Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Available online at www.sciencedirect.com R Earth and Planetary Science Letters 212 (2003) 1^14 www.elsevier.com/locate/epsl 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 EPSL 6683 19-6-03 Cyaan Magenta Geel Zwart 2 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 EPSL 6683 19-6-03 Cyaan Magenta Geel Zwart C. Chopin / Earth and Planetary Science Letters 212 (2003) 1^14 5 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 EPSL 6683 19-6-03 Cyaan Magenta Geel Zwart 4 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. EPSL 6683 19-6-03 Cyaan Magenta Geel Zwart C. Chopin / Earth and Planetary Science Letters 212 (2003) 1^14 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 EPSL 6683 19-6-03 Cyaan Magenta Geel Zwart 6 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 EPSL 6683 19-6-03 Cyaan Magenta Geel Zwart 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 EPSL 6683 19-6-03 Cyaan Magenta Geel Zwart 8 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 EPSL 6683 19-6-03 Cyaan Magenta Geel Zwart 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] References [1] B.W. Evans, V. Trommsdor¡, W. Richter, Petrology of an eclogite-metarodingite suite at Cima di Gagnone, Ticino, Switzerland, Am. Mineral. 64 (1979) 15^31. [2] C.A. Heinrich, Eclogite facies regional metamorphism of hydrous ma¢c rocks in the central alpine Adula nappe, J. Petrol. 27 (1986) 123^154. [3] C. Chopin, Coesite and pure pyrope in high-grade blueschists of the western Alps: a ¢rst record and some consequences, Contrib. Mineral. Petrol. 86 (1984) 107^118. [4] D.C. Smith, Coesite in clinopyroxene in the Caledonides and its implications for geodynamics, Nature 310 (1984) 641^644. [5] N.V. Sobolev, V.S. Shatsky, Diamond inclusions in garnet [17] [18] [19] [20] 11 from metamorphic rocks: a new environment for diamond formation, Nature 343 (1990) 742^746. R.J. Hemley (Ed.), Ultrahigh-pressure Mineralogy: Physics and Chemistry of the Earth’s Deep Interior, MSA Rev. Mineral. 37 (1998). Ph. Gillet, J. Ingrin, C. Chopin, Coesite in subducted continental crust: P-T history deduced from an elastic model, Earth Planet. Sci. Lett. 70 (1984) 426^436. C.D. Parkinson, I. Katayama, Present-day ultrahigh-pressure conditions of coesite inclusions in zircon and garnet: Evidence from laser Raman microspectroscopy, Geology 27 (1999) 979^982. C.D. Parkinson, Coesite inclusions and prograde compositional zonation of garnet in whiteschist of the HPUHPM Kokchetav massif, Kazakhstan: a record of progressive UHP metamorphism, Lithos 52 (2000) 215^233. J.L. Mosenfelder, S.R. Bohlen, Kinetics of coesite to quartz transformation, Earth Planet. Sci. Lett. 153 (1997) 133^147. J.G. Liou, R.Y. Zhang, Occurrences of intergranular coesite in ultrahigh-P rocks from the Su-Lu region, eastern China: implications for lack of £uid during exhumation, Am. Mineral. 81 (1996) 1217^1221. B. Ghiribelli, M.L. Frezzotti, R. Palmeri, Coesite in eclogites of the Lanterman Range (Antarctica): Evidence from textural and Raman studies, Eur. J. Mineral. 14 (2002) 355^360. E.D. Mposkos, D.K. Kostopoulos, Diamond, former coesite and supersilicic garnet in metasedimentary rocks from the Greek Rhodope: a new ultrahigh-pressure metamorphic province established, Earth Planet. Sci. Lett. 192 (2001) 497^506. O. Beyssac, C. Chopin, Comment on ‘Diamond, former coesite and supersilicic garnet in metasedimentary rocks from the Greek Rhodope: a new ultrahigh-pressure metamorphic province established’ by Mposkos and Kostopoulos, Earth Planet. Sci. Lett., accepted for publication. H.J. Massonne, H.J. Bernhardt, D. Dettmar, E. Kessler, O. Medenbach, T. Westphal, Simple identi¢cation and quanti¢cation of microdiamonds in rock thin-sections, Eur. J. Mineral. 10 (1998) 497^504. L. Nasdala, H.J. Massonne, Microdiamonds from the Saxonian Erzgebirge, Germany: in situ micro-Raman characterization, Eur. J. Mineral. 12 (2000) 495^498. S. Xu, A.I. Okay, S. Ji, A.M.C. Sengo«r, W. Su, Y. Liu, L. Jiang, Diamond from the Dabie Shan metamorphic rocks and its implications for tectonic setting, Science 256 (1992) 80^82. A. Wain, New evidence for coesite in eclogites and gneisses: De¢ning an ultrahigh-pressure province in the Western Gneiss Region of Norway, Geology 25 (1997) 927^930. A. Wain, D. Waters, A. Jephcoat, H. Olijynk, The highpressure to ultrahigh-pressure eclogite transition in the Western Gneiss Region, Norway, Eur. J. Mineral. 12 (2000) 667^687. D.G. Pearson, G.R. Davies, P.H. Nixon, H.J. Milledge, EPSL 6683 19-6-03 Cyaan Magenta Geel Zwart 12 [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] C. Chopin / Earth and Planetary Science Letters 212 (2003) 1^14 Graphitized diamonds from a peridotite massif in Morocco and its implication for anomalous diamond occurrences, Nature 338 (1989) 60^62. M.L. Leech, W.G. Ernst, Graphite pseudomorphs after diamond? A carbon isotope and spectroscopic study of graphite cuboids from the Maksyutov Complex, south Ural Mountains, Russia, Geochim. Cosmochim. Acta 62 (1998) 2143^2154. J.G. Liou, R.Y. Zhang, W.G. Ernst, D. Rumble III, S. Maruyama, High-pressure minerals from deeply subducted metamorphic rocks, Rev. Mineral. 37 (1998) 33^ 96. D.A. Carswell, R.Y. Zhang, Petrographic characteristics and metamorphic evolution of ultrahigh-pressure eclogites in plate-collision belts, Int. Geol. Rev. 41 (1999) 781^798. B. Sto«ckhert, J. Duyster, C. Trepmann, H.-J. Massonne, Microdiamond daughter crystals precipitated from supercritical COH plus silicate £uids included in garnet, Erzgebirge, Germany, Geology 29 (2001) 391^394. S.L. Hwang, P. Shen, H.T. Chu, T.F. Yui, C.C. Lin, Genesis of microdiamonds from melt and associated multiphase inclusions in garnet of ultrahigh-pressure gneiss from Erzgebirge, Germany, Earth Planet. Sci. Lett. 188 (2001) 9^15. R.Y. Zhang, J.G. Liou, W.G. Ernst, R.G. Coleman, N.V. Sobolev, V.S. Shatsky, Metamorphic evolution of diamond-bearing and associated rocks from the Kokchetav Massif, northern Kazakhstan, J. Metamorph. Geol. 15 (1997) 479^496. Y. Ogasawara, K. Fukasawa, S. Maruyama, Coesite exsolution from supersilicic titanite in UHP marble from the Kokchetav Massif, northern Kazakhstan, Am. Mineral. 87 (2002) 454^461. H.L.M. Van Roermund, M.R. Drury, Ultra-high pressure (P s 6 GPa) garnet peridotites in Western Norway: exhumation of mantle rocks from s 185 km depth, Terra Nova 10 (1998) 295^301. H.L.M. Van Roermund, M.R. Drury, A. Barnhoorn, A. De Ronde, Relict majoritic garnet microstructures from ultra-deep orogenic peridotites in Western Norway, J. Petrol. 42 (2001) 117^130. M. Akaogi, S. Akimoto, Pyroxene-garnet solid-solution equilibria in the systems Mg4 Si4 O12 -Mg3 Al2 Si3 O12 and Fe4 Si4 O12 -Fe3 Al2 Si3 O12 at high pressures and temperatures, Phys. Earth Planet. Int. 15 (1977) 90^106. S.E. Haggerty, V. Sautter, Ultradeep (greater than 300 kilometers), ultrama¢c upper mantle xenoliths, Science 248 (1990) 993^996. K. Ye, B. Cong, D. Ye, The possible subduction of continental material to depths greater than 200 km, Nature 407 (2000) 734^736. H.L.M. Van Roermund, D.A. Carswell, M.R. Drury, T.C. Heijboer, Micro-diamonds in a megacrystic garnetwebsterite pod from Bardane on the island of FjTrtoft, western Norway, Geology 30 (2002) 959^962. H.K. Brueckner, D.A. Carswell, W.L. Gri⁄n, Paleozoic diamonds within a Precambrian peridotite lens in UHP [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] gneisses of the Norwegian Caledonides, Earth Planet. Sci. Lett. 203 (2002) 805^816. A. Liati, D. Gebauer, R. Wysoczanski, U-Pb SHRIMPdating of zircon domains from UHP garnet-rich ma¢c rocks and late pegmatoids in the Rhodope zone (N Greece); evidence for Early Cretaceous crystallization and Late Cretaceous metamorphism, Chem. Geol. 184 (2002) 281^299. R. Knoche, R. Angel, F. Seifert, T. Fliervoet, Complete substitution of Si for Ti in titanite Ca(Ti1 x Six )SiO5 , Am. Mineral. 83 (1998) 1168^1175. P.J. O’Brien, N. Zotov, R. Law, M.A. Khan, M.Q. Jan, Coesite in Himalayan eclogite and implications for models of India-Asia collision, Geology 29 (2001) 435^438. B.K. Mukherjee, H.K. Sachan, Discovery of coesite from Indian Himalaya: A record of ultra-high pressure metamorphism in Indian Continental Crust, Curr. Sci. (India) 81 (2001) 1358^1361. C.D. Parkinson, A. Motoki, C.T. Onishi, S. Maruyama, Ultrahigh-pressure pyrope-kyanite granulites and associated eclogites in Neoproterozoic nappes of southeast Brazil, Abstr. UHPM Workshop Waseda University, 2001, pp. 87^90. C. Chopin, H.P. Schertl, The UHP unit in the DoraMaira massif, Western Alps, Int. Geol. Rev. 41 (1999) 765^780. B.R. Hacker, L. Ratschbacher, L. Webb, T. Ireland, D. Walker, D. Shuwen, U/Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling-Dabie Orogen, China, Earth Planet. Sci. Lett. 161 (1998) 215^230. D. Rumble, Stable isotope geochemistry of ultrahigh-pressure rocks, in: B.R. Hacker, J.G. Liou (Eds.), When Continents Collide: Geodynamics and Geochemistry of Ultrahigh-Pressure Rocks, Kluwer, 1998, pp. 241^259. H. Tabata, K. Yamauchi, S. Maruyama, Tracing the extent of a UHP terrane: Mineral-inclusion study of zircons in gneisses from the Dabie Shan, in: B.R. Hacker, J.G. Liou (Eds.), When Continents Collide: Geodynamics and Geochemistry of Ultrahigh-Pressure Rocks, Kluwer, 1998, pp. 261^273. K. Ye, Y.P. Yao, I. Katayama, B. Cong, Q. Wang, S. Maruyama, Large areal extent of ultrahigh-pressure metamorphism in the Sulu ultrahigh-pressure terrane of East China: new implications from coesite and omphacite inclusions in zircon of granitic gneiss, Lithos 52 (2000) 157^ 164. F.L. Liu, Z. Xu, J.G. Liou, I. Katayama, H. Masago, S. Maruyama, J. Yang, Ultrahigh-pressure mineral inclusions in zircons from gneissic core samples of the Chinese Continental Scienti¢c Drilling Site in eastern China, Eur. J. Mineral. 14 (2002) 499^512. S. Cuthbert, D.A. Carswell, E.J. Krogh-Ravna, A. Wain, Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides, Lithos 52 (2000) 165^195. H.J. Massonne, First ¢nd of coesite in the ultrahigh-pressure metamorphic area of the central Erzgebirge, Germany, Eur. J. Mineral. 13 (2001) 565^570. EPSL 6683 19-6-03 Cyaan Magenta Geel Zwart C. Chopin / Earth and Planetary Science Letters 212 (2003) 1^14 [48] J. Hermann, D. Rubatto, A. Korsakov, V.S. Shatsky, Multiple zircon growth during fast exhumation of diamondiferous, deeply subducted continental crust (Kokchetav Massif, Kazakhstan), Contrib. Mineral. Petrol. 141 (2001) 66^82. [49] I. Katayama, S. Maruyama, C.D. Parkinson, K. Terada, Y. Sano, Ion micro-probe U-Pb zircon geochronology of peak and retrograde stages of ultrahigh-pressure metamorphic rocks from the Kokchetav massif, northern Kazakhstan, Earth Planet. Sci. Lett. 188 (2001) 185^198. [50] J. Yang, Z. Xu, S. Song, J. Zhang, C. Wu, R. Shi, H. Li, M. Brunel, Discovery of coesite in the North Qaidam Early Palaeozoic ultrahigh pressure (UHP) metamorphic belt, NW China, C. R. Acad. Sci. Paris 333 (2001) 719^ 724. [51] C.D. Parkinson, K. Miyazaki, K. Wakita, A.J. Barber, D.A. Carswell, An overview and tectonic synthesis of the pre-Tertiary very-high-pressure metamorphic and associated rocks of Java, Sulawesi and Kalimantan, Indonesia, Island Arc 7 (1998) 184^200. [52] J.A. Giletti, E.J. Krogh-Ravna, First evidence for ultrahigh-pressure metamorphism in the North-East Greenland Caledonides, Geology 30 (2002) 551^554. [53] J.M. Lardeaux, P. Ledru, I. Daniel, S. Duche“ne, The Variscan French Massif Central - a new addition to the ultrahigh pressure metamorphic ‘club’: exhumation processes and geodynamic consequences, Tectonophysics 332 (2001) 143^167. [54] G.R. Tilton, W. Schreyer, H.P. Schertl, Pb-Sr-Nd isotopic behavior of deeply subducted crustal rocks from the Dora-Maira-Massif, Western Alps-II: what is the age of the ultrahigh-pressure metamorphism?, Contrib. Mineral. Petrol. 108 (1991) 22^33. [55] D. Gebauer, H.P. Schertl, M. Brix, W. Schreyer, 35 Ma old ultrahigh-pressure metamorphism and evidence for very rapid exhumation in the Dora-Maira-Massif, Western Alps, Lithos 41 (1997) 5^24. [56] D. Rubatto, J. Hermann, Exhumation as fast as subduction?, Geology 29 (2001) 3^6. [57] S. Maruyama, J.G. Liou, Initiation of ultrahigh-pressure metamorphism and its signi¢cance on the ProterozoicPhanerozoic boundary, Island Arc 7 (1998) 6^35. [58] B. Jahn, R. Caby, P. Monie¤, The oldest UHP eclogites of the world: age of UHP metamorphism, nature of protoliths and tectonic implications, Chem. Geol. 178 (2001) 143^158. [59] A. Mo«ller, P. Appel, K. Metzger, V. Schenk, Evidence for a 2 Ga subduction zone Eclogites in the Usagaran belt of Tanzania, Geology 23 (1995) 1067^1070. [60] D.R. Snoeyenbos, M.L. Williams, S. Hanmer, Archean high-pressure metamorphism in the western Canadian Shield, Eur. J. Mineral. 7 (1995) 1251^1272. [61] A.L. Wain, D.J. Waters, H. Austrheim, Metastability of granulites and process of eclogitisation in the UHP region of western Norway, J. Metamorph. Geol. 19 (2001) 609^ 626. [62] M. Bruno, R. Compagnoni, M. Rubbo, The ultra-high [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] 13 pressure coronitic and pseudomorphous reactions in a metagranodiorite from the Brossasco-Isasca Unit, DoraMaira Massif, western Italian Alps: a petrographic study and equilibrium thermodynamic modelling, J. Metamorph. Geol. 19 (2001) 33^43. A.K. Straume, H. Austrheim, Importance of fracturing during retro-metamorphism of eclogites, J. Metamorph. Geol. 17 (1999) 637^652. G. Simon, C. Chopin, Enstatite-sapphirine crack-related assemblages in ultrahigh-pressure pyrope megablasts, Dora-Maira massif, western Alps, Contrib. Mineral. Petrol. 140 (2001) 422^440. R. Bousquet, B. Go¡e¤, P. Henry, X. Le Pichon, C. Chopin, Kinematic, thermal and petrological model of the Central Alps: Lepontine metamorphism in the upper crust and eclogitisation of the lower crust, Tectonophysics 273 (1997) 105^127. M.P. Doin, P. Henry, Subduction initiation and continental crust recycling the roles of rheology and eclogitization, Tectonophysics 342 (2001) 163^191. E. Burov, L. Jolivet, L. Le Pourhiet, A. Poliakov, A thermomechanical model of exhumation of high pressure (HP) and ultra-high pressure (UHP) metamorphic rocks in Alpine-type collision belts, Tectonophysics 342 (2001) 113^136. J.L. Paquette, J.M. Montel, C. Chopin, U-Pb-Th dating of the Brossasco ultrahigh-pressure metagranite, DoraMaira massif, western Alps, Eur. J. Mineral. 11 (1999) 69^77. G.R. Tilton, L. Ames, H.P. Schertl, W. Schreyer, Reconnaissance isotopic investigations on rocks of an undeformed granite contact within the coesite-bearing unit of the Dora-Maira-Massif, Lithos 41 (1997) 25^36. S. Scaillet, K-Ar (40 Ar/39 Ar) geochronology of ultrahigh pressure rocks, in: B.R. Hacker, J.G. Liou (Eds.), When Continents Collide: Geodynamics and Geochemistry of Ultrahigh-Pressure Rocks, Kluwer, 1998, pp. 161^201. D. Giorgis, M. Cosca, S. Li, Distribution and signi¢cance of extraneous argon in UHP eclogite (Sulu terrain, China): insight from in situ 40 Ar/39 Ar UV-laser ablation analysis, Earth Planet. Sci. Lett. 181 (2000) 605^615. D. Rumble, D. Giorgis, T. Ireland, Z. Zhang, H. Xu, T.F. Yui, J. Yang, Z. Xu, Low N18 O zircons, U-Pb dating, and the age of the Qinglongshan oxygen and hydrogen isotope anomaly near Donghai in Jiangsu Province, China, Geochim. Cosmochim. Acta 66 (2002) 2299^2306. H. Bureau, H. Keppler, Complete miscibility between silicate melts and hydrous £uids in the upper mantle: Experimental evidence and geochemical implications, Earth Planet. Sci. Lett. 165 (1999) 187^196. W. Schreyer, Experimental aspects of UHP metamorphism: granitic systems, Int. Geol. Rev. 41 (1999) 701^ 710. B. Sto«ckhert, J. Renner, Rheology of crustal rocks at ultrahigh pressure, in: B.R. Hacker, J.G. Liou (Eds.), When Continents Collide: Geodynamics and Geochemistry of Ultrahigh-Pressure Rocks, Kluwer, 1998, pp. 57^95. EPSL 6683 19-6-03 Cyaan Magenta Geel Zwart 14 C. Chopin / Earth and Planetary Science Letters 212 (2003) 1^14 [76] B.R. Hacker, L. Ratschbacher, L. Webb et al., Exhumation of ultrahigh-pressure continental crust in east central China: Late Triassic-Early Jurassic tectonic unroo¢ng, J. Geophys. Res. 105 (2000) 13,339^13,364. [77] W.G. Ernst, S. Maruyama, S. Wallis, Buoyancy-driven, rapid exhumation of ultrahigh-pressure metamorphosed continental crust, Proc. Natl. Acad. Sci. USA 94 (1997) 9532^9537. [78] W.G. Ernst, Subduction, ultrahigh-pressure metamorphism, and regurgitation of buoyant crustal slices - implications for arcs and continental growth, Phys. Earth Planet. Int. 127 (2001) 253^275. [79] A. Michard, C. Henry, C. Chopin, Structures in UHPM rocks: a case study from the Alps, in: R.G. Coleman, X. Wang (Eds.), Ultrahigh Pressure Metamorphism, Cambridge University Press, New York, 1995, pp. 132^158. [80] D. Avigad, C. Chopin, R. Le Bayon, Thrusting and extension in the southern Dora-Maira ultra-high-pressure massif (Western Alps): view from below the coesite-bearing unit, J. Geol. 111 (2003) 57^70. [81] A.I. Chemenda, M. Mattauer, A.N. Bokun, Continental subduction and a mechanism for exhumation of highpressure metamorphic rocks: new modelling, ¢eld data from Oman, Earth Planet. Sci. Lett. 143 (1996) 173^182. [82] G.T. Roselle, M. Engi, Ultra high pressure (UHP) terrains: lessons from thermal modelling, Am. J. Sci. 302 (2002) 410^441. [83] N. Merveilleux du Vignaux, L. Fleitout, Stretching and mixing of viscous blobs in Earth’s mantle, J. Geophys. Res. 106 (2001) 30893^30908. [84] P.J. O’Brien, J. Ro«tzler, High-pressure granulites: forma- tion, recovery of peak conditions and implications for tectonics, J. Metamorph. Geol. 21 (2003) 3^20. [85] S.L. Hwang, P.Y. Shen, H.T. Chu, T.F. Yui, Nanometersize K-PbO2 -type TiO2 in garnet: A thermobarometer for ultrahigh-pressure metamorphism, Science 288 (2000) 321^324. [86] E. Schma«dicke, W.F. Mu«ller, Unusual exsolution phenomena in omphacite and partial replacement of phengite by phlogopite+kyanite in an eclogite from the Erzgebirge, Contrib. Mineral. Petrol. 139 (2000) 629^642. [87] T.V. Gerya, B. Sto«ckhert, A.L. Perchuk, Exhumation of high-pressure metamorphic rocks in a subduction channel: A numerical simulation, Tectonics 21 (2002) 10.1029/ 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