Lu–Hf and Ar–Ar chronometry supports extreme rate of subduction zone metamorphism deduced from geospeedometry

Tectonophysics 342 (2001) 23 – 38 www.elsevier.com/locate/tecto

Lu–Hf and Ar–Ar chronometry supports extreme rate of subduction zone metamorphism deduced from geospeedometry Pascal Philippot a,*, Janne Blichert-Toft b, Alexei Perchuk a,c, Sylvie Costad, Vladimir Gerasimovc a

Laboratoire de Ge´osciences Marines (FRE 1326), Institut de Physique du Globe, Universite´ Paris-Jussieu, Case 89, T26-E3, 4 place Jussieu, 75252 Paris Cedex 05, France b Laboratoire des Sciences de la Terre (UMR 5570), Ecole Normale Supe´rieure de Lyon, 46, Alle´e d’Italie, 69364 Lyon Cedex 7, France c Institute for the Geology of Ore Deposits, Petrology, Mineralogy, and Geochemistry, Russian Academy of Sciences, Staromonetny 35, Moscow, 109017, Russia d Department of Earth Sciences, Monash University, Clayton, Vic 3168, Australia Received 18 July 2000; accepted 3 December 2000

Abstract Recent diffusion modeling of eclogitic garnets from the Great Caucasus, Russia, and Yukon, Canada, have shown that the preservation of garnet growth zoning in rocks that have equilibrated at high temperature (680 – 700
C) is possible only if rates of pressure and temperature change on the burial and/or exhumation paths are in the order of several cm/year and several hundreds of
C/Ma. In order to confirm this observation, we performed Lu – Hf and Sm – Nd dating of garnet and Ar – Ar dating of mica on the same samples that were used for geospeedometry measurements in an earlier study. In both localities, garnet grew during prograde metamorphism at 690 ± 40
C and > 1.5 GPa (Yukon) and 680 ± 40
C and >1.6 GPa (Great Caucasus). In contrast, phengite formed soon after the main eclogitic foliation at 520 ± 50
C (Yukon) and 600 ± 40
C (Great Caucasus). Garnet of the Yukon samples yielded Lu – Hf ages of 252 ± 7, 255 ± 7, 257 ± 6 and 264 ± 6 Ma that fall within error of phengite Ar – Ar integrated ages of 261 ± 2 (laser spot date) and 256 ± 3 Ma (age of mineral separates). No Sm – Nd ages were measured on the Yukon samples. For Great Caucasus samples, all Sm – Nd ages with the exception of one garnet – whole rock pair yielding a Sm – Nd age of 311 ± 22 Ma are poorly constrained. In contrast, the Lu – Hf garnet chronometer yields ages of 322 ± 14, 316 ± 5 and 296 ± 11 Ma that again fall within error of the phengite Ar – Ar mean age of 303 ± 5 Ma. Because the geospeedometry approach provides information on cooling rates, information on the closure temperature of a given isotopic system can be extracted from the analytical solution of Dodson [Contrib. Mineral. Petrol. 40 (1973) 259] using appropriate sets of experimentally determined diffusion data. The results of these calculations indicate that uncertainties of more than 200
C are to be expected for the Sm – Nd and Lu – Hf closure temperatures for both the Great Caucasus (750 ± 150
C) and Yukon samples (710 ± 120
C). In all cases, calculated closure temperatures are equivalent to or in the upper range of peak metamorphic temperatures. With respect to Ar, calculated closure temperatures of 570
C for the Yukon eclogites and 560 – 600
C for the Great Caucasus eclogites are within error of the temperatures of the early stage of cooling and/or exhumation. These results indicate that the eclogitic rocks experienced a minimum cooling and exhumation of about 150
C and 25 km in a time interval smaller than the errors on the ages. The fact that garnet and phengite yield indistinguishable Lu – Hf and Ar – Ar ages is in good

*

Corresponding author. Tel.: +33-1-44-27-71-42; fax: +33-1-44-27-39-11. E-mail address: [email protected] (P. Philippot).

0040-1951/01/$ - see front matter D 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 0 1 ) 0 0 1 5 5 - X

24

P. Philippot et al. / Tectonophysics 342 (2001) 23–38

agreement with the observation deduced from geospeedometry that the time elapsed at eclogitic conditions should be extremely short (of the order of 1 Ma). Considering the exceptional precision of the age information obtained on eclogitic garnet using the Lu – Hf technique and that Lu – Hf, Ar – Ar and geospeedometry approaches were carried out the same samples, these results suggest that the time-scale resolution required for unraveling rates of high-pressure metamorphism remains out of reach of current thermochronological methods. D 2001 Elsevier Science B.V. All rights reserved. Keywords: Lu – Hf; Ar – Ar; Geospeedometry

1. Introduction Among the most important controls on metamorphism is the rate at which tectonic processes operate. With regards to the evolution of high-pressure (HP) and low-temperature (LT) rocks (blueschist and eclogite), direct constraints on the rate of burial arise from the observations of modern subduction zones. Much speculation persists, however, concerning the rates of exhumation of high-pressure rocks during continental collision. Although forward analogue models (Chemenda et al., 1995) and numerical simulations (Burg and Podladchikov, 1999; Burov et al., 2001) of continental collision zones have predicted that the rate of exhumation/cooling can be comparable to that of burial/heating (1 to 15 cm/ year and several hundreds
C/Ma), thermo-mechanical insights inferred from P – T –t reconstruction and structural studies of high-pressure terranes have failed to reproduce the trajectories and the velocity field of mass transport in the crust during the entire orogenic period. Recently, much progress has been made toward the measurement of rates and timing of prograde and early retrograde processes in high-pressure metamorphic rocks. First, efforts have been made to constrain the relative duration of metamorphic events by temperature-time dependent diffusion modeling of highpressure minerals (Perchuk et al., 1996, 1998, 1999; Perchuk and Philippot, 1997, 2000; O’Brien, 1997; Ducheˆne et al., 1998). These diffusion models all converge to the fact that the preservation of zoning in garnets of rocks that have equilibrated at high temperature (650 – 700
C) is only possible if the time spent at these conditions is extremely short (less than 1 Ma). Second, a major advance for the absolute dating of eclogitic garnet has been the development of the Lu –Hf technique (Blichert-Toft et al., 1997,

1999; Ducheˆne et al., 1997). Indeed, Sm – Nd and U –Pb isotope systematics of garnet have relentlessly been plagued by the complication that the inventories of radiogenic isotopes measured in the garnet separates may be located in inclusions in the garnet rather then in the garnet lattice itself. Inclusions of old refractory minerals, such as zircon and light rare earth element rich minerals, such as monazite and apatite, may bias the ages to the extent that they are rendered meaningless in terms of elucidating P –T histories (e.g., DeWolf et al., 1996; Scherer et al., 2000a). In the present paper, we present new results of Lu – Hf and Sm – Nd dating of garnet and Ar – Ar dating of phengite from two different eclogitic rocks of the Great Caucasus, Russia, and Yukon, Canada. These rocks have been the subject of detailed diffusion modeling by Perchuk and collaborators (see below), who showed that the preservation of garnet growth zoning is possible only if rates of pressure and temperature change on the burial and/or exhumation path are on the order of several cm/year and several hundreds of
C/Ma. Thus, the use of thermometric sensors having distinct blocking temperatures allows us to obtain age information for different temperature intervals along the P – T paths of interest. In addition, by adopting a dual garnet dating approach, combining both Lu – Hf and Sm – Nd on the same samples and/or mineral grains, it is possible to evaluate the advantages and limitations of the two methods. The conclusions to be drawn from our Lu – Hf and Ar – Ar data agree well with the extreme rates of high-pressure metamorphism predicted by diffusion modeling of the Great Caucasus and Yukon samples. In contrast, the ages obtained using the Sm – Nd method are not geologically relevant, which we ascribe primarily to the presence of mineral inclusions in the core of the eclogitic garnets that have significantly affected their isotopic signature.

P. Philippot et al. / Tectonophysics 342 (2001) 23–38

2. Previous work 2.1. Geological background, timing constraints and P –T path reconstruction 2.1.1. Great Caucasus The Great Caucasus is characterized by widespread crystalline basement rocks and metasediments of Variscan age that are located in three main pre-Alpine zones (the Bechasyn, the Fore Range and the Main Range zones) separated by subvertical faults (Somin, 1991). The eclogitic rocks analyzed in this study were collected in the Red Cliff area of the Fore Range zone (Perchuk and Philippot, 1997). This zone exhibits an internal stratigraphy of interlayered gneisses, epidoteand garnet-bearing amphibolites, garnet-bearing mica schists with minor massive and banded mafic eclogites and ultramafic rocks. The eclogites are found as slab-like bodies of 10 to 100 m in length and 1 to 10 m in thickness, embedded in garnet mica schists. Both massive and layered eclogites, containing omphacite, garnet, kyanite, phengite, paragonite and epidote, were identified. Some eclogites display a foliation defined by the average flattening plane of omphacite, white mica and kyanite. Locally, quartz-, kyanite- and white mica-bearing veins, a few centimeters wide and several tens of centimeters long, occur at high angles to the foliation plane. The veins consist of an intimate association of mm- to cm-scale kyanite grains and phengite flakes oriented normal to the vein walls. The observation that vein minerals show no evidence of internal strain indicates that the veins formed during or soon after the main eclogitic foliation. Most of the samples studied are fresh eclogites and show no, or only minor, textural and mineralogical evidence for retrogression following their crystallization at high pressure. The age of the eclogite facies metamorphism is unknown. The amphibolite facies episode is dated at 374 ± 30 Ma based on K – Ar mineral age data (amphibole, biotite and white mica; Afanas’ev et al., 1973). Texturally, large garnets (>100 mm in diameter) display inclusion-rich cores composed of quartz, epidote, omphacite, paragonite, amphibole (taramite and glaucophane) and rutile. A characteristic feature of the large garnets is the presence of radially oriented linear zoning forming micro-channels or stringers (see Perchuk and Philippot, 1997, 2000). In addition, garnet

25

exhibits different degrees of prograde growth zoning depending on the size of the grain (100 mm to several mm in diameter). Both concentric and stringer zoning patterns are mainly expressed in terms of Fe– Mg exchange, with 100
Mg/(Mg + Fe) increasing from 18 –20 to 33 – 37 from core to rim. Geothermobarometry calculations indicate temperature – pressure conditions of 680 ± 40
C and a minimum of 1.6 ± 0.2 GPa for the high-pressure equilibration stage and 600 ± 40
C, and 600 ± 40
C and 0.8 ± 0.2 GPa for the retrograde stage (Fig. 1a; Perchuk and Philippot, 1997). 2.1.2. Yukon The Yukon– Tanana terrane is the innermost terrane of the western Canadian Cordillera. It includes both pericratonic rocks of likely North American affinity and allochthonous slices of volcano-sedimentary, oceanic, and granitic material, inferred to have been emplaced onto the North American margin during the Mesozoic collision (Erdmer and Helmstaedt, 1983). In Yukon, blueschist- and eclogitefacies rocks are known in at least six widely separated localities (see Erdmer et al., 1998). Eclogite occurs generally as meter- to several hundred meter-sized lenses within siliceous or mafic and ultramafic rocks. Existing age data for the Yukon eclogites consist of multiple concordant Rb – Sr ages of approximately 246 Ma, K – Ar ages around 260 Ma for white mica, and a concordant U – Pb age of 269 ± 2 Ma (Wanless et al., 1978; Erdmer and Armstrong, 1989; Creaser et al., 1997). Recently, Erdmer et al. (1998) presented a variety of Ar – Ar ages ranging from 228 to 344 Ma for the different eclogite and blueschist rocks of the Yukon – Tanana terrane, which they interpreted as reflecting a multi-episodic and diachronous process associated with either the existence of several subduction zones or continuous convergence and episodic exhumation above a single zone. Samples for the present study were obtained from a single meter-sized outcrop near Faro (Faro lens III of the nomenclature of Erdmer and Helmstaedt, 1983; see also Perchuk et al., 1999 for a description). This eclogite displays a porphyroblastic texture characterized by euhedral garnet grains 1 to 2 mm across homogeneously distributed within a fine-grained matrix of elongated omphacite lying parallel to the foliation plane, with minor quartz and rutile. Garnet

26

P. Philippot et al. / Tectonophysics 342 (2001) 23–38

Fig. 1. Pressure – temperature diagram showing P – T trajectories of the Yukon and Great Caucasus eclogites (after Perchuk, 1993; Perchuk et al., 1999). Shown in inset are modeled compositional profiles illustrating the progressive homogenization of a garnet inclusion – garnet host interface (Yukon sample) and a garnet concentric zoning and a stringer in garnet from the Great Caucasus (after Perchuk and Philippot, 2000). Initial growth zoning profile at t = 0 was assumed as a perfect step (garnet inclusion), a unmodified concentric zoning displayed by 1 mm diameter large garnet (concentric zoning) and a semi-infinite slab geometry (stringer). Doted lines and thick continuous lines labeled 0.1, 0.2 and 0.5 Ma represent best fit to true compositional data. Thin dashed lines represent successive relaxation time at different times. See text for discussion.

contains inclusions of omphacite, clinozoisite, quartz, rutile, and titanite. A remarkable feature of the garnet cores is the presence of tiny (20 – 50 mm in diameter) garnet inclusions of distinctly different composition than the host garnet. The latter shows prograde chemical zoning with 100
Mg/(Mg + Fe) increasing from 11 to 42 from core to rim. In contrast, the garnet inclusions are markedly enriched in Ca and depleted in Fe and to a lesser extent, in Mg and Mn as well. Perchuck et al. (1999) proposed that the inclusions represent relicts of an earlier metamorphic event predating the eclogitic episode. The eclogite shows only local textural and mineralogical evidence of retrogression following peak P – T equilibration. The main alteration feature is the replacement of the matrix of omphacite and quartz by local segregation of clinozoisite ± paragonite, phengite, and amphibole. The segregation occurs either as thin layers oriented parallel to the foliation plane, or as randomly distributed ‘‘patches’’ overprinting the rock fabric. Garnet is surrounded by

haloes of phengite and minor chlorite. All hydrous phases appear to have formed at the expense of an earlier anhydrous eclogite assemblage; they show no evidence of internal strain or preferred orientation and postdate the eclogite-facies foliation. This observation indicates that late-stage re-equilibration of the eclogite was controlled by a water-rich fluid. It is notable that no plagioclase-clinopyroxene symplectites at the expense of omphacite – quartz occur in partially altered domains. Garnet concentric zoning and inclusion patterns (omphacite) support a prograde evolution from 520 ± 50
C and 1.1 ± 0.2 GPa to 690 ± 40
C and 1.5 ± 0.2 GPa. In the partially altered domains, the presence of blueschist hydrous minerals (glaucophane and phengite) and the absence of plagioclase-clinopyroxene symplectites after omphacite – quartz indicate that high pressures prevailed during cooling to at least 540
C (Fig. 1). Phengite in the Faro eclogite yielded a consistent K –Ar age of 258 Ma (Wanless et al., 1978), an Ar – Ar plateau age of 260 ± 3 Ma with an integrated age of

P. Philippot et al. / Tectonophysics 342 (2001) 23–38

256 ± 3 Ma, and an integrated laser spot age of 261 ± 2 Ma (Erdmer et al., 1998). 2.2. Results of diffusion modeling The monocrystalline geospeedometry approach developed by Perchuk and collaborators (see Perchuk and Philippot, 2000 and references therein) is based on two models. These are the model of diffusion in a sphere of Crank (1975) and Jiang and Lasaga (1990), and the model of diffusion along an infinite layer of known thickness and uniform initial concentration (Carslaw and Jaeger, 1986). It allows the evaluation of the time scale of metamorphic events using the extent of relaxation of garnet growth zoning along a particular P – T trajectory. The approach is critically dependent on two main assumptions, namely the initial garnet composition and closed-system behavior. Several strategies have been developed to minimize the first source of uncertainty (see Jiang and Lasaga, 1990 for an example). For the Great Caucasus, considering that diffusion rates are primarily dependent on the size of the grains, the recognition that garnet of different sizes display similar compositional gradients led Perchuk and Philippot (1997) to suggest that the larger grains were not affected by diffusional processes and hence could be used as initial profiles. In Yukon, owing to the distinctly different compositions of the garnet inclusions and garnet porphyroblasts, Perchuk et al. (1999) regarded the original contact between the two garnet populations as a stair-like geometry (see also Ganguly et al., 1996 and Perchuk et al., 1998 for similar interpretation of garnet core/ mantle interfaces). With regards to the second source of uncertainties, the use of stringers (Great Caucasus) and of garnet inclusions (Yukon) rules out the possibility of cation exchange with surrounding matrix minerals, thus satisfying the boundary condition for closed-system behavior. A final source of uncertainty inherent to all geospeedometric approaches is related to the diffusion coefficient data. A detailed discussion of this source of uncertainty can be found in Perchuk and Philippot (1997, 2000) and Perchuk et al. (1999) and will not be repeated here. 2.2.1. Great Caucasus The relaxation of garnet zoning profiles was modeled using a polynomial function (concentric zoning)

27

and a semi-infinite slab geometry (stringers) for a variety of model P – T – t trajectories (Perchuk and Philippot, 1997, 2000). Among the model P – T – t paths, a regular P – T path consisting of decompression from 1.6 to 0.8 GPa at regularly decreasing temperature from 680 to 600
C indicates that the garnet zoning patterns can be preserved if duration of the post-growth history is less than 0.5 Ma using concentric zoning profiles and as low as 0.1 Ma using stringers. Considering a duration of 0.5 Ma, a pressure change of 0.8 GPa and a temperature change of 80
C yield minimum rates of exhumation and cooling rates on the order of 6 cm/year and 170
C/Ma. Using a time scale of 0.1 Ma yields exhumation and cooling rates of 28 cm/year and 800
C/Ma. 2.2.2. Yukon The garnet inclusion – garnet host interface was modeled as a ‘‘stair-like’’ initial distribution. Results of the calculation indicate that the grossular profile along the garnet interface can be preserved for a metamorphic history spanning about 0.2 Ma. Although the relative duration of the prograde and retrograde P – T segments shown in Fig. 1 is unknown, it is possible to determine minimum values of burial-heating and cooling rates above which the garnet-interface chemical profile cannot be preserved. Assuming that the Faro eclogite experienced a continuous P –T evolution, and considering a duration of 0.2 Ma, a P –T change of 0.4 GPa and 180
C during the prograde metamorphic event yields minimum rates of burial and heating of 7 cm/year and 950
C/Ma, respectively. Conversely, a temperature change of 150
C and a duration of 0.2 Ma for the isobaric cooling event results in a minimum cooling rate of 750
C/Ma.

3. Samples With the exception of the Ar – Ar dating of phengite from the Great Caucasus sample, all the mineral ages presented in this study were carried out on the same samples that we used in previous P – T and diffusion modeling studies. The Great Caucasus sample used for Ar – Ar dating is a kyanite-bearing vein, which was collected a couple of meters away from the samples for which Lu – Hf, Sm –Nd and petrological data are available. The vein consists of intimately

28

P. Philippot et al. / Tectonophysics 342 (2001) 23–38

intergrown kyanite and phengite fibers one to several millimeters in length and oriented normal to the vein wall. 3.1. Great Caucasus All the samples analyzed are fresh eclogites showing no textural or mineralogical evidence for retrogression following high-pressure metamorphic conditions. Garnet and whole rock were analyzed for two eclogitic samples (GC53 and GC57/6) using the Lu – Hf isotope technique. For sample GC53, two populations of garnet of different sizes (5 and 9 mm) were separated and analyzed for their rim composition. To cut the garnet rims, we used a low-speed diamond saw with a 150-mm-thick blade. For sample GC57/6, garnets were analyzed as a whole with no distinction of grain size. Garnet, omphacite and whole rock samples from three eclogitic samples (GC57/6, GC60/5 and GC61/ 2) were analyzed for their Sm – Nd isotopic compositions. No rim – core or size distinctions were made for any of the garnet separates used in the Sm –Nd study. One- to two-millimeter-large phengite flakes collected from a kyanite – phengite – quartz vein were used for single grain Ar –Ar analysis (sample GCB67). We chose to analyse phengite from veins rather than from the matrix to avoid any problems of excess radiogenic argon common in polymetamorphic rocks (see Scaillet, 1998 for a review; also see below). Because phengite –kyanite-bearing veins were formed during or soon after high-pressure metamorphism, vein phengite is most likely to retain a minimum age (i.e., youngest) for the temperature of eclogite metamorphism, and therefore should provide valuable information for constraining the P – T exhumation history of the rock. 3.2. Yukon Garnet, omphacite and whole rock from two samples of the Faro III eclogite were analyzed for their Lu – Hf isotopic compositions. The analyzed garnet and omphacite separates were about 2 mm in size. For Ar – Ar ages, we used the data of Erdmer et al. (1998), which were obtained on the same samples as those used previously for diffusion modeling (Perchuk et al., 1999) and in this study for Lu –Hf geochronology.

We point out that all hydrous phases in the Faro III eclogite appear to have formed during a late-stage reequilibration event under blueschist-facies conditions, thus minimizing, although not completely avoiding, the ‘‘excess argon’’ problem. No Sm – Nd dating were performed on these rocks. Further details about geological setting and field relations, as well as petrological characteristics and mineral chemistry of the Great Caucasus and Yukon eclogites can be found in the papers by Erdmer and Helmstaedt (1983), Perchuk (1993), Perchuk and Philippot (1997), Erdmer et al. (1998) and Perchuk et al. (1999).

4. Analytical procedures 4.1. Lu – Hf Lu – Hf isotope analyses were carried out by multiple-collector magnetic-sector inductively coupledplasma mass spectrometry using the VG Plasma 54 instrument in Lyon (France) following sample dissolution in steel-jacketed Teflon bombs and one- and two-stage column separations, respectively, for Lu and Hf using a mixed 176Lu – 180Hf spike (BlichertToft et al., 1997). The only modification relative to the procedure described in Blichert-Toft et al. (1997) consisted in reducing the abundant Cr in garnets and omphacites from Cr6 + (oxidized from natural Cr3 + by fuming with HClO4 during sample digestion) to Cr3 + using H2O2 in an acid environment before the first-stage Hf anion-exchange column. This prevents erratic loss of Hf on this column caused by the strong complexation of Hf with bichromate combined with the partial destruction of the resinous exchangers by chromic acid. Prior to dissolution, the mineral separates were leached with 10% HF and 6 N HCl. Total procedural Hf and Lu blanks were less than 25 and 20 pg, respectively, or at least a factor 1000 lower than the processed Hf and Lu for the smallest of the samples. The JMC-475 Hf standard yielded a value of 0.28216 ± 1 for 176Hf/177Hf and was analyzed every two to three samples. The Lu –Hf isotope data are presented in Table 1. Contrary to the Sm –Nd system, there is currently no consensus on which decay constant to use for 176 Lu. We chose that of Sguigna et al. (1982)

P. Philippot et al. / Tectonophysics 342 (2001) 23–38

29

Table 1 Lu – Hf isotope data for eclogites from Great Caucasus, Russia, and Yukon, Canada Samplea

Lu (ppm)b

Hf (ppm)b

176

176

Great Caucasus GC53 wr GC53 gt (5mm) GC53 gt (9 mm) GC57/6 wr GC57/6 gt

0.488 1.92 2.55 0.102 0.179

1.45 0.733 1.15 0.383 0.173

0.04763 0.3710 0.3132 0.03793 0.1470

0.283271 ± 3 0.285122 ± 63 0.284895 ± 4 0.283160 ± 7 0.283840 ± 26

Yukon Faro IIIa Faro IIIa Faro IIIa Faro IIIc Faro IIIc Faro IIIc

0.365 2.12 0.0889 0.448 2.03 0.0776

3.11 2.63 3.34 3.07 2.20 1.09

0.01666 0.1146 0.00377 0.02067 0.1306 0.01012

0.283054 ± 3 0.283520 ± 5 0.282974 ± 4 0.283093 ± 4 0.283615 ± 4 0.283000 ± 6

wr gt omp wr gt omp

Lu/177Hf b

Hf/177Hf c

Age T (Ma)d

Age based on

296 ± 11 316 ± 5

wr – gt (5 mm) wr – gt (9 mm)

322 ± 14

wr – gt

252 ± 7 255 ± 7

wr – gt – omp gt – omp

257 ± 6 264 ± 6

wr – gt – omp gt – omp

eHf (T )e + 14.5 + 14.7 + 14.5 + 12.7 + 12.7

+ 12.8 + 12.5 + 12.3 + 13.6 + 12.7 + 12.2

a

wr = Whole-rock; gt = garnet; omp = omphacite. 2s errors: Lu concentrations and 176Lu/177Hf < 1.0%. Hf concentrations < 0.5%. c Uncertainties reported on Hf measured isotope ratios are 2s/ n analytical errors in last decimal place, where n is number of measured isotopic ratios. Normalized for mass fractionation to 179Hf/177Hf = 0.7325. 176Hf/177Hf of JMC-475 Hf standard = 0.28216 ± 1. d Errors on the ages were estimated assigning 1% error to 176Lu/177Hf and ± 0.00001 (the external reproducibility) to 176Hf/177Hf except for the two samples (GC53 gt (5 mm) and GC57/6 gt) for which the internal error exceeds 0.00001 in which case the internal error was used. e eHf values were calculated at age T using 176Hf/177HfCHUR(0) = 0.282772 and 176Lu/177HfCHUR(0) = 0.0332. b



(1.93
10  11 year  1) for this study because it agrees most closely with that of Tatsumoto et al. (1981) (1.94
10  11 year  1), which is the decay constant that so far has been most commonly used in Hf isotope studies. It does, however, have a higher reported uncertainty than that of Sguigna et al. (1982) (4% vs. 1.5%), which was our reason for using the latter. Recent results from both direct-counting measurements (Dalmasso et al., 1992; Nir-el and Lavi, 1998) and calibrations of the 176Lu decay constant against those of 235U and 238U in concordant geological samples (Scherer et al., 2000b) suggest that this value is about 3– 4% too high. If these latest results are confirmed, the Lu – Hf ages presented here will therefore be older than reported by about 3 – 4%. However, since this is largely within the quoted uncertainty of these ages, it does not change our general conclusion of concordance between Lu – Hf and Ar – Ar ages. 4.2. Sm –Nd Mineral separates of garnet and omphacite were used in combination with host whole rocks to establish two- or multiple point isochrons. The mineral

separates and whole-rock powders were prepared at IGEM in Moscow (Russia) and the Sm – Nd isotopic analyses were completed at the Max-Planck Institute for Chemistry in Mainz (Germany). Mineral grains were concentrated using heavy liquids and a magnetic separator, and subsequently handpicked under a binocular microscope to a high degree of purity. Coarse grain size fractions were crushed in an agate mortar under alcohol. Prior to dissolution, the omphacite grains were repeatedly leached in 6 N HNO3 and 6 N HCl, washed in an ultrasonic bath and rinsed in distilled water. The garnet grains were treated in a similar way, except that 2.5 N HCl was used instead of 6 N HCl. After spiking with a mixed 149Sm – 150Nd tracer, the omphacite and garnet separates were digested in screw-top Teflon1 beakers on a hot plate, and the whole rocks in Teflon1 bombs in an oven at 200
C, using a mixture of concentrated HF, HNO3 and HClO4. Sm and Nd were separated using a chemical procedure similar to that described by White and Patchett (1984). Total procedural blanks were less than 60 pg for both Sm and Nd. Isotopic measurements were performed on a MAT 261 mass spectrometer, equipped with multiple collectors operating in static mode. 143Nd/144Nd ratios were normalized to

30

P. Philippot et al. / Tectonophysics 342 (2001) 23–38

146

Nd/144Nd = 0.7219. In the course of our analyses, repeated measurements of the La Jolla Nd standard provided a mean 143Nd/144Nd ratio of 0.511836 ± 15 (2 m; n = 20). For isochron calculations, the uncertainty for the 147Sm/144Nd ratios was estimated to a precision of 0.3%, and a minimum uncertainty of 0.003% was assumed for the 143Nd/144Nd ratios, based on the reproducibility of the La Jolla standard.



corrected by on-line measurements of air. 40Ar/39Ar ages were calculated incorporating the decay constants given in Steiger and Jager (1977) and an initial trapped 40 Ar/36Ar value of 295.5.

4.3. Ar– Ar The 40Ar/39Ar isotopic dating were carried out using the laser mass-spectrometry facility established at IGEM, Moscow. As preparations we used special transparent rock thin sections (6
8
0.04 mm) with assembled quartz glass and the built-in monitors of neutron flow. Such preparations contain small amount of substance and have low activity after irradiation. They are convenient for phase diagnosis in polarised light, which allows controlling the homogeneity of Kbearing minerals, their crystalline orientation as well as possible diffusion loss of argon from core to rim of the grains. Sandwich thin sections were cleaned in methanol before being wrapped in aluminium foil for irradiation. The samples were irradiated with 2.1018 neutrons/cm2 at the wet canal of the MEFI reactor (IRT) in Moscow and monitored by the Black Salma muscovite standard 1790 ± 20 Ma (Starik, 1961; Shanin et al., 1983). The sample was loaded into the ultrahigh-vacuum laser port and heated to 200
C to reduce atmospheric blank levels. Argon was extracted from small areas of the phengite grains partially melted by YAG pulse infrared (1.064 mm) laser. The beam was directed into the optic system of a LMA-1 microscope and focused at the sample surface via the objective lens. Gases released by the laser were gathered for 2 min, purified by exposure to activated Ti getters for 3 min and noble gases then were sorbed for 3 min on a cold carbon ‘‘finger’’ cooled with nitrogen. After 3 min heating, the argon was transferred into a MI1201 IG noble gas mass spectrometer and analyzed statically on a faraday collector. Peaks between 35 and 41 were scanned eight times and amounts extrapolated back to the inlet time. Blanks were analyzed before and after every sample extraction. Results were corrected for blanks, 39Ar decay and mass discrimination. The calculated J value of the neutron flux for the sample was 0.005539. Mass discrimination was periodically

Fig. 2. Lu/Hf isochron diagrams for Great Caucasus (a) and Yukon (b, c) eclogite samples. Analysis of whole rock (wr), garnet (gt) and omphacite (omp) are shown.

P. Philippot et al. / Tectonophysics 342 (2001) 23–38

31

2b,c). Since the two ages obtained for each eclogite are identical within error, including the whole rock in the age calculation does not affect the result. Whole rocks may be more prone to grain boundary contamination by exotic phases than pure mineral separates. The MSWD for the Faro IIIa and Faro IIIc garnet – whole rock – omphacite Lu – Hf isochrons are 0.7 and 1.7 (Fig. 2b,c), thus indicating that the isochron ages are statistically significant. The initial eHf values of the Yukon eclogites vary from + 12.2 to + 13.6 (Table 1) which, as for the Great Caucasus eclogites, falls within the range of ocean island basalts.

5. Results 5.1. Lu – Hf The Great Caucasus eclogites give Lu – Hf garnet – whole rock ages of 296 ± 11 Ma (for 5 mm sized garnets), 316 ± 5 Ma (for 9 mm sized garnets) and 322 ± 14 Ma (Table 1 and Fig. 2a). The difference in ages of the two differently sized garnet fractions is consistent with the result of Scherer et al. (2000a) that the closure temperature of Lu –Hf in garnet correlates positively with the garnet grain size so that larger garnets yield older ages. The initial eHf values of the Great Caucasus eclogites vary from + 12.7 to + 14.7 (Table 1), which is within the range of ocean island basalts. The Yukon eclogites yield garnet –whole rock – omphacite and garnet – omphacite ages of 252 ± 7 and 255 ± 7 for the Faro IIIa eclogite and 257 ± 6 and 264 ± 6 Ma for the Faro IIIc eclogite (Table 1 and Fig.

5.2. Sm – Nd Sm and Nd concentrations and isotopic compositions of garnet, omphacite and whole rock of the three eclogite samples analyzed in this study are given in Table 2 and shown graphically in Fig. 3. Because of the small differences recorded between the Sm and Nd

Table 2 Sm/Nd analytical results and age information for Great Caucasus eclogite samples 147

143

eNd (T )

Calculated ages (Ma)

2.31 2.17 2.26 2.59 1.93 1.87 4.37 4.40 0.28

0.2150 0.2146 0.2143 0.2103 0.2102 0.2116 0.1798 0.1795 0.2269

0.512985 ± 22 0.512980 ± 16 0.513013 ± 55 0.512942 ± 39 0.512974 ± 30 0.512992 ± 14 0.512822 ± 17 0.512828 ± 63 0.513149 ± 39

+ 6.0 + 6.0 + 6.6

omp – grt: 1716 ± 184 Ma (mswd = 1.7)

3.78 4.76 4.85 0.56

11.07 16.00 16.32 1.10

0.2065 0.1799 0.1797 0.3103

0.512911 ± 37 0.512920 ± 15 0.512922 ± 20 0.513144 ± 72

+ 4.9

wr – grt: 338 ± 31 Ma (mswd = 1.4) omp – grt: 261 ± 22 Ma (mswd = 0.1)

3.38 3.35 3.23 2.85 0.27

10.95 10.81 10.57 9.32 0.66

0.1869 0.1873 0.1846 0.1848 0.2460

0.512915 ± 8 0.512887 ± 9 0.512955 ± 33 0.512941 ± 7 0.513022 ± 62

+ 5.8

wr – grt: 313 ± 48 Ma (mswd = 1.8) omp – grt: 184 ± 46 Ma (mswd = 0.5)

Sample

Sm (ppm)

GC 57/6 wr-1 wr-2 wr-3 omp-1 (ul) omp-2 (l) omp-3 (l) grt-1 (ul) grt-2 (ul) grt-3 (l)

0.82 0.77 0.80 0.90 0.67 0.66 1.30 1.30 0.10

GC 60/5 wr omp-1 (l) omp-2 (l) grt (l) GC 61/2 wr-1 wr-2 omp-1 (l) omp-2 (l) grt (l)

Nd (ppm)

Sm/144Nd

Nd/144Nda

eNd values calculated at T = 310 Ma. wr = Whole rock; omp = omphacite; grt = garnet; ul = unleached; l = leached. a Normalized to 146Nd/144Nd = 0.7219; errors are 2 Sm.

32

P. Philippot et al. / Tectonophysics 342 (2001) 23–38

data points and the leached garnet fraction (difference in ratio of only 0.01) gives rise to a poorly constrained age of 1716 ± 184 Ma, to which little geological significance can be given. The other two samples, GC 60/5 and GC 61/2, both have 143Nd/144Nd ratios of omphacite that are higher than those of the whole rock, despite the lower 147 Sm/144Nd ratios in omphacite (Fig. 3b). As a consequence, only two-point isochrons, using wr – grt or omp – grt, have been calculated for these samples. The wr – grt pairs give ages of 338 –313 Ma, while the omp –grt pairs provide younger and more scattered values of 261 – 184 Ma (Table 2). When the data of both samples are considered together, a wr – grt isochron age of 311 ± 22 Ma (MSWD = 1.6) and an omp – grt date of 245 ± 19 Ma (MSWD = 0.8) are obtained (Fig. 3b). Because of the anomalous isotopic characteristics of the omphacite separates, the wr –grt ages are preferred here to that provided by the omp –grt pairs. The wr –grt ages obtained are very similar to those revealed by the Lu– Hf and Ar – Ar methods and could be of geological significance. 5.3. Ar – Ar Fig. 3. Sm/Nd isochron diagrams for Great Caucasus eclogite samples (a) GC 57/6 (circle), and (b) GC 60/5 (square) and GC 61/2 (triangle), with analysis of whole rock (wr) samples, and garnet (grt) and omphacite (omp) separates. Other symbols used: l = leached; ul = unleached; Ndi = initial 143Nd/144Nd ratio of isochron.

isotopic ratios of garnet and that of omphacite or whole rock, the Sm – Nd isochron data of all three Great Caucasus samples are very poorly constrained. Only the isotopic data of leached garnet, omphacite and the whole rock of sample GC 57/6 define an isochron, with all the data points falling on the isochron line within error bars (Fig. 3a). Unfortunately, the very small range in 147Sm/143Nd values recorded between the pool of whole rock –omphacite Table 3 Phengite

The 40Ar/39Ar isotopic data from the phengites analyzed in this study are given in Table 3. Laser spot ages on two phengite grains 2 mm in length from sample GC-B67 show no correlation of age with location and yielded an integrated age of 309 ± 5 and 297 ± 3. Considering these data together yield a weighted mean value of 303 ± 5 Ma.

6. Discussion Garnets typically are characterized by high 147Sm/ Nd ratios (usually well in excess of 0.6), while

144

40

Ar/39Ar isotopic data from a kyanite-bearing vein of the Red Cliff eclogite

Sample

Run

36

37

GC-B67

16 17

0.096 0.111

 0.030 0.127  0.041 0.152 Weighted mean

Ar (mV)

Ar (mV)

J = 0.005539, D = 1.0069 (correction for

39

38

Ar (mV)

Ar decay).

39

Ar (mV)

3.162 4.815

40

%40Ar (atm)

40

Age (Ma)

129.689 183.402

7.2 7.6

33.86 ± 0.40 32.29 ± 0.26 33.1 ± 0.80

309 ± 5 297 ± 3 303 ± 5

Ar (mV)

Ar/39Ar, corrected

P. Philippot et al. / Tectonophysics 342 (2001) 23–38

minerals such as omphacite, paragonite and amphibole have much lower ratios (usually < 0.2), thus allowing high-precision isochron ages to be determined for eclogitic rocks using the Sm/Nd dating technique. In the present case, however, the presence of numerous inclusions of the latter minerals in the cores of some of the garnets analyzed may explain their abnormally low isotopic ratios, generally well below 0.4 (Table 2). One of the primary advantages of the Lu – Hf isotope system over the Sm –Nd isotope system is its comparatively much larger parent – daughter fractionation for garnet, yielding more extreme parent – daughter ratios that help to provide more precise ages due to the larger spread of Lu/Hf ratios. In addition, not only do the high Lu/Hf ratios of garnet lead to more significant radiogenic ingrowth of 176Hf over time, but the decay constant of 176Lu is also about three times larger than that of 147Sm making radiogenic ingrowth of 176Hf three times faster than that of 143 Nd and thereby the Lu –Hf chronometer relatively more sensitive, especially for resolving young ages. Moreover, due to the high Sm and Nd and low Lu and Hf contents in monazite and apatite, inclusions of these accessory phases in garnet have little or no measurable effect on the Lu – Hf ages of garnet, whereas they may severely affect the Sm – Nd systematics of garnet (Scherer et al., 2000a). By contrast, inclusions of Hf-rich zircon in garnets may severely affect their Lu – Hf systematics (Scherer et al., 2000a). However, the absence of zircons in mafic eclogites such as those analyzed in the present study makes this problem of no concern here. With regards to the Sm – Nd results of sample GC 57/6, for which both leached and unleached omphacite and garnet separates were analyzed, the leaching procedure decreased the Sm and Nd concentrations and significantly increased the isotopic compositions of garnet, but did not noticeably affect omphacite (Fig. 3a). The isotopic composition of garnet is still abnormally low after leaching, which indicates that this procedure failed to reveal the true isotopic composition of garnet. Leaching of the garnet grains probably removed surface or rim contamination, but was not efficient enough to dissolve the mineral inclusions present in the cores. It is disturbing to notice that the unleached garnet fractions have higher Sm and Nd concentrations and significantly lower isotopic com-

33

positions than the whole rock that contained them. This implies the presence in the garnet cores of an additional mineral with such Sm – Nd isotopic characteristics. A large amount of epidote inclusions relative to omphacite, paragonite or amphibole, could explain the anomalous Sm and Nd concentrations and Nd isotopic ratios observed in the unleached garnets, since epidote is characterized by high Sm and Nd concentrations and unradiogenic Nd isotopic compositions (e.g., Amatto et al., 1999). The results obtained for Lu – Hf and Ar – Ar are exceptional in terms of the concordance between the two systems. The agreement for the Yukon samples is particularly noteworthy, with mean values of 257 ± 7 and 258 ± 3 Ma for the Lu – Hf and Ar – Ar ages, respectively. The critical interpretation of this data requires a knowledge of the effective closure temperature of each system. Temperature – time data derived from different mineral pairs having different closure temperatures allow, together with pressure and temperature calibrations of reconstructed P –T paths, inferences to be made on exhumation rates of HP and UHP rocks (e.g., Zeck et al., 1992; Christensen et al., 1994; Gebauer et al., 1997; Ducheˆne et al., 1997; de Sigoyer et al., 2000). As discussed by Scaillet (1998) and Villa (1998), however, significant deviations from preconceived sequence of mineral closure temperatures are to be expected owing to dependence of the closure of isotopic chronometers to phenomena like deformation processes and fluid infiltration. For example, there has been much debate about the closure temperature for Sm and Nd diffusion in garnet. Some authors suggested that garnet has high closure temperatures for Sm and Nd diffusion of up to 800
C (Jagoutz, 1988; Hensen and Zhou, 1995) which makes the Sm – Nd chronometer very suitable for dating high-temperature metamorphic events. Others, however, have argued for lower closure at  600
C under slow cooling conditions (Mezger et al., 1992). Although little is known about the diffusion of Hf in garnet, comparison with the closure temperature of major cations (Fe – Mg) suggests that the closure temperature of the Lu – Hf system is not very different from that of the Sm – Nd system (Ducheˆne et al., 1997). Recently, Scherer et al. (2000a) measured Lu –Hf, Sm –Nd, and U – Pb isotope compositions for a number of petrologically

34

P. Philippot et al. / Tectonophysics 342 (2001) 23–38

diverse garnet-bearing samples of a wide range of ages for which the general geochronology and thermal history had been previously established. They proposed that the closure temperature for Lu – Hf in garnet is greater than that for Sm – Nd by up to 100
C, the actual values for Lu – Hf in garnet depending principally on the garnet grain size. The situation is most problematic for Ar in HP minerals such as phengite for which a wide range of closure temperatures is to be expected due to the strong dependence of Ar isotopes

on fluid and mica compositions, local fluid – rock exchange, recrystallization processes and pressure (Harrison et al., 1985; Scaillet et al., 1992; Hodges et al., 1994; Dahl, 1996; Scaillet, 1998). Accordingly, the absence of reliable calibrations of closure temperatures imposes great restrictions upon the common held usage of interpreting temperature –pressure – time data in terms of exhumation and cooling rates. Given that the geospeedometry approach presented above provides independent information on cooling rates, information on closure temperature can be extracted from the analytical solution of Dodson (1973) using appropriate sets of experimentally determined diffusion data. In doing so, we recognize that isotopic behavior in minerals can be complex, implying that the considerations presented below should be regarded with caution. Nevertheless, all the geospeedometry results and geochronological data presented here were extracted from the same samples implying that the thermal aspect of each metamorphic event is intimately linked to the deformation and fluid flow history experienced by the rocks. In the mathematical formalism by Dodson (1973), transport of isotopes by diffusion during cooling is described by the closure temperature (Tc) of the mineral, the cooling rate (s), the activation energy (Ea) and the length scale for diffusion (a): Tc ¼

Ea =R ; ln½AD0 RTc2 =a2 sEa 

where R is the gas constant and A is a geometric factor (for the one-dimensional case, A = 8.66 and a

Fig. 4. Isotope closure temperatures vs. mineral size diagram calculated for different cooling rates using Dodson (1973) formalism. (a) Sm – Nd (and Lu – Hf) closure temperature of garnet calculated for two cooling rates (100 and 1000
C/Ma) and using the Sm diffusion coefficient in grossular (solid line) and in pyrope (dashed line) from Harrison and Wood (1980). Assumed cooling rates are 750
C/Ma for Yukon eclogites and 170 to 800
C/Ma for the Great Caucasus. Lu diffusion data was considered same as Sm. Hatched area represents the peak temperature of equilibration for both localities. Black circle = Yukon garnet; empty circle = Great Caucasus garnet. (b) Ar closure temperature of phengite calculated for cooling rates of 1, 10, 100 and 1000
C/ Ma and using the Ar diffusion data of Kirschner et al. (1996). Dashed lines show the temperature of late stage equilibration for each locality. Black circle = Yukon phengite; empty circle = Great Caucasus phengite.

P. Philippot et al. / Tectonophysics 342 (2001) 23–38

is a half width; for a cylinder (phengite), A = 27 and a is the radius; for a sphere (garnet), A = 55 and a is the radius). Extrapolating the high-temperature Sm and Nd diffusivities of Harrison and Wood (1980) and Coghlan (1990) to the temperature region of interest, it is possible to quantify the dependence of Sm blocking temperature using cooling rates deduced from geospeedometry and measured garnet grain size (Fig. 4a; Perchuk and Philippot, 2000). Fig. 4a shows that uncertainty on Sm closure temperature of more than 200
C is to be expected for both the Great Caucasus and Yukon samples). The critical interpretation of these results requires that the diffusion rates of Lu and Sm are not very different from each other and that they are comparable to those for major cations. In all cases, calculated closure temperatures are equivalent or in the upper range of peak metamorphic temperatures estimated from geothermobarometry. As a consequence, the Sm – Nd whole rock – garnet age, and by inference the Lu – Hf whole rock –(omphacite) –garnet ages, are interpreted to closely approximate the time of garnet growth during eclogite facies metamorphism (see Fig. 5).

35

40

Ar – 39Ar laser spot ages on single phengite grains from the Great Caucasus showed no correlation of age with location and yielded an integrated age of 303 ± 5 Ma, which we interpreted to be representative of the age of the vein micas. With respect to the Yukon eclogites, laser spot ages on a single phengite of 1.5 mm in diameter yielded an integrated age of 261 ± 2 Ma, identical within error to the plateau age obtained using the conventional furnace technique on mica separates (256 ± 3 Ma; see Erdmer et al., 1998). For this reason, we assumed that the half width of analyzed micas (0.75 and 1 mm, respectively) can be used in Dodson’s equation to calculate the closure temperature of the Ar system for both localities (see also Perchuk and Philippot, 2000). The results of the calculation performed using the Ar diffusivity data of Kirschner et al. (1996) is shown in Fig. 4b. Calculated closure temperatures of 570
C for the Yukon eclogite and 580
C for the Great Caucasus eclogite are within error of the temperatures of late stage equilibration of about 520 ± 50 and 600 ± 40
C, respectively, deduced from mineral phase analysis. This, together with the observation that phengite formed at the expense of an earlier

Fig. 5. Summary of the pressure – temperature – time evolution for the analyzed eclogites. In the Great Caucasus P – T diagram, Lu – Hf ages of 296 ± 11 and 316 ± 5 Ma are for garnet separates of 5 and 9 mm diameter, respectively (sample GC53). Lu – Hf age of 322 ± 14 is from sample GC57/6 for which no rim – core or size distinction was made for the garnet separates. Whole rock – garnet Sm – Nd isochron age of 311 ± 22 Ma is for samples GC 60/5 and GC 61 considered together. Error bar represents uncertainties on calculated closure temperature (see Fig. 4).

36

P. Philippot et al. / Tectonophysics 342 (2001) 23–38

anhydrous eclogite assemblage, imply that the Ar –Ar dates are best interpreted to closely approach the age of the early cooling events following peak temperature equilibration (see Fig. 5). Because of the large uncertainties on Sm and Hf closure temperatures and of the high Ar closure temperature (Fig. 4), it remains difficult to establish a clear relationship between mineral reaction history and the timing of mineral growth. Considering that the eclogitic garnets analyzed are intermediate in composition between pure grossular and pyrope end-members, for which the Sm diffusivities were determined (see Fig. 4), implies that the mean closure temperature values shown as filled (Yukon) and empty circles (Great Caucasus) in Figs. 4 and 5 can be regarded as geologically relevant. In assuming so, the sequence of Lu –Hf and Ar –Ar ages recorded by coexisting garnet and phengite can be used to place limits on the cooling and exhumation rates. In both localities, garnet preserves a Lu – Hf age which is indistinguishable from the Ar – Ar age obtained on phengite. This is particularly clear for the Yukon eclogite for which good precision ages have been recorded using both techniques. This implies that the eclogitic rocks experienced a minimum cooling of 150
C and a pressure change of 0.8 GPa (i.e., 25 km depth) in a very short time span. This agrees well with the results of the diffusion modeling, which show that the duration of the high-pressure metamorphic event should not exceed 1 Ma or the garnet growth zoning patterns would have been significantly modified or eradicated. The recognition that garnet and phengite yield indistinguishable ages implies that the time resolution needed to constrain the exhumation history of the eclogites is smaller than the uncertainties on the ages. In saying this, we contend that the time scale of exhumation should be of the order of 1 Ma. This value is to be considered as an ‘‘order of magnitude’’ value not as a real duration. This conclusion is in good agreement with recent thermo-mechanical modeling of continental collision zones (e.g., Burov et al., 2001).

7. Conclusions One novel aspect of this paper is that here for the first time we combine the Lu – Hf and Ar – Ar techniques on the same samples. Another is that isotopic

dates are related to the mineral metamorphic history throughout their closure temperatures. The crux of the present work is to show that there is an incompatibility between the time-scale resolution of what is considered to be some of the most precise geochronological techniques and the rates of subduction zone metamorphism deduced from geospeedometry. In doing so we realize that the approach is subject to large uncertainties, although the salient conclusion that the time scale of exhumation events are comparable to that of subduction may not be in error. Turning this argument around, it implies that even in a highly optimized analytical situation, we failed to verify what numerical and analogue modeling of plate convergence environments have long suggested.

Acknowledgements We thank two anonymous reviewers for constructive criticism. This work benefited from financial support of CNRS-INSU program ‘‘Inte´rieur de la Terre’’ (Contribution No. 278), a CNRS-RAS project (No. 7768) to P. Philippot and A. Perchuk, and Russian Foundation for Basic Research grant No. 9705-64418 to A. Perchuk and No. 00-15-98519 to L.L. Perchuk (leading scientific schools of Russia). A. Perchuk also acknowledges support in the form of a seven-month Invited Professor fellowship at the Universite´ Paris 7. J. Blichert-Toft thanks Philippe Te´louk for assistance with the Plasma 54.

References Afanas’ev, G.D., Benet, K., Boiko, A.K., 1973. Preliminary results of ChSSR – SSSR collaboration in the problem of correlation precambrian formations and development of orogenic belts. Izv. Akad. Nayk SSSR 11, 3 – 15. Amatto, J.M., Johnson, C.M., Baumgartner, L.P., Beard, B.L., 1999. Rapid exhumation of the Zermatt-saas ophiolite deduced from high-precision Sm – Nd and Rb – Sr geochronology. Earth Planet. Sci. Lett. 171, 425 – 438. Blichert-Toft, J., Chauvel, C., Albare`de, F., 1997. Separation of Hf and Lu for high-precision isotope analysis of rock samples by magnetic sector-multiple collector ICP-MS. Contrib. Mineral. Petrol. 127, 248 – 260. Blichert-Toft, J., Albare`de, F., Kornprobst, J., 1999. Lu – Hf isotope systematics of garnet pyroxenites from Beni Bousera, Morocco: implications for basalt origin. Science 283, 1303 – 1306.

P. Philippot et al. / Tectonophysics 342 (2001) 23–38 Burg, J.P., Podladchikov, Y., 1999. Lithospheric scale folding and rock exhumation: numerical modelling and application to the Himalayan syntaxes. In: Khan, M.A., Treloar, P.J., Searle, M.P., Jan, M.Q. (Eds.), Tectonics of the Nanga Parbat Syntaxis and the Western Himalaya. Geol. Soc. Special Pub., London, pp. 219 – 236. Burov, E., Jolivet, L., Le Pourhiet, L., Poliakov, A., 2001. A thermomechanical model of exhumation of HP and UHP metamorphic rocks in Alpine mountain belts. Tectonophysics 342, 113 – 136. Carslaw, H.S., Jaeger, J.C., 1986. Conduction of Heat in Solids. Oxford Univ. Press, p. 510. Chemenda, A.I., Mattauer, M., Malavieille, J., Bokun, A.N., 1995. A mechanism for syn-collisional deep rock exhumation and associated normal faulting: results from physical modeling. Earth Planet. Sci. Lett. 132, 225 – 232. Christensen, J.N., Selverstone, J., Rosenfeld, J.L., DePaolo, D.J., 1994. Correlation by Rb – Sr geochronology of garnet growth histories from different structural levels within the Tauern Window, Eastern Alps. Contrib. Mineral. Petrol. 118, 1 – 12. Coghlan, R.A.N., 1990. Studies of diffusional transport: grain boundary transport of oxygen in feldspars, diffusion of oxygen, strontium and REEs in garnet and thermal histories of granitic intrusions in south-Central Maine using oxygen isotopes. PhD Thesis, Brown Univ., Providence, RI. Crank, J., 1975. The Mathematics of Diffusion. Oxford Univ. Press, 414 p. Creaser, R.A., Heaman, L.H., Erdmer, P., 1997. Timing of high pressure metamorphism in the Yukon – Tanana terrane, Canadian Cordillera: constraints from U – Pb zircon dating of eclogite from the Teslin tectonic zone. Can. J. Earth Sci. 34, 709 – 715. Dahl, P.S., 1996. The crystal – chemical basis for Ar retention in micas: inferences from interlayer partioning and implications for geochronology. Geol. Soc. Am. Bull. 106, 942 – 951. Dalmasso, J., Barci-Funel, G., Ardisson, G.J., 1992. Reinvestigation of the decay of the long-lived odd – odd 176Lu nucleus. Appl. Radiat. Isot. 43, 69 – 76. De Sigoyer, J., Chavagnac, V., Blichert-Toft, J., Villa, I.M., Luais, B., Guillot, S., Cosca, M., Mascal, G., 2000. Dating the Indian continental subduction and collisional thickening in the northwest Himalaya: Multichronology of the Tso Morari eclogites. Geology 28, 487 – 490. DeWolf, C.P., Zeissler, C.J., Halliday, A.N., Mezger, K., Essene, E.J., 1996. The role of inclusions in U – Pb and Sm – Nd garnet geochronology: stepwise dissolution experiments and trace uranium mapping by fission track analysis. Geochim. Cosmochim. Acta 60, 121 – 134. Dodson, M.H., 1973. Closure temperature in cooling geochronological and petrological systems. Contrib. Mineral. Petrol. 40, 259 – 274. Ducheˆne, S., Blichert-Toft, J., Luais, B., Telouk, P., Lardeaux, J.-M., Albarede, F., 1997. The Lu – Hf dating of garnets and the ages of the Alpine high-pressure metamorphism. Nature 387, 586 – 588. Ducheˆne, S., Albarede, F., Lardeaux, J.-M., 1998. Mineral zoning and exhumation history in the Munchberg eclogites (Bohemia). Am. J. Sci. 298, 30 – 59. Erdmer, P., Armstrong, R.L., 1989. Permo-Triassic isotopic dates

37

for blueschists, Ross River area, Yukon. In: Yukon Geology, Exploration and Geological Services Division, Yukon. Indian North. Aff. Can. 2, 33 – 36. Erdmer, P., Helmstaedt, H., 1983. Eclogite from central Yukon: a record of subduction at the western margin of ancient North America. Can. J. Earth Sci. 20, 1389 – 1408. Erdmer, P., Ghent, E.D., Archibald, D.A., Stout, M.Z., 1998. Paleozoic and Mesozoic high-pressure metamorphism at the margin of the ancestral North America in central Yukon. Geol. Soc. Am. Bul. 110, 615 – 629. Ganguly, J., Chakraborty, S., Sharp, T.G., Rumble, D., 1996. Constraint on the time scale of biotite-grade metamorphism during Acadian orogeny from a natural garnet – garnet diffusion couple. Am. Mineral. 81, 1208 – 1216. Gebauer, D., Schertl, H.P., Brix, M., Schreyer, W., 1997. 35 Ma old ultrahigh-pressure metamorphism and evidence for very rapid exhumation in the Dora – Maira Massif, western Alps. Lithos 41, 5 – 24. Harrison, T.M., Wood, B.J., 1980. An experimental investigation f the partitioning of REE between garnet and liquid with reference to the role of defect equilibria. Contrib. Mineral. Petrol. 82, 145 – 155. Harrison, T.M., Duncan, I., McDougall, I., 1985. Diffusion of 40Ar in biotite: temperature, pressure and compositional effects. Geochim. Cosmochim. Acta 49, 2461 – 2468. Hensen, B.J., Zhou, B., 1995. Retention of isotopic memoy in garnets partially broken down during an overprinting granulite-facies metamorphism: implications for the Sm – Nd closure temperature. Geology 23, 225 – 228. Hodges, K.V., Hames, W.E., Bowring, S., 1994. A 40Ar/39Ar age gradients in micas from high temperature – low pressure metamorphic terrain: evidence for very slow cooling and implications for the interpretation of age spectra. Geology 22, 55 – 58. Jagoutz, E., 1988. Nd and Sr systematics in an eclogite xenolith from Tanzania: evidence fro frozen mineral equilibria in the continental lithosphere. Geochim. Cosmochim. Acta 52, 1285 – 1293. Jiang, J., Lasaga, A.C., 1990. The effect of post-growth thermal events on the growth-zoned garnet: implications for metamorphic P – T history calculations. Contrib. Mineral. Petrol. 105, 454 – 459. Kirschner, D.L., Cosca, M.A., Masson, H., Hunziker, J.C., 1996. Staircase 40Ar/39Ar spectra of fine-grained white mice: timing and duration of deformation and empirical constrains on argon diffusion. Geology 24, 747 – 750. Mezger, K., Essene, E.J., Halliday, A.N., 1992. Closure temperatures of the Sm – Nd system in metamorphic garnets. Earth Planet. Sci. Lett. 113, 397 – 409. Nir-El, Y., Lavi, N., 1998. Measurement of the half-life of 176Lu. Appl. Radiat. Isot. 49, 1653 – 1655. O’Brien, P.J., 1997. Garnet zoning and reaction textures in overprinted eclogites, Bohemain Massif, European Variscides: a record of their thermal history during exhumation. Lithos 41, 119 – 133. Perchuk, A.L., 1993. Petrology of the kyanite eclogites from the Red Cliff area, Fore Range Zone, Great Caucasus. Petrology 1, 88 – 98.

38

P. Philippot et al. / Tectonophysics 342 (2001) 23–38

Perchuk, A.L., Philippot, P., 1997. Rapid cooling and exhumation of eclogitic rocks from the Great Caucasus, Russia. J. Metamorph. Geol. 15, 299 – 310. Perchuk, A.L., Philippot, P., 2000. Geospeedometry and time scales of high-pressure metamorphism. Int. Geol. Rev. 42, 207 – 223. Perchuk, A.L., Gerasimov, V.Yu., Philippot, P., 1996. Theoretical modeling of eclogite uplift. Petrology 4/5, 518 – 532. Perchuk, A.L., Yapaskurt, V.O., Podlesskii, S.K., 1998. Genesis and exhumation dynamics of eclogites in the Kokchetav massif near mount Sulu-Tjube, Kazakhstan. Geochem. Int. 36/10, 877 – 885. Perchuk, A.L., Philippot, P., Erdmer, P., Fialin, M., 1999. Rates of thermal equilibration at the onset of subduction deduced from diffusion modeling of eclogitic garnets, Yukon – Tanana terrain, Canada. Geology 27, 531 – 534. Scaillet, S., 1998. K – Ar (40Ar/39Ar) geochronology of ultrahigh pressure rocks. In: Hacker, B.R., Liou, J.G. (Eds.), When Continents Collide: Geodynamics and Geochemistry of UltrahighPressure Rocks. Kluwer Academic Publishing, Dordrecht, pp. 161 – 201. Scaillet, S., Fe´raud, G., Balle`vre, M., Amouric, M., 1992. Mg/Fe and (Mg,Fe)SiAl2 compositional control on argon behaviour in high-pressure white micas: a 40Ar/39Ar continuous laser-probe study from the Dora Maira nappe of the internal western Alps, Italy. Geochim. Cosmochim. Acta 56, 2851 – 2872. Scherer, E.E., Cameron, K.L., Blichert-Toft, J., 2000a. Lu – Hf garnet geochronology: closure temperature relative to the Sm – Nd system and the effects of trace mineral inclusions. Geochim. Cosmochim. Acta 64, 3413 – 3432. Scherer, E.E., Mu¨nker, C., Mezger, K., 2000b. The decay constant of 176Lu determined by calibration against the U – Pb system in the Phalaborwa carbonatite. J. Conf. Abs. 5, 887.

Sguigna, A.P., Larabee, A.J., Waddington, J.C., 1982. The half-life of 176Lu by g – g coincidence measurement. Can. J. Phys. 60, 361 – 364. Shanin, L.L., Arakelyants, M.M., Pupiyrev, U.G., Kolesnikov, A.G., 1983. New model of the metallic equipment for K – Ar method of datingMass-Spectrometry and Isotope Geology. Nauka Publishing, Moscow, 164 pp. Somin, M.L., 1991. Geological characteristics of the Forerange and Main Range zones, Great Caucasus. In: Korikovskii, S.P. (Ed.), Petrology of Metamorphic Complexes of the Great Caucasus. Nauka Press, Moscow, pp. 18 – 45. Starik, I.E., 1961. Nuclear Geochronology. AS USSR Publishing, Moscow, 630 pp. Steiger, R.H., Jager, E., 1977. Subcommission on geochronology: convention on the use of dicey constant s in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359 – 361. Tatsumoto, M., Unruh, D.M., Patchett, P.J., 1981. U – Pb and Lu – Hf systematics of Antarctic meteorites. Proc. 6th Symp. Antarctic Meteorites. Natl. Inst. Polar Res., Tokyo, pp. 237 – 249. Villa, I.M., 1998. Isotopic closure. Terra Nova 10, 42 – 47. Wanless, R.K., Stevens, R.D., Lachance, G.R., Delabio, R.N., 1978. Age determinations and geological studies: K – Ar isotopic ages. Geol. Survey Canada Paper 13, 77-2, 60 pp. White, W.M., Patchett, P.J., 1984. Hf – Nd – Sr isotopes and incompatible element abundances in island arcs, implications for magma origin and crust – mantle evolution. Earth Planet. Sci. Lett. 67, 167 – 185. Zeck, H.P., Monie´, P., Villa, I.M., Hansen, B.T., 1992. Very high rates of cooling and uplift in the Alpine belt of the Betic Cordilleras, southern Spain. Geology 2, 79 – 82.