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Geological Society, London, Special Publications

Metamorphic patterns in orogenic systems and the

geological record
Michael Brown

Geological Society, London, Special Publications 2009, v.318;

p37-74.
doi: 10.1144/SP318.2

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Metamorphic patterns in orogenic systems and the geological record

MICHAEL BROWN
Laboratory for Crustal Petrology, Department of Geology, University of Maryland,
College Park, MD 20742-4211, USA (e-mail: mbrown@umd.edu)

Abstract: Regional metamorphism occurs in plate boundary zones. Accretionary orogenic

systems form at subduction boundaries in the absence of continent collision, whereas collisional
orogenic systems form where ocean basins close and subduction steps back and flips (arc
collisions), simply steps back and continues with the same polarity (block and terrane collisions)
or ultimately ceases (continental collisions). As a result, collisional orogenic systems may be
superimposed on accretionary orogenic systems. Metamorphism associated with orogenesis pro-
vides a mineral record that may be inverted to yield apparent thermal gradients for different meta-
morphic belts, which in turn may be used to infer tectonic setting. Potentially, peak mineral
assemblages are robust recorders of metamorphic P and T, particularly at high P –T conditions,
because prograde dehydration and melting with melt loss produce nominally anhydrous mineral
assemblages that are difficult to retrogress or overprint without fluid influx. Currently on Earth,
lower thermal gradients are associated with subduction (and early stages of collision) whereas
higher thermal gradients are characteristic of back-arcs and orogenic hinterlands. This duality of
thermal regimes is the hallmark of asymmetric or one-sided subduction and plate tectonics on
modern Earth, and a duality of metamorphic belts will be the characteristic imprint of asymmetric
or one-sided subduction in the geological record. Accretionary orogenic systems may exhibit
retreating trench– advancing trench cycles, associated with high (.750 8C GPa21) thermal gradi-
ent type of metamorphism, or advancing trench–retreating trench cycles, associated with low
(,350 8C GPa21) to intermediate (350–750 8C GPa21) thermal gradient types of metamorphism.
Whether the subducting boundary advances or retreats determines the mode of evolution.
Accretionary orogenic systems may involve accretion of allochthonous and/or para-autochthonous
elements to continental margins at subduction boundaries. Paired metamorphic belts, sensu
Miyashiro, comprising a low thermal gradient metamorphic belt outboard and a high thermal
gradient metamorphic belt inboard, are characteristic and may record orogen-parallel terrane
migration and juxtaposition by accretion of contemporary belts of contrasting type. A wider
definition of ‘paired’ metamorphism is proposed to incorporate all types of dual metamorphic
belts. An additional feature is ridge subduction, which may be reflected in the pattern of high
dT/dP metamorphism and associated magmatism. Apparent thermal gradients derived from inver-
sion of age-constrained metamorphic P –T data are used to identify tectonic settings of ancient
metamorphism, to evaluate the age distribution of metamorphism in the rock record from the
Neoarchaean Era to the Cenozoic Era, and to consider how this relates to the supercontinent
cycle and the process of terrane export and accretion. In addition, I speculate about metamorphism
and tectonics before the Mesoarchaean Era.

Forty years ago, the introduction of the plate At the same time, the relationship between plate tec-
tectonics paradigm provided a robust framework tonics and metamorphism was addressed by Ernst
within which to understand the tectonics of the (1971, 1973, 1975), Oxburgh & Turcotte (1971),
lithosphere, the strong outer layer of Earth above Miyashiro (1972) and Brothers & Blake (1973).
the softer asthenosphere (Isacks et al. 1968), during Currently, the circum-Pacific and Alpine –Hima-
the Cenozoic and Mesozoic Eras (the maximum layan –Indonesian orogenic systems define two
lifespan of the ocean floors before return to the orthogonal great circle distributions of the conti-
mantle via subduction). Orogenesis, the process of nents (Fig. 1), each of which has a different type
forming mountains, was one of a number of funda- of orogenic system along the convergent plate
mental geological processes that became under- boundary zone (Dickinson 2004). These orogenic
standable once placed within a plate tectonics systems record the two main zones of active
context (Dewey & Bird 1970). Within a few years, subduction into the mantle, the circum-Pacific
Dewey et al. (1973) had demonstrated that the evol- and the Alpine –Himalayan –Indonesian subduction
ution of young orogenic systems could be unra- systems (Collins 2003). Complementary to these are
velled by inverting geological data in combination two major P- and S-wave low-velocity structures
with ocean-floor magnetic anomaly maps by follow- (superswells or superplumes) in the lower mantle,
ing the kinematic principles of plate tectonics. under southern Africa and the South Pacific

From: CAWOOD , P. A. & KRÖNER , A. (eds) Earth Accretionary Systems in Space and Time.
The Geological Society, London, Special Publications, 318, 37–74.
DOI: 10.1144/SP318.2 0305-8719/09/$15.00 # The Geological Society of London 2009.
 Downloaded from http://sp.lyellcollection.org/ at University of Calgary on June 5, 2012

38 M. BROWN

Savostin et al. 1986). The collision of an island

arc with a rifted continental margin involves
choking the subduction zone, subducting slab
breakoff and a reversal or flip in the subduction
polarity if subduction is initiated behind the arc.
These ‘soft’ collisions generate only a short period
of orogenesis because the forces opposing shorten-
ing are relieved by the renewed subduction (Dewey
2005). A similar process occurs where allochtho-
nous blocks and terranes are sutured to an active
continental margin, except that the subduction
boundary steps back and continues to subduct
towards the overriding plate and the associated oro-
Fig. 1. Distribution of continents in relation to the genic event is minimal (e.g. van Staal et al. 2008). In
Alpine–Himalayan and Circum-Pacific orogenic belts
(Cordilleran orogen is cross-hatched) in ‘circular’
contrast, orogens in which two continents become
projection. BI, British Isles; F, Fiji; G, Greenland; GA, sutured involve significant thickening of the conti-
Greater Antilles; J, Japan; NZ, New Zealand; PI, nental lithosphere over a wide zone, perhaps up to
Philippine Islands. Reprinted, with permission, from 1500 km across, and far-field shortening of the
Dickinson, W. R. 2004. Evolution of the North American crust, which generates a mountain front and a
Cordillera. Annual Review of Earth and Planetary wide orogenic plateau surrounded by internal
Sciences, 32, 13–45. # 2004 by Annual Reviews (www. basins, which themselves are bordered by thrust
annualreviews.org). belts (Dewey 2005). Features such as deformation,
metamorphism and magmatism may vary in inten-
sity along and across the length and breadth of
(Montelli et al. 2006; Tan & Gurnis 2007). They are these systems because the continental margin
interpreted to record upwelling responsible for being subducted need not be rectilinear.
advective heat transport through the lower into the Previously these orogenic systems have been
upper mantle (Nolet et al. 2006). This arrangement called ‘Himalayan-type’ (Liou et al. 2004) or
of subduction and upwelling reflects a simple pattern ‘Turkic-type’ (Şengör & Natal’in 1996). Turkic-
of long-wavelength mantle convection, a view that type collisional orogenic systems are characterized
is supported by a variety of geophysical data by incorporation into the suture of large subduction–
(Richards & Engebretson 1992). accretion complexes with associated magmatic arcs
Accretionary orogenic systems form above a and terrane elements that evidence a complex history
subduction boundary during continuing plate con- prior to terminal collision (Şengör et al. 1993; Moix
vergence in the absence of continental collision, as et al. 2008). In some orogenic systems, such as the
exemplified by the evolution of the Pacific Ocean Alpine –Himalayan –Cimmerian orogenic system,
rim during the Phanerozoic Eon (Coney 1992). These the principal suture, in this case the Palaeo-Tethyan
systems vary according to whether the subduction suture, divides the system into two parts on either
boundary or trench is retreating, neutral or advan- side of which the subducting boundary zones,
cing. They may exhibit cyclic behaviour in which which were active accretionary orogens prior to col-
a retreating trench changes to an advancing trench lision, had very different character (Şengör 1992).
or in which an advancing trench changes to a retreat- Accretionary orogenic systems may experience
ing trench (Lister et al. 2001; Lister & Forster 2006). multiple ‘soft’ collisions. The modern example of
Accretion of arcs and/or allochthonous terranes, such a system involves the Cenozoic evolution of
some of which may be far-travelled, is a common southeastern Asia and the southwestern Pacific
feature of accretionary orogenic systems, making (Hall 2002). Here there were three system-wide
the distinction from collisional orogenic systems events, at c. 45, 25 and 5 Ma, each involving changes
somewhat arbitrary with the exception of terminal in plate boundaries and motions as a result of col-
continent–continent collisions. Previously these lisions along some part of the subduction boundary
orogenic systems have been called ‘Pacific-type’ system that led to subducting slab breakoff (Cloos
(e.g. Matsuda & Uyeda 1971) or ‘Cordilleran-type’ et al. 2005).
(e.g. Coney et al. 1980), and more recently In the older geological record, the early history
Maruyama (1997) has proposed that they should of the Appalachian–Caledonian orogenic system
be called ‘Miyashiro-type’. involved peri-Laurentian arcs accreted during the
Collisional orogenic systems are those in which Taconic (e.g. Newfoundland: Lissenberg et al. 2005;
an ocean is closed and arcs and/or allochthonous Zagorevski et al. 2006) or Grampian (e.g. western
terranes and/or continents collide, as exemplified Ireland: Friedrich et al. 1999; Dewey 2005) oro-
by the Tethysides (Dercourt et al. 1985, 1986; genies. In these cases, the period from initial
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METAMORPHIC PATTERNS IN OROGENS 39

subduction of continental margin sediments to the evolving for c. 50 Ma (Rowley 1996) and was pre-
end of shortening was no more than 20 Ma. The ceded by a longer period of subduction-related
peak of metamorphism occurred within a few orogenesis involving accretion of arc terranes.
million years after initial collision, suggesting that This is by no means a long period of orogeny. In
emplacement of mafic magma below and into the the older geological record, the orogenic system
evolving orogen advected the necessary heat. The that sutured Laurentia with Amazonia (Grenville)
generation of mafic magma might be achieved via evolved over c. 500 Ma (Rivers & Corrigan 2000).
subducting slab breakoff and decompression A continental-margin magmatic arc existed on the
melting of asthenospheric mantle as it upwelled southeastern margin of Laurentia from c. 1500
through a tear (Davies & von Blanckenburg 1995) to 1230 Ma, with part of the arc subsequently incor-
or through gaps between drips (Lister et al. 2008), porated into the 1190–990 Ma collisional orogen.
as shown, for example, for the evolution of New The arc oscillated between extension and shorten-
Guinea during the Neogene Era (Cloos et al. 2005). ing several times with back-arc deposits of several
Two of the largest accretionary orogenic systems ages; closure of the back-arc basins occurred during
on Earth are the Terra Australis orogenic system, two accretionary orogenies at c. 1495– 1445 and
which was active from the Edicaran Period into 1250– 1190 Ma, as well as during three crustal short-
the Triassic Period, and the Central Asian Orogenic ening events associated with the 1190– 990 Ma
Belt (CAOB), which was active from the beginning collisional orogeny.
of the Neoproterozoic Era to the end of the Mesozoic Young mountain chains on Earth are clearly
Era. The Terra Australis orogenic system, which associated with linear belts of distinctive sedi-
extended some 18 000 km along the Gondwanan mentation, deformation, magmatism and meta-
margin and up to 1600 km inboard, includes the morphism, as well as the high relief. In contrast,
Tasman, Ross and Tuhua orogens of Australia, ancient orogenic belts must be identified based on
Antarctica and New Zealand in East Gondwana, sedimentation and stratigraphy, particularly the
and the Cape Basin of Southern Africa and the occurrence of unconformities in the rock record,
Andean Cordillera of South America in West Gond- the type of associated volcanism and plutonism,
wana (Cawood 2005; Cawood & Buchan 2007). commonly using chemical fingerprinting, and the
Subduction along the Pacific margin of Gondwana style of tectonic deformation and regional meta-
was established c. 580 –550 Ma (Cawood & morphism, simply because the mountains were
Buchan 2007) contemporaneously with a major eroded long ago.
global plate reorganization associated with the last A characteristic feature of subduction boundary
steps in the assembly of Gondwana by final closure zones is the development of dual thermal environ-
of ocean basins and termination of intra-Gondwana ments (Oxburgh & Turcotte 1970, 1971), represent-
subduction, and with opening of the Iapetus Ocean ing the subduction zone or collisional suture
between Laurentia and Baltica. The Terra Australis (cooler) and the arc –back-arc system or orogenic
orogenic system has a protracted history of con- hinterland (warmer). This feature is the hallmark
tinuing subduction and associated episodic plate of asymmetric or one-sided subduction on modern
boundary orogenesis, but terrane accretion and arc Earth (one-sided subduction is defined in Fig. 2).
collisions are rare and continent–continent col- Brown (2006) showed that different types of meta-
lisions are absent. In contrast, the CAOB formed morphism would be registered in each of these
by accretion of ophiolitic mélange zones, accre- thermal environments, and proposed that the
tionary wedges, oceanic plateaux, island arcs and record of metamorphism in ancient orogens may
microcontinents in a manner comparable with be inverted to determine when this style of sub-
circum-Pacific Mesozoic –Cenozoic accretionary duction boundary zone first was registered in the
orogens (Windley et al. 2007). The CAOB evolved geological record. This simple approach to the
over a period of more than 700 Ma culminating in geological record of metamorphism may appear
terminal collision between the Siberian and North incompatible with the complexity of evolution
China cratons at around 250 Ma (Windley et al. implied by the discussion of orogenic systems
2007; Kröner et al. 2008). Closure of small ocean above. However, prograde metamorphism involves
basins between accretion events means that ridge– dehydration of the crust leading ultimately to
trench interactions were likely (Windley et al. nominally anhydrous peak mineral assemblages.
2007). Furthermore, the CAOB represents a major In the absence of rehydration, which is generally
site of juvenile magmatic additions to the crust linked with localized deformation (Boundy et al.
during the Neoproterozoic and Palaeozoic Eras 1992; Austrheim & Boundy 1994; Blattner 2005;
(Şengör et al. 1993; Jahn et al. 2000; Jahn 2004). Camacho et al. 2005; Clarke et al. 2005; Fitzherbert
The Himalayan orogenic system is generally et al. 2005; Bjornerud & Austrheim 2006; Glodny
regarded as the ‘type’ example of a collisional oro- et al. 2008), the initial record of metamorphism
genic system. Here the collisional phase has been is likely to be preserved, at least partially, through
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40 M. BROWN

belts for evaluating tectonic setting back through

time. Next, I review types of metamorphism in
relation to accretionary and subduction-to-collision
orogenic systems as they occur on Earth since the
Cretaceous Period, before considering the imprint
of different types of metamorphism in the rock
record back to the Neoarchaean Era using a
dataset for high-temperature and high-pressure
metamorphism compiled in 2005 (Brown 2007a).
Using this dataset, I define three types of meta-
morphism characterized by different ranges of
apparent thermal gradient, which I relate to the
changing tectonics of orogenic systems through
Earth history. Finally, I evaluate the age distribution
of metamorphism from the Neoarchaean Era to
Fig. 2. Various one-sided (a, b) and two-sided (c, d)
the Cenozoic Era and consider how this relates
subduction geometries. In the case of the one-sided
geometries, the overriding plate does not subduct. to the supercontinent cycle and the process of
Depending on the relative plate motions, the overriding terrane export and accretion (see Brown 2007b,
plate may be extending (a) or shortening (b). In the case 2008). Excluded from consideration in this review
of two-sided geometries, both plates subduct together. are intraplate orogenies such as the Ediacaran –
Depending on the relative thicknesses (ages) of the Cambrian Petermann and the Devonian –
plates, two-sided subduction may be symmetric (c) or Carboniferous Alice Springs orogenies in Central
asymmetric (d). Reprinted from Gerya, T. V., Connolly, Australia (e.g. Sandiford & Hand 1998; Hand &
J. A. D. & Yuen, D. A. 2008. Why is terrestrial Sandiford 1999; Sandiford 2002).
subduction one-sided? Geology, 36, 43– 46, with
permission. # 2008 The Geological Society of America
(GSA). Tectonics of orogenic systems on Earth
At a global scale on modern Earth, subduction
overprinting events such as may occur during at convergent plate boundaries is continuous. Oro-
subduction-to-collision orogenesis. Also, as shown genic systems occur along convergent plate bound-
by Hollis et al. (2006), the prograde history may aries, where they act as buffer zones (Lister et al.
be preserved through subsequent high-temperature 2001) to accommodate deformation (Kreemer et al.
events. 2003). Although deformation is continuing at con-
The P –T–t evolution of orogens, the formation vergent plate boundaries, generally involving
of paired metamorphic belts, and the temporal dis- strain accumulation at low strain rates, orogenic
tribution of different types of metamorphism have systems may also exhibit episodic behaviour of
been reviewed in a series of papers by Brown various types at all scales (Dewey 1981); some of
(1993, 1998a, b, 2001, 2002b, 2007a) that comp- these episodic events have been inferred to be glob-
lement those by Ernst & Liou (1995), Maruyama ally significant (Lister et al. 2001). Global events
et al. (1996), Maruyama & Liou (1998, 2005), may be associated with switches in tectonic mode,
Liou et al. (2004) and Ernst (2005). Here I consider from contraction to extension and vice versa (Lister
metamorphism in orogenic systems in general, et al. 2001). These switches will effect change in
giving this paper a broader perspective than a the thermal structure of orogenic systems, and
simple discussion of metamorphism in accretionary therefore they should be registered in the metamor-
orogenic systems. This is justified by the complexity phic record.
induced in many ancient orogens by collision- Orogenesis in accretionary orogenic systems
related phenomena overprinting subduction-related may be driven by reorganization of plate motions
phenomena, where the collisions vary from terrane (Dewey 1975), ridge– trench interactions (Brown
accretion to arc accretion to continental margin sub- 1998b; Wakabayashi 2004), subduction of ocean-
duction. The additional complexity that may result floor debris (Cloos 1993; Collins 2002), or terrane
from this overprinting, which may vary with the accretion (Jones et al. 1983; Howell et al. 1985;
style of collision involved, provides the foundation Coney 1992). Processes that change the state of
of fertile disagreements over the interpretation of stress along one plate boundary will affect the
ancient orogenic systems. balance of torques on all other plate boundaries
First, I review the tectonics of accretionary (Coblentz & Sandiford 1994; Coblentz et al. 1994,
orogenic systems and consider when this style of 1995, 1998; Richardson & Coblentz 1994; Coblentz
orogenic system first appeared in the geological & Richardson 1996). To maintain the torque
record. I also consider how to classify metamorphic balance, kinematic adjustments must be made
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METAMORPHIC PATTERNS IN OROGENS 41

elsewhere along the plate boundary system in Lister et al. 2001; Lister & Forster 2006; Schellart
response to the change along the first plate bound- 2007b, 2008a, b; Schellart et al. 2007; Lallemand
ary. For this reason processes such as ridge– et al. 2008; but see Doglioni (2008) for an alterna-
trench interactions, subduction of ocean-floor debris tive perspective). The mode of evolution of the
and terrane accretion that have a significant impact system is primarily determined by whether the
along one plate boundary will affect the plate trench advances (is pushed back by the advance of
boundary torque balance and promote changes in the overriding plate) or retreats (is pulled back
plate kinematics that may be imprinted on the rock faster than the overriding plate is able to adjust).
record as a global event. Of course, this is also Trench migration is also influenced by the velocity
true for terminal continent collision in subduction- of the subducting plate (Fig. 3), which largely
to-collision orogenesis. depends on its age at the trench (Lallemand et al.
For accretionary orogenic systems, the behavi- 2008), and proximity to lateral slab edges (Schellart
our of the subduction boundary or trench in relation et al. 2007; Schellart 2008b). Shortening of the
to the overriding plate is important (Uyeda & overriding plate and formation of an orogenic
Kanamori 1979; Uyeda 1982; Royden 1993a, b; plateau in the hinterland behind a mountain chain

Fig. 3. Overriding plate absolute motion v. subducting plate absolute motion for 166 transects across trenches
considered by Lallemand et al. (2008). Vup, Vt and Vsub are the absolute upper plate, trench and subducting plate motions
counted positive landward (velocities in HS3 reference frame; references frames were discussed by Lallemand et al.
2008). The regression line Vup ¼ 0.5 Vsub 2 2.3 in cm a21 (or Vsub 2 2 Vup ¼ 4.6) is valid only for the neutral
subduction zone transects (light grey dots). Quality factor R 2 ¼ 0.37. Along the neutral line, the trench velocity Vt
equals the upper plate velocity Vup because there is no deformation across the arc system. All transects characterized
by active shortening (medium grey dots) are located below this line, and all transects characterized by active spreading
(¼ extension; dark grey dots) are located above the line (with the single exception of four transects across the New
Hebrides trench, discussed by Lallemand et al. 2008, p. 7). Republished from Lallemand, S., Heuret, A., Faccenna, C. &
Funiciello, F. 2008. Subduction dynamics as revealed by trench migration. Tectonics, 27, TC3014, doi:10.1029/
2007TC002212, with permission. # 2008 American Geophysical Union.
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42 M. BROWN

in the absence of terminal continent collision, such kinematics to regain the plate boundary torque
as occurs in the modern Andes, requires resistance balance. Low thermal gradient type metamorphic
to trench retreat, a condition that is met far from rocks in accretionary orogenic systems dominated
lateral slab edges (Schellart et al. 2007; Schellart by cycles in which an advancing trench changes
2008b), and overriding plate motion towards the to a retreating trench come back to the surface
trench (van Hunen et al. 2002), which is most prob- relatively soon after their burial at rates comparable
ably driven by ridge-push forces. In addition, in with plate boundary velocities. Rapid return to
these advancing systems, extensional collapse of shallow crustal levels is also the case for low
the orogenic suprastructure may be a factor caused thermal gradient type metamorphic rocks exhumed
by the gravitational potential energy of the orogen, by extrusion, such as in the Thompson et al.
as described for collisional orogenic systems by (1997) model.
Dewey (1988). In the Tethysides, the occurrence of low thermal
Thompson et al. (1997) treated orogenic systems gradient type metamorphic rocks in several belts
along subduction boundaries as complex transpres- related to a single subduction system suggests that
sive systems in which the degree of obliquity or these cycles represent recurrent transient events
ratio of pure shear to simple shear components con- (Lister et al. 2001). Exhumation of these meta-
trols the style of metamorphism, whereas Lister & morphic rocks appears to have been associated in
Forster (2006) argued that switches in tectonic time and space with subduction–accretion of conti-
mode are primarily responsible for differences in nental ribbon terranes that triggered slab rollback or
style of metamorphism. Thompson et al. (1997) slab step-back outboard of the accreted terrane and
proposed that a low ratio of pure shear to simple created the necessary space for exhumation (Brun
shear, typical for wrench-dominated plate bound- & Faccenna 2008). This is not the only mechanism
aries, implies a larger component of horizontal by which low thermal gradient type metamorphic
transport from a position deep in a transpressive rocks are exhumed, but a detailed discussion is
orogenic system, which allows for a longer time outside the scope of this review and the interested
during which heating may occur. In contrast, a reader is referred to papers by Platt (1993), Gerya
high ratio of pure shear to simple shear, typical for et al. (2002) and Warren et al. (2008a, b) and ref-
convergence-dominated plate boundaries, implies erences therein.
a smaller component of horizontal transport from
a position deep in a transpressive orogenic system, How far back in time are we able to
which drives more rapid exhumation and allows less recognize these orogenic systems?
time during which heating may occur. Thompson
et al. (1997) proposed that high dT/dP, intermediate Since the early days of the new global tectonics,
dT/dP and low dT/dP metamorphism are associ- there has been much debate about when Earth
ated with an increasing angle of subduction obli- might have adopted a mobile lid mode of convec-
quity, respectively. However, the complexity of tion, whether once one-sided subduction was estab-
natural obliquely subducting plate boundary zones lished it was maintained to the present day, and
suggests that this analysis may be far too simple whether the formation and stabilization of the conti-
(e.g. Baldwin et al. 2004). nental lithosphere and the change to one-sided sub-
In contrast, Lister & Forster (2006) identified duction were related to each other. Alternative
two end-member types of orogenic system based geodynamic scenarios for the early history of the
on cyclicity of tectonic mode switches. Some Earth include an alternation between plate tectonics
systems may exhibit a cycle in which a retreating and some other mode (Sleep 2000) and episodic
trench changes to an advancing trench (‘pull–push (O’Neill et al. 2007) or intermittent (Silver &
inversion cycles’ of Lister & Forster 2006), and Behn 2008) plate tectonics.
these systems are associated with high thermal Consensus is emerging that Earth had adopted,
gradient type of metamorphism (.750 8C GPa21). either partially or completely, one-sided subduction
Other systems may exhibit a cycle in which an and a form of plate tectonics akin to that on modern
advancing trench changes to a retreating trench Earth by sometime in the Archaean Eon (Brown
(‘push–pull inversion cycles’ of Lister & Forster 2006, 2007a, b, 2008). However, there are those
2006), and these systems are associated with low who argue for plate tectonics as early as the
(,350 8C GPa21) to intermediate (350 –750 8C Hadean Eon (Davies 2006; Harrison et al. 2006;
GPa21) thermal gradient types of metamorphism. Shirey et al. 2008), consistent with the null hypoth-
Metamorphic imprints in orogens are likely esis that plate tectonics was the mode of convection
to be the result of discrete events caused loc- throughout Earth history. There are also those who
ally by ridge–trench interactions, subduction of argue against worldwide modern-style subduction
ocean-floor debris, terrane accretion and terminal before the Neoproterozoic Era (Stern 2005, 2007,
continent collision or globally by changes in plate 2008), requiring an alternative paradigm to plate
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METAMORPHIC PATTERNS IN OROGENS 43

tectonics for the period from the Hadean Eon to the belt of the jadeite –glaucophane type and/or high-
Mesoproterozoic Era. pressure intermediate group occurred together as a
The style of subduction is the key. It is the pair. The latter belt was inferred to develop at a
change from symmetric to asymmetric or one-sided site of low heat flow such as a subduction zone
subduction sometime in the Archaean Eon (Fig. 2; whereas the former belt was inferred to develop at
Gerya et al. 2008) that marks the beginning of a site of high heat flow such as that beneath an
what I have called a ‘Proterozoic plate-tectonics associated volcanic arc or back-arc basin. Miyashiro
regime’; that is to say, a regime with dual thermal (1961) suggested that these belts formed in different
environments, one on the oceanward side that phases of the same cycle of orogenesis, whereas
is cooler and another on the hinterland side that subsequently Brown (1998a, b, 2002b) proposed
is warmer (Brown 2006, 2007a, b, 2008). The they might have formed along different sectors of
Neoarchaean Era is when many of the characteristic the same subducting margin, and became juxta-
features of plate tectonics become widespread in the posed only by later orogen-parallel translation.
geological record (Condie & Pease 2008), the conti- Miyashiro (1961) perspicaciously observed (in rela-
nental crust became stabilized, and the continental tion to secular change) ‘regional metamorphism
lithosphere was created as crustal fragments aggre- under higher rock pressures appears to have taken
gated into several supercratons (Bleeker 2003). place in later geological times’.
Since the formation of supercratons in the Since 1961 not only has the plate tectonics para-
Neoarchaean Era (Bleeker 2003), the continents digm revolutionized our view of orogenesis and its
have grown laterally through time. This growth attendant processes but also we have seen the
has occurred via the processes of magmatism in limits of regional metamorphism pushed to press-
arc –back-arc systems, accretion at continental ures of up to 10 GPa (ultrahigh-pressure meta-
margin trenches of ocean plate materials, including morphism, UHPM) and temperatures of at least
offscraping and underplating of ocean-floor topo- 1050 8C (ultrahigh-temperature metamorphism,
graphic features, and collision with incoming ter- UHTM); the field between these types of metamor-
ranes and arc systems that also may have been phism is one in which eclogite and high-pressure
displaced laterally by orogen-parallel motions at granulite mineralogies overlap according to the
active continental margins. These processes all con- dictates of bulk composition in relation to P–T con-
tribute to the complexity recorded by accretionary ditions (Brown 2007a, 2008). In 2007 I highlighted
orogenic systems at convergent plate margins three types of metamorphism (namely, granulite –
(Dewey et al. 1990; Şengör et al. 1993; Collins ultrahigh-temperature metamorphism (G– UHTM),
2002; Dickinson 2004; Cawood 2005), prior to eclogite –high-pressure granulite metamorphism
eventual incorporation of these accretionary oro- (E –HPGM) and high-pressure– ultrahigh-pressure
genic systems into collisional orogenic systems metamorphism (HPM –UHPM)) because I was
during supercontinent formation. Whether accre- interested in the different thermal regimes they
tionary orogenic systems are confined to the post- record (Brown 2007a). Close-to-peak P –T con-
Mesoarchaean period or whether they predate the ditions for single samples from granulites and
stabilization of the continental lithosphere during ultrahigh-temperature metamorphic belts and high-
the Mesoarchaean to Neoarchaean Eras periods pressure to ultrahigh-pressure metamorphic belts
remains an open question (Condie & Kröner 2008). define two well-delineated sectors of P–T space
(Fig. 4), with a gap that is represented in the rock
The classification of metamorphic belts record by eclogite–high-pressure granulite meta-
morphic belts (e.g. O’Brien & Rotzler 2003).
The principal tectonic settings for regional meta- Miyashiro used the metamorphic field gradients
morphism are zones of continent extension, conti- determined for a limited number of metamorphic
nent margins associated with subduction and belts to place each of them into one of his five types
collision, island arcs and ocean plateaux (excluding of metamorphism based on increasing pressure
the ocean ridge systems for the purpose of this (Miyashiro 1961, fig. 4). The concept of grouping
review). In a classic paper, Miyashiro (1961) classi- metamorphic belts by pressure is a useful one.
fied metamorphic facies series characteristic of However, the lower grade and/or prograde history
regional metamorphic belts into the following of many higher temperature belts commonly is not
five types with increasing pressure: andalusite– recorded. For this reason, in this review I use
sillimanite type; low-pressure intermediate group; apparent thermal gradient defined by close-to-peak
kyanite–sillimanite type; high-pressure interme- P –T conditions for classification rather than
diate group; jadeite –glaucophane type. Miyashiro metamorphic field gradients. As with Miyashiro’s
(1961) observed that in some regions a metamorphic approach, the resulting classification into high
belt of the andalusite –sillimanite type and/or dT/dP (synonymous with low-P– high-T or LP –
low-pressure intermediate group and a metamorphic HT), intermediate dT/dP (similar to ‘Barrovian’)
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44 M. BROWN

Fig. 4. P– T diagram to show the results of internally consistent thermobarometry of single rock samples for high-
pressure and ultrahigh-pressure metamorphism (black squares; P– T calculated using Grt–Cpx–Ky –Pag– Qtz/Coe
thermobarometry, mostly from Ravna & Terry (2004) with additional data from Hacker (2006) and Tsujimori et al.
(2006)) and granulite and ultrahigh-temperature metamorphism (grey circles; P –T calculated by Pattison et al. (2003)
using Grt– Opx thermobarometry with Al-solubility in Opx corrected for retrograde Fe–Mg exchange). Uncertainties
on the P– T data are likely to be of the order of 0.2–0.3 GPa and 30–60 8C. Thermal gradients of ,150 8C GPa21 are
not recorded in crustal rocks on Earth.

and low dT/dP (synonymous with high-P –low-T records crustal evolution across the changing spec-
or HP –LT) metamorphism coincides with major trum of transient metamorphic geotherms character-
changes in facies series. These three types of istic of one or more thermal environments.
metamorphism culminate in ultrahigh-temperature, This observation about the dynamic evolution
eclogite–high-pressure granulite or ultrahigh- of metamorphic belts is important because classifi-
pressure metamorphism, respectively. They register cation is based on an apparently static view of meta-
the imprint of three different thermal environments morphism based on the metamorphic facies concept.
in the geological record, each with a characteristic The close-to-peak P–T conditions recorded by the
range of dP/dT or apparent thermal gradients. equilibrium mineralogy and the implied apparent
Of course, metamorphism is a dynamic and not thermal gradients derived by linear extrapolation
a static process and metamorphic rocks record the back to the surface do not preclude a P–T evolution
evidence of this dynamic process in changes in that transgresses from one thermal environment
mineralogy and/or mineral chemistry that track to another. Thus, a metamorphic belt may evolve
the pressure (P) and temperature (T ) with time (t). from a low dT/dP to intermediate dT/dP type or
Once this encoded information is decoded and from an intermediate dT/dP to a high dT/dP type.
inverted the result is commonly represented as a This may or may not be registered in the rock by
P–T –t path. This is the record of burial and features such as disequilibrium microstructures or
exhumation in a particular thermal environment mineral chemistries that identify the change from
or combination of thermal environments in which one environment to the other; for example, from
the minerals crystallized and/or re-equilibrated the HPM –UHPM facies series to the E –HPGM
(e.g. Brown 1993, 2001). In effect, the P –T– t path facies or from the E –HPGM facies to the
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METAMORPHIC PATTERNS IN OROGENS 45

G –UHTM facies series, respectively. Thus, the granulite metamorphism (E–HPGM; e.g. O’Brien
metamorphic facies concept, the apparent thermal & Rötzler 2003; Brown 2007a). On modern Earth,
gradient and the metamorphic P–T –t path record the different types of metamorphic facies series
different characteristics of a metamorphic belt. leading to granulite–ultrahigh-temperature meta-
morphism (G –UHTM), medium-temperature
The metamorphic realm eclogite –high pressure granulite metamorphism
(E –HPGM) and high-pressure metamorphism–
The P–T field of conditions recorded by crustal ultrahigh-pressure metamorphism (HPM –UHPM)
rocks traditionally has been divided into facies, each are generated in different tectonic settings with con-
represented by a group of mineral assemblages trasting thermal regimes at convergent plate bound-
associated in space and time that are inferred to ary zones (Brown 2006, 2007a). Based on apparent
register equilibration within a limited range of thermal gradients (discussed further below), these
P–T conditions (Fig. 5a). The limits of P and T metamorphic facies series correspond to high
recorded by crustal rocks are higher than in 1961. dT/dP, intermediate dT/dP and low dT/dP meta-
Some rocks characteristic of the granulite facies morphism, respectively.
record temperatures .900 8C at pressures of Recently, Stüwe (2007) has introduced an alter-
0.6–1.3 GPa (ultrahigh-temperature metamor- native division of P –T space, based on whether
phism (UHTM); e.g. Harley 1998, 2008; Brown thermal conditions implied by close-to-peak meta-
2007a; Kelsey 2008) and some eclogite facies morphic mineral assemblages in orogenic crust
rocks record pressures up to 10 GPa at temperatures were warmer or cooler than a normal (conductive)
of 600 –1000 8C (ultrahigh-pressure metamor- continental geotherm (Fig. 5b). On this P– T
phism (UHPM); e.g. Chopin 2003; Liu et al. diagram, we may distinguish P–T fields that are
2007; Brown 2007a). In addition, we now recognize reached as a function of different tectonic processes.
a transition between these two facies, referred For thermal conditions warmer than a normal
to as medium-temperature eclogite–high-pressure continental geotherm, a thermal gradient of

Fig. 5. (Left) P –T diagram to show the principal metamorphic facies in P –T space and the P –T ranges of different
types of extreme metamorphism. HP– UHP metamorphism includes the following: BS, blueschist; AEE, amphibole –
epidote eclogite facies; ALE, amphibole lawsonite eclogite facies; LE, lawsonite eclogite facies; AE, amphibole
eclogite facies; UHPM, ultrahigh-pressure metamorphism; GS, greenschist facies; A, amphibolite facies; E-HPG,
medium-temperature eclogite– high-pressure granulite metamorphism; G, granulite facies, whereas UHTM is the
ultrahigh-temperature metamorphic part of the granulite facies. (Right) An alternative division of P –T space based on
whether thermal conditions implied by peak metamorphic mineral assemblages were warmer or cooler than a normal
(conductive) continental geotherm, which is constructed to pass through 500 8C at 1 GPa and 1200 8C at 3 GPa (see
Stüwe 2007). This diagram distinguishes P– T fields that are reached as a function of different tectonic processes. The
boundary between the warmer than normal and ultrahigh temperature fields reflects the conductive limit; it has dT/dP of
1000 8C GPa21. The boundary between the cooler than normal and ultra-low-temperature fields represents half the
normal geothermal gradient. It should be noted that the field boundaries in this figure are simpler than the facies
boundaries based on equilibrium mineral assemblages.
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46 M. BROWN

c. 1000 8C GPa21 is a practical limit for a conductive by the presence of a diagnostic mineral assemblage
response, based on the unlikely scenario of astheno- in an appropriate bulk composition and oxidation
spheric mantle being located at the base of normal state, such as assemblages with sapphirine þ quartz,
thickness crust (Stüwe 2007). Metamorphic belts spinel þ quartz or corundum þ quartz. The current
that record apparent thermal conditions hotter than state-of-the-art in robust thermobarometry for gran-
this limit require a component of advection associ- ulite metamorphism in general for a variety of bulk
ated with mantle-derived magma in addition to compositions is either the method proposed by
thinning the subcrustal lithospheric mantle (e.g. Pattison et al. (2003) or the average P–T mode in
Sandiford & Powell 1991), consistent with an arc – THERMOCALC (Powell et al. 1998) with the intern-
back-arc setting (Hyndman et al. 2005a, b; Currie ally consistent thermodynamic dataset of Holland
& Hyndman 2006, 2007; Schellart 2007a; Currie & Powell (1998, most recent update).
et al. 2008), or internal heat generation by radio- The transition from granulite- to high-pressure
active decay in over-thickened crust (Le Pichon granulite-facies conditions is not well defined, but
et al. 1997; McKenzie & Priestley 2008), consistent I recommend the sillimanite to kyanite transform-
with an orogenic hinterland setting. ation in metapelitic rocks as a simple rule of
For thermal conditions cooler than a normal thumb. Furthermore, the field of high-pressure
continental geotherm, Stüwe (2007) suggested a granulite-facies metamorphism overlaps the trans-
two-fold division into a ‘cooler than normal’ and formation of amphibolite to eclogite in basaltic
an ‘ultra-low-temperature’ P –T field. The cooler rocks, which occurs over more than 1 GPa accord-
than normal P –T field is a thermal regime that ing to the chemical composition of the protolith
may be reached by thickening both the crust and (O’Brien & Rötzler 2003). These two features
mantle components of the lithosphere for an accep- define the intermediate E–HPGM field.
table range of thermal parameters (Sandiford & Peak mineral assemblages are potentially robust
Dymoke 1991; Stüwe 2007). This observation recorders of metamorphic P and T, particular at
suggests that at least some low dT/dP metamorphic high P– T conditions (HPM –UHPM, E –HPGM
belts within orogenic systems may reflect the and G –UHTM), because prograde dehydration
thermal response to lithospheric thickening and and melting with melt loss produce nominally
may not necessarily reflect thermal regimes attend- anhydrous mineral assemblages that are difficult
ant on subduction or in response to unusually rapid to retrogress or overprint without fluid influx.
exhumation. In contrast, processes other than None the less, the effects of overprinting of
lithospheric thickening are required to reach the younger orogenic events on older orogens leading
ultra-low-temperature P–T field; subduction is one to polymetamorphism must be avoided wherever
possible process by which rocks may enter this possible, or where present or suspected must be dis-
thermal regime (Stüwe 2007). tinguished (e.g. Hensen & Zhou 1995; Hensen et al.
In this review, the range of P–T conditions 1995). Without attention to the possible degradation
recorded by HPM –UHPM, E–HPGM and of the dataset by incorporation of potentially over-
G– UHTM is the same as those specified by Brown printed P– T information, use of the geological
(2007a). High-pressure metamorphism (HPM) record of metamorphism in relation to secular
and granulite-facies (G) metamorphism are well- change will be potentially flawed. Furthermore, in
understood terms in the literature and are not evaluating secular change in patterns of metamorph-
redefined here. ism it is essential to use precise P –T–age relations.
For UHPM, pressure must have exceeded the The P –T conditions should be assessed based on
stability field of quartz, recognized by the presence robust thermobarometry or the presence of a diag-
of coesite or diamond in crustal rocks or by equi- nostic mineral assemblage in an appropriate bulk
valent P–T conditions determined using robust composition and oxidation state, and the age should
thermobarometry. Examples of robust thermobaro- be determined using a robust chronometer and
metry currently in use, as recently evaluated by should be related to a specific P–T point along the
Hacker (2006), include the method of Ravna & P– T–t evolution; if possible, close to peak P –T.
Terry (2004) and the average P –T mode in Today we are confident in our ability as forensic
THERMOCALC (Powell et al. 1998) with the intern- scientists correctly to interpret the evidence recov-
ally consistent thermodynamic dataset of Holland ered from eroded orogens (e.g. O’Brien 1997,
& Powell (1998, most recent update). 1999; Brown 2001, 2002a, 2007a; White et al.
For UHTM, I follow Harley (1989, 1998) in 2002; O’Brien & Rötzler 2003; Johnson & Brown
using 900 8C as an arbitrary lower temperature 2004; Baldwin et al. 2005; Powell et al. 2005); this
limit and Brown (2007a) in using the stability field confidence was lacking in the early days of studies
of sillimanite in rocks of appropriate composition of metamorphism under extreme P –T conditions
to define the pressure. UHTM in crustal rocks may (Green 2005). Our confidence is in part due to impro-
be recognized either by robust thermobarometry or vements in our ability to interrogate rocks and to
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METAMORPHIC PATTERNS IN OROGENS 47

recognize the effects of overprinting; for example, Ultrahigh-pressure metamorphic rocks generally
by using chemical mapping of mineral grains and register deep subduction and exhumation of conti-
high spatial resolution in situ geochronology. nental crust (lithosphere) during the early stage of
The wider availability of high-precision ages collision, as evidenced by coesite-bearing eclogites
on accessory phases linked to specific microstruc- of Eocene age at Tso Morari in the frontal Hima-
tural sites and robustly determined P –T conditions, layas (Sachan et al. 2004; Leech et al. 2005).
commonly through the identification of mineral However, strong coupling also may drag down
assemblages preserved as micro-inclusions in the hanging-wall lithosphere during the subduction
core, mantle or rim of zircon grains, has increased process, as might be suggested for the coesite-
our confidence that we know the true ‘age’ of bearing eclogites of Pliocene age in Eastern Papua
close-to-peak metamorphism or at least a particular New Guinea (Baldwin et al. 2004). Additionally,
P–T point or range in many metamorphic belts. Searle et al. (2001) have argued that deep earth-
However, for UHTM rocks accessory phase geo- quakes of the Hindu Kush seismic zone represent
chronology most commonly records crystallization a tracer of contemporary coesite and diamond
along a segment of the high-temperature cooling formation in crustal rocks, and Rondenay et al.
path (Harley et al. 2007; Baldwin & Brown 2008; (2008) have used seismic imaging to reveal the
Harley 2008; Kelsey 2008). With these caveats in release of metamorphic fluids during subduction
mind we may now assess reliably the imprint of zone metamorphism.
metamorphism through Earth history. Lawsonite blueschists and lawsonite eclogites
generally record ultra-low-temperature subduction
Metamorphism since the in comparison with typical HPM –UHPM. We
know this from thermodynamic modelling of
Cretaceous Period Eocene lawsonite blueschists and lawsonite eclo-
In this section, I discuss the relationship between gites exhumed to the Earth’s surface in the Pam
the three types of metamorphism based on ranges Peninsular, New Caledonia (Clarke et al. 1997,
of apparent thermal gradient identified above and 2006), and xenoliths of lawsonite eclogite of
the variety of tectonic settings that are associated Eocene age inferred to record ultra-low-temperature
with convergent plate boundary zones during the subduction to .3.5 GPa brought up in Tertiary
interval of Earth history for which information kimberlite pipes at Garnet ridge in the Colorado
about plate kinematics may be retrieved from the Plateau, USA (Usui et al. 2003, 2006).
magnetic anomaly patterns in the ocean basins. In
addition, I review likely sources of heat to drive Medium-temperature eclogite – high-
metamorphism in general, and to drive high
dT/dP metamorphism in particular. pressure granulite metamorphism
(intermediate d T/d P)
High-pressure metamorphism – Modern examples of medium-temperature eclo-
ultrahigh-pressure metamorphism gite –high-pressure granulite metamorphism are
(low d T/d P metamorphism) not common at outcrop in orogens of Cenozoic
age. One example within the appropriate range of
High-pressure metamorphism–ultrahigh-pressure apparent thermal gradients occurs in the upper
metamorphism (blueschist –eclogite –ultrahigh- units of the Nevado –Filabride Complex in the
pressure facies series; Fig. 5) most commonly Betic Cordillera of southern Spain (Platt et al.
occurs associated with subduction, particularly in 2006), where eclogite lenses sheathed by blueschist
the subduction zone prior to and during collision. within pelitic schist yield P– T of 1.1– 1.8 GPa
Evidence includes samples of glaucophane schist and 550–700 8C yielding dP/dT around 390 8C
entrained in serpentine mud volcanoes in the GPa21. These units record clockwise P– T paths,
Mariana forearc (e.g. Shipboard Scientific Party and the evolution from HPM conditions to E–
2002), where incipient blueschists with the assem- HPGM conditions records the transition from
blage lawsonite–pumpellyite –hematite yield T of subduction to collision. Another example from a
150 –250 8C at P of 0.5– 0.6 GPa (Maekawa et al. collision zone is provided by Miocene age crustal
1993) and dT/dP of 300 –400 8C GPa21 (approxi- xenoliths from the southern Pamir (Hacker et al.
mately equivalent to 9–11 8C km21). This apparent 2005), where basaltic eclogite, sanidine eclogite
thermal gradient is consistent with thermal models and high-P felsic granulite record P–T of
for subduction zones (e.g. Hacker et al. 2003) and 2.5 –2.8 GPa and 1000–1100 8C yielding dP/dT
apparent thermal gradients derived from Eocene around 400 8C GPa21.
low-grade blueschist-facies series rocks (e.g. Potel High-P granulites also are reported from two
et al. 2006). exhumed Cretaceous arcs. The first example is the
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48 M. BROWN

Jijal complex in northern Pakistan, part of the (Hyndman et al. 2005a, b), as suggested for the Var-
Kohistan arc, which cooled from magmatic con- iscan evolution of the Moldanubian Zone in the
ditions to E –HPGM conditions of 2.0–1.5 GPa Vosges Mountains by Schulmann et al. (2002), or
and 1100–800 8C, based on relict magmatic assem- by extension of the lithosphere and advection
blages within overprinting metamorphic assem- of heat with magma (Dewey et al. 2006). This is
blages (Ringuette et al. 1999; Yamamoto et al. discussed further below.
2005). The second example is the Arthur River
complex in Fiordland, New Zealand, which yields What is the source of the heat? The mechanism
E–HPGM conditions of 1.4 GPa and .750 8C that provides the heat necessary to drive high
(Clarke et al. 2000; Daczko et al. 2001; Hollis dT/dP metamorphism remains elusive in most cir-
et al. 2003, 2004). Both arcs yield apparent thermal cumstances. Radiogenic heating is important and
gradients of 530 –550 8C GPa21. sometimes is argued to be sufficient, particularly if
I interpret E–HPGM to register collisional time for incubation is available between thickening
orogenesis, most probably identifying the suture and reaching peak metamorphic conditions (Le
where elements of the subduction complex and the Pichon et al. 1997; Gerdes et al. 2000; Jamieson
overriding plate become tectonically juxtaposed et al. 2002; Andreoli et al. 2006; McLaren et al.
and commonly become structurally attenuated. 2006; McKenzie & Priestley 2008). The observation
Thus, E –HPGM registers the subduction-to-col- that belts of regional metamorphism typically
lision process. In Phanerozoic orogens E–HPGM contain abundant crust-derived intrusive rocks
may be closely associated with HPM –UHPM. leads to the postulate that intracrustal magmatism
increases the regional thermal gradient at shallow
Granulite – ultrahigh-temperature crustal levels, consistent with observations from
metamorphism (high d T/dP) several metamorphic belts that medium-pressure
regional metamorphism grades with decreasing
Granulite–ultrahigh-temperature metamorphism crustal depth into regional-scale contact meta-
(granulite–ultrahigh-temperature facies series) morphism (e.g. Brown & Solar 1999). In addition,
occurs in the deeper parts of Mesozoic oceanic pla- in accretionary orogenic systems ridge subduction
teaux (e.g. Gregoire et al. 1994; Shafer et al. 2005) may introduce hot asthenospheric mantle to the
and in exposed middle to lower crust of young con- base of the overriding plate to generate anomalous
tinental arcs (e.g. Lucassen & Franz 1996). In high dT/dP conditions in the forearc leading to ana-
addition, metapelitic xenoliths retrieved from texis at shallow crustal levels (Brown 1998a, b;
Neogene volcanoes in central Mexico (Hayob Groome & Thorkelson 2008).
et al. 1989, 1990) and at El Joyazo in SE Spain Apparent thermal gradients that significantly
(Cesare & Gomez-Pugnaire 2001) record evidence exceed 750 8C GPa21 are retrieved from G –
of Cenozoic G –UHTM during crustal extension, UHTM terranes (see below), but these gradients
and evidence of melt-related processes in lower cannot be sustained to mantle depths in overthick-
crustal garnet granulite xenoliths from Kilbourne ened crust without exceeding the peridotite
Hole, Rio Grande Rift, suggests contemporary solidus. Two alternatives are implied by this
G– UHTM in the lower crust of rifts (Scherer observation. The first is that the asthenosphere was
et al. 1997). It is likely that G –UHTM conditions close to the Moho, which could lead to transient
are being generated today in orogenic hinterlands, thermal gradients up to the conductive limit
such as under Tibet and the Altiplano, based on (1000 8C GPa21, potentially higher over a plume
the interpretation of multiple geophysical datasets head), which might occur, for example, if a litho-
(e.g. Nelson et al. 1996; Schilling & Partzsch spheric root was removed as a result of convective
2001; Unsworth et al. 2005) that suggest the pre- instability (Platt & England 1994; Platt et al.
sence of melt, inferred to have been derived from 1998, 2003) or if the subducting slab breaks off
mica breakdown melting, and new inferences from (Davies & von Blanckenburg 1995). The second is
seismic data and expectations from numerical mod- that there was sufficient advection of heat into the
elling (Le Pichon et al. 1997; McKenzie & Priestley crust with mantle-derived magma (Sandiford &
2008). The rarity of young G –UHTM rocks at the Powell 1991; Stüwe 2007), for which evidence is
Earth’s surface probably reflects the general expec- generally scarce (Harley 2004). An example occurs
tation that granulites are exhumed in a subsequent in the Bohemian Massif, where there appears to be a
orogenic cycle (Harley 1989). Metasedimentary relationship between the late Variscan ultrapotassic
protoliths may attain G –UHTM P –T conditions magmatism and G –UHTM in the Moldanubian
by internal heating if sufficient time is available Zone (Janousek & Holub 2007; Leichmann et al.
(Le Pichon et al. 1997; McKenzie & Priestly 2007). Underplating by basaltic magma commonly
2008) or by inversion of thinned lithosphere with is implicated to provide heat (e.g. Dewey et al.
high heat flow such as occurs in back-arcs 2006), but the temporal relations between extension,
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METAMORPHIC PATTERNS IN OROGENS 49

crustal melting and emplacement of basic magma The reason subduction zone back-arcs are hot may
sometimes are ambiguous (e.g. Barboza et al. be principally related to thin lithosphere and
1999; Peressini et al. 2007). Modelling studies shallow convection in the mantle wedge astheno-
suggest that heating by multiple intraplating by sphere, where convection is inferred to result from
dykes and sills may be effective at melting the a reduction in viscosity induced by water from the
lower crust in continental arcs (e.g. Jackson et al. underlying subducting plate, with additional
2003; Dufek & Bergantz 2005; Annen et al. 2006). effects from local extension (Currie & Hyndman
Heating by viscous dissipation may be important 2006, 2007; Schellart 2007a; Currie et al. 2008).
in some subduction-to-collision orogenic systems. Most circum-Pacific mountain belts are broad
Although viscous dissipation may generate heat zones of long-lived tectonic activity because they
(e.g. Kincaid & Silver 1996; Stüwe 1998; Leloup remain sufficiently weak to deform by the forces
et al. 1999; Burg & Gerya 2005), it requires initially developed at plate boundaries.
strong lithosphere (e.g. a differential stress of
100 –300 MPa); England & Houseman (1988) sug- Summary
gested that a differential stress of 100– 200 MPa is
necessary to generate a plateau, consistent with I conclude that metamorphism since the Cretaceous
this requirement. The strong positive correlation Period occurs in several different thermal environ-
between overall intensity of viscous heating in ments. One environment is characterized by low
crustal rocks and the instantaneous convergence dT/dP, corresponding to the subduction zone or to
rate suggests that a significant contribution the transition from subduction to collision. The
(.0.1 mW m23) of viscous heating into the crustal subduction-to-collision setting also appears to be
heat balance may be expected when the convergence appropriate for intermediate dT/dP metamorphism,
rate exceeds 1 cm a21, particularly if the lower crust where some components are related to subduction
is strong (Burg & Gerya 2005). Therefore, heating by (medium-temperature eclogite) but other compo-
viscous dissipation may become a dominant heat nents record thickening of the crust in arcs (high-
source in the early stage of subduction-to-collisional pressure granulite). These elements have become
orogenesis if convergence rates are rapid (e.g. the juxtaposed during the collision process. Another
Himalayas), and it may be significant in the heat environment is characterized by high dT/dP, corre-
budget of orogens (e.g. Stüwe 2007). sponding to crustal extensional settings or the
Currently, the major mountain belts of the back-arc or the orogenic hinterland. The back-arc
circum-Pacific orogenic systems are located in and orogenic hinterland may be inverted or
former subduction zone back-arcs, which are charac- thickened during collision and may be modified
terized by high heat flow of 69 + 16 to 85 + subsequently by orogenic collapse or extension.
16 mW m22 for continental crust with average
radiogenic heat production (Hyndman et al.
2005a; Currie & Hyndman 2006; Currie et al. Metamorphism earlier in the
2008), similar to the average value of Phanerozoic Eon
65 + 10 mW m22 for Variscan crust of the Iberian
mainland (Fernandez et al. 1998) but lower than In this section, I discuss the relationship between the
the range of 90 –150 mW m22 for a limited different types of metamorphism and orogenic
number of measurements from Tibet (Francheteau systems during the interval of Earth history for
et al. 1984). Currie & Hyndman (2006) reported which there is no disagreement about the operation
observations that indicate Moho temperatures of of modern plate tectonics (Stern 2005; Brown 2006).
800 –900 8C, uniformly high temperatures of
1200 8C in the shallow mantle and a thin lithosphere Accretionary orogenic systems
c. 60 km thick over back-arc widths of 250 to
.900 km, compared with Moho temperatures of High dT/dP metamorphism in accretionary oro-
400 –500 8C and lithosphere 200 –300 km thick genic systems associated with retreating trench–
for cratons; the difference results in back-arc litho- advancing trench tectonics, such as occurred in the
sphere being at least an order of magnitude Tasmanides of eastern Australia (Collins 2002) or
weaker than cratons. Similar high temperatures are the Acadian of northeastern North America (Solar
inferred for extensional back-arcs of the western & Brown 2001a, b), commonly follows counter-
Pacific and southern Europe, but the thermal struc- clockwise P–T –t paths in which deformation
tures are complicated by extension and spreading (thickening) and metamorphism (heating) proceed
(Currie & Hyndman 2006). contemporaneously and peak metamorphism is
Following termination of subduction by col- late syntectonic (e.g. Solar & Brown 1999). Evi-
lision, the high temperatures decay over a time dence of extreme metamorphism generally is
scale of about 300 Ma (Currie & Hyndman 2006). absent from the level of erosion. None the less,
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50 M. BROWN

blueschists may occur sporadically early in the oro- Ridgway & Flesch 2007). The Mesozoic– Cenozoic
genic cycle (e.g. the southern Lachlan orogen in the Cordilleran orogen of the western USA (Fig. 1)
Tasmanides; Spaggiari et al. 2002a, b) or may be includes multiple tectonostratigraphic terranes prin-
exposed during core complex formation late in the cipally composed of subduction complexes and
orogenic cycle (e.g. the New England orogen in interoceanic island arcs that were accreted sequen-
the Tasmanides; Little et al. 1992, 1993, 1995; Hol- tially to the continental margin since the Triassic
combe & Little 1994). G –UHTM may be implied at Period (Dickinson 2008). Because of this terrane
depth (e.g. the Lachlan orogen; Collins 2003). accretion, the continental margin of the western
Short-lived phases of orogenesis may relate to inter- USA my have grown by as much as 800 km
ruptions in the continuity of subduction by topo- during Mesozoic –Cenozoic Cordilleran orogenesis.
graphic features on the ocean plate, particularly Paired metamorphic belts sensu Miyashiro
oceanic plateaux (Cloos 1993; Koizumi & Ishiwa- (1961) are a characteristic that results from orogen-
tari 2006), as has been suggested for the Lachlan parallel terrane migration and juxtaposition by
orogen (Collins 2002). Conversely, orogen-wide accretion of contemporary belts of contrasting
change from a retreating trench to an advancing type (Brown 1998a, b, 2002b). These paired belts
trench may relate to changes in plate kinematics comprise a low dT/dP metamorphic belt outboard
and advance of the subduction hinge (e.g. the and a high dT/dP metamorphic belt inboard, as
Acadian orogen in the Appalachians; Solar et al. exemplified by the Mesozoic metamorphic belts
1998; Solar & Brown 2002b). of Japan (Miyashiro 1961) and the Palaeozoic –
In contrast, we may consider the Andes, where Mesozoic metamorphic belts of the western USA
the Late Palaeozoic accretionary prism of the (Ernst et al. 1990; Patrick & Day 1995). In some
Coastal Cordillera in Chile (Willner 2005) and systems, an important additional feature was ridge
the Cretaceous Diego de Almagro Metamorphic subduction, which may be reflected in both the
Complex in Chilean Patagonia (Willner et al. pattern of high dT/dP metamorphism and the asso-
2004) are characterized by low dT/dP metamorph- ciated magmatism (Hole & Larter 1993; Thorkelson
ism, including blueschists, registering a thermal 1996; Brown 1998b; Sisson et al. 2003; Farris &
environment with an apparent thermal gradient Paterson 2009), although the thermal effect of
around 350 8C GPa21. Such conditions may be ridge subduction may vary with orientation of the
more typical of an advancing subduction hinge in ridge segment in relation to the trench and
the early stage of an advancing trench –retreating whether separation continues as the ridge is sub-
trench cycle. It should be noted, however, that the ducted (e.g. Daniel et al. 2001; Farris & Paterson
modern Andes probably was developed as a conse- 2009; Groome & Thorkelson 2008).
quence of two factors (Schellart 2008b), the large Granulites may occur at the highest grade of
slab width (i.e. trench-parallel extent) and the tren- metamorphism in the high dT/dP belt (Osanai
chward motion of the overriding plate (i.e. trench et al. 1998; Miyazaki 2004). Although high-pressure
advance). rocks such as lawsonite eclogites are common
In accretionary orogenic systems that involve (Tsujimori et al. 2006), UHPM generally appears
terrane accretion, allochthonous and/or para- to be absent in the outboard low dT/dP belt.
autochthonous elements become accreted to conti- Examples with UHPM registered include the
nental margins in convergent systems that involved Sambagawa Belt in Japan (Ota et al. 2004) and the
oblique relative plate motion vectors (e.g. Dewey western USA at Garnet ridge, on the Colorado
et al. 1990). Accretion at trenches has played a Plateau (Watson & Morton 1969; Helmstaedt &
major role in the growth and evolution of many seg- Schulze 1988; Usui et al. 2003, 2006); additional
ments of the circum-Pacific margin, as revealed by studies elsewhere may reveal UHPM as the norm
investigations of the geology around the Pacific in terrane accretion orogenic systems formed
rim since the 1970s. These studies identified the during the Phanerozoic Eon. Rapid loading and
importance of tectonostratigraphic terranes in unloading in some sectors of contractional (trans-
understanding Pacific rim geology (Jones et al. pressive) continental arcs is an additional feature
1983; Howell et al. 1985; Coney 1992) and the in some terrane accretion orogenic systems, ident-
metamorphic patterns in accretionary orogens ified by close-to-isothermal increase followed by
(Ernst et al. 1990). Much of the Cordillera in close-to-isothermal decrease in P, which yields a
western North America comprises a collage in distinctive P– T–t path (Hiroi et al. 1998; Whitney
which terranes have been either accreted to the con- et al. 1999).
tinent or displaced along the trench by orogen-
parallel motions during the Mesozoic and Cenozoic Subduction-to-collision orogenic systems
(Coney et al. 1980; Jones et al. 1983; Ernst et al.
1990; Johnston 2001, 2008; Dickinson 2004; In subduction-to-collision orogenic systems, meta-
Johnston & Borel 2007; Redfield et al. 2007; morphism associated with continental subduction
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METAMORPHIC PATTERNS IN OROGENS 51

and terminal continental collision follows standard record generated by the complex interactions and
subduction-related metamorphism of accreted diversity of processes along plate boundary zones.
materials; initial subduction of the continental My aim now is to extend this analysis back through
margin may generate HPM– UHPM of continental the Precambrian using the record of metamorphism
crust as it is being subducted until the subduction exposed at the surface in continents.
zone is choked. Conductive relaxation associated
with thickening across the suture may lead to Metamorphism since the
E–HPGM of suture-related rocks, and deformation Neoarchaean Era
of the hinterland may generate clockwise meta-
morphic P –T– t paths in granulite-facies series As discussed above, metamorphic belts are classi-
rocks, possibly leading to UHTM conditions, fied into three types according to the characteristic
according to the mechanisms and relative rates of metamorphic facies series and peak P –T conditions
thickening and thinning versus heat transfer by registered by the belt. In this section, I review the
conduction and advection. In many subduction-to- geological record of metamorphism since the
collision orogenic systems penetrative deformation Neoarchaean Era in relation to this typology using
largely precedes the peak of metamorphism because the dataset from Brown (2007a).
rates of continental deformation typically are about HPM –UHPM is characterized by lawsonite
an order of magnitude faster than rates of thermal blueschist- to lawsonite eclogite-facies series rocks
equilibration over the length scale of the crust and blueschist- to eclogite- to ultrahigh-pressure-
(Stüwe 2007). facies series rocks, where T plotted in Figure 6a is
The European Variscides represent a classic that registered at maximum P. E– HPGM is charac-
example of a subduction-to-collision orogenic terized by facies series that reach peak P –T in the
system. Here early (Devonian) subduction-to- eclogite –high-pressure granulite facies (O’Brien &
collision-related HPM – UHPM is registered in the Rötzler 2003), where maximum P and T generally
Galicia –Trás-os-Montes allochthon of NW Iberia are achieved sequentially, but close enough to be
(Martı́nez Catalán et al. 1997), the Champtoceaux considered as contemporaneously for this analysis
and Essarts complexes of southern Brittany (Bosse (Fig. 6a). G –UHTM is characterized by granulite-
et al. 2000; Godard 2001), the Monts du Lyonnais facies series rocks that may reach ultrahigh-
unit in the eastern French Massif Central (Lardeaux temperature metamorphic conditions, where P
et al. 2001) and the Saxothuringian belt across plotted in Figure 6a is that registered at maximum
the German –Czech border (Massonne 2001; T. Each metamorphic belt is represented by a
Konopásek & Schulmann 2005). This is followed single datum that is my best estimate of representa-
by later (Carboniferous) high dT/dP metamorphism tive peak P –T conditions, which defines an apparent
and anatexis in the foreland in the Central Iberian thermal gradient, and close-to-peak age as defined
Zone (Pereira & Bea 1994), the southern Brittany above. These arbitrary simplifications ignore the
migmatite belt (Johnson & Brown 2004) and dynamic nature of metamorphism, recorded by
the central French Massif Central (Macaudière P –T– t paths, and the evolution of geotherms in
et al. 1992; Montel et al. 1992), and E –HPGM to orogens with time. A future analysis could be
G –UHTM in the Saxothuringian belt and the extended to take account of the spatial and temporal
Bohemian Massif (e.g. Willner et al. 1997; variation within each metamorphic belt. Bearing in
O’Brien & Rötzler 2003; Štı́pská & Powell 2005). mind these caveats, let us examine the rock record
At a larger scale, the variation in metamorphic of metamorphism.
style of subduction-to-collision orogenic systems The P– T value for each terrane shown in
is well illustrated by the variation along the length Figure 6b records a point on a metamorphic (transi-
of both the Appalachian/Caledonian–Variscide – ent) geotherm, and different apparent thermal
Altaid and Alpine– Himalayan–Cimmerian chains. gradients are implied by each type of metamorphism.
These apparent thermal gradients are inferred to
Summary reflect different tectonic settings. HPM –UHPM is
characterized by apparent thermal gradients of
Each of these orogenic systems that formed before 150–350 8C GPa21 (approximately equivalent to
the Cretaceous Period in the Phanerozoic Eon pre- 4 –10 8C km21), and plots across the boundary
serves metamorphic belts with contrasting types of between the ‘cooler than normal’ and ‘ultra-low-
metamorphism that may be inferred to record temperature’ fields. About half of these terranes
contrasting thermal regimes. These examples are require a process other than simple thickening
consistent with the premise that plate tectonics to achieve such cold gradients. We know from
may be extrapolated back through at least three the global context that all of these terranes were
ocean lithosphere turnover cycles (to c. 600 Ma); associated with subduction, so it is likely that
they also highlight the variable imprint in the rock subduction was the process that created the
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52 M. BROWN

Fig. 6. Left. Metamorphic patterns based on representative ‘peak’ metamorphic P– T conditions of metamorphic belts
in relation to the metamorphic facies. Common granulite and granulite-facies ultrahigh-temperature metamorphism
(G–UHTM belts; P at maximum T; light grey circles are common granulite belts and dark grey circles are granulite-
facies ultrahigh-temperature metamorphic belts; data from tables 1 and 2 of Brown 2007a); medium-temperature
eclogites– high-pressure granulites (E– HPGM belts; peak P– T; diamonds; data from table 3 of Brown 2007a);
lawsonite blueschists– lawsonite eclogites and ultrahigh-pressure metamorphic rocks (HPM– UHPM belts; T at
maximum P; light grey squares are lawsonite-bearing rocks and black squares are ultrahigh-pressure belts; data from
tables 4 and 5 of Brown 2007a). Right. Peak P –T values for each terrane (from Fig. 3a) in relation to a normal
(conductive) continental geotherm. Each of these data records a point on a metamorphic (transient) geotherm, and the
different apparent thermal gradients implied by each type of metamorphism are inferred to reflect different tectonic
settings and thermal regimes.

ultra-low-temperature environment. E–HPGM is expect that the high dT/dP metamorphism would
characterized by apparent thermal gradients of be of similar age to or younger than the low to
350– 750 8C GPa21 (10–20 8C km21), and plots intermediate dT/dP metamorphism, as the latter
across the normal continental geotherm but mostly records the transition from subduction to collision
in the field where heating may be a conductive whereas the former records inversion of a back-arc
response to thickening. G –UHTM is characterized basin (Hyndman et al. 2005a, b) or heating by radio-
by apparent thermal gradients 750 8C GPa21 active decay in a thickened orogenic hinterland
(20 8C km21), and mostly plots across the bound- (Le Pichon et al. 1997; McKenzie & Priestley
ary into the field that requires an advective com- 2008). At present we do not have data to test this
ponent of heating. prediction.
Figure 7 illustrates apparent thermal gradient, Analysis of the data in Figure 7 provides a set of
which is inferred to relate to tectonic setting, plotted compelling first-order observations from which to
against age of peak metamorphism. Each type of argue that the modern era of ultra-low-temperature
metamorphism has a distinct range of apparent subduction began in the Neoproterozoic Era, as
thermal gradient, as anticipated from Figure 6, and registered by the occurrence of HPM –UHPM, but
HPM– UHPM is restricted to the later part of the that ultra-low-temperature subduction alone is not
Neoproterozoic Era and the Phanerozoic Eon. the hallmark of plate tectonics. In contrast, G –
However, what is now clear is the dual nature of UHTM and E–HPGM are present in the exposed
the thermal regimes represented in the metamorphic rock record back to at least the Neoarchaean Era,
record since the Neoarchaean Era. The period from registering a duality of thermal regimes, which has
the Neoarchaean Era to the Neoproterozoic Era been argued to represent the hallmark of plate tec-
is characterized by G –UHTM and E– HPGM, tonics (Brown 2006). Based on this observation,
whereas the period from late in the Neoproterozoic plate tectonics processes probably were operating
Era and through the Phanerozoic Eon is character- in the Neoarchaean Era as recorded by the imprints
ized by HPM– UHPM and E–HPGM together of dual types of metamorphism in the rock record,
with G –UHTM, although the latter crops out only and this may manifest the first record of a global
sporadically after the Cambrian Period. We might plate tectonics mode on Earth.
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METAMORPHIC PATTERNS IN OROGENS 53

Fig. 7. Apparent thermal gradient (in 8C GPa21) plotted against age of peak metamorphism (in Ma) (data from tables
1– 5 of Brown 2007a) for the three main types of extreme metamorphic belt (G– UHTM, circles; E –HPGM, diamonds;
HPM– UHPM, squares; for further details see Fig. 6) for two time intervals: (a) Phanerozoic and Neoproterozoic;
(b) Mesoproterozoic to Neoarchaean.
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54 M. BROWN

Fig. 8. Plot of apparent thermal gradient (in 8C GPa21) v. age of peak metamorphism (in Ma) for the three main types
of extreme metamorphic belt (G– UHTM, circles; E –HPGM, diamonds; HPM–UHPM, squares; for further details see
Fig. 6); an approximate conversion to 8C km21 is also shown (data from tables 1– 5 of Brown 2007a). Also shown are
the timing of supercraton amalgamation (Vaalbara, Superia and Sclavia) and supercontinent amalgamation (Nuna
(Columbia), Rodinia and Pannotia and Gondwana as steps to Pangaea), and the start of the Proterozoic plate tectonics and
modern plate-tectonics regimes in relation to thermal gradients for the three main types of extreme metamorphic belt.

The distribution of these types of metamorph- observations are inconsistent with a progressively
ism throughout Earth history based on the age of degraded record with increasing age. Extrapolation
close to peak metamorphism as defined above is back in time also raises questions about partial to
displayed in Figure 8 together with periods of complete overprinting by younger orogenic events
supercontinent amalgamation. Changes in the on older orogens, which is a concern in any meta-
metamorphic record broadly coincide with the tran- morphic study. However, our ability to recognize
sitions from the Archaean to Proterozoic Eons and the effects of overprinting has improved signifi-
the Proterozoic to Phanerozoic Eons, and imply a cantly (discussed above), and overprinting has
different style of tectonics in the Archaean Eon in been avoided in compiling the dataset used for this
comparison with the Proterozoic Eon and in the analysis (Brown 2007a). Finally, as discussed above
Proterozoic Eon in comparison with the Phanero- it is likely that some Phanerozoic G –UHTM rocks
zoic Eon. Overall, the restricted timespan of differ- have not yet been exposed at Earth’s surface,
ent types of metamorphism through Earth history leading to possible bias in the younger part of
and the periods of metamorphic quiescence the record.
during the Proterozoic Eon suggest a link with
the supercontinent cycle and major events in the The transition from the Archaean Eon
mantle. This issue is discussed in more detail to the Proterozoic Eon
later in this review.
Granulite-facies ultrahigh-temperature metamor-
Caveats There are several caveats about possible phism (G–UHTM) is documented in the rock record
bias in this record. It is commonly argued that going predominantly from the Neoarchaean Era to the
back through time increases loss of information by Cambrian Period, although it may be inferred at
erosion of the older record. However, the data in depth in some younger orogenic systems (Le
Figures 7 and 8 plot in particular periods and there Pichon et al. 1997; Brown 2007a; McKenzie &
is a clear distinction between the period before the Priestley 2008). The first occurrence of G –UHTM
Neoproterozoic Era, where UHPM does not occur, in the rock record signifies a change in geodynamics
and during the Neoproterozoic Era and the Phaner- that generated transient sites of very high heat flow,
ozoic Eon, where UHPM is common. These perhaps analogous to modern subduction zone
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METAMORPHIC PATTERNS IN OROGENS 55

back-arcs (and possible deep in arcs) or orogenic worldwide during the Palaeoproterozoic Era as
hinterlands. In the Neoarchaean Era, G– UHTM evidenced in orogenic belts that suture Nuna
occurs within the Kaapvaal Craton and the Southern (Columbia), unless this distribution is an artefact
Marginal Zone of the Limpopo Belt, southern of (lack of ) preservation or thorough overprinting
Africa, and in the Lewisian Complex of the Assynt of eclogite in the exposed Archaean cratons.
terrane, Scotland (the Badcallian event), at c. 2.72 – The western Superior craton in Laurentia rep-
2.69 Ga, and within the Napier Complex, East resents a prime example of craton formation by
Antarctica, and the Andriamena Unit, Madagascar, terrane accretion. Although elements of the geology
at c. 2.56 –2.46 Ga (references in Brown 2007a, extend back perhaps to the Eoarchaean Era (e.g.
2008). Böhm et al. 2000), the western Superior craton
Although well-characterized examples are was created by a sequence of five orogenic events
rare in the transition from the Neoarchaean Era that assembled continental and oceanic terranes
to the Palaeoproterozoic Era, medium-temperature into a coherent unit, part of the supercraton
eclogite– high-pressure granulite metamorphism Sclavia (Bleeker 2003), during the Mesoarchaean
(E–HPGM) also is first recognized in the Neoarch- and Neoarchaean Eras (Percival et al. 2006). The
aean Era and occurs at intervals throughout the rock constituent terranes exhibit similar sequences of
record during the Proterozoic and Palaeozoic Eons events comprising termination of arc magmatism,
(Brown 2007a). E –HPGM belts are inferred to early deformation, synorogenic sedimentation,
record subduction-to-collision orogenesis and are sanukitoid magmatism, main-phase shortening
complementary to G– UHTM belts. However, deformation and regional metamorphism to granulite
there are not yet sufficient localities for these facies (e.g. Pikwitonei granulite domain; Mezger
types of metamorphism in the Neoarchaean Era et al. 1990; Böhm et al. 2004), late transpression,
to determine whether they are contemporary or not and emplacement of crust-derived granites. Seismic
in any particular orogen. The oldest occurrence of reflection and refraction images indicate north-
E–HPGM is represented by eclogite blocks within dipping structures, interpreted as a stack of discrete
mélange in the Gridino Zone of the Eastern 10 –15 km thick terranes. A model of progressive
Domain of the Belomorian Province, White Sea, accretion by early plate tectonics is suggested by
Russia. The eclogite-facies metamorphism appears the presence of a slab of high-velocity material,
to have been reliably dated at c. 2.72 Ga and the inferred to represent subcreted oceanic lithosphere,
P–T data of 1.40 –1.75 GPa and 740 –865 8C are and Moho offsets, consistent with shallow subduc-
well characterized. This occurrence is one of the tion (Percival et al. 2006).
earliest records of E –HPGM within a suture zone The transition to a Proterozoic plate-tectonics
and is critical in evaluating the start of plate tec- regime resulted in stabilized lithosphere in which
tonics, but the fact that the ages come from blocks cratons form the cores of continents that sub-
in a mélange cannot be avoided and does not neces- sequently grew dominantly by marginal accretion.
sarily undermine the antiquity of these eclogites. Furthermore, the transition coincides with the first
The occurrence of both G –UHTM and E– occurrence of ophiolite-like complexes in suture
HPGM belts since the Neoarchaean Era manifests zones during the Proterozoic Eon (Moores 2002)
the onset of a ‘Proterozoic plate-tectonics regime’, and with the increase in d18O of magmas through
which may have begun locally during the Mesoarch- the Palaeoproterozoic Era, which may reflect matu-
aean and Neoarchaean Eras and may have become ration of the crust, the beginning of recycling of
global only during the transition to the Palaeoproter- supracrustal rocks and their increasing involvement
ozoic Eon (Brown 2007b, 2008). This premise is in magma genesis via subduction (Valley et al.
consistent with aggregation of continental crust 2005). Although the style of subduction during the
into progressively larger units to form supercratons, Proterozoic Eon remains cryptic, the change in
perhaps indicating a change in the pattern of tectonic regime whereby interactions between dis-
mantle convection during the transition to the crete lithospheric plates generated tectonic settings
Proterozoic Eon. with contrasting thermal regimes was a landmark
The emergence of plate tectonics requires forces event in Earth history (Brown 2006).
sufficient to initiate and drive subduction, and litho- As an example of accretion during the Palaeo-
sphere with sufficient strength to subduct coher- proterozoic Era, we may consider the metasedi-
ently. These requirements probably were met as mentary belts within the Lewisian complex of
basalt became able to transform to eclogite. Secular northwestern Scotland. Although the Lewisian
change in thermal regimes to allow this transforma- complex has been correlated with the intercontinen-
tion appears to have been gradual, occurring region- tal collisional belts of Palaeoproterozoic age that
ally first during the Mesoarchaean to Neoarchaean suture Laurentia, identifying an appropriate tectonic
Eras, leading to the successive formation of the setting for amalgamation of the various components
supercratons Vaalbara, Superia and Sclavia, and of the Lewisian complex has been elusive because
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56 M. BROWN

of uncertainty about the age and origin of the supra- Paired metamorphic belts revisited
crustal rocks (the Loch Maree Group on the
mainland and the Leverburgh Belt in South Harris, Orogens may be composed of belts with contrasting
Outer Hebrides) and the paucity of subduction- types of metamorphism that record different appar-
related intrusive rocks. The Loch Maree Group is ent thermal gradients. Classic paired metamorphic
made up of two components (Park et al. 2001), belts in which an inboard high dT/dP metamorphic
one oceanic (plateau basalts or primitive arcs, plus belt is juxtaposed against an outboard low dT/dP
associated abyssal sediments, ferruginous hydro- metamorphic belt along a tectonic contact are
thermal deposits, and platform carbonates) and the found in accretionary orogens of the circum-Pacific
other continental (deltaic flysch, greywacke shale), (Miyashiro 1961). Generally, they appear to result
and probably represents an accretionary complex from tectonic juxtaposition of terranes with differ-
similar to the Shimanto belt in Japan. Subsequently, ent metamorphic facies series that may or may not
the various elements of the Loch Maree Group be exactly contemporaneous and that may or may
became tectonically intermixed and subject to not be far-travelled (Brown 1998a, b, 2002b;
extreme deformation under amphibolite-facies con- Tagami & Hasebe 1999). This is an inevitable
ditions during accretion to an overriding Lewisian consequence of the difference between contem-
terrane in the Orosirian Period. Metamorphism in poraneous subduction, generating low dT/dP blues-
the Leverburgh Belt was more extreme, reaching chists and eclogites in the subduction zone and
high-pressure granulite-facies conditions (transi- extension in an arc – back-arc system with formation
tional UHTM to E –HPGM), also in the Orosirian of depositional basins, compared with the timing of
Period (Baba 1998, 1999; Hollis et al. 2006). The events such as ridge subduction (e.g. Brown 1998b,
preservation of evidence of prograde increase in 2002b) or deformation and thickening of the arc –
P, high P–T mineral assemblages, and chemical back-arc system (e.g. Collins 2002), generating
disequilibrium features in these rocks is attributed high dT/dP metamorphism. Thus, metamorphic
to a short-lived tectonometamorphic cycle that ter- belts of contrasting type may be formed during a
minated in rapid exhumation after a collisional single orogenic cycle along different sectors of a
event. Overall, the Lewisian complex is best inter- common convergent margin by multiple processes,
preted as a collage of terranes resulting from but juxtaposition may have been due to the obliquity
subduction-to-collision orogenesis, with the accre- of convergence and lateral translation along the
tionary complexes (all probably part of the same margin late in the orogenic event (e.g. Brown
unit) sandwiched between the terrane elements. 1998a).
However, the complex as a whole contains com- In Miyashiro’s original classification of types of
paratively little juvenile material of Palaeoprotero- metamorphism (Miyashiro 1961), an intermediate
zoic age, limited to possible arc remnants in the type of metamorphism was included for unpaired
Outer Hebrides (Roineabhal and Nis terranes) and belts such as those in the Scottish Highlands and the
the volcano-sedimentary assemblage at Loch Maree Northern Appalachians, although in both cases the
(Park et al. 2005). intermediate dT/dP metamorphic belt (Barrovian
The absence of HPM –UHPM terranes before type) is juxtaposed against a high dT/dP meta-
the Ediacaran may relate to weakness of the morphic belt (Buchan type). Miyashiro (1973) sub-
subducting lithosphere, which might have been sequently suggested that
strong enough to allow shallow subduction but the
rheology may have been too weak to allow deep ‘paired and unpaired (single) metamorphic belts form by
the same mechanism, and an unpaired belt represents
subduction of coherent slabs and/or provide a
paired belts in which the contrast between the two belts
mechanism for eduction of continental crust if, is obscure, or in which one of the two belts is undeveloped
indeed, it was ever subductable before the Ediacaran or lost’.
(Burov & Watts 2006; Brun & Faccenna 2008; van
Hunen & van den Berg 2008). This does not One issue to consider is whether to extend the
conflict with subduction of ocean lithosphere late concept of ‘paired metamorphic belts’ more widely
during the Archaean Eon and during the Proterozoic than accretionary orogens, outside the original usage
Eon, as early slab breakoff and slab breakup still by Miyashiro (1961), to subduction-to-collision oro-
might have transported materials with a surface genic systems, as perhaps was implied by Miyashiro
chemical signature deep into the mantle as recorded in his later publication (Miyashiro 1973). This issue
in some diamonds. Thus, subduction zone meta- is raised in part by the use of the term ‘paired meta-
morphism and the transition from subduction to morphic mountain belt’ by Goscombe & Hand
collision are recorded by eclogites and high- (2000) to refer to the eastern Himalaya in Nepal,
pressure granulites (intermediate dT/dP meta- where two sectors of the mountain belt are charac-
morphism) during the Neoarchaean Era and the terized by contrasting types of P–T path, clockwise
Proterozoic Eon. versus counter-clockwise in P –T space, but the
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METAMORPHIC PATTERNS IN OROGENS 57

metamorphism is of ‘Barrovian’ (intermediate dT/ lithosphere, we may anticipate longer wavelength

dP) and ‘Buchan’ (high dT/dP) type, respectively, convection and global variations in mantle heat
without a low dT/dP metamorphic belt at all. This loss and mantle potential temperature (Grigné
is similar to the Scottish Highlands, from where et al. 2005; Lenardic et al. 2005, 2006; Coltice
these two types of metamorphism were identified, et al. 2007).
and the two types of metamorphic belt in the North-
ern Appalachians (Spear et al. 2002). Correlations
The modern plate-tectonics regime is charac-
terized by a duality of thermal environments, the The temporal relationship between metamorphism
subduction zone and the suture zone of subduc- at extreme P –T conditions, plate tectonics and the
tion-to-collision orogens, and the back-arc or oro- supercontinent cycle is shown in Figure 8. There
genic hinterland, in which contrasting types of are five principal periods of crustal amalgamation
regional-scale metamorphic belts are being formed since the Neoarchaean Era (Bleeker 2003), as
contemporaneously. I consider this duality to be follows: formation of the supercratons Vaalbara,
the hallmark of one-sided subduction. Thus, the Superia and Sclavia from older superterranes and
characteristic imprint of one-sided subduction and juvenile crustal additions during the Mesoarchaean
perhaps of plate tectonics in the geological record and Neoarchaean Eras (Vaalbara slightly older and
will be the broadly contemporaneous occurrence Sclavia slightly younger); and, formation of the
of two contrasting types of metamorphism reflect- supercontinents Nuna (Columbia; 2.0– 1.8 Ga),
ing this duality of thermal environments (Brown Rodinia (1.2–1.0 Ga) and Pangaea (320–250 Ma;
2006). On this basis, I propose the following: via the intermediate steps of Pannotia (650 –
570 Ma) and Gondwana (570 –510 Ma)). There is
paired metamorphic belts are penecontemporaneous belts a correspondence between the timing of extreme
of contrasting type of metamorphism that record different
metamorphism in the rock record and suturing of
apparent thermal gradient, one warmer and the other
colder, juxtaposed by plate tectonics processes. continental lithosphere by subduction-to-collision
orogenesis into supercratons and supercontinents.
By this definition, the combination of G –
UHTM with E –HPGM (from the Neoarchaean The Phanerozoic Eon
Era to the Neoproterozoic Era) or HPM – UHPM
and/or E –HPGM with G –UHTM (during the Pha- The Phanerozoic Eon has been distinguished by
nerozoic Eon) may be described as a paired meta- rearrangement of the continental lithosphere within
morphic belt. This extends the original concept of a continent-dominated hemisphere in a series of
Miyashiro (1961) beyond the simple pairing of steps leading to the assembly of Pangaea during
high dT/dP and low dT/dP metamorphic belts in the Palaeozoic Era, followed by the early stage of
circum-Pacific accretionary orogens, and makes it supercontinent breakup and dispersal during the
more useful in the context of our better understand- Mesozoic –Cenozoic (Brown 2008). The formation
ing of the relationship between thermal regimes and of Pangaea involved successive subduction-to-
tectonic setting. This is particularly useful in sub- collision orogenic systems (both accretionary oro-
duction-to-collision orogenic systems, where an gens, involving ribbon continent terranes and/or
accretionary phase is overprinted by a collision arcs, and collisional orogens, involving multiple
phase that will be registered by the imprint of pene- continents) within a single (Pangaean) convection
contemporaneous low to intermediate dT/dP and cell, resulting in the Appalachian/Caledonian–
high dT/dP metamorphism in the rock record Variscide –Altaid and the Alpine –Himalayan –
(Brown 2006). Cimmerian orogenic systems. In contrast, a comp-
lementary (Panthalassan) convection cell, centred
The supercontinent cycle on the South Pacific low-velocity structure, is com-
posed of ocean lithosphere and defined at its outer
The continental lithosphere has been amalgamated edge by the circum-Pacific accretionary orogenic
into a supercontinent at several intervals during systems (Coney 1992; Collins 2003).
Earth history with dramatic effects on both surface The Panthalassan convection cell became estab-
and deep Earth processes. Aggregated continental lished towards the end of the Neoproterozoic Era,
lithosphere influences mantle heat loss by acting centred on the former Rodinia and most probably
as an insulator to the convecting mantle and by reflecting a slab ‘graveyard’ as a result of Rodinia
imposing its own wavelength on mantle convection assembly, and reached a maximum horizontal
by impeding downwelling (Le Pichon & Huchon extent around the Devonian –Carboniferous bound-
1984; Gurnis 1988; Guillou & Jaupart 1995; ary. It has been in decline since, as the Pangaean
Cooper et al. 2006; Coltice et al. 2007). There- cell, centred on the former Gondwana and most
fore, during periods of aggregated continental probably reflecting a slab ‘graveyard’ as a result of
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58 M. BROWN

Gondwana assembly, began to expand in the Examples of Wilson-type cycles and

Jurassic –Cretaceous Eras as excessive heating of intrasupercontinent orogenesis during
the upper mantle beneath Pangaea led to breakup
(Le Pichon & Huchon 1984; Gurnis 1988). the Phanerozoic Eon
Slab ‘graveyards’ might form after continent During Wilson-type cycles, ‘expansion’ of a
aggregation by collision if stagnant slabs break off continent-dominated hemisphere may be limited
and avalanche into the lower mantle. They are by the persistence of an opposed ocean-dominated
thought to be associated with P- and S-wave low- hemisphere, as has been the case for the Pacific
velocity structures (LVS; Christensen & Hofmann Ocean through the Phanerozoic Eon. In this case,
1994; McNamara & Zhong 2004; Tan & Gurnis modification of the way the continents are aggre-
2007). LVS coincide with positive anomalies in the gated requires splitting the continental lithosphere
geoid, are inferred to be warmer than surrounding to generate an internal ocean, such as the Iapetus
mantle and are interpreted to be high-bulk-modulus Ocean (Murphy & Nance 2005), which separated
domes composed of pyroxenite (perovskite-rich Laurentia from Baltica and Gondwana, or the
material in the lower mantle), supporting the link Rheic Ocean, which separated the Avalonian and
with subducted mid-ocean ridge basalt (MORB) Hunic terranes from Gondwana (von Raumer &
(Tan & Gurnis 2007). Confirmation of crustal recy- Stampfli 2008), or the closure of the Palaeotethys
cling in the Pacific mantle comes from Sr, Nd, Hf and the formation of the Neotethys as ‘Cimmeria’
and Pb isotope data from garnet-clinopyroxenite was sutured to Eurasia (Metcalfe 1996).
xenoliths in lavas from Malaita, Solomon Islands In the case of splitting off a terrane, the terrane
(Ishikawa et al. 2007). migrates across a closing ocean before suturing at
a new location on the opposite margin (commonly
Wilson-type cycles and Hoffman-type referred to as ‘terrane export’). Calving of another
breakups terrane from the parent continent by splitting
inboard from the margin and initiation of subduction
There are two contrasting models for the behaviour behind the newly accreted terrane on the opposite
of supercontinents (Brown 2008), based on different side of the new ocean in turn allows a second
concepts introduced by Wilson (1966, closing and terrane to migrate across a closing ocean to be
reopening an ocean) and Hoffman (1991, turning accreted on the opposite margin. The process,
a supercontinent inside out), respectively. In one which may continue to repeat, is best exemplified
model, continental lithosphere simply fragments by the dispersion of terranes from Gondwana
and reassembles along the same (internal) contacts across the eastern Tethys to form Asia by accretion
or introverts (the ‘Wilson cycle’ sensu stricto; (e.g. Metcalfe 1996). It is the successive closure of
Wilson 1966), whereas in the alternative model a these newly generated but relatively short-lived
supercontinent fragments, disperses and reassem- oceans by subduction and terrane export from one
bles by closure of the complementary superocean continental aggregate to another that leads to a
(Hoffman 1991). Wilson-type cycles operated during new arrangement of the continental lithosphere
the Phanerozoic Era, when the continental litho- within the continental hemisphere (e.g. Stampfli
sphere was restricted to one hemisphere. In contrast, et al. 2002; Collins 2003; von Raumer et al. 2003;
a Hoffman-type breakup was the process by which von Raumer & Stampfli 2008).
the Gondwanan elements of Rodinia were dispersed HPM –UHPM, which may be linked with or
and reassembled, possibly first as part of the ephem- transitional to E–HPGM, is the metamorphic style
eral Pannotia (Dalziel 1997) before final amalgama- generally associated with Wilson-type cycles during
tion as Gondwana (Collins & Pisarevsky 2005; the Phanerozoic Eon (Carswell & Compagnoni
Tohver et al. 2006). In addition, a Hoffman-type 2003; Liou et al. 2004), reflecting the short-lived
breakup was most probably the process by which subduction and terrane collision style of tectonics
Nuna (Columbia) was reconfigured into Rodinia, characteristic of the Caledonian–Variscide –Altaid
although in this case the continental fragments and the Alpine –Himalayan –Cimmerian orogenic
were fewer and larger (Condie 2002). Whether any systems (Brun & Faccenna 2008). These also corre-
Wilson-type cycles occurred within Rodinia or spond to the ‘push– pull’ cycles of Lister & Forster
Nuna remains to be confirmed, but there is no a (2006).
priori reason to suppose that Wilson-type cycles Numerical modelling of the minimum subduc-
did not occur during the Proterozoic Eon and the tion necessary to generate thermal conditions suit-
Trans-Hudson orogen in North America has been able for blueschist metamorphism is consistent
interpreted in terms of a Wilson-type cycle with limited subduction of short-lived ocean
(Murphy & Nance 2005). basins (Maresch & Gerya 2005). Because of the
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METAMORPHIC PATTERNS IN OROGENS 59

geometric interplay between the nose of the sub- Supercontinent fragmentation, dispersal and
ducting ocean lithosphere and the evolving array reassembly by Hoffman-type breakups
of isotherms, subduction of younger and hotter
ocean lithosphere may lead to earlier formation of Continental lithosphere tends to amalgamate over
blueschists than subduction of older and cooler cold downwellings to form a supercontinent, which
ocean lithosphere. Maresch & Gerya (2005) ident- inhibits subduction and mantle cooling (Gurnis
ified an optimum age of 40–60 Ma for subducted 1988). The mantle beneath the supercontinent may
ocean lithosphere to generate blueschist conditions. eventually overheat and become the site of a new
The thermal structure also explains the rarity of arc upwelling that fragments the overlying continental
volcanic rocks in these orogens because the necess- lithosphere under tension to dissipate the thermal
ary flux of water into the mantle wedge to drive anomaly. Thus, the breakup of Rodinia was
melting will be weak or even absent. centred over the Pacific LVS and the breakup of
The Appalachian orogen is rather different, the Gondwanan sector of Pangaea was centred
particularly in the northern sector, where older over the Africa LVS. The next supercontinent amal-
Barrovian and younger high dT/dP metamorphism gamates above areas of mantle downwelling by
may be a consequence of subduction and arc –back- subduction-to-collision orogenesis, as did the
arc formation on one or sometimes both margins of Gondwanan elements of Rodinia by the end of the
sutures between peri-Laurentian and peri-Gondwa- Cambrian Period. This is the basic principle that
nan terranes and intervening Iapetan elements. leads to breakup of one supercontinent and for-
UHPM rocks have not yet been identified in the mation of another. As a result of Hoffman-type
southern Appalachians, but HPM rocks are present breakup, the internal rifted margins of the old
(Dilts et al. 2006) and are a characteristic feature supercontinent become the external margins of the
of the final (Alleghanian) phase of orogenesis. new supercontinent, and the external accretionary
The Iapetus Ocean was closed along the margins of the old supercontinent become deformed
Appalachian– Caledonian orogenic system; the and smeared out along the internal sutures of the
intervening internally generated lithosphere was new supercontinent.
consumed during assembly of Laurentia, Avalonia Let us consider the transformation of Rodinia
and Baltica in the Early Devonian forming Lau- to Gondwana, which is the original example
russia (Murphy & Nance 2005). UHPM rocks are of a Hoffman-type breakup (Hoffman 1991). This
a feature of the Norwegian Caledonides (e.g. transformation involved the fragmentation, dis-
Carswell & Compagnoni 2003; Liou et al. 2004). persal and reassembly of the continental litho-
The Rheic Ocean was closed along the Variscide– sphere by subduction-to-collision orogenesis to
Altaid orogenic system. In the Variscide sector, form the network of Brasiliano and Pan-African
the intervening internally generated lithosphere belts that suture Gondwana (e.g. Cordani et al.
was consumed by subduction during clockwise 2003; Collins & Pisarevsky 2005), possibly via the
rotation of Gondwana and collision with Laurussia. ephemeral supercontinent Pannotia (Dalziel 1997).
UHPM rocks are a common feature of the European The orphaned Laurasian continental fragments
Variscides (e.g. Carswell & Compagnoni 2003; combined with each other at a later time and then
Liou et al. 2004). The Altaid Central Asian with Gondwana to form Pangaea by the Permian
Orogenic Belt is something of an enigma; although Period. The internal geometry of Rodinia changed
it includes some of the oldest formed and most considerably during its few hundred million years
extreme UHPM rocks known (at Kotchatev; of existence. Geological and palaeomagnetic data
Kaneko et al. 2000) it also includes high dT/dP suggest that the supercontinent consolidated at
metamorphic belts, inferred to record ridge–trench 1100– 1000 Ma and most probably disintegration
interactions, and arc collisions (Windley et al. began between 850 and 800 Ma (e.g. Cordani
2007; see also Filippova et al. 2001; Collins 2003). et al. 2003; Torsvik 2003). Reassembly to form
Within the Mesozoic –Cenozoic Tethysides, the Gondwana occurred by destruction of parts of the
Central Orogenic Belt of China (the Tienshan– complementary superocean as Rodinia progress-
Qinling –Dabie –Sulu orogenic system; e.g. Yang ively disintegrated, although the exact configuration
et al. 2005) and the Alpides (the Alpine –Zagros– and global location of the different continental
Himalayan orogenic system; e.g. Şengör 1987; lithosphere fragments in Rodinia at the time of
Hafkenscheid et al. 2006) are products of multiple breakup is uncertain (e.g. Murphy et al. 2004).
accretion events or collisions. In each case, the oro- Although G – UHTM and E–HPGM are associated
genic event involved the destruction of a relatively with the Brasiliano and Pan-African orogenic belts,
short-lived ocean, and the sutures commonly are the Trans-Saharan segment of the Pan-African also
decorated with occurrences of HPM and/or records the first coesite-bearing eclogites, and
UHPM rocks. sutures within the Anti-Atlas and the South China
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60 M. BROWN

block record the first blueschists. These features 2005a); and plate tectonics processes operating on
point to a transition caused by secular cooling Earth (Brown 2006, 2007b).
and the changeover to the ‘modern plate-tectonics Although part of the Isua Supracrustal Belt
regime’ characterized by colder subduction. appears to represent early ocean-floor and interocea-
nic island arc material (Furnes et al. 2007), it is by
no means clear that claims that the Isua Supracrustal
Ancient Earth Belt is dominantly a subduction–accretion complex
Metamorphism before the Neoarchaean are valid (Komiya et al. 2002). Early plate-like
behaviour may have allowed crust to float to
The rock record from the Eoarchaean Era through form thick stacks above zones of boundary-layer
the Mesoarchaean Era generally records P –T downwelling that ‘drip’ into the mantle, or, for a
conditions characteristic of low-to-moderate-P– slightly cooler Earth, thick stacks above zones
moderate-to-high-T metamorphic facies, yielding of ‘sub-lithospheric’ subduction, in which the
dT/dP in the range for G –UHTM but without mantle part of the ‘lithosphere’ is rigid enough
achieving the extreme temperatures that appear in to subduct (Fig. 9; Davies 1992; van Hunen et al.
the record during the Neoarchaean Era. In green- 2008). These thick stacks might be called
stone belts metamorphic grade varies from pre- ‘pseudo-accretionary complexes’, but neither
hnite– pumpellyite facies through greenschist boundary-layer downwelling nor ‘sub-lithospheric’
facies to amphibolite facies and, rarely, into the subduction is likely to have created the dual
granulite facies; ocean-floor metamorphism of the thermal environments at the site of tectonic stacking
protoliths is common. In high-grade terranes, argued to be characteristic of plate tectonics (Brown
granulite-facies metamorphism at temperatures 2006). It is possible, however, that thickening (with
below 900 8C and multiple episodes of anatexis or without delamination of (?) eclogite sinkers)
are the norm.
In southern West Greenland, in the Isua
Supracrustal Belt the metamorphism is polyphase,
occurring during the Eoarchaean and Neoarchaean
Eras, and an age of c. 3.7 Ga has been argued for
the early metamorphism. P –T conditions of 0.5–
0.7 GPa and 500 –550 8C (or up to 600 8C) have
been retrieved from the Isua Supracrustal Belt
(Hayashi et al. 2000; Rollinson 2002). These data
yield warm apparent thermal gradients of 800 –
1000 8C GPa21 (c. 23– 28 8C km21), which is just
within the range for a purely conductive response
to thickening, but may reflect thinner lithosphere
with higher heat flow than later in Earth history.
Also in southern West Greenland, in mafic rocks
on the eastern side of Innersuartuut Island in the
Itsaq Gneiss Complex of the Færingehavn terrane,
where the gneisses record several events in the
interval c. 3.67 –3.50 Ga, early orthopyroxene þ Fig. 9. (a) Sketch of a conjectured regime that might
plagioclase assemblages record poorly defined precede plate tectonics proper (from Davies 1992). The
mantle part of the ocean lithosphere is thick enough to
but rather ordinary granulite-facies conditions
behave rigidly. Although the ocean lithosphere as a
whereas overprinting garnet þ clinopyroxene þ whole is buoyant, the lower mantle part of the lithosphere
quartz assemblages probably record widespread is negatively buoyant and detaches from the crust to
high-pressure granulite-facies metamorphism asso- subduct into the deeper mantle (sublithospheric
ciated with final terrane assembly in the interval subduction). The crust may thicken and differentiate
2.715– 2.650 Ga (Friend & Nutman 2005b, 2007). internally, and the deeper part may transform to eclogite
This is generally consistent with an evolution that may drip into the mantle. (b) Sketch of a conjectured
during the Mesoarchaean and Neoarchaean Eras pre-plate regime (from Davies 1992). The buoyant crust
involving: a postulated island arc complex in the decouples from a denser mantle boundary layer, and
eastern Akia terrane of southern West Greenland, accumulates above the downwelling mantle boundary
layer as it drips into the deeper mantle. The crust may
built on a volcanic substrate in the interval thicken and differentiate internally, and the deeper part
c. 3.07 –3.00 Ga and subsequently thickened and may transform to eclogite that may drip into the mantle.
metamorphosed in the interval c. 3.00 –2.97 Ga Reprinted from Davies, G. F. 1992. On the emergence of
(Garde 2007); progressive terrane assembly during plate tectonics. Geology, 20, 963– 966, with permission.
the interval c. 2.95 –2.65 Ga (Friend & Nutman # 1992 The Geological Society of America (GSA).
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METAMORPHIC PATTERNS IN OROGENS 61

might have induced melting deep in the pile to gen- to the paper by van Hunen et al. (2008), where a
erate tonalite –trondhjemite–granite-type magmas variety of possible models for tectonics during the
in a manner that might resemble future arcs. In Archaean Eon are summarized and discussed.
principle, the metamorphic imprint imposed by Tonalite –trondhjemite –granite suites (TTGs)
either of these behaviours during the early part of dominate the rock record of the Archaean Eon, but
Earth history should be distinguishable from that their origin has been controversial. One recent
imposed by a plate tectonics mode of convection. view suggests that many older TTGs formed by
In South Africa, recent work on the c. 3.23 Ga melting of garnet amphibolite of broadly basaltic
metamorphism of the Barberton and related green- composition hydrated by interaction with seawater,
stone belts has yielded the following P–T data: in whereas melting of eclogite increased in importance
the Onverwacht Group greenstone remnants, P–T through the Mesoarchaean and Neoarchaean Eras,
conditions of 0.8–1.1 GPa and 650 –700 8C as shown by an increase in Nb/Ta in TTGs (Foley
(Dziggel et al. 2002); in the southern Barberton et al. 2002, 2003). Melting of garnet amphibolite
greenstone belt, P–T conditions of 0.9–1.2 GPa may be achieved in the lower part of thickened
and 650 –700 8C (Diener et al. 2005, 2006); and, basaltic crust or by subduction on a warmer Earth.
in amphibolite-dominated blocks of supracrustal Conceptually, this is consistent with early plate-like
rocks in tectonic mélange from the Inyoni Shear behaviour that allowed crust to float to form thick
Zone, P –T conditions of 1.2–1.5 GPa and 600– stacks above zones of boundary-layer downwelling,
650 8C (Moyen et al. 2006). Apparent thermal gradi- or, for a slightly cooler Earth, thick stacks above
ents for these belts are in the range 450– zones of ‘sub-lithospheric’ subduction, and this
700 8C GPa21 (c. 13 –20 8C km21), which are not indeed may have been the convective mode during
outside the limit for a conductive response to the Eoarchaean and Palaeoarchaean Eras (Fig. 9;
thickening. Consequently, an earlier conclusion van Hunen et al. 2008). By the Mesoarchaean and
that this intermediate dT/dP metamorphism rep- Neoarchaean Eras, and possibly as early as the
resents evidence for subduction-driven tectonic pro- Palaeoarchaean Era, subduction was operating in
cesses during the evolution of the early Earth may some regions of the Earth; this subduction was
be premature. probably characterized by warmer geotherms that
allowed melting of subducting garnet amphibolite
Tectonics before the Neoarchaean Era and, with secular cooling, a change to melting of
subducting eclogite.
Blueschists are not documented in the Archaean Worldwide, the oldest surviving crustal rem-
Eon and there is no metamorphic imprint of subduc- nants from the Palaeoarchaean Era generally are
tion of continental crust to mantle conditions and composed of juvenile TTGs formed prior to a
return to crustal depths. However, the chemistry of period of polyphase granulite-facies metamorphism
eclogite xenoliths of Mesoarchaean to Neoarchaean in the interval 3.65 –3.60 Ga; extreme thermal con-
age and the chemistry and paragenesis of diamonds ditions typical of G– UHTM are not generally regis-
of Neoarchaean age in kimberlites within cratons tered before the Neoarchaean Era. This appears
suggest that some process associated with supercra- to be counterintuitive, as we might expect that the
ton formation at convergent margins was operating higher abundance of heat-producing elements might
to take basalt and other supracrustal material into have led to higher crustal heat production and hotter
the mantle by the Neoarchaean Era (Brown 2008). orogens. However, it is possible that contemporary
This may have been some form of subduction. heat loss through oceans and continents was higher
However, subduction as we know it on modern and lithosphere rheology was generally weaker on
Earth requires strong lithosphere and appropriate early Earth. This is permissive for a ‘crème brûlée’
driving forces. The hotter lithosphere in the lithosphere rheology structure. Also, it is consistent
Archaean Eon is likely to have been weaker with: (1) modelling suggesting dominance of
because of lower viscosity, perhaps weakening coup- unstable subduction for plate collision regimes
ling across the subduction boundary and perhaps with very hot geotherms in which convergence is
leading to more frequent slab breakoff and/or slab accommodated by pure shear thickening and devel-
breakup (e.g. Burov & Watts 2006). Thus, it may opment of gravitational (Rayleigh –Taylor) instabil-
be that weaker lithosphere prevented exhumation ities (Burov & Watts 2006); (2) modelling that rules
of subducted crust by mechanisms related to subduc- out the commonly proposed flat subduction model
tion and plate kinematics (e.g. Brun & Faccenna for early Earth tectonics (van Hunen et al. 2004);
2008; van Hunen & van den Berg 2008). Regardless (3) the proposition that early plate-like behaviour
of the tectonic style, an essential requirement for the may have allowed crust to float to form thick
return of ocean crust into the mantle is the efficient stacks above zones of boundary-layer downwelling,
transformation of basalt to eclogite (van Hunen & or, for a slightly cooler Earth, thick stacks above
van den Berg 2008). The interested reader is referred zones of ‘sub-lithospheric’ subduction (Davies
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62 M. BROWN

1992; van Hunen et al. 2008); (4) modelling of hinterlands. The oldest record of E–HPGM also
thinner ocean lithosphere suggesting that plate coincides with the formation of supercratons; in
motions in the Archaean Eon might have been addition to occurrences formed during the Pro-
faster than on modern Earth, which may explain terozoic Era, E–HPGM is common in the Caledo-
the scarcity of accretionary prisms in the rock nides and Variscides of the Palaeozoic Era.
record during the Archaean Eon, as high conver- E– HPGM belts complement G –UHTM belts, and
gence rates will favour subduction erosion over sub- are generally inferred to record subduction-to-
duction accretion (Hynes 2008); and (5) the absence collision orogenesis.
of E–HPGM and HPM –UHPM in the early Earth The occurrence of G –UHTM and E –HPGM
rock record, inferred herein to be more typical of belts since the Neoarchaean Era signifies change
plate collisions in the Proterozoic and Phanerozoic to globally ‘subductable’ oceanic lithosphere, poss-
Eons (Brown 2006). ibly beginning as early as the Palaeoarchaean Era,
and transfer of water into upper mantle. The first
Concluding discussion appearance of dual metamorphic belts also registers
operation of a global ‘Proterozoic plate-tectonics
Metamorphism associated with orogenesis provides regime’, which evolved during a transition in the
a mineral record that may be inverted to yield Neoproterozoic Era to the ‘modern plate-tectonics
ambient apparent thermal gradients. Apparent regime’ characterized by subduction of continental
thermal gradients derived from inversion of age- crust deep into the mantle and its (partial) return
constrained metamorphic P –T data are used to from depths of up to 300 km. This transition in the
identify tectonic settings of ancient metamorphism Neoproterozoic Era has implications for transfer of
and evaluate geodynamic regimes. On modern water deeper into mantle. Age distribution of meta-
Earth, tectonic settings with lower thermal gradients morphic belts that record extreme conditions is not
are characteristic of subduction zones whereas those uniform; they occur in periods that correspond to
with higher thermal gradients are characteristic of amalgamation of continental lithosphere into super-
back-arcs and orogenic hinterlands. If a duality of cratons (e.g. Superia/Sclavia) or supercontinents
thermal regimes is the hallmark of plate tectonics, (e.g. Nuna (Columbia), Rodinia, and the Gondwana
then a duality of metamorphic belts is the character- segment of Pangaea). Accretionary orogenic systems
istic imprint of plate tectonics in the record. Ideally, form at sites of subduction of oceanic lithosphere;
these belts should be ‘paired’ broadly in time and these systems dominate during the lifetime of a
space, although the spatial arrangement may be supercontinent and during break-up and dispersal.
due to late orogenic juxtaposition. Given this Collisional orogenic systems form where ocean
characteristic imprint, I propose broadening the basins close and subduction ultimately ceases;
definition of paired metamorphic belts to the follow- these systems dominate during crustal aggregation
ing: paired metamorphic belts are penecontempora- and assembly of supercontinents. It follows that
neous belts of contrasting type of metamorphism collisional orogenic systems may be superimposed
that record different apparent thermal gradient, on accretionary orogenic systems.
one warmer and the other colder, juxtaposed by For accretionary orogenic systems where a
plate tectonics processes. retreating plate boundary is dominant, high dT/dP
The first global occurrences of granulite-facies metamorphism is typical, commonly with counter-
ultrahigh-temperature metamorphism (G– UHTM) clockwise P –T– t paths and peak metamorphic
are recorded during the Neoarchaean Era and mineral growth syn- to late in relation to tectonic
contemporaneous but more sparse occurrences of fabrics. G –UHTM and E– HPGM rocks generally
medium-temperature eclogite –high-pressure gran- are not exposed, and although rare, blueschists
ulite metamorphism (E –HPGM), signifying changes may occur early to record HPM, but UHPM is not
in global geodynamics that generated transient sites found. Short-lived contractional phases of orogen-
of higher and lower heat flow than required by esis probably relate to interruptions in the continuity
apparent thermal gradients recovered from the of subduction caused by topographic features on the
earlier record. G –UHTM is dominantly a phenom- ocean plate, particularly ocean plateaux. Extensive
enon limited to the period from the Neoarchaean granite magmatism accompanies metamorphism.
Era to the Cambrian Period and is documented Examples include the Lachlan orogen, Australia, the
during four distinct periods, the Neoarchaean Era, Acadian orogen, NE USA and Maritime Canada,
the Orosirian Period, the Ectasian and Stenian and the orogens of the Proterozoic Eon in the SW
Periods, and the Ediacaran and Cambrian Periods, USA. In contrast, for accretionary orogenic systems
synchronous with formation of supercratons, super- where an advancing plate boundary is dominant,
continents and the Gondwana segment of Pangaea. such as the accretionary prism of the Coastal Cordil-
Many G –UHTM belts may have developed in set- lera in Chile, which formed late during the Late
tings analogous to modern back-arcs and orogenic Palaeozoic Era, and the Diego de Almagro
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METAMORPHIC PATTERNS IN OROGENS 63

Metamorphic Complex in Chilean Patagonia, which Anápolis –Itauçu Complex, southern Brası́lia Belt,
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