Contact metamorphism

Contact metamorphism is caused by the temperature rise in rocks adjacent to magmatic intrusions of local extent that penetrate relatively shallow, cold regions of the earth’s crust. Many economically important metallic mineral deposits occur in contact metamorphic zones.

Contact Metamorphic Facies

Contact metamorphic rocks are moderately widespread; they occur at or near the earth’s surface, where magmas of all kinds intrude low-temperature rocks. Minerals in contact metamorphic rocks are similar to those in regional metamorphic rocks of comparable metamorphic grade. Contact metamorphic effects are divided into facies (zones), each of which is characterized by a small number of concentric indicator mineral rings surrounding an intrusive rock. Nonsymmetrical zones imply special conditions, such as less (or more) chemically or thermally reactive rocks or a nonvertical intrusive body.

Development of contact metamorphic facies reflects both the history of pressure and temperature changes and the bulk-rock chemistry. Thus, by stating that a rock belongs to a particular facies, scientists convey much about the rock’s history. This information is vital to exploration for metallic and industrial minerals that commonly occur in contact metamorphic aureoles and for general understanding of regional geology.

Contact metamorphic mineral facies have counterparts with regional metamorphic zones. In addition to the bulk chemical composition of a rock, temperature and pressure are two variable factors. These two factors can be independent. For example, one can find low-pressure, high-temperature facies or, alternatively, high-pressure, low-temperature facies. With contact metamorphism, these facies occur very close to the intrusive rock. With regional metamorphism, however, the effect is widespread and may not be related to an intrusive rock. It can be hard to tell if metamorphism around deep intrusive rocks is contact or regional metamorphism, or a combination of the two.

Aureoles

Contact metamorphic rocks are recognized by their location adjacent to igneous bodies and by evidence indicating a genetic temporal relationship. Contact metamorphic rocks are commonly massive. Granitic rocks are the most common intrusive material. The most frequent depth of the solidification of a granitic magma is 3 kilometers, corresponding to a load pressure of 800 to 2,100 bars (a bar represents approximate atmospheric pressure, or pressure under 10 meters of water). There are intrusions that solidify at a greater or shallower depth; a depth of 1 kilometer corresponds to a load pressure of 250 bars. Consequently, the load pressures effective during contact metamorphism range from 200 to 2,000 bars in most cases. In contrast, load pressures prevailing during regional metamorphism are generally greater.

When a magma intrudes into colder regions, the adjacent rocks are heated. If the heat content of the intruded magma is high and the volume of the magma is not too small, there will be a temperature rise in the bordering rock that lasts long enough to cause mineral reactions to occur. The rocks adjacent to small intrusions of dikes and sills are not metamorphosed (only baked), whereas larger plutonic rocks give rise to a distinct contact aureole of metamorphic rocks. Several zones of increasing temperature are recognized in contact aureoles.

The contact metamorphic zones surround the intrusion in generally concentric rings that approximate the shape of the intrusion. Those zones that correspond to the highest-temperature minerals are closest to the intrusion; the zone corresponding to the lowest temperature is located farthest from the intrusion. The lowest-temperature contact metamorphic zone gradually grades into unmetamorphosed country rock.

Mineral Development

Contact metamorphic rocks are characteristically massive because of lack of deformation; most are fine-grained except for a special variant called a skarn. Skarns may contain metallic mineralization in sufficient concentration that they can be worked for a profit. Such mineral concentrations are called ores.

Contact metamorphic rocks lack the schistosity or platy texture characteristic of most metamorphic rocks. The very fine-grained, splintery varieties are called hornfels. The large metamorphic gradient, decreasing from the hot intrusive contact to the unaltered country rock, gives rise to zones of metamorphic rocks differing markedly in mineral constituents. The intensity and mineral assemblage in contact metamorphic zones depend on several factors: the chemical composition of the intrusive rock; its temperature and volatile content; the composition and permeability of the host units; the structural or spatial relationship of the reactive units to intense contact effects and solutions conduits, or traps; and the pressure or depth of burial.

Argillaceous (clay rich) limestone is usually susceptible to contact metamorphism, as its diverse rock chemistry allows mineral development over a broad set of physical and chemical conditions. A typical contact metamorphic mineral assemblage formed from rocks originally of this composition is a plagioclase-garnet-epidote rock. This kind of rock frequently hosts important ore deposits. Shale usually converts to hornfels, which is characterized by the minerals cordierite, biotite, and chlorite and is essentially nonreactive and nonpermeable. This lithology usually does not host ore deposits. Porous limestone is often exceptionally susceptible to solution and to replacement by ore. Rocks formed from this material contain magnetite, garnet, and pyroxene.

Pure, massive limestone has only thin, contact metamorphic zones developed in it. Pure limestone merely recrystallizes, forming coarse-grained marble in the highest-temperature contact metamorphic zones adjacent to an intrusion. Dolomite is usually poorly mineralized and nonreactive. Siliceous dolomite, however, may act as a good host, with assemblages characterized by tremolite, diopside, serpentine, and talc. Mafic rocks, such as andesites, diabases, and diorites, can be reactive hosts capable of producing some types of ore deposits. Secondary biotite is the key alteration mineral in mafic rocks and may be associated with ore. The biotite zone may be very broad.

Diagnostic Minerals

Contact metamorphic facies are identified by the mineral assemblages that are developed in the metamorphosed rocks. From lowest to highest temperature, the contact metamorphic facies are called albite-epidote, hornblende hornfels, pyroxene hornfels, and sanidinite.

Albite and chlorite are the contact metamorphic minerals restricted to the albite-epidote hornfels facies. Calcite, epidote, and talc also occur in this facies. Andalusite may occur in the highest-temperature part of the albite-epidote hornfels.

The amphiboles anthophyllite and cummingtonite are restricted to the hornblende hornfels facies. Muscovite is present in this facies as well as in the albite-epidote hornfels facies, and sometimes in the lowermost pyroxene hornfels facies. Grossular-andradite garnet and idocrase, sometimes with biotite and almadine garnet, are present here as well as in the pyroxene hornfels facies. Sillimanite may occur in the highest-temperature part of the facies at higher pressure; staurolite may occur in high-pressure, iron-rich rocks. Calcite is present, but not with tremolite-actinolite, epidote, or plagioclase. Under certain chemical compositions of the original rocks, other minerals that may be present include anthophyllite, cummingtonite, phlogopite, biotite, diaspore, and scapolite.

Orthoclase with andalusite or sillimanite is restricted to the pyroxene hornfels facies. Sillimanite is present here and also in the upper hornblende hornfels facies. Hyperstine and glass present in the pyroxene hornfels may also be present in the sanidinite facies. Muscovite is present only in the lowest-temperature part of the facies. For silica-deficient rocks in the pyroxene hornfels facies, dolomite, magnesite, and talc may occur only in the lowest-temperature part of the facies or at pressures of high carbon dioxide. The pyroxene hornfels facies is characterized by the breakdown of the last water-bearing minerals, muscovite and amphibole.

Diagnostic minerals of the sanidinite facies are sanidine, mullite, tridymite, and pigeonite. In silica-deficient rocks at this facies, wollastonite, grossularite, and plagioclase are present. Other minerals that may occur under special conditions are perovskite, spinel, diopside, and pseudobrookite. This is the highest- temperature facies of contact metamorphism.

Study and Exploration

Contact metamorphic zones are studied by standard geologic techniques, including the preparation of geologic maps through field study. Aerial photographs and satellite images are frequently used to identify contact metamorphic zones through detecting rock alteration in the contact metamorphic zone. Satellite sensors measure, analyze, and interpret electromagnetic energy reflected from the earth’s surface for subsequent computer analysis. Geophysical techniques (gravity and electrical methods) are used to locate mineralized zones containing relatively heavy metallic ore minerals. The gravity contrast of these zones can be measured with surface instruments and then mapped. Some of these same minerals, particularly the ore minerals, transmit electric current in an anomalous manner. These anomalies plotted on base maps may be an additional clue to the presence of ore deposits in contact metamorphic zones.

Subsequent laboratory work involves the determination of mineral relationships by studying thin slices of rock through which light passes under a microscope (petrography). Frequently, the chemistry of entire rock samples is determined to help scientists understand the presence or absence of minerals as a guide to mineral exploration and to the composition of the original rock. Because of significant advances in laboratory instruments, detailed mineral chemistry analyses are routinely performed that determine chemical makeup of microvolumes of minerals. These determinations permit earth scientists to understand the conditions under which the contact metamorphic zones formed and to interpret the history of the contact metamorphic zone.

This information provides critical insight into the potential for metallic ore deposits within the contact metamorphic zone. Such deposits are explored by means of surveys that detect and record variations in geochemistry, gravity, and plant types and abundances. Anomalously high element concentrations in rocks, soils, and plant tissues may indicate mineral deposits that are not exposed. Many ore deposits have an anomalous gravity signature because of heavy associated silicate minerals and metallic minerals. Maps of localized variations may lead to subsurface exploration, which is conducted by drilling techniques. Continuous cylindrical samples are taken from depths of up to several thousand feet and the study of such subsurface samples leads to the evaluation of possible minable concentrations of certain metals and other elements used by modern civilizations.

Economic and Geologic Value

Contact metamorphic zones frequently contain metallic mineral deposits without which modern civilization could not exist. Many significant mineral deposits worldwide occur in these zones. The metals extracted from deposits of this type include tin, tungsten, copper, molybdenum, uranium, gold, silver, and, in some cases, refractory industrial metals. Specialized surveys assess the potential of metallic ore deposits before expensive subsurface sampling by drilling.

Recognition and understanding of the contact metamorphic environment can lead to a significantly improved understanding of regional geology and the regional geologic history. In some areas, contact metamorphic zones are associated with igneous intrusives of only one geologic age. Specialized laboratory studies provide supporting data to determine the geologic history and potential for economic mineralization.

Principal Terms

aureole: a ring-shaped zone of metamorphic rock surrounding a magmatic intrusion

contact metamorphic facies: zones of contact metamorphic effects, each of which is characterized by a small number of indicator minerals

facies: a part of a rock or group of rocks that differs from the whole formation in one or more properties, such as composition, age, or fossil content

hornfels: the hard, splintery rocks formed by contact metamorphism of sediments and other rocks

lithology: the general physical type of rocks or rock formations

Bibliography

Best, Myron G. Igneous and Metamorphic Petrology. 2d ed. Malden, Mass.: Blackwell Science Ltd., 2003.

Bucher, Kurt, and Martin Frey. Petrogenesis of Metamorphic Rocks. 8th ed. New York: Springer-Verlag Berlin Heidelberg, 2011.

Cheviet, Alban, et al. "Contact Metamorphic Reactions and Fluid-Rock Interactions Related to Magnetic Still Intrusion in the Guayamas Basin." European Journal of Mineralogy, vol. 36, no. 6, 16 Nov. 2023, doi.org/10.5194/ejm-35-987-2023. Accessed 28 July 2024.

"Contact Metamorphism: What is Contact Metamorphism? How It Formed?" Geology Page, 22 May 2019, www.geologypage.com/2019/05/contact-metamorphism.html. Accessed 25 July 2024.

Deer, W. A., and R. A. Howie. An Introduction to Rock-Forming Minerals. 3rd ed. Mineralogical Society of Great Britain and Ireland, 2013.

Fettes, Douglas, and Jacqueline Desmons, eds. Metamorphic Rocks: A Classification and Glossary of Terms. New York: Cambridge University Press, 2007.

Hyndman, D. E. Petrology of Igneous and Metamorphic Rocks. 2d ed. New York: McGraw-Hill, 1985.

Oldershaw, Cally. Rocks and Minerals. New York: DK, 1999.

Perchuk, L. L., ed. Progress in Metamorphic and Magmatic Petrology. New York: Cambridge University Press, 1991.

Philpotts, Anthony, and Jay Aque. Principles of Igneous and Metamorphic Petrology. 2d ed. New York: Cambridge University Press, 2009.

Pough, F. H. A Field Guide to Rocks and Minerals. 5th ed. Boston: Houghton Mifflin, 1998.

Sorrella, C. A. Minerals of the World. Racine, Wis.: Western Publishing, 1973.

Vernon, Ron H., and Geoffrey Clarke. Principles of Metamorphic Petrology. New York: Cambridge University Press, 2008.

Voll, Gerhard, ed. Equilibrium and Kinetics in Contact Metamorphism: The Ballachulish Igneous Complex and Its Aureole. New York: Springer-Verlag, 1991.

Winter, John D. An Introduction to Igneous and Metamorphic Petrology. Upper Saddle River, N.J.: Prentice Hall, 2001.