Batholiths

Batholiths are gigantic bodies of granitic rock located in mobile belts surrounding the ancient cores of the continents. The growth of continental crust during the past 2.5 billion years is intimately related to the origin and emplacement of major volumes of granitic magma that solidify as batholiths.

Batholith Formation

Batholiths are large composite masses of granitoid rock formed by numerous individual bodies of magma that have risen from deep source areas in molten form and solidified near enough to the surface to be exposed by erosion. The resulting rocks are relatively coarse-grained in texture and markedly heterogeneous in chemical and mineralogical composition. A well-studied example is the coastal batholith of Peru, which forms an almost continuous outcrop 1,100 kilometers long and 50 kilometers wide along the western flank of the Andes. This enormous body has steep walls and a flat roof. It is composed of more than one thousand individual plutons emplaced along a narrow belt parallel to the present coastline during a volcanic-plutonic event that extended over a period of 70 million years. Many such batholiths are known in the mountainous areas of the world, but few are as large or as magnificently exposed to view as that in Peru. Geological glossaries often define a batholith as a “coarse-textured igneous mass with an exposed surface area in excess of 100 square kilometers.” This description has the virtue of simplicity, but it is misleading; the description encompasses granitic plutons, and even nongranitic plutons, which form under conditions quite removed from those associated with the world’s major batholiths.

In most instances, there is evidence to indicate that the individual plutons of a batholith were emplaced as hot, viscous melts containing suspended crystals. This molten material is called magma. Cooling and crystallization occur during the ascent of magma toward the surface and gradually transform it to solid rock, which prevents further upward movement. The depth at which total solidification occurs varies and is strongly dependent upon the initial temperature and water content of the magma. Extreme levels of ascent, within 3 to 5 kilometers of the surface, are possible only for very hot magmas with very low initial water contents. Most granitic plutons complete their crystallization at depths in the range of 8 to 20 kilometers. The characteristic coarse textures observed in most granitic plutons are the result of slow cooling, which, in turn, implies that the rate of magma ascent is also slow. These traits distinguish plutonic rocks from their volcanic counterparts. As would be expected, the formation of a batholith is a complex and lengthy event that is the sum of the processes responsible for the emplacement of each member pluton. Each member pluton has an individual history involving the generation of magma in the source region, ascent and partial crystallization, the physical displacement of overlying solid rock, chemical interaction with the solid rocks encountered during ascent, and the terminal crystallization phase. Consequently, each member pluton of a batholith can be expected to exhibit a unique combination of textural, mineralogical, and chemical variations.

Mobile Belts

It has long been recognized that major batholiths are confined to narrow zones elongated parallel to present or former continental margins. In such zones, granitic melts intrude either thick sequences of chemically related volcanic rocks or highly deformed and metamorphosed sedimentary rocks. These granite-dominated zones are called mobile belts. The ancient cores of continents are all more than 2.5 billion years old. They are surrounded by mobile belts that become successively younger away from the core. The most recent mobile belts form major mountain chains along continental margins. The resulting age pattern clearly shows that continents grow larger with time by the marginal accretion of mobile belts. In the late 1960’s, the emergence of plate tectonic theory provided a basis for understanding how mobile belts form and are accreted to preexisting continent margins. The impetus provided by this theory sparked intensive study of the world’s mobile belts. These studies amply show that logical time-space relationships exist between plate collisions, deformation styles, and rock types that occur in mobile belts. Two distinct types of mobile belts are now recognized, and each is dominated by granitic batholiths. These batholiths, however, are very different in terms of granitic rock types, modes of pluton emplacement, rock associations, metamorphic effects, and the metallic ores they host. The batholiths of the two mobile belt types are called I-type and S-type batholiths. I-type batholiths are those that seem to have derived mostly from igneous sources—that is, by melting of the mantle—and S-type batholiths seem to have derived from remelting of metamorphic rocks that were originally sedimentary.

Mobile belts along the eastern margin of the Pacific Ocean contain I-type batholiths exclusively. Their size and collective volume is staggering. The Peruvian batholith is an example already mentioned. Others of this type include the Sierra Nevada batholith, the Idaho batholith, and the tremendous Coast Range batholith, which extends from northern Washington to the Alaska-Yukon border. In contrast, the western margin of the Pacific Ocean is dominated by mobile belts with S-type batholiths, although some I-types are also present. The batholiths of Western Europe are also mainly of the S-type. In southeastern Australia, where the two types of batholiths were first recognized, S-type and I-type granitoids form a paired belt parallel to the coastline. Although their geographical distribution is uneven, both I-type and S-type batholiths occur worldwide.

I-Type Batholiths

The most distinctive trait of I-type batholiths is the broad range of granitic rock types they contain. In these batholiths, the rock types gabbro-diorite, quartz diorite-granodiorite, and granite occur in the approximate proportions of 15:50:35. This means that quartz diorite (also called tonalite) and granodiorite jointly compose 50 percent of I-type batholiths, and true granite is a subordinate component in them. This wide compositional spectrum not only characterizes an entire I-type batholith but also is typical of the individual member plutons. Usually the major plutons are concentrically zoned with small central cores of true granite enveloped by extensive zones of granodiorite, which grade outward into margins of quartz diorite. Small plutons in the compositional range of gabbro-diorite are common but subordinate to the zoned granitic bodies. Most member plutons of I-type batholiths have domal or cylindrical shapes and very steep contacts with the surrounding rock. Others may have a steeply titled sheetlike form, but regardless of shape most I-type plutons cut through the preexisting rock layers at a steep angle. The emplacement of these plutons appears to be controlled by near-vertical fractures that may extend downward to the base of the crust. In younger I-type batholiths such as that in Peru, the granitoids have intruded into a roof of chemically related volcanic rocks that show the same compositional spectrum as the granitic plutons. This volcanic pile, dominated by andesite, may be 3 to 5 kilometers thick at the time of pluton emplacement. Gradually, this volcanic roof is stripped away by erosion, so that volcanic rocks are generally absent in older, deeply eroded batholiths. The grade of regional metamorphism in the rocks enclosing I-type batholiths is relatively low, and there is little evidence of large-scale horizontal compression or crustal shortening. Structural displacements and the movements of rising plutons are dominantly vertical and typically occur over a time span of 50 to 100 million years.

S-Type Batholiths

S-type batholiths contrast with I-types in almost every respect. To begin with, the ratio of gabbro-diorite to quartz diorite-granodiorite to granite is 2:18:80 in S-type batholiths. These plutonic complexes are very much dominated by true granite, and gabbro-diorite plutons are rare or absent. In many cases, S-type granites are the distinctive “two mica granites,” which contain both biotite and muscovite, and are frequently associated with major tin and tungsten ore deposits. The batholiths of northern Portugal are typical examples of this association. S-type batholiths, as well as their member plutons, lack the concentric zoning that characterizes I-type plutons. Compositional homogeneity is their trademark. S-type plutons are intruded into thick sequences of regionally metamorphosed sedimentary rocks. The metamorphic grade ranges from moderate to very high, and, frequently, the granites are located within the zone of highest metamorphic grade. In such cases, migmatites are often present. The enclosing metamorphosed rocks are intensely folded in response to marked crustal compression, and volcanic rocks are conspicuously absent. S-type batholiths are smaller in volume and form over a shorter period of time (usually less than 20 million years) than their I-type counterparts.

Differing Origins of Batholith Types

The many contrasting traits of I-type and S-type batholiths are an indication that the conditions of magma generation and emplacement are very different in the mobile belts in which they are found. In the case of I-type batholiths, it appears that magmas are generated at relatively great depths and above the subduction zones that form at destructive plate boundaries. The magmas are derived by partial melting of upper-mantle basic igneous rocks and, perhaps, lower crustal igneous rocks. The melts rise along the steep fractures produced by crustal tension over the subduction zone. The igneous ancestry of these melts is the reason for calling the resulting plutons “I-type.” The hottest and driest of these I-type magmas will reach the surface to produce extensive fields of volcanic rocks and large calderas. In some cases, like the Peruvian batholith, the rise of magma was “passive” in the sense that room was provided for the rising plutons by gravitational subsidence of the overlying roof rock. This is the process of cauldron subsidence. In the case of the Sierra Nevada batholith, however, it appears that I-type magmas were emplaced by “forceful injection.” In this process, rising magma makes room for itself by shouldering aside the surrounding solid rock.

Conversely, the evidence suggests that S-type magmas originate by partial melting of metamorphosed sedimentary rocks. This sedimentary parentage of the magmas is the reason for designating them as “S-type.” Melting is made possible by dehydration of water-bearing minerals under conditions of intense metamorphism. The frequent presence of migmatites (the complex intermingling of igneous and metamorphic rocks) is evidence for this transition from metamorphic conditions to magmatic conditions. The essential requirements for relatively high-level crustal melting are high temperatures, intense horizontal compression to produce deep sedimentary basins, and a thick pile of sediments to fill these basins. Such conditions are best met in back-arc basins, which form at subduction margins but considerably inland from the volcanic-plutonic environment of I-type batholiths. This may explain why some I-type and S-type batholiths occur in paired belts parallel to a continental margin, as in southeastern Australia. The collision environment that arises when continent meets continent in the terminal stage of subduction may also provide suitable conditions for S-type magma production. The magmas that result will be relatively cool and wet and will not be able to rise far above their zone of melting. During this limited ascent, the S-type magmas tend to assume the shape of a light bulb, with a neck tapering down to the zone of melting. Because of their limited capacity for vertical movement, S-type plutons require no special mechanisms to provide additional space for them.

Field Study

The study of a batholith begins with the study of its individual member plutons. This always involves fieldwork, laboratory analysis of rock samples returned from the field, and comparison of the resulting data with those obtained from other batholiths. Because of their great size, batholiths present special problems for field study. The most informative studies are those in mountainous areas, such as the Peruvian Andes, where erosion has exposed the batholith roof contact and cut steep canyons between and through individual plutons. High topographic relief is desirable if the geologist is to learn anything about the variation in shape and composition within the plutons.

Study of even a small portion of a major batholith requires several well-trained geologists working intensively during short field seasons over a period of several years. Geologists traverse on foot across and around the individual plutons as topography permits, and they record the textural, mineralogical, and structural features observed on maps or aerial photographs of the area. These maps eventually reveal the overall shape of plutons and the patterns of concentric zoning within them. Special maps are prepared to show the distribution of fractures and flow structures within individual plutons. These features indicate how fluid the magma was at the time of emplacement. Contacts between the plutons and older enclosing rocks are closely examined for deformation effects and evidence of thermal and chemical interaction with the magma. Fragments of older rock engulfed by magma are often preserved in a recrystallized state, and these are scrutinized carefully, since they provide clues as to whether the emplacement process was passive or forceful. As the end of the field season draws near, the mapped plutons are sampled. Large, fresh samples must be collected from each recognized zone of each pluton for subsequent laboratory study. The number of samples collected from a single pluton depends upon its size and homogeneity but is frequently in the range of one hundred to five hundred samples. A smaller number of samples is collected from the host rocks at varying distances from the plutonic contact in order to study the thermal effects produced by the pluton. The field description, identifying number, and exact location of each sample site must be meticulously recorded. If, at the end of the field season, several plutons have been studied and sampled, there may be several thousand rock samples to label, pack securely, and ship to the laboratory, where they will receive further study.

Laboratory Study

At the laboratory, the samples are usually cut in half and labeled in a permanent fashion. One half of each sample is stored for future reference, and the other is prepared for the laboratory procedures. Paper-thin slices of each sample are glued to glass slides for examination under a petrographic microscope. The microscopist identifies the mineral phases present in each slide and determines the abundance of each. The texture of each rock, as revealed under the microscope, is carefully described and interpreted in terms of crystallization sequence and deformation history. When the microscopic study is complete, certain samples, perhaps fifty to one hundred, are chosen for chemical analysis. Most will be analyzed because they are judged to be representative of major zones of a pluton; a few may be analyzed because they exhibit unusual minerals or some peculiar trait not explained by the microscopic study. If the age of a pluton is not known, a few samples (one to ten) will be shipped to a laboratory that specializes in age determinations by radioisotope methods.

Determining Origin and Emplacement

Finally, on the basis of the field observations, microscopic examinations, and chemical data, the investigators will assemble rival hypotheses or scenarios for the origin and emplacement of the plutons that have been studied. Any scenario that conflicts seriously with known facts is discarded. Those remaining are compared with well-known laboratory melting-crystallization experiments on synthetic and natural rock systems. The size and shape of the plutons, as determined by the field mapping, can be compared with those of “model plutons” derived through sophisticated, but idealized, centrifuge experiments in laboratory settings. The investigators will compare their data in detail with data reported in the geological literature by workers in other parts of the world. They will also compare their results with earlier studies of the same plutons, or studies in the same region, if they exist. As additional plutons are studied in detail, more constraints on the mode of origin and emplacement of the batholith are obtained.

Role in Crustal Growth

Mobile belts, dominated by immense granitic batholiths, have been systematically accreted to the ancient continental cores for the last 2.5 billion years of Earth's history. Modern plate tectonic theory has provided the basis for understanding the periodic nature of mobile belt accretion and the ways in which crustal and mantle materials are recycled. It is evident that the emplacement of batholiths is at present—and has been for at least 2.5 billion years—the major cause for progressive crustal growth. It is also clear that the rate at which batholiths formed during this lengthy period has far exceeded the rate of continental reduction by erosion. The generation of large volumes of granitic magma and its subsequent rise to form batholiths a few kilometers below the crustal surface must be viewed as fundamental to crustal growth. Batholiths play a major role in the formation of mountain systems and are the most important element in the complex rock and metallic ore associations of mobile belts. The very existence of continents is, in fact, a result of the long-standing process of batholith emplacement.

Principal Terms

crystallization: the solidification of molten rock as a result of heat loss; slow heat loss results in the growth of crystals, but rapid heat loss can cause glass to form

granitic/granitoid: descriptive terms for plutonic rock types having quartz and feldspar as major mineral phases

I-type granitoid: granitic rock formed from magma generated by partial melting of igneous rocks in the upper mantle or lowermost crust

magma: molten rock material that crystallizes to form igneous rocks

migmatite: a rock exhibiting both metamorphic and plutonic textural traits

mobile belt: a linear belt of igneous and deformed metamorphic rocks produced by plate collision at a continental margin; relatively young mobile belts form major mountain ranges; synonymous with orogenic belt

partial melting: a process undergone by rocks as their temperature rises and metamorphism occurs; magmas are derived by the partial melting of preexisting rock; also known as ultrametamorphism or anatexis

pluton: a generic term for an igneous body that solidifies well below the earth’s surface; plutonic rocks are coarse-grained because they cool slowly

S-type granitoid: granitic rock formed from magma generated by partial melting of sedimentary rocks within the crust

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