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*
Present address: Department of Geosciences, National Taiwan University, P.O. Box 13-318, Taipei 106, Taiwan; e-mail: jahn@ccms.ntu.edu.tw

Abstract

After more than a decade of research, the concept of subduction of light continental rocks into the denser upper mantle has become a well-established fact. Primary evidence for this phenomenon is found in the similarity of lithologic successions of UHP metamorphic terranes with stratigraphic sequences observed in upper continental rocks. Interlayered quartzite, marble, mica schist, and paragneiss resemble sedimentary sequences of sandstone, limestone, shale and graywacke deposited along continental margins. Concordant layers of eclogite in quartzite, marble, schist, and biotite gneiss suggest basaltic sills or lava flows. More importantly, based on chemical and isotopic analyses, eclogites from many UHP metamorphic terranes, regardless of their occurrence types, can be proven to have a continental affinity, hence a part of continental crust.

Introduction

After more than a decade of research, the concept of subduction of light continental rocks into the denser upper mantle has become a well-established fact. Primary evidence for this phenomenon is found in the similarity of lithologic successions of UHP metamorphic terranes with stratigraphic sequences observed in upper continental rocks. Interlayered quartzite, marble, mica schist, and paragneiss resemble sedimentary sequences of sandstone, limestone, shale and graywacke deposited along continental margins. Concordant layers of eclogite in quartzite, marble, schist, and biotite gneiss suggest basaltic sills or lava flows. More importantly, based on chemical and isotopic analyses, eclogites from many UHP metamorphic terranes, regardless of their occurrence types, can be proven to have a continental affinity, hence a part of continental crust.

Eclogites are volumetrically minor in UHP metamorphic terranes, but they are the principal rock type that bears evidence of UHP metamorphism. If eclogites were produced by metamorphism of a subducted oceanic lithosphere, as documented in the Hercynian and Alpine orogenic chains (Bernard-Griffiths & Cornichet, 1985; Bernard-Griffiths et al., 1985; Stosch & Lugmair, 1990; Beard et al., 1992; Thöni & Jagoutz, 1992; Miller & Thöni, 1995), their contact relation with associated continental rocks, i.e., granitic gneisses, must be tectonic. Thus, even if the eclogites possess UHP parageneses, it does not necessarily indicate that the country gneisses have also undergone the same UHP metamorphism. However, if the eclogites can be identified as part of ancient continental crust, then their presence must imply deep subduction of a continental block. This chapter will deal mainly with eclogites and particularly those from the Dabie orogen of east-central China.

Eclogites from the UHP metamorphic terranes of Dabieshan, China, have three occurrence types: (1) gneiss-hosted, in which eclogites occur as enclaves within granitic gneisses, (2) marble-associated, in which eclogites form interlayers or lenses within marble or calc-silicate rocks, and (3) layered intrusion, in which eclogites form part of a magmatic differentiation series (e.g. Bixiling and Maowu). Geochemical as well as Sr-Nd and oxygen isotopic analyses have clearly established their continental tectonic settings (Jahn, 1998, 1999; Zheng et al., 1998, 1999). On the other hand, the implication of ultramafic rocks in UHP metamorphic terranes may be more controversial. In some cases, mantle blocks could have been emplaced by an earlier tectonic process into continental crust, then subjected to subduction to mantle depths together with the host continental crust. Such a process is difficult to identify if the rocks have chemical and isotopic compositions comparable with those of upper mantle rocks. This may be represented by the Rizhao clinopyroxenites (Hiramatsu & Hirajima, 1995; Zhang et al., 2000). However, if mantle peridotites have recorded a history of crustal contamination as revealed by Nd-Sr-O isotopic analyses and/or petrological evidence, then their UHP assemblages would imply deep continental subduction. This is exemplified by garnet peridotites of Donghai-Zhimafang (Yang & Jahn, 2000).

Identification of the origin of mafic and ultramafic rocks in an UHP metamorphic terrane is based much on geochemical and isotopic fingerprints. Geochemistry also plays an important role in recognising Earth surface features in subducted rocks. For example, the low δ18O and low δD rocks in the Dabie and Su-Lu terranes demonstrate that UHP gneisses, eclogites and quartzites were altered by meteoric water from a cold climate in a geothermal system at Earth’s surface prior to subduction (Yui et al., 1995, 1997; Zheng et al., 1996, 1998, 1999; Baker et al., 1997; Rumble & Yui 1998; Fu et al., 1999; Rumble et al., 2002).

In this chapter, emphasis will be placed on the use of chemical and isotope tracers to identify the nature of protoliths of eclogites and unravel their pre-metamorphic to post-metamorphic tectonic evolution. The tracers to be used include radiogenic (Nd-Sr) and stable (oxygen) isotopes. Some problems concerning the dating of UHP metamorphic events using the isochron methods will be discussed. Finally, an example will be given to illustrate how the isotope tracers could be used to constrain the tectonic evolution of the celebrated Dabie orogen.

II. Chemical compositions of eclogites and ultramafic rocks

Eclogites are mainly of basaltic composition, but they show a wide range of major and trace element abundances. This suggests that they may have multiple origins and derived from heterogeneous mantle sources. However, unusual compositions could also be an artifact resulted from biased analyses conducted on banded rocks produced by metamorphic segregation. Eclogites and ultramafic rocks that have been recrystallised in UHP metamorphic conditions often show coarse-grained texture with distinct mineral banding of variable scale. It is often very difficult to obtain a truly representative bulk composition for the protolith of a banded eclogite. Fine-grained or homogeneous textured eclogites also occur, but they seem to be minor in comparison with heterogeneous textured facies. Eclogites from the Dabie and Su-Lu terranes have been shown to cover a wide range of SiO2 contents, from 36 to 60%, although the majority (> 70%) still have basaltic or gabbroic compositions (SiO2 = 45-52%, Fig. 1.; Jahn, 1998).

Fig. 1.

Major element variations of eclogites and associated ultramafic rocks from the Su-Lu and Dabie terranes of east-central China. Eclogites are generally basaltic and quartz-normative, with some showing the cumulative nature of their protoliths. Type I is gneiss-hosted, Type II is marble-associated, and Type III is a member of layered intrusion or associated with ultramafic rocks.

Fig. 1.

Major element variations of eclogites and associated ultramafic rocks from the Su-Lu and Dabie terranes of east-central China. Eclogites are generally basaltic and quartz-normative, with some showing the cumulative nature of their protoliths. Type I is gneiss-hosted, Type II is marble-associated, and Type III is a member of layered intrusion or associated with ultramafic rocks.

Higher silica eclogites (SiO2 = 53%) suggest that their protoliths are more differentiated, and lower silica ones (= 45%) imply a “cumulate” nature of their protoliths or a portion of rock rich in low SiO2 phases (e.g. garnet, epidote, rutile etc.) from a banded eclogite. Garnetite is a good example of metamorphic differentiation; it cannot be used to discuss the nature of its protolith. It is common to find that the chemical composition of an eclogite cannot be matched by any reasonable magmatic protoliths. In any case, eclogite protoliths cannot be easily classified with the commonly used classification schemes for igneous rocks because most of them, such as AFM or TAS (Middlemost, 1994), involve the use of alkali elements which are known to be mobile in metamorphic processes. This subject is discussed below.

Numerous petrological studies have established that the protoliths of eclogites underwent progressive dehydration during prograde metamorphism, and many of them were later rehydrated to some extent during retrograde metamorphism when the rocks were exhumed (e.g. Liou et al., 1998; Zheng et al., 1999; Hirajima & Nakamura, 2003). These processes are not isochemical. In fact, some gneissic rocks of Dabieshan show textural and mineralogical evidence of transformation from retrograde eclogites through hydration, recrystallisation and metasomatism (Zhang et al., 2003). In this retrograde metamorphism, CO2-rich and Na+ and K+-bearing low salinity aqueous fluids reacted with the UHP eclogites (Zheng et al., 2000; Fu et al., 2001) and resulted in partial to complete transformation to biotite- and amphibole-bearing gneisses at greenschist to low amphibolite facies conditions. LIL elements (K, Rb, Cs, Sr, Ba), U and to some extent, La and Ce, could be mobilised along with volatile species such as H2O and CO2. In a normal subduction environment, dehydration of subducted oceanic crust (amphibolite and serpentinite) creates a hydrous curtain, in which hydrous fluid moves upward carrying these elements, but leave behind insoluble Nb and Ta, and metasomatizes the mantle wedge (Tatsumi & Eggins, 1995). Magmas produced by melting of such a metasomatized mantle would show enrichment in LIL elements, U and probably also La-Ce, but would be depleted in Nb and Ta. This is the characteristic geochemical signature of arc magmas formed in zones of oceanic lithosphere subduction.

Subduction of a continental crust has a somewhat different chemical consequence because the continental crust is globally less hydrated, especially the lower crust which is composed mainly of anhydrous granulite facies rocks. The protolith of an eclogite could equally be a part of the upper to middle crust. It may occur as a basic dike or enclave, or as a layered intrusion (e.g. Bixiling or Maowu), within granitic gneisses; and together metamorphosed in greenschist to amphibolite facies rocks prior to the subduction to mantle depths. This scenario is often observed in Precambrian terranes. During continental subduction, greenschist or amphibolite of basaltic composition would then follow the similar route as the oceanic crust, and engaged in dehydration reactions. The pattern of elemental loss should be rather similar to that in a subducted oceanic lithosphere, except that the amount of fluid released from the continental subduction is comparatively insignificant.

The present abundances of LIL elements and U, as well as REE patterns in eclogites cannot be easily used to argue for or against their mobility, because these elements show a great variation in basaltic rocks from diverse tectonic settings. Consequently, any difference in their concentrations could reflect the original difference in the protoliths, but not necessarily related to the gain-loss process during metamorphism. Nevertheless, the clearest evidence of elemental loss during eclogitization is from studies of the Rb-Sr isotopic systematics. Analyses on eclogites from Western Norway (Griffin & Brueckner, 1985) and from the Dabie and Su-Lu terranes (Ames et al., 1996; Jahn, 1998) indicate that many eclogites have highly radiogenic 87Sr/86Sr ratios that are “unsupported” by their Rb/Sr ratios (Fig. 2). This is particularly true for eclogites that occur as enclaves within quartzofeldspathic gneisses (Griffin & Brueckner, 1985; Jahn, 1998). The most plausible explanation is that Rb is preferentially lost relative to Sr during prograde

Fig. 2.

87Sr/86Sr vs. 87Rb/86Sr diagram for UHP eclogites from the Su-Lu and Dabie terranes. Three reference isochrons based on the protolith ages of the Weihai eclogites (1.7 Ga, Jahn et al., 1996), granitic gneisses of the Su-Lu and Dabie terranes (~ 800 Ma) and the UHP metamorphic event (~ 220 Ma) are shown for assessment of Rb-Sr isotopic systems in different types of eclogites. Sources: Jahn (1998), Li (2003), and Ames et al. (1996) for those occurrences “not defined”. Note that the data are highly dispersed and many show “unsupported” high Sr isotopic ratios (data left of the 1.7 Ga isochron), implying depletion of Rb and alkali elements during the metamorphic processes.

Fig. 2.

87Sr/86Sr vs. 87Rb/86Sr diagram for UHP eclogites from the Su-Lu and Dabie terranes. Three reference isochrons based on the protolith ages of the Weihai eclogites (1.7 Ga, Jahn et al., 1996), granitic gneisses of the Su-Lu and Dabie terranes (~ 800 Ma) and the UHP metamorphic event (~ 220 Ma) are shown for assessment of Rb-Sr isotopic systems in different types of eclogites. Sources: Jahn (1998), Li (2003), and Ames et al. (1996) for those occurrences “not defined”. Note that the data are highly dispersed and many show “unsupported” high Sr isotopic ratios (data left of the 1.7 Ga isochron), implying depletion of Rb and alkali elements during the metamorphic processes.

eclogite facies metamorphism. By analogy, elements of similar geochemical behaviour, such as K, Cs and probably Ba, could have been lost along with Rb in the same process.

During the exhumation of deeply subducted continental crust, retrograde metamorphism overprints HP-UHP eclogites to variable extents. Retrograde metamorphism took place most efficiently in the presence of water. The process would rehydrate eclogites and probably rehabilitate the lost mobile elements in the course. However, the process should not be considered as reversible to prograde metamorphism. In the Dabie orogen, many examples have shown that during the entire course of continental subduction to its final exhumation, aqueous fluid phase has played little role as a whole. This is evidenced by the preservation of the world’s lowest δ18O values (−10%o) in high temperature rocks (Yui et al., 1995, 1997; Zheng et al., 1996, 1998, 1999; Rumble & Yui 1998; Fu et al., 1999; Rumble et al., 2002), the world’s highest ϵNd values (+270) in eclogites of the Weihai area (Jahn et al., 1996) and the general absence of syntectonic granites in the orogen. This is in line with the frequent occurrence of coesite in the Dabieshan eclogites. On the contrary, if the water activity was elevated, coesite would not survive due to its easy conversion to quartz. However, localised and channelised fluid activity has also been recorded in the same metamorphic terrane (Li et al., 2001; Li, 2003).

Eclogites from the European Hercynides and the Alpine chain are known to have an affinity with oceanic basalts (or MORB); whereas those from the Caledonides and the Dabie orogen have an affinity with continental basalts (see references cited by Jahn, 1999). REE patterns are invariably LREE-enriched in the eclogites of Dabieshan and Su-Lu (Jahn, 1998), but LREE-depleted, MORB-like patterns are abundant in the Hercynian and Alpine chains. Although LREE, especially La, might be susceptible to mobilisation in the presence of hydrous fluids, the general absence of such fluids in Dabieshan makes the identification of the protolith nature via the use of REE patterns credible (Fig. 3).

Fig. 3.

REE distribution patterns of eclogites from the Su-Lu, Dabie and Hong’an terranes. All eclogites are gneiss-hosted except two from Rongcheng (Su-Lu). The common feature is the enrichment in light REE, which contrast with N-MORB but resemble continental mafic rocks (basalt, amphibolite, or basic granulite).

Fig. 3.

REE distribution patterns of eclogites from the Su-Lu, Dabie and Hong’an terranes. All eclogites are gneiss-hosted except two from Rongcheng (Su-Lu). The common feature is the enrichment in light REE, which contrast with N-MORB but resemble continental mafic rocks (basalt, amphibolite, or basic granulite).

III. Sm-Nd and Rb-Sr isochron ages and Nd-Sr isotope tracers

Geochronology plays a crucial role in our understanding of the processes of subduction and exhumation of continental crustal rocks. When dealing with a UHP metamorphic rock, the key questions on the timing of events include: the age of protolith, the ages of peak and retrograde metamorphism, the time of later thermal disturbance, the episode of fluid-rock interaction etc. Rate of exhumation could be estimated if precise age information can be obtained and linked to a specific stage of an orogenic process. Moreover, for the entire metamorphic terrane, comprehensive age patterns would be needed to reconstruct a scenario of tectonic evolution (e.g. Hacker et al., 1998, 2000).

Different events can be dated by different chronometers. The most commonly used chronometers include U-Pb on zircon or monazite, Sm-Nd on garnet, Ar-Ar on white mica or hornblende, and Rb-Sr on biotite or white mica. Lu-Hf has also been used for garnet dating, and it was shown that this method may be superior to Sm-Nd in some cases (Duchêne et al., 1997; de Sigoyer et al., 2000; Scherer et al., 2000). However, analytical facilities of Hf isotopes are more limited, and are practiced only with MC-ICP-MS on eclogitic minerals (Blichert-Toft et al., 1997). Until now only a few resultshave been published. The U-Pb systems of zircon and monazite have yielded a great amount of chronological information on a variety of thermal events. However, as pointed out cogently by Thöni (2002) in his excellent summary of geochronological problems of the Alpine metamorphic rocks, even though the zircon and monazite U-Pb chronometers usually provide exact time information, they are somewhat hampered by the fact that the mechanisms of zircon and monazite (re)crystallisation are still poorly understood in the context of the P-r evolution of metamorphic rocks. In HP and UHP metamorphic terranes, zircon may grow during the prograde as well as retrograde recrystallisation (Tilton et al., 1991; Gebauer, 1996; Thöni, 2002). Metamorphic recrystallisation of zircon and resetting of its U-Pb systematics could be enhanced by fluids even at moderate temperatures (< 600 °C, Rubatto et al., 1999, 2003; Liati et al., 2000). Partial recrystallisation may even result in apparent concordant ages without geological meaning for any particular events (Pidgeon, 1992; Pidgeon et al., 1998; Li, 2003). Consequently, zircon may yield no clear petrological record in the P-r path, and it is sometimes difficult to link “punctual” U-Pb age data (e.g. SHRIMP dates) with a specific stage of metamorphic evolution.

The technique of mineral isochron is based on the in situ isotopic evolution of an equilibrated mineral assemblage. In dating HP-UHP eclogite, only minerals that are formed in the eclogite facies metamorphism are useful; these include garnet, omphacite, phengitic mica etc. Presence of relict minerals or retrograde metamorphic products would likely disturb the isochron relationship between the different phases and whole-rock. The mineral isochron method not only provides age information but also yields initial isotopic ratios that are very valuable in tracing the evolution of its protolith as well as the genetic relationship between UHP metamorphic rocks and spatially associated post-metamorphic rocks. Among the high-grade metamorphic minerals, garnet is known to be the most useful one in metamorphic geochronology. Garnet is ubiquitous, it occurs in a wide range of metamorphic and igneous rocks of both crustal and mantle origins. In metamorphic petrology, garnet is often used in thermobarometry, thus is most useful in the reconstruction of P-r paths. Since the mid-1980’s, garnet has become one of the most powerful metamorphic chronometers because of its high rate of radiogenic growth of Nd isotope ratios (Griffin & Brueckner, 1985; Vance & O’Nions, (1990), 1992; Mezger et al., 1992; Li et al., 1993; Chavagnac & Jahn, 1996).

As emphasised by Thöni (2002), the major advantage of garnet dating is the ability to extract microstructural and thermobarometric information together. The combination of textural evidence, P-T estimates and age data allows a much better understanding of the evolution of a metamorphic terrane and hence the orogenic processes. If zircon U-Pb and Rb-Sr isochron ages are jointly used, the constraint would be much more enhanced. In any case, garnet is most useful in its Sm-Nd isotopic system, but the interpretation of garnet Sm-Nd age (or date) is not always straightforward. The most important factors that influence the garnet-based isochron chronometer are (1) isotope disequilibrium and (2) presence of high-LREE inclusions. Before the two factors are discussed, it would be instructive to understand the concentration ranges of Sm and Nd in a few most important HP-UHP minerals (garnet, omphacite, phengite etc.) and the REE partition coefficients between these minerals.

Ranges of Sm and Nd concentrations

333 garnet analyses by isotope dilution method are compiled from the literature and our unpublished data. The ranges of Sm and Nd concentrations in garnets are shown in Fig. 4. A few observations could be made: (1) Sm and Nd concentrations are limited to 14 and 44 ppm, respectively. However, the majority of data points (250 out of 333) fall within the range of 2 ppm for both elements (Fig. 4b). (2) Overall, garnets from ultramafic rocks (solid circle and diamond) have concentrations lower than those from eclogites and granitic gneisses. (3) Sm/Nd ratios cover a wide range from 0.3 to 10. A small number of garnets (25/333) have Sm/Nd ratios less than the chondritic value of ca. 0.325; most of these have exceptionally high Nd (5 to 44 ppm), suggesting that the garnets contain significant amounts of LREE-rich inclusions, such as monazite, epidote or apatite. The same has also been underlined by Thöni (2002) in his study of eclogites from the Eastern Alps. (4) “Clean” garnets, like those from the Bixiling complex of Dabieshan, tend to have low Nd concentrations (= 0.5 ppm) and high Sm/Nd ratios (about 2). These observations suggest that LREE-rich inclusions could exert important influence on garnet geochronology (Luais et al., 2001; Thöni, 2002).

Fig. 4.

Sm and Nd concentrations in garnet, clinopyroxene, orthopyroxene and amphibole. All data were obtained by the isotope dilution method reported in the literature (references too numerous to be cited herein). The two charts at the top represent the total ranges observed. Data of more restricted concentration ranges are shown in the two diagrams in the middle and the two at the bottom. The majority of garnet data have both Sm and Nd less than 2 ppm, and most have Sm/Nd ratios falling between 1 and 2. Higher Nd concentrations are often accompanied by lower Sm/Nd ratios, which suggest a presence of REE-rich inclusions, such as epidote, zoisite, or apatite, in garnet. Most clinopyroxenes have Sm/Nd ratios falling about the chondritic ratio of 0.325, and their Nd concentrations less than 6 ppm.

Fig. 4.

Sm and Nd concentrations in garnet, clinopyroxene, orthopyroxene and amphibole. All data were obtained by the isotope dilution method reported in the literature (references too numerous to be cited herein). The two charts at the top represent the total ranges observed. Data of more restricted concentration ranges are shown in the two diagrams in the middle and the two at the bottom. The majority of garnet data have both Sm and Nd less than 2 ppm, and most have Sm/Nd ratios falling between 1 and 2. Higher Nd concentrations are often accompanied by lower Sm/Nd ratios, which suggest a presence of REE-rich inclusions, such as epidote, zoisite, or apatite, in garnet. Most clinopyroxenes have Sm/Nd ratios falling about the chondritic ratio of 0.325, and their Nd concentrations less than 6 ppm.

Figure 4 also shows Sm-Nd concentrations of Omp-Cpx and a few Opx and Amp. Some observations: (1) Most data points have Nd ≤ 10 ppm and Sm ≤ 3 ppm. (2) The majority have Sm/Nd ratios falling about the chondritic value of 0.325 (0.2 to 0.5); only a few have ratios as high as 1.0. (3) Like garnets, higher Nd (≥ 10 ppm) Cpx tend to have lower Sm/Nd ratios, suggesting a contribution of LREE-rich inclusions.

Equilibrium partition coefficients (Kd values) between Cpx and Grt

Several recent studies of element abundances in eclogite minerals have established some interesting patterns of trace element partition coefficients (Harte & Kirkley, 1997; Bocchio et al., 2000; Sassi et al., 2000). These studies were mainly realised using the analytical techniques of secondary ion mass spectrometry (SIMS) and laser ablation ICP-MS. The data of the Maowu eclogite-pyroxenite body from Dabieshan is used to illustrate this point because the coherent U-Pb and Sm-Nd age data seem to indicate that REE in coexisting Cpx and Grt have attained chemical and Nd isotopic equilibrium (Rowley et al., 1997; Jahn et al., 2003). The Maowu eclogites and pyroxenites have been dated at 221 to 236 Ma by the Sm-Nd mineral isochron method (Jahn et al., 2003), and at 220 to 230 Ma by zircon and monazite U-Pb analyses (Rowley et al., 1997; Ayers et al., 2002). In addition, the metamorphic conditions have been determined at P = 40 ± 10 kbar and T = 750 ± 50 °C (Wang et al., 1990; Okay, 1994; Fan et al., 1996; Liou and Zhang, 1998). The general patterns for whole-rock and minerals observed for the Maowu rocks are very similar to those of the Norwegian eclogites (Griffin & Brueckner, 1985), the diamondiferous eclogites from Yakutia, Siberia (Jerde et al., 1993; Taylor et al., 1996) and the eclogites from the Adula Nappe of the central Alps reported by Bocchio et al. (2000). They are known to represent chemically equilibrated assemblages.

Cpx/Grt partition coefficients (= Kd values) of the Maowu rocks are shown in Figure 5. The Kd’s of LREE have a variation much greater than that of HREE - up to three orders of magnitude for La, but all Kd’s decrease regularly from La (or Ce) to Lu.Similar patterns have been observed for eclogites from the central Alps (Bocchio et al., 2000). The Kd’s are also quite comparable with those obtained in a high pressure (20-30 kbar) and high temperature (1300-1470 °C) experiment of Johnson (1994). Consequently, our results strongly indicate that chemical equilibrium was achieved during the UHP metamorphism. The only green amphibole shows a REE pattern almost identical to that of the coexisting Cpx, causing the REE Kd values between Amp and Cpx to be close to unity. Thus, the amphibole appears to have been transformed entirely from Cpx during retrograde metamorphism.

Fig. 5.

REE partition coefficients (Kd) between clinopyroxene and garnet. The data were obtained on the eclogite-pyroxenite complex of Maowu (data source: Jahn et al., 2003). Kd values for LREE vary by three orders of magnitude, whereas those for HREE by only one order.

Fig. 5.

REE partition coefficients (Kd) between clinopyroxene and garnet. The data were obtained on the eclogite-pyroxenite complex of Maowu (data source: Jahn et al., 2003). Kd values for LREE vary by three orders of magnitude, whereas those for HREE by only one order.

Figure 6 shows Cpx/Grt Kd values for a few selected elements. While Mg shows little preference for Cpx or Grt, Ca and Sr are, as expected, strongly partitioned into Cpx, but Fe, Y and Zr are partitioned into Grt. The partitioning of Y is easily understood as it

Fig. 6.

Some selected element partition coefficients between clinopyroxene and garnet. The data were obtained on the eclogite-pyroxenite complex of Maowu (data source: Jahn et al., 2003). Note that the Kd(REE) values are positively correlated with Kd(Ca), and Zr-Y, like HREE, are preferentially partitioned in garnet.

Fig. 6.

Some selected element partition coefficients between clinopyroxene and garnet. The data were obtained on the eclogite-pyroxenite complex of Maowu (data source: Jahn et al., 2003). Note that the Kd(REE) values are positively correlated with Kd(Ca), and Zr-Y, like HREE, are preferentially partitioned in garnet.

behaves like HREE. However, the behaviour of Zr is worth some comment. It has been shown that Kd(Zr) values are consistently below 1 (0.1 to 0.6) for other Dabieshan eclogites (Sassi et al., 2000), but are greater than unity (1.6 to 2.2) for eclogites from the central Alps (Bocchio et al., 2000). In a study of mantle-derived pyroxenites and eclogites, O’Reilly & Griffin (1995) observed that Zr contents are generally higher in Grt (< 1 to 80 ppm) than in Cpx (< 1 to 50 ppm). Kd(Zr) for most samples range from 0.1 to 3, but Zr partitioning is temperature dependent (Kd proportional to 1/7). They found that Kd(Zr) decreases with increasing XJd(Cpx) and possibly XCa(Gt), but shows no P effect. Because of the relatively large Kd(Zr) in Grt, Degeling et al. (2001) suggest that metamorphic zircon could be produced by Zr-bearing Grt breakdown reaction: 2 Zr-bearing Grt + 4 Sillimanite + 13 Qtz = 3 Cordierite + 4 Zircon.

In summary, the REE Kd values for Cpx/Grt show a dependence on the Ca content of the host phases, with Kd’s decreasing with decreasing Kd(Ca), as shown by comparing Figures 5 and 6. This behaviour has also been found in mantle eclogites (O’Reilly & Griffin, 1995; Harte & Kirkley, 1997) which have much higher equilibrium temperatures of 1000-1200 °C than the Maowu UHP rocks (700-800 °C). The temperature difference appears to have little effect on the Kd values, but this view is not shared by Bocchio et al. (2000) from their analysis on alpine eclogites. Sassi et al. (2000) also observed that the patterns of REE Kd’s for all UHP eclogites from Dabieshan are similar, but the absolute values vary a great deal, from three orders of magnitude for La to about one order for Lu. Furthermore, a weak correlation between Kd(Zr) and Kd(Ca) for both mantle and Dabieshan eclogites was noticed by Harte & Kirkley (1997) and Sassi et al. (2000).

Meaningful Sm-Nd and Rb-Sr isochron ages: some examples

Meaningful Sm-Nd and Rb-Sr isochron ages have been obtained for the Bixiling and Maowu complexes (Figs. 7 and 8). The metamorphic ages of ca. 220 Ma for the Bixiling Complex are in perfect agreement with the SHRIMP U-Pb zircon ages obtained by Cheng et al. (2000) and Li (2003). The Maowu eclogites and pyroxenites appear to have recorded slightly older ages at ca. 230 Ma, and the results are also consistent with those obtained by the conventional (Rowley et al., 1997) as well as ion probe (CAMECA ims-1270, Ayers et al., 2002) zircon analyses. Moreover, meaningful Sm-Nd and Rb-Sr isochron ages of about 620 Ma have been obtained for the Pan-African coesite-bearing eclogites from Mali (Jahn et al., 2001a). The Mali eclogites are the oldest UHP eclogites identified so far. Note that all the rocks have been recrystallised in UHP metamorphic conditions with temperatures well over 700 °C.

Fig. 7. (a)

Meaningful Sm-Nd and (b) Rb-Sr (next page) mineral isochron ages were obtained for the Bixiling Complex (data source: Chavagnac & Jahn, 1996). All rocks have very similar metamorphic initial isotope ratios (ϵNd(T) = 0 to −2, ISr 0.704). The amphibolite has a much younger biotite age of 179 Ma and a very different ISr of 0.707, as a result of retrograde metamorphism.

Fig. 7. (a)

Meaningful Sm-Nd and (b) Rb-Sr (next page) mineral isochron ages were obtained for the Bixiling Complex (data source: Chavagnac & Jahn, 1996). All rocks have very similar metamorphic initial isotope ratios (ϵNd(T) = 0 to −2, ISr 0.704). The amphibolite has a much younger biotite age of 179 Ma and a very different ISr of 0.707, as a result of retrograde metamorphism.

Fig. 7. (b)

See previous page.

Fig. 7. (b)

See previous page.

Fig. 8.

Meaningful Sm-Nd mineral isochron ages obtained for an eclogite, a garnet websterite, and a garnet clinopyroxenite from the Maowu Complex of Dabieshan (data source: Jahn et al., 2003).

Fig. 8.

Meaningful Sm-Nd mineral isochron ages obtained for an eclogite, a garnet websterite, and a garnet clinopyroxenite from the Maowu Complex of Dabieshan (data source: Jahn et al., 2003).

Failure of producing correct ages

Excess Ar in phengitic mica of HP-UHP rocks have been frequently documented and it produced aberrant ages (e.g. Li et al., 1994, 2000; Jahn et al., 2001). Similarly, aberrant Sm-Nd mineral isochron ages have also been obtained, particularly for “low temperature” (= 600 °C) HP-UHP metamorphic rocks. The most notorious examples are from the Alps and the Himalayas (e.g. Luais et al., 2001; Thöni, 2002). Most recently, the same phenomenon became better known to eclogitic rocks of the Hong’an Block in the western part of the Dabie orogen (Jahn & Liu, 2002). The main cause for this chronometric problem is the lack of isotopic equilibrium between garnet and its coexisting minerals. Garnet is the major Al-carrying phase in eclogite and is believed to be transformed mainly from plagioclase, whereas omphacite is derived from magmatic pyroxenes. Of course, the detail of phase reactions leading to the formation of these two minerals is much more complicated and beyond the scope of this chapter.

Granting the simplified case, during the “low temperature” metamorphic transformation from plagioclase to garnet, the reconstitution of lattice-forming major elements may not be closely followed by REE’s due to their smaller diffusion coefficients. Isotopic equilibrium would not be expected to occur when chemicalequilibrium is not reached. In numerous cases, garnet crystals show major element zoning of prograde metamorphism. This is clear evidence for non-equilibrated growth zones. In this case, trace element and isotopic equilibrium is probably out of question.

The phenomenon of Sm-Nd isotopic equilibrium/disequilibrium is most dependent on the prime factor of temperature, but little on pressure as far as goes the present knowledge. Other factors, such as the intensity of deformation, remains to be further evaluated. The temperature effect leads to the concept of blocking temperature(TB; Dodson, 1973), which does not have a unique value but is also influenced by several other factors including mineral grain size, rate of cooling, duration of metamorphic reaction at temperatures higher than TB, presence of fluid phase and its composition, and penetrative deformation (e.g. Thöni, 2002). The literature data indicate that TB values range from 850 to 650 °C for garnet Sm-Nd isotope chronometer (Humphries & Cliff, 1982; Jagoutz, 1988; Mezger et al., 1992; Burton et al., 1995; Zhou & Hensen, 1995; Günther & Jagoutz, 1997; Thöni, 2002). Garnet out of isotopic equilibrium is often accompanied by non-equilibrated trace element patterns. In some cases, Sm/Nd ratios of garnets are so unusual that they are smaller than that of whole-rock samples; whereas in others, all co-existing minerals show little difference in Sm/Nd ratios so they form a cluster in an isochron diagram. In still other cases, garnet and omphacite may have established equilibrium Sm/Nd partition coefficients but not their Nd isotopic compositions, thus resulting in a negative slope “futurechron” relationship. This has been found in eclogites of Tso Morari of the Himalayas and the Sesia zone of western Alps (Duchêne et al., 1997; de Sigoyer et al., 2000; Luais et al., 2001), and the Hong’an Block of China (Jahn & Liu, 2002).

Porphyroblastic garnets often contain mineral inclusions. In addition to the celebrated coesite, many of them are REE-rich phases, such as monazite, zoisite, epidote, allanite, titanite, apatite, zircon etc. Except zircon, all these phases are highly enriched in LREE with very low Sm/Nd ratios. Thus, a tiny amount of such inclusions could significantly lower the Sm/Nd ratio in garnet, hence affecting an isochron construction based on Grt-Cpx-WR and other minerals. Inclusions must have undergone the same metamorphic P-T path as garnet, Cpx and other major phases. If the metamorphic temperature exceeds the TB of the inclusions (moderate TB minerals, such as epidote, apatite and titanite), the Nd isotopic compositions of inclusions could be expected to attain isotopic equilibrium with host garnet. In this case, a correct isochron age may be expected, but the range of data spread would be reduced, and hence the statistical error in age increased. This has been most frequently observed in the “high-temperature” eclogites of the Dabie and Su-Lu terranes. On the other hand, mineral inclusions of very high TB (e.g. zircon and monazite) may not be easily reset isotopically to keep pace with growing garnet. In this case, it may produce a disequilibrated isochron relationship.

In conclusion, the failure of producing a correct Sm-Nd isochron age is due to isotope disequilibrium between garnet and its coexisting minerals. The disequilibrium results from two distinct processes: (1) garnet preserves the isotopic composition of precursor mineral (plagioclase) due to the fact that the rate of Nd isotope exchange is more sluggish than that of chemical and mineral phase reconstitution; and (2) garnet contains high-temperature refractory LREE-rich inclusions, which have not been isotopically re-equilibrated with host garnet, though the garnet may have attained equilibrium with coexisting Cpx and other principal phases. The above disequilibrium does not include the open system behaviour occurring in retrograde metamorphism with strong influence of hydrothermal activity. As it will be discussed later, oxygen isotope study can provide a test of equilibrium or disequilibrium between Sm-Nd isochron minerals (Zheng et al., 2002).

An example from the Hong’an Block in western Dabieshan

The Hong’an Block exposes a series or high P/T metamorphic rocks, with a S–N distribution from blueschist/blueschist-greenschist, amphibolite, kyanite-free and kyanite-bearing eclogites to coesite-eclogite facies rocks. The Block has been considered to provide better archives for the understanding of the tectonic evolution of the Qinling-Dabie orogen for three main reasons (e.g. Eide & Liou, 2000): (1) it is least affected by the thermal and structural overprint imposed on much of the Dabie terranes during the voluminous Cretaceous granitoid intrusion; (2) HP eclogites are widespread in the Hong’an Block and they often preserve prograde metamorphism, and can be directly linked to the blueschist/blueschist-greenschist rocks; (3) in comparison with the Dabie terrane, the better exposure of blueschist/blueschist–greenschist facies rocks offer opportunities for simultaneous structural and metamorphic analysis.

Six basaltic eclogites were analysed. Two of them (Gaoqiao and Qiliping) come from the Hong’an Unit; they recorded the lowest temperatures (ca. 500 °C) and a pressure range of 16–20 kbar. The next two (Qianjinhepeng and Xuanhuadian) are from the Huwan Unit of the Sujiahe Group; their thermobarometric data are 620 °C (rim) and > 12 kbar for sample QJH01–1 and 680–700 °C and 14–18 kbar for XHD07–1. The last two were collected from within the coesite-bearing Xinxian Unit of the Dabie Group; their P-T conditions are ca. 640–680 °C and < 27 kbar. In general, the metamorphic temperatures are lower than those recorded in the Su-Lu and Dabie terranes at corresponding pressures (Zhang & Liou, 1994;, Eide & Liou, 2000). The eclogites from the Hong’an and Huwan units have often been referred to as “cold eclogites” and no coesite has been identified. Published white mica Ar–Ar analyses provide cooling ages from 225 to 205 Ma for the Hong’an block, and support the peak UHPM event at about 230 Ma (Eide et al., 1994; Webb et al., 1999; Eide & Liou, 2000).

Rb–Sr and Sm–Nd isotopic analyses on mineral separates (Fig. 9) yielded the following results. (1) Phengite-based Rb–Sr isochrons gave 225 ± 34 (2a) Ma for Qiliping, 212 ± 7 Ma for Tianpu, and 171 ± 17 Ma for another sample (P260) of the Xinxian Unit. The other three rocks yielded no age information as the data of coexisting minerals and WR are highly scattered. The ages of 225–212 Ma are comparable with many Ar/Ar ages obtained on a variety of rock types from the Hong’an Block (Eide et al., 1994; Webb et al., 1999; Eide & Liou, 2000). They can be interpreted as cooling ages. The meaning of the younger age of 171 Ma is not clear. (2) The pattern of Rb–Sr isotope disequilibrium is random and is independent of the petrological temperatures. (3) Sm–Nd data show even more disastrous chronological information. None of them gave meaningful ages. Negative isochron relationship is observed for two eclogites: one from the cold eclogite unit (Qiliping) and the other from the coesite-bearing unit (# P260). (4) Grt–Omp Sm–Nd tie-lines yielded “isochron ages” of 182 Ma (Gaoqiao), 260 Ma (Xuanhuadian), 420 Ma (Qianjinhepeng), and 1057 Ma (Tianpu), whereas Grt–WR tie-lines gave 218, 337, 250, and 820 Ma, respectively. Quite visibly, Devonian and Silurian ages, as claimed to exist by some workers, can be produced from such disequilibrated isotopic systems. (5) WR data points are always found to lie outside the Grt–Omp tie-lines. This indicates that isotopic compositions of coexisting minerals were not homogenised during the metamorphism and that WR must be mass-balanced by non-analysed accessory phases. (6) The 147Sm/144Nd ratios of garnets range from 0.17 to 0.32. This is much lower than the “normal” garnet values of 0.5 to 2.0. Besides, two garnets contain high Nd concentrations (3.5 and 6.8 ppm), which likely resulted from the presence of LREE-rich micro-inclusions (e.g. monazite minerals).

Fig. 9.

Isotopic disequilibrium in eclogites of the Hong’an Block of western Dabieshan. The Rb–Sr systems are shown in the left diagrams and the Sm–Nd systems in the right. The non-radiogenic Sr of phengite and dispersed data points for XHD–07–1 suggests a total disequilibrium despite of its high petrological temperature of 680–700 °C. Likewise, its Sm–Nd data yield grt–omp or grt–WR ages much higher than the supposed 220 Ma. For P 260, the Rb–Sr phengite age of ca. 170 Ma is too young, and a Sm–Nd “futurechron” is obtained. Garnet is completely out of equilibrium not only for its low 143Nd/144Nd ratio, but also for its unusually low 147Sm/144Nd (less than chondritic value). Sample TP03–2 and QLP08–1 appear to give “correct” Rb–Sr ages of 212 and 225 Ma, but no meaningful Sm–Nd ages were obtained.

Fig. 9.

Isotopic disequilibrium in eclogites of the Hong’an Block of western Dabieshan. The Rb–Sr systems are shown in the left diagrams and the Sm–Nd systems in the right. The non-radiogenic Sr of phengite and dispersed data points for XHD–07–1 suggests a total disequilibrium despite of its high petrological temperature of 680–700 °C. Likewise, its Sm–Nd data yield grt–omp or grt–WR ages much higher than the supposed 220 Ma. For P 260, the Rb–Sr phengite age of ca. 170 Ma is too young, and a Sm–Nd “futurechron” is obtained. Garnet is completely out of equilibrium not only for its low 143Nd/144Nd ratio, but also for its unusually low 147Sm/144Nd (less than chondritic value). Sample TP03–2 and QLP08–1 appear to give “correct” Rb–Sr ages of 212 and 225 Ma, but no meaningful Sm–Nd ages were obtained.

The above observations raise a serious question about the validity of the published Sm–Nd mineral isochron ages on the Hong’an Block. Such disequilibrated Sm–Nd systems are rarely encountered for eclogites from the Su–Lu and Dabie terranes (e.g. Li et al., 1993, 2000; Chavagnac & Jahn, 1996; Chavagnac et al., 2001; Jahn et al., 2003) except two localities (Yakou and Yangkou) of the Su–Lu terrane (Zheng et al., 2002). In the Hong’an Block, the Xiongdian eclogite (locality identical to our Xuanhuadian sample, N31°45.14’, E114°28.50’) has been subjected to several zircon U–Pb and garnet Sm–Nd geochronological investigations. However, all the published results failed to produce consistent and interpretable ages. Clearly, the isotopic systems (Sm–Nd and U–Pb) are largely out of equilibrium in these “cold eclogites”, and some of the apparent concordant zircon dates of 300 to 480 Ma could have been produced by partial recrystallisation effect (Pidgeon, 1992; Pidgeon et al., 1998; Hoskin & Black, 2000).

Radiogenic isotope tracers

The protoliths of metasedimentary rocks and metagranitoids are clearly of continental origin. However, those of mafic eclogites could be controversial with regard to their oceanic or continental heritage. Yet, eclogites are the most important rock type that provides the best evidence of UHP metamorphism. Subduction of the oceanic crust leads to progressive metamorphism and formation of eclogite when the plate descends beyond the hydrous curtain. If an oceanic eclogite happens to be exhumed by any tectonic process and melanged into a continental gneiss complex, then the presence of UHP minerals such as coesite in the eclogite is not sufficient to argue for a process of continental subduction. Formation of eclogite from a subducted oceanic lithosphere is a natural consequence of plate tectonics; it is nothing special. However, if protoliths of UHP eclogite can be proven to be of continental origin, then the case of continental subduction can be established. The long-standing controversy on the tectonic relationship, “in situ” vs. “foreign”, between eclogites and country gneisses very much hinges on the identification of oceanic or continental affinity of the protoliths. Broadly, eclogites produced by metamorphism of subducted oceanic crust and mantle eclogite enclaves in kimberlites do not imply continental subduction. Whereas UHP eclogites formed from the mafic components (greenschists, amphibolites, basic granulites) of an old and stable continent would adamantly argue for subduction of a continental block. The distinction of oceanic vs. continental origin for an eclogite must rely on the analyses of trace element and radiogenic isotope compositions.

Isotope tracer studies have made important contributions to the understanding of the processes involved in deep burial and exhumation of continental rocks. For instance, the very low oxygen isotope ratios of UHP rocks (eclogites and country rocks) from Su–Lu and Dabieshan have been taken to imply that these rocks were once exposed at the Earth’s surface, and the preservation of such low ratios indicate that the later deep subduction and following exhumation have not re-equilibrated the oxygen isotopes (Yui et al., 1995, 1997; Zheng et al., 1996, 1998, 1999; Baker et al., 1997; Rumble, 1998). On the other hand, very spectacular ultra-high ϵNd values of ca. +170 to +260 have been determined for six retrograded eclogites from Weihai (Su–Lu region; Jahn et al., 1996). These are the highest values ever measured in terrestrial rocks (not minerals). They require a very long time lapse for any reasonable Sm/Nd systems to develop, and such values allowed us to estimate rather precisely the protolith age of about 1.7 Ga (Jahn et al., 1996). The preservation of extraordinary low δ18O and extremely high ϵNd values in UHP rocks indicates that a pervasive fluid phase was absent and the rate of subduction and exhumation was too fast to re-equilibrate oxygen isotopes (Zheng et al., 1998, 1999) and to rehomogenise the extreme Nd isotope compositions. Furthermore, isotope tracer studies also provide information on the behaviour of various elements and isotopes in rocks that have undergone UHP metamorphism. Until present, such behaviours are not well understood.

A rather comprehensive survey of whole-rock Nd isotopic compositions of eclogites from worldwide occurrences was presented by Jahn (1999). A typical diagram to be presented is shown to have Nd isotopic composition, expressed as ϵNd value, as the ordinate, and 147Sm/144Nd ratio as the abscissa, which is roughly equivalent to the chemical nature of rocks (Fig. 10). 147Sm/144Nd ratios = 0.20 (or chondritic value of 0.1967) imply light rare earth element (LREE) depletion, whereas lower ratios indicate LREE enrichment. In each set of data, two companion plots of ϵNd(0) vs. 147Sm/144Nd and ϵNd(T) vs. 147Sm/144Nd are presented. The ϵNi(T) values in most cases are calculated based on the metamorphic ages, hence they are “metamorphic initial ratios”. The time interval (ΔT) between the protolith formation and metamorphism are not known in most cases. True ϵNd(T) for the protoliths require further correction based on ΔT values.

Fig. 10.

ϵNd(0) vs. 147Sm/144Nd plots (left) and ϵNd(T) vs. 147Sm/144Nd plots (right) for eclogites and garnet peridotites from the Alpine, Hercynian and Caledonian belts. Data source: see Appendices of Jahn (1999). For the Alpine belt, except the UHP metasediments of Dora-Maira, all the rocks show positive ϵNd(T) values. Most eclogites and garnet peridotites from the Hercynian belt show positive ϵNd(T) values, suggesting their oceanic affinity. Those having negative values have been interpreted as due to crustal contamination by reinjection of crustal rocks (oceanic clays) into the upper mantle. As for the eclogites and garnet peridotites from the European Caledonides, the majority of the rocks show negative ϵNd(T) values, in strong contrast to those of the Alpine and Hercynian chains. These rocks appear to have resided in the continental crust for a lengthy period of time before their UHP metamorphism.

Fig. 10.

ϵNd(0) vs. 147Sm/144Nd plots (left) and ϵNd(T) vs. 147Sm/144Nd plots (right) for eclogites and garnet peridotites from the Alpine, Hercynian and Caledonian belts. Data source: see Appendices of Jahn (1999). For the Alpine belt, except the UHP metasediments of Dora-Maira, all the rocks show positive ϵNd(T) values. Most eclogites and garnet peridotites from the Hercynian belt show positive ϵNd(T) values, suggesting their oceanic affinity. Those having negative values have been interpreted as due to crustal contamination by reinjection of crustal rocks (oceanic clays) into the upper mantle. As for the eclogites and garnet peridotites from the European Caledonides, the majority of the rocks show negative ϵNd(T) values, in strong contrast to those of the Alpine and Hercynian chains. These rocks appear to have resided in the continental crust for a lengthy period of time before their UHP metamorphism.

Figures 10 and 11 present the isotopic characteristics of eclogites from five classic orogenic belts: Alpine, Hercynian, Caledonian, and Dabie–Su–Lu. Some interesting observations:

Fig. 11.

ϵNd(0) vs. 147Sm/144Nd plots (left) and ϵNd(T) vs. 147Sm/144Nd plots (right) for eclogites from the Su–Lu and Dabie terranes of China. All types I & II eclogites (gneiss and marble-hosted) are characterised by negative ϵNd(T) values, suggesting their continental affinity. The data of the Weihai eclogites are out of the scale, and their ϵnd and 147Sm/144Nd ratios are given in the parentheses. Data sources: see Appendices of Jahn (1999).

Fig. 11.

ϵNd(0) vs. 147Sm/144Nd plots (left) and ϵNd(T) vs. 147Sm/144Nd plots (right) for eclogites from the Su–Lu and Dabie terranes of China. All types I & II eclogites (gneiss and marble-hosted) are characterised by negative ϵNd(T) values, suggesting their continental affinity. The data of the Weihai eclogites are out of the scale, and their ϵnd and 147Sm/144Nd ratios are given in the parentheses. Data sources: see Appendices of Jahn (1999).

  1. 1.

    The most celebrated UHP rocks, jadeite-kyanite or garnet quartzites (also termed as whiteschists) of Dora-Maira, are shown to have negative ϵNd(T) values of −5 to −8 (Figs. 10a, b). This is consistent with their ultimate derivation from middle Proterozoic continental crust as estimated from their TDM ages of 1.3 to 1.8 Ga (Tilton et al., 1989, 1991). Except these metasediments, the entire population of eclogites from the Alpine chain possess positive ϵNd(T) values, suggesting that all of them originated in subducted oceanic crust, or directly from the depleted upper mantle such as the garnet peridotites of the Alpe Arami (Becker, 1993). This conclusion is largely supported by available geochemical and petrological data (Paquette et al., 1989; Thöni & Jagoutz, 1992; Miller & Thöni, 1995; Dobrzhinetskaya et al., 1996; von Quadt et al., 1997).

  2. 2.

    In the Hercynian belt, most eclogites possess positive ϵNd(T) values (Figs. 10c, d), suggesting their dominant oceanic derivation (Bernard-Griffiths & Cornichet, 1985; Bernard-Griffiths et al., 1985; Stosch & Lugmair, 1990; Beard et al., 1992, 1995; Medaris et al., 1995). One eclogite from Münchberg and two from Schwarzwald with low ϵNd(T) values of about 0 to −3 are due probably to crustal contamination (Stosch & Lugmair, 1990; Kalt et al., 1994). A few eclogites from the Bohemian Massif also show negative ϵNd(T) values (−3 to −6, Medaris et al., 1995), but such isotopic signature was interpreted as due to involvement of oceanic clay in the melting of subducted oceanic lithosphere, and the eclogites were considered to represent high-pressure cumulate of the “enriched” basaltic liquid (Medaris et al., 1995). In any case, the oceanic crust origin of the eclogites from the Hercynian belt is clearly demonstrated, and the negative ϵNd(T) values do not imply a deep subduction of the continental crust.

  3. 3.

    Eclogites from the Caledonides have a much more complex evolution. Two eclogites from NW Scotland (Sanders et al., 1984) and over half of the UHP rocks (including about equal proportions of eclogites and garnet pyroxenites; Griffin & Brueckner, 1980, 1985; M0rk & Mearns, 1986; Mearns, 1986; Jamtveit et al., 1991) show negative ϵNi(T) values down to as low as −10 (Figs. 10e, f). It appears that the Norwegian and Scottish eclogites have both oceanic and “continental” affinities. It merits a further explanation about the meaning of “continental” affinity. From the compositional point of view, all the analysed rocks (eclogites, garnet pyroxenites and garnet peridotites) are clearly of mantle origin. Some, if not all, of them were emplaced into the continental crust in Precambrian time(s) and became an integral part of the continental crust since then. This represents the first stage of evolution. However, the process of the first stage emplacement could not be precisely determined, as it is a problem apart, a controversy about the styles of the early Precambrian tectonics. Note that many eclogites have 147Sm/144Nd ratios lower than the chondritic values of 0.1967, suggesting that they were likely emplaced first as LREE-enriched continental basalts, later metamorphosed to amphibolites, and finally became an integral part of the continental crust. This further implies that an ancient crustal segment was subducted to mantle depths and UHP metamorphic assemblages were produced during the Caledonian orogeny. In the Western Gneiss Region of Norway, eclogites recorded only the Caledonian thermal event of 410–440 Ma (Griffin & Brueckner, 1980, 1985; Mork & Mearns, 1986; Mearns, 1986; Carswell & Cuthbert, 2003), whereas garnet peridotites and garnet pyroxenites often yielded a Proterozoic event of about 1.7 Ga (Jamtveit et al., 1991). It is possible that the isotopic systems registered at 1.7 Ga in garnet peridotites at mantle P–T conditions have not been erased or homogenised during the Caledonian event. It is equally possible that these garnets were not isotopically equilibrated hence the ages of 1.7 to 1.0 Ga render no strict geological significance. Nevertheless, newly formed garnets from basaltic protoliths appear to have faithfully recorded the Caledonian orogeny at 410–400 Ma. Thus, the evolution of the European Caledonides is clearly distinguished from that of the Alpine and Hercynian chains of western Europe.

  4. 4.

    The Dabie and Su–Lu terranes show quite a distinguished “continental” isotopic signature (Fig. 11). Eclogites occurring as enclaves or blocks in granitic gneisses have low ϵNd(T) values (−6 to −20) at the time of peak metamorphism at ca. 220–230 Ma. These values are among the lowest of the known eclogites and are significantly different from those of the Alpine and Hercynian chains in western Europe. Moreover, most rocks have 147Sm/144Nd ratios less than the chondritic value, suggesting their LREE-enriched geochemical characteristics. To some extent, only eclogites from the Caledonian belt (e.g. NW Scotland and western Norway) have Sm–Nd isotopic compositions comparable with those of the Dabie Orogen. The isotopic data and geochemical characteristics undoubtedly indicate that the eclogite protoliths of the Dabie Orogen have the “continental” affinity and could not be produced from the subducted Tethyan oceanic slab. The low ϵNd(T) values further require the protoliths of these eclogites to have formed long before the Triassic collision, probably in middle to late Proterozoic times. It appears that, in addition to the UHP metasedimentary rocks of Dora-Maira, Dabieshan (including Su–Lu) and the Western Gneiss Region (WGR) are two regions where coesite and diamond-bearing eclogites and ultramafic rocks show clear evidence of subduction of ancient and cold continental crustal blocks to mantle depths.

The eclogite-ultramafic suites in the Su–Lu and Dabie terranes have two different tectonic origins. The first suite, represented by the Bixiling and Maowu complexes, comprises layered intrusions initially emplaced in crustal levels and later subjected to UHP metamorphism as a result of continental subduction (Okay, 1994; Zhang et al., 1995a; Chavagnac & Jahn, 1996; Fan et al., 1996; Liou et al., 1996, 1997). The second suite is made up of mantle rocks exhumed together with crustal UHP metamorphic rocks; they are considered here as “tectonic enclaves of mantle origin” within granitic gneisses. A probable example is the garnet pyroxenites of Rizhao. Even though these pyroxenites represent igneous cumulate rocks from a mantle derived liquid, the magma differentiation probably occurred within the mantle. The isotopic systematics of the Maowu layered intrusion (ϵNd(T) ≈ −5 to −6, ISr ≈ 0.707–0.708; Jahn et al., 2003) are distinguished from that of the Bixiling complex. Upper crustal contamination during magma chamber processes is likely to have occurred in the Maowu intrusion (Jahn et al., 2003), whereas the Bixiling complex has undergone an AFC process in the lower crustal condition (Chavagnac & Jahn, 1996; Li, 2003).

In conclusion, basaltic (or gabbroic) eclogites of continental origin are best shown in the Dabie and Su–Lu terranes in east-central China and the Western Gneiss Region of Norway. Their identification provides strong evidence for subduction of large blocks of continental crust. On the other hand, most eclogites from the Alps and Hercynian belts are of oceanic origin and many have a MORB-like affinity. Their occurrence is not sufficient to indicate that the eclogite-bearing gneiss terranes have also been subducted to mantle depths, unless it can be proven that the oceanic rocks were tectonically emplaced in a continental setting, and the ensemble was subducted and metamorphosed at great depths. Nevertheless, the evidence for continental subduction in the western Alps comes from the coesite-bearing metasedimentary rocks of Dora-Maira.

IV. Oxygen isotope tracer

Two major contributions have been made by studies of oxygen isotope compositions of UHP rocks and constituent minerals, particularly those from the Dabie and Su–Lu terranes. (1) Revelation of strong water-rock interactions prior to the Triassic subduction; the aqueous fluid was extremely depleted in 18O and the preservation of such low δ18O isotopic signature implies a very low water mobility during the entire process of subduction and exhumation (Yui et al., 1995, 1997; Baker et al., 1997; Rumble, 1998; Rumble et al., 2002; Zheng et al., 1996, 1998, 1999; Fu et al., 1999). In fact, this has been used as a palaeoclimatic proxy, tracing the Neoproterozoic palaeoenvironment (e.g. Rumble, 1998; Rumble et al., 2002, 2003; Zheng et al., 2003). (2) Better understanding of O isotope fractionation between UHP minerals, and identification of isotopic disequilibrium in O and Nd, and hence validation of the Sm–Nd isochron chronometer (Zheng et al., 2002).

δ18O values of eclogites from UHP metamorphic terranes – a summary

Figure 12 summarises the oxygen isotope compositions of the principal eclogitic minerals from the Dabie and Su–Lu UHP terranes. Additional data from mantle xenoliths and eclogite facies rocks of Dora-Maira (Alps) and the Western Gneiss Region (Norway), as well as common metamorphic quartz are also shown for reference. In general, crustal rocks have a wide range of δ18O values but the majority of them are greater than zero. Mantle peridotites and mantle-derived basalts have rather homogeneous values between +5 and +6%o. Seawater is defined to have both δ18O and ϵD equal to zero. Meteoric waters (rain, groundwater, snow, ice etc.) have negative δ18O and δD values. A comprehensive review of published δ18O values of quartz from metamorphic rocks worldwide by Sharp et al. (1993) shows that the lower limit of metamorphic quartz is +7%o, and the entire range extends to about +30%o (Fig. 12). The discovery of very low δ18O values for garnet and omphacite (ca. −10%o) and quartz (−7%o) in coesite-bearing eclogites of Qinglongshan, China, was most surprising because they are among the lowest ever recorded in any high-temperature rocks (igneous or metamorphic; Yui et al., 1995; Zheng et al., 1996; Blattner et al., 1997). Zircons from granitic gneisses of Qinglongshan have also recorded low δ18O values, from 0 to −7.5%o (Rumble et al., 2002). Garnets and omphacites from many localities of Dabieshan also show negative values, though they are not as depleted in 18O as in Qinglongshan eclogites. Unusually low δD values of−113 to −124%o (VSMOW) were found in phengites of Qinglongshan eclogite and quartzite (Rumble & Yui, 1998). The SD values are not as spectacular as the low δ18O, but they are at the low end of the total range of natural variation in metamorphic micas (Fig. 8 of Sharp et al., 1993). Masago et al. (2003) reported negative δ18O values (−3.9%o) for minerals in eclogite and whiteschist of the Kokchetav Massif in Kazakhstan. The Kokchetav Massif is the second recognised UHP region that preserves a significant effect of such water-rock interaction prior to subduction.

Fig. 12.

Range of δ18O values for metamorphic minerals and rocks. Modified after Rumble (1998) with additional data of Zheng et al. (1998, 1999). The Qinglongshan data show the lowest delta18O values (~ −10%o) for any magmatic and metamorphic rocks. For eclogites from other localities (Shuanghe, Bixiling, Huangzhen and Maowu), all minerals seem to have low δ18O values about 0%o, but preserved equilibrium high-temperature isotope fractionation with the order of quartz > omphacite > garnet > rutile.

Fig. 12.

Range of δ18O values for metamorphic minerals and rocks. Modified after Rumble (1998) with additional data of Zheng et al. (1998, 1999). The Qinglongshan data show the lowest delta18O values (~ −10%o) for any magmatic and metamorphic rocks. For eclogites from other localities (Shuanghe, Bixiling, Huangzhen and Maowu), all minerals seem to have low δ18O values about 0%o, but preserved equilibrium high-temperature isotope fractionation with the order of quartz > omphacite > garnet > rutile.

What do these oxygen isotopic data tell us? Many recent studies on the Dabie and Su–Lu terranes seem to have reached the following conclusions: (1) the protoliths of eclogites and associated gneisses were subjected to hydrothermal alteration involving very light meteoric waters prior to Triassic subduction; (2) the preservation of such distinctive isotopic signature for both eclogites and their host gneisses supports the hypothesis of in situ tectonic relationship between them, with a scale of structural coherence of at least 100 km; (3) the persistence of pre-metamorphic isotopic distinction for the UHPM rocks implies a very limited H2O activity, hence no or little pervasive fluid free to infiltrate the rocks that have undergone subduction, UHP metamorphism and exhumation; and (4) the preservation of high-temperature equilibrium oxygen isotope fractionation between constituent minerals, as shown by the same descending order in δ18O from quartz, omphacite/garnet, to rutile (Fig. 12), also indicates a low fluid activity so that the high-temperature isotopic fractionation was not re-equilibrated after the peak metamorphism (see also below). The above points are also in support of a high rate of subduction and exhumation of these UHP terranes. However, this generality does not exclude some localised high fluid activity leading to the formation of kyanite-bearing quartz veins in Dabieshan (Li et al., 2001) and hydrous mineral veins (e.g. epidote) in eclogites of Qinglongshan (Li, 2003).

Limited fluid activity

Fluids of high pressure (HP) and UHP metamorphic rocks are known to exist as intergranular phases, and, indeed, must be present to facilitate the attainment of mm to cm-scale element and stable isotope exchange equilibrium between adjacent mineral grains during prograde metamorphism (Philippot & Rumble, 2000; Scambelluri & Phillipot, 2001). Fluids participate in many metamorphic reactions; they act as catalyst in solid-solid reactions, lead to compositional change (metasomatism) and control rheological properties. In the Dabie and Su–Lu UHP terranes, deeply subducted rocks contain minor fluids in hydrous and carbonate phases, there is little evidence for H2O to be a separate phase (e.g. Liou et al., 1997). In such fluid-deficient environments, metamorphic reaction rates are severely retarded, and UHP index minerals and geochemical signature of the protoliths may be preserved.

Fluid mobility is limited during prograde UHP metamorphism. Owing to the diversity of protoliths undergoing subduction, there is ample textural, chemical, and isotopic evidence of fluid behaviours. Evidence of limited fluid mobility during prograde HP and UHP metamorphism is provided by the failure of rocks to attain textural equilibrium. Preservation of gabbroic textures and primary igneous minerals including plagioclase, orthopyroxene and biotite in the core of a 3 m coesite-bearing eclogite block from Su–Lu (Zhang & Liou, 1997) and the occurrence of intergranular coesite (Liou & Zhang, 1996) suggest a lack of fluid to facilitate equilibration. Similarly, cover-basement relationships in the Dabie UHP terrane, which in part preserve a primary unconformity, pillow structures, and mineral fabrics, have been suggested (Dong et al., 2002; Oberhänsli et al., 2002). In addition, a magmatic mingling or partial melting texture has been preserved in a Neoproterozoic bimodal suite in the margin of the Bixiling Complex.

Evidence of limited fluid mobility during prograde HP and UHP metamorphism is also seen in the failure of dissimilar protoliths to achieve stable isotope equilibrium on mm, cm, and m scales despite extreme metamorphic conditions (Rumble & Yui, 1998; Baker et al., 1997; Zheng et al., 1998, 1999; Fu et al., 1999; Philippot & Selverstone, 1991; Selverstone et al., 1992; Getty & Selverstone, 1994; Nadeau et al., 1993; Fruh-Green, 1994; Barnicoat & Cartwright, 1997; Miller et al., 2001). In fact, the failure to equilibrate confers an advantage to unravel the geologic history of HP and UHP rocks because remnants of pre-metamorphic stable isotopic signatures, diagnostic of protolith origins, may have survived. Vestiges of distinctive Earth surface environments and processes have been preserved in HP and UHP metamorphic rocks, and they greatly facilitate the interpretation of the geodynamic cycle of continental collision (Rumble, 1998; Rumble et al., 2003; Zheng et al., 2003). In the European Alps, stable isotopic evidence of ocean floor and ophiolitic hydrothermal alteration has been observed in HP eclogites (Philippot et al., 1998; Barnicoat & Cartwright, 1997; Miller et al., 2001; Scambelluri & Philippot, 2001). A record of Neoproterozoic surface conditions has been found in Chinese UHP rocks including low δ18O and δD values indicating a cold climate (Yui et al., 1995; Zheng et al., 1996, 1998, 1999, 2003; Baker et al., 1997; Rumble & Yui, 1998; Fu et al., 1999). Dabieshan marbles carry the high δ13C values of carbonate sediments that typically accompany Neoproterozoic tillites (Yui et al., 1997; Baker et al., 1997; Zheng et al., 1998; Rumble et al., 2000). New U–Pb dating and δ18O analyses of zircons from orthogneisses show that a cold-climate geothermal system covering hundreds of square kilometres existed during the Neoproterozoic, consistent with snowball Earth conditions (Rumble et al., 2002). The limited fluid activity is indicated not only by stable isotope data but also by fluid inclusion studies. Low salinity, primary fluid inclusions reported from low δ18O eclogites at Qinglongshan, China, may be samples of Neoproterozoic meteoric water that survived subduction and exhumation (Fu et al., 2002).

Preservation of equilibrated high-temperature isotope fractionation

Oxygen isotopic compositions of eclogite minerals have attained isotope equilibrium at their eclogite facies temperatures. The equilibrium is indicated by the fractionation between individual minerals, as expressed by ∆ values (δ18OA – δ18OB). As shown in Figure 12, the sequence of 18O enrichment in eclogite minerals is consistent with the empirically, experimentally and theoretically determined values for equilibrium fractionation (see references cited in Zheng et al., 1998). In fact, calculated oxygen isotope temperatures have been shown to be quite comparable with petrological temperatures (Yui et al., 1995; Zheng et al., 1998, 1999). Figure 13 (Zheng et al., 1998) shows that for the Shuanghe eclogites of Dabieshan (solid circles), oxygen geothermometry yields 545–735 °C for quartz-garnet pairs and 555–680°C for quartz-omphacite pairs. These temperatures have ca. 30–50 °C of uncertainty due to analytical error and fractionation curve calibration. Generally, they are slightly lower than, but close to, the petrological temperatures of 600–750 °C (Okay, 1993; Wang et al., 1995; Cong, 1996; Cong et al., 1995). Similarly, for the Donghai eclogites (Qinglongshan included; solid triangles), the oxygen geothermometry gives 650–765 °C for quartz-garnet pairs, 655–760 °C for quartz-omphacite pairs, 620–755 °C for quartz-phengite pairs, and 665–755 °C for quartz-kyanite pairs (Yui et al., 1995; Zheng et al., 1998). These temperatures are also slightly lower than, or pretty close to, the petrological temperatures of 700–850 °C (Hirajima et al., 1990, 1992; Zhang et al., 1995b). In both cases, the peak metamorphic oxygen isotope equilibrium has been preserved and this suggests little isotope resetting during later exhumation. This, in turn, argues that water-rock interaction, which resulted in low δ18O values in eclogites as mentioned above, must have taken place before the UHP metamorphism, and most likely, soon after the protoliths were formed about 700–800 Ma ago.

Fig. 13.

Plots of δ18O values of quartz vs. coexisting minerals in eclogites from Shuanghe in the Dabie Mountains (solid dots) and Donghai of the Su-Lu terrane (solid triangles). Equilibrium isotope fractionation lines at different temperatures are shown for reference (after Zheng et al., 1998).

Fig. 13.

Plots of δ18O values of quartz vs. coexisting minerals in eclogites from Shuanghe in the Dabie Mountains (solid dots) and Donghai of the Su-Lu terrane (solid triangles). Equilibrium isotope fractionation lines at different temperatures are shown for reference (after Zheng et al., 1998).

Figure 14 illustrates oxygen isotope fractionation values (∆18O) between garnet and omphacite vs. δ18O of garnet in eclogites from numerous localities of the Su–Lu and Dabie terranes. The range of isotope equilibrium fractionation at eclogitic temperatures (∆18O = 0 to 2) is shown by grey area for reference. Garnet covers a wide range of δ18O values from +8 to −10, but the majority of eclogites (70%) fall in the area of isotope equilibrium. Other rocks of positive fractionation (A = 2.2) are out of equilibrium, whereas many eclogites (ca. 30%) show negative or reverse fractionation, resulting in quartz-omphacite isotope temperatures lower than 400 °C (Zheng et al., 1999). This phenomenon of disequilibrium has been ascribed to retrograde hydration reactions, in which the responsible fluids were probably derived from the exsolution of dissolved hydroxyl in UHP metamorphic minerals (Zheng et al., 1999), or the fluids that had equilibrated with low δ18O country gneisses (Yui et al., 1997; Zheng et al., 1999).

Fig. 14.

Oxygen isotope fractionation between omphacite and garnet (∆;18O) vs. δ18O values of garnet from eclogites of the Dabie and Su–Lu terranes. The reference lines of ∆18O = 0 and 2%o represent the limit of equilibrium fractionation between these two minerals. Data outside of this range are out of equilibrium. Data sources: Yui et al. (1997), Zhang et al. (1998), Zheng et al. (1998, 1999).

Fig. 14.

Oxygen isotope fractionation between omphacite and garnet (∆;18O) vs. δ18O values of garnet from eclogites of the Dabie and Su–Lu terranes. The reference lines of ∆18O = 0 and 2%o represent the limit of equilibrium fractionation between these two minerals. Data outside of this range are out of equilibrium. Data sources: Yui et al. (1997), Zhang et al. (1998), Zheng et al. (1998, 1999).

Garnet is very resistant to oxygen isotope exchange during cooling. Consequently, it has presumably preserved original δ18O values acquired before or during the eclogite facies metamorphism. The large range of δ18O in garnet shown in Figure 14 roughly correspond to the range of whole-rock values, and it can be explained by three possible processes (Zheng et al., 1998): (1) inhomogeneous water-rock interaction prior to the UHP metamorphism; (2) isotope exchange with crustal fluids during subduction (prograde metamorphism) and/or exhumation (retrograde metamorphism); and (3) heterogenous crust-mantle interaction during the residence of subducted rocks at mantle depths. Of the three listed possibilities, water-rock interaction prior to or during subduction is the most likely explanation for the heterogeneity of garnet δ18O values. Possibility (3) may be excluded because there is no evidence of oxygen isotope exchange between mantle peridotites and their UHP wall rocks (Zhang et al., 2000). As for possibility (2), retrograde metamorphism could exert a significant isotope exchange (both stable and radiogenic) in clinopyroxenes (Xie et al., 2003), but the maintenance of high-temperature garnet-quartz 18O/16O fractionation indicates that the range in oxygen isotope composition in garnet has little connection with retrograde metamorphism.

Conclusions from oxygen isotope tracer studies

Oxygen isotope tracer studies have provided the following important conclusions. (1) Garnet, and hence bulk rock, of UHP eclogites from the Dabie orogen shows a very large variation in oxygen isotope composition (δ18O = −10 to +8%o), with the majority lower than +6%o, i.e. the presumed value of pristine mantle-derived basaltic rocks. Such variation is considered to have been inherited from the protoliths as a result of uneven water-rock interaction, in which the hydrothermal system was charged with meteoric water from a cold climate. Rumble et al. (2002) suggested that the Qinglongshan’s cold climate could be a manifestation of Neoproterozoic “snowball” Earth. (2) Despite the large variation in oxygen isotope composition of the protoliths, most rocks show equilibrium isotope fractionation between constituent minerals. The preservation of protolith isotope signatures implies little hydrous fluid activity during subduction. Thus, the lowering of δ18O values must have taken place before the subduction. On the other hand, the safeguarding of equilibrium isotope fractionation of eclogite facies temperatures equally indicate the absence of pervasive fluid activity during exhumation. However, channelised fluid flows were present locally as witnessed by the formation of kyanite-bearing quartz vein at Huangzhen and epidote-rich veins at Qinglongshan. (3) The presence of disequilibrium isotope fractionations in some eclogites indicates the effect of retrograde metamorphism (re-hydration reactions). The responsible fluids were probably equilibrated with low-δ18O country gneisses (Yui et al., 1997) or came from structural hydroxyls originally bound to anhydrous minerals (Zheng et al., 1999).

Coupled Nd and O isotopic disequilibrium

One of the most interesting results in isotopic studies of UHP eclogites is the demonstration of a direct correspondence in equilibrium (or disequilibrium) state between the Sm-Nd and O isotopic systems in eclogitic minerals (Zheng et al., 2002). The state of O isotope equilibrium may provide a critical test for the validity of the Sm–Nd mineral isochron ages. Zheng et al. (2002) analysed O and Sm–Nd isotopic compositions for four eclogites from southern Su–Lu in China. In the first two cases where Omp-Grt oxygen isotope fractionation is normal (∆ = 1.4 to 0.8%o at 600 to 900 °C; Zheng, 1993) and produces oxygen isotope temperatures compatible with those estimated from petrological geothermometry, their Sm–Nd isotope data also yield reasonable isochron ages of about 220 Ma. By contrast, the third eclogite from Yakou shows an Omp–Grt oxygen isotope temperature of 545 °C, much lower than the petrological temperature of 700–800 °C. The low temperature is likely due to isotope re-equilibration during retrograde metamorphism at amphibolite facies conditions. Its Grt and Omp Sm–Nd systems produced an aberrant age of 280 Ma. The fourth eclogite from Yangkou, near Qingdao city, shows a reversed Omp–Grt isotope fractionation (∆ = −0.5%o), clearly indicating oxygen isotope disequilibrium. In this case, the Sm–Nd systematics of Grt, Omp, Phg and WR fail to yield any isochron relationship. The correspondence of equilibrium state between the two isotope systems (Sm–Nd and O) is illustrated by Figure 15 (Fig. 5 of Zheng et al., 2002).

Fig. 15.

Relationships between the Nd and O isotope ratios of garnet and omphacite from eclogites of the Su–Lu terrane (data and diagram from Zheng et al., 2002).

Fig. 15.

Relationships between the Nd and O isotope ratios of garnet and omphacite from eclogites of the Su–Lu terrane (data and diagram from Zheng et al., 2002).

Interpretation of such a correspondence is similar to that for the correlation of equilibrium states between the chemical (trace element) and Nd isotope compositions as discussed earlier. As explained by Zheng et al. (2002), the transformation of a basaltic rock to an eclogite may be expressed by a simplified reaction: Pl + Diop = Grt + Q + Omp. In this reaction, the Si2O6 structural unit in diopside is conveyed to omphacite, whereas the Si3O8 and Si2O8 units in plagioclase are reorganised to form SiO4 and SiO2 in garnet and quartz, respectively. In an incomplete isotope exchange as observed in the Yangkou eclogite, Grt inherits the oxygen isotope signature of the plagioclase structural units and thus has higher δ18O than coexisting Omp, whereas Omp inherits the oxygen isotope composition of the Si2O6 structural unit in its precursor diopside with little change in δ18O value.

This work and subsequent studies (Zheng et al., 2002) provide an additional insight into the kinetics of isotopic disequilibrium. Aberrant Sm–Nd isochron ages are shown to be accompanied by oxygen isotope disequilibrium. Based on the literature data (see summary by Zheng et al., 2002), the rates of O diffusion in Grt and Omp are lower than, or close to, those of Nd diffusion at the same temperatures. Consequently, an attainment of O isotopic equilibrium in the Grt–Omp pair suggests that Nd isotopic compositions are homogenised at metamorphic temperatures. Aberrant Sm–Nd isochron ages, or no ages at all, are expected to show disequilibrated oxygen isotope fractionation. This is illustrated by the Hong’an eclogites (Jahn & Liu, 2002).

V. Application of isotope constraints to tectonic evolution – example of the Dabie orogen

This section is here to show how radiogenic isotopes can be used to constrain the tectonic evolution of the Dabie orogen. Some of the opinions, conclusions and implications reached in the followings are not necessarily shared by co-authors Rumble and Liou.

It is generally agreed that the Qinling-Dabie Orogen was produced by a Triassic collision between two Precambrian cratons in China. The traditional and unchallenged concept about the pre-collisional plate movement is that the Yangtze craton was subducted northward underneath the Sino-Korean craton, and the exhumed UHP metamorphic rocks and subjacent “basement” gneisses and derivative Cretaceous granitoids represent part of the Yangtze craton. At present, the overwhelming arguments (geological, structural, and oxygen isotopes) are in favour of northward subduction of the Yangtze craton. However, our Sr-Nd-Pb isotopic tracer analysis indicates that the Cretaceous granitoids and mafic-ultramafic rocks of Dabieshan have a very close affinity with the Sino-Korean craton, and are quite distinguished from the Yangtze craton. This may suggest an opposite polarity of the Triassic subduction, and has a drastic consequence for all tectonic models.

Despite some consensus, the tectonic evolution of the Dabie orogen has been controversial. While most agree that the UHP terrane (= Southern Dabie Complex or SDC) represents a “thin” slice exhumed from great depths (= 100 km), the Northern Dabie Complex (NDC) has been variously considered as a migmatite terrane originally presented as the upper part of subducted plate (Maruyama et al., 1994; Ernst & Liou, 1995), as a high-temperature part of a vertical extrusion complex which includes UHP and HP rocks (Hacker et al., 1996), as a high-temperature metamorphic terrane formed in the hanging wall of the Sino-Korean craton, intruded by crustal melts and mafic-ultramafic rocks (Zhang et al., 1996a), as a magmatic complex created during the Cretaceous N–S extension (Hacker et al., 1998), as a migmatitic dome (= core complex) formed by decompressional partial melting of an exhuming slice, soon after the plate collision but not in the Cretaceous (Faure et al., 1999), or as an asymmetric Cordilleran-type extensional complex (= core complex) formed in the Cretaceous (Hacker & Peacock, 1995; Ratschbacher et al., 2000). Because the present orogenic architecture of Dabieshan is dominated by late Mesozoic structures, it is important to determine the origin of the gneiss terranes of the NDC as well as the Cretaceous mafic and granitic intrusions. Mafic magmas provide information about the characteristics of their upper mantle source(s), whereas granitic magmas reveal their source characteristics at lower to middle crustal levels. Here we employ a geochemical and Sr–Nd–Pb isotopic tracer technique to characterise the “source rocks” beneath Dabieshan and unravel the possible genetic relationship between different terranes and lithologic units. The result of the isotope tracer analysis reveals a severe problem to the traditionally held northward subduction and tectonic evolution of the Dabie orogen (e.g. Jahn et al., 2001b).

Lithological and geochemical characteristics of the NDC and SDC gneisses and Cretaceous intrusions

The Northern Dabie Complex (NDC) consists dominantly of granitic gneisses and subordinate migmatite, amphibolite, garnet granulite, marble and some conspicuous trains of mafic-ultramafic rocks. The presence of mafic granulites at several localities suggests that the NDC may have reached the granulite facies metamorphism and was strongly overprinted by the amphibolite facies and a later thermal event when massive Cretaceous granites were emplaced. The granitic gneisses of the NDC cover a wide range of chemical compositions (SiO2 = 53–80%). Grey gneisses with TTG composition appear to dominate over gneisses of granitic composition. In the TAS diagram of Middlemost (1994), the NDC gneisses plot in the fields following the sub-alkaline trend of intrusive rocks.

Quartzofeldspathic gneisses of the ultrahigh-pressure SDC have been separated into paragneiss and orthogneiss based on their mineral assemblages (e.g. Zhang et al., 1996b; Carswell et al., 1997). The paragneisses (Pl + Qtz + Phe + Bt + Ep ± Gt ± Ttn ± Rt) are intimately associated with eclogites, often interlayered with marble, phengitic schists and jadeitic quartzite, and have undergone UHP metamorphism. The orthogneisses (Pl + Kf+ Qtz + Mica + Ep ± Grt ± Ttn ± Am) are granitic and trondhjemitic; they are distinguished from paragneisses by the absence of rutile and presence of K-feldspar. Zircon dating revealed that most protoliths of the gneisses were formed in the Late Proterozoic (ca. 700–800 Ma; Ames et al., 1996; Rowley et al., 1997; Hacker et al., 1998; Zheng et al., 2003; Li, 2003), but a few, such as those occurring at Shuanghe, have definitely much older Late Archaean ages (Chavagnac et al., 2001; Ayers et al., 2002). Together with granulites from the Luotian gneiss dome and trondhjemitic gneisses and metapelites of the Kongling group (zircon ages up to 3.2 Ga; Chen et al., 1996; Wu et al., 2002; Qiu et al., 2000), these rocks represent the only known relics of Archaean rocks in the Yangtze craton.

The Cretaceous granitic intrusions encompass a wide spectrum of rock types including monzonite, quartz monzonite, syenite and granite. They plot in the mid-alkaline field in the TAS diagram, and form a roughly continuous trend with alkali gabbros of Dabieshan (Chen et al., 2002). The granitic intrusions, like mafic-ultramafic rocks, were emplaced indiscriminately in all tectonic subunits of Dabieshan. In the N Huaiyang belt, the intrusions are dominated by Sr, Ba and REE-rich syenitic magmas (Zhou et al., 1995a). Overall, the Cretaceous granitoids have geochemical compositions indicative of an origin from enriched sources. They are not crustal melts produced in a continental collision zone, but are liquids formed in an extensional setting with significant input of metasomatised mantle.

In addition to granitic and syenitic intrusions, numerous small mafic/ultramafic bodies were also emplaced in Dabieshan during the Cretaceous, practically contemporaneously with the granitoids. These mantle-derived rocks (gabbro and pyroxenite) show an astonishing isotopic signature of the continental crust, which led Jahn et al. (1999) to propose a model of crust-mantle interaction and production of the mafic magmas by melting of a metasomatised mantle source.

Isotope test of the existing tectonic models

In all the published tectonic models, the Yangtze craton is assumed to subduct northwards beneath the Sino-Korean craton (e.g. Maruyama et al., 1994; Ernst & Liou, 1995; Hacker et al., 1996, 1998; Faure et al., 1999). The exhumed blocks of UHP and HP units are thin slices underlain by unknown “basement” rocks. Cretaceous doming in the NDC (Hacker et al., 1998, 2000; Faure et al., 1999; Ratschbacher et al., 2000) stretched and separated the thin slice of the UHP unit and resulted in two disproportional entities, now represented in majority by the UHP terrane of the SDC and a tiny part in the north of the NDC (Xu et al., 1999). Recently, microdiamond inclusions in garnet have been identified in eclogites from the NDC, thus concluding the UHP metamorphic history of the eclogite-bearing unit in the NDC (Xu et al., 2003). The Cretaceous doming in the NDC was accompanied by intense magmatism, resulting in voluminous granitoid intrusions and ubiquitous mafic/ultramafic stocks and dikes. Note that the magmatism took place within all the tectonic units of Dabieshan.

The northward subduction models predict that the Cretaceous magmatism would show isotopic signature of the lower crust and upper mantle (= lithosphere) of the Yangtze craton. The thin slice of UHP/HP metamorphic rocks could have been part of a microcontinent (= Neoproterozoic island arc) lying between the two cratons, so whether it belonged to the Yangtze craton is debatable. In any case, the northward subduction models can be tested by the isotope tracers of the post-collisional Cretaceous magmatic rocks that were formed by melting of the subducted-and-exhumed lithosphere.

The Sr–Nd–Pb isotopic data used in the present analysis are from our own work and the literature (references too many to cite individually; they are available upon solicitation). The key points are given as follows.

  1. (1)

    Nd–Sr isotopic compositions of granitic gneisses (Figs. 16a, b). At 220 Ma (Fig. 16a), all granitic gneisses have evolved into negative εNd values. However, the data sets for the UHP (SDC) and high–T terranes (NDC) of Dabieshan are easily distinguished, with ϵNi(T) values of −2 to −13 (except one) for SDC and −11 to −23 (except Huangtuling) for NDC. The SDC gneisses have more dispersed 87Sr/86Sr ratios (= ISr), up to 0.740, whereas the NDC gneisses are more restricted in the range of 0.706 to 0.715. The gneisses at Shuanghe locality are evidently extraordinary in all aspects with regard to other gneisses of the SDC. They have the lowest εNd(T) values, the highest TDM model ages, and the oldest zircon ages (Late Archaean) among the SDC gneisses. A granulite from Huangtuling village, near Yinshan, has a very low ϵNd(T) value of −30 and very high 87Sr/86Sr of 0.742. Together with some known Archaean zircon ages for granulites from the same region, it indicates the presence of late Archaean relics in Dabieshan. The granitic gneisses of the Su–Lu UHP terrane seem to differ from those of the Dabie UHP terrane (SDC) in terms of their lower range of ϵNd(T) values, but are similar in the wide range of Sr isotope compositions. At 120 Ma (Fig. 16b) Cretaceous mafic/ultramafic rocks were derived from the upper mantle. Their restricted ϵNd(T) of −15 to −20 and ISr (0.706–0.710) suggest a metasomatised mantle presumably produced by interaction of subducted lower crust with mantle peridotites (Jahn et al., 1999). However, this interpretation is not unique as we will discuss later. The coeval but much more voluminous granitic intrusions are shown to have isotopic compositions remarkably similar to both the mafic rocks and NDC granitic gneisses. The Cretaceous granitoids were emplaced in all the tectonic subunits of Dabieshan (North Huaiyang, NDC, SDC, and Susong Blueschist terrane), yet their isotopic compositions are conspicuously uniform.

  2. (2)

    Sm–Nd isotopic and chemical characteristics (Figs. 17a, b). While the ϵNd(T) value represents time-integrated isotopic evolution of a rock, 147Sm/144Nd ratio is a parameter characteristic of its chemical composition. The majority of the SDC granitic gneisses have Sm/Nd ratios higher than the average upper continental crust (UCC) value, whereas the opposite is true for the NDC gneisses. This indicates that NDC gneisses have more fractionated REE and greater enrichment in LREE. The Su–Lu gneisses have intermediate characteristics, showing data points straddling the SDC and NDC fields (Fig. 17a). At 120 Ma, the Cretaceous granitic magmas are shown to have both Nd isotopic and REE patterns similar to the NDC gneisses, and clearly distinguished from the SDC gneisses.

  3. (3)

    Depleted mantle-based model ages (TDM, Figs. 18a, b). The SDC gneisses have TDM ranging from 1.1 to 2.4 Ga, except two with high fSm/Nd values and those from the Shuanghe locality (Fig. 18a). On the other hand, the NDC gneisses have older TDM from 1.5 to 3.0 Ga, including two granulites from Huangtuling. The Cretaceous granitoids and mafic intrusions also have a range of TDM similar to the NDC gneisses. Fig. 18b shows the Nd isotopic distinction between the Sino-Korean Craton (SKC) and the South China Block (Yangtze Craton and Cathaysia included) based on Sm–Nd isotope data of Mesozoic granitoids and felsic volcanic rocks. The two blocks are roughly separated at εNd = −13. The field of Dabieshan rocks overlap much of the granitoids from the SKC, but have little in common with those from the South China Block.

  4. (4)

    Pb isotopic compositions (Figs. 19a, b). Based on data from the literature (e.g. compilations of Zhang, 1995; Chen & Jahn, 1999), the Pb isotope fields of Mesozoic granitoids from the SKC are contrasted with that from the Yangtze craton. The data of Mesozoic granitoids from Dabieshan and Su–Lu have Pb isotopic compositions similar to those of the SKC, but very different from the Yangtze craton. Granitic gneisses of Dabieshan at 120 Ma show the same isotopic characteristics.

Discussion and tectonic implications

The geochemical and isotopic constraints to the tectonic evolution of the entire Dabie orogen may be summarised below:

  1. (1)

    The Cretaceous mafic–ultramafic magmas in Dabieshan were derived from a metasomatised mantle source with EM1 signature. Their isotopic characteristics are similar to the coeval mafic rocks from the SKC (e.g. Qiu et al., 1997).

  2. (2)

    The Cretaceous granitoids (felsic volcanics included) have Sr–Nd isotopic and elemental characteristics similar to the mafic-ultramafic rocks, suggesting their possible genetic relationship. The isotopic signature is “typical” of the lower continental crust of moderately depleted granulite facies rocks. This is also supported by Pb isotopic data (Zhang, 1995; Zhou et al., 1995b; Chen & Jahn, 1999; Zhang et al., 2002).

  3. (3)

    Recent isotopic investigations on Cretaceous mafic and felsic volcanic rocks from western Shandong, eastern Shandong (Jiaodong peninsula), Xishan and Nankou (near Beijing) reveal that these rocks also have very similar isotopic signature as the contemporaneous intrusions of Dabieshan. The similar isotopic signature is also found in Cretaceous granites of the Taihang Mountains (Chen et al., 2003).

  4. (4)

    The quartzofeldspathic gneisses of the NDC and SDC are distinguished both in isotopic and trace element compositions. Sr–Nd isotopic data suggest that the SDC gneisses have no role in the genesis of post-orogenic granitoids, but the data allow the NDC gneisses to be a likely source for Cretaceous granitoids.

  5. (5)

    A core complex interpretation for the NDC (e.g. Faure et al., 1999; Ratschbacher et al., 2000) is supported by the geochemical and isotopic data, as well as by the occurrence of Triassic eclogites in the northern margin of the NDC (Xu et al., 1999).

Fig. 16. (a)

ϵNd(T) vs. 87Sr/86Sr ratios, recalculated at 220 Ma, for the granitic gneisses from the Southern and Northern Dabie Complex (SDC, NDC), and the Su–Lu terrane. Shuanghe gneisses and Huangtuling granulite are exceptions. (b) ϵNd(T) vs. 87Sr/86Sr ratios, recalculated at 120 Ma, for the granitic gneisses and Cretaceous mafic/ultramafic and granitic rocks. Note the similarity between the Cretaceous mafic/ultramafic rocks, Cretaceous granites/syenites, and the Neoproterozoic NDC gneisses.

Fig. 16. (a)

ϵNd(T) vs. 87Sr/86Sr ratios, recalculated at 220 Ma, for the granitic gneisses from the Southern and Northern Dabie Complex (SDC, NDC), and the Su–Lu terrane. Shuanghe gneisses and Huangtuling granulite are exceptions. (b) ϵNd(T) vs. 87Sr/86Sr ratios, recalculated at 120 Ma, for the granitic gneisses and Cretaceous mafic/ultramafic and granitic rocks. Note the similarity between the Cretaceous mafic/ultramafic rocks, Cretaceous granites/syenites, and the Neoproterozoic NDC gneisses.

Fig. 17. (a)

ϵNd(T) vs. 147Sm/144Nd, recalculated at 220 Ma, for the granitic gneisses from the Southern and Northern Dabie Complexes (SDC, NDC) and the Su–Lu terrane. The NDC and SDC gneisses are separated at ϵNd(T) = −13, except for Shuanghe and Huangtuling. (b)ϵNd(T) vs. 147Sm/144Nd, recalculated at 120 Ma, for the granitic gneisses and Cretaceous mafic/ultramafic and granitic rocks. Note the similarity between the Cretaceous mafic/ultramafic rocks, Cretaceous granites/syenites, and the Neoproterozoic NDC gneisses.

Fig. 17. (a)

ϵNd(T) vs. 147Sm/144Nd, recalculated at 220 Ma, for the granitic gneisses from the Southern and Northern Dabie Complexes (SDC, NDC) and the Su–Lu terrane. The NDC and SDC gneisses are separated at ϵNd(T) = −13, except for Shuanghe and Huangtuling. (b)ϵNd(T) vs. 147Sm/144Nd, recalculated at 120 Ma, for the granitic gneisses and Cretaceous mafic/ultramafic and granitic rocks. Note the similarity between the Cretaceous mafic/ultramafic rocks, Cretaceous granites/syenites, and the Neoproterozoic NDC gneisses.

Fig. 18. (a)

ϵNd(T) vs. model age TDM for the granitic gneisses from the Southern and Northern Dabie Complexes (SDC, NDC), and the Su–Lu terrane. (b) ϵNd(T) vs. model age TDM for Mesozoic granitic rocks from the SKC and South China Block (Yangtze craton & Cathaysia). The two blocks can be separated at εNd = +13.

Fig. 18. (a)

ϵNd(T) vs. model age TDM for the granitic gneisses from the Southern and Northern Dabie Complexes (SDC, NDC), and the Su–Lu terrane. (b) ϵNd(T) vs. model age TDM for Mesozoic granitic rocks from the SKC and South China Block (Yangtze craton & Cathaysia). The two blocks can be separated at εNd = +13.

Fig. 19. (a)

207Pb/204Pb vs. 206Pb/204Pb plot for Cretaceous granitoids and granitic gneisses of Dabieshan, and Mesozoic granitoids of the Su–Lu terrane. The distinction of isotope fields for the Yangtze and Sino-Korean cratons is clear. Most Cretaceous granitoids and gneisses (calculated at 120 Ma) show their close affinity with the Sino-Korean craton, and completely distinguished from the Yangtze craton. (b) 208Pb/204Pb vs. 206Pb/204Pb plot for Cretaceous granitoids and granitic gneisses of Dabieshan, and Mesozoic granitoids of the Su–Lu terrane.

Fig. 19. (a)

207Pb/204Pb vs. 206Pb/204Pb plot for Cretaceous granitoids and granitic gneisses of Dabieshan, and Mesozoic granitoids of the Su–Lu terrane. The distinction of isotope fields for the Yangtze and Sino-Korean cratons is clear. Most Cretaceous granitoids and gneisses (calculated at 120 Ma) show their close affinity with the Sino-Korean craton, and completely distinguished from the Yangtze craton. (b) 208Pb/204Pb vs. 206Pb/204Pb plot for Cretaceous granitoids and granitic gneisses of Dabieshan, and Mesozoic granitoids of the Su–Lu terrane.

The grossly similar Nd–Sr–Pb isotopic signature between Cretaceous mafic-ultramafic rocks, Cretaceous granitic rocks and the NDC gneisses is intriguing. The derivation of mafic and ultramafic rocks from a metasomatised mantle is beyond any doubt. However, it is not clear how the metasomatism was effected. Jahn et al. (1999) proposed that the process occurred in the post-collisional epoch, and between the hot asthenosphere and “trapped” lower continental crust after most subducted continental slices were uplifted. Implicitly, the lower crust dominated the Sr–Nd isotope budget for both the mafic liquids produced from the mantle and the granitic rocks formed by the melting of the lower crust. While this interpretation may be suitable for a terrane with deep subduction of continental blocks, it becomes difficult to explain the similar isotopic signature observed for other contemporaneous rocks emplaced in other parts of the Sino-Korean craton (SKC). Consequently, the mantle metasomatism cannot be regarded as a local Dabieshan phenomenon; it is of a cratonic scale.

Moreover, a comparison of chemical compositions between the NDC gneisses and Cretaceous granitoids poses another problem. Petrological experiments constrain that melting of felsic gneisses at lower crustal conditions would produce liquids of silica-rich granitic composition, and in no way syenitic liquids could be formed (Litvinovsky et al., 2000). The occurrence of syenitic rocks in the North Huaiyang belt probably indicate an important additional input of the mantle component (Litvinovsky et al., 2001). Besides, high Sr–Ba concentrations in Cretaceous granitoids cannot be explained by melting of ordinary felsic gneisses. Consequently, in addition to the NDC gneisses, input from metasomatised mantle is required for the production of Cretaceous granitoids.

If the lower crust and the upper mantle sources of Cretaceous magmatic rocks in Dabieshan are identical to that of the SKC, then it implies that the continental lithosphere underneath Dabieshan is the same as that underneath the SKC. Consequently, the traditional view that the Yangtze craton subducted northwards beneath the SKC may no longer be acceptable. The subduction polarity could have been just opposite. At present, we are working on an appropriate tectonic model to explain the isotopic features. Nevertheless, a recent detailed structural and deformation analyses on UHP metamorphic rocks at Yangkou in the Su–Lu terrane lend a strong argument for such a reversed subduction polarity (Zhao et al., 2002).

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Acknowledgements

Constructive comments and suggestions on an earlier version by Yong-Fei Zheng (Hefei, China) have greatly improved the final version of this article. The editorial assistance of Tamás Váczi is much appreciated. We thank the European Mineralogical Union for invitation to present this work in the 5th EMU School and Symposium on Ultrahigh Pressure Metamorphism held in Budapest, Hungary (21–25 July 2003). The preparation of this review was supported in part by the National Science Council (NSC) of Taiwan (Jahn), and the U.S. National Science Foundation, EAR Continental Dynamics Program, EAR-0003276 (Rumble) and EAR−0003355 (Liou).

Figures & Tables

Fig. 3.

REE distribution patterns of eclogites from the Su-Lu, Dabie and Hong’an terranes. All eclogites are gneiss-hosted except two from Rongcheng (Su-Lu). The common feature is the enrichment in light REE, which contrast with N-MORB but resemble continental mafic rocks (basalt, amphibolite, or basic granulite).

Fig. 3.

REE distribution patterns of eclogites from the Su-Lu, Dabie and Hong’an terranes. All eclogites are gneiss-hosted except two from Rongcheng (Su-Lu). The common feature is the enrichment in light REE, which contrast with N-MORB but resemble continental mafic rocks (basalt, amphibolite, or basic granulite).

Fig. 4.

Sm and Nd concentrations in garnet, clinopyroxene, orthopyroxene and amphibole. All data were obtained by the isotope dilution method reported in the literature (references too numerous to be cited herein). The two charts at the top represent the total ranges observed. Data of more restricted concentration ranges are shown in the two diagrams in the middle and the two at the bottom. The majority of garnet data have both Sm and Nd less than 2 ppm, and most have Sm/Nd ratios falling between 1 and 2. Higher Nd concentrations are often accompanied by lower Sm/Nd ratios, which suggest a presence of REE-rich inclusions, such as epidote, zoisite, or apatite, in garnet. Most clinopyroxenes have Sm/Nd ratios falling about the chondritic ratio of 0.325, and their Nd concentrations less than 6 ppm.

Fig. 4.

Sm and Nd concentrations in garnet, clinopyroxene, orthopyroxene and amphibole. All data were obtained by the isotope dilution method reported in the literature (references too numerous to be cited herein). The two charts at the top represent the total ranges observed. Data of more restricted concentration ranges are shown in the two diagrams in the middle and the two at the bottom. The majority of garnet data have both Sm and Nd less than 2 ppm, and most have Sm/Nd ratios falling between 1 and 2. Higher Nd concentrations are often accompanied by lower Sm/Nd ratios, which suggest a presence of REE-rich inclusions, such as epidote, zoisite, or apatite, in garnet. Most clinopyroxenes have Sm/Nd ratios falling about the chondritic ratio of 0.325, and their Nd concentrations less than 6 ppm.

Fig. 5.

REE partition coefficients (Kd) between clinopyroxene and garnet. The data were obtained on the eclogite-pyroxenite complex of Maowu (data source: Jahn et al., 2003). Kd values for LREE vary by three orders of magnitude, whereas those for HREE by only one order.

Fig. 5.

REE partition coefficients (Kd) between clinopyroxene and garnet. The data were obtained on the eclogite-pyroxenite complex of Maowu (data source: Jahn et al., 2003). Kd values for LREE vary by three orders of magnitude, whereas those for HREE by only one order.

Fig. 7. (a)

Meaningful Sm-Nd and (b) Rb-Sr (next page) mineral isochron ages were obtained for the Bixiling Complex (data source: Chavagnac & Jahn, 1996). All rocks have very similar metamorphic initial isotope ratios (ϵNd(T) = 0 to −2, ISr 0.704). The amphibolite has a much younger biotite age of 179 Ma and a very different ISr of 0.707, as a result of retrograde metamorphism.

Fig. 7. (a)

Meaningful Sm-Nd and (b) Rb-Sr (next page) mineral isochron ages were obtained for the Bixiling Complex (data source: Chavagnac & Jahn, 1996). All rocks have very similar metamorphic initial isotope ratios (ϵNd(T) = 0 to −2, ISr 0.704). The amphibolite has a much younger biotite age of 179 Ma and a very different ISr of 0.707, as a result of retrograde metamorphism.

Fig. 7. (b)

See previous page.

Fig. 7. (b)

See previous page.

Fig. 8.

Meaningful Sm-Nd mineral isochron ages obtained for an eclogite, a garnet websterite, and a garnet clinopyroxenite from the Maowu Complex of Dabieshan (data source: Jahn et al., 2003).

Fig. 8.

Meaningful Sm-Nd mineral isochron ages obtained for an eclogite, a garnet websterite, and a garnet clinopyroxenite from the Maowu Complex of Dabieshan (data source: Jahn et al., 2003).

Fig. 9.

Isotopic disequilibrium in eclogites of the Hong’an Block of western Dabieshan. The Rb–Sr systems are shown in the left diagrams and the Sm–Nd systems in the right. The non-radiogenic Sr of phengite and dispersed data points for XHD–07–1 suggests a total disequilibrium despite of its high petrological temperature of 680–700 °C. Likewise, its Sm–Nd data yield grt–omp or grt–WR ages much higher than the supposed 220 Ma. For P 260, the Rb–Sr phengite age of ca. 170 Ma is too young, and a Sm–Nd “futurechron” is obtained. Garnet is completely out of equilibrium not only for its low 143Nd/144Nd ratio, but also for its unusually low 147Sm/144Nd (less than chondritic value). Sample TP03–2 and QLP08–1 appear to give “correct” Rb–Sr ages of 212 and 225 Ma, but no meaningful Sm–Nd ages were obtained.

Fig. 9.

Isotopic disequilibrium in eclogites of the Hong’an Block of western Dabieshan. The Rb–Sr systems are shown in the left diagrams and the Sm–Nd systems in the right. The non-radiogenic Sr of phengite and dispersed data points for XHD–07–1 suggests a total disequilibrium despite of its high petrological temperature of 680–700 °C. Likewise, its Sm–Nd data yield grt–omp or grt–WR ages much higher than the supposed 220 Ma. For P 260, the Rb–Sr phengite age of ca. 170 Ma is too young, and a Sm–Nd “futurechron” is obtained. Garnet is completely out of equilibrium not only for its low 143Nd/144Nd ratio, but also for its unusually low 147Sm/144Nd (less than chondritic value). Sample TP03–2 and QLP08–1 appear to give “correct” Rb–Sr ages of 212 and 225 Ma, but no meaningful Sm–Nd ages were obtained.

Fig. 10.

ϵNd(0) vs. 147Sm/144Nd plots (left) and ϵNd(T) vs. 147Sm/144Nd plots (right) for eclogites and garnet peridotites from the Alpine, Hercynian and Caledonian belts. Data source: see Appendices of Jahn (1999). For the Alpine belt, except the UHP metasediments of Dora-Maira, all the rocks show positive ϵNd(T) values. Most eclogites and garnet peridotites from the Hercynian belt show positive ϵNd(T) values, suggesting their oceanic affinity. Those having negative values have been interpreted as due to crustal contamination by reinjection of crustal rocks (oceanic clays) into the upper mantle. As for the eclogites and garnet peridotites from the European Caledonides, the majority of the rocks show negative ϵNd(T) values, in strong contrast to those of the Alpine and Hercynian chains. These rocks appear to have resided in the continental crust for a lengthy period of time before their UHP metamorphism.

Fig. 10.

ϵNd(0) vs. 147Sm/144Nd plots (left) and ϵNd(T) vs. 147Sm/144Nd plots (right) for eclogites and garnet peridotites from the Alpine, Hercynian and Caledonian belts. Data source: see Appendices of Jahn (1999). For the Alpine belt, except the UHP metasediments of Dora-Maira, all the rocks show positive ϵNd(T) values. Most eclogites and garnet peridotites from the Hercynian belt show positive ϵNd(T) values, suggesting their oceanic affinity. Those having negative values have been interpreted as due to crustal contamination by reinjection of crustal rocks (oceanic clays) into the upper mantle. As for the eclogites and garnet peridotites from the European Caledonides, the majority of the rocks show negative ϵNd(T) values, in strong contrast to those of the Alpine and Hercynian chains. These rocks appear to have resided in the continental crust for a lengthy period of time before their UHP metamorphism.

Fig. 12.

Range of δ18O values for metamorphic minerals and rocks. Modified after Rumble (1998) with additional data of Zheng et al. (1998, 1999). The Qinglongshan data show the lowest delta18O values (~ −10%o) for any magmatic and metamorphic rocks. For eclogites from other localities (Shuanghe, Bixiling, Huangzhen and Maowu), all minerals seem to have low δ18O values about 0%o, but preserved equilibrium high-temperature isotope fractionation with the order of quartz > omphacite > garnet > rutile.

Fig. 12.

Range of δ18O values for metamorphic minerals and rocks. Modified after Rumble (1998) with additional data of Zheng et al. (1998, 1999). The Qinglongshan data show the lowest delta18O values (~ −10%o) for any magmatic and metamorphic rocks. For eclogites from other localities (Shuanghe, Bixiling, Huangzhen and Maowu), all minerals seem to have low δ18O values about 0%o, but preserved equilibrium high-temperature isotope fractionation with the order of quartz > omphacite > garnet > rutile.

Fig. 13.

Plots of δ18O values of quartz vs. coexisting minerals in eclogites from Shuanghe in the Dabie Mountains (solid dots) and Donghai of the Su-Lu terrane (solid triangles). Equilibrium isotope fractionation lines at different temperatures are shown for reference (after Zheng et al., 1998).

Fig. 13.

Plots of δ18O values of quartz vs. coexisting minerals in eclogites from Shuanghe in the Dabie Mountains (solid dots) and Donghai of the Su-Lu terrane (solid triangles). Equilibrium isotope fractionation lines at different temperatures are shown for reference (after Zheng et al., 1998).

Fig. 14.

Oxygen isotope fractionation between omphacite and garnet (∆;18O) vs. δ18O values of garnet from eclogites of the Dabie and Su–Lu terranes. The reference lines of ∆18O = 0 and 2%o represent the limit of equilibrium fractionation between these two minerals. Data outside of this range are out of equilibrium. Data sources: Yui et al. (1997), Zhang et al. (1998), Zheng et al. (1998, 1999).

Fig. 14.

Oxygen isotope fractionation between omphacite and garnet (∆;18O) vs. δ18O values of garnet from eclogites of the Dabie and Su–Lu terranes. The reference lines of ∆18O = 0 and 2%o represent the limit of equilibrium fractionation between these two minerals. Data outside of this range are out of equilibrium. Data sources: Yui et al. (1997), Zhang et al. (1998), Zheng et al. (1998, 1999).

Fig. 15.

Relationships between the Nd and O isotope ratios of garnet and omphacite from eclogites of the Su–Lu terrane (data and diagram from Zheng et al., 2002).

Fig. 15.

Relationships between the Nd and O isotope ratios of garnet and omphacite from eclogites of the Su–Lu terrane (data and diagram from Zheng et al., 2002).

Fig. 16. (a)

ϵNd(T) vs. 87Sr/86Sr ratios, recalculated at 220 Ma, for the granitic gneisses from the Southern and Northern Dabie Complex (SDC, NDC), and the Su–Lu terrane. Shuanghe gneisses and Huangtuling granulite are exceptions. (b) ϵNd(T) vs. 87Sr/86Sr ratios, recalculated at 120 Ma, for the granitic gneisses and Cretaceous mafic/ultramafic and granitic rocks. Note the similarity between the Cretaceous mafic/ultramafic rocks, Cretaceous granites/syenites, and the Neoproterozoic NDC gneisses.

Fig. 16. (a)

ϵNd(T) vs. 87Sr/86Sr ratios, recalculated at 220 Ma, for the granitic gneisses from the Southern and Northern Dabie Complex (SDC, NDC), and the Su–Lu terrane. Shuanghe gneisses and Huangtuling granulite are exceptions. (b) ϵNd(T) vs. 87Sr/86Sr ratios, recalculated at 120 Ma, for the granitic gneisses and Cretaceous mafic/ultramafic and granitic rocks. Note the similarity between the Cretaceous mafic/ultramafic rocks, Cretaceous granites/syenites, and the Neoproterozoic NDC gneisses.

Fig. 17. (a)

ϵNd(T) vs. 147Sm/144Nd, recalculated at 220 Ma, for the granitic gneisses from the Southern and Northern Dabie Complexes (SDC, NDC) and the Su–Lu terrane. The NDC and SDC gneisses are separated at ϵNd(T) = −13, except for Shuanghe and Huangtuling. (b)ϵNd(T) vs. 147Sm/144Nd, recalculated at 120 Ma, for the granitic gneisses and Cretaceous mafic/ultramafic and granitic rocks. Note the similarity between the Cretaceous mafic/ultramafic rocks, Cretaceous granites/syenites, and the Neoproterozoic NDC gneisses.

Fig. 17. (a)

ϵNd(T) vs. 147Sm/144Nd, recalculated at 220 Ma, for the granitic gneisses from the Southern and Northern Dabie Complexes (SDC, NDC) and the Su–Lu terrane. The NDC and SDC gneisses are separated at ϵNd(T) = −13, except for Shuanghe and Huangtuling. (b)ϵNd(T) vs. 147Sm/144Nd, recalculated at 120 Ma, for the granitic gneisses and Cretaceous mafic/ultramafic and granitic rocks. Note the similarity between the Cretaceous mafic/ultramafic rocks, Cretaceous granites/syenites, and the Neoproterozoic NDC gneisses.

Fig. 18. (a)

ϵNd(T) vs. model age TDM for the granitic gneisses from the Southern and Northern Dabie Complexes (SDC, NDC), and the Su–Lu terrane. (b) ϵNd(T) vs. model age TDM for Mesozoic granitic rocks from the SKC and South China Block (Yangtze craton & Cathaysia). The two blocks can be separated at εNd = +13.

Fig. 18. (a)

ϵNd(T) vs. model age TDM for the granitic gneisses from the Southern and Northern Dabie Complexes (SDC, NDC), and the Su–Lu terrane. (b) ϵNd(T) vs. model age TDM for Mesozoic granitic rocks from the SKC and South China Block (Yangtze craton & Cathaysia). The two blocks can be separated at εNd = +13.

Fig. 19. (a)

207Pb/204Pb vs. 206Pb/204Pb plot for Cretaceous granitoids and granitic gneisses of Dabieshan, and Mesozoic granitoids of the Su–Lu terrane. The distinction of isotope fields for the Yangtze and Sino-Korean cratons is clear. Most Cretaceous granitoids and gneisses (calculated at 120 Ma) show their close affinity with the Sino-Korean craton, and completely distinguished from the Yangtze craton. (b) 208Pb/204Pb vs. 206Pb/204Pb plot for Cretaceous granitoids and granitic gneisses of Dabieshan, and Mesozoic granitoids of the Su–Lu terrane.

Fig. 19. (a)

207Pb/204Pb vs. 206Pb/204Pb plot for Cretaceous granitoids and granitic gneisses of Dabieshan, and Mesozoic granitoids of the Su–Lu terrane. The distinction of isotope fields for the Yangtze and Sino-Korean cratons is clear. Most Cretaceous granitoids and gneisses (calculated at 120 Ma) show their close affinity with the Sino-Korean craton, and completely distinguished from the Yangtze craton. (b) 208Pb/204Pb vs. 206Pb/204Pb plot for Cretaceous granitoids and granitic gneisses of Dabieshan, and Mesozoic granitoids of the Su–Lu terrane.

Contents

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