Contact Metamorphism


by Rex Biggers - Date: 2006-12-17 - Word Count: 2740 Share This!

Here's an old paper I had kicking around on the computer. The topic is contact metamorphism, a sort of dry and lifeless topic but it might be possible to apply a little of the information to detecting when it comes to when, how and under what conditions minerals form in these environments. It's possible that there might be some weird formatting issues since this was originally in wordperfect, then switched to a text file and then to microsoft word. If you get something out of it, then enjoy. I'd forgotten some of the stuff in this paper myself.

Rex

You can download a Microsoft Word version double-spaced with paragraphs below for the time being: http://www.verzend.be/v/9171867/geo_cont_meta.doc.html or select the text above and put it in a program such as Microsoft Word and double space it for easier reading.

INTRODUCTION

The topic of contact metamorphism is very broad, thus, this report will only focus on the factors which affect the process of contact metamorphism and review the various types of contact metamorphism. The igneous intrusion near Horseshoe Canyon in Idaho will also be considered in this report. The factors involved within the process of contact metamorphism include: the temperature of the igneous intrusion, the water content at the intrusion, the variety of countryrock, and the size of the intrusion. The process of contact metamorphism occurs mainly due to the high temperatures of intrusive igneous bodies which surround local countryrock. The exertion of pressure is a minimal factor in contact metamorphism compared to regionally metamorphosed rocks.

FACTORS IN CONTACT METAMORPHISM

The temperature of the intrusion is one principal factor in determining a specific metamorphic facies. For example, temperatures at the intrusive contact as well as that of the surrounding countryrock are partially responsible for the extent of metamorphic aureoles. They also help determine general mineralogical composition at the site of contact metamorphism (Malamed, et al. 648). The duration of an elevated temperature also affects the size of the metamorphic aureole and the resulting mineral assemblage of the intrusion. In fact, it appears that for contact metamorphism at greater depths, each increase of 100 degrees Celsius in the magma, will increase the size of the aureole by fifteen percent (Malamed et al. 650).

Higher temperatures also usually help facilitate rapid nucleation of new minerals, thus, producing faster crystal growth (Con Gillen 29). Water content at the intrusion also significantly influences contact metamorphism evolution. In fact, water is the most important crustal fluid to affect contact metamorphic aureoles (Kerrick 49, 10). Water sources may be "internally derived" from the hydrated countryrock or from within the magma itself (Harte et al. 421). The structure of water facilitates the dissolution of compounds (Kerrick 49), and it also affects the rate of recrystallization (Con Gillen 31).

Even though water plays such an important role in contact metamorphism, more often than not, contact metamorphism tends to be isochemical in nature. Therefore, most contact metamorphism transfers heat by way of conduction and not convection in fluid (Melamed et al. 650). However, when isochemical metamorphism occurs, there is no great alteration in the chemistry of the rock formation in contact with the pluton (Kerrick 9). Therefore, if the bulk of the rock is to alter in chemistry it is important that there be a way for the proper percolation of water. The heat transfer by conduction is probably still within water saturated rocks. Yet, these rocks only allow a little or no percolation due to its' impermeability or inherent limited fracture systems (Melamed et al. 650). However, when fluids freely percolate, calculations show that the heating of rock overwhelmingly occurs by conduction (Melamed et al. 650). When water percolates, it may help alter the equilibrium assemblages and the composition of rock in advanced stages. This advanced stage of rock compositional replacement is a process known as metasomatism (Harte et al. 406).

It must also be noted that water is an important factor in the formation of ore deposits at areas of contact metamorphism. This is because skarns, which are the host rocks for ore deposits, are caused by metasomatism (Kerrick 9). Skarns form by metasomatism between intrusives and carbonate rocks. Water may also allow for hydrothermal vents to form which contain ore deposits in areas of contact metamorphism.

The countryrock around the igneous intrusion is another important factor which determines the formation of localized minerals that occur as a consequence of contact metamorphism. Countryrock may be divided into three generalized categories. They are: basites, elites, and carbonate deposits (Reverdatto 1226). Basites include argillaceous sediments such as shales, mudstones (Con Gillan 47), and tuffaceous rocks (Reverdatto 1227). Elites include granitoids while carbonate deposits include various dolomites, marls, and limestones (Reverdatto 1227).

First, we will examine the effects of argillaceous sediments within the countryrock on contact metamorphism. Argillaceous sediments such as shale and mudstone are composed of very small sediments, therefore, the grains have a large surface area. Since chemical reactions take place mainly at these boundaries the reactions become more rapid in fine grained shale and mudstone (Con Gillen 48). This is especially true in the presence of water (Con Gillen 48). Furthermore, shales and mudstones inherently contain water in their mineral composition. This further increases the possible reaction efficiency to form new minerals (Con Gillen 47). Shales and mudstones also contain a variety of mineral grains which are decomposed products of feldspar, mica, pyroxene, olivine, iron ore, organic matter, and quartz. These minerals contain compounds such as: sulfides, sulfates, chlorides, chlorine, carbon dioxide, carbon, etc. (Con Gillen 47). This assemblage of decomposed minerals and chemical compounds in shale and mudstone creates a wide array of possible new mineral which may result from this metamorphosed argillaceous sediment (Con Gillen 47).

Carbonate rocks also have an effect on the mineral types that result from contact metamorphism. Carbonate rocks metamorphose to form marbles When carbonate rocks are thermally altered they also form skarns (Con Gillen 53). Impurities in carbonate rocks may also react to form other minerals such as periclase, or wollastonite (Con Gillen 53). The minerals which generally indicate the grade of metamorphism from low to high in this type of countryrock are: talc, amphibole, pyroxene, olivine, periclase, and wollastonite (Con Gillen 55).

The intrusion near Horseshoe Canyon is a good example of carbonate rocks coming into contact with a pluton. The intrusion near Horseshoe Canyon has been dated as occurring during the Eocene epoch approximately 45 million years ago (Kowallis). The Jefferson Dolomite of this region was metamorphosed to form marble (Kowallis). The organic material in the Jefferson dolomite may have contributed to the variety of minerals found at this site of contact metamorphism.

The intrusion near Horseshoe Canyon may also contain ore deposits if the resulting mineralogical compositions would not allow ores such as copper or tungsten, etc. to bind with them (Kowallis). If these metallic ores could not be incorporated in the mineralogical assemblage they are expelled into fissures within the countryrock. Also, if the intrusion near Horseshoe Canyon is a skarn (which forms from metasomatism or from the significant elemental replacement of carbonate rocks) (Kerrick 9), the area surrounding the intrusion would be even more likely to include ore deposits since skarns characteristically allow the movement of fluids during contact metamorphism. The pluton near Horseshoe Canyon does conform to the criteria for contact carbonate rocks which are an ingredient in skarns, since they are present at this site of contact metamorphism. However, this report does not find sufficient evidence to label the intrusion near Horseshoe Canyon as producing the formation of a skarn.

Pluton size is another factor which also affects contact metamorphism. A large pluton helps to form a contact aureole of metamorphosed countryrock (Con Gillen 47), while a very thin dike or sill in contact with the countryrock will not allow mineral reactions to take place. Instead, the rocks in contact with the pluton will only become more hardened and baked by the heat (Con Gillen 46).

TYPES OF METAMORPHISM

There are six types of contact metamorphism (Malamed, Reverdatto, Sharapov 648). The first four types form at shallow depths of 8-10 kilometers and occur within the low pressure of 2-3 kilobars (Reverdatto 1226). The last two facies occur at greater depths with increased pressures (Malamed, Reverdatto, Sharapov 648).

Type One Metamorphism

The first type of contact metamorphism includes the Spurrite-merwinite or pyroxene-hornfel facies. It is a high temperature, single facies type of contact metamorphism (Reverdatto 1225). It is usually has an intrusive body composed of dolerites and basalts. The dimensions of their resulting metamorphic aureoles are only a few meters to some tens of meters across (Reverdatto 1226). This type of metamorphism occurs in sill, stocks, dikes and other such subvolcanic bodies (Reverdatto 1227).

Even though five of the types of contact metamorphism can be explained by quiet cooling of magma and crystallization, the first type of metamorphism cannot be explained by this origin, since temperatures often exceed 900 Celsius (Malamed et al. 649). Such high temperatures may indicate a certain velocity which forms a "heat wave"(Malamed et al. 649). This hypothesis may explain why this type of metamorphism occurs within intrusive bodies of basic or gabbroid magma (Reverdatto 21). However, gabbroid magma is already a hotter magma, so it may be inferred that this alone may be sufficient to cause a 900 degree Celsius heat source. This level of contact metamorphism also requires a constant source of intense heat (Reverdatto 90).

Type Two Metamorphism

Type two metamorphism is a high temperature contact metamorphism which is slightly below type one metamorphism in regard to temperature. It includes three possible mineralogical facies. These facies include: pyroxene-hornfels, amphibole-hornfels, and muscovite hornfels. The type two variety of contact metamorphism results from gabbroid and dioritic stocks, thick dikes, lopoliths, and thick sills (Reverdatto 1227). Type two metamorphism is associated more than 65-70 percent of the time with gabbros and other intermediate intrusive rocks (Reverdatto 1225).

The maximum temperature for this type of contact metamorphism is 1700 degrees Celsius while 700 degrees Celsius is the approximate minimal temperature for its' formation (Reverdatto 92). The size of their aureoles are dependant upon the size and temperature of the intrusion. They extend hundreds of meters up to 2 kilometers (Reverdatto 1226). Type two metamorphism does not require a steady transfer of heat unlike type one contact metamorphism. Instead, it forms due to transient heat at the exocontact of the intrusion (Reverdatto 90).

Type Three Metamorphism

Type three metamorphism is composed of amphibole-hornfels, and muscovite-hornfels (Reverdatto 1226). It is a medium temperature three facies type (Reverdatto 1225). Approximately eighty percent of the intrusions associated with this type of contact metamorphism are granitic in origin (Reverdatto 1225). Type three contact metamorphism is formed by medium to larger sized plutons which develop aureoles of hundreds of meters up to 3 kilometers in diameter (Reverdatto 1226). The minimal required temperature for this type of contact metamorphism is 550-600 degrees Celsius (Reverdatto 195). This minimal temperature is requisite to melt the country rock at the given parameters of pressure within this metamorphic facies.

Type three contact metamorphism is particularly interesting because it appears to correspond with the metamorphism occurring at the pluton near Horseshoe Canyon. Some minerals listed as occurring in type three metamorphism include: magnetite, plagioclase, tremolite, diopside, garnet, etc. (Reverdatto 181). Each of these minerals were also observed or are known to occur at the contact area of the pluton occurring near Horseshoe Canyon. In fact, Grossular garnet which occurs at this site, has a optimal temperature formation range of 500-700 degrees Celsius (Reverdatto 223). This temperature range also corresponds to the temperatures requisite for type three metamorphism.

The association of minerals grouped above also are diagnostic of a type three metamorphic intrusion that is in contact with adjacent carbonate rocks. The intrusion near Horseshoe Canyon is also an example of an intrusion which does come into contact with carbonate rocks. The contact of the intrusion near Horseshoe Canyon with carbonate rocks provides calcium for the formation of minerals such as diopside. Diopside is a pyroxene which is composed of calcium and magnesium (Kowallis).

The tremolite, a white amphibole, which formed at the pluton near Horseshoe Canyon, may have formed at lower temperatures than the diopside; since diopside it is a pyroxene that forms at higher temperatures (Reverdatto 179). These two minerals constitute the separation of a subfacies in the amphibole-hornfels facies (Reverdatto 179) and both are found at the igneous intrusion near Horseshoe Canyon.

In this type of low pressure environment Almandine garnet will form if the country rock is composed of metapelitic hornfelses (Reverdatto 181). The pluton near Horseshoe Canyon is not in contact with metapelitic hornfelses, therefore almandine garnet was not observed as an accessory mineral at this site of contact metamorphism. Some additional accessory minerals associated with this facies include forsterite, diopside, and spinel (Reverdatto 181).

Type Four Metamorphism

Type four metamorphism formation is not dependent upon magma composition but rather the size of the intrusive body and the rapid cooling of the magma near the surface. Even though type four contact metamorphism is not dependant upon magma composition, this type of contact metamorphism chiefly occurs with granitoids and not gabbroid magma (Reverdatto 1225). This type of metamorphism is a low temperature single facies type composed of muscovite-hornfels (Reverdatto 1225). Their aureole sizes are much smaller than those contained in the previously discussed contact metamorphism types. They range in size from centimeters and reach rarely up to one meter across (Reverdatto 1226).

The last two types of contact metamorphism only result from granitoid intrusions (Reverdatto 1226). They also occur at increased pressures and therefore are called contact metamorphism only because an igneous intrusion is still a the primary catalyst in the process of this type of metamorphism (Melamed et al. 648 ).

Type Five Metamorphism

Type five metamorphism is composed of a complex of regional metamorphism at moderate pressures (Reverdatto 1225). It is a "plutonometamorphosed" or contact-regional metamorphic type (Reverdatto 1226). Type five contact metamorphism aureoles are several kilometers up to ten kilometers in diameter (Reverdatto 1226).

Type Six Metamorphism

The sixth type of contact metamorphism has a facies of biotite-sillimanite gneisses, muscovite-staurolite and kyanite schists of regional metamorphism (Reverdatto 1226). It is a type of "prezometamorphosed" rock formed under moderately low temperatures and locally increased pressure (Reverdatto 1225). Higher temperature contact metamorphism would alter these minerals forming other minerals such as andalusite and cordierite (Ferguson and Al-Ameen 506).

Type six metamorphism has the lowest temperature and in fields of low pressure the threshold for its' metamorphism is about 450-500 Celsius (Reverdatto 216). These very low metamorphic temperatures also prevent rapid mineral growth (Reverdatto 217). At this metamorphic phase almandine garnet does not occur but instead is replaced by chlorite, quartz, and magnetite (Reverdatto 217). Even though type six metamorphism has relatively low temperatures the aureoles within it are up to three kilometers in width and they can be even broader in some instances Reverdatto 1226).

Conclusion

The temperature and pressure of the intrusion during contact metamorphism may be considered together as the most important factors affecting metamorphic grade. These factors help determine mineral facies as well as aureole size. Water content as well as the surrounding countryrock also are important variables within the process of contact metamorphism. They affect mineral dissolution and mineral recrystallization. Water also contributes to the formation of hydrothermal vents containing ore deposits.

Argillaceous sediments may increase the quantity of minerals in an aureole and the formation of ore deposits. Water supply, temperature, and argillaceous sediment all contribute to the rapid formation of minerals. The interpretation of the pluton near Horseshoe Canyon is that of an intrusion displaying minerals within the amphibole-hornfel facies or a type three contact metamorphic pluton. By analyzing the local minerals and conditions required for their formation as well as the size of an aureole and the structure of a pluton metamorphic grade can be successfully determined by the geologist.

REFERENCES

Con Gillen, C., Metamorphic Geology, George Allen and Unwin , Boston, 1982.

Ferguson, C.C., and S.I. Al-Ameen, Contact metamorphism in Omey granite aureole, Mineral. Mag., 49, 509, 1985.

Harte, B., D.R.M. Pattison, S. Heus-Albichler, S. Hoernes, L. Masch, and S. Weiss, Evidence of fluid phase behaviour and controls in the intrusive complex and its aureole, Equilibrium and Kinetics an Contact Metamorphism: The Ballachulish Igneous Complex and its Aureole, Springer-Verlag, New York, 1991.

Kerrick, D.M., Overview of Contact Metamorphism, Contact Metamorphism: Reviews in Mineralogy, 26, 10, 1991.

Labotka, T.C., Chemical and Physical Properties of Fluids, Contact Metamorphism: Reviews in Mineralogy, 26, 49, 1991.

Melamed, V.G., V.V. Reverdatto, and V.N. Sharapov, Factors in contact metamorphism, Internat'l Geo. Rev., 15, 6, 648-651, 1973.

Reverdatto, V.V., Types of Contact Metamorphism, Internat'l Geo. Rev., 13, 8, 1225-1227, 1971.

Reverdatto V.V., The Facies of Contact Metamorphism, D. A. Brown, 1973.

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Related Tags: contact, metamorphism

Rex Biggers
Professor Kowallis
Geology 210
1 October 1999

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