When the Continental Crust Melts
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When the Continental
Crust Melts
Edward W. Sawyer1, Bernardo Cesare2 and Michael Brown3
1811-5209/11/0007-0229$2.50 DOI: 10.2113/gselements.7.4.229
P
artial melting of the continental crust has long been of interest to Ga) continental crust appears to be
petrologists as a small-scale phenomenon. Mineral assemblages in the sl ig ht ly more felsic t ha n
Proterozoic (2.5 – 0.5 Ga) or
cores of old, eroded mountain chains that formed where continents Phanerozoic (< 0.5 Ga) crust
collided show that the continental crust was buried deeply enough to have (Rudnick and Gao 2003). Thus,
melted extensively. Geochemical, experimental, petrological and geodynamic juvenile material added to the
crust must be modified in order to
modelling now show that when the continental crust melts the consequences b e come cont i nent a l c r ust.
are crustal-scale. The combination of melting and regional deformation is Evidence from modern arcs indi-
critical: the presence of melt on grain boundaries weakens rocks, and weak cates that more felsic compositions
arise because the mafic magmas
rocks deform faster, influencing the way mountain belts grow and how rifts fractionate and because they cause
propagate. Tectonic forces also drive the movement of melt out of the lower the crust to partially melt.
continental crust, resulting in an irreversible chemical differentiation of Consequently, a layer of mafic
cumulate and residual material
the crust. develops at the base of arc crust.
KEYWORDS : continental crust, partial melting, microstructures, As the arc crust thickens, this
metamorphic petrology cumulate and residual part at the
base converts to denser material,
detaches (a process called delami-
INTRODUCTION
nation) and sinks into the mantle. Thus, the bulk composi-
The continental crust is 41.4 km thick on average and tion of the remaining continental crust becomes more
covers 39% of the Earth’s surface. Information from the felsic. The residual and cumulate material that returns to
isotopic and trace element composition of >4-billion-year- the mantle contains, and hence is enriched by, a small
old (Ga) zircon grains and the evolution of mantle isotopic proportion of felsic melt and becomes the Enriched Mantle
reservoirs indicates that 75%, and possibly more, of the I (EMI) isotopic reservoir (Tatsumi 2005).
continental crust was created before 2.5 Ga (Harrison 2009;
Belousova et al. 2010). Thus, the continental crust is much
longer-lived than oceanic crust and, consequently, has
EVIDENCE THAT THE CONTINENTAL CRUST
acquired considerable complexity. This is reflected in the PARTIALLY MELTED
petrological and structural characteristics of the rocks At the beginning of the last century, extensive mapping
within it. was done in the shield areas of Scandinavia, Canada and
elsewhere. This pioneering work revealed that large parts
The continental crust began to form in the Hadean, more of the continental crust have been metamorphosed to a
than 4.0 billion years ago, first as the mantle differentiated, higher degree and more strongly deformed than adjacent
then from thickened oceanic crust above “hotspots” and areas. We now know that the structures in these highly
at shallow levels (~15 km) above convergent margins deformed regions are similar to those in modern orogens
(Harrison 2009). Since the late Archean (from ca 2.8 Ga), where continents have collided and that the metamorphic
most new, or juvenile, continental crust has formed in temperature in these regions was high enough (> 700 oC)
magmatic arcs above subduction zones, but about 10% was for large areas to partially melt. Some continental crust
formed where mantle magmas were added to existing crust has experienced repeated episodes of modification by
by hotspots or plumes. If new, juvenile continental crust intense deformation, high-temperature metamorphism and
is formed from mantle magma in magmatic arcs and at partial melting: examples occur in the Grenville Province
hotspots or plumes, then its average composition should of Canada, in southern West Greenland, in the Western
be mafic. It is not. The average composition of the conti- Gneisses of Norway and in East Africa. Different terms are
nental crust is broadly andesitic, although Archean (>2.5 used to describe this modification. It is simply called
reworking by petrologists and structural geologists, but from
a geochemist’s perspective, it is intracrustal differentiation.
1 Département des Sciences Appliquées,
Université du Québec à Chicoutimi The largest and most intensely reworked regions of conti-
Chicoutimi, Québec G7H 2B1, Canada nental crust are located where continents collided and
E-mail: ewsawyer@uqac.ca major mountain chains were formed, for example, the East
2 Dipartimento di Geoscienze, Università di Padova African Orogen. Reworking is not restricted to thickened
Via Gradenigo 6, I-35131 Padova, Italy orogens. Mantle melts emplaced into the continental crust
E-mail: bernardo.cesare@unipd.it at rifts or in large igneous provinces associated with
3 Department of Geology, University of Maryland hotspots can result in high-temperature metamorphism.
College Park, MD 20742-4211, USA Partial melting in such settings can lead to intense, local
E-mail: mbrown@umd.edu
E LEMENTS , V OL . 7, PP. 229–234 229 A UGUS T 2011
reworking of the continental crust, but such thermal the source of the heat for melting, what happens at the
reworking is not generally accompanied by intense grain scale during anatexis, or how felsic melt moves from
deformation. the lower to the upper crust. Nor is it concerned with the
broader consequences of partial melting, such as its effect
The deformed and metamorphosed continental crust is not on the rheology of the continental crust and how this
uniform. The upper part is approximately granodioritic in affects the way mountain chains are built when continents
composition and is richer in SiO2 and K 2O relative to the collide. These and other questions are the subject of this
lower part, which is more mafic and richer in Al2O3, FeO, issue of Elements on the theme “When the Continental
MgO and CaO (Rudnick and Gao 2003). These differences Crust Melts.”
as well as the considerable enrichment in light rare earth
elements and the large negative Eu anomaly in the upper
crust relative to the lower crust are best explained by partial TYPES OF MELTING IN
melting, a process that is also called anatexis. Thus intra- THE CONTINENTAL CRUST
crustal differentiation occurs by partial melting of the Rock types such as metapelite, metagreywacke and granite
lower part of the continental crust and migration of the may begin to partially melt when the metamorphic temper-
melt to the upper part, leaving the lower crust with a more ature exceeds 650 oC (FIG. 3), and the melt they produce is
mafic and residual bulk composition (FIG. 1 AND 2). In addi- granitic in composition. Whether they melt or not and the
tion to these geochemical differences, this process imparts quantity of melt produced depend on the availability of
a layered structure to the continental crust, which is H2O. Melting may occur if H 2O is present as a free fluid in
revealed by an increase in seismic P- and S-wave velocities the pores and grain boundaries of the rock; this is called
with depth. Seismic profi les across young continental crust H 2O fluid-present melting and takes place at the lowest
affected by late Paleozoic collision and mountain building temperatures. Melting may also occur when hydrous
in western Europe show the same sub-horizontal Moho minerals (hydrates), such as muscovite, biotite and amphi-
and internal velocity structure as old crust in northern bole, melt incongruently (see glossary); other minerals,
Europe that was reworked by mountain building events in most commonly quartz and feldspar, may also participate
the Proterozoic and Archean. Thus, the acquisition of a in these melting reactions. Incongruent melting may be
sub-horizontal layered structured must happen soon after either H 2O fluid-present or, at higher temperature, H 2O
mountains stop growing. This same basic pattern of modi- fluid-absent. Crystalline rocks have very low porosity and
fication to continental crust has been going on since the so contain very little fluid H 2O; thus the amount of melt
late Archean, at least. produced from H 2O in the pores is too small to be easily
detected. Consequently, the production of large volumes
The geochemical approach has revealed that the large-scale
of granitic melt in continental crust is widely thought to
process of intracrustal differentiation occurs by partial
occur by fluid-absent incongruent melting, except for
melting, but it does not address other concerns, such as
instances where large volumes of aqueous fluid were intro-
duced into rocks already at high temperature, as discussed
below.
Schematic representation of the reworking of conti-
FIGURE 2 nental crust by partial melting. Partial melting occurs
Sill and dike network in stromatic metatexite migma- in the lower part of the crust where temperatures exceed the
FIGURE 1 tite at Maigetter Peak (height 480m) in the Fosdick solidus and migmatites are formed (brown). Melt is formed on
Mountains of West Antarctica (76°26’38”S, 146°30’00”W). The grain boundaries but segregates from the residual solids along a
image is looking to the SE and was taken from the air (Twin Otter progressively more focussed pathway (shown in red), first through
wing tip in upper right). From the aerial perspective and also upon leucosomes then dykes. The melt collects to form plutons, typically
close examination in outcrops, intersecting dikes do not appear to at the transition from ductile middle crust (yellow) to brittle upper
truncate or displace each other; the sills and dikes of granite crust (green); some felsic lavas may be erupted. It is not yet clear
crosscut foliation but may be continuous with or discordant to whether melt ascent is uninterrupted or whether melt ponds at
leucosomes in the migmatite. The leucosomes contain peritectic intermediate levels, shown by the question marks. The ascent of
garnet and cordierite (see Figure 1 in Brown et al. this issue). some melt ends in the middle crust as dyke complexes, without
forming plutons.
E LEMENTS 230 A UGUS T 2011
Pelitic rocks contain a large amount of muscovite and
biotite – 30 to 50 vol% is not unusual – and will produce
melt progressively as the temperature rises above the
temperatures of the incongruent melting reactions
involving these minerals, typically ~720 oC and ~820 oC,
respectively. Other rock types also undergo fluid-absent
incongruent melting. Metagreywackes and meta-andesites
begin to melt between 750 oC and 800 oC. Amphibolites
follow at about 850 oC, but they produce melt of tonalitic
composition. Fluid-absent incongruent melting of micas
in metapelites and metagreywackes can produce as much
as 50 vol% melt. After all the mica is consumed at about
925 o C, the rate of melt production decreases, and the
composition of the melt is no longer granitic.
Fluid-absent incongruent melting of micas and amphibole
describes the melting of metapelite, metagreywacke and
mafic rocks quite well. It explains both the volumes of melt
generated and the granulite facies, residual mineral assem- Types of melting in P–T space for continental crust
blages found deep in the crust that are left behind after FIGURE 3 thickened to 71 km. The base of average (41.4 km)
melt has been extracted. However, it is not a good descrip- crust is shown by the blue dashed line. The red curve is the
tion of melting in hydrate-poor quartzofeldspathic rocks, H2O-present solidus in the haplogranite system; subsolidus condi-
tions occur in the yellow field to its left, and partial melting can
such as leucocratic granites, trondhjemites and tonalites. occur in the pink field. Fields for melting by hydrate breakdown are
Recent studies in metamorphic terranes, ranging in age shown: blue for muscovite (Ms), brown for biotite (Bt) and green
from Archean to Phanerozoic, show far higher degrees of for amphibole (Amp). The purple line marks the start of ultrahigh-
partial melting in granitic rocks than can be accounted for temperature (UHT) metamorphism. Two equilibrium geotherms for
crust of normal thickness are shown as dotted black lines. Crustal
by H2O in pores or by the breakdown of their mica and
radiogenic heat production (0.61 µW·m -3) and a mantle heat flux at
amphibole. Melting in these rocks occurred because an the Moho (30 mW·m -2) are the same for both, but thermal conduc-
aqueous fluid infi ltrated them and led to what is called tivity is 3.0 W·m -1·K-1 for geotherm A and 2.0 for B; hence
water-fluxed melting at low temperature, around 700 oC. geotherm B is hotter but still does not reach UT conditions.
Such an influx of H2O is now recognised as being respon-
sible for melting of metapelitic, metapsammitic and PETROLOGICAL ASPECTS OF MELTING
metamafic rocks in some anatectic terranes (Ward et al. THE CONTINENTAL CRUST
2008; Berger et al. 2008). Oxygen stable isotope studies The rocks in the continental crust that have partially
reveal diverse sources for this H2O. In some terranes it came melted are called migmatites; the nomenclature specific to
from dehydration reactions in nearby metapelites or from these rocks and the means by which they are identified in
crystallizing plutons, whereas in others it originated as the field are outlined by Sawyer (2008a, b). Migmatites are
deeply penetrating seawater or meteoric water, and in yet basically simple rocks with two components. One, which
others it came from the mantle. It is not surprising, there- is partially melted, is called neosome, and consists of the
fore, that many of the places where water-fluxed melting crystallized products from the melt and the complementary
has occurred in the continental crust are adjacent to major residual material. The second, called paleosome, consists of
crustal-scale shear zones that provided the pathways for rock that did not melt. In most cases, however, the melt
the H2O to infi ltrate the continental crust (Sawyer 2010). and residual solid have segregated from each other,
although not completely. The neosome then consists of
THE HEAT PROBLEM two petrologically different parts, one derived from the
The temperature required for H2O fluid–present or water- melt and called leucosome, and the other derived from the
fluxed melting (700 oC) might be reached as a result of residual solid material and, if dark coloured, called melano-
mantle heat entering the base of the crust and radiogenic some, otherwise simply residue. In most cases this simple
heat generated in a continental crust thickened by orogen- petrological framework is made morphologically complex
esis (FIG. 3). However, large granulite terranes that under- by deformation during the melting process. Deformation
went melting at temperatures well above 850 oC and appear results in the translation, rotation and distortion of the
to have lost substantial volumes (>600,000 km3 for the constituents parts. If the strain is high enough, the migma-
Ashuanipi subprovince in Quebec; Guernina and Sawyer tite becomes attenuated, resulting in a banded or layered
2003) of granitic melt as determined from the composition appearance (FIG. 4) typically seen in the deep parts of orogens.
of their residual rocks are problematic in that they required
a great deal of heat. The average continental crust does not EXPERIMENTS AND PETROGENETIC
contain enough K, Th and U to produce sufficient radio- MODELLING
genic heat to sustain this degree of melting on the required The pressure and temperature conditions retrieved from
timescale. Other sources of heat are required. The mantle granulites and migmatites tell us how deep in the conti-
is an obvious source, and strain heating may be significant nental crust melting occurred and provide minima that
in some circumstances. New measurements (Whittington must be achieved by any proposed mechanism of heating.
et al. 2009) indicate that the thermal diffusivity of rocks Basic information for determining the pressure and temper-
at high temperature is low; consequently, the middle and ature (P–T) history comes from well-controlled experi-
lower crust may retain heat better than previously thought. ments on the partial melting of rocks such as pelite,
Identifying the source of heat and the combination of greywacke and amphibolite. Phase equilibria modelling
parameters or circumstances required to focus the heat using internally consistent thermodynamic datasets
into thickening crust and produce a high degree of partial derived from experiments has now been added to the set
melting remains a major problem. Hence, the article by of tools available for understanding the P–T conditions for
Clark et al. (2011 this issue) is the starting point for “When partial melting in the continental crust. The article by White
the Continental Crust Melts.” et al. (2011 this issue) compares the results from both
E LEMENTS 231 A UGUS T 2011
approaches to better understand the conditions and petro- Since leucosome cannot be considered as representative of
logical processes that occur when the continental crust melts. the initial melt composition, because of crystal fraction-
ation and contamination for example, the chemical compo-
Dating the time of formation of metamorphic minerals sition of quenched glass from melting experiments has
and adding this time constraint to P–T information results been the principal source of information on the composi-
in a P–T–t trajectory, which charts the movement of rocks tion of anatectic melts. This situation is changing: micron-
through the continental crust. These trajectories provide sized inclusions of glass and “nanogranite” (FIG. 5), believed
a powerful tool for testing numerical models that investi- to be respectively quenched anatectic melt and its crystal-
gate the combination of parameters governing the develop- lization products, have been found in minerals from
ment of orogens. migmatite terranes (Cesare et al. 2011). These inclusions
could provide the major, trace and isotopic compositions
MELTED ROCKS UNDER THE MICROSCOPE of natural anatectic melts; such “starting-point” composi-
The microstructure in rocks continually readjusts to tions are required to understand what changes occur to
changes in conditions. Minerals disappear, new ones grow, anatectic melts in the crust. How can anatectic melt remain
and grain boundaries move, driven by the need to reduce as glass in slowly cooled rocks from deep in the continental
energy (e.g. Holness 2008), whether that is lattice, inter- crust? This and other questions are addressed in the contri-
facial or surface energy. The extent to which microstructure bution by Holness et al. (2011 this issue), which outlines
reaches the equilibrium state, often thought of as uniform what recent studies of the microstructure in partially
grain size and polygonal grain shapes, contains informa- melted rocks tell us about the processes that occur when
tion on driving forces and the kinetics of grain-boundary the continental crust melts and subsequently cools.
migration. These factors could be related to such diverse
and interesting parameters as the cooling and deformation TECTONIC AND GEODYNAMIC
histories of the rocks. The type of microstructural informa- IMPLICATIONS OF PARTIAL MELTING
tion sought must be matched to the rock sampled. It is
The onset of partial melting has a profound effect on the
fruitless, for example, to attempt to understand the melting
continental crust. The types of structures that form change
reactions or mineral–melt equilibration microstructures
and strain rates increase when the temperature of the conti-
by examining the paleosome, since it did not melt.
nental crust passes the solidus temperature. Because
Similarly, the microstructure of a leucosome contains infor-
anatectic melt is less dense and less viscous than either the
mation about the crystallization of anatectic melt rather
protolith or the solid residue, it is more mobile than the
than the melt-producing reactions. The correct identifica-
solid fraction and will separate from it. Buoyancy is a
tion of each petrological part of a migmatite is necessary
driving force, but differential stress acting on an inevitably
because each contains information about processes specific
anisotropic crust induces pressure gradients, and these
to its origin.
constitute another, locally stronger, driving force for the
movement of melt. Differential stress in anisotropic rocks
A results in the formation of many different types of dilatant
structures, the space between boudins being one well-known
example. Melt migrates to and collects in these structures.
The transfer of heat in the continental crust is largely by
the slow process of conduction, so the deep parts of the
crust are slow to heat up and slow to cool. Consequently,
metamorphic temperatures can remain above the solidus
(650 oC) for times as long as 30 million years, e.g. in the
Himalayan–Tibetan system. In that period melt can move
from one set of dilatant structures to the next as the crust
progressively deforms, crystallizing partially in each and
creating a complex network of leucosomes.
B C
Examples of partially melted rocks. (A) Migmatite migmatite derived from metatonalite partially melted under granu-
FIGURE 4 derived from pelite and psammite protoliths, lite facies conditions in the Limpopo Mobile Belt, a deeply eroded
Nemiscau subprovince, Quebec. The lightest-coloured parts are orogen. Penknife is 11 cm long. (C) Migmatite in which the garnet-
leucosome and the darkest parts, rich in biotite and conspicuous bearing neosomes have been highly strained, creating a banded or
red garnet, are residual material; together these are the neosome. layered structure typical of shear zones developed in melt-bearing
The medium-grey-coloured part is a psammite that did not partially rocks. Scale is 15 cm long.
melt; it is paleosome. Scale is 15 cm long. (B) Highly strained
E LEMENTS 232 A UGUS T 2011
produced when and where rocks become hot and melt.
Strain and advected heat may be focussed into a narrow
zone between a reverse-sense shear zone at the bottom and
a normal-sense one at the top, in a phenomenon called
channel flow. Over the past two decades, advances in under-
standing these topics and other tectonic and geodynamic
consequences of “When the Continental Crust Melts” have
occurred through the use of highly sophisticated numerical
models, and the article by Jamieson et al. (2011 this issue)
presents the state of the art in this critical field.
MOVING THE MELT TO DIFFERENTIATE
THE CONTINENTAL CRUST
Granites are accumulations of anatectic melt, albeit melt
that has had its composition changed through contamina-
tion – by residuum (peritectic phases), wall rocks, or mixing
with different magmas – and through fractional crystalliza-
tion. Melting takes place deep (>25 km) in the continental
Backscattered electron image of a “nanogranite” crust. However, most plutons of granite are emplaced in
FIGURE 5 derived from a small (6 µm) inclusion of granitic melt its upper part, mostly at depths of 12 to 15 km where the
trapped in a garnet (Grt) crystal from a migmatite at Ronda transition from ductile to brittle rheology occurs (FIG. 2).
(Spain). The melt inclusion has a typical polyhedric shape (“nega- To accomplish the differentiation of the continental crust,
tive crystal”; see Cesare et al. 2011) and crystallized into a fine-
grained aggregate of quartz (Qtz), biotite (Bt), K-feldspar (Kfs), anatectic melt must migrate from the grain boundaries
apatite (Ap) and plagioclase (not visible in this image). where it was formed and become progressively concen-
IMAGE COURTESY OF O MAR BARTOLI, U NIVERSITY OF PARMA , ITALY trated into a more focussed flow pattern. Thus, the melt is
able to traverse rocks that are at subsolidus temperatures
Approximately 80% of grain boundaries have melt on them in the middle crust without freezing as dykes. In other
when the melt reaches ~7 vol%, and this results in a loss words the flow of granite melt must become organised.
of about 80% of the pre-melting strength of the protolith How this happens “When the Continental Crust Melts” is
(Rosenberg and Handy 2005). Rocks become very weak discussed by Brown et al. (2011 this issue) in the fi nal
long before melting advances enough (~26 vol%) to turn article.
them into magma, i.e. a suspension of crystals in melt. The
onset of melting and the weakening it causes have a ACKNOWLEDGMENTS
profound effect on the rheology of the continental crust,
Constructive reviews and comments by principal editor
on the way it deforms and on how orogens develop. The
Hap McSween and reviewers Tracy Rushmer, Nick Petford
location of weak rocks is controlled by where the heat
and Gary Stevens have greatly improved this contribution.
source is and by the rate at which hot rocks and cold rocks
On behalf of all the contributors we would like to express
are moved to advect heat and mass. These factors are
our collective thanks to Pierrette Tremblay for her encour-
controlled in part by isostasy, by the development of a
agement and help at all stages in the development of this
ductile root at the bottom of the continental crust and by
issue.
erosion at the top of it. A weak region in the crust is
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microstructures. In: Sawyer EW, Brown of migmatites in the field. In: Sawyer
Belousova EA, Kostitsyn YA, Griffi n WL, M (eds) Working with Migmatites. EW, Brown M (eds) Working with
Begg GC, O’Reilly SY, Pearson NJ (2010) Mineralogical Association of Canada, Migmatites. Mineralogical Association
The growth of the continental crust: Short Course Volume 38, pp 57-76 of Canada, Short Course Volume 38, pp
Constraints from zircon Hf-isotope 29-36
data. Lithos 119: 457-466 Holness MB, Cesare B, Sawyer EW (2011)
Melted rocks under the microscope: Sawyer EW (2010) Migmatites formed by
Berger A, Burri, T, Alt-Epping P, Engi M Microstructures and their interpreta- water-fluxed partial melting of a leuco-
(2008) Tectonically controlled fluid flow tion. Elements 7: 247-252 granodiorite protolith: Microstructures
and water-assisted melting in the in the residual rocks and source of the
middle crust: An example from the Jamieson RA, Unsworth MJ, Harris NBW, fluid. Lithos 116: 273-286.
Central Alps. Lithos 102: 598-615 Rosenberg CL, Schulmann K (2011)
Crustal melting and the flow of moun- Tatsumi Y (2005) The subduction factory:
Brown M, Korhonen FJ, Siddoway CS tains. Elements 7: 253-260 How it operates in the evolving Earth.
(2011) Organizing melt flow through GSA Today 15: 4-10
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E LEMENTS 233 A UGUS T 2011
GLOSSARY
Anatectic front – The surface marking the beginning processes such as fractional crystallization and
of partial melting in the continental crust. It corre- contamination may have modified its composition.
sponds to the fi rst occurrence of neosome in the
Melanosome – A type of residuum composed predomi-
direction of increasing metamorphic grade.
nantly of dark-colored minerals, such as biotite,
Anatectic melt – A melt, generally granitic in composi- garnet, cordierite, amphibole or pyroxene
tion, produced by anatexis
Metatexite – A type of migmatite in which coherent
Anatexis – Partial melting of the continental crust, irre- pre–partial melting structures, such as bedding, folia-
spective of the degree of partial melting tion and folds, are preserved
Brittle–elastic fracturing – Open-mode fracturing by Migmatite – A metamorphic rock formed by partial
crack propagation normal to the direction of melting. At the outcrop scale migmatites are hetero-
minimum compression. It occurs when stresses at the geneous. In addition to two petrogenetically related
crack tips equal fracture toughness, or when reduced parts called leucosome and residuum, migmatites can
stresses lead to subcritical crack growth. also contain rocks, called paleosome, which did not
melt.
Constrictional strain – Deformation resulting in
prolate fabrics in which linear structures dominate Neosome – The part of a migmatite formed by partial
over planar structures melting and consisting of melt-derived and residual
fractions. The neosome may, or may not, have under-
Diatexite – A migmatite in which neosome dominates gone segregation.
and pre–partial melting structures (bedding, folia-
tion, folds) have been destroyed and commonly Orogenesis – The process of forming a mountain chain
replaced by syn-anatectic flow structures in the Earth’s continental crust due to the conver-
gence and collision of tectonic plates
Ductile fracturing – Fracturing due to creep and
growth of microscale voids—fi lled with either fluid Paleosome – The non-neosome part of a migmatite that
or melt in rock—that become interconnected leading was not affected by partial melting because of its bulk
to rupture. composition
Ductile-to-brittle transition zone – The depth in the Peritectic mineral(s) – A new mineral (or minerals)
Earth’s crust where the brittle strength equals the produced in addition to melt during incongruent partial
ductile strength. It occurs in the range of 12 to 18 km. melting of a rock, mineral or mineral assemblage
Flattening strain – A deformation resulting in oblate Protolith or parent rock – The rock from which the
fabrics in which planar structures dominate over neosome in a migmatite was derived
linear structures Pseudosection – A map of phase assemblages for two
Haplogranite system – A simplification of the composi- specified intensive and or/extensive variables (for
tion of granite to just albite + orthoclase + quartz + example, pressure and temperature) and a specified
H2O components (the Ab–Or–Qz system). Adding an bulk composition
anorthite component creates the haplogranodiorite Residuum – The solid fraction left in a migmatite after
system. partial melting and the extraction of some or all of
Incongruent melting – The process by which partial the melt
melting of a rock, mineral or mineral assemblage Segregation – The overall process in which anatectic
produces one or more new (peritectic) minerals, in melt is separated from the residuum in a migmatite
addition to melt
Solidus – The boundary separating the solid (± fluid)
Leucosome – The part of a migmatite derived from segre- phase assemblage fields (generally at lower tempera-
gated partial melt. Leucosome does not necessarily ture) from the melt-bearing phase fields (generally at
have the composition of an anatectic melt because higher temperature) in a P–T phase diagram
Stromatic migmatite – A type of metatexite migmatite
in which the leucosome and melanosome, or just the
leucosome, occur as laterally continuous, parallel
layers called stroma, which are commonly oriented
along the compositional layering or the foliation
Supercontinent – A large continental landmass created
from the collision of several continental cores or
cratons
Ultrahigh-temperature (UHT) metamorphism –
Metamorphism that occurred at temperatures
above 900 oC and pressures compatible with the
stability of sillimanite
E LEMENTS 234 A UGUS T 2011
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