitre, R. W., et al. (1989). “A Classification of Igneous Rocks and Glossary of Terms,” Blackwell, Oxford, Fig. B9.]

mica and quartz to produce feldspar). Next to the intrusion the surrounding rock may be completely reconstituted, with a new texture and new minerals. This thermal metamorphism creates a baked zone that preserves information about the interactions between magma and the intruded rock, including any interchange of fluids between the two, and the thermal history of the pluton. Magma moving through the crust can incorporate fragments of the rocks traversed. The fate of these depends on the time that elapses before the magma solidifies, the thermal stability of the rock types at the temperature of the magma, and the chemical stability of the crystals in the rock when in contact with the magma. Thus, some rocks will melt or partially melt and the melt blend with the magma, whereas other rocks will slowly dissolve in the magma, and yet others will react with the magma to produce new minerals that are stable in the magma. Time, reaction kinetics, and size of the fragment will all dictate the extent to which these reactions proceed, resulting in more or less complete assimilation of the fragments. Residual fragments and crystals from the original rock are commonly preserved in plutons and are known as xenoliths and xenocrysts, literally rocks and crystals that are “foreign” to the igneous rock.

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FIGURE 7 Block diagrams showing the form of some common types of intrusion and illustrations of the mechanism of ring-dike formation and how that relates to the mechanism of formation of flat-bottomed volcanic craters (calderas). The thinly ruled space above the ring dike represents a bell-jar intrusion. Note that the subsiding block can be symmetrical or asymmetrical, resulting in variation in the thickness of a ring dike.

B. The Forms of Plutons “Pluton” is used for any intrusion, regardless of its shape, size, or composition. Many special names were coined in the 1900s for intrusions of particular shape and/or relationship with enclosing rocks, but most have fallen into disuse, either because of scarcity of examples or because they are recognized as variants of other, more common types. These common types include dikes (dykes in the UK), sills, lopoliths, laccoliths, cone sheets, ring dikes and bell-jar intrusions, funnel-shaped intrusions, batholiths, stocks, and plugs (Fig. 7). Some of the most common types of intrusive bodies are dikes and sills. Both are tabular, parallel-sided bodies that are very much thinner than their lateral extent. Most are a few to a few hundred meters thick. When exposed by erosion they may extend for tens to hundreds of kilometers in extent. The difference between the two types is relationship to their host rocks. Like the banks or walls created to prevent flooding which cut across a landscape and which the intrusion type is named after, dikes crosscut bedding and mineral alignment structures within the country rock. Sills, on the other hand, are concordant bodies which generally lie between bedding planes within the country rocks. Consequently most dikes are vertical or steeply inclined, whereas sills are horizontal or of low inclination. Both bodies are normally emplaced by dilation of the country rocks induced by excess pressure of the magma, but faulting may sometimes be involved. Mapping of an area commonly reveals tens to many hundreds of dikes in a parallel array, a dike swarm. Some dikes display evidence of the movement of several magma batches

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through them, with later ones cutting earlier ones; these form multiple dikes. So-called sheeted dike complexes, consisting of a vast number of such dikes, characterize much of the oceanic crust. These form at mid-ocean ridges and act as feeders to the overlying lava flows that erupt in the axial rift valley. As concordant bodies, laccoliths and lopoliths are variants of sills. Laccoliths are lens-shaped and normally 1– 2 km at the thickest. They have a planar base but a domed upper surface, above which the country rocks are arched up. On the other hand, lopoliths have a saucer form, implying sagging of the underlying rocks under the weight of emplaced rock; most are several kilometers thick and can be very extensive in area, covering thousands of square kilometers, as in the case of the Bushveld Complex, South Africa. Laccoliths and lopoliths can be produced from the amalgamation of sills and have normally been fed by several dikes which, unable to rise higher, spread their magma laterally along bedding planes and coalesce. Cone sheets, ring dikes, and funnel intrusions are all discordant bodies. A cone sheet is a thin dike (from less than 1 meter to several meters) with the form of a downwardpointing cone, causing it to display a circular outcrop pattern (Fig. 7). The diameter of the cone sheet may vary from several hundreds of meters to several kilometers. It is usual for large numbers of such sheets to be concentrically arranged. The apex of the cones is considered to be located at the top of a former magma chamber. Each sheet is formed by overpressure of magma in the chamber, causing fracturing of the overlying rocks and forcing magma into the fracture. Ring dikes are also circular in outcrop, reflecting their upward-pointing, truncated conical form in three dimensions. They are typically inclined outward at a steep angle. Dikes vary in thickness from meters to hundreds of meters and their diameters range from several kilometers to several tens of kilometers. Ring dikes are commonly surmounted by a bell-jar intrusion, which is effectively a disk-shaped sill. The ring-dike plus bell-jar combination results from the vertical subsidence into an underpressured magma chamber of the block of country rock at the center of a ring dike. As this so-called cauldron subsidence proceeds, the resulting space is filled by magma displaced from the chamber. If the ring fracture penetrates to the earth’s surface, a circular crater known as a caldera is formed and magma erupts within the crater (Fig. 7). Funnel-shaped intrusions are mostly occupied by basic and ultrabasic rocks. One of the best studied is the Skaergaard intrusion in east Greenland (see below, at the end of Section VI.A). This body has the shape of a champagne glass with two feeder pipes at the base. In some cases funnel intrusions have the long, linear form of a dike but are V-shaped in cross section and narrow downward. These

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intrusions are described as funnel dikes. Although uncommon, they can be very large; for example, the Great Dike in Zimbabwe is over 500 km long, several kilometers wide and up to 3 km thick; it contains at least 35,000 km3 of rock. A batholith is the collective name for a group of plutons of various shapes, sizes, and rock types that have accumulated and intruded one another over a long interval of time. They typically form a linear belt up to hundreds of kilometers long and tens of kilometers wide, as in the case of the coastal batholith of Peru and the Sierra Nevada batholith of California. Most batholiths have an overall granitoid composition but can include gabbros and even scarce ultramafic rocks. The term is also used for a single, steep-sided, granitoid intrusion, circular-ovoid in plan, and of great vertical and areal extent (>100 km2 in outcrop area). Similar-shaped granitoid intrusions that are smaller than this are known as stocks. Eroded volcanic landscapes are often characterized by upstanding hills and knolls formed of the hard, resistant plutonic rock that solidified inside a volcanic pipe, blocking further eruption of the volcano. These are referred to as plugs. The Castle Rock in the city of Edinburgh, Scotland, and the towering rock pillars of the Puy region of France are well-known examples. C. Intrusion Processes of Granitoid Magmas The intrusion of granitoid magmas shows a close spatial link to regional and crustal scale structures, such as deep faults, fractures, shear zones, and lineaments, which clearly act as controls on the movement of magma. Two processes need to be clearly defined in the consideration of intrusion: ascent is the vertical transport of magma from the source region (generally the lower to middle crust) to the point of emplacement (middle to upper crust); emplacement is the higher crustal level process of pluton construction, which is manifested as “lateral spreading” or “ballooning” and amalgamation of ascended magma batches to produce discrete crystallized plutonic bodies. These are usually gently inclined to flat-lying in cross section and composed of many sheets, some of which may show evidence of the mixing of magmas. 1. The Problems of Granitoid Intrusions One of the major problems associated with the presence of large volumes of granitic material as intrusive bodies within the earth’s crust is what is described as the “space problem” or “room problem.” Simply put, how can the intrusion of large volumes of granitoid magma through and into the crust, especially the upper crust, be facilitated? Some intrusions represent vast quantities of granitic magma emplaced into the crust over a relatively

short time span, for example, the central Sierra Nevada Batholith, California, comprises of 106 km3 of intrusive material which was intruded during the Mesozoic Period (250–60 million years ago). Historically, granitoid bodies (particularly batholiths) have been depicted on geological maps and cross sections as large, bloblike features (typically colored red) that extend to the base of the crust. However, much recent research indicates that individual intrusions in a batholith are rarely more than 2–4 km thick. An early way around the space problem was the view that granite forms in situ, deep in the crust, as a result of fluids reacting with a variety of rocks at high temperature and pressure, converting all to a granite mineralogy and composition but without any melt being created. This is the theory of granitization or transformism. However, it is now clearly evident that granitoids are discrete bodies which have traveled through the crust as magmas. Therefore all subsequent models proposed for the intrusion of granitoids have involved the movement of granitic melt during intrusion through the crust. It is now generally accepted that granitoid intrusion is a complex interplay between the buoyancy of the magma (which is generally less dense than the surrounding country rock) and regional earth movements. 2. Intrusion Mechanisms Mechanisms for granitoid intrusion have been historically grouped into two end-member categories: 1. Forceful intrusion mechanisms. These involve the granitoid body pushing aside its wall rock; processes such as diapirism, ballooning, and doming have been cited as examples of such mechanisms. 2. Passive intrusion mechanisms. These involve transport of the granitoid magmas along/through fractures/voids created by tectonic stresses. These include the processes of cauldron subsidence, stoping, sheeting, and diking. These concepts, however, do not distinguish between ascent and emplacement as defined above. The reality is that in many cases the field evidence for so-called “intrusion mechanisms” actually describes local emplacement phenomena and gives little or no information about the ascent process. It is therefore useful to consider ascent and emplacement processes separately, although it is important to note that the first mechanism to be discussed, diapirism, incorporates both the process of ascent and that of emplacement. Diapirism is the process of rupturing or piercing of domed or uplifted rocks by an igneous intrusion. The concept envisages a large plutonic mass moving up through

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from batches of melt as they arrive at a certain crustal level. Most plutons are filled or inflated laterally to produce flatlying or gently inclined sheets which are silllike in nature. Typically these plutons may only be several kilometers in vertical thickness. Detailed examination of many plutons in the field shows clear evidence that these sheets have been constructed by complex mixing processes involving many magma batches amalgamating during emplacement. A full understanding of emplacement is hindered due to the lack of three-dimensional exposure of complete granitoid bodies; in particular, the floors of granitoid bodies are seldom exposed. Much geophysical work has now confirmed the common field observation that most are generally tabular or laccolithic in overall shape. 3. Fabrics in Intrusive Bodies Mineral fabrics have long been recognized within intrusive bodies. These fabrics are defined by the alignments of the minerals within the rock and record flow and/or deformation during intrusion. Fabrics and structures are also often developed within the country rock during emplacement. Analysis of both sets of structures can provide useful information about stress regimes and deformation accompanying emplacement. The rheological behavior of granitoid magma changes dramatically during crystallization. As crystallization proceeds and a framework of touching crystals is created, the viscosity will increase rapidly. This transition is known as the rheological critical melt percentage, or RCMP, and occurs at between 20% and 40% of the remaining melt. In the field and in thin section, fabrics can be recognized as having formed in either the magmatic or solid state. Magmatic fabrics are produced early in the crystallization history when crystals are free to align within a melt. Solid-state fabrics are more akin to those which form in metamorphic rocks, due to the recrystallization of minerals and growth of new ones. Recent classification schemes refine these ideas and define fabrics as occurring pre-RCMP (minerals are free to rotate and move within the surrounding melt) or postRCMP (minerals have undergone some internal deformation). Much useful information regarding the temperatures of crystallization and recrystallization of the various minerals in granitoids can be deduced from such studies.

V. PLATE TECTONIC OCCURRENCE OF PLUTONS Wherever magma is created in the earth, plutons will inevitably form when rising magma is trapped (Fig. 1b). The

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FIGURE 8 Schematic cross section illustrating the sites of magmatism at plate boundaries and within plates.

principal locations of magma production are at plate margins where two plates have a boundary (processes 1 and 2 below), but lesser volumes of magma are also intruded within plates (processes 3 and 4) (Fig. 8): 1. Magma is produced at divergent plate margins (constructive plate boundaries). Here new lithosphere is created under mid-ocean ridges due to intrusion of basaltic magma into the mantle and crust (see below, at the end of this section). These are the most prolific sites on earth of magma production, with 18 km3 per annum estimated to be intruded globally. The magma itself is the product of decompressive partial melting of peridotite mantle rock flowing upward to fill the space created by the continuous thinning of the lithosphere as it is pulled apart. 2. Magma is generated at convergent plate margins (destructive plate boundaries). Here oceanic lithosphere is subducted back into the mantle, releasing fluids that trigger partial melting of the overlying peridotite mantle and gabbroic–granodioritic crustal rocks. Some 8.5 km3 of magma is estimated to be generated each year at these margins. Most of the rising magma is then trapped as intrusions in the lithosphere of the overriding oceanic plate beneath a volcanically active island arc, or in the lithosphere of the overriding continental plate beneath a line of active volcanoes (e.g., the Cascades volcanoes of Oregon– Washington). 3. Magma is emplaced within plates at active continental rifts, such as the East African rift. Thinning of the crust triggers ascent of mantle rock and its decompressive melting. Most of the ca. 1 km3 of magma is trapped as plutons beneath the rift valley. 4. Magma is also generated anywhere within plates to produce ocean islands anywhere on a plate (e.g., Hawaii

is in the middle of the Pacific Plate, whereas Tristan da Cunha lies close to the mid-Atlantic ridge). These are huge volcanic edifices built upon the ocean floor. Geophysical surveys of active volcanoes and examination of eroded extinct ones show that the fundaments of these volcanoes are intruded by magmas over thousands of years as these seek to erupt at the volcano summit. The geological record contains possible within-plate continental analogues of the ocean island in the form of vast outpourings of lava on continental crust that at the time of eruption had no connection with either constructive or destructive plate margins or with continental rifts. Examples include the Deccan Traps of northern India, erupted 65 million years ago, and the Columbia River plateau of the northwest United States, erupted roughly 10 million years ago. Again geophysical evidence points to the presence of major gabbroic intrusions at the base of the crust from which the eruptions were fed. Erosion also exposes high-level dikes, sills, and occasional lopoliths of gabbro. Both continental and oceanic magmatic events are commonly ascribed to the ascent of a narrow column (“plume”) of mantle rock, possibly from the mantle–core boundary at 3900 km, which partially melts to create gabbroic magma at shallow levels (100– 200 km). Different kinds and sizes of pluton characterize different environments. For example, where continental crust is located above a convergent margin a plexus of numerous minor–major intrusions may be created over millions of years by repeated intrusion of magmas of various compositions to create a vast batholithic complex. By contrast, at a divergent margin, small, shallow-level, elongate intrusions develop (see below). Different again is the situation involving a plume. Here vast numbers of dikes

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504 cut through the oceanic volcano or the continental crust to release magma to the surface. Locally some magma may be trapped, forming a thick, tabular-funnel-shaped intrusion. Intrusions at Mid-Ocean Ridges Inaccessibility has meant that the precise nature of intrusions at mid-ocean ridges has been obscure. Until recently understanding was based on drilling that penetrated only the very top of a few intrusions and on observations of slices of oceanic crust that have slid over continental crust, such as in eastern Oman, to form so-called “ophiolite” complexes. In the last decade detailed seismic studies of small segments of mid-ocean ridge have led to a new geophysical model which holds that an intrusion approximately 3 km across, several tens of kilometers long (parallel to the ridge axis), ca. 4 km thick, and located just 1–2 km below the axial valley is occupied by partially crystallized magma. Only at the very top of the intrusion is there crystal-free magma. Both the lithosphere and the intrusion are being pulled apart here, orthogonal to the ridge axis, and new pulses of magma continually enter the intrusion, so that although the pluton contains only a little magma at any time, it has a long life and cools slowly to produce coarse-grained rock. Hundreds of such intrusions must be located along the length of the world’s ocean ridges, and periodically basalt erupts from them onto the ocean floor, building low, ephemeral volcanoes.

VI. THE ORIGIN AND EVOLUTION OF MAGMA IN THE EARTH—KEY PLUTONIC PROCESSES A. Bowen’s Hypothesis The Earth’s continental crust is made predominantly of gabbro (dominating the lower crust) and granitoid rocks (i.e., granite + granodiorite + diorite) (concentrated in the upper crust).5 In 1915 N. L. Bowen proposed that gabbro is the only magma produced by melting of rock and that all other magma types derive from it by processes of “crystallization differentiation,” that is, crystal-liquid fractionation, during cooling of a pluton, and notably by crystal settling (see below). He considered the common magmas to be linked in the sequence gabbro → diorite → granodiorite → granite by progressive fractionation of crystals from them, with gabbroic magma as 5 The oceanic crust is made of ultramafic rocks, gabbro, and basalt, but next to no granitoid rocks.

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the ultimate progenitor. Through high-temperature experiments with simplified rock melts Bowen demonstrated that this sequence of liquids could be expected in nature. In 1928 he suggested that gabbroic magma is created by partial melting of the ultramafic rock peridotite in the mantle. Forty years later this idea became testable when laboratory apparatus capable of reaching the pressures and temperatures of the upper mantle was constructed and peridotite partial melting experiments were conducted. Rapid cooling of the products of experiments quenches the melt to glass which can be chemically analyzed with the electron probe microanalyzer. As Bowen had predicted, gabbroic melt is indeed produced in this manner. The experiments also reveal that the melt formed spreads over all the crystal boundaries creating an interconnected network of melt in the partially molten rock. This is an unstable situation, however, because of the weight of overlying rock pressing down on the crystal– melt mixture. Compaction drives the melt out from between the crystals, like water squeezed from a sponge, and because it is less dense than the crystals the melt escapes upward (Fig. 1b) Separate aliquots of melt aggregate and ascend into the crust, either as a discrete mass (diapir) or in fissures that may feed an intrusion. Layered Plutons and the Evidence for Magmatic Differentiation by Fractional Crystallization While some intrusions are homogeneous masses of rock, many are not. In addition to xenoliths that may be present, some contain two or more rock types in which bulbous masses of one in the other and lobate contacts between the two rock types indicate that two magmas were emplaced in the pluton but failed to blend before solidifying. Inhomogeneity in the form of layers of rock characterizes another, small group of plutons, mostly gabbroic and ultramafic ones, which are generally of lopolithic/funnel form. The layers are broadly conformable with the floor of the intrusion, range in thickness from a few millimeters to tens of meters, and vary in extent from a few centimeters to many kilometers. Layers may differ in texture and/or in relative proportions of the minerals making up the rock. The differences between layers may be striking or subtle and contacts between layers can be knife sharp or gradational (Fig. 9). Repeated patterns of layers suggest that a cycle of repeated conditions caused the layers to form. Layer contacts are usually planar but may show erosion hollows in an underlying layer that have been infilled by the overlying layer. A small number of such intrusions were known to petrologists in the 1920s and Bowen considered them to

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FIGURE 9 Layering in the Skaergaard intrusion. [Photo reproduced with permission from original account of the geology by Wager, L. R., and Deer, W. A. (1939). “Geological Investigations in East Greenland, Part III, The Petrology of the Skaergaard Intrusion, Kangerdlugssuaq, East Greenland,” Medd. Grønland 105(4), Plate 8, Fig. 1.]

support his belief that crystals can settle in magmas during prolonged cooling of a pluton. This view was strengthened in the 1930s with the discovery in eastern Greenland of a spectacularly layered gabbroic pluton, the Skaergaard intrusion. Its discoverer, British petrologist Lawrence Wager, concluded that successive crops of crystals from the cooling magma were carried to the floor regions of the intrusion by magmatic currents, and as the current crossed the floor, crystals denser than the surrounding melt settled to build up a precipitate. Since dark minerals are denser than light ones (e.g., pyroxene is ca. 30% denser than plagioclase), they were assumed to settle more rapidly to build a layer dominated by dark minerals, while the slower settling, lighter colored crystals would subsequently accumulate to create a color-contrasted layer. This crystal-sorting mechanism leant support to Bowen’s argument that crystal settling is the principal cause of magma differentiation. Layered intrusions, and especially the Skaergaard, have been subject to detailed scrutiny and assessment of the physical and chemical processes causing layering. Opinion is divided about whether crystals indeed have settled in individual intrusions to create layering. An alternative view holds that all crystals have grown in situ and have experienced no transport and no segregation from one another during crystallization of the magma body. This opin-

ion considers that the layering has a chemical origin, with episodes in which crystallization of dark-colored minerals oscillate with ones in which light-colored minerals crystallize, and with others in which both light and dark minerals can crystallize in situ. Other explanations of layering include: r The possibility that during the late stages of

crystallization, movement of nearly consolidated rock, with a few percent of interstitial melt, can cause segregation of melt into layers. r The migration of films of melt through the crystal–liquid mush can bring about recrystallization of the crystals by a solution-precipitation-like mechanism, with local movement of constituents to produce layers enriched in one or more minerals. Analysis of the compositions of minerals in layered plutons reveals that they are of high-temperature types in the lower reaches of an intrusion and become of progressively lower temperature types upward. So regardless of whether or not crystals settled in these plutons as Bowen supposed, some process causing fractional crystallization of magma undoubtedly took place. Layered plutons are sites of unequivocal magmatic processing where new compositions of magma are created

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origin of granite, such that most geologists would now accept that a majority of granitoid plutons are products of partial melting and only a minority form by extensive gabbroic magma differentiation, and even these generally involve some assimilation of crustal rocks as an additional process of differentiation.

B. The Problem of Granite Magma Formation Bowen’s gabbro →granite differentiation scheme predicts that the amount of melt remaining in a pluton during solidification progressively decreases such that 100 units of original gabbro magma can yield ca. 10 units of diorite and just 1 unit of granite magma. Critics of the scheme pointed to the relative amounts of gabbro and granite in the intrusions that make up the continental crust, ca. 66:34, to make the point that while some granite may be created by extreme fractional crystallization of gabbroic magma, this cannot be the only mechanism by which it is generated. From his experience as a field geologist mapping in the Canadian Shield in 1908–1911, Bowen was acutely aware of this weakness in his hypothesis but never found a different explanation of the “surplus” of granite. Ironically, it was another program of crystal–melt experiments conducted in the last years of his life that eventually provided the solution. These experiments were conducted by O. F. Tuttle and Bowen in the early 1950s and published in 1958, shortly after Bowen’s death. Their purpose was to test the transformism hypothesis of the origin of granite (see Section IV.C.1). Tuttle and Bowen found that a very narrow range of compositions of melt can coexist in equilibrium with quartz and feldspar, the principal minerals of granite, and demonstrated that the compositions of granites from around the world match those of the experimental melts. Since a transformist origin of granite would produce rock compositions that did not involve crystal–melt equilibria, it was clear that granites are normally magmatic rocks. Just as significantly, in his account and discussion of their experimental findings, Tuttle pointed out that the experiments were consistent with two possible magmatic origins of granite magma: r That of Bowen, whereby a granite melt, able to

crystallize quartz and feldspars, is the end product of extreme fractional crystallization of gabbroic magma. r By the partial melting of any rock which contains quartz and feldspars. Subsequent laboratory testing of the second mechanism showed that a wide range of continental crustal rocks, including sedimentary, metamorphic, and igneous types, partially melt to produce granitoid liquid. Detailed geochemical investigations of individual granite plutons have now demonstrated the validity of Tuttle’s partial melting

C. Granite and Orogenic Belts Granite is most abundantly preserved in batholiths in orogenic belts, linear, arcuate zones on a regional scale that have undergone compressional tectonics causing deformation and metamorphism of the rocks and production of a fold mountain belt (e.g., the Alps). In ancient orogenic belts that have been deeply eroded, removing 15–30 km of rock, the roots of orogenic belts can be examined. In the katazone of Buddington, for example, granite is seen to intimately vein the plastically deformed banded gneisses, to form pods in the gneisses, and, where particularly abundant, to enclose blocks of gneiss. These mixed igneous– metamorphic rocks are known as migmatites. They have been variously assigned to the action of fluids permeating the host rock (granitization), to magma intrusion (injection gneiss), and to partial melting of the gneiss with incomplete segregation of the melt. Most geologists regard the third interpretation as the usual origin of migmatite. The importance of migmatites is that they preserve evidence of crustal rocks that underwent ultrametamorphism to the point of partial melting deep in the earth’s crust, in support of Tuttle’s second mechanism of granite creation. Modern orogenic belts develop at convergent plate margins, where a continental margin on one plate is undercut by oceanic lithosphere of a second plate that is descending into the earth. Magma is produced in this setting by: r Partial melting of the subducting plate, including some

ocean-floor sediment that is carried into the earth, to create basalt/andesite. r Heating of the subducting plate causing decomposition of hydrous minerals (clays, amphiboles, and micas), with the fluid migrating upward into the overlying mantle triggering partial melting by lowering the temperature needed for fusion; Basalt magma is the product. r Basaltic magmas created in the mantle rising into the base of the continental crust, passing heat to the surrounding rocks and raising their temperature to values at which crustal rocks can partially melt to create granitoid magmas. This new magma either mixes with the mantle-derived magmas or moves away as a discrete magma body. Differentiation of the basaltic magmas by fractional crystallization produces yet more granitoid magma.

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This is the situation at present in the Cordilleran mountains that extend parallel to the west coasts of North and South Americas and in the local development of chains of stratovolcanoes, including the Cascades and the Andes. Subduction and magma production have been operating here for millions of years and erosion has exposed batholiths and their constituent plutons. Detailed mapping of groups of plutons shows various intrusion types, a variety of rock types ranging from gabbro to granite, and cross-cutting relationships that indicate the relative ages of intrusions. The Himalayas represent an additional complication in which the subducting oceanic plate carries a continent, India. When that northward-moving continent met the Asian continent, its leading edge pushed under the Asian plate. The consequence of placing continental crust beneath continental crust has been to cause major uplift of both the Himalayas and the Tibetan Plateau. It has also resulted in the creation of granite magma by partial melting of the sunken Indian crust and the overlying Asian crust. Some of the resultant plutons created are already exposed by the rapid erosion of the Himalayas that has accompanied their spectacular uplift. It is now widely believed that magmatism in orogenic belts/subduction zones6 is a key crust-building process at the present day and probably has been for the last 2.500 million years.7 This continental crust is the product of a series of chemical refining processes which have ensured that magmas emplaced above subduction zones include high-density mafic ones (gabbroic) along with intermediate (dioritic) and acid (granite) ones. By contrast, in the other main region of magmatism, the mid-ocean ridge, it is almost exclusively “unrefined” gabbroic magma that is emplaced.

VII. ECONOMIC USES OF PLUTONIC ROCKS The interlocking texture and consequent hard nature of plutonic rocks, together with their great strength in compression, has made them ideal for use in the construction industry. Many plutonic rocks have been employed as building stones, often with the external surface polished to add to their visual appeal. In the past these were used as integral, load-bearing building blocks [e.g., the old London Bridge (now rebuilt at Lake Havasu City in Arizona), which was quarried from the Ross of Mull granite intrusion of western Mull, Scotland]. In modern construction, 6 Together with the accretion of island arcs when these collide with a continent at a subduction zone. 7 The origin of continental crust formed prior to this (ca. 60%) remains conjectural.

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507 polished rock is more likely to be used in the form of 1-m2 , 1- to 3-cm-thick plates that clad the exterior of a building. Very large but unshaped quarry blocks dumped along a coastline are widely used to slow erosion. They are also used to line and protect the banks of major rivers that are prone to breaching during flooding. Crushed plutonic rock is also widely used as aggregate in the manufacture of concrete and is a major source of roadstone. Polished granite and gabbro in various colours are two popular choices for gravestones, in part because of their durability, but also because of their resistance to acid rain in urban settings. A minority of pegmatites provide economic concentrations of exotic minerals and low-abundance chemical elements. Unusually high abundances of certain exotic minerals (e.g., beryl, topaz, spodumene, and tourmaline) and exceptional concentrations of elements that are usually scarce in rock (e.g., of Li, B, Be, Nb, Ta, Cs, U, and rareearth elements) may be worth mining. The principal localities where rare-metal pegmatites are mined are Tanco in Manitoba, Canada, several sites in South Carolina, and Greenbushes in Queensland, Australia. Some very large granite plutons have small but significant concentrations of minerals that make for vast mineral deposits. Examples include the copper, tin, and silver ore deposits found in granite stocks in the Sierra Nevada of the western United States and in the Andes of Bolivia. Here concentrations of hundreds to several thousands of parts per million are enough to make it economically feasible to process the vast volumes of granite needed. The element concentrations in such granites are due to former hydrothermal circulation of fluids through a pluton as it cooled. The fluid dissolved the element concerned from either the country rock or from a deeper level of the pluton and carried it through the pluton to shallower and cooler regions where supersaturation-induced precipitation left it as a secondary mineral in cracks and fractures in the rock. Particularly extensive alteration of granite plutons by circulating acidic fluids can result in dissolution of quartz and conversion of the remaining feldspar to the clay mineral kaolinite. Very pure kaolin deposits, e.g., in southwest England, central Germany, and in the Georgia–South Carolina clay belt in southeastern United States, are exploited for ceramics and china production. In contrast, a few gabbro and ultramafic plutons host economic deposits of minerals that are of primary origin. Foremost among these are the chromite (iron + chromium oxide) and magnetite (iron oxide) deposits found as discrete layers, 1–5 cm thick, in a number of intrusions, including the huge Bushveld layered gabbro of South Africa (9 km thick and over 100 km in extent) and the Stillwater intrusion of Montana. It is widely believed that because of the high density of chromite and magnetite crystals

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508 (ca. 5.1–5.2 g cm−3 ) compared to the basaltic magma from which they grew (ca. 2.7 g cm−3 ), they would have sunk quickly through the magma to concentrate as a deposit on the intrusion floor. The Bushveld pluton is also host to concentrations of small grains of Pt, Pd, and Rh, and of Ni and Cu sulfides for the same reason. This simple chemical process of element concentration, which occurred about 2000 million years ago in the Bushveld pluton, is a major source of South Africa’s mineral wealth.

SEE ALSO THE FOLLOWING ARTICLES CONTINENTAL CRUST • CRYSTALLIZATION PROCESSES • GEOLOGY, EARTHQUAKE • HEAT FLOW • IGNEOUS GEO-

Plutonic Geology

• MINERAL PROCESSING • PLATE TECTONICS • ROCK MECHANICS • VOLCANOLOGY

LOGY

BIBLIOGRAPHY Bowes, D. R. (ed.) (1989). “The Encyclopedia of Igneous and Metamorphic Petrology,” Van Nostrand Reinhold, New York. Dietrich, R. V., and Skinner, B. J. (1979). “Rocks and Minerals,” Wiley, New York. Duff, D. (1993). “Holmes’ Principles of Physical Geology,” 4th ed., Chapman and Hall, London. Lamb, S., and Sington, D. (1998). “Earth Story. The Shaping of Our World,” BBC, London. Le Maitre, R. W., et al. (1989). “A Classification of Igneous Rocks and Glossary of Terms,” Blackwell, Oxford. Press, F., and Siever, R. (1998). “Understanding Earth,” Freeman, New York. Raymond, L. A. (1995). “Petrology,” Wm. C. Brown, Dubuque, IA.