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Abstract

Ultrahigh pressure (UHP) rocks are extraordinary rocks that recorded extreme metamorphic conditions. Their study has been an exciting part of geosciences and in the last few years has emerged as a distinct discipline. Only few UHP terranes have been recognised worldwide so far and this only over the last ca. 20 years. They occur within major continental collision belts, predominantly in Eurasia and Africa (see e.g. Ernst & Liou, 2000; Liou, 2000; Carswell & Compagnoni, 2003 for a general review). Even though UHP rocks have been subject to intense studies in the last two decades, many questions regarding their formation and evolution remain open. Geochronology can assist this search for information in providing the absolute time of their formation and several other parameters regarding their evolution.

Introduction

The aims and the challenge

Ultrahigh pressure (UHP) rocks are extraordinary rocks that recorded extreme metamorphic conditions. Their study has been an exciting part of geosciences and in the last few years has emerged as a distinct discipline. Only few UHP terranes have been recognised worldwide so far and this only over the last ca. 20 years. They occur within major continental collision belts, predominantly in Eurasia and Africa (see e.g. Ernst & Liou, 2000; Liou, 2000; Carswell & Compagnoni, 2003 for a general review). Even though UHP rocks have been subject to intense studies in the last two decades, many questions regarding their formation and evolution remain open. Geochronology can assist this search for information in providing the absolute time of their formation and several other parameters regarding their evolution.

Geochronology determines the absolute time of formation of UHP units and nearby units within the same chain recording different pressure-temperature (PT) conditions. This can in turn serve to identify the dynamics and extension of the tectonic processes responsible for the formation and preservation of UHP terranes.

Age determination of several stages of the evolution of UHP rocks provides the time of exhumation and, rarely, that of subduction. Prograde ages are difficult to obtain mainly because of later re-equilibration at higher temperatures and age resetting of the minerals of interest. The ages, in combination with petrological data, are used to infer exhumation and subduction rates. These rates are crucial for the construction of geodynamic models and to discern between the different tectonic processes responsible for bringing rocks to great depth and exhume them to the surface.

A detailed geochronological investigation of UHP assemblages, applying different mineral chronometers, can provide earth scientists with an estimate of the duration of peak metamorphism. This information would have important implications for the thermal and rheological regimes present along the subduction slab at great depth.

For each of these tasks the correlation between time, pressure, temperature and, ideally, fluids and deformation is crucial. This is a big challenge for geochronology, a discipline that has long operated relatively separate from petrology and to some extent even geology. It is only in the last years that increasing efforts in crossing these boundaries have been made.

The problems

The main problem in dating UHP and HP rocks and metamorphic rocks in general is, once a suitable mineral chronometer is found, to establish the relationships between ages and metamorphic conditions under which the minerals used for dating were formed. In UHP rocks, more than in other rock types, several factors (see below) can hamper the achievement of this goal, as is also reported in the summary of Gebauer (1990) for dating of HP rocks.

The best way to correlate age and PT of metamorphism is to date major metamorphic minerals for which conditions of formation are known. In HP rocks in particular, this can be best achieved by dating garnet, which can be used for thermobarometry. Micas and amphiboles are other datable minerals, together with titanite. More problems are encountered when dating accessory minerals such as zircon and monazite, which are the pillars of U-Pb dating. The metamorphic behaviour of these minerals is much less known and, thus, additional information is required to define a correlation with P and T conditions (see below).

Because of the extreme conditions that UHP rocks experienced, they often display a complex evolution, during prograde and retrograde stages, as widely reported in other chapters of this volume. This is reflected in the wide presence of zoned minerals (e.g. garnet) and/or multiple generations of minerals (e.g. micas) as well as in prograde and/or peak metamorphic inclusions. Modern geochronology has to take advantage of this mineralogical complexity by dating single mineral zones or generations. This task is best achieved with beam techniques that allow isotopic measurements, either in thin section or mineral separates, of such mineral zones (domains), a discipline that has seen enormous progress in the last decade (e.g. Claoué-Long et al., 1991; Gebauer, 1990; Kelley, 2002; Müller, 2003).

A consequence of the complex mineralogy of UHP rocks is the widespread occurrence of only local equilibrium, with several parageneses (e.g. peak and retrograde) being preserved in the same rock sample (e.g. Hermann et al., 2001; Reinecke, 1991; Schertl et al., 1991). This becomes a problem when using isotopic systems that rely on large mineral separates, multi-domain single minerals or whole rock data (e.g. Rb-Sr, Sm-Nd and Lu-Hf isochrons or Ar-Ar step-wise degassing). Whole rock data points always present a risk and should thus be avoided. Mineral data can be improved using careful mineral separation and chemical characterisation of the separate to check for heterogeneity or inclusions. Modern leaching techniques have sometimes produced promising results in the elimination, or minimisation, of inclusions (e.g. Amato et al., 1999; Anczkiewicz et al., 2002; Scherer et al., 2000), a problem particularly important in dating garnet. Oxygen isotopes have been proposed as a tool to establish equilibrium between HP minerals (Zheng et al., 2002).

A characteristic of UHP rocks are steep PT paths in which short T intervals correspond to great depth variations, with the extreme condition being isothermal decompression (e.g. Gebauer, 1996; Gebauer et al., 1997; Hermann et al., 2001; Sánchez-Rodríguez & Gebauer, 2000; Terry et al., 2000). This feature hampers the use of thermochronometers, which assume a known temperature as closure for the diffusion of a certain isotope in a given mineral. Closure temperatures are not absolute values, but are a function of cooling rates, mineral grain size, fluid-rock interaction and mineral chemistry (e.g. Kelley, 2002; Scherer et al., 2000). Therefore, closure temperatures can vary from rock to rock, and they always bear an intrinsic error. Alternatively, if radiometric mineral ages are considered to be primarily caused by fluid-induced recrystallisation (e.g. Villa, 1998), temperature is only one of a series of parameters that influence the closure of an isotopic system. Thus, a precise definition of a cooling path, with respect to absolute ages, remains difficult and the application of fission track data, mainly on zircon and apatite, is more promising. Because during fast exhumation of UHP rocks small T intervals are covered in short periods of time, even small errors in temperature-time correlation can produce large inaccuracy in cooling profiles.

Together with extreme pressures, UHP rocks often display relatively high peak temperatures (> 600 °C). Such temperatures make it impossible to constrain the age of peak metamorphism with conventional thermochronometers (e.g. Ar-Ar, Rb-Sr), with closure temperatures generally below ~ 600 °C (e.g. Kelley, 2002; Scherer et al., 2000). However, closure temperatures are - amongst others - a function of cooling rates (i.e. residence time at high T), and in UHP rocks, fast exhumation and short residence time at high T can push them towards higher values (e.g. Rubatto & Hermann, 2001).

There is good indication that rocks which preserve HP and UHP minerals were exhumed fast and/or have not suffered very strong deformation. This is rather the exception, since in most cases (U)HP minerals are obliterated due to metamorphic overprint and the rock preserves no memory of the (U)HP event. Fast geological processes require dating with high age resolution in order to date the different metamorphic stages that the rock experienced. This is a challenge for some geochronological techniques more than others, for example those relying on small sample sizes such as beam techniques (laser ablation ICP-MS, ion microprobe and electron microprobe U-Pb dating, and laser spot Ar-Ar dating).

The method

Accurate and precise dating of distinct stages during the evolution of UHP rocks requires a combination of techniques that

  • can take advantage of polyphase growth of minerals,

  • have good to high time resolution,

and that are based on minerals that
  • can be related to metamorphic conditions,

  • are robust to retrogression and to HT perturbations, and

  • are sensitive to pressure changes.

All these features are hardly met in a single chronometer. There are more chances to achieve the above requirements if mineral chronometers are combined. Moreover, techniques that help to understand the conditions of formation, the petrology, the composition, and the metamorphic behaviour of the dated minerals need to be applied.

Even though the ideal chronometer that fulfils all theoretical requirements may not yet be available, a number of studies have successfully managed to describe and constrain the P–T–t evolution of some UHP units throughout the world. These studies accomplished their goal combining different techniques, which may vary from case to case. They all present some degree of innovation that can contribute to accurate dating of UHP metamorphism. In the following, we present two case studies (the Dora-Maira and the Kokchetav massifs) to illustrate how modern geochronology can deal with the problem of dating UHP metamorphism. We go through the major burdens encountered in each case and refer particularly to the studies that introduced innovations and marked a significant advance. A brief review on studies that have dealt with other important UHP terranes is following, with the intention to stress what geochronology is still lacking to reach the goal of dating UHP metamorphism. We then discuss the tectonic and geological implications of the P–T–t paths of UHP units, which information can they provide to geologists and how this gives an insight on the tectonic processes involving UHP rocks. A final chapter is addressing the challenges that lie ahead in the geochronology of UHP rocks.

Case studies

In this section we present a review of the geochronological work done on two extensively studied UHP units: the Dora-Maira Massif in the Western Alps and the Kokchetav Massif in Kazakhstan. In these localities there is a relatively complete knowledge of P–T–t evolution. Focussing particularly on the more recent works, we discuss the major improvements in dating UHP rocks, both in terms of techniques and reliability.

The Dora-Maira

The Dora-Maira Massif in the Western Alps contains a UHP unit (see the review of Compagnoni & Rolfo, 2003 in this volume for a complete presentation of the geological setting) that forms a coherent body (ca. 10 × 5 × 1 km) of continental crust. It consists of a lithologically heterogeneous basement (metapelites, para- and orthogneisses with intercalated eclogites, calc-silicate rocks, and marbles) that was intruded by Permo-Carboniferous granites (Chopin et al., 1991; Compagnoni et al., 1995; Gebauer et al., 1997). There is evidence through the whole unit of a widespread recrystallisation in the coesite stability field during the Alpine orogeny at pressures of ~ 3.5 GPa and temperatures of ~ 750 °C (Chopin et al., 1991; Compagnoni et al., 1995; Schertl et al., 1991). In a recent work, Hermann (2003) suggests that the unit reached conditions of even 4.1 GPa, within the diamond stability field.

The first geochronological investigations of the UHP rocks were carried out in the late 80’s using classical techniques, such as Ar-Ar and Rb-Sr on micas, and U–Pb isotope dilution, multigrain analysis on zircon. The first results of these studies reported ages scattering between 120 and 95 Ma (Monié & Chopin, 1991; Paquette et al., 1989). These figures fitted the view supporting a Cretaceous Alpine subduction, which was generally accepted at that time and also claimed for other Alpine areas (e.g. Hunziker et al., 1992). However, at the same time Ar-Ar data in the Dora-Maira and the nearby HP unit of Monviso (Monié & Philippot, 1989; Monié et al., 1989) already contradicted this picture reporting younger Tertiary ages around 50 Ma. Even though these were interpreted as cooling ages, the authors postulated two episodes of HP metamorphism in the area. Further evidence for the possibility of Tertiary subduction came from the work of Tilton et al. (1989, 1991). These authors obtained U-Pb data on zircon, ellenbergerite and monazite interpreted to reflect metamorphic ages in the range of 30-38 Ma, not necessarily corresponding to UHP metamorphism. The same authors reported the first Sm-Nd errorchron data from a pyrope garnet of the UHP unit, interpreted to correspond to an “age” of ca. 38 Ma. Despite the clear scattering of the Tertiary ages, their dubious reliability and different significance, these works opened the possibility of Tertiary UHP metamorphism in the Dora-Maira and the Alps in general. From this set of data also arose the problem of which technique to use for best dating such complicated rocks involved in unusual metamorphic conditions.

The reliability of conventional Ar-Ar stepwise heating dating technique in HP and UHP minerals was questioned by Arnaud & Kelley (1995), who documented anomalously old ages in HP minerals from several localities including the Dora-Maira. The introduction of in situ laser spot extraction techniques made possible Ar-Ar studies of within crystal age variations and distribution of argon between mineral phases. This led to the discovery of concentration of excess argon at the grain boundaries and the close correlation of excess argon with fluid mediated alteration in HP and UHP terrains (e.g. Giorgis et al., 2000; Kelley, 2002; Scaillet, 1998). The problems described remain major obstacles in dating the cooling path of HP rocks with the Ar-Ar technique.

It was the work of Gebauer et al. (1997) to sign a breakthrough in the determination of the P–T–t path of the Dora-Maira UHP unit. The authors applied the technique of ion microprobe (SHRIMP) dating of single zircon domains supported by cathodoluminescence (CL) imaging of the zircons dated, a technique that was proven to be successful in several localities with polymetamorphic (U)HP rocks (e.g. Gebauer, 1996). This represented an important innovation because it allowed dating of narrow zircon domains (20-30 μm wide) that recorded UHP metamorphism separately from crystal domains that preserved pre-metamorphic ages (Fig. 1). This was most important for zircon, a mineral that does not generally recrystallise completely during HP or UHP metamorphism, unless fluids are present in abundance. Zircon has the capacity to preserve its original U-Pb composition despite high-T overprint. With the assistance of CL imaging, Gebauer et al. (1997) could identify metamorphic domains in zircon crystals included in prograde garnets and associated with UHP minerals. These domains, generally forming crystal rims or aureoles around UHP inclusions (Fig. 1), were characterised by relatively homogeneous CL patterns contrasting with the oscillatory zoning of the older magmatic cores, and Th/U ratios lower than their magmatic precursor. Newly formed Alpine zircon developing oscillatory zoning during precipitation from a supercritical fluid/melt could also be dated (Fig. 1 in this study or Fig. 3 in Gebauer et al., 1997). Thus, based on the above information, and despite the existence of many discordant oscillatory zoned domains, the age of the metamorphic zircon domains could be well constrained at 35.4 ± 1.0 Ma (Gebauer et al., 1997). In this study the link to metamorphic conditions was possible because metamorphic zircons and/or zircon domains were extracted from large (ca. 15 cm in diameter) pyrope megablasts (Gebauer et al., 1997). These megablasts grew at the expense of chlorite + talc + kyanite at about 700 °C and 30 kbar, i.e. at UHP conditions shortly before the UHP peak (e.g. Schertl et al., 1991). The enclosed ca. 35 Ma old newly formed zircons and zircon domains (Fig. 1 this study and Fig. 3 in Gebauer et al., 1997) were thus interpreted to predate the UHP peak as they must have crystallised during or very slightly before the growth of the garnet megablasts, probably from a supercritical fluid/melt. Additionally, because of the analytical error of the SHRIMP technique, ages that may correspond to different stages of metamorphism appear as identical (e.g. Kröner et al., 2000). In this respect, minimisation of the analytical error would be a step forward in distinguishing between ages acquired during different metamorphic stages and therefore better constrain subduction and exhumation rates. In the specific case of the Dora-Maira, the zircon-bearing rocks mainly preserved a UHP assemblage and, importantly, some of the ca. 35 Ma old metamorphic zircon domains occur in contact with and around pseudo-inclusions of UHP minerals such as phengite and coesite (Fig. 1). The UHP “inclusions” found within oscillatory magmatic (Permian) zircon must be secondary: they either filled fractures or replaced magmatic domains during UHP metamorphism. The process that produced these pseudo-inclusions also favoured recrystallisation of zircon during the UHP stage. These UHP “inclusions” were then armoured by the surrounding zircon and thus could survive retrogression during exhumation. Based on these observations, the age of 35.4 ± 1.0 Ma was attributed to the UHP peak and, combined with a zircon fission track age of 29.9 ±1.4 Ma (Gebauer et al., 1997), a two-point P–T–t path was constructed. This combination of dating techniques, together with the petrological information, allowed the calculation of an average exhumation rate for the UHP Dora-Maira unit on the order of 20-24 km/m.y. (2-2.4 cm/year) with a cooling rate of about 85-100 °C/m.y. (Gebauer et al, 1997).

Fig. 1.

Cathodoluminescence image of zircon crystals from Dora-Maira UHP rocks. (a) Crystal include in a 15 cm large, prograde megablast, in which the distinction between oscillatory-zoned, magmatic zircon core and metamorphic rim developing oscillatory zoning against the outer surface (increasing amounts of fluids/melts) is particularly evident. Ion microprobe analysis (analysed spots represented by the circles) is the only technique that allows dating the different domains separately. (b) Crystal, separated from a fine-grained pyrope quartzite, containing an “inclusion” of coesite partly surrounded by metamorphic zircon. For more details see text.

Fig. 1.

Cathodoluminescence image of zircon crystals from Dora-Maira UHP rocks. (a) Crystal include in a 15 cm large, prograde megablast, in which the distinction between oscillatory-zoned, magmatic zircon core and metamorphic rim developing oscillatory zoning against the outer surface (increasing amounts of fluids/melts) is particularly evident. Ion microprobe analysis (analysed spots represented by the circles) is the only technique that allows dating the different domains separately. (b) Crystal, separated from a fine-grained pyrope quartzite, containing an “inclusion” of coesite partly surrounded by metamorphic zircon. For more details see text.

The Dora-Maira was subject of another milestone study of (U)HP rocks representing the first example of Lu-Hf dating (Duchêne et al., 1997). The Lu-Hf system is based on the same principles as the more commonly used Sm-Nd dating. The main advantage of this new technique is the high Lu/Hf ratio of garnet, the main mineral used, which results in much better precision of an isochron age. Thanks to the introduction of plasma source mass spectrometry, the precision in measuring low Hf concentrations improved significantly, thus making Lu-Hf dating suitable for low Hf minerals such as garnet. For a Dora-Maira coesite-pyrope quartzite, Duchêne et al. (1997) reported an age of 32.8 ± 1.2 Ma, based on an isochron defined by two garnet and one whole rock analyses. The same authors also obtained a ca. 31.8 Ma Sm-Nd age from garnets of a similar sample. However, it is worth mentioning that Nd isotopic disequilibrium within the Dora-Maira garnets was later reported, and related to the presence of REE rich, Hf poor micro-inclusions (Luais et al., 2001). On the basis of a suggested closure temperature in excess of 600 °C for Lu-Hf in garnet, Duchêne et al. (1997) interpreted their ages as dating the early stage of exhumation after peak metamorphism at around 35 Ma. Unfortunately no pressure estimate was obtained from the dated garnet, an information that would have strengthened the importance of this age. The authors proposed an exhumation rate “of the order of 3 cm/year”, in line with the one suggested by Gebauer et al. (1997).

The P–T–t path of the Dora-Maira UHP rocks, first established by Gebauer et al. (1997), was constrained in further detail by Rubatto & Hermann (2001), who applied in situ ion microprobe U-Pb dating to titanite from calc-silicates. The importance of this study lies in the fact that the above authors combined geochronological data with petrological information and dated a metamorphic mineral (titanite), for which conditions of formation are much easier to establish when compared to zircon. By using textural relationships, the composition of titanite itself, as well as that of numerous mineral inclusions in it, the existence of three generations of titanite could be established. These three generations of titanite formed at different PT conditions, based on their different major and trace element composition (Fig. 2; Rubatto & Hermann, 2001 and unpublished data). With the additional support of some thermodynamic equilibrium calculations involving titanite, it was inferred that the oldest titanite formed at the metamorphic peak at 35.1 ± 0.9 Ma, confirming the data of Gebauer et al. (1997). In addition, it was demonstrated that another two titanite generations formed during two distinct decompressional stages: at 32.9 ± 0.9 Ma and 31.8 ± 0.5 Ma. The combination of geochronology with metamorphic petrology and the addition of the fission track data from Gebauer et al. (1997) allowed the definition of a four-point P–T–t path. This represents one of the best-documented exhumation trajectories of a UHP unit (Fig. 3). More than confirming fast exhumation, Rubatto & Hermann (2001) could identify changes in exhumation rates from 3.4 to 0.5 cm/year during the evolution of the Dora-Maira. This new piece of information bears important implications for tectonic models (see below).

Fig. 2.

Dora-Maira titanites. (a, b) Backscattered electron (BSE) images of dated titanite crystals from a UHP and a LP calc-silicate, respectively. Four generations of titanite are distinguishable on the base of their BSE emission, which reflects different chemical composition. (c, d) Variation in major and trace element compositions, respectively, between the four generations of titanites. Major element data from Rubatto & Hermann (2001) and unpublished trace element data.

Fig. 2.

Dora-Maira titanites. (a, b) Backscattered electron (BSE) images of dated titanite crystals from a UHP and a LP calc-silicate, respectively. Four generations of titanite are distinguishable on the base of their BSE emission, which reflects different chemical composition. (c, d) Variation in major and trace element compositions, respectively, between the four generations of titanites. Major element data from Rubatto & Hermann (2001) and unpublished trace element data.

Fig. 3.

P–T–t path for the Dora-Maira UHP unit, modified after Rubatto & Hermann (2001). Geochronological data from: 1) Rubatto & Hermann (2001); 2) Gebauer et al. (1997); 3) Duchêne et al. (1997).

Fig. 3.

P–T–t path for the Dora-Maira UHP unit, modified after Rubatto & Hermann (2001). Geochronological data from: 1) Rubatto & Hermann (2001); 2) Gebauer et al. (1997); 3) Duchêne et al. (1997).

Apart from tight constraints on the last part of the prograde, as well as on the retrograde P–T–t trajectory of these unique rocks, also a series of further indications on their pre-metamorphic history, as well as the availability of fluids during metamorphism could be gained (Gebauer et al., 1997). 1) The first order Permian protolith of granitic composition was probably metasomatised at low P-T to form Mg-rich leucophyllites during Permo-Triassic rifting. 2) Zircon formed metamorphic domains at 35 Ma only in the fluid-rich, second order protoliths leucophyllites of the UHP white-schists and not in the much drier country rocks. 3) Minor partial melting probably occurred during UHP conditions in the white-schists as suggested by rare, newly formed, metamorphic zircon domains that grade into oscillatory-zoned domains (Fig. 1). 4) Jadeite-rich layers within the white-schists probably represent former Permian aplite veins within the first order granitic protolith and are not a product of UHP melting.

In summary, the geochronological investigation of the Dora-Maira UHP unit comprises a large variety of isotopic systems and methods, including ion microprobe U-Pb analysis, Sm-Nd and Lu–Hf mineral isochrons, Ar-Ar and fission track data. This is the most complete data set acquired on a UHP unit so far. The combination of different dating techniques, some more reliable than others, led to a good knowledge of the age of the UHP metamorphic peak and of the fast exhumation history (Fig. 3).

The Kokchetav Massif

The Kokchetav Massif in Kazakhstan forms part of the Caledonian Central Asiatic fold belt and is situated between the Siberian platform and the East European platform. This massif has become worldwide known because of the finding of metamorphic micro-diamonds, providing evidence for UHP (> 40 kbar) metamorphism (see Shatsky & Sobolev, 2003 in this volume for a review). The diamondiferous rocks are present in the Zerenda Series, which consists mainly of garnet-biotite-kyanite gneisses and schists with intercalated dolomitic marbles, calc-silicates and eclogites.

The first geochronological investigation of the Kokchetav Massif UHP rocks was carried out soon after the finding of diamonds in the area. Claouè-Long et al. (1991) dated zircons from a biotite schist in a work that belongs to the first generation of SHRIMP ion microprobe study of metamorphic rocks and was the first work on HP/UHP rocks. On the basis of optical microscopy only of the zircon crystals (cathodoluminescence was not yet a routine technique at the time), Claouè-Long et al. (1991) identified rounded, inherited zircon cores of likely detrital origin and zircon rims. The apparent U-Pb ages of the cores scattered between 558 and 1981 Ma, confirming their detrital origin. The rims were attributed to the UHP stage because a diamond aggregate was found at the core-rim boundary. Sixteen analyses on rims yielded a mean age of 530 ± 7 Ma, which was interpreted as the age of the UHP metamorphic peak.

Several years later, the Kokchetav rocks were again investigated with the same technique (SHRIMP dating of zircon domains) by Katayama et al. (2001) and by Hermann et al. (2001). This latter study extended dating to other rock types including several gneisses and a dolomitic metacarbonate. The main innovation introduced by these studies was the use of zircon imaging to assist SHRIMP dating (CL and backscattered electron images) and the detailed investigation of mineral inclusions in zircon (Fig. 4). The inclusions, which were analysed by Raman spectroscopy (mainly Katayama et al., 2001) and by electron microprobe (Hermann et al., 2001), proved to be not exclusively of UHP origin, but also prograde and retrograde (diamond, graphite, quartz, coesite, kyanite, rutile, biotite, phengite, K-feldspar, plagioclase, chlorite, clinopyroxene). Thus, the zircon crystals in the Kokchetav rocks must have formed (or recrystallised) during different stages of the evolution of the UHP unit. With a detailed petrological study, Hermann et al. (2001) could correlate the composition of the inclusions to the variable rock mineral assemblages from which P-T estimates were obtained.

Fig. 4.

Photomicrograph (transmitted light) (a) and CL image (b) of different zircon crystals from a Kokchetav UHP gneiss. Inclusions belonging to different P–T stages are present in crystals from the same rock and even within the same crystal, indicating that zircon formed over a range of P-T conditions. Surprisingly, the CL zoning of the domains with different inclusions is quite similar.

Fig. 4.

Photomicrograph (transmitted light) (a) and CL image (b) of different zircon crystals from a Kokchetav UHP gneiss. Inclusions belonging to different P–T stages are present in crystals from the same rock and even within the same crystal, indicating that zircon formed over a range of P-T conditions. Surprisingly, the CL zoning of the domains with different inclusions is quite similar.

The inclusion study was coupled with trace element analysis of zircon domains by laser ablation ICP-MS (Hermann et al., 2001). The trace element composition of zircon was revealed to be another powerful tool allowing distinction between zircon domains formed at different PT conditions. In particular, zircon formed at HP/UHP had a peculiar signature characterised by a flattening of the HREE pattern and a less pronounced negative Eu anomaly, when compared to zircon formed at lower pressure (Fig. 5). The different zircon chemistry was attributed to changes in the rock paragenesis during different stages of metamorphism. The depletion in HREE is due to the presence of garnet as a HREE sink during HP metamorphism and part of the early decompression history. On the other hand, zircon domains enriched in HREE with respect to the MREE and with a marked negative Eu anomaly are related to the breakdown of garnet and the formation of feldspar (a known sink for Eu) during amphibolite facies overprint. These changes in the trace element chemistry of zircon and their correlation with different stages of metamorphism have been since investigated more in detail by Rubatto (2002) and Rubatto & Hermann (2003), who confirmed the typical signature of eclogite-facies zircon. In the Kokchetav rocks, Hermann et al. (2001) identified four different types of zircon domains. According to the composition of their mineral inclusions and trace element chemistry, three of these zircon domains were attributed to metamorphism, although most of them showed apparent similarities in CL patterns. The accurate determination of P–T conditions for zircon (re)crystallisation indicated that zircon formed during the peak of metamorphism and mainly in the early stages of the decompressional path. This observation implies caution in assuming that zircon generally forms at the metamorphic peak.

Fig. 5.

Trace element patterns of zircon crystals from a Kokchetav UHP biotite gneiss. Note the difference in composition between zircon domains formed in different conditions. Data from Hermann et al. (2001).

Fig. 5.

Trace element patterns of zircon crystals from a Kokchetav UHP biotite gneiss. Note the difference in composition between zircon domains formed in different conditions. Data from Hermann et al. (2001).

Ion microprobe U-Pb dating of the different zircon domains identified by Hermann et al. (2001) on the basis of mineral inclusions and trace element chemistry, produced a narrow range of ages, confirming the data of Claoué-Long et al. (1991). Through statistical analysis it was concluded that the ages of the zircon domains formed from the UHP peak to amphibolite facies overprint were indistinguishable within analytical uncertainty (527 ± 5, 528 ± 8 and 526 ± 5 Ma; error at 95% c. l.). This provided evidence for a fast exhumation of the Kokchetav UHP rocks. On the other hand, Katayama et al. (2001) produced SHRIMP U-Pb data with large errors and spreading over a wide range (442 ± 40 to 563 ± 43 Ma). These authors argued for UHP peak at 537 ± 9 Ma, low-pressure overprint at 507 ± 8 Ma and a post orogenic thermal event at 456-461 Ma. The discrepancy between the two studies could be partly due to different sample localities (Kumdy Kol for Katayama et al., 2001, and Barchi Kol for Hermann et al., 2001) and the effect of a late thermal perturbation in the Katayama et al. (2001) samples. Additional disagreement could be related to analytical differences in the two laboratories or different statistical treatments of the data. Despite the different age results and interpretation, these studies once again pointed to a fast exhumation of UHP rocks (> 1.8 cm/year from a pressure peak of 43 kbar in Hermann et al., 2001; 0.5 cm/year from an uncertain peak of over 60 kbar in Katayama et al., 2001).

The geochronological study of the Kokchetav rocks indicates how even an apparently simple zircon population, yielding an apparent single age, can bear important information about the time evolution of the host rocks. In this case, dating of crystal domains showing different CL patterns was not enough to unravel the complex history of zircon growth. The determination of metamorphic conditions of zircon formation through mineral inclusions and trace element composition could unravel the fast exhumation history of the unit.

Other localities

Besides the excellent examples of the Dora-Maira and the Kokchetav Massif, a number of other UHP terranes have been the subjects of geochronological investigation. In this session we will review some of these studies. The aim is to point out on one hand the interesting approaches used, and on the other hand the problems that remain open in dating UHP rocks in general.

The Dabie Shan metamorphic belt, the largest block of UHP rocks documented so far (e.g. Hirajima & Nakamura, 2003), has been dated using a variety of techniques including U-Pb dating of zircon and monazite, Sm-Nd mineral and whole rock isochrons, and Ar-Ar mineral dating (for a review see Hacker et al., 2000). In this area, early isotope dilution multigrain zircon analysis yielded an age of ~ 220 Ma, which has been later suggested to date a fluid influx during retrogression (Ayers et al., 2002; Hacker et al., 1998). The age of the metamorphic peak was determined at 230-240 Ma only when ion microprobe zircon dating was used (Ayers et al., 2002; Hacker et al., 1998; Hacker et al., 2000). Studies that applied the Sm-Nd techniques to eclogites reported both ages, but failed to correctly interpret their significance (for a review see Hacker et al., 2000). Ar-Ar dating of hornblende, micas and K-feldspar was partly compromised by excess argon and, similarly to the Sulu terrain (Giorgis et al., 2000), provided an extremely complex picture (for a review see Hacker et al., 2000). These cooling ages apparently suggest a relatively slow cooling and exhumation (only 2 mm/year) when compared to the Dora-Maira or Kokchetav massifs. Despite the large number of isotopic data, the geochronological investigation of the Dabie Shan UHP belt suffers the lack of a systematic attempt to relate ages with metamorphic conditions. The recognition and identification of a few UHP inclusions in zircon is the main ground on which the 230-240 Ma age was attributed to UHP metamorphism (Ayers et al., 2002). Neither zircon nor monazite were ever screened in detail for inclusions, or analysed for trace elements. Similarly, Sm-Nd dating was not coupled with the determination of P-T conditions of the analysed minerals. The large spectrum of Ar-Ar data would have greatly benefited from a more detailed chemical characterisation of the minerals to understand the conditions of formation.

Good results in terms of P–T–t correlation have been reached for the UHP rocks of the Western Gneiss Region, Norway (e.g. Carswell & Cuthbert, 2003). Terry et al. (2000) carried out in situ dating of monazite by ion microprobe and electron microprobe. The above authors combined dating with sample textural analysis, petrography, and chemical mapping of the monazite crystals. By dating monazite in different textural relationships (crystals included in garnet and crystals in the matrix) and comparing their Th content, three monazite generations with different ages were recognised. Despite a certain degree of assumption in attributing age to metamorphic conditions, Terry et al. (2000) concluded that the peak occurred at 407 ± 2 Ma, the end of isothermal decompression at 395 ± 2 Ma, and a later shearing event at 375 ± 3 Ma. Similar to the Dora-Maira, precise ages obtained with the same technique and linked to metamorphism and deformation allowed description of the exhumation path of this unit.

A new UHP terrane has been recently identified in the Rhodope zone of N Greece (Liati & Gebauer, 2001a; Mposkos & Kostopoulos, 2001). In the eastern part of this UHP terrane, U-Pb ion microprobe dating (SHRIMP), assisted by CL imaging, was carried out on zircon domains from garnet-rich rocks (Liati et al., 2002a). The zircon crystals contain Early Cretaceous magmatic cores (117.4 ± 1.9 Ma) and Late Cretaceous (73.5 ± 3.4 Ma) metamorphic domains, that were interpreted as dating the (U)HP metamorphic peak. A retrograde metamorphic stage that corresponded to the intrusion of cross-cutting pegmatites was dated at 62 ± 2 Ma. According to the different tectonic scenarios and the peak pressure considered, Liati et al. (2002a) proposed average minimum exhumation rates of 2.2-2.8 mm/year or 5.2 mm/year. These figures should rather be considered as an approximation, because of the strong T-overprint of the rocks studied, which makes the calculation of precise peak UHP conditions difficult.

Tectonic implications

Dating UHP rocks is not only aimed to establish the absolute age of the rocks. It should also tackle other issues such as the correlation of ages with distinct metamorphic stages along a P–T–t path, the age of nearby units, exhumation-, subduction-, heating- and cooling rates, and possibly the duration of metamorphic peak. Some of these important parameters have been determined in different localities. Their implications for tectonic and geodynamic models are reviewed in the following sections.

Exhumation rates

The most complete information in terms of exhumation rates resulted from recent studies that made an effort to relate ages with metamorphic conditions (Table 1). The best constrained data so far remain the ones obtained by Rubatto & Hermann (2001) for the Dora-Maira rocks. The exhumation of this unit is described by three P–T–t points, all obtained with the same dating technique (in situ ion microprobe U-Pb analysis). In this unit, average exhumation from 120 to 35 km depth has been estimated to be as fast as 3.4 cm/year. Relevant is also the fact that exhumation slowed down (1.6 cm/year) when the UHP rocks reached the base of the crust and were assimilated with other HP nappes. A further decrease of exhumation rates is recorded when the rocks reached mid crustal levels (0.5 cm/year) and exhumation probably changed from tectonic to erosion dominated. In the Kokchetav Massif, although less precisely defined, initial exhumation from depths of more than 140 km appears to be faster than 1.8 cm/year (Rubatto & Hermann, 2001).

Table 1.

Summary of exhumation and cooling rates of selected UHP terranes

UHP terraneAge range (Ma)Depth range (Km)Cooling rates (°C/m.y.)Exhumation rates(cm/year)Data source
Dora Maira35.4–32.9–31.8–29.9120–10 / 750–250 °C~ 603.4–0.5Gebauer et al. (1997)
Lago di44-38 to 3590-301-0.5Rubatto et al. (1998)
CignanaAmato et al. (1999)
Barnicoat et al. (1995)
Rondaca. 20-19340-2003.1Sánchez-Rodríguez & Gebauer (2000)
E’ Rhodope73.5-61.9>75-15>0.52Liati et al. (2002a)
Kokchetav528 ± 3>140-35~ 60>1.8Hermann et al. (2001)
WGR407-401125-60<1.1Terry et al. (2000)
Dabie Shan240- ~220125->0.2Hacker et al. (2000)
UHP terraneAge range (Ma)Depth range (Km)Cooling rates (°C/m.y.)Exhumation rates(cm/year)Data source
Dora Maira35.4–32.9–31.8–29.9120–10 / 750–250 °C~ 603.4–0.5Gebauer et al. (1997)
Lago di44-38 to 3590-301-0.5Rubatto et al. (1998)
CignanaAmato et al. (1999)
Barnicoat et al. (1995)
Rondaca. 20-19340-2003.1Sánchez-Rodríguez & Gebauer (2000)
E’ Rhodope73.5-61.9>75-15>0.52Liati et al. (2002a)
Kokchetav528 ± 3>140-35~ 60>1.8Hermann et al. (2001)
WGR407-401125-60<1.1Terry et al. (2000)
Dabie Shan240- ~220125->0.2Hacker et al. (2000)

WGR: Western Gneiss Region

Variations in exhumation rates have been identified also in the Western Gneiss Region (Terry et al., 2000): initial fast exhumation from depths of 125 km slowed down when the unit reached 35-40 km depth, probably corresponding to the base of the crust. The same tendency is seen at shallow crustal levels where exhumation is slower. Even though in the Western Gneiss Region the absolute values are different and generally lower (1.1 cm/year, ~ 0.4 cm/year and < 0.1 cm/year; Terry et al., 2000) compared to Dora-Maira or Kokchetav they repeat the pattern already described in the Dora-Maira.

A remarkably high exhumation rate of 3.1 cm/year (Sánchez-Rodriguez & Gebauer, 2000) has been suggested for rocks in and around the Ronda peridotite (Southern Spain), a unit that is considered to be the equivalent of the diamond-bearing Beni Bousera peridotite (Marocco). In this area, however, lack of good correlation between ages and metamorphic conditions, as well as extremely high cooling rates (200-340 °C/m.y.) cause considerable uncertainty on the exhumation rate and require some caution on the use of this data. In fact, a more recent study demonstrated that metamorphic zircon in a pelitic granulite adjacent to the Ronda peridotite grew during decompression (Whitehouse & Platt, 2003).

A relatively slow exhumation rate has been proposed for the large Dabie Shan UHP belt, where, however, only a minimum estimate of 0.2 cm/year (Hacker et al., 2000) was obtained because of the large scatter in cooling ages. At Lago di Cignana (Zermatt-Saas Fee ophiolites, Western Alps) exhumation rates are difficult to calculate because of the uncertainty in the age and conditions of retrogression. The best estimate on the age of the metamorphic peak, which corresponds to a depth of ~ 90 km (28-30 kbar; Reinecke, 1991), is 44.1 ± 0.7 Ma (Rubatto et al., 1998). Greenschist facies overprint probably occurred at around 38-35 Ma according to a Rb-Sr whole rock-phengite isochron (38 ± 2 Ma, Amato et al., 1999) and a U-Pb age on retrograde titanite (35 ± 3 Ma, Barnicoat et al., 1995). It results in a rate in the range 0.5-1.5 cm/year, another apparently low value.

In summary, despite the numerous studies reporting exhumation rates, well-documented estimates are only obtained where a precise age is linked to P-Tconditions, a difficult task to achieve. The data available (Table 1) overall indicate fast exhumation rates of UHP rocks (on the order of centimetres per year, equivalent to tens of km per m.y.), at least for the first part of their decompressional history. These rates might be restricted to rock units of limited extent (a maximum of few km in thickness) such as the Dora-Maira. The limited dimensions are probably a crucial feature in justifying such fast movements of rocks in the upper mantle and the lower crust. Such movements of rock units on the order or a few cm/year are only comparable to the speed of plate motion. They are generally faster than average convergence rates (mm to cm/year) and much higher than average erosion rates (0.1 to 5 mm/year). This observation favours the view that the rise of UHP rocks from great depth is not related to surface erosion, but rather to the interplay at depth of tectonic processes such as buoyancy, slab break off, faults and detachment zones (Chemenda et al., 1995; England & Holland, 1979; Ernst et al., 1997; von Blanckenburg & Davies, 1995). Larger units such as the Dabie Shan and the Western Gneiss Region might have reached mid crustal levels at a slower pace.

Duration of UHP metamorphism

We have good indication that UHP rocks rise to upper crustal levels in short time, but how long do they reside at great depth? Establishing the duration of the peak of metamorphism requires good knowledge not only of the time of distinct exhumation stages, but also of those of subduction. Constraining the timing of prograde metamorphism is made difficult by the fact that prograde minerals or structures are rarely preserved in metamorphic rocks. Armoured inclusions in garnet are the most common prograde mineral relics, also reported in UHP rocks (e.g. Chopin et al., 1991; Compagnoni et al., 1995; Hermann et al., 2001; Reinecke, 1991; Schertl et al., 1991). Even assuming that they preserved their original isotopic composition through the peak and later overprint, their dating has never been achieved. Monazite inclusions in garnet appear to generally preserve the age of peak metamorphism (e.g. Foster et al., 2000; Terry et al., 2000). However, zircon from the Dora-Maira included in a large pyrope megablast could so far be successfully dated on the prograde P–T–t path (Gebauer et al., 1997). Nevertheless, as the garnet megablasts formed at UHP conditions (ca. 700 °C and 30 kbar) shortly before the UHP peak, the early prograde path remains still to be dated.

Another avenue attempted so far for dating prograde metamorphism is the determination of ages in minerals robust to HT overprint and isotopic resetting, such as zircon, that formed in prograde veins. This approach has been first used by Rubatto et al. (1999) who dated zircon domains interpreted to have grown during prograde metamorphism in quartz veins within the HP sediments of the Sesia Zone. In the Rhodope massif, a similar study determined the age of zircon contained in a prograde quartz vein concordant with the schistority of HP rocks (Liati & Gebauer, 1999). Even though the main mineral assemblage in the vein reflected rather upper amphibolite facies conditions acquired during the post-peak metamorphic overprint, the hydrothermal zircon domains were found to be ~ 3 Ma older than the domains interpreted as having formed close to the HP peak. On this basis, Liati & Gebauer (1999) proposed subduction rates of at least 1.5 cm/year (for depths between ca. 10 km and ca. 60 km). The prograde age implies that HP metamorphism was short living, a not surprising conclusion given the extreme tectonic condition in which these rocks formed. However, these are only few data and more needs to be done to understand how fast rocks can be subducted and how long they reside at depths in excess of 90 km.

Age variations within an orogen: The case of the Western Alps

The large number of geochronological studies in metamorphic belts where UHP rocks are present allows identification of relative differences in age of metamorphism between the UHP units and the nearby nappes. The Alps are one of the best orogens where this comparison can be made.

As presented above, the Dora-Maira reached peak metamorphism at around 35 Ma (Gebauer et al., 1997; Rubatto & Hermann, 2001). To the west of the Dora-Maira Massif is the Monviso Unit, a slice of ophiolitic material comparable to the Zermatt-Saas ophiolite. The Monviso recorded eclogite facies metamorphism at lower PT conditions: ~ 600 °C and 20-24 kbar (e.g. Messiga et al., 1999; Schwartz et al., 2000). A Tertiary age of metamorphism in the Monviso was first proposed using the Ar-Ar technique (Monié & Philippot, 1989) and then supported by Lu-Hf (49.2 ± 1.2 Ma, Duchêne et al., 1997) and Sm-Nd (60 ± 12 and 62 ± 9 Ma, Cliff et al., 1998). The discrepancy between these ages is probably due to the fact that they rely on mineral-whole rock isochrons and thus assume isotopic equilibrium, a very doubtful assumption (see above). A more recent work by Rubatto & Hermann (2003) applied ion microprobe U-Pb dating to zircon that could be proven of HP origin via mineral inclusions and trace element composition. The above authors constrained an age of 45 ± 1 Ma for the HP stage, 10 Ma older than the nearby Dora-Maira.

The other Alpine unit where UHP conditions have been well documented is found at Lago di Cignana, within the Zermatt-Saas ophiolites, some 150 km to the north of the Dora-Maira (see Compagnoni & Rolfo, 2003 in this volume, for a description of the area). At Lago di Cignana, zircon crystals, some of which contained rutile inclusions, yielded an age of 44.1 ± 0.7 Ma (Rubatto et al., 1998), which was interpreted as dating the metamorphic peak. A multi-mineral Sm-Nd isochron provided an age of 40.6 ± 2.6 Ma (Amato et al., 1999) for an eclogite of the same unit. A Lu-Hf isochron on inclusion-rich garnet-whole rock yielded an age of 49 ± 3 Ma (Lapen et al., 2002). Given the relatively large errors, the presence of inclusions in garnet, and the common problem of disequilibrium and/or open system behaviour these latter ages are not straight forward. they are, however in general agreement with a metamorphic peak at ca. 44 Ma. Also in the case of Zermatt-Saas ophiolites, the nearby units, which preserve HP assemblages, recorded peak metamorphism at different times (Fig. 6).

As shown in Figure 6, in the Western and Central Alps, the age of peak metamorphism in different UHP and HP nappes varies over more than 30 Ma, from the Cretaceous-Tertiary boundary in the Sesia-Lanzo Zone to the Late Eocene-Early Oligocene in the Dora-Maira and Voltri Massifs. Whatever is the tectonic scenario that can explain this variability, it implies that different UHP and HP units were subducted and exhumed at different times. For example, the UHP Zermatt-Saas ophiolite was already exhumed at upper crustal levels when the Dora-Maira was subducted to more than 120 km depth. It follows that, during the Tertiary, exhumation and subduction processes were contemporaneously active within this part of the orogen.

The challenge ahead

In the last years substantial progress has been achieved in dating HP-UHP rocks, particularly in the field of correlation of time with P-T conditions. However, geochronology has only partly achieved the wishful goal of precisely dating distinct stages along UHP metamorphic paths. A number of challenges lie ahead, most of them related to improvement of techniques.

As seen in the examples reported, ion microprobe U-Pb dating of single zircon, monazite and titanite domains is the most reliable and successful method to date UHP metamorphism. A better understanding of the behaviour of these minerals during metamorphism is a requisite in order to interpret better the ages obtained with U-Pb dating. Structural and textural analyses of mineral assemblages, CL and backscattered electron imaging of dated crystals, identification and petrological study of mineralinclusions, and chemical analysis (mapping and trace elements) of zircon, monazite and titanite must become routine techniques. They have the potential to allow the critical link between age and stage of metamorphism - pressure, temperature, presence of fluid and melt, and deformation. Another important step would be the generation of a consistent and reliable set of data on the trace element partitioning between zircon or monazite and key metamorphic minerals such as garnet. This data set, in part already available for zircon-garnet pairs (Rubatto, 2002; Rubatto & Hermann, 2003), would allow determining the coexistence between datable minerals and P-T indicators in a more rigorous way. Thermobarometers involving U-Pb minerals such as the monazite-garnet proposed by Pyle et al. (2001) represent another important step toward T-t correlation.

Because of the relatively large analytical errors of the ion microprobe technique, ages that correspond to different stages of metamorphism may appear as identical (e.g. Hermann et al., 2001; Kröner et al., 2000). In this respect, minimisation of the analytical errors would be a step forward in distinguishing between ages acquired during different metamorphic stages and therefore would constrain better subduction and exhumation rates.

Dating prograde metamorphism remains a challenge, not only for UHP terranes. The approach of dating minerals robust to HT resetting that formed in prograde veins (e.g. Liati & Gebauer, 1999) is certainly valid, but limited by the rare occurrence and the identification of other hydrothermal minerals in such rocks. More promising for the future of dating prograde metamorphism would be inclusions of minerals such zircon and monazite armoured in garnet (e.g. Catlos et al., 2002; Foster et al., 2000; Gebauer et al., 1997; Terry et al., 2000). This is best achieved with beam techniques (ion microprobe, electron microprobe and laser ablation ICP-MS) that allow dating directly in thin section, were textural relationship can be controlled. However, the crucial point is to prove that the dated mineral really formed during prograde metamorphism and did not re-equilibrate at the peak or during later overprint. It is not enough to observe textural relationships in order to establish equilibrium between an inclusion and the host garnet. To achieve this task, a series of data acquired by means of all the methods discussed above with respect to zircon and monazite dating are necessary.

Garnet is definitely the most important mineral in eclogite facies rocks, not only because it is almost always present, but also because it has the capability - under certain conditions - to record and preserve successive metamorphic events. Therefore, garnet dating is an important component for constraining prograde and cooling paths. Improvement in Sm-Nd and particularly Lu-Hf dating of garnet is thus desirable. Leaching and separation techniques need to be improved in order to eliminate or at least minimise the influence of inclusions such as zircon, monazite and apatite (e.g. Amato et al., 1999; Anczkiewicz et al., 2002; Scherer et al., 2000). Additionally, a better knowledge and modelling of the effects of inclusions in garnet, as indicated by Scherer et al. (2000), can assist the data interpretation. The closure temperature, particularly for Lu-Hf, needs to be better constrained and its variations according to chemistry, grain size and cooling history accounted for (e.g. Scherer et al., 2000; Scherer et al., 2001). Ideally, in situ dating of garnet should be achieved in order to fully exploit the potential of this mineral as chronometer. Finally, a constant effort to chemically characterise the grains dated and obtain P-Testimates directly from garnet needs to be made.

Dating deformation is also a crucial task for understanding the late history of metamorphic rocks (see e.g. Müller, 2003 for a review), including UHP rocks. Micas, nearly ubiquitous in eclogite facies assemblages, are suitable for Rb-Sr and Ar-Ar dating, However, the success of these techniques in obtaining cooling ages for UHP terranes has been hampered by disequilibrium and excess argon (e.g. Giorgis et al., 2000; Kelley, 2002; Müller, 2003; Scaillet, 1998). These problems have to be carefully monitored with a combination of thermal ionisation and/or step-wise heating dating, micro-sampling or in situ laser isotope analyses and chemical characterisation of the target. This should be aimed to understand its (in)homogeneity and the conditions of formation (e.g. Villa et al., 2000).

Finally, for any of the dating techniques mentioned above, an effort has to be made to determine the pressure at which the mineral dated formed. In fact, in UHP rocks, which generally have a steep P–T path, determining the age of different pressure stages, and thus variations in depth, is more relevant than thermochronology.

Summary

Geochronology provides important information for unravelling the evolution of UHP terranes. In order to obtain useful time constraints on UHP rocks, the geochronological information must be integrated with other geological parameters such as pressure, temperature, deformation and presence of fluid/melt. Complex mineral assemblages that include multiple generations of minerals, as well as local disequilibrium hamper the use of techniques that rely on mineral separates or bulk rock analysis. Common thermochronometers (e.g. K-Ar, Ar-Ar, Rb-Sr on micas and amphiboles) are often of little use because of relatively high peak temperatures (usually > 600°C) and steep P–T paths recorded by UHP units.

Accurate and precise geochronology of UHP rocks thus requires dating minerals that can be related to metamorphic conditions, that are robust to retrogression, and possibly sensitive to pressure changes. The dating technique (or combination of techniques) has to allow the distinction of different mineral domains, must use minerals that are robust to HT perturbations, and have good time resolution. Recent improvements have been made in sample preparation and isotopic measurements using beam techniques. Such techniques offer the best solution for achieving dating of different metamorphic stages within complex P–T paths.

In key UHP terranes, most of the time constraints on the peak of metamorphism have been obtained by ion microprobe U-Pb dating of zircon, monazite and titanite. Isotopic measurements have been assisted by textural analysis, imaging techniques, study of inclusions and chemical characterisation (trace element spot analysis or mapping) of the dated mineral, in order to determine the P–T conditions at which the mineral formed. To constrain the retrograde and cooling path, U-Pb dating of minerals formed at low temperature (e.g. titanite), Sm-Nd and Lu-Hf isochrons on mineral separates (mainly garnet) and Ar-Ar dating of micas have been the major techniques used.

Reliable exhumation rates of UHP units are still rare because of the problems in relating pressure, and thus depth, with age. Exhumation rates obtained so far in UHP units, such as the Dora-Maira and the Kokchetav massifs, are on the order of centimetres per year, much faster than erosion rates. These values suggest that tectonic processes including buoyancy, slab break-off, faults and detachment zones are responsible for the exhumation of UHP units of limited dimensions. The exhumation of larger units, such as the Dabie Shan and the Western Gneiss Region, might proceed at a lower pace and be related to other tectonic processes.

In the Western Alps, one of the regions with the highest occurrence of UHP and HP rocks, the large number of geochronological data allow a regional perspective on the relative timing of subduction and exhumation. It appears that subduction was diachronous across the area, and that exhumation and subduction were contemporaneously active within different parts of the orogen.

The future of dating UHP metamorphism, and metamorphism in general, lies in the capability of improving metamorphism-time correlation. This requires also improvement of the dating techniques in terms of error minimisation, so that metamorphic stages with apparent identical ages can be resolved. For U-Pb dating, metamorphism-time correlation requires structural and textural analysis of rock assemblages, cathodoluminescence and backscattered electron imaging of dated crystals, characterisation of mineral inclusions, and chemical analysis (mapping and trace elements) of zircon and monazite. Sm-Nd and particularly Lu-Hf dating of garnet will have to focus on elimination of mineral inclusions, quantification of their effect on isotopic ratios, and better estimation of closure temperatures considering also the effect of fluid phases in the loss of radiogenic isotopes. In Ar-Ar and Rb-Sr dating of micas, so far the major source for cooling ages, the detection of excess argon and isotopic disequilibrium needs to be improved.

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Acknowledgements

The work presented in this chapter has benefited from the collaboration with a number of colleagues. The contribution of Roberto Compagnoni, Mark Fanning, Jörg Hermann, Andrei Korsakov and Ian Williams is kindly acknowledged. Stephanie Duchêne is thanked for a constructive review.

Figures & Tables

Fig. 1.

Cathodoluminescence image of zircon crystals from Dora-Maira UHP rocks. (a) Crystal include in a 15 cm large, prograde megablast, in which the distinction between oscillatory-zoned, magmatic zircon core and metamorphic rim developing oscillatory zoning against the outer surface (increasing amounts of fluids/melts) is particularly evident. Ion microprobe analysis (analysed spots represented by the circles) is the only technique that allows dating the different domains separately. (b) Crystal, separated from a fine-grained pyrope quartzite, containing an “inclusion” of coesite partly surrounded by metamorphic zircon. For more details see text.

Fig. 1.

Cathodoluminescence image of zircon crystals from Dora-Maira UHP rocks. (a) Crystal include in a 15 cm large, prograde megablast, in which the distinction between oscillatory-zoned, magmatic zircon core and metamorphic rim developing oscillatory zoning against the outer surface (increasing amounts of fluids/melts) is particularly evident. Ion microprobe analysis (analysed spots represented by the circles) is the only technique that allows dating the different domains separately. (b) Crystal, separated from a fine-grained pyrope quartzite, containing an “inclusion” of coesite partly surrounded by metamorphic zircon. For more details see text.

Fig. 2.

Dora-Maira titanites. (a, b) Backscattered electron (BSE) images of dated titanite crystals from a UHP and a LP calc-silicate, respectively. Four generations of titanite are distinguishable on the base of their BSE emission, which reflects different chemical composition. (c, d) Variation in major and trace element compositions, respectively, between the four generations of titanites. Major element data from Rubatto & Hermann (2001) and unpublished trace element data.

Fig. 2.

Dora-Maira titanites. (a, b) Backscattered electron (BSE) images of dated titanite crystals from a UHP and a LP calc-silicate, respectively. Four generations of titanite are distinguishable on the base of their BSE emission, which reflects different chemical composition. (c, d) Variation in major and trace element compositions, respectively, between the four generations of titanites. Major element data from Rubatto & Hermann (2001) and unpublished trace element data.

Fig. 3.

P–T–t path for the Dora-Maira UHP unit, modified after Rubatto & Hermann (2001). Geochronological data from: 1) Rubatto & Hermann (2001); 2) Gebauer et al. (1997); 3) Duchêne et al. (1997).

Fig. 3.

P–T–t path for the Dora-Maira UHP unit, modified after Rubatto & Hermann (2001). Geochronological data from: 1) Rubatto & Hermann (2001); 2) Gebauer et al. (1997); 3) Duchêne et al. (1997).

Fig. 4.

Photomicrograph (transmitted light) (a) and CL image (b) of different zircon crystals from a Kokchetav UHP gneiss. Inclusions belonging to different P–T stages are present in crystals from the same rock and even within the same crystal, indicating that zircon formed over a range of P-T conditions. Surprisingly, the CL zoning of the domains with different inclusions is quite similar.

Fig. 4.

Photomicrograph (transmitted light) (a) and CL image (b) of different zircon crystals from a Kokchetav UHP gneiss. Inclusions belonging to different P–T stages are present in crystals from the same rock and even within the same crystal, indicating that zircon formed over a range of P-T conditions. Surprisingly, the CL zoning of the domains with different inclusions is quite similar.

Fig. 5.

Trace element patterns of zircon crystals from a Kokchetav UHP biotite gneiss. Note the difference in composition between zircon domains formed in different conditions. Data from Hermann et al. (2001).

Fig. 5.

Trace element patterns of zircon crystals from a Kokchetav UHP biotite gneiss. Note the difference in composition between zircon domains formed in different conditions. Data from Hermann et al. (2001).

Table 1.

Summary of exhumation and cooling rates of selected UHP terranes

UHP terraneAge range (Ma)Depth range (Km)Cooling rates (°C/m.y.)Exhumation rates(cm/year)Data source
Dora Maira35.4–32.9–31.8–29.9120–10 / 750–250 °C~ 603.4–0.5Gebauer et al. (1997)
Lago di44-38 to 3590-301-0.5Rubatto et al. (1998)
CignanaAmato et al. (1999)
Barnicoat et al. (1995)
Rondaca. 20-19340-2003.1Sánchez-Rodríguez & Gebauer (2000)
E’ Rhodope73.5-61.9>75-15>0.52Liati et al. (2002a)
Kokchetav528 ± 3>140-35~ 60>1.8Hermann et al. (2001)
WGR407-401125-60<1.1Terry et al. (2000)
Dabie Shan240- ~220125->0.2Hacker et al. (2000)
UHP terraneAge range (Ma)Depth range (Km)Cooling rates (°C/m.y.)Exhumation rates(cm/year)Data source
Dora Maira35.4–32.9–31.8–29.9120–10 / 750–250 °C~ 603.4–0.5Gebauer et al. (1997)
Lago di44-38 to 3590-301-0.5Rubatto et al. (1998)
CignanaAmato et al. (1999)
Barnicoat et al. (1995)
Rondaca. 20-19340-2003.1Sánchez-Rodríguez & Gebauer (2000)
E’ Rhodope73.5-61.9>75-15>0.52Liati et al. (2002a)
Kokchetav528 ± 3>140-35~ 60>1.8Hermann et al. (2001)
WGR407-401125-60<1.1Terry et al. (2000)
Dabie Shan240- ~220125->0.2Hacker et al. (2000)

WGR: Western Gneiss Region

Contents

GeoRef

References

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