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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
Fig. 1 Migmatite at outcrop. Light material is
granite that records the location of melt. From the ancient Acadian mountain
belt of west-central
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
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
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
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
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.
- M. Brown, From microscope to mountain belt: 150 years of petrology and its contribution to understanding geodynamics, particularly the tectonics of orogens, J. Geodyn., vol. 32, 2001
- M. Brown, The generation, segregation, ascent and emplacement of granite magma: The migmatite-to-crustally-derived granite connection in thickened orogens, Earth Sci. Rev., 36:83–130, 1994
- M. Brown and G. S. Solar, The mechanism of ascent and emplacement of granite magma during transpression: A syntectonic granite paradigm, Tectonophysics, 312:1–33, 1999
- W. G. Ernst and J. G. Liou, Overview of UHP metamorphism and tectonics in well-studied collisional orogens, Int. Geol. Rev., 41:477–493, 1999
- S. L. Harley, The occurrence and characterization of ultrahigh-temperature (UHT) crustal metamorphism, in P. J. Treloar and P. O'Brien (eds.), What Drives Metamorphism and Metamorphic Reactions?, Geol. Soc. Spec. Publ., no. 138, 1998
- S. L. Harley and D. A. Carswell, Ultra-deep crustal metamorphism—A prospective view, J. Geophys. Res.—Solid Earth, 100:8367–8380, 1995
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