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Earth and Planetary Science Letters 212 (2003) 1^14
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Frontiers
Ultrahigh-pressure metamorphism:
tracing continental crust into the mantle
Christian Chopin
Laboratoire de Ge¤ologie, UMR 8538 du CNRS, Ecole normale supe¤rieure, 24 rue Lhomond, 75005 Paris, France
Received 16 December 2002; received in revised form 6 May 2003; accepted 6 May 2003
Abstract
More and more evidence is being discovered in Phanerozoic collision belts of the burial of crustal rocks to
previously unsuspected (and ever increasing) depths, presently on the order of 150^200 km, and of exhumation from
such depths. This extends by almost one order of magnitude the depth classically ascribed to the metamorphic cycling
of continental crust, and demonstrates its possible subduction. The pieces of evidence for this new, ultrahigh-pressure
(UHP) metamorphism exclusively occur in the form of relics of high-pressure minerals that escaped backtransformation during decompression. The main UHP mineral indicators are the high-pressure polymorphs of silica
and carbon, coesite and microdiamond, respectively; the latter often demonstrably precipitated from a metamorphic
fluid and is completely unrelated to kimberlitic diamond or any shock event. Recent discoveries of pyroxene
exsolutions in garnet and of coesite exsolutions in titanite suggest a precursor garnet or titanite containing six-fold
coordinated silicon, therefore still higher pressures than implied by diamond stability, on the order of 6 GPa. The
UHP rocks raise a formidable geological problem: that of the mechanisms responsible for their burial and, more
pressingly, for their exhumation from the relevant depths. The petrological record indicates that large tracts of UHP
rocks were buried to conditions of low T/P ratio, consistent with a subduction-zone context. Decompression occurred
in most instances under continuous cooling, implying continuous heat loss to the footwall and hangingwall of the
rising body. This rise along the subduction channel ^ an obvious mechanical discontinuity and weak zone ^ may be
driven by buoyancy up to mid-crustal levels as a result of the lesser density of the acidic crustal rocks (even if
completely re-equilibrated at depth) after delamination from the lower crust, in a convergent setting. Chronological
studies suggest that the rates involved are typical plate velocities (1^2 cm/yr), especially during early stages of
exhumation, and bear no relation to normal erosion rates. Important observations are that: (i) as a result of strain
partitioning and fluid channelling, significant volumes of subducted crust may remain unreacted (i.e. metastable) even
at conditions as high as 700‡C and 3 GPa ^ with implications as to geophysical modeling; (ii) subducted continental
crust shows no isotopic or geochemical evidence of interaction with mantle material. An unknown proportion of
subducted continental crust must have escaped exhumation and effectively recycled into the mantle, with geochemical
implications still to be explored, bearing in mind the above inefficiency of mixing. The repeated occurrence of UHP
metamorphism, hence of continental subduction, through time and space since at least the late Proterozoic shows that
it must be considered a common process, inherent to continental collision. Evidence of older, Precambrian UHP
metamorphism is to be sought in high-pressure granulite-facies terranes.
8 2003 Elsevier Science B.V. All rights reserved.
E-mail address: [email protected] (C. Chopin).
0012-821X / 03 / $ ^ see front matter 8 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0012-821X(03)00261-9
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C. Chopin / Earth and Planetary Science Letters 212 (2003) 1^14
Keywords: metamorphism; ultrahigh pressure; continental subduction; exhumation; mantle^crust interaction; collision; buoyancy;
eclogite; coesite; diamond; majorite; kinetics; £uids
1. Introduction
One of the key features in the theory of plate
tectonics is the opposition of a young, continuously renewed and subducted oceanic lithosphere,
to an old and everlasting continental lithosphere.
In this paradigm, continental crust was assigned,
because of its low density, the role of a £oating
object which could be accreted, deformed, thickened and eroded in collision zones, but the fate of
which was restricted to an erosion^sedimentation^
metamorphism/melting cycle encompassing at
most the 30 or 40 outer kilometers of the solid
Earth. The existence of crustal mountain roots as
deep as 70 km was evident from gravity and seismic data, but it was unclear whether samples
could ever be brought from such depth to the
surface by tectonic processes.
A ¢rst hint was given by the recognition that
rocks of oceanic origin in the central Alps, after
they were hydrothermally transformed under
near-surface conditions, were metamorphosed to
conditions of ca. 3 GPa (i.e. near 100 km depth)
[1] and that the enclosing continental basement
may have shared the same burial [2]. In the early
80s the successive discoveries of coesite, a highpressure polymorph of quartz, in the western Alps
[3] and the Caledonian chain of Norway [4] led to
the inescapable conclusion that continental crust
can be buried to 100 km depths and be brought
back to outcrop during the course of orogenic
processes. The geological problems at stake became even more formidable after the discovery
of demonstrably metamorphic diamond in the
Cambrian orogen of the Kokchetav massif, Kazakhstan [5], implying minimum formation pressures of about 4 GPa, i.e. burial to depths exceeding 120 km.
These ¢ndings, which rely entirely on careful
microscopic observation, have been the incentive
for a blooming ¢eld-oriented high-pressure research activity. A wealth of new ¢ndings over a
decade con¢rms worldwide the generality of the
early discoveries, brings answers to the more
pressing questions but also pushes ever deeper
the limits of continental metamorphism, through
the recognition of ever more tenuous indicators of
ever higher pressures ^ and so raises new questions. We examine here these recent developments
^ made in parallel with those of mantle mineralogy [6] as revealed by high-pressure experimental
work and by mineral inclusions in ‘super-deep’
kimberlitic diamonds ^ as well as their limitations
and implications.
2. The force of mineralogical evidence
2.1. The classics : simple polymorphic indicators
The recognition of ultrahigh-pressure metamorphic rocks has essentially relied on the identi¢cation of the high-pressure polymorphs of silica
(coesite) or of carbon (diamond). The simplicity
of the ¢rst-order phase transitions involved indeed
makes the very presence of one such phase, whatever its grain size or abundance, an immediate indicator of minimum pressures attained, on the
order of 3 and 4 GPa, respectively (Fig. 1). The
underlying assumption that these phases formed
in their stability ¢eld is borne out by independent
lines of evidence, as shown below.
The rock mineralogy tending to adapt to
changing pressure (P) and temperature (T) conditions, such UHP phases normally disappear during decompression, i.e. during the movement of
the rock toward the surface ( = exhumation), this
the more so if the rock enters the stability ¢eld of
the respective low-pressure phase at high temperature. As a matter of fact, the metastable persistence of these indicators up to surface conditions
is commonly the result of their inclusion in mechanically strong host minerals like garnet or zircon (Fig. 2A,B,D). These act as pressure vessels
around each inclusion, insulating it from metamorphic £uid and preventing the volume increase
of the back-transformation, i.e. maintaining the
inclusion pressure on the relevant transition curve
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C. Chopin / Earth and Planetary Science Letters 212 (2003) 1^14
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3
obstacle than kinetics to the complete transformation of the high-P phase into the lower-P polymorph, i.e., for coesite/quartz, temperatures lower
than about 375^400‡C [10] or a completely dry
system [11].
4
3
2.2. Relics and pseudomorphs : the limits of
evidence
2
The tiniest relic of a UHP indicator in a metamorphic rock is in itself compelling evidence that
the rock passed at some stage through UHP conditions, and this may imply entire reconsideration
of the geological or structural record for the area.
Given these far-reaching implications, a prerequisite is that the mineralogical evidence is unambiguously characterized ^ and demonstrably in situ.
Techniques with high spatial resolution, such as
Raman microspectroscopy, are invaluable complements of optical observation in this respect.
Yet, the limit of what can be done convincingly
has probably been reached with a weak coesite
Raman signal in what is optically a polycrystalline
quartz inclusion in Antarctic eclogite [12], or with
the report of diamond from the Rhodope, Greece
[13], in which the Raman spectrum of ‘diamond’,
usually an intense sharp band, can be interpreted
as well as that of a defective graphite [14]. Besides, diamond is so prone to artifact, to be
plucked o¡ or introduced into a sample during
processing (sawing, grinding) that utmost care
and special techniques are required to ensure its
identi¢cation in situ (e.g. [15]), the safest way
being to work exclusively with crystals that are
entirely included in another mineral, small size
and confocal techniques permitting [5,16]. This
may be the reason why the original reports of
large UHP diamond from Dabie Shan, eastern
China [17], have not yet been independently con¢rmed by micro-¢ndings.
However, absence of evidence is not evidence of
absence, and this may be a golden rule for UHP
studies. The original coesite report from the Caledonides [4] could not be duplicated for a decade
in spite of the e¡orts of several expert groups ^
until a Ph.D. student was able to demonstrate the
regional occurrence of coesite there [18,19]. New
eyes are sometimes required to open frontiersT
1
0
0
200
400
600
800
1000
Fig. 1. P^T diagram showing the stability ¢eld of the main
UHP index minerals, coesite and diamond, and the decompression trajectory common to most UHP terranes (broad
arrow). By de¢nition, UHP metamorphic conditions are
reached above the coesite^quartz transition. For comparison,
one of the coldest T/P ratios achievable in subduction zones
is shown (5‡C/km), as well as the 20‡C/km line, which may
be seen as a lower boundary value for common, low- to intermediate-pressure metamorphism. Incipient partial melting
of granitic to granodioritic compositions in the presence and
absence of a hydrous £uid is also shown. The thin solid line
(arrowed) is the P^T path computed by Roselle and Engi
[82] for the top of a 5 km thick continental crustal fragment
subducted to 100 km depth with a plate velocity of 3 cm/yr
and a 30‡ dip, detached from the subducting plate, held at
constant depth for 3 Ma, then exhumed along the subduction shear zone at a constant exhumation rate of 1 cm/yr
(frictional shear heating is considered).
as long as the tensile strength of the host mineral
can sustain the di¡erence between the internal (inclusion) and external pressure. Although early anticipated [7], the e¡ectiveness of this mechanism
was only recently demonstrated for small (10^20
Wm) coesite inclusions in garnet and zircon, in
which Raman microspectroscopy revealed residual internal pressures that could exceed 2 GPa
[8,9] ! Such residual pressures are also evidence
against metastable coesite formation. Note that,
once the host mineral is fractured (Fig. 2A) ^ or
in the absence of a ‘container’ ^ there is no other
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C. Chopin / Earth and Planetary Science Letters 212 (2003) 1^14
Fig. 2. Photomicrographs of characteristic or potential mineral indicators of UHP conditions (plane-polarized light unless otherwise speci¢ed). (A) Coesite (coes) inclusion in pyrope garnet (gt); note the incipient transformation into quartz (qz) and, as a result of the related volume increase, the radial cracks in the host garnet [3]; Parigi, Dora-Maira massif. (B) Ellenbergerite (Elb) included in pyrope garnet along with rutile (ru); Parigi, Dora-Maira massif. This Mg^Al^(Ti,Zr)-silicate contains 8 wt% H2 O and
is stable at pressures exceeding 2.7 GPa (and T 6 725‡C). (C) Oriented quartz needles precipitated within omphacitic clinopyroxene (cpx); Blumenau eclogite, Erzgebirge [86]. This feature is often encountered in UHP eclogite. (D) Microdiamond (diam) inclusions in zircon (zr); garnet gneiss, Seidenbach, Erzgebirge [16]. (E) Oriented orthopyroxene (opx) precipitates in garnet, evidence of a former super-silicic, majoritic garnet; Ugelvik peridotite, OtrTy island, western Norway [28]. (F) Oriented K-feldspar
(Kfs) precipitates in clinopyroxene; crossed nicols; pyroxenite in marble, Kumdi Kol, Kokchetav massif.
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The evidence may also become more elusive,
when the indisputable relics of UHP minerals
have disappeared and only pseudomorphs thereof,
i.e. breakdown products retaining the original
shape of the parent mineral, are present. Graphite
octahedra [20] are a straightforward example of
pseudomorphs after diamond, but what is the diagnostic value of graphite cuboids in schist [21]?
Likewise, the texture of quartz pseudomorphing
coesite (Fig. 2A) is characteristic [7] but tends to
disappear through recrystallization upon prolonged thermal annealing. What then is the diagnostic value of a well-annealed polycrystalline
quartz inclusion in fractured garnet? Clearly at
some stage the force of the evidence declines,
and only an array of converging hints may then
convince the sceptical petrologist.
2.3. Beyond the polymorphic record:
deeper and deeper with other UHP indicators
The intensive study of ascertained or potential
UHP rocks has revealed a number of uncommon
mineralogical features and assemblages, of more
or less de¢nite UHP value. Some of these mineralogical hints for UHP are illustrated in Fig. 2 ;
many have recently been reviewed [22,23], so only
the most salient or new features are addressed
here. Importantly, none of them involves a polymorphic transition, thus avoiding the suspicion
bearing on such phase transitions, namely that
they may be a¡ected by high deviatoric stresses
(e.g. [3]) ^ which is de¢nitely not the case for
microdiamond in the Erzgebirge UHP gneisses,
which precipitated as a daughter mineral in supercritical COH £uid/melt inclusions in garnet [24,
25].
2.3.1. K-bearing clinopyroxene
The occurrence, in calc^silicate rocks of the diamond-bearing Kokchetav massif, of clinopyroxene with extremely high potassium contents (up to
1.5 wt% K2 O) [5] was a con¢rmation of stable
diamond occurrence. So far solely known from
a few eclogitic xenoliths in kimberlite, this feature
was experimentally shown to be stable at pressures of 4^10 GPa. Upon decompression this potassic pyroxene shows characteristic textures, with
5
oriented precipitates of K-feldspar (Fig. 2F) [5,
26], sometimes of a later K-bearing mica [27].
2.3.2. Majoritic garnet
Another most signi¢cant discovery is, in the
peridotite body of OtrTy Island, western Norway,
that of microtextural evidence for exsolution of
orthopyroxene in coarse garnet, implying a
‘super-silicic’ precursor garnet, i.e. the presence
of several mol% majorite component [28,29]. It
was known from experiment [30] that with increasing pressure pyropic garnet can incorporate
more and more MgSiO3 as majorite component
(to ¢t the garnet formula, better written Mg3 Mg
VI
Si IV Si3 O12 , in which one-fourth of the silicon
atoms is six-fold coordinated). Modal estimates of
the majorite component originally present in the
Norwegian garnet, i.e. before exsolution, lead to
formation pressures exceeding 6 GPa ( s 200 km).
Such sensational exsolution had been discovered
in garnet from kimberlite xenoliths a few years
earlier [31] but it was completely unexpected
that the delicate textures (Fig. 2E) could be preserved during the long ascent and slow cooling
of an orogenic peridotite body, as opposed to
quenching by explosive kimberlite sampling. Admittedly, mantle rather than crustal material was
involved in either case, but the demonstration was
made that, even with the rates attending metamorphic processes, such evidence could be preserved ^ opening the prospect that it may be
found in UHP crustal rocks too.
Since then, indeed, three reports of related textures were made in UHP terranes. In garnet^peridotite of the Su-Lu terrane, eastern China [32],
the textural evidence of pyroxene exsolution
from garnet is convincing but also involves oriented micrometer-size rods of rutile and apatite;
however, it is unclear whether the extreme conditions recorded by the ultrama¢c body were shared
by the associated UHP (coesite-grade) crustal
rocks. As in Norway ([28], cf. [33,34]), the extreme
conditions may record an earlier, deeper mantle
stage before the body was amalgamated with
crustal material during ‘normal’ UHP metamorphism. In two other reports from the Greek Rhodope [13,35], the crustal nature of the rocks is
clear but the evidence of exsolution from a former
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C. Chopin / Earth and Planetary Science Letters 212 (2003) 1^14
Fig. 3. The UHP metamorphic terranes presently known, on the basis of coesite or metamorphic diamond occurrence. Adapted
from [57]; references in text.
majoritic garnet is less compelling [14]: in garnet
from kyanite^biotite gneiss, no pyroxene occurs
but oriented rods of quartz and rutile [13]; and
in garnet from metabasite, with oriented rutile
rods, clinopyroxene occurs as blebs [35], which
may or may not represent recrystallized exsolution products (alternatively they could be inclusions trapped during garnet growth).
2.3.3. ‘Super-silicic’ titanite
Other spectacular evidence for the metamorphism of crustal material at unsuspected depths
is the discovery of coesite precipitates in titanite,
CaTiSiO5 , in marble from the Kokchetav massif
[27]. This suggests the existence of a super-silicic
precursor titanite, as experimentally anticipated in
a high-pressure study of the series CaTiSiO5 ^
CaSi2 O5 , in which the Ca-silicate end-member is
stable above 8 GPa and has the titanite structure
with 50% six-fold coordinated silicon [36]. The
precursor compositions reconstructed by integration of the exsolved phases (also minor calcite and
apatite) point again to pressures that may have
exceeded 6 GPa [27], i.e. near 200 km burial. Paradoxically, coesite in such a case plays the role of
a decompressional, lower-pressure phase!
A general feature of these UHP conditions,
whether they approach the 3 GPa/650^800‡C
range in ‘standard’ coesite-bearing terranes or
the 4^6 GPa/900^1000‡C range in the diamondbearing terranes, is that they represent a low T/P
ratio at mantle depths. The most likely if not the
sole geodynamic context in which such a low ratio
is realized is subduction zones.
3. UHP through space and time
After an incubation period of 5 years after the
¢rst coesite reports, more and more evidence for
UHP metamorphism is being discovered worldwide (e.g. [23]), even in orogens where high-pressure metamorphic rocks were so far virtually absent or terribly underestimated, like the Himalaya
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C. Chopin / Earth and Planetary Science Letters 212 (2003) 1^14
[37,38], Indonesia [8], Antarctica [12] or South
America [39]. In short, de¢nite evidence of UHP
metamorphism is now missing only from Australia and North America (Fig. 3).
3.1. Coherent UHP terranes
A major result concerns the actual extent of
UHP rocks within a given metamorphic terrane.
Pieces of de¢nite evidence for UHP remain rare
and occur in rock types (eclogitic metabasite,
magnesian schist, marble or pelitic metasediment)
that are subordinate with respect to the overwhelming country rock of felsic gneiss (metagranite), in which evidence for UHP has long remained elusive. Yet the areal extent of UHP
occurrences is over kilometers in the Alps [40],
tens of kilometers in Norway [19] and the Kokchetav massif [9], and hundreds of kilometers
along the Hong’an-Dabie-Su-Lu Triassic belt of
east central China (e.g. [22]).
The key point is whether the UHP rocks and
their country-rock gneiss, which bears only lowgrade assemblages, share the same UHP history
or were tectonically amalgamated during a later
stage, i.e. whether the metamorphic terrane as a
whole forms a coherent UHP terrane or a mixture
of UHP and low-pressure rocks. New problem,
new methods: beside extensive zircon U^Pb
point-dating [41] and stable-isotope data [42],
the systematic study by Raman spectroscopy of
the minute mineral inclusions in zircon concentrates extracted from the country-rock gneisses
has proved to be most e¡ective in unraveling the
former extent of UHP conditions in rocks that
bear now only low-grade mineral assemblages, especially in the huge Dabie-Su-Lu belt [43^45].
There the resulting evidence is that the gneisses,
whether of sedimentary derivation (paragneiss associated with eclogite and marble), or of magmatic derivation (orthogneiss, former granite
forming large tracts of country rock), both commonly bear coesite relics in zircon, thereby demonstrating the coherency of the UHP terrane. A
similar conclusion was reached in the small UHP
terrane (15U5U1 km) of the Dora-Maira massif,
western Alps [40] and might hold in Norway
[18,19,46], as well as in the Erzgebirge [47] and
7
the Kokchetav massif [9], in both of which microdiamonds are also gneiss-hosted. However, the
implications are particularly formidable in China,
given the scale of the UHP terrane: several hundred kilometers in length, a few tens of kilometers
lateral exposure, and some kilometers thickness!
Obviously large tracts of continental crust were
involved in UHP metamorphism.
3.2. Shaping the continents
One of the most fascinating aspects of recent
¢ndings is the repeated occurrence through time
of UHP metamorphism of continental crust. The
composite Eurasian continent bears the scars of
the successive orogenies that make it a collage of
microcontinental blocks. In each of these scars
UHP rocks do occur, marking the steps of continental accretion (Fig. 3): from the Cambrian
Kokchetav massif in Kazakhstan (530 Ma
[48,49]), to the Early Paleozoic central China
belt (Altun-Qaidam and Qinling; ca. 500 Ma
[50], Jingsui Yang et al., article in revision), to
the Triassic Hong’an-Dabie-Su-Lu belt (220^240
Ma, e.g. [41]), to the Cretaceous merging of the
Lhassa block prolongation in Indonesia (110^120
Ma [51]), to the Tertiary Himalayan belt (45 Ma?
[37]). On the western side of the Eurasian block,
UHP crustal metamorphism is likewise well represented in the Caledonian chain (Laurentia^Baltica collision) from eastern Greenland [52] to Norway, in the Variscan belt (Laurentia^Gondwana
collision) from Central to Western Europe (Bohemian massif [16] and Monts du Lyonnais, French
Massif Central [53], respectively), but also in the
Meso-Cenozoic Alpine chain (Europe^Apulia collision), from the Rhodope massif (70^75 Ma [35])
to the western Alps (38^35 Ma [54^56]). By contrast, the processes active along the circum-Paci¢c
margins seem to be unsuitable to produce or,
more likely, to exhume UHP rocks. In the case
of Eurasia, the UHP material involved may be of
oceanic origin in a few instances (Sulawesi, Monts
du Lyonnais, Cignana) but continental in most
cases, attesting to the general rule that continental
subduction has operated at least since the late
Proterozoic.
This apparent age limit as well as the paucity
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C. Chopin / Earth and Planetary Science Letters 212 (2003) 1^14
of Precambrian high-pressure, low-temperature
rocks (blueschists and eclogites) have led to the
suggestion of a rapid decrease of the subductionzone thermal gradient during the late Proterozoic,
after a massive heat release made possible by the
dislocation of the Rodinia supercontinent [57].
However, the existence of Proterozoic UHP metamorphism in the Pan-African belt of Mali (ca. 600
Ma [58]) and Brazil [39], of 2 Ga old eclogites in
the Usagaran belt of Tanzania [59] and of 2.6 Ga
high-pressure granulites in the western Canadian
shield [60] indicates that zones of low T/P ratios
did exist early in the Earth’s history. The preservation of the HP/UHP rocks formed was admittedly more di⁄cult because of the Earth’s higher
heat production, and the likely higher average
geothermal gradient and lesser plate thickness.
This may be su⁄cient to account for the paucity
of preserved Precambrian blueschists and eclogites.
The important point is that in collision belts ^
as opposed to Paci¢c-type margins ^ subduction
of continental crust to depths reaching 100^200
km appears to be the rule since at least the Proterozoic. The implications in terms of tectonics
and geodynamics are far-reaching. Were it simply
for the Himalaya, most of what has previously
been considered relevant for the archetype of the
collision belts ^ in terms of thermal regime, metamorphism, partial melting, uplift and cooling ^
actually pertains to a late stage of the chain evolution : metamorphism of continental crust started
about 20 Ma earlier with the subduction of the
leading edge of the Indian plate to coesite-forming
depths (90^100 km); the present-day shallow dip
of India below southern Tibet bears no relation to
an initially much steeper dip, and the average exhumation rates are twice as high as previously
thought (ca. 10 mm/yr) [37].
4. A window into subduction-zone processes
Another important aspect of these ¢ndings is
that the exposure of large tracts of HP/UHP
rocks represents windows opening into subduction-zone processes. Aspects thus accessible to observation and measurement include the extent of
Fig. 4. Undeformed UHP metagranite sample, Brossasco,
Dora-Maira massif. Would you believe that this Hercynian
granite underwent conditions of 700^750‡C at 100 km depth
during Alpine times? Yet it did, according to de¢nite mineralogical evidence [62].
mineral transformation and the role of £uids and
deformation under such conditions, the degree of
interaction with mantle material ^ in the footwall
of the subduction zone or in the overlying mantle
wedge ^ and its possible geochemical signature.
Admittedly, the message is in essence a palimpsest
that has to be deciphered, since the features acquired under UHP conditions may have been altered, overprinted or erased during the later decompression history. Yet it is a unique record.
4.1. Extent of rock transformation at UHP
A repeated observation in HP/UHP terranes is
that signi¢cant rock volumes may escape deformation and mineral equilibration at the prevailing
P^T conditions, despite temperatures as high as
650^750‡C for UHP rocks. Famous examples are
the metagranite body of Flatraket in the Western
Gneiss Region, Norway [61], or the Brossasco
metagranite in the Dora-Maira massif [62] (Fig.
4). Pure volume di¡usion is clearly an ine⁄cient
process; the partitioning of deformation and the
related £uid access, both triggering nucleation and
mass transfer [63,64], are actually much more ef¢cient than temperature.
This has led thermomechanical modelers to
consider not only the changing mineralogy according to P^T conditions, in order to account
for changing physical properties like density or
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C. Chopin / Earth and Planetary Science Letters 212 (2003) 1^14
rheology, but also the e¡ectiveness of this transformation (e.g. [65^67]), i.e. metastable persistence or absence thereof. The consequences in
terms of density changes and buoyancy are significant (see below).
4.2. Isotopic behavior at UHP
This contrast between reactive and metastable
systems, regardless of the high temperatures attained, is also entirely re£ected in their geochemical and isotopic behavior, thereby questioning
the meaning and the applicability of closure temperatures. Monazite U^Pb ages in undeformed
facies of the Brossasco metagranite are intermediate between the Hercynian intrusion age and the
Alpine age of the UHP event, whereas they are
de¢nitely Alpine in nearby UHP rocks like magnesian schists [68] which were completely reworked during Alpine times. The same holds for
U^Pb in titanite, for which formation ages allow
one to date the successive steps of UHP metamorphism, incipient overprint and low-grade overprint, depending on the rock type addressed
[56]. Evidently the rocks showing the largest mineral metastability under UHP conditions show
Rb^Sr disequilibrium among their phases [69]
and are the worst target for Ar^Ar dating (e.g.
[70]).
Independent of the problem of the equilibration
scale, there is an inherent problem to K^Ar systematics in UHP terranes. Under UHP conditions
the partial pressure of argon is expected to be one
order of magnitude higher than in low-pressure
terranes, leading to signi¢cant argon incorporation during the growth of some UHP phases, in
particular white mica. The result is commonly
that the K^Ar ages of UHP micas (even Ar^Ar
plateau ages) are paradoxically higher than those
derived from the more retentive U^Pb or Rb^Sr
systems, and geologically meaningless [70,71].
4.3. Fluid behavior: open vs. closed system,
mantle^crust interaction, rheology
In contrast to reports of low-pressure terranes
£ushed by ‘oceans’ of hydrothermal £uids, the
experience gained in HP and UHP terranes is
9
that of very limited £uid £ow and at best centimeter-scale isotopic equilibration, in spite of the
uncommon P^T conditions. As summarized by
Rumble [42], ‘stable isotope evidence denies the
existence of a pervasive £uid free to in¢ltrate
across lithologic contacts during UHP metamorphism. Furthermore, the preservation of hightemperature oxygen isotope fractionations among
minerals argues against the presence of free £uid
after peak metamorphism, during exhumation
and cooling. Residence in the upper mantle had
no discernible metasomatic e¡ect on the stable
isotope composition of crustal rocks subducted
during continental collision’. The best evidence
for this is the preservation, throughout the UHP
event in Dabie-Su-Lu, of a huge oxygen and hydrogen isotope negative anomaly that dates back
to a premetamorphic, Neoproterozoic hydrothermal system involving meteoric water from a cold
climate [72]. This does not mean the rocks were
dry: most UHP assemblages bear hydrous minerals like micas or epidote that are stable under
these conditions, and therefore evolve little £uid.
The presence or absence of a £uid also directly
a¡ects the temperature of partial melting (Fig. 1).
Therefore, depending on both £uid availability
and the P^T path followed, in particular during
decompression, melting may be expected to be
minimal or quite general. Interestingly, under
UHP conditions there may be complete miscibility
between silicate melts and hydrous £uids for a
range of compositions, with the consequence
that the supercritical £uids generated at UHP by
dehydration reactions during subduction can dissolve considerable amounts of silicate material
with increasing T, and therefore be of restricted
mobility (as compared to the more hydrous £uids
produced at lower pressure, in the subcritical
range) [73,74]. Besides, the presence of such £uids,
whenever water is available, has a decisive weakening e¡ect on the rheological strength of rocks
during deep subduction, precluding notable shear
heating [75].
5. Exhumation : rates and processes
The realization that crustal segments may re-
EPSL 6683 19-6-03 Cyaan Magenta Geel Zwart
10
C. Chopin / Earth and Planetary Science Letters 212 (2003) 1^14
turn to the surface from depths exceeding 120 km
raises a di⁄cult geological problem: that of the
mechanisms responsible for their burial and, more
pressingly, for their exhumation from such
depths. The petrological record indicates that
large tracts of crustal rocks were buried to UHP
conditions of low T/P ratios, which is consistent
with and best accounted for by a subduction-zone
context. It has indeed long been recognized by
modeling that several hundred kilometers of the
leading edge of a continental plate can be entrained into a subduction zone by the sole e¡ect
of body forces, all the more so as the continental
margin is thinner and the cohesion between continental and sinking oceanic lithosphere is higher.
Continental subduction during incipient collision
is therefore an easy way to account for the relevant burial.
As to exhumation, the petrological record in
most instances constrains decompression to have
occurred under continuous cooling [3,9,23,40,64],
implying continuous heat loss to the footwall and/
or hangingwall of the rising body. In addition,
isotopic dating shows that formation ages and
cooling ages obtained on UHP rocks span a narrow range of a few million years in most instances
[35,41,48,49,54^56,76], implying high cooling
rates and, through the petrological constraints,
uncommonly high decompression rates, especially
during early stages of exhumation. The exhumation rates obtained (up to 1^2 cm/yr) are indeed
more akin to plate velocities and bear no relation
to normal erosion rates. It was soon recognized
that erosion and tectonic processes like extensional faulting or ‘corner £ow’ are unable to account
for the critical, early stage of extrusion, even if
they may become e¡ective during later stages, at
higher structural levels. Besides, it is certainly not
fortuitous that most UHP terranes presently exposed are essentially made of light, upper-crustal
material, thus pointing to the role of buoyancy as
a main (?) factor of exhumation (e.g. [77]). The
constraint of continuous cooling precludes any
mechanism like diapiric rise across the overlying
mantle wedge and makes the return back along
the subduction channel ^ a ‘two-way street’ and
obvious mechanical discontinuity ^ the most
likely exhumation path. The process is essentially
driven by buoyancy up to mid-crustal levels, as a
result of the smaller density of the acidic crustal
rocks (even if completely re-equilibrated at
depth), after delamination from the lower crust.
Field data (e.g. [76,79,80]) as well as physical [81],
thermal [82] or thermo-mechanical [66,67,87]
modeling support the idea that most of the exhumation of UHP rocks may be achieved by such
‘extrusion’ of light crustal material along the subduction channel, in a continuously convergent setting, this without the need to resort to slab breako¡ as a ‘deus ex machina’.
6. Consequences and prospects
A new picture emerges for the fate of continental margins and microcontinents. Continental subduction must now be considered a normal process, inherent to continental-plates convergence,
collision and orogeny ^ at least since the late Proterozoic. Part of (most of?) the subducted upper
crust may be delaminated and exhumed, bringing
testimony of subduction-zone processes back to
the surface, with a clear geochemical message :
the ine⁄ciency of mixing. Such a message could
help us shape our view of the deeper mantle in
terms of its heterogeneity and inheritance from
subducted slabs (e.g. [83]). An open question is
still the extent to which continental lithosphere,
upper and lower crust possibly included, may actually disappear into subduction zones and contribute to magma generation [78], mantle geochemistry, geophysical signature, etc. Another
concern is that our present view of UHP metamorphism might be intrinsically biased by the
necessary preservation of a mineralogical record.
It may well be that subducted continental fragments were exhumed and decompressed under
near-isothermal or even increasing temperature
conditions, leading to dehydration reactions and
partial melting (Fig. 1). This would likely erase
most records of the early UHP stage. High-grade,
granulite-facies metamorphic terranes are therefore also open to UHP detective work [84].
Given the exploding number of UHP ¢ndings
during the past few years, the author is con¢dent
that UHP metamorphism will sooner or later be
EPSL 6683 19-6-03 Cyaan Magenta Geel Zwart
C. Chopin / Earth and Planetary Science Letters 212 (2003) 1^14
recognized on all continents and back into the
Precambrian as well. An important issue will be
increasing chronological and spatial resolution in
the dating of the highest-pressure and early-exhumation stages, for instance in the dating of successive growth zones of crystals. Conversely, the
ever-increasing spatial resolution of characterization techniques also hints at one of the limits to
be encountered. More and more polymorphs having a UHP thermodynamic stability ¢eld will be
identi¢ed at the submicrometer scale, down to
‘crystals’ having only a few unit-cells extension;
thereby the limit is reached under which surface
energies may become preponderant over enthalpic
properties, casting some doubt as to the actual
P^T formation conditions. The recent ¢ndings
of nanometer-size TiO2 with the K-PbO2 structure
in Erzgebirge [85] show that this concern is already no longer a purely theoretical one.
[6]
[7]
[8]
[9]
[10]
[11]
[12]
Acknowledgements
Sharing the excitement of ‘high-pressure hunting’ with Bruno Go¡e¤ and Werner Schreyer has
been a life experience. The help of B. Go¡e¤ and
A. Michard through comments on an early version, of H.-J. Massonne, H.-P. Schertl and E.
Schma«dicke for photographs or rock samples,
and of M. Engi, W. Schreyer and an anonymous
referee through helpful reviews are gratefully acknowledged, as well as the editorial patience of
A.N. Halliday.[AH]
[13]
[14]
[15]
[16]
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2002TC001406
Christian Chopin, 48, has a CNRS
researcher position at the Ecole
normale supe¤rieure, Paris. His
long-standing interests are in the
phase equilibria, mineral chemistry
and transformations of rocks under
high-pressure conditions, and the
implications they have for mountain-building and geological processes in general. This led him to
the ¢rst discovery of metamorphic
coesite. Trying to maintain a balance between experimental
and ¢eld-oriented petrological work, he is a ¢rm believer in
the unsurpassable value of observational data: Nature as the
largest laboratory.
EPSL 6683 19-6-03 Cyaan Magenta Geel Zwart