Early Cambrian eclogites in SW Mongolia: evidence that the Palaeo‐Asian Ocean suture extends further east than expected

Newly discovered eclogites, Early Cambrian carbonates and chloritoid‐bearing metapelites form the Tsakhir Uul accretionary wedge, which was thrust during the Early Cambrian over the Mesoproterozoic Dzabkhan‐Baydrag continent. The rock association of the wedge forms a tectonic window emerging through the hangingwall Khantaishir ophiolite unit, which preserves a typical Tethyan‐type ophiolitic sequence. The eclogites correspond geochemically to T‐MORB modified by fluid circulation. They are composed of garnet, omphacite, amphibole, rutile ±muscovite ±quartz ±epidote and exhibit well‐equilibrated matrix textures. Jadeite content of the omphacite reaches up to 45 mol.%, the Si content of muscovite is between 3.40 and 3.45 p.f.u., amphibole is winchite to barroisite, but reaches tschermakitic composition at some rims, and garnet composition is grs0.24–0.36, alm0.43–0.56, py0.05–0.18, sps0.00–0.18, . The peak assemblage, together with the composition of garnet rims, omphacite, amphibole and muscovite, correspond in a pseudosection to 20−22.5 kbar and 590−610 °C. The tschermakitic rim of amphibole is interpreted as partial reequilibration on decompression below 16 kbar and ∼600−630 °C. Two muscovite separates from the eclogite yielded an Ar–Ar plateau age of 543.1 ± 3.9 Ma (1σ) and a mean age of 547.9 ± 2.6 Ma (1σ), whereas muscovite from an interbedded garnet‐chloritoid micaschist yielded an Ar–Ar plateau age of 536.9 ± 2.7 Ma (1σ); these ages are interpreted as cooling ages. The P–T data, geochemistry of eclogites and cooling ages suggest an affinity between the Tsakhir Uul wedge and the Gorny Altai and the north Mongolian blueschist belt, which are believed typical for subduction of warm oceanic lithosphere and closure of small oceanic basins. Thus, the discovery of the Tsakhir Uul eclogites represents an important finding suggesting extension of the Early Cambrian subduction system of the Central Asian Orogenic Belt far to the east in a region where it was not expected.


INTRODUCTION
Since the end of the Late Proterozoic till the Mesozoic, the Central Asian Orogenic Belt (CAOB), also known as Altaids (S¸engo¨r et al., 1993;S¸engo¨r & NatalÕin, 1996), has been the largest region of crustal growth on the Earth (Jahn et al., 2009). The CAOB evolved through accretion of magmatic arcs, back-arc terranes, accretionary complexes and continental blocks. Accretion is considered to have lasted from the latest Mesoproterozoic to the late Permian (Kro¨ner et al., 2007;Windley et al., 2007;Xiao et al., 2009). Two main periods of continental growth related to accelerated accretion in the CAOB are recognized (Zonenshain et al., 1976): (i) the Late Proterozoic to Early Palaeozoic period (reported also as ÔCaledo-nianÕ), when Late Proterozoic ophiolites of the Palaeo-Asian Ocean have been accreted to the Siberia craton and to Proterozoic continental basement fragments; and (ii) the Late Palaeozoic to Mesozoic period, when a vast Siluro-Devonian oceanic domain of the Ôsouth MongolianÕ ocean was accreted during the so-called ÔHercynianÕ phase. The first accretion period is evidenced by a number of ophiolitic bodies and highpressure rock occurrences, namely in the central Kazakhstan, Gorny Altai and North Mongolia (Khain et al., 2003;Volkova & Sklyarov, 2007). However, the shape of the Palaeo-Asian Ocean suture is unknown because of the scattered character of ophiolitic and high-pressure units developed during a long period of time between 760 and 490 Ma (Volkova & Sklyarov, 2007). In contrast, the Late Palaeozoic to Mesozoic suture of the south Mongolian ocean is well defined, being parallel to the northern termination of the North Chinese and Tarim cratons (e.g. Xiao et al., 2009).
In this article, we present petrological and geochronological data for the eclogites and associated schists that suggest subduction of the Early Cambrian passive margin sequences of the Palaeo-Asian Ocean in SW Mongolia. Major and trace element geochemistry is used to discuss the character of the eclogite protolith and the geodynamic setting of formation. Thermodynamic modelling is used to characterize P-T conditions and the metamorphic field gradient of the eclogite and 40 Ar-39 Ar geochronology on muscovite is applied to date exhumation of the high-pressure rocks and by inference the age of the Palaeo-Asian Ocean suture. Finally, we discuss the significance of the newly discovered eclogites for the geographic extension of the Palaeo-Asian Ocean far to the east compared to the existing model of Zonenshain & Kuzmin (1978), which may have important consequences for established tectonic models for the evolution of the CAOB by S¸engo¨r et al. (1993) and Windley et al. (2007).

FIELD SETTING
SW Mongolia is a region affected by both Early Palaeozoic (also called ÔCaledonianÕ) and late Palaeozoic (ÔHercynianÕ) tectonic events (Mossakovsky et al., 1993). In agreement with the early models for the tectonic evolution of the CAOB (Zonenshain, 1973;Ruzhentsev & Pospelov, 1992;Ruzhentsev, 2001), the geology of Mongolia is divided into two major tectonic domains that differ in tectonic style and ages of geological formation: (i) the Mesoproterozoic northern domain (the Mongolian continent of Zonenshain,1973), corresponding to the Dzabkhan-Baydrag continent of Badarch et al. (2002) that was predominantly affected by Early Palaeozoic orogenesis; and (ii) the Early Palaeozoic, southern, mostly oceanic domain (south Mongolian Ocean domain) that was predominantly affected by late Palaeozoic orogenesis (Fig. 1). The two domains are separated by the Fig. 2. Detailed geological map of the studied area with Landsat image as background (compiled from Rauzer, 1987;Hanzˇl & Aichler, 2007 and with some spatial extent of the lithological units inferred from the examination of Landsat imagery). Sample locations used for Ar-Ar dating and petrology and position of the structural profile from Fig. 3 (Tomurtogoo, 1997).
The study area is located in the Zamtyn range belonging to the Dzabkhan-Baydrag continent in the northern Mesoproterozoic domain, which forms a large mountain crest $20 km NE from the village of Chandman (Fig. 3). It is flanked in the north by a Cretaceous basin and in the south by the E-W trending Main Mongolian Lineament. The newly discovered eclogite belt belongs to the so called ÔLake ZoneÕ of Rauzer (1987), which shows the following geological units and formations from bottom to top.
The structurally lowest and eastern part is represented by coarse-grained augen-gneiss, banded amphibolites, amphibolitic gneisses and marbles of the Zamtyn Nuuru complex, which was recently dated as late Mesoproterozoic and attributed to the Dzabkhan basement (c. 950 Ma, Kro¨ner et al., 2010). These Dzabkhan basement rocks are affected by amphibolite facies metamorphism and are locally thermally reworked by Late Cambrian (c. 500 Ma) magmatic and migmatitic event (Hrdlicˇkova´et al., 2008;Kro¨ner et al., 2010).
The Dzabkhan basement rocks are tectonically overlain by the Tsakhir Uul formation (Fig. 3), composed of fossiliferous volcano-sedimentary, sedimentary, and tuffaceous siliceous schists and marbles, which were dated in the studied area, and in the type locality of the Khantaishir ridge some 120 km to the west, as Early Cambrian by Archaeocyatha macrofossils (Markova, 1975;Hanzˇl & Aichler, 2007). The Tsakhir Uul formation in the study area essentially consists of marble, enclosing large boudins of eclogite and metapelite up to hundreds of metres across in the north. The central part of the unit is composed of a NE-SW trending belt of metagabbro and amphibolite, overlain by a large body of peridotite, whereas the southern part is dominated by marble and metapelite. The eclogites of this study are associated with garnet-and chloritoid-bearing metapelites. However, metagabbros show only amphibolite facies mineral assemblages, suggesting that the unit did not reach eclogite facies conditions as a whole. The metasedimentary rocks in the southern part of the Tsakhir Uul formation (south of the peridotite body) have amphibolite facies assemblages.
Further north occurs an ophiolitic unit composed of serpentinized peridotites, a tonalite-trondhjemite suite, and abundant tholeiitic to calc-alkaline mafic to intermediate volcanic rocks (Hanzˇl & Aichler, 2007). This unit is interpreted as a lateral equivalent of the Khantaishir ophiolite unit (Zonenshain & Kuzmin, 1978), which was dated at 568 ± 4 Ma using U-Pb method on zircon from a plagiogranite (Gibsher et al., 2001). The uppermost part of the section studied is represented by clastic sequences recently dated as Devonian and Early Carboniferous (Kro¨ner et al., 2010). On the southwestern foothill of the Zamtyn Range, a volcaniclastic formation of bimodal volcanic rocks and volcaniclastic sediments was dated as Permian (Hanzˇl & Aichler, 2007).
The Tsakhir Uul formation was thrust over the Zamtyn Nuuru basement complex as a me´lange of eclogite, gabbro and high-to medium-pressure pelite, surrounded by carbonate matrix (Lehmann et al., 2010). The basement metamorphic foliation S1 was determined as Late Proterozoic based on 40 Ar-39 Ar cooling ages of c. 570 Ma on intrafolial coarse muscovite . The eclogites and marbles reveal polyphase deformation marked by an eclogite facies fabric (S1) reworked by a dominant amphibolite facies fabric (S2) developed in impure marbles and metapelites, and by heterogeneous amphibolite facies shear zones in the eclogites. This metamorphic foliation shows variable dip indicating an early steep folding event probably associated with emplacement of the Tsakhir Uul eclogites. This early D2 thrusting event is marked by shear fabrics in metavolcanics underlying peridotite sheet in the Erdene Uul mountains suggesting that the Khantasihir ophiolites experienced a similar thrusting event. Although kinematic data are scarce, a SE-NW shortening direction is likely based on a few L2 lineations and the orientation of F2 fold hinges.
The whole sequence of the Tsakhir Uul accretionary wedge is deformed by major E-W trending large scale folds. In the metasedimentary rocks, these folds are associated with a very low grade but intense, steep E-W-trending cleavage. The same cleavage affects also late Permian volcaniclastic rocks and is dated farther east as mid-Permian (c. 270 Ma, 40 Ar-39 Ar method on synkinematic muscovite; Lehmann et al., 2010). The late cleavage affects also the Khantaishir ophiolite and basement rocks thereby modifying the original emplacement geometry of the eclogite-ophiolite system.

ANALYTICAL PROCEDURES AND ABBREVIATIONS
The whole-rock inductively coupled plasma mass spectrometry analyses were preformed in the Acme laboratories, Canada. Mineral analyses were per-formed on an electron microprobe CAMECA SX-100 at the Institute of Mineralogy at the University of Stuttgart in point beam mode at 15 kV and 15 nA. The petrography is documented in Figs 5 and 6, representative mineral analyses are summarized in Tables 1 and 2, garnet and amphibole chemistry are shown in Figs 7, 8 and 9. The sign Ô=>Õ is used for a trend in mineral composition or for zoning and the sign Ô-Õ for a range of mineral compositions; p.f.u. = per formula unit. For the amphibole, the variables and isopleth notations are used as in Dale et al. (2005) and Diener et al. (2007) to compare the chemical variability of the amphibole with the modelled pseudosections.

PETROGRAPHY
Macroscopically, the eclogites appear as isotropic, fine-grained massive rocks, with rarely distinguishable foliation. They are composed of a light to dark greenish, fine-grained matrix that may have dark green  (Sun & McDonough, 1989) multi-element diagram plotted with GCDkit (Janousˇek et al., 2006). OIB, N-and E-MORB compositions are according to Sun & McDonough (1989) and T-MORB is represented by basalts from the Isabela island, Galapagos (sample DG43A, Geist et al., 1995). Eclogite compositions show an affinity to T-MORB, assumed to be the magmatic protolith of the eclogite.
to black patches, with $40 vol.% of garnet porphyroblasts, usually $2)4 mm in size. Some eclogites contain macroscopically visible muscovite. Locally, the eclogites show compositional layering at centimetric to decimetric scale marked essentially by variable amount and size of garnet.
Under the microscope the common assemblage is garnet, clinopyroxene, colourless to light blue amphibole that may be light to dark green at the rims, garnet, rutile and quartz. Some samples contain also epidote and ⁄ or muscovite. In thin section, a majority of the eclogites show a strong foliation that is defined by the preferred orientation of clinopyroxene and amphibole, and sometimes by muscovite. Some of the rocks sampled are garnetiferous amphibolites as they contain no pyroxene or only very small amounts and have a matrix dominated by colourless to light blue amphibole; these rocks were not included in this study. Two samples of the most typical eclogites were chosen for this petrological study, the muscovite-bearing eclogite M133p17 ⁄ 06 (45°24,276N, 98°13,828E) and the muscovite-absent, epidote-bearing eclogite M134p45 ⁄ 06 (45°24,597N, 98°14,902E). These samples were chosen for their well-equilibrated matrix textures to determine peak conditions of metamorphism.
Eclogites contain on average (n = 5) 48 wt% SiO 2 , 2.17 wt% Na 2 O and 0.54 wt% K 2 O, which corresponds to a basaltic composition according to the SiO 2 v. K 2 O + Na 2 O relation. Primitive mantle normalized multi-element patterns (plotted with GCDkit, Janousˇek et al., 2006) are very similar to the pattern for T-MORB, transitional between E-MORB and OIB (Geist et al., 1995), except for highly fluid-mobile large ion lithophile element as Cs, Rb and K (Fig. 4). Therefore, the precursor of the eclogite is interpreted to have the composition of T-MORB basalt modified by fluid circulation.

Sample M134p45 ⁄ 06
Sample M134p45 ⁄ 06 is composed of garnet (40%), clinopyroxene (30%), colourless to pale blue amphibole (20%) that is in places pale green at the rim, minor epidote (3%), and accessory quartz, albite, rutile and zircon (Fig. 6). Garnet (3 mm) commonly includes amphibole, rutile, quartz, and rarely epidote and albite (Fig. 6a). Clinopyroxene and amphibole in the matrix are strongly preferentially oriented parallel to the foliation (Fig. 6b), they exhibit mutual contacts and straight boundaries with garnet, epidote and rutile that is interpreted as textural equilibrium attained at peak metamorphic conditions. Garnet is slightly zoned from core to rim as follows grs 0.30=>0.36=>0.24 , alm 0.43=>0.55=>0.50 , py 0.05=>0.18 , sps 0.18=>0.00 and X  The pseudosections were calculated using THERMOCALC THERMOCALC 3.3 2009 version) and the data set 5.5 November 2003 upgrade) Sˇtı´pska´et al., 2006). The range of P-T conditions shown is chosen to discuss the peak P-T conditions and the retrogression of the eclogite (Figs 10 & 11). The pseudosections are contoured with the calculated compositional isopleths for muscovite, clinopyroxene, garnet and amphibole.  Fig. 10. The major features involve the stability of muscovite over the whole calculated P-T range, epidote stability below 21 kbar and at maximum temperature of 640°C at 16 kbar, lawsonite stable above 19.5 kbar and below 620°C at 25 kbar. Amphibole stability is up to 660°C at 17 kbar and up to 585°C at 25 kbar. Glaucophane is stable below $580°C and between 1 and 20 kbar, hornblende above 575°C at 16 kbar and to a maximum pressure of 18 kbar at 630°C, actinolite up to 590°C at 25 kbar and to the maximum temperature of 630°C at 18 kbar. The observed well-equilibrated peak assemblage for the eclogite M133p17 ⁄ 06 is muscovite, garnet, clinopyroxene, amphibole, rutile and quartz that corresponds to the narrow field between the epidote-out and actinolite-out lines, outside the stability of lawsonite. The composition of muscovite with Si = 3.40)3.45 (p.f.u.) and composition of clinopyroxene with 35)45% of jadeite corresponds closely to the calculated compositional ranges in the upper pressure part of the g-o-act-ru field. The x(o) isopleths range in this region between 0.17 and 0.19 (not shown) differing slightly from the measured X Fe = 0.04)0.12 of the matrix pyroxene. Garnet rim composition with $27)29% of grossular are close to the calculated z(g) values in the upper pressure part of the g-o-act-ru field, X Fe at the rim (0.78) is close to calculated range of x(g) between 0.73 and 0.74 in the same region (not shown). The compositional isopleths of amphibole in the upper pressure part of the g-o-act-ru field correspond closely to the winchitic to barroisitic amphibole cores [Z = NaM4 ⁄ 2 = 0.20)0.28, Y = Al VI ⁄ 2 ¼ 0.15)0.25, A = Na(A) = 0.02)0.15], therefore, these amphibole compositions are considered to correspond to the pressure peak. By comparing the observed assemblage and mineral compositions with the calculated model, the peak P-T conditions for the eclogite sample M133p17 ⁄ 06 may be estimated at 21)22.5 kbar and 590)600°C. The compositional variables of the thin barroisitic to tschermakitic rims (Z = 0.20)0.33, Y = 0.26)0.42, A = 0.15)0.40, X Fe = 0.1)0.45) tend to be close to the calculated isopleths $18 kbar and 630°C and are therefore interpreted as modified on decompression.  Fig. 11. The major features of the pseudosection are similar to the pseudosection of the sample M133p17 ⁄ 06 (Fig. 10), except that muscovite is not stable, glaucophane is stable to slightly higher temperature of $600°C, epidote is stable to slightly higher temperature, from 635°C at 16 kbar to 635°C at 19 kbar and to 600°C at 22 kbar, which is its upper pressure limit. Actinolite and hornblende are also stable to higher temperature, from 650°C at 19 kbar to 610°C at 25 kbar.
The observed well-equilibrated assemblage of garnet, clinopyroxene, amphibole, epidote, rutile and quartz corresponds to the calculated field g-o-act-epru, situated between the glaucophane-out and epidoteout lines. The composition of pyroxene with up to 42% of jadeite and garnet rim compositions with grossular between 24% and 27% are close to the calculated isopleths j(o) and z(g) in the upper pressure part of the g-o-act-ep-ru field. (a-f) See chapter Analytical procedures and abbreviations for notation and abbreviations. The ellipse in (f) shows peak P-T conditions. For discussion see text.
of the g-o-hb-ep-ru field and are therefore interpreted as compositions modified on decompression at 600)630°C and below 16 kbar. Mineral separates of single mica grains ranging in size from 0.160 to 500 mm were obtained after sample crushing by handpicking under a binocular microscope. After acetone, alcohol and distilled water washing, single muscovite grains were irradiated in the nuclear reactor of McMaster University in Hamilton, Canada. The total neutron flux density during irradiation was 8.8 · 10 18 n cm )2 with a maximum flux gradient estimated at 0.2% in the volume where the samples were included. Irradiation lasted for 70 h, corresponding to a total of nearly 110 MWh. Hb3gr hornblende was used as neutron flux monitor. J-values were calculated relative to an age of 1072 Ma for Hb3gr (Turner et al., 1971) and the decay constants of Steiger & Ja¨ger (1977) were used. The irradiated separates were measured in the Geosciences Azur Laboratory at Nice University. The step-heating procedure is described in detail by Ruffet et al. (1991). Fig. 11. H 2 O-saturated pseudosection for the composition of muscovite-bearing eclogite M133p17 ⁄ 06 contoured for compositional isopleths (a-f). See Analytical procedures and abbreviations section for notation and abbreviations. The ellipse in (f) shows peak P-T conditions. For discussion see text.
Heating was carried out by a CO 2 Synrad 48-5 laser, and isotopic measurements were performed in a VG 3600 mass spectrometer equipped with a Daly detector system. Blanks of the extraction and purification laser system were measured after every third heating step and subtracted from each argon isotope from the subsequent gas fraction. Typical blank values were in the range of 6)18, 0.3)2.0, 0.4)1.2 and 0.6)1.4 · 10 )13 ccSTP for masses 40, 39, 37 and 36, respectively. The criteria for defining a plateau age were as follows: (i) a plateau age should contain at least 70% of the released 39 Ar; (ii) there should be at least three successive heating steps in the plateau; and (iii) the integrated age of the plateau should agree with each apparent age of the plateau within the 1r error confidence interval. Errors on plateau ages are given at the 1r level and do not include the errors on the age of the monitor. However, the error in the 40 Ar* ⁄ 39 Ar k ratio of the monitor is included in the calculation of the plateau age error bar.
Elongated chloritoid porphyroblasts (up to 5 mm) occur preferentially aligned within the mica layers. Garnet (up to 5 mm) contains inclusions of quartz, rutile, ilmenite and rare muscovite. This sample yielded a plateau age of 536.9 ± 2.7 Ma (1r) (Fig. 13, Table 3). Lehmann et al. (2010) obtained 40 Ar-39 Ar cooling ages for micaschists from the same outcrop which overlap, within error, with the age for the micaschist. Therefore, c. 540 Ma is interpreted as the age when the eclogite complex passed through the $360°isotherm during or after its emplacement.

DISCUSSION AND CONCLUSIONS
In this work, new geological, geochemical, petrological and 40 Ar-39 Ar geochronological data are presented from an eclogite bearing Tsakhir Uul unit in southwestern Mongolia. The Tsakhir Uul unit, spatially associated with Khantaishir ophiolites overlying the Dzabkhan-Baydrag Mesoproterozoic basement, may represent a type tectonic assemblage for accretionary complexes of the eastern part of the CAOB. We discuss the lithotectonic position of the Tsakhir Uul eclogite bearing unit, P-T evolution and 40 Ar-39 Ar geochronology of the eclogites and associated micaschists in comparison with other high-pressure units of the CAOB. Finally, the geographic position of the eclogites is discussed as a marker of the major suture zone of the Palaeo-Asian Ocean in its easternmost extremity.

Structure of the Tsakhir Uul accretionary wedge and the Khantaishir ophiolite
The eclogite boudins and marbles are enclosed in a metapelite bearing unit that is overlying a continental Mesoproterozoic basement. The carbonate rocks locally contain Archeocyatha fossils of Early Cambrian age providing an important chronological boundary for onset of the subduction process. These carbonates constitute continuous sequences east and west of the study region, being deposited on Precambrian granitoids and high-grade rocks. The metasedimentary rocks are rich in K 2 O ($3 wt%) and Al 2 O 3 ($20 wt%) and contain chloritoid, suggesting an Al-rich pelitic composition with high content of continental material. Within the metapelites dark schists and fine-grained quartzites also occur locally, which may indicate minor intercalations of deep marine sediments. The MORB chemistry of the eclogites suggests that these rocks originated in oceanic environment, probably formed at a mid-ocean ridge. The whole sequence can be interpreted as follows. The Early Cambrian carbonate rocks probably represent relicts of a platform, which rimmed an Early Cambrian passive margin of the Dzabkhan-Baydrag continent (Rauzer, 1987). The eclogites represent sub-ducted, slightly differentiated MORB incorporated within the metasedimentary wedge during exhumation. Therefore, this complex unit can be interpreted as an accretionary wedge, which contains both continental and oceanic elements. The Tsakhir Uul accretionary wedge constitutes a tectonic nappe overlying paraautochthonous continental margin represented by orthogneiss and migmatites belonging to the Dzabkhan-Baydrag continent (Fig. 3). The structural mapping of Lehmann et al. (2010) also suggests that the Khantaishir ophiolite resides structurally above the high-pressure unit. This inference is based on the existence of the Khantaishir ophiolite in lowlands north of the Tsakhir Uul accretionary wedge, which occupies topographic highs, and on the existence of a peridotite and amphibolite sheet located structurally above the southern part of the wedge. Therefore, we suggest that the Dzabkhan basement is covered by part of the Tsakhir Uul accretionary wedge, which itself is overlain by the Khantaishir ophiolite nappe. Based on  lithology and geochemical data, the rock assemblage of the Khantaishir ophiolite in its type section was interpreted as a supra-subduction island arc complex (Zonenshain & Kuzmin, 1978;Khain et al., 2003). In the section of the Khantaishir ophiolite nappe studied here, it is represented by rocks of the southeastern Erdene Mountain piedmont where serpentinized peridotites, a tonalite-trondhjemite suite, and abundant tholeiitic to calc-alkaline mafic to intermediate volcanic rocks were interpreted as an arc assemblage (Hanzˇl & Aichler, 2007). This tectonic sequence shows a number of similarities with Tethyan-type ophiolites, which may be best exemplified by the Oman ophiolitic sequence, such as: similar ages for the ophiolites (c. 570 Ma) and the high-pressure rocks (c. 540 Ma), the presence of an amphibolite sole underneath peridotite (southern peridotite sheet and associated amphibolites) and metamorphic units exhibiting different pressure histories (northern eclogitic and southern non-eclogitic part of the wedge). The Tethyan model implies subduction of an oceanic domain together with a continental margin, forming eclogites and high-pressure metapelites, and thrusting of these units back over the continent, which is then followed shortly after by thrusting of an ophiolite unit over the continental margin units (Goffe et al., 1988;Yamato et al., 2007;Agard et al., 2009). Based on the Tethyan ophiolite analogue, it is possible that the exhumation of the eclogite bearing Tsakhir Uul accretionary wedge and emplacement of the Khantaishir ophiolitic nappe are quasi-synchronous processes (Montigny et al., 1988). P-T gradients and age of metamorphism of the CAOB high-pressure rocks There are three major regions in the CAOB containing blueschist and eclogite bearing sequences of Mesoproterozoic and Early Palaeozoic age. The westerly Kokchetav subduction-collision zone (Kazakhstan) is characterized by the presence of diamond (40-70 kbar, 1000)1200°C, in the Kumdy-KolÕ and Barchi units) and coesite (30 kbar, 700°C, in the Kulet unit) in gneisses and eclogites, and records an exceptionally steep P-T gradient of 6)7°C km )1 (see Dobretsov et al., 2006;for review;Fig. 14). The formation of ultra high pressure and high pressure rocks in the Kokchetav subduction-collision zone is interpreted as a result of subduction of the Palaeo-Asian Ocean lithosphere and blocks of continental crust underneath the Ediacaran to Early Cambrian island arc system at c. 535 Ma. The high-pressure rocks have been exhumed at c. 528 Ma to mid-crustal depths and reached the base of the accretionary wedge. Subsequently, the subduction zone sequences were interlayered with micaschists, mylonites and schists, forming together the Kokchetav subduction-collision zone.
The second area of high-pressure rocks is represented by numerous occurrences of blueschists and eclogites in the Gorny and Rudny Altai and in northern Mongolia, where protoliths have common Normal Mid-Ocean Ridge Basalt (N-MORB) tholeitic chemistry (Volkova & Sklyarov, 2007). These rocks are represented by thrust slices associated with me´lange zones and common olistostromes containing blueschist blocks with P-T conditions of 6)9 kbar and 380)520°C (Fig. 14), and ophiolite slices, for example in the Oka and the Kurtushiba units from the Sayan mountains, the Uimon unit from the Gorny Altai mountains and the Huegyn unit from the north Mongolia (Volkova & Sklyarov, 2007). Less common eclogite blocks are contained in a serpentinite me´lange and show P-T conditions of 15-20 kbar and 660-770°C in the Chagan-Uzun unit from the Gorny Altai or in the Chara unit from the NE Kazakhstan (Fig. 14) (Volkova & Sklyarov, 2007). These high-pressure rocks are characterized by warmer P-T gradients of $8)20°C km )1 compared to the colder Kokchetav subduction zone (Fig. 14). Geochronological data show that the subduction started at 700 Ma with formation of eclogites at c. 650-630 Ma (Sm-Nd ages, Buslov et al., 2001) and exhumation and cooling at c. 535 Ma (K-Ar ages, Buslov & Watanabe, 1996). In the Gorny Altai, separate basins were closed during the Ordovician as shown by numerous 40 Ar-39 Ar ages on phengite and Na-amphibole (Volkova et al., 2005). The blueschists are interpreted to result from subduction of an oceanic plate beneath an island arc in an intraoceanic fore-arc environment (Volkova & Sklyarov, 2007).
The Tsakhir Uul accretionary wedge shows remarkable lithological similarity with typical Gorny Altai blueschist bearing units that are composed of siliceous rocks, carbonate rocks and N-MORB basalts (the Uimon unit in the Gorny Altai and the Chara unit in the Rudny Altai). Recorded P-T conditions of $21 kbar and 600°C and mineral assemblage of the Tsakhir Uul eclogites are similar to eclogites associated with serpentinite me´langes of the Gorny Altai and plot on a P-T gradient of $8°C km )1 (Fig. 14). These data suggest that the Tsakhir Uul accretionary wedge eclogites originated by subduction of a young and ÔwarmÕ oceanic crust (Volkova et al., 2005). It is suggested that the eclogites of the SW Mongolia are connected to the same closure of small oceanic basins typical for the Gorny Altai subduction system, while the major subduction system operated to the west in the region of Kokchetav subduction-collision zone. The 40 Ar-39 Ar cooling ages of c. 540 Ma reported in this study show that the Tsakhir Uul accretionary wedge was cooled earlier than the Gorny Altai-Mongolian blueschists but at the same time as some eclogites of the Gorny Altai accretionary complex (Buslov & Watanabe, 1996) and at the same time as the high-pressure rocks of the Kokchetav subduction system. The true age of eclogites facies metamorphism remains presently unknown but we speculate that it is not older than the c. 570 Ma age of type locality of the Khantaishir ophiolite close to Altai city (Gibsher et al., 2001). This is also the Ar-Ar cooling age of muscovite from a greenschist facies orthogneiss basement underlying the Tsakhir Uul eclogite and Khantaishir ophiolite nappe pile .

Palaeo-Asian Oceanic suture in Mongolia
Our study shows that the eclogites from SW Mongolia show geochemical, structural and P-T affinity to the Gorny Altai blueschist and eclogite belts. These belts are aligned along the Tuva Mongol continental fragment in the Gorny Altai. However, so far no highpressure rocks were reported to occur along the Dzabkhan-Baydrag continental block. The discovery of the Tsakhir Uul eclogites thus represents an important finding suggesting extension of the Early Cambrian subduction system far to the east in the region where it was not expected (Zonenshain, 1973). The lithological character of the accretionary wedge associated with ophiolites, the chemistry of the eclogite protoliths and the cooling ages of the Tsakhir Uul accretionary wedge indicate the existence of a subduction system rimming both the Dzabkhan and the Tuva Mongol continental blocks and filling a gap in knowledge of the extension of the Proterozoic to the Cambrian Palaeo-Asian Ocean in the eastern part of the CAOB. Coeval exhumation of the Tsakhir Uul accretionary wedge and the Kokchetav high-pressure rocks may indicate a common tectonic evolution of the eastern and western terminations of the Palaeo-Asian Ocean in the Early Cambrian.