UHP terrane, western China: Petrological and isotopic constraints

Lithos 84 (2005) 51 – 76 www.elsevier.com/locate/lithos

Two contrasting eclogite cooling histories, North Qaidam HP/UHP terrane, western China: Petrological and isotopic constraints J.X. Zhanga,T, J.S. Yanga, C.G. Mattinsonb, Z.Q. Xua, F.C. Menga, R.D. Shia b

a Institute of Geology, CAGS Beijing, 100037, PR China Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA

Received 15 August 2002; accepted 1 February 2005 Available online 7 April 2005

Abstract Felsic gneisses near Yuka, western North Qaidam Mountains (NQD), and (60 km SE) near Xitieshan, central NQD, enclose eclogite-bearing mafic boudins. Inclusions in prograde-zoned garnets from a Yuka eclogite record pre-eclogite facies conditions of 12.1 F 2.1 kbar, 603 F 36 8C. The phengite-bearing matrix assemblage records peak conditions near or in the coesite stability field (23–28 kbar, 600–730 8C), but no coesite or coesite pseudomorphs have been found. Amphibolitized eclogite in the boudin margin records retrograde conditions of 10.9 F 1.2 kbar, 630 F 44 8C. Garnets from the Xitieshan eclogite lack prograde inclusions and compositional zoning. The bimineralic matrix assemblage records minimum eclogite-facies conditions of N 14 kbar, 730–830 8C. Symplectites of plagioclase and clinopyroxene record early retrograde conditions of 10–14 kbar, 750–865 8C. Positive e Nd values from two Yuka and five Xitieshan eclogites indicate a depleted mantle source, consistent with formation of the eclogite protoliths in a continental rift or incipient oceanic basin setting. A Sm–Nd WR–Grt–Cpx isochron age of 435 F 49 Ma obtained from one Xitieshan eclogite represents a cooling age. Zircon U–Pb TIMS and SHRIMP geochronology, combined with CL and inclusion analysis indicates a magmatic protolith age of ca. 750–800 Ma, and eclogite–facies zircon growth at 486–488 Ma. For the Yuka eclogite, exhumation to lower-crustal depths occurred before 477 F 8 Ma (amphibole 40 Ar–39Ar), and a 466 F 5 Ma phengite 40Ar–39Ar age indicates an average cooling rate of 13–19 8C/Ma. In contrast, the 407 F 4 Ma amphibole 40Ar–39Ar age from the Xitieshan eclogite indicates a much slower cooling rate of 3–4 8C/Ma. HP/UHP rocks of similar age 200 km SE near Dulan, eastern NQD, and in the Altun Mountains on the NW side of the Altyn Tagh fault suggest that the Altun, Yuka, Xitieshan, and Dulan localities are part of the same early Paleozoic HP/UHP metamorphic belt. D 2005 Elsevier B.V. All rights reserved. Keywords: North Qaidam Mountains; Eclogite; PT paths; Tectono–thermal history

1. Introduction

T Corresponding author. E-mail address: [email protected] (J.X. Zhang). 0024-4937/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2005.02.002

Eclogites and associated rocks are volumetrically minor components in orogenic belts, but provide invaluable information about orogenic processes. In

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Fig. 1. Geological sketch map of the northern Qaidam Mountains (a) and distribution of eclogites in the Yuka area (b) and Xitieshan area (c). Locations of selected samples for geochronological analyses are shown in b and c; the size of eclogite bodies is exaggerated for clarity.

J.X. Zhang et al. / Lithos 84 (2005) 51–76

recent years, considerable progress has been made in the study of the high-pressure/ultrahigh-pressure (HP/ UHP) metamorphic rocks in the North Qaidam Mountains (NQD), western China. Yang et al. (1994) described garnet peridotites enclosed in gneisses in the Luliangshan of the NQD, and subsequently, eclogites of the North Qaidam Mountains (NQD) were reported from the Yuka area, 50 km NW of Da Qaidam (Yang et al., 1998), and from the Dulan area (Yang et al., 2000) (Fig. 1a). The two localities are about 350 km apart, and so far no eclogites have been reported in the intervening area. This contribution reports on both the Yuka eclogite and recently discovered, moderately to strongly retrogressed eclogites in the Xitieshan area, about 60 km SE of Da Qaidam in the middle segment of the NQD. In order to constrain the age, P–T path, and protolith nature of the Xitieshan eclogites, and compare them to the Yuka eclogites, this contribution focuses on the comparison of the field relations, petrology, mineralogy, U–Pb, Sm–Nd and 40Ar–39Ar isotopic systematics between eclogites from the Yuka (three samples) and Xitieshan (six samples) areas. Mineral abbreviations of Kretz (1983) are used except Amp (amphibole), Phe (phengite), and as noted. The case for a different metamorphic evolution in the two parts of the NQD HP/UHP terranes has important significance for constraining the tectonic model of the formation and exhumation of HP/UHP rocks, and will also contribute to a better understanding of the geodynamic history in an important part of the Qilian–Altun orogenic belt.

2. Geological setting The North Qaidam Mountains are bounded by the Qaidam basin to the southwest, the Altyn Tagh fault to the northwest, and the Qilian block to the northeast (Fig. 1a). The Qaidam basin is a Cenozoic sedimentary basin underlain by an accreted foldbelt complex of the Qaidam basement (Wang and Coward, 1990). The basement consists of a Paleozoic fold belt in the southwest and a Proterozoic to Paleozoic complex in the northeast, based on published borehole, geophysical, and field data from the surrounding mountains (BGMQ, 1991). The Qilian block comprises mainly Paleozoic sedimentary rocks deposited over Precambrian metamorphic basement. The basement

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consists of granitic gneiss, pelitic gneiss, schist and marble (Wan et al., 2001). The Qilian block was previously considered to be a fragment rifted from the North China craton, but recent geochronological data demonstrate that the metamorphic basement of the Qilian block formed during 800–1000 Ma, indicating a close affinity to the Yangtze craton (South China plate; Guo et al., 1999; Wan et al., 2001). The North Qaidam Mountains mainly comprise amphibolites (commonly including eclogite blocks), para- and orthogneiss, schist, and marble of the Dakendaban metamorphic group thrust over the lower Paleozoic volcanic and sedimentary rocks of the Tanjianshan Group (Fig. 1b and c). The Dakendaban Group is intruded by early Paleozoic ultrabasic rocks and gabbros in the Yuka area, and granites in the Xitieshan area (Fig. 1b and c). Recent petrologic and geochronologic investigations show that the Dakendaban Group is a complex related to early Paleozoic subduction and collision (Zhang et al., 2001b; Wan et al., 2001).

3. Field relations of eclogites 3.1. Yuka eclogite Eclogites occur as boudins from one to tens of metres in width within granitic gneisses and muscovite schists (Figs. 1b and 2). Most eclogites are well preserved, but eclogite lenses commonly are rimmed by foliated amphibolites parallel to the foliation of the enclosing gneisses, and their margins show a strong shear deformation linked to amphibolitization of eclogite. Asymmetric boudins indicate a consistent top to the W or NW shear deformation (Fig. 2a and b). The amphibolite rims of the eclogite lenses are isofacial with the surrounding gneisses, which implies that the eclogites and the gneisses experienced a common late-stage metamorphism and deformation. The country rock foliations dip northeast, with E to SE plunging lineations (Fig. 1b). The granitic gneisses consist of Qtz + Pl + Kfs + Czo + Ms (3.1–3.2 Si p.f.u.), and the micaschists of Qtz + Phe (3.35–3.45 Si p.f.u.) + Grt + Pl + Rt, both with minor apatite and zircon. Although the country rocks are highly retrograded compared to the eclogites, eclogitic assemblages composed of Grt + Phe + Ky + Cld + Pg + Qtz have

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J.X. Zhang et al. / Lithos 84 (2005) 51–76

Fig. 2. Field views showing occurrence of eclogite in Yuka and Xitieshan area. a and b are from the Yuka area, c and d are from the Xitieshan area. The length of the rock hammer in a, b and d is 32 cm; the length of the pen used in c is 14 cm.

been recognized recently in the micaschists (Zhang et al., 2003). 3.2. Xitieshan eclogite The Dakendaban Group in the Xitieshan area mainly consists of garnet–sillimanite–biotite gneiss (paragneiss) and granitic gneiss (orthogneiss). These gneisses are folded by SSE–NNW trending open folds (Fig. 1c). Sub-horizontal stretching lineations marked by oriented sillimanite and biotite (Fig. 1c) suggest high-temperature deformation, possibly related to exhumation. Eclogites occur as boudins and lenses in gneisses (Fig. 2c and d), and are more extensively retrogressed than the Yuka eclogites. All eclogites are retrogressed to various degrees: some eclogites contain relict omphacites or Cpx–Pl symplectites in addition to garnet, but most eclogites are retrogressed to garnet amphibolites. Well-preserved eclogites are only found in a few outcrops preserved in the centers of large boudins (thicknesses up to 50 m). The successive transition from eclogite to garnet granulite (Grt + Cpx + Pl), then to garnet amphibolite (Grt + Amp + Pl), and finally to amphibolite (Amp + Pl) is visible from the center to the margin of the boudins. The gneisses are

characterized by upper amphibolite-facies mineral assemblages with local granulite-facies mineral assemblages, and show extensive migmatization. However, no eclogite-facies assemblages have been recognized thus far in the quartzofeldspathic gneisses.

4. Analytical methods More than 100 samples of eclogites and retrograded eclogites have been collected from the Yuka and Xitieshan area. Thirty thin sections from the Yuka area were studied, and 20 from the Xitieshan area. The following description focuses on representative samples of the well-preserved eclogites from the Yuka and Xitieshan area. Description of the retrograded eclogites is also included to illustrate retrograde conditions. Minerals were analyzed at the Institute of Geology and Geophysics, Chinese Academy of Sciences, using a CAME-CAUISX-51 electron probe (15 keV accelerating voltage, 20 nA beam current, and with counting time of 100 s), and at the University of Montpellier-2, Montpellier, France, using a CAMECA SX 100 electron probe (15 keV accelerating voltage, 15 nA beam current).

J.X. Zhang et al. / Lithos 84 (2005) 51–76

X-ray maps of garnets were made at the Laboratory of Continental Dynamics, Institute of Geology, CAGS, China, using a JEOL JSM-5610 LV scanning microscope equipped with an EDS Oxford ISIS analytical system. Operating conditions were an accelerating potential of 20 keV and a beam current of 45 nA. Amphiboles are classified according to Leake et al. (1997). Ferric iron recalculation for garnet and clinopyroxene follows Droop (1987).

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5. Petrography 5.1. Yuka eclogite Yuka eclogites are composed of garnet and omphacite with varying amounts of barroisite, zoisite (clinozoisite in some samples), phengite, rutile and quartz. These minerals are in direct contact with each other along sharp grain boundaries. Idioblastic to xenoblastic garnet grains are surrounded by oriented

Fig. 3. Photomicrograghs of eclogites in the Yuka area (a, b) and Xitieshan are (c–f). (a) Idioblastic garnet is surrounded by oriented omphacite, zoisite and phengite (97A25-1, crossed polarizers). (b) Garnet contains S-shaped inclusion trails defined by very small amphibole, quartz, epidote, and plagioclase. The outermost garnet rim is inclusion-free (QZ16-3-1, crossed polarizers). (c): Omphacites have been partly replaced by Cpx + Pl, and garnets rimmed by thin Cpx–Pl coronas (QZ19-9, crossed polarizers). (d) Cpx forms coronas around quartz and garnet grains (QZ19-2). (e) and (f): Exsolution quartz needles within omphacites (Qz19-9, e: plane polarized light, f: crossed polarizers). Symp=symplectite of Cpx + Pl.

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Table 1 Representative mineral compositions of eclogites from the Yuka area 97A25-1 R-Grt

39.10 0.02 22.41 0.01 22.51 0.29 7.28 8.14 0.02 0.00 99.78 3.00 0.00 2.03 0.00 0.00 1.44 0.02 0.83 0.70 0.00 0.00 8.01

38.10 0.10 21.73 0.00 23.07 0.94 4.58 11.05 0.03 0.00 99.60 2.97 0.01 1.99 0.00 0.05 1.45 0.06 0.53 0.92 0.01 0.00 8.01

37.23 0.14 22.00 0.00 23.25 3.55 2.02 11.35 0.03 0.00 99.57 2.95 0.01 2.05 0.00 0.03 1.51 0.24 0.24 0.94 0.01 0.00 7.99

54.61 0.07 9.38 0.00 3.03 0.04 10.45 16.09 5.74 0.00 99.41 1.95 0.00 0.39 0.00 0.01 0.08 0.00 0.56 0.61 0.40 0.00 4.00 38

Phe

AmpI AmpII Pl-C

54.79 53.58 52.87 50.61 49.16 0.04 0.35 0.34 0.28 0.33 8.43 27.68 27.34 10.99 10.91 0.01 0.00 0.02 0.00 0.05 3.91 1.52 1.56 7.73 9.85 0.02 0.03 0.00 0.05 0.06 10.00 4.29 4.16 15.57 15.00 16.29 0.01 0.00 9.44 8.37 5.18 0.44 0.48 3.13 3.55 0.00 9.58 9.64 0.30 0.33 98.67 97.48 96.41 98.10 97.61 1.99 3.45 3.44 7.04 6.88 0.00 0.02 0.02 0.03 0.04 0.36 2.10 2.10 1.80 1.80 0.00 0.00 0.00 0.00 0.01 0.03 0.04 0.02 0.20 0.43 0.08 0.04 0.07 0.70 0.73 0.00 0.00 0.00 0.01 0.01 0.54 0.41 0.42 3.23 3.13 0.63 0.00 0.00 1.41 1.26 0.36 0.06 0.06 0.74 0.96 0.00 0.79 0.80 0.05 0.06 3.99 6.91 6.93 15.30 15.28 34

65.97 0.00 21.67 0.00 0.36 0.00 0.00 2.50 10.44 0.01 100.93 2.88 0.00 1.11 0.00 0.01 0.00 0.00 0.00 0.12 0.88 0.00 5.00

Ep-C Grt

QZ16-3-1 AmpIII Pl

38.96 39.24 49.50 0.13 0.12 0.12 28.26 22.21 8.26 0.13 0.02 0.01 6.33 26.36 12.24 0.05 0.74 0.09 0.04 2.91 11.95 23.94 9.68 12.80 0.01 0.03 2.35 0.00 0.00 0.15 97.86 101.31 97.67 3.03 3.03 7.36 0.01 0.01 0.01 2.59 2.02 1.45 0.01 0.00 0.00 0.29 0.00 0.00 0.09 1.70 1.52 0.00 0.05 0.01 0.01 0.34 2.65 2.00 0.80 2.04 0.00 0.00 0.68 0.00 0.00 0.03 8.03 7.95 15.75

R-Grt

R-Grt

C-Grt

Omp Omp Phe

Phe

64.40 37.88 37.82 37.86 55.12 55.51 50.93 50.25 0.00 0.06 0.13 0.11 0.03 0.03 0.69 0.48 19.73 21.70 21.87 21.39 9.96 10.45 25.99 26.91 0.00 0.02 0.02 0.02 0.00 0.09 0.08 0.11 0.56 27.07 25.39 26.70 4.94 4.28 2.05 1.96 0.00 0.85 0.90 0.56 0.03 0.04 0.08 0.04 0.74 4.86 4.43 2.48 9.19 9.34 4.24 3.91 4.85 8.03 9.69 11.45 14.29 13.60 0.03 0 9.52 0.03 0.01 0.05 6.01 6.32 0.68 0.65 0.03 0.01 0.00 0.00 0.00 0.00 10.13 10.17 99.83 100.51 100.26 100.62 99.55 94.91 94.48 2.87 2.95 2.96 2.97 1.98 1.98 3.41 3.38 0.00 0.00 0.01 0.01 0.00 0.00 0.04 0.03 1.03 1.99 2.02 1.98 0.42 0.44 2.05 3.13 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.02 0.10 0.09 0.07 0.04 0.03 0.00 0.00 0.00 1.65 1.57 1.67 0.10 0.09 0.12 0.11 0.00 0.06 0.06 0.04 0.00 0.00 0.01 0.00 0.05 0.56 0.52 0.29 0.49 0.50 0.42 0.39 0.23 0.67 0.81 0.96 0.55 0.52 0.00 0.00 0.82 0.01 0.00 0.01 0.42 0.44 0.09 0.08 0.00 0.00 0.00 0.00 0.00 0.00 0.87 0.87 5.02 7.99 8.04 8.00 4.00 4.00 7.01 7.00 39 42

Garnet normalized to 12 oxygens; pyroxenes to 6 oxygens; phengites to 11 oxygens; amphiboles to 23 oxygens; plagioclases to 8 oxygens; epidote to 12.5 oxygens. C-Grt: core composition; M-Grt:middle composition; R-Grt: rim composition; Pl-C: plagioclase occurring as inclusion in garnet core; Ep-C: epidote occurring as inclusion in garnet core. The approach of calculation used for Fe3+ in garnet, clinopyroxence and amphibole is according to Droop (1987).

J.X. Zhang et al. / Lithos 84 (2005) 51–76

SiO2 39.32 0.01 TiO2 Al2O3 22.83 Cr2O3 0.01 FeO 22.09 MnO 0.42 MgO 7.72 CaO 8.46 Na2O 0.03 0.00 K2O Total 100.88 Si 2.98 Ti 0.00 Al 2.04 Cr 0.00 Fe3+ 0.01 1.39 Fe2+ Mn 0.03 Mg 0.87 Ca 0.69 Na 0.00 K 0.00 Total 7.98 Jd (mol%)

97A26

R-Grt M-Grt C-Grt Omp Omp Phe

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Fig. 4. X-ray compositional maps showing the distribution of (a) MgO, (b) CaO, (c) MnO and (d) FeO in a garnet from eclogite 97A25-1 from the Yuka area.

omphacite, barroisite, zoisite and phengite, which define the foliation and lineation (Fig. 3a). This assemblage is volumetrically dominant and is interpreted to correspond to the metamorphic peak.

In thin section, large garnets (N 2 mm) show conspicuous cores with inclusions of epidote, amphibole, quartz and plagioclase surrounded by inclusion-poor margins. S-shaped inclusion trails in

Fig. 5. BSE image (a) and composition profile (b) of garnet of the Yuka eclogite 97A25-1 (this garnet is same as that in Fig. 4).

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Fig. 6. Representative garnet (a) and pyroxene (b) compositions of the Yuka eclogites and Xitieshan eclogites plotted after Coleman et al. (1965) and Essene and Fyfe (1967), respectively.

some garnet porphyroblasts indicate syntectonic growth (Fig. 3b). In retrograded domains, omphacite is partially replaced by crypto-crystalline symplectites (b 1 Am) of Cpx + Pl, and in intensely amphibolitized eclogite, symplectites of Cpx + Pl are replaced by Amp + Pl, and phengite is replaced by Bt + Pl symplectite. Three types of amphibole are recognized: the first type (AmpI) is magnesiohornblende, and occurs as inclusions associated with epidote and plagioclase within garnet cores; the second type (AmpII) is barroisite, which exhibits sharp grain boundaries with intergrown omphacite, phengite and garnet, and occurs associated with phengite and omphacite as inclusions in garnet margins, indicating that it formed in equilibrium with garnet rim, omphacite and phengite; the third type (AmpIII) is edenitic to pargasitic, and occurs in kelyphitic rims around garnet, or in vermicular symplectite with plagioclase near garnet (Table 1). The third type of amphibole mainly occurs in intensely amphibolitized eclogite (97A26) surrounding well-preserved eclogite (97A25-1). The textural relations observed suggest the following sequences of mineral assemblages: (1) garnet (core)–epidote–amphibole–plagioclase– quartz (prograde stage) (2) garnet (rim)–omphacite–phengite–barroisite F zoisite or clinozoisite–quartz (peak stage) (3) garnet (garnet in intensely amphibolitized eclogite)–amphibole–plagioclase–quartz (retrograde stage).

5.2. Xitieshan eclogite The peak assemblage of the well-preserved eclogites consists of Grt + Omp (CpxI) + Rt + Qtz (Fig. 3c).

Fig. 7. BSE image and composition profile of garnet of the Xitieshan eclogite QZ19-9.

J.X. Zhang et al. / Lithos 84 (2005) 51–76

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Table 2 Representative mineral compositions of eclogites from the Xitieshan area QZ19-9 SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total Si Ti Al Cr Fe2+ Fe3+ Mn Mg Ca Na K Total Jd (mol%)

QZ19-2

C-Grt

M-Grt

R-Grt

Omp

Omp

C-Grt

R-Grt

CpxII

CpxIII

Pl

Amp

41.14 0.02 22.72 0.08 14.50 0.32 13.55 7.87 0.03 0.00 100.23 3.02 0.00 1.96 0.01 0.84 0.05 0.02 1.48 0.62 0.00 0.00 7.98

40.78 0.03 23.38 0.05 14.80 0.27 13.38 8.07 0.02 0.00 100.78 2.98 0.00 2.01 0.00 0.87 0.03 0.02 1.46 0.63 0.00 0.00 8.00

40.54 0.04 22.30 0.26 18.12 0.37 10.85 8.47 0.02 0.00 100.97 2.99 0.00 1.94 0.02 1.11 0.06 0.02 1.19 0.67 0.00 0.00 8.00

55.43 0.05 7.43 0.00 2.15 0.00 12.26 18.83 3.63 0.04 99.82 1.99 0.00 0.31 0.00 0.07 0.00 0.00 0.65 0.72 0.25 0.00 3.99 25.9

54.29 0.14 7.60 0.15 2.51 0.09 12.91 19.20 3.04 0.00 99.93 1.95 0.00 0.32 0.00 0.08 0.00 0.00 0.69 0.74 0.21 0.00 3.99 22.0

39.51 0.03 22.01 0.06 21.27 0.38 8.28 8.16 0.00 0.00 99.70 3.00 0.00 1.97 0.00 1.31 0.04 0.02 1.00 0.66 0.00 0.00 8.00

39.17 0.00 22.05 0.04 20.84 0.36 7.88 9.01 0.01 0.00 99.36 2.98 0.00 1.98 0.00 1.32 0.07 0.02 0.89 0.73 0.00 0.00 7.99

54.01 0.05 2.29 0.05 6.51 0.09 13.88 21.78 1.11 0.00 99.77 1.99 0.00 0.10 0.00 0.20 0.00 0.00 0.76 0.86 0.08 0.00 3.99 8.4

54.75 0.06 0.96 0.05 7.95 0.14 13.94 22.68 0.48 0.00 101.01 2.002 0.002 0.042 0.001 0.248 0.000 0.005 0.774 0.995 0.021 0.000 4.000 3.4

60.68 0.02 24.31 0.00 0.19 0.00 0.00 6.06 8.41 0.00 99.70 2.71 0.00 1.28 0.00 0.00 0.07 0.00 0.00 0.29 0.73 0.00 5.01

48.896 1.573 7.542 0.129 10.555 0.089 15.053 11.949 1.022 0.271 96.979 7.05 0.17 1.28 0.02 1.13 0.14 0.01 3.23 1.85 0.29 0.05 15.23

Garnet normalized to 12 oxygens, pyroxenes to 6 oxygens, amphiboles to 23 oxygens, plagioclases to 8 oxygens. C-Grt: core composition; MGrt: middle composition; R-Grt: rim composition. The approach of calculation used for Fe3+ in garnet, clinopyroxence and amphibole is according to Droop (1987).

Idioblastic to xenoblastic garnets (b1 mm) are surrounded by retrograde Pl and Cpx + Pl or Amp (pargasite) + Pl coronas. Most omphacites contain crystallographically oriented quartz needles which are about 2–3 Am wide and 50–200 Am long (Fig. 3e and f; confirmed with the EMP). Omphacite is

typically replaced by fine-grained vermicular symplectite (5–20 Am) and coarse-grained sieve-texture symplectite (20–100 Am) of CpxII + Pl. The symplectites are much coarser than those in the Yuka eclogite, and are overgrown by coarser bspotsQ of Cpx (CpxIII). CpxIII also forms a corona around quartz grains near

Table 3 Average P–T according THERMOCALC v3.1 Sample

T (8C)

P (kbar)

a H2O

System

End member

97A25-1 (core)

603 F 36 565 F 37 637 F 56 699 F 56 565 F 46 574 F 47 635 F 44 594 F 40

12.1 F1.3 11.5 F 1.4 25.0 F 2.4 25.7 F 2.2 23.7 F 2.4 25.0 F 2.3 10.9 F 1.2 10.2 F 1.1

1.0 0.5 1.0 1.0 1.0 1.0 1.0 0.5

NCFMASH

py gr alm cz ep fep fact ts parg gl an ab q H2O

KNCFMASH

py gr alm di hed jd mu pa cel q H2O

KNCFMASH

py gr alm di hed jd mu pa cel q H2O

NCFMASH

py gr alm tr fact ts gl an ab q H2O

97A25-1 QZ16-3-1 97A26

Abbreviation of end members is after Holland and Powell (1998); activities are from ax 1998 program (Holland and Powell, 1998); quartz is considered to be in excess.

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6. Mineral chemistry

uniform compositions (Prp11–12Alm57–58Grs27–28 Sps1.5–2.0). Omphacite has a content of 34–42 mol% jadeite (Table 1, Fig. 6b). Clinopyroxene in symplectites is too fine (b1 Am) to analyze. Phengite contains 3.41–3.45 Si p.f.u. (Table 1). Composition of AmpI is magnesiohornblende, AmpII is barroisite, and AmpIII is edenite to pargasite. Plagioclase inclusions in garnet cores have an anorthite component of 0.1–0.2. Plagioclase in intensely amphibolitized eclogite has slightly higher anorthite content (0.2–0.3).

6.1. Yuka eclogite

6.2. Xitieshan eclogite

Most garnets of the well-preserved eclogites (Prp8–29Alm45–54Grs23–32Sps0.9–9.0) show a prograde bell-shaped MnO profile, with a very sharp rise in MgO and an equally sharp fall in CaO at the core/ rim boundary (Figs. 4, 5, and 6a). This pattern can result from prograde garnet growth at relatively low temperature (core zone) followed by a second major growth stage at higher temperature and pressure (O’Brien and Sachan, 2000). Garnets of the intensely amphibolitized eclogites have rather

Most garnets of the well-preserved eclogite have homogeneous garnet cores surrounded by rims with increasing Fe/(Fe + Mg) (Fig. 7). These are interpreted to reflect homogenization at high temperature, followed by retrograde Fe/Mg re-equilibration related to volume diffusion in garnet and adjacent minerals. The CpxI have a Jd content of 22–26 mol% (Table 2; Fig. 6b) decreasing slightly from core to rim. The jadeite content is much lower than that of the Yuka eclogite, and may reflect the low sodium content of the bulk

the garnet (Fig. 3d). The coexistence of plagioclase with Cpx heralds the transition from the eclogite facies to the high-pressure granulite facies. Orthopyroxene is conspicuously absent in the retrograde assemblage, possibly due to the fact that temperatures of the post-eclogite facies stage were never high enough nor pressures low enough to stabilize Opx. Subsequent growth of Amp + Pl indicates an amphibolite-facies overprint.

Fig. 8. Proposed P–T path for the Yuka eclogite (a) and the Xitieshan eclogite (b). a is based on P–T estimates of sample 97A25-1 and 97A26; b is based on P–T estimates of sample Qz19-9 and QZ19-2. Ab = Jd + Qtz and stability of clinopyroxene (Jd = 25 and 35) are after Holland (1980); Grt–Cpx geothermometer used in eclogite facies stage of Fig. 10b is after Krogh, (1988) and Powell, (1985); Al-silicates is after Holdaway (1971); coesite=quartz: calculated with program THERMOCALC v3.1.

J.X. Zhang et al. / Lithos 84 (2005) 51–76

61

Fig. 9. Thermobarometry results for garnet–omphacite–phengite equilibria in the Yuka eclogites (97A25-1 and QZ16-3-1). 1–3: garnet omphacite Fe–Mg exchange geothermometer (1: Ravna, 2000; 2: Krogh, 1988; 3: Powell, 1985); 4: garnet phengite Fe–Mg exchange geothermometer (Green and Hellman, 1982); 5–7: 2Grs + Prp + 3cel = 6Di + 3Ms (Grt–Omp–Phe barometer) (5: Ravna and Terry, 2001; 6: Waters and Martin, 1993; 7: calculated with program THERMOCALC v3.1, related data from Holland and Powell, 1998; Powell et al., 1998); coesite=quartz: calculated with program THERMOCALC v3.1.

Fig. 10. Thermobarometry results for mineral equilibria in retrograde eclogite (QZ19-2) 1–4: Prp + 3Hd = Alm + 3Di (Grt–Cpx thermometer) (1: Krogh, 1988; 2: Powell, 1985; 3: TWQ v1.02 program; 4: Ravna, 2000); 5–6: 3Hd + 3An = 2Grs + Alm + 3Qtz (5: TWQ v1.02 program; 6: THERMOCALC 3.1); 7–9: 3Di + 3An = Prp + 2Grs + 3Qtz (Grt–Cpx–Pl-Qtz barometer) (7: TWQ v1.02 program; 8: THERMOCALC 3.1; 9: Newton and Perkins, 1982). TWQ v1.02 program is based on data set Jun92.gsc. The solid solution models used in the calculations are from Berman (1990) for garnet, Fuhrman and Lindsley (1988) for plagioclase, and ideal models are used for clinopyroxence.

62

J.X. Zhang et al. / Lithos 84 (2005) 51–76

Table 4 Sm–Nd analytical results for the Yuka eclogites (97A25-1 and QZ16-3-1) and Xitieshan eclogites (QZ19-9 and XT-33, 37, 49, 51) Sample no.

Sm (ppm)

Nd (ppm)

147

Sm/144Nd

143

F 2j

Yuka eclogite 97A25-1 Wr 97A25-1 Cpx 97A25-1 Grt QZ16-3-1 Wr

4.550 2.806 0.9058 2.666

16.316 10.506 3.052 8.518

0.1687 0.1616 0.1808 0.1893

0.512677 0.512636 0.512776 0.512837

6 7 13 8

Xitieshan eclogite QZ19-9 Wr QZ19-9Cpx QZ19-9 Grt XT-37 XT-49 XT-51 XT-33

1.004 3.078 0.499 0.9722 3.3030 0.9898 1.126

2.617 12.760 0.200 2.745 11.660 2.649 2.649

0.2319 0.1458 1.5120 0.2142 0.1714 0.2260 0.2570

0.513175 0.512695 0.516594 0.513201 0.512767 0.513194 0.513110

11 6 9 17 16 9 18

rock. CpxII of symplectite have a Jd content of 7–9 mol%, CpxIII overgrowing symplectite have a Jd content of 3–5 mol%, and the composition of amphibole is pargasite.

7. PT estimations Three approaches were used to make estimates of metamorphic conditions for elcogites and retrograded eclogites in the Yuka and Xitieshan area. The first is conventional geothermobarometry, discussed below. The second uses the THERMOCALC v3.1 program described by Powell et al. (1998) and the related internally consistent thermodynamic database (Holland and Powell, 1998). The third uses the TWQ v2.02 program as described by Berman (1991) using the end-member properties and thermodynamic database of Berman (1990). Where possible, multiple approaches are used, and the similarity of the different estimates is consistent with equilibrium.

Nd/144Nd

e Nd(0)

e Nd (800 Ma)

0.76

3.62

3.88

4.64

10.47

6.88

10.98 2.52 10.85 9.21

9.20 5.10 7.86 3.05

strates that garnet does not display significant Fe or Mg zoning adjacent to the inclusions. Application of THERMOCALC v3.1 program to the coexisting Grt core–Amp–Pl–Ep–Qtz inclusion assemblage of sample 97A25-1 (for a H2O = 1.0) gives a P–T condition of P = 12.1 F 2.1 kbar and T = 603 F 36 8C (Table 3, Fig. 8a). A lower a H2O(0.5) shifts the PT estimates to lower temperature (30–40 8C) and lower pressure (0.6 kbar; Table 3). Similar results are given by the TWQ program ( P = 12.6 F 2.0 kbar and T = 612 F 32 8C, assuming a H2O = 1). Peak P–T conditions are estimated using garnet– omphacite–phengite barometry (Waters and Martin, 1993; Ravna and Terry, 2001), and garnet–omphacite

7.1. Yuka eclogite The preservation of amphibole + plagioclase + epidote inclusions indicates that garnet growth began in the epidote–amphibolite facies. Thermobarometry using inclusion suites requires that garnet and the included minerals represent an original equilibrium assemblage, and have not changed compositions subsequent to entrapment. X-ray mapping demon-

Fig. 11. Sm–Nd isochron diagram for the Xitieshan eclogite (QZ199) (data from sample 97A25-1 also was showed in the diagram). Grt=garnet, Cpx=clinopyroxene (omphacite), WR=whole rock.

Table 5 TIMS U–Pb isotopic data for zircons from eclogites in the Yuka and Xitieshan area Sample features

Concentrations

Apparent ages F 2 r (Ma)

Common Isotopic atomic ratios 206

204

208

206

206

238

207

235

207

206

206

238

207

235

207

206

Grains no. Colour and morphology

Wt U Pb Pb (ng) (Ag) (ppm) (ppm)

QZ16-3-1 1

10

174

18

0.100

187

0.2051

0.0791 F19

0.624 F 22

0.0572 F 13

491 F12

492 F 13

501 F 9

10

345

30

0.017

1033

0.1566

0.0780 F 21

0.613 F 23

0.0570 F 13

484 F 13

484 F 14

491 F 9

15

220

19

0.014

1138

0.1417

0.0786 F 13

0.618 F 14

0.0570 F 8

488 F 7

489 F 7

492 F 4

20

153

20

0.130

156

0.04402

0.0943 F 21

0.874 F 27

0.0672 F 12

581 F13

638 F 15

844 F 9

20

205

21

0.050

517

0.06795

0.0947 F 17

0.912 F 22

0.0698 F 10

583 F 10

658 F 12

923 F 5

20

86

14

0.006

2316

0.5383

0.1190 F 25

1.044 F 30

0.06359 F 111

725 F 15

726 F 18

728 F 10

10

74

17

0.006

1011

1.081

0.1217 F 58

1.086 F 68

0.06469 F 237

740 F 31

746 F 36

764 F 21

10 20

68 165

14 45

0.003 0.160

1702 187

0.7435 0.9120

0.1260 F 63 0.1326 F 21

1.130 F 74 1.214 F 29

0.06509 F 245 0.06638 F 106

765 F 35 803 F 13

768 F 37 807 F 15

777 F 22 819 F 10

20

132

31

0.160

130

0.7200

0.1118 F 24

0.987 F 30

0.06404 F 126

683 F 14

697 F 16

743 F 11

20 20

63 123

16 16

0.019 0.043

456 299

1.463 0.5614

0.1068 F 56 0.0784 F 28

0.941 F 66 0.623 F 32

0.06392 F 264 0.05761 F192

654 F 30 487 F 16

673 F 33 492 F 18

739 F 24 515 F 17

20

165

43

0.210

136

0.7601

0.1233 F 19

1.183 F 25

0.06959 F 88

749 F 12

793 F 15

916 F 8

10

807

69

0.008

5098

0.1926

0.0784 F 39

0.6126 F 42

0.05669 F 23

486.5 F 2.4

485.2 F 2.7

479.3 F 1.7

10

335

32

0.055

312

0.1319

0.0781 F 205 0.6121 F 233 0.05685 F 140 484.8 F 12.7 484.9 F 13.8 485.6 F 14.1

10

381

33

0.022

870

0.1234

0.0794 F 187 0.6149 F 205 0.05689 F 120 486.5 F 11.6 486.7 F 12.4 487.3 F 12.1

2 3 4

QZ19-9 1 2 3 4 5 6 7 8

QZ19-2 1 2 3 206

Colourless, long-prism Colourless, rounded Colourless,prism Colourless, rounded Light-red, clear prism Colourless,prism Light-red, rounded Light-brown, prism

Light-yellow, long prism Light-purplish, short prism Light-purplish, short prism

Pb

Pb/

Pb

Pb

U

Pb/

U

Pb/

Pb

Pb/

U

Pb/

U

Pb/

Pb

J.X. Zhang et al. / Lithos 84 (2005) 51–76

5

Light-yellow, rounded Light-yellow, rounded Light-yellow, rounded Light-purplish, rounded Light-yellow, short-prism

Pb/

Pb/204 Pb corrected for blank (Pb = 0.05 ng, U = 0.002 ng), the other ratios involve radiogenic Pb. 63

64

J.X. Zhang et al. / Lithos 84 (2005) 51–76

and garnet–phengite thermometry (Krogh, 1988; Powell, 1985; Ravna, 2000; Green and Hellman, 1982). X-ray and compositional profiles of garnet do not show evidence for volume diffusion between the garnet rim and adjacent minerals. Garnet rim compositions and rims of adjacent omphacite and phengite or omphacite and phengite inclusions in garnet rims indicate equilibrium conditions of P = 23–28 kbar and T = 600–730 8C (Figs. 8a and 9), close to or in the coesite field, although no coesite or coesite pseudomorphs have been discovered in these rocks. Similar conditions are indicated by THERMOCALC v3.1 program ( P = 24–25 kbar and T = 570–700 8C, assuming a H2O = 1; Table 3). A lower a H2O(0.5) shifts the PT estimate to unrealistically low temperature

(T b 500 8C). The TWQ program was not used to constrain the peak conditions due to the absence of phengite from the Berman (1990) database. Compositions of garnet, amphibole and plagioclase of intensely amphibolitized eclogite (97A26) around well-preserved eclogite (97A25-1) were used for P–T estimations of retrograde conditions. Using THERMOCALC v3.1 program (for a H2O = 1.0), a P–T condition P = 10.9 F 1.2 kbar and T = 630 F 44 8C is obtained (Table 3) if we assume domain equilibrium among the re-equilibrated garnet and newly formed amphibole + plagioclase. A lower a H2O(0.5) shifts the PT estimate to lower temperature (30–40 8C) and lower pressure (0.7 kbar; Table 3). Similar results are given by the TWQ

Fig. 12. U–Pb concordia diagrams of zircon analyses from eclogite of the Yuka (a) and Xitieshan area (b, c and d). Error bars in a, c and d are 2sigma errors (at 95% confidence level), and that in b is 1-sigma error (at 95% confidence level).

J.X. Zhang et al. / Lithos 84 (2005) 51–76

program ( P = 11.5 F 1.3 kbar and T = 631 F 40 8C, assuming a H2O = 1). Petrographic observations, textural relations, and PT estimates of eclogitic metapelites reported by Zhang et al. (2003) indicate a PT path similar to that of the eclogites: P = 10.7 F 3.1 kbar, T = 564 F 22 8C (prograde stage); P = 23–31 kbar, T = 615–700 8C (peak stage); and P = 12.2 F 2.6 kbar, T = 581 F 20 8C (retrograde stage). 7.2. Xitieshan eclogite For the Xitieshan eclogites, a quantitative estimate of the peak pressure is not possible due to the absence of minerals other than garnet and clinopyroxene in the peak metamorphic assemblage. The garnet with the lowest Fe/(Fe + Mg) ratio and the clinopyroxene with the most jadeite-rich composition are used for estimation of eclogite facies temperature in order to get a bnear-peakQ condition because the increasing Fe/ (Mg + Fe) ratios in the garnet rims is probably related

65

to retrograde volume diffusion, and the clinopyroxene is replaced partly by symplectites of Cpx + Pl. The garnet–omphacite thermometer (Powell, 1985; Krogh, 1988; Ravna, 2000), and the geobarometer based on the jadeite content of omphacite in the presence of quartz (Holland, 1980) indicate eclogite facies conditions of T = 730–830 8C and P N 14 kbar (minimum pressure estimate due to the absence of plagioclase; Fig. 8b). No prograde garnet zoning or inclusions are retained: these were probably obliterated during hightemperature eclogite-facies recrystallization. The omphacite contains abundant oriented quartz needles (Fig. 3e and f; confirmed with the EMP). Parallel SiO2 needles within Omp have been suggested to be an exsolution product from a precursor, non-stoichiometric Cpx which contained excess silica (Zhang and Liou, 1999). Although these exsolution textures do not provide unequivocal proof of UHP metamorphism, some authors (e.g.Carswell and Zhang, 1999; Tsai and Liou, 2000) suggest that P N 25 kbar is required to stabilize supersilicic Cpx.

Fig. 13. Mineral inclusions within zircons from Sample 97A25-1 (these mineral inclusions were identified by Laser Raman spectrophotometry).

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J.X. Zhang et al. / Lithos 84 (2005) 51–76

Compositions of Cpx + Pl symplectite and adjacent garnet in a retrograde eclogite (QZ19-2) were chosen for geothermobarometry of post-eclogite P–T conditions because local equilibrium between garnet rims and newly formed clinopyroxene and plagioclase is hypothesized. Two generations of Cpx formed in local equilibrium with Grt, Pl and Qtz. Pressure–temperature conditions were evaluated using the Grt–Cpx geothermometer (Powell, 1985; Krogh, 1988), Grt– Cpx–Pl–Qtz barometer (Newton and Perkins, 1982), TWQ program as described by Berman (1991), and the THERMOCALC v3.1 program (Holland and

Powell, 1998). Temperatures of 750–865 8C and pressures of 10–14 kbar were obtained for both CpxII and CpxIII (Fig. 10).

8. Isotopic data of the eclogites Two eclogites from the Yuka area were selected for Sm–Nd isotope mineral–whole rock analysis, and one sample for zircon U–Pb analysis by TIMS (conventional isotope dilution thermal ionization mass spectrometry) and 40Ar–39Ar dating of amphibole and

Table 6 40 Ar–39Ar data for phengite and amphibole from eclogites in the Yuka and Xitieshan area Temperature (8C)

(40Ar/39Ar)m

(36Ar/39Ar)m

(37Ar/39Ar)m

39

Ar  10

14

(mol)

Cumulative

39

Ar (%)

Age F 2r (Ma)

Phengite from sample 97A25-1 W = 94.40 mg J = 0.013851 500 26.0961 0.0109 0.0092 610 23.9524 0.0060 0.0083 730 21.7763 0.0022 0.0008 810 21.6044 0.0009 0.0006 890 21.9859 0.0016 0.0049 950 21.7829 0.0019 0.0042 1010 22.1084 0.0021 0.0236 1070 21.9498 0.0018 0.0258 1140 21.9126 0.0017 0.0237 1240 22.1045 0.0036 0.0529 1320 19.9871 0.0084 0.1058 1400 19.1347 0.0214 0.2111

133.49 218.62 1056.85 2276.37 1059.74 324.95 172.92 552.85 162.65 190.87 80.85 50.38

2.13 5.61 22.43 58.68 75.55 80.73 83.48 92.28 94.87 97.91 99.20 100.00

496.7 F 5.4 483.1 F 5.6 463.0 F 4.1 466.9 F 4.2 470.2 F 4.2 464.5 F 4.2 469.9 F 4.2 468.5 F 5.3 468.2 F 4.2 461.5 F 5.3 391.8 F 9.0 295.1 F 8.7

Amphibole from sample 97A25-1 W = 117.1 mg J = 0.013605 500 57.2727 0.0909 3.0657 610 37.5000 0.0429 2.1679 700 26.8781 0.0171 2.1933 800 24.5325 0.0075 2.2847 900 25.2791 0.0128 2.3527 1000 22.8824 0.0044 2.2812 1100 24.5143 0.0071 0.7226 1200 23.8667 0.0065 3.1225 1300 22.4085 0.0070 3.1664 1400 28.5294 0.0323 19.8368

11.00 28.00 61.50 73.80 43.00 34.00 70.00 54.00 21.30 17.00

2.66 9.42 24.40 42.27 52.66 60.87 77.78 90.82 95.89 100.00

629.6 F 9.2 528.7 F 7.7 472.8 F 7.4 482.3 F 5.8 466.7 F 6.7 468.1 F 5.6 481.1 F 5.7 476.6 F 6.1 445.7 F 6.0 447.6 F 12.6

Amphibole from sample QZ19-2 W = 2195.85 mg J = 0.014000 500 482.0782 1.5400 5.7896 600 110.8159 0.3306 4.2872 700 30.0280 0.0346 13.2049 800 65.7471 0.1756 10.7389 900 26.2008 0.0297 10.5806 1000 19.4437 0.0075 10.4631 1100 19.2819 0.0066 10.6697 1200 18.8091 0.0058 11.0064 1300 19.4104 0.0037 10.6429 1400 21.4040 0.0095 42.5600

41.05 49.80 97.79 75.91 158.08 82.36 310.49 335.68 962.05 66.84

1.88 4.17 8.65 12.14 19.39 23.16 37.41 52.80 96.93 100.00

587.9 F 89.0 311.5 F 18.3 464.3 F 5.5 338.4 F 6.7 411.9 F 5.9 407.6 F 4.2 410.0 F 4.5 406.3 F 4.0 430.0 F 4.1 491.7 F 5.4

J.X. Zhang et al. / Lithos 84 (2005) 51–76

phengite. Five fresh and retrograded eclogites from the Xitieshan area were selected for Sm–Nd isotope anlyses, and one fresh eclogite and one retrograded eclogite for zircon U–Pb analysis (TIMS and

67

SHRIMP) and 40Ar–39Ar dating of amphiboles. TIMS dating was conducted at Tianjin Institute of Geology and Mineral Resources, CAGS; SHRIMP dating was carried out in the Ion Microprobe Laboratory at

Fig. 14. Age spectrum and isochron diagram for phengite and amphibole of the eclogite in the Yuka area (a, b, c, d), and for amphibole of the eclogite in the Xietieshan area (e, f).

68

J.X. Zhang et al. / Lithos 84 (2005) 51–76

Stanford University. Sm–Nd isotope analyses and 40 Ar–39Ar dating were conducted at the Institute of Geology, CAGS (isotope analytical procedures are attached in the Appendix). 8.1. Yuka eclogite 8.1.1. Sm–Nd isotopic data The results of whole-rock Sm–Nd isotope analyses of samples 97A25-1 and QZ16-3-1 and of mineral Sm–Nd isotope analyses of 97A25-1 are given in Table 4. Similar Sm/Nd ratios and 143 Nd/144Nd ratios of the whole rock, garnet, and clinopyroxene from sample 97A25-1 (Fig. 11) imply that the REE have not achieved equilibrium partitioning at the millimeter scale. An isochron age representing eclogite facies metamorphism was not obtained due to this lack of isotopic homogenization during metamorphism (Griffin and Brueckner, 1985; Li et al., 1999). The e Nd(T) values (based on an 800 Ma U–Pb protolith age, Zhang et al., in review) and e Nd(0) values are intermediate between a chondritic uniform reservoir and the depleted mantle.

8.1.2. Zircon U–Pb dating Clear, inclusion-free, spheroidal zircon grains from eclogite sample QZ16-3-1 were analyzed by the U–Pb TIMS method (Table 5, Fig. 12a). The apparent ages of No.1–3 zircon fractions are concordant within error, and yield a weighted mean 206 Pb/238U age of 488 F 6 Ma, which is indistinguishable from the 495 F 7 Ma age previously reported for sample 97A25-1 (Yang et al., 2001a; Zhang et al., 2000). The discordia through No.4 and No.5 zircon fractions yields a poorly constrained early Proterozoic age. Although only inclusion-free zircons were used for TIMS dating, abundant Omp, Grt, and Rt inclusions identified by laser Raman spectroscopy in many other grains (Fig. 13) indicate that 488 F 6 Ma represents the age of eclogite-facies zircon growth. 8.1.3. 40Ar–39Ar isotopic data Amphibole and phengite were separated from sample 97A25-1 for 40Ar–39Ar dating. In this sample, only AmpII (barroisite) in equilibrium with garnet, omphacite and phengite was present. The amphibole isochron age is 477 F 8 Ma, and the plateau age of

Fig. 15. Cathodoluminescence (CL) images of eclected zircon grains from sample QZ19-9. Analysis numbers in each image correspond to Table 6.

J.X. Zhang et al. / Lithos 84 (2005) 51–76

476 F 6 Ma includes 80% of 39Ar released (Table 6, Fig. 14a, b). The phengite isochron age is 466 F 5 Ma, and the plateau age of 466 F 1 Ma includes 90% of 39 Ar released (Table 6, Fig. 14c, d). The trapped 40 Ar/36Ar ratio for both samples is statistically indistinguishable from the atmospheric value of 295.5 (Fig. 14b, d). 8.2. Xitieshan eclogite 8.2.1. Sm–Nd isotopic data The large spread in Sm/Nd among whole rock, garnet, and clinopyroxene of sample QZ19-9 indicates an equilibrium distribution of REE, and yields an isochron age of 436 F 49 Ma (Table 4 and Fig. 11). Some workers propose that the closure temperature of the Sm–Nd system depends mainly on the cooling rate and the mineral grain size (e.g. Mezger et al., 1992). The slower cooling rate (4 8C/Ma, see below) and the fine grain size of minerals (b1 mm) suggest a closure temperature of ca. 600 8C for Grt and Cpx in our

69

sample (Mezger et al., 1992). Peak temperatures estimated for this sample are significantly higher than the closure temperature, so the Sm–Nd isochron is interpreted as a cooling age. Sample QZ19-9 and the four additional samples selected for whole rock Sm– Nd isotope analyses have e Nd(T) values from 3.05 to 9.20 (based on the 800 Ma U–Pb protolith age, see Fig. 12b) and e Nd(0) values from 2.52 to 10.98, implying that their protoliths may have been derived from a depleted mantle reservoir (Table 4). 8.2.2. Zircon U–Pb dating Small (ca. 50 Am), multifaceted, clear, spherical zircon grains from eclogite QZ19-9 and retrograded eclogite QZ19-2 were analyzed by the U–Pb TIMS method (Table 5, Fig. 12b, c). One zircon fraction from sample QZ19-9 gave a 206Pb/238U apparent age of 487 F 17 Ma, and seven zircon fractions yielded 206 Pb/238U apparent ages of 725–802 Ma, with a concordia upper intercept age of 807 F 48 Ma. All three zircon fractions from Sample QZ19-2 are

Fig. 16. Mineral inclusions within zircons from sample QZ19-9 (these mineral inclusions were identified by Laser Raman spectrophotometry).

70

J.X. Zhang et al. / Lithos 84 (2005) 51–76

concordant within error and yield a weighted mean 206 Pb/238U age of 486.4 F 2.3 Ma (Fig. 12c). Cathodoluminescence (CL) investigations of QZ19-9 zircons selected for SHRIMP dating reveal weak oscillatory and/or sector zoning in most grains (Fig. 15). Omp, Grt, Ky(?), Rt and Zo inclusions identified by laser Raman spectroscopy (Fig. 16) indicate eclogite-facies zircon growth. SHRIMP data from seven unzoned or weakly zoned zircons, with Th/U b 0.03 (except for one spot), gave ages of 450– 500 Ma (Table 7). Low U and radiogenic Pb concentrations result in large uncertainties, but these ages overlap the 487 F 17 Ma TIMS age of this sample, and the 486.4 F 2.3 Ma TIMS age of sample QZ19-2. Based on inclusions, CL patterns, low Th and U contents, and low Th/U values similar to those reported from HP and UHP eclogitic zircons elsewhere (Rubatto et al., 1999; Bingen et al., 2001; Liati and Gebauer, 1999), we interpret 486 Ma to be the age of eclogite-facies zircon growth. Two SHRIMP analyses of CL dark cores with oscillatory zoning, Th/U N 1, and high U and Th contents (Table 7), gave ages of 755 and 768 Ma, which overlap the TIMS ages of fractions 1–4 (725–803 Ma), and are within error of the concordia upper intercept age of 807 F 48 Ma. We interpret these ages to represent the magmatic age of the eclogite protolith. Although there is a strong granulite-facies overprint in sample QZ19-2, this evidently was insufficient to reset the zircon (e.g. Hanchar and Miller, 1993). 8.2.3. Ar–Ar isotopic data The amphibole (pargasite) from sample QZ19-2 yielded an isochron age of 407 F 4 Ma and a plateau age of 409 F 1 Ma that includes 33.4% of 39Ar

released (Table 6, Fig. 14e and f). The trapped 40 Ar/36Ar ratio of 305.5 F 6.5 is close to—but statistically different from—the atmospheric value of 295.5.

9. Discussion and conclusion 9.1. Different cooling rates Zircon U–Pb dating indicates that both eclogites have identical 486–488 Ma eclogite-facies ages within error, but petrographic and 40Ar–39Ar data suggest that they cooled differently. Based on an argon diffusion closure temperature of 500 8C for amphibole and 350 F 50 8C for white mica (Boundy et al., 1996), the Yuka eclogite cooled from peak conditions of 600–730 to 500 8C within ca. 10 Ma, resulting in an average cooling rate of 10–23 8C/Ma, and to 350 8C within ca. 20 Ma, resulting in an average cooling rate of 13–19 8C/Ma (Fig. 17). The amphibolite-facies overprint occurred above the amphibole closure temperature, so exhumation from 70–90 km depths (23–28 kbar) to lower-crustal levels (ca. 11 kbar) occurred prior to 477 F 8 Ma, which implies minimum exhumation rates faster than 2–5 km/Ma for the first stage of exhumation. In retrograded domains, omphacite is partially replaced by crypto-crystalline symplectites of Cpx + Pl, which presumably formed when the rocks were at relatively low temperatures. The decompressional path after the eclogitic peak was therefore characterized by decreasing temperature (Godard, 1988; Joanny et al., 1991). The preservation of a sharp compositional difference between garnet core and rim (Figs. 4 and 5) is also consistent with fast cooling and exhumation rates for

Table 7 U, Th and Pb SHRIMP zircon data for eclogite QZ19-9 Spot name

U (ppm)

Th (ppm)

Th/U

Pb* (ppm)

207

Pb/206Pb

238

U/206Pb

207

Pb/235U

206 Pb/238U age (Ma)

1.1 2.1 3.1 4.1 5.1 6.1 7.1 8.1 9.1

1 3 2 1 5 51 3 195 212

b1 b1 b1 b1 b1 31 b1 247 665

0.02855 0.01188 0.02665 0.0293 0.02105 0.60869 0.02293 1.2618 3.1357

b1 b1 b1 b1 b1 5 b1 31 48

0.06900 F 0.02715 0.10765 F 0.02832 0.06688 F 0.01699 0.05713 F 0.01764 0.08361 F 0.01091 0.11157 F 0.01000 0.07803 F 0.01409 0.07610 F 0.00317 0.06820 F 0.00127

13.761 F1.8407 13.578 F 1.7933 13.822 F 1.2389 12.349 F 1.4980 13.174 F 0.72863 12.603 F 0.41476 13.612 F 1.6098 8.0457 F 0.1704 7.9091 F 0.1035

0.69109 F 0.29809 1.0932 F 0.33751 0.66714 F 0.18663 0.63792 F 0.22047 0.87509 F 0.12945 1.2206 F 0.12119 0.79037 F 0.1799 1.304 F 0.064 1.1889 F 0.0286

452 F 59 458 F 59 450 F 39 502 F 59 472 F 25 492 F 16 457 F 52 755 F 15 768 F 9

J.X. Zhang et al. / Lithos 84 (2005) 51–76

71

Fig. 17. Temperature–time path illustrating the thermochronologic history of the eclogites in the Yuka area (a) and Xitieshan area (b).

the Yuka eclogite, because at high temperature, the abrupt compositional discontinuity would be smoothed out over tens of millions of years (O’Brien and Sachan, 2000). The amphibole from Xitieshan eclogite sample QZ19-2 yields a disturbed age spectrum and an isochron age of 407 F 4 Ma, obtained from only 33% of the 39Ar released. Cooling time from peak conditions of 730–830 to 500 8C is about 80 Ma, giving an average cooling rate of 3–4 8C/Ma (Fig. 17). Although the cooling rate is poorly constrained, the high post-eclogite temperature estimates, coarse Cpx + Pl symplectites, high-pressure granulite facies overprint, and presence of sillimanite and extensive migmatization in the surrounding gneisses also suggest that eclogites in the Xitieshan area maintained high temperatures for a longer time than eclogites in the Yuka area. However, more 40Ar–39Ar thermochronology is needed to better constrain the cooling rate in the Xitieshan area. 9.2. Similar protolith nature and age Nd isotopic abundances and ratios are important tracers of the eclogite protolith sources (Jahn, 1999). Nd isotopic data of two eclogite samples from the Yuka area and five eclogite samples from the Xitieshan area gave positive e Nd values, suggesting that their protoliths were derived from a depleted mantle reservoir. In combination with published and unpublished geochemical data (Zhang et al., 2000; Zhang et al., unpublished data), these protoliths have chemical characteristics similar to present-day bTQ

type or bEQ MORB formed in continental/oceanic transitional zones or in extensional domains of attenuated continental crust (Paquette et al., 1989). The association of eclogites together with continentaltype metasediments and granitic gneisses suggests a continental rift or an incipient oceanic basin as the tectonic setting of the eclogite protoliths. The zircon U–Pb data indicate a magmatic protolith age of 750– 800 Ma for both Yuka and Xitieshan eclogites, suggesting that they both formed in the same Late Proterozoic rifting event. 9.3. Different metamorphic evolution and P–T conditions Peak metamorphic assemblages, mineral inclusions, garnet zoning, and retrograde reaction textures in the Yuka eclogites indicate a clockwise, hairpinshaped P–T path with a coincidence of the pressure and the thermal peak of metamorphism which reflects a low geothermal gradient for the prograde as well as the retrograde path (Fig. 8). The preservation of prograde mineral relics and garnet zoning might be due to rapid burial and exhumation accompanying continued subduction, which may also explain cooling during decompression. In contrast, the Xitieshan eclogite was subjected to higher eclogite-facies and post-eclogite facies temperatures, and the tentative P– T path suggests that the eclogite pressure peak preceded the thermal culmination. This reflects slower exhumation under a higher geothermal gradient for the Xitieshan eclogites compared with the Yuka eclogites.

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The different retrograde paths of the Yuka and Xitieshan eclogites suggest that they did not maintain structural coherence (at the 60 km scale) during exhumation. This contrasts with the large-scale structural coherence proposed for the Dabie (e.g., Hacker et al., 2000), and may support the model of independent diapirs proposed by Yin et al. (2001) for the North Qaidam gneisses. A similar situation has been also described in the Dulan area of the eastern NQD, in which the P–T paths of the North Dulan units indicate near-adiabatic decompression to amphibolite-facies, but South Dulan units indicate heating during decompression and granulite-facies overprinting (Song et al., 2003). In summary, HP/ UHP units which record contrasting P–T paths are significant features of the metamorphic pattern of the whole NQD HP/UHP terrane, implying that the terrane may be a tectonic collage of multiple HP/ UHP units with similar eclogitic metamorphic age but different post-eclogite metamorphic history. More detailed structural data, geological mapping, and geochronology are needed in order to constrain the relationship among different HP/UHP units along the NQD.

(Hanson et al., 1995; Yang et al., 1998, 2000; Liu et al., 1996, 2002; Zhang et al., 1999, 2001a; 2002). The belt is distributed in the Altun Mountains along a near-EW strike and is truncated by the ENE–WSW trending Altyn Tagh fault, the longest sinistral strikeslip fault in Eurasia (Peltzer and Tapponnier, 1988; Zhang et al., 2001a). Polycrystalline quartz inclusions within eclogite garnets, P–T estimates from phengitebearing eclogites, and unusual exsolution textures suggest UHP conditions of eclogite formation (Zhang et al., 2002). Zircon U–Pb and Sm–Nd isochron analyses from these eclogites indicate eclogite-facies ages of 500–504 Ma (Zhang et al., 2001a). The similarity of age, occurrence, and P–T conditions of the NQD and Altun HP/UHP rocks suggests that the Altun Mountains are the northwestward extension of the NQD metamorphic belt, formed by early Paleozoic subduction and collision of the Qaidam and Qilian blocks, and subsequently offset by left-lateral slip on the Altyn Tagh fault. This situation might be similar to the Dabie–Sulu HP–UHP metamorphic zone in eastern China, which was split into Dabie and Sulu regions by the sinistral Tanlu strike-slip fault.

9.4. An early Paleozoic HP–UHP metamorphic belt cut by the Altyn Tagh fault

Acknowledgements

In addition to the Yuka and Xitieshan eclogites described in this paper, garnet–peridotite lenses occur within quartzofeldspathic gneisses in the Luliangshan between Xitieshan and Yuka (Fig. 1a) (Yang et al., 1994), and along strike, UHP eclogites occur 200 km SE near Dulan, eastern NQD (Fig. 1a) (Yang et al., 2000), where coesite inclusions have been discovered in zircon from the host paragneisses (Yang et al., 2001b, 2002; Song et al., 2003). Zircon U–Pb and Sm–Nd isochron ages from the eclogites and garnet peridotites indicate similar early Paleozoic eclogitefacies metamorphism (Zhang et al., 2000, 2001a; Yang et al., 2002). These eclogites and associated rocks constitute an early Paleozoic HP/UHP metamorphic belt which extends from the Dulan in the east to the Altyn Tagh fault to the west along the NQD (Fig. 1). To the northwest of the Altyn Tagh fault, a HP/ UHP metamorphic belt consisting of eclogites and garnet peridotites has also been recognized recently

We also highly appreciate the detailed and critical review by Daniela Rubatto and an anonymous referee. This study was financially supported by the National Natural Science foundation of China (40272095, 40472102) and the Key Project of the Ministry of Land and Resources of China (20010201). C.G.M. thanks the Geological Society of America for partial support of this project (Grant No. 7466-03). Dr. C.L. Wu performed the SHRIMP analyses with the valuable assistance of J.L. Wooden. Drafts of this manuscript were reviewed by W.G. Ernst and C.A. Snow.

Appendix A. Appendix explaining the isotopic methods A.1. Sm–Nd method Garnet and omphacite were hand-picked under a binocular microscope. The garnets without inclusions

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and the omphacites without rims of symplectites were separated. For Sm–Nd analyses these minerals were washed by 1:1 HCl at 100 8C for 1 h before dissolution to remove any surface contamination, and then ultrasonically clean in H2O. Sm–Nd isotopic determination was made at the isotopic laboratory, Institute of Geology, Chinese Academy of Geological Sciences (CAGS). Isotope analyses were conducted with Finnigan MAT-261 mass spectrometer. Blanks for the whole chemical procedures are 5  10 11 g for Sm and Nd. Mass fractionation was corrected against 146 Nd/144Nd = 0.7219. All 143Nd/144Nd values are given relative 0.512643 F 8 (2r) for standard BCR1. The following factors and constants were used: GBW04419 1 4 3 Nd/ 1 4 4 Nd = 0.512728 F 9 (2r), k 147Sm = 6.54  10 12 a 1 d e(Nd)(t) was calculated based on present-day reference values for CHUR (chondritic uniform reservoir): (143Nd/144Nd)CHUR = 0.51264, ( 147Nd/144Nd)CHUR = 0.1967. Isochron ages were calculated with York II weighted regression. A.2. TIMS U–Pb method Zircon samples were obtained with using heavy liquids and finally with hand picking under microscope to remove all visible impurities. Selected zircons were dissolved in HF in 0.25 ml Teflon microcapsules. U–Pb isotope analyses were conducted with a VG354 thermal ion mass spectrometer. The measurements were analyzed at Tianjing Institute of Geology and Mineral Resources. Age uncertainties are given at the 95% (2r) confidence level. Analytical procedures and methods have been illustrated in other papers (Lan et al., 2001). A.3. SHRIMP U–Pb method Zircon U–Pb geochronology was carried out on the SHRIMP-RG (sensitive high-resolution ion microprobe-reverse geometry) ion microprobe at Stanford University. Zircon separates or crystal fragments were mounted in epoxy, polished, and coated with gold before analysis. Analytical spots ~30 Am in diameter were sputtered using an ~10-nA O2 primary beam. The primary beam was rastered across the analytical spot for 90 s before analysis to reduce common Pb, resulting in analyses with 204Pb generally being b0.01% of total Pb. Concentration data were stand-

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ardized against the Sri Lankan zircon standard SL-13 and the Duluth Gabbro zircon standard AS57. Isotope ratios were calibrated against AS57, with an assumed age of 1099 Ma (Paces and Miller 1993). Data reduction followed procedures described by Williams (1997). Errors on Concordia intercepts were calculated at the 95% confidence level. See also Clement and Compston (1994) and Barth et al. (1995). A.4. Ar–Ar method Ar–Ar samples were analyzed at Institute of Geology, CAGS. Minerals separated were examined carefully to remove visible impurities under a microscope after gravity/magnetic separation and then they were cleaned by using ultrasonic boths in ethanol. The samples were wrapped in aluminum foil and irradiated for 55 h in the nuclear reactor (The Swimming Pool Reactor, Chinese Institute of Atomic Energy, Beijing). The reactor delivers a neutron flux of 7.6  1012 n cm 2 s 1, the integrated neutrom flux is about 1.5 1018 n cm 2. Ar purified was trapped on activated charcoal fingers at liquid-nitrogen temperature, and then released into the MM-1200B Mass Spectrometer to analyze Ar isotope. The measured isotopic ratios were corrected for the mass discrimination, atmospheric Ar component, blanks and irradiation induced mass interference. The correction factors of interfering isotopes produced during irradiation were determined by analysis of irradiated K2SO4 and CaF4 pure salts and their values are: (40Ar/39Ar)K = 4.478  10 3, ( 36 Ar/ 37 Ar) Ca = 2.40 10 4 ; ( 39Ar/ 37Ar) Ca = 8.06  10 4 the blank of the m/e = 40 gave a value of 2  10 13 mol at 1400 8C. The decay constant used is k = 5.543  10 11 a 1. All 37Ar were corrected for radiogenic decay (half-life 35.1 days). The errors are 2r deviations and correspond to the 95% confidence level. The monitor used in this work is an internal standard: Fangshan biotite whose age is 133.5 Ma and potassium content is 7.6%.

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