دگرگونی

Metamorphism

Metamorphism is the process whereby the mineralogy, microstructure, and chemistry of rocks are changed due to changing conditions of pressure and temperature. The information contained in metamorphosed rocks thus sheds light on the burial and thermal history of Earth's crust and underlying mantle in response to plate tectonics. Geologists who study metamorphism are concerned with processes at scales that range from atomic (in minerals) to global (interactions between tectonic plates). These processes occur at (1) convergent plate boundaries, where subduction (such as produced the Andes) or continental collision after a phase of subduction (such as produced the Himalayas) takes rocks deep into Earth's mantle during mountain building (orogeny); (2) at plate boundaries that involve predominantly lateral displacement (such as produced the San Andreas fault system); and (3) at divergent plate boundaries, where oceanic crust is metamorphosed immediately after formation by hydrothermal circulation of seawater at high temperature. These diverse tectonic environments generate different, perhaps unique metamorphic signatures in rocks that span a range in temperature from several hundred to more than a thousand degrees Celsius and in depth from near-surface to several hundred kilometers.

Exciting new discoveries in metamorphism have come in part from the development of improved instrumentation and the introduction of new techniques to investigate the composition and structure of Earth materials, and in part from better methods to quantify metamorphic conditions and time scales and the rates and kinetics of processes. Measured time scales have become shorter as the precision in age determination has increased. As a result, rates are now accepted that would have been thought far too fast only a few years ago. On the other hand, the slow kinetics of many metamorphic processes, especially along the decreasing temperature part of the metamorphic cycle during exhumation, which also involves decreasing depth (and, therefore, pressure), means that equilibrium commonly is not achieved. Disequilibrium features can be used to calculate the speed of tectonic processes, using diffusion rates that are calibrated based on laboratory data. The whole metamorphic cycle involving burial (increasing pressure and temperature) and exhumation is called a pressure-temperature-time-deformation (P-T-t-d) path.

During the last decade, much of the attention in metamorphic petrology was focused on the extremes of pressure and temperature apparently recorded by crustal rocks involved in mountain building. The fields of ultrahigh-pressure (UHP) and ultrahigh-temperature (UHT) metamorphism have evolved in the past 15 years. UHP and UHT metamorphism were identified as a direct result of the improved ability to quantify the conditions of metamorphism. UHP metamorphism is related to the subduction of continental materials to great depths during collisional mountain building [greater than 100 km (62 mi) depth, with some arguments to suggest that crustal material may have been buried to >300 km (186 mi) depth in some places]. UHT metamorphism is related to the exhumation from depth of overthickened continental crust that commonly has been invaded extensively by magmas to generate temperatures in excess of 1000°C (1832°F) at lower crustal depths (35–40 km; 22–25 mi). Because of an improved understanding of the extent to which lower crustal rocks have been affected by metamorphism, it is now believed that the deep crust has contained melt for a significant part of the history associated with mountain building and erosion. The presence of melt has been postulated in both the Andes and the Himalayas to explain geophysical observations, and is evidenced in the exposed deep levels of ancient mountain belts as migmatites (Fig. 1) and residual granulites, rocks that preserve the physical evidence of melt flow pathways.

Fig. 1  Migmatite at outcrop. Light material is granite that records the location of melt. From the ancient Acadian mountain belt of west-central Maine.

Alpe Arami enigma

In 1996, a controversial proposal was made that the Alpe Arami peridotite in Switzerland contains mineralogical evidence—in the form of abundant titanate (FeTiO3) rods embedded within olivine crystals—implying an origin at a depth >300 km (186 mi). The original depth estimate was based on the crystallography of these titanate rods, and this has been supported by experiments that demonstrate an increased solubility of titanium dioxide (TiO2) in olivine with pressure. Other corroborating evidence for UHP conditions includes exsolution lamellae of clinoenstatite (a low-calcium pyroxene) within diopside (a calcic pyroxene) crystals in the same rock as the olivine with titanate rods. Of the five different polymorphs known for low-calcium pyroxenes, the only precursor for the exsolution lamellae that is consistent with all of the crystallographic and geologic evidence is high-pressure clinoenstatite (HPclen). The conditions necessary for exsolution of HPclen provide independent evidence of a minimum depth of origin of the Alpe Arami peridotite of 250 km (155 mi). In spite of the increasing amount of apparent corroborating evidence to support the original postulate for a deep origin, others have argued against such great depth, although all agree that the peridotite formed at >100 km (62 mi).

Examples are known from other continental collision zones that imply very great depth of exhumation of mantle rocks, or subduction of crustal rocks to such depths. These include the Sulu terrane of eastern China, the Western Gneiss region of Norway, and the Erzgebirge of Germany. Thus, deep subduction and exhumation has perhaps occurred multiple times during the Phanerozoic, which would indicate that this phenomenon is a normal part of subduction and collisional mountain building. Whether such exhumation is a single- or multiple-step process is an important question for future research.

UHT metamorphism

It used to be thought that the temperatures necessary for melting crustal rocks in the absence of a free water-rich metamorphic volatile phase were unlikely to be achieved. However, scientists now know differently, and the temperatures recorded by the mineral assemblages in some crustal rocks are 300–400°C (570–750°F) above the beginning of melting due to the breakdown of hydrate minerals (micas and amphiboles). In particular, the aluminum content in solution in the mineral orthopyroxene increases with temperature, and a high aluminum content in orthopyroxene coexisting with garnet, cordierite, sillimanite, or sapphirine indicates UHT metamorphic conditions. Recently, peak temperatures of at least 1120°C (2050°F) have been calculated for a granulite from the Tula Mountains of Enderby Land, East Antarctica, based on the aluminum content of orthopyroxene coexisting with sapphirine and quartz.

Successively overprinted coronitic [successive rings of new material products replacing the outer part of a large grain of a reacting (unstable) mineral] and intergrowth reaction microstructures in granulites have allowed them to be used to deduce reaction histories and, from these, to infer P-T-t-d paths of UHT granulite terranes (Fig. 2). However, since crustal rocks under UHT metamorphic conditions generally are melt-bearing, misreading the record of microstructural features due to back reaction with coexisting melt must be avoided. After the peak temperature of metamorphism has been achieved, both close-to-constant pressure (isobaric) cooling and close-to-constant temperature (isothermal) decompression are documented in different UHT granulites. An important but commonly ambiguous issue concerns the increasing temperature path during burial and heating to the peak metamorphic conditions, which may follow a clockwise path in P-T space (Fig. 2) or, much less likely, a counterclockwise path involving heating before substantial burial. Different P-T-t-d paths help to constrain the tectonic environment within which metamorphism proceeds. For example, three different P-T paths are illustrated in Fig. 2: (I) from a deep contact metamorphic zone around a body of hot magma, (II) from a continental collision zone, and (III) from a UHT terrane. Composite isothermal decompression–isobaric cooling–isothermal decompression postpeak temperature P-T paths may record periods of exhumation separated by a period of stability.

Fig. 2  P-T diagram of the anatectic zone, that region in P-T space above the wet granite melting curve in which melt may be present in many common crustal rocks. The symbol Xmw is used to denote the mole fraction of H2O in the melt and is considered to equal the activity of H2O in the melt. Schematic P-T paths: (I) Isobaric heating—cooling path characteristic of deep contact metamorphism (for example, around deep granites of the Cascades of Washington state). (II) Stepped clockwise path, characteristic of collisional metamorphism (for example, the Himalayas or the ancient Variscan belt of Europe). (III) Stepped clockwise path at UHT (for example, ancient rocks in peninsular India).

Geochronology

Although establishing rates and time scales of processes such as heating has recently come within scientists' grasp, determining the age of peak P-T conditions of metamorphism in ancient mountain belts remains elusive, although it can be achieved. During the past decade, techniques have been developed, principally using the common metamorphic mineral garnet, both to date close-to-peak P-T and to constrain the period of heating by knowing the time scale for growth of garnet. There are several advantages to using garnet as a chronometer, including its common occurrence in metamorphosed sedimentary rocks and the fact that garnet chemistry, when garnet grows in equilibrium with appropriate other minerals, can be used to obtain a good measure of the P-T conditions of metamorphism (using a technique called thermobarometry). Information on the time scale of metamorphism during burial and heating and exhumation and cooling also can be gained by careful use of multiple isotope systems and different methods on accessory phases, such as the minerals zircon, monazite, rutile, titanite, and apatite, although there remains the issue of being sure of what we are dating when we do not understand fully the exact crystallization history of some of these minor minerals.

Ideally, in situ dating is to be preferred in P-T-t-d studies so that all metamorphic and deformation information is related to a specific interval of time during the evolution. An important development in relation to structural and metamorphic studies is the rapid chemical dating of monazite, a thorium-rich accessory mineral common in metasedimentary rocks (rocks originally deposited at the Earth's surface as sediments before burial during mountain building). This rapid dating method has been enabled by the development of in situ mapping of thorium, uranium, and lead concentrations using the electron-probe microanalyzer (an instrument more commonly used for analyzing major element concentrations in minerals and only rarely used to analyze elements present in trace quantities). For metasedimentary rocks, which are commonly used to track the burial and thermal history of ancient mountain belts, this technique offers a rapid method to obtain age information as part of routine petrology. This is particularly useful in Precambrian belts where the lower precision on ages obtained in comparison with conventional isotope techniques is sufficient to distinguish between major episodes of mountain-building activity. However, rapid chemical dating of monazite using the electron-probe microanalyzer does not eliminate the need for more time-consuming but higher-resolution and higher-precision geochronological studies (such as using isotope dilution mass spectrometry, secondary ion mass spectrometry, and laser ablation–inductively coupled mass spectrometry).

Postpeak thermal histories are better known than prograde thermal histories because there are a number of minerals from which information can be retrieved along the whole exhumation P-T path. These data are also commonly used to constrain models for the tectonic evolution of ancient mountain belts. It has become clear during the past decade that the time-integrated rates of cooling of orogens vary considerably. Thus, exhumation may vary from very slow, for example ∼1.5°C Ma−1 for at least 150 million years following the last phase of high-grade metamorphism, to extremely rapid, with rates of up to 100°C Ma−1 during 1–10 million years. This variation suggests a range of behaviors between limited vertical tectonic displacement and approximate isostatic equilibrium, and significant vertical tectonic displacement probably reflecting tectonic exhumation. Further, rates of cooling change during the period of exhumation.

New tools in petrology

Most new tools relate to microscopy or in situ analysis, although the increase in computer power and its availability has allowed more complex tectonic models of mountain belts to be developed. Advances in image analysis have also opened up new research directions. The development of techniques for mapping in situ distributions of elements within minerals has allowed advances in understanding processes reflected by differences in element distributions. For example, serial sectioning and three-dimensional reconstruction of compositional zoning from electron backscatter images and quantitative x-ray elemental maps can be used to examine the three-dimensional growth history of large crystals grown during metamorphism. In the mineral garnet, there is a coupling between major and accessory phases during reaction progress, and because trace elements are sensitive to changes in accessory mineral assemblage or fluid composition, these features can be used to calibrate trace-element thermobarometers, identify changes in reacting assemblage, and reveal information not recorded by the major elements.

High-resolution x-ray computed tomography (HR x-ray CT) has revolutionized the analysis of the three-dimensional spatial relationships among features in rocks without destroying them, as would occur in serial sectioning. In hand samples of migmatite, for example, HR x-ray CT illustrates well the three-dimensional distribution of the melt flow network represented by granite due to the mass density contrast between it and the residual matrix (Fig. 3); this distribution is seen to be similar to meter-scale magma transfer sheets observed at outcrops (Fig. 4). In partially melted rocks, scanning electron microscope cathodoluminescence (SEM-CL) allows identification of textural features related to melting, crystallization, and melt movement that are not resolvable with the petrographic microscope. These two novel techniques offer exciting potential to characterize the nature and distribution of petrologic features, especially as related to evolution of melt-bearing rocks.

Fig. 3  Projection of three-dimensional image of layered migmatite, derived from a stack of two-dimensional representations of high-resolution x-ray computed tomography scans, created using VoxBlast.

Fig. 4  Oblique view of outcrop of layered migmatite with sheetlike bodies of granite that record the magma extraction pathways through the crust. From the ancient Acadian mountain belt of west-central Maine.

Outlook

Metamorphism has benefited from dramatic advances in both analytical capabilities and the development of quantitative methods to determine the depth-time evolution of rocks in orogens. P-T-t-d paths are the link between petrology and tectonics, and between the small-scale interactions between deformation and metamorphism, and mountain building. Thus, although metamorphic petrology is an essential tool available to the geologist, it is a valuable tool only if it forms part of an integrated study linked with structural geology and chronology. Indeed, the regional scale availability of P-T-t-d information in an orogen will enable us to constrain this history and thus discriminate among different tectonic models for the evolution of that orogen. Ultimately, the P-T-t-d path of UHP and UHT rocks will enable us to discriminate between hypotheses for the formation and preservation of these rocks. In the case of UHP metamorphism it is the process of exhumation that remains enigmatic, whereas in the case of UHT metamorphism it is the source of the extreme heat at modest depths of crustal thickening that must be explained.