Lithos 112 (2009) 342–350

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Zircon Hf isotope signature of the depleted mantle in the Myanmar jadeitite: Implications for Mesozoic intra-oceanic subduction between the Eastern Indian Plate and the Burmese Platelet Guang-Hai Shi a,b,⁎, Neng Jiang b, Yan Liu a, Xia Wang a, Zhi-Yu Zhang a, Yong-Jing Xu a a b

State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

a r t i c l e

i n f o

Article history: Received 5 November 2008 Accepted 6 March 2009 Available online 21 March 2009 Keywords: Zircon Hf isotope Jadeitite Depleted mantle Myanmar Indo-Burman Range

a b s t r a c t This study systematically investigated the Lu–Hf isotope signatures of zircons in jadeitite from the North Myanmar, which was formed from jadeite-forming hydrothermal fluids and is always sourced in serpentinite as a mark for the subduction zone. Results show that the three group zircons have positive εHf(t) values. Group-I zircons (163.2 ± 3.3 Ma) have relatively higher 176Lu/177Hf ratios (N 0.0004) and the corresponding εHf(t) values range from 15.5 to 20.0 at t = 163 Ma. In contrast, Group-II zircons (146.5 ± 3.4 Ma) exhibit highly variable 176Lu/177Hf ratios from 0.000027 to 0.001398, but still have εHf(t) values of 15.6 to 18.5 at t = 147 Ma, which resemble to those of the Group-I zircons. The Group-III zircon (122.2 ± 4.8 Ma) has a 176Lu/ 177 Hf ratio of 0.000578 and a εHf(t) value of 15.8 at t = 122 Ma. Such highly positive εHf(t) values for all jadeitite zircons indicate that they were derived from rapid reworking of the very juvenile crust. Therefore, zircon in jadeitite can be used as a valuable mineral to constrain the age of serpentinization/rodingitization, and even the age of formation of ultramafic rock within ophiolites. The results also suggest the presence of the Mesozoic intra-oceanic subduction within the Indo-Burman Range, and further suggest that the hydrothermal fluids were derived from dehydration of seawater-altered oceanic juvenile crust that had been hydrated during and/or after the formation of the oceanic crust, and additionally from serpentine minerals at greater depth. Consequently, the fluids carry the Hf isotope signature of the depleted mantle that can be later imparted to the jadeitite zircons. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Zircon is a common accessory mineral occurring widely in sedimentary, igneous, metamorphic and metasomatic rocks. As a significant carrier of U, Th and rare earth elements (REEs) at ppm level and hafnium at the percent level, zircon hosts a remarkable number of long-lived radioactive isotopes. Consequently, the stable decay products of zircon have been widely used to study the geochronology and petrogenesis of its host rock and the related tectonic terrain (e.g., Zheng et al., 2005; Chu et al., 2006; Skora et al., 2006; Wu et al., 2006a; Liu et al., 2008; Thoni et al., 2008). However, in some rocks, for example, ultramafic igneous rocks, zircon is rare. Fortunately, in rocks associated with oceanic peridotite, such as rodingite (Dubinska et al., 2004) and jadeitite (Tsujimori et al., 2005; Shi et al., 2008), zircons are abundant, providing the opportunity to extract information on the prehistory and genesis of these rocks and their associated ultramafic rocks, as well as relationships among them. Zircon-bearing jadeitite always occurs in serpentinite and is clearly associated with the ⁎ Corresponding author. China University of Geosciences, Beijing 100083, China. Tel.: +86 10 82321502; fax: +86 10 82322227. E-mail address: [email protected] (G.-H. Shi). 0024-4937/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.03.011

subduction zone. Therefore, geochronological studies of jadeitite allow us to better understand the geological processes within the subduction zone. There are currently three jadeitite locations reported to contain zircons: southwest Japan (Tsujimori et al., 2005), Guatemala (Harlow, 1994) and Myanmar (Shi et al., 2008). As the biggest jadeite deposit in the world, the Myanmar jadeitites and their related rocks, as well as their affiliated tectonic regime, the Indo-Burma Range, have attracted the attention of geologists and geophysical scientists (e.g., Bertrand et al., 1999; Hughes et al., 2000; Bertrand and Rangin, 2003; Satyabala, 2003; Shi et al., 2003; Morley, 2004; Nielsen et al., 2004; Mitchell et al., 2004; Shi et al., 2005a,b; Mitchell et al., 2007; Cummins, 2007; Gahalaut and Gahalaut, 2007; Hu et al., 2008; Shi et al., 2008). Studies of this deposit will most certainly improve our understanding of the metasomatic, deformation and metamorphic processes within the subduction zone, as well as the tectonic framework and geological evolution between the Indian Plate and the Burmese (Myanmar) Platelet. Importantly, in the Myanmar jadeitite area, several features indicate that the jadeitites were deposited from metasomatic fluids. These include the occurrence of CH4-bearing fluid inclusions, complex rhythmical zoning patterns revealed by cathodoluminescence (CL) images of the primary jadeitite with mosaic texture and vein

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Fig. 1. (a)Tectonic map of Northern Myanmar; (b) The simplified geological map of the Myanmar jadeite area (modified after Bender, 1983; Morley, 2004; Mitchell et al., 2007).

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Fig. 1 (continued).

structure, as well as geochemical features (Shi et al., 2000, 2005b; Sorensen et al., 2006; Shi et al., 2008). The related rocks, such as metasomatic kosmochlor rocks, omphacitites and sodic- to sodic– calcic-amphibolites, were formed slightly later than jadeitite (Harlow and Olds, 1987; Shi et al., 2003, 2005a; Yi et al., 2006). The relationship between metasomatic events and zircon growth/overgrowth, the origin of the zircons and the derivation of the jadeite-forming fluids, however, still need further studies. Therefore, in this study, we will investigate Hf isotope signatures of the dated zircons in the Myanmar jadeitite, and further determine the origin of the zircon. In the context of geodynamics and tectonics, the growth and overgrowth of zircon, implications for petrogenesis of the jadeitite will also be discussed. 2. Geological setting The Myanmar jadeite deposit straddles the western part of the Sagaing fault belt in the Hpakan area of Kachin State (Bertrand et al.,

1999; Bertrand and Rangin, 2003). It belongs to the Indo-Burma Range (Mitchell et al., 2004), interpreted as the eastern subduction zone where the oceanic crust of the Indian plate was overridden by the Burmese platelet (Fig. 1a). The Indo-Burman Range stretches between the Myanmar central basin and the western Myanmar border. The eastern boundary of the Range is defined generally by a discontinuous line of ophiolite and ophiolite-derived blocks. The accumulation and deformation of rocks in this terrain took place within a zone where the oceanic crust of the Indian plate was subducted beneath the Burmese platelet (Holt et al., 1991; Mitchell, 1993, 2004, 2007; Searle et al., 2007). The rocks in the Indo-Burma Range are progressively younger from east to west. The forming age of the Myanmar jadeitite, namely the subducted age, and the forming or serpentinized/rodingitized age of its host rock are ca. 147 Ma and 163 Ma, respectively (Shi et al., 2008). This age is much older than the reported 40Ar/39Ar ages (80 to 30 Ma) of phengites in mélange tectonic blocks (Goffé et al., 2000). With our unpublished 40Ar/39Ar data obtained from phengites and

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glaucophanes from blueschist facies rocks (Shi et al., in preparation), we infer that the two high-pressure and low-temperature belts of different ages possibly occurred around the Myanmar jadeitite area. Primary jadeitite occurs as a massive vein (called “dikes” by Chhibber, 1934) cross the Hpakan ultramafic body (Fig. 1b). The jadeitite veins are almost vertical, strike N–S, and are 1.5 to 5 m wide, and 5 to 100 m long. A vein at Nant Maw #109 Mine starts as a vein dipping about 70° W, striking near N–S, but undulating, with an initial width of about 5 cm before it dilates (Harlow et al., 2007; Harlow, per comm.). The boundary zone between the serpentinized peridotite body and the jadeitite vein is a metasomatic amphibole association consisting of eckermannite, magnesiokatophorite, Nyböite, glaucophane, richterite and winchite (Shi et al., 2003). Within or beside the amphibole zone, kosmochlor, Cr-bearing jadeite and some Cr-bearing omphacite occur as coronal aggregates or small blocks (Shi et al., 2005a). Small blocks of omphacitite are also present (Yi et al., 2006). The jadeitite veins are crosscut in some places by fine veins of latestage albitite, which are commonly b5 mm wide. The Hpakan ultramafic rocks are serpentinized ophiolite complexes (Chhibber, 1934; Bender, 1983), which contain serpentinized dunite (Shi et al., 2001). Outside the ultramafic bodies, high-pressure rocks such as phengite-bearing glaucophane schists and stilpnomelane-bearing quartzites, and amphibolite facies rocks such as garnet-bearing amphibolites and diopside-bearing marbles occur as tectonic intercalations (Shi et al., 2001).

3. Petrography of the jadeitite and zircon features Zircons for Hf isotope analysis were selected from about 5.8 kg of white to slightly grey jadeitite block in a primary deposit vein near the Tawmaw tract from Myanmar (Fig. 1b). The jadeitite is comprised predominately of nearly pure jadeite with very minor omphacite, amphibole and very rare accessory minerals of zircon, pyrite and galena. Sodic- and sodic–calcic-amphiboles were formed under pressure of more than 1.0 GPa and temperatures of about 250– 370 °C (Shi et al., 2003). Most jadeites are very pure, with jadeite (XJd) contents of more than 98 mol% (Shi et al., 2003, 2005b), whereas some jadeites contain a component of kosmochlor (Shi et al., 2005a) and some consist of diopside component (normally XDi b 10 mol%) (Shi et al., 2005b). The jadeitite is texturally inhomogeneous, and most jadeite grains are deformed and fine-grained, with a few being coarse-grained and either somewhat slightly deformed or undeformed jadeites. In coarsegrained jadeite crystals, rhythmic zoning patterns are distinct in cathodoluminescence CL) images (Fig. 2a), similar to observations by Harlow (1994) and Sorensen et al. (2006). Primary fluid inclusions were also found in the coarse-grained jadeites (Fig. 2b). In finegrained jadeitites, shape pronounced orientation and crystallographic orientation are obvious under polarized microscopy, indicating that the jadeite grains had undergone ductile deformation (Fig. 2c). About 200 zircon grains or fragments were extracted following the separation procedure (crushing, sieving, gravity separating, electromagnetic separation and then binocular microscopic selection) at the Langfang Lab, Hebei Geology and Resource Bureau. The selected zircons, together with standard zircons, were mounted in an epoxy resin, and were abraded and then polished for CL, backscattered electron (BSE) and secondary electron (SE) imaging. Prior to Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analysis, CL and BSE images of zircons, and chemical compositions of mineral inclusions in zircons were obtained using a CAMECA-SX-51 electron microprobe at the Institute of Geology and Geophysics, Chinese Academy of Sciences under the same analytical conditions as Shi et al. (2005a). Zircon U–Th–Pb analyses were performed on SHRIMP II at the Beijing SHRIMP Lab, Institute of Geology, Chinese Academy of Geological Sciences. The

Fig. 2. (a) Cathodoluminescence image of a white jadeitite from Myanmar showing the rhythmic zoning pattern of the coarse-grained jadeite crystals from the undeformed domain; (b) microphotograph showing fluid inclusions in the coarse-grained jadeite crystals from the undeformed domain; (c) microphotograph of a deformed domain within a jadeitite from Myanmar. The jadeite grains had experienced ductile deformation with partial recrystallization, producing a shape and/or crystallographic preferred orientation.

results have been presented previously (Shi et al., 2008), the features of zircon in the Myanmar jadeitite were summarized below. Three groups of zircons are identified according to their growth patterns, CL brightness, mineral inclusions and SHRIMP analyses: Group-I zircons are typically zoned and have mineral inclusions of Nafree Mg-rich silicates and the highest U and Th contents and Th/U ratios with a weighted mean age of 163.2 ± 3.3 Ma. Group-II zircons are much brighter than those in Group-I and have no obvious oscillatory zoning. They contain mineral inclusions of jadeite and jadeite-rich pyroxene without any Na-free mineral inclusions, and have lower U and Th contents and Th/U ratios with a weighted mean

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excimer laser. Typical ablation times for 200 cycles of each measurement were 30 to 90 s with a 10 Hz repetition rate and a laser power of 100 mJ/pulse. A stationary spot was used for analyses with a beam diameter of about 32 µm. Ar and He carrier gases were used to transport the ablated sample from the laser-ablation cell via a mixing chamber to the ICPMS torch. Zircon 91500 was used as the reference standard during our routine analyses, with a recommended 176Hf/ 177 Hf ratio of 0.282293 ± 28 from laser analyses (Woodhead et al., 2004). Raw count rates for 172Yb, 173Yb, 175Lu, 176(Hf + Yb + Lu), 177Hf, 178 Hf, 179Hf, 180Hf and 182W were collected. 175Lu was calibrated using 176 Yb on 176Hf. The 176Yb/172Yb value of 0.5887 and mean βYb value obtained during Hf analysis on the same spot were applied for the interference correction of 176Yb on 176Hf (Xu et al., 2004; Wu et al., 2006b). We have used a decay constant for λLu = 1.867 × 10− 11 year− 1 (Soderlund et al., 2004) and the 176Hf/177Hf and 176Lu/177Hf ratios of average chondrite and the estimated depleted mantle at the present day are 0.282772 and 0.0332, and 0.28325 and 0.0384, respectively (Blichert-Toft and Albarede, 1997). The results are listed in Table 1, with initial Hf isotope ratios being calculated following the parameters used in Zheng et al. (2006). Representative εHf(t) results, corresponding spots and published SHRIMP U–Pb ages are plotted in Fig. 3. Relationships between 176Lu/177Hf and initial 176Hf/177Hf ratios and between 206Pb/238U age and εHf(t) are plotted in Fig. 4.

age of 146.5 ± 3.4 Ma. The Group-III zircon has the lowest U and Th content and the Th/U ratio, occurring as small late-stage veins crosscutting Group-I and Group-II zircons, and aged at 122.2± 4.8 Ma. Group-II zircons are coeval with jadeitite formation, whereas Group-I zircons crystallized during an earlier igneous (formation of oceanic crust) or hydrothermal (serpentinization and/or rodingitization of the oceanic crust) event (Shi et al., 2008). Large variations of Th and U contents and Th/U ratios for the zircons, as well as different mineral inclusions in Group-I and II zircons, suggest that while some of the variations indicate zircon precipitation from aqueous fluid similar to those reported by Zheng et al. (2007) from high-pressure quartz vein within ultrahighpressure eclogite, a few zircons from Group-I, which have very high Th and U contents, as well as oscillatory zoning, were formed possibly from hydrous melt. 4. Method and results In situ Hf isotopic analyses on the dated zircons in jadeitite from Myanmar were conducted using the Neptune Multicollector ICP-Mass Spectrometer, equipped with a Geolas-93 laser-ablation microprobe, at the Institute of Geology and Geophysics, Chinese Academy of Sciences. The laser system delivers a beam of 193 nm UV light from an

Table 1 LA-MC-ICPMS Lu–Hf isotope data of zircon grains in jadeitite from Myanmar. 176

Yb/177Hf

176

2σ

176

2σ

(176Hf/177Hf)i

εHf(t)

2σ

TDM (Ma)

Group-I zircon (t = 163.2 ± 3.3 Ma) 1.1 0.037141 0.000858 2.1 0.030626 0.000433 3.1 0.044991 0.000075 5.2 0.017012 0.000301 6.1 0.010850 0.000182 15.2 0.010993 0.000181 19.2 0.017737 0.000074 H1.10 0.015448 0.000242 H1.13 0.012709 0.000050 H1.14 0.064573 0.000442 H1.16 0.018677 0.000204 H1.17 0.022442 0.000414 H1.18 0.013287 0.000321 H1.19 0.029810 0.000127 H1.20 0.017981 0.000368 H1.4 0.052877 0.000074 H1.5 0.016687 0.000675 H1.6 0.064094 0.000422 H1.8 0.050429 0.000420 H1.9 0.036863 0.000083 H2-11 0.034219 0.000725 H2-12 0.041100 0.000059 H2-13 0.039904 0.000069 H2-14 0.032635 0.000328 H2-19 0.027877 0.000391 H2-20 0.033611 0.000281 H2-21 0.044087 0.000866 H2-4 0.042079 0.000331 H2-5 0.016592 0.000344 H2-6 0.016640 0.000345

0.001478 0.001220 0.002007 0.000644 0.000508 0.000447 0.000663 0.000602 0.000521 0.002895 0.000741 0.000817 0.000471 0.001134 0.000667 0.002330 0.000679 0.002800 0.002228 0.001372 0.001558 0.001922 0.001850 0.001489 0.001254 0.001452 0.001874 0.001857 0.000688 0.000742

0.000035 0.000019 0.000004 0.000009 0.000008 0.000007 0.000002 0.000009 0.000002 0.000019 0.000008 0.000014 0.000016 0.000003 0.000014 0.000005 0.000028 0.000020 0.000015 0.000002 0.000034 0.000003 0.000002 0.000014 0.000018 0.000013 0.000035 0.000013 0.000015 0.000014

0.283158 0.283153 0.283208 0.283133 0.283222 0.283118 0.283110 0.283115 0.283125 0.283151 0.283200 0.283151 0.283216 0.283214 0.283123 0.283129 0.283131 0.283244 0.283192 0.283125 0.283137 0.283169 0.283215 0.283117 0.283139 0.283185 0.283122 0.283124 0.283147 0.283153

0.000014 0.000014 0.000013 0.000015 0.000024 0.000017 0.000020 0.000018 0.000017 0.000020 0.000020 0.000017 0.000021 0.000019 0.000019 0.000015 0.000012 0.000026 0.000017 0.000017 0.000017 0.000022 0.000036 0.000023 0.000017 0.000025 0.000020 0.000017 0.000018 0.000018

0.283153 0.283149 0.283202 0.283131 0.283221 0.283117 0.283108 0.283114 0.283124 0.283142 0.283198 0.283148 0.283215 0.283211 0.283121 0.283122 0.283129 0.283235 0.283185 0.283121 0.283133 0.283164 0.283210 0.283113 0.283135 0.283181 0.283117 0.283119 0.283145 0.283151

17.1 16.9 18.8 16.3 19.1 15.8 15.5 15.7 16.0 16.7 18.6 16.9 19.2 19.1 15.9 16.0 16.2 20.0 18.2 15.9 16.3 17.4 19.1 15.6 16.4 18.0 15.8 15.8 16.8 17.0

0.5 0.5 0.5 0.5 0.9 0.6 0.7 0.6 0.6 0.7 0.7 0.6 0.7 0.7 0.7 0.5 0.4 0.9 0.6 0.6 0.6 0.8 1.3 0.8 0.6 0.9 0.7 0.6 0.6 0.6

134 140 61 166 40 186 198 190 176 149 71 141 48 51 180 179 169 10 86 181 165 119 51 193 160 94 187 184 146 138

Group-II zircon (t = 146.5 ± 3.4 12.1 0.032423 15.1 0.023643 16.1 0.010868 18.1 0.014167 19.1 0.023357 H2-1 0.001269 H2-16 0.000808 H2-18 0.001437

Ma) 0.000339 0.000450 0.000399 0.000229 0.000508 0.000044 0.000022 0.000035

0.001398 0.000950 0.000412 0.000591 0.000878 0.000028 0.000027 0.000045

0.000017 0.000019 0.000013 0.000008 0.000013 0.000001 0.000001 0.000001

0.283157 0.283151 0.283122 0.283161 0.283185 0.283137 0.283159 0.283205

0.000020 0.000020 0.000016 0.000011 0.000039 0.000016 0.000019 0.000019

0.283153 0.283149 0.283121 0.283159 0.283183 0.283137 0.283159 0.283205

16.7 16.5 15.6 16.9 17.8 16.1 16.9 18.5

0.7 0.7 0.6 0.4 1.4 0.5 0.7 0.7

134 141 181 126 93 158 126 63

Group-III zircon (t = 122 ± 5 Ma) 1.2 0.014046 0.000298

0.000578

0.000011

0.283143

0.000019

0.283142

15.8

0.7

151

2σ

Zircon U–Pb ages are after Shi et al. (2008).

Lu/177Hf

Hf/177Hf(corr)

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Fig. 3. Representative Hf isotope analysis spot numbers (corresponding to Table 1), εHf(t) and published 206Pb/238U ages (Shi et al., 2008) are plotted on cathodoluminescence images of zircons grains from the Myanmar jadeitite.

Analyses of thirty-nine Lu–Hf spots were obtained from 21 zircon grains, including 30 spots on Group-I zircons, 8 spots on Group-II zircons and 1 spot on the Group-III zircon. Group-I zircons are characterized by 176Lu/177Hf N 0.0004, ranging from 0.000447 to 0.002895, and εHf(t) values of 15.5 to 20.0 at t = 163 Ma. Group-II zircons show lower 176Lu/177Hf ratios, generally from 0.000027 to 0.001398, having εHf(t) values of 15.6 to 18.5 at t = 147 Ma, which fall into the value variety range of Group-I. The Group-III zircon has a 176 Lu/177Hf ratio of 0.000578 and a εHf(t) value of 15.8 at t = 122 Ma. Most zircons in this Lu–Hf isotope analysis show low 176Lu/177Hf ratios which indicate that they grew from metamorphic fluid (Zheng et al., 2005). A majority of the εHf(t) values are close to, or coeval with the depleted mantle Hf isotope ratios (Fig. 4b). Therefore, they can be reasonably interpreted to be derived from prompt reworking of the juvenile crust because jadeite in the jadeitite is a mineral precipitated from hydrothermal fluid rather than from mafic melt (e.g., Shi et al., 2005b; Sorensen et al., 2006). Furthermore, the depleted asthenospheric mantle is unsaturated with Zr, therefore, its partial melting is not able to produce mafic melt from which the Zr saturation is attained in order to precipitate zircon (Zheng et al., 2006). However, a few extremely high εHf(t) values (e.g., spots #3.1, H1.6 and H1.8) are even higher than the coeval εHf(t) values for the depleted mantle (Fig. 4b). They also show good correspondence between high 176Yb/177Hf and

176

Hf/177Hf ratios and have very young Hf model ages (Table 1), suggesting either the undercorrection of 176Yb to 176Hf or the introduction of 176Hf-rich fluid (Zheng et al., 2005).

5. Discussion 5.1. Constraints on dating serpentinization Most ultramafic rocks contain less than 20 ppm Zr (Erlank et al., 1978) and zircon in ultramafic rocks is very rare. However, rocks within Zr-depleted environments are not necessarily rare for zircon, and many zircon occurrences in jadeitite and rodingite sourced in serpentinized mantle peridotites have been documented (e.g., Dubinska et al., 2004; Tsujimori et al., 2005). Jadeitite and rodingite are vein assemblages in serpentinized peridotite and thus represent a mixture of the introduced foreign material plus some cross-lithology metasomatic exchange. All authors have emphasized contributions of hydrothermal fluids on zirconium transfer and zircon formation. Myanmar Jadeitites contain Zr from 18.7 to 44.8 ppm (Shi et al., 2008), the same magnitude as those of ultramafic rocks. However, major components of jadeitite did not come from peridotite but rather from components of the down-sloping slab, i.e., the altered oceanic crust

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Fig. 4. (a) 176Hf/177Hf(t) vs. 176Lu/177Hf (triangle for Group-I, square for Group-II, circle for Group-III), (b) εHf(t) vs.206Pb/238U ages; (c) 176Hf/177Hf values vs. 206Pb/238U ages diagrams for zircon in jadeitite from Myanmar.

and sediment (Sorensen et al., 2006; Harlow et al., 2007; Simons et al., 2007, 2008; Shi et al., 2008). It is possible that serpentinization and jadeite crystallization occurred simultaneously, as evident partly by the absence of quartz in most jadeitites and the minor and trace-element chemistry of the jadeitite itself. Abundant zircons in the Myanmar jadeitite, as well as the distinct characteristics and the complex growth patterns of the three zircon Groups as mentioned above support a dual hypothesis. On the one hand, zircons could have crystallized directly from a hydrothermal fluid during jadeitite formation in serpentinites as suggested by Tsujimori et al. (2005), on the other hand, they could have crystallized from fluids generated during serpentinization (Dubinska et al., 2004), or they could have even previously existed as inherited phases during formation of ultramafic rock within ophiolites. Zircon in jadeitite can be used to constrain the age of serpentinization/rodingitization, or even the age of formation of ultramafic rock

within ophiolites. Before the U–Pb dating of serpentinization on zircons selected in rodingite (Dubinska et al., 2004), serpentinization was considered to be a process that does not produce minerals suitable for direct isotopic dating and that the age of serpentinization is often estimated according to the age of different rocks, including plagiogranites (e.g., Graham et al., 1996), deep-sea sediments containing microfossils (e.g., Hopper and Smith, 1996; Schellekens, 1998), or tectonic relations between serpentinized peridotite and adjacent rocks (e.g., Froitzheim and Rubatto, 1998). Therefore, the ability to isolate and characterize zircons in jadeitite associated with serpentinite confirms that zircon-bearing jadeitite can be used as a reliable and feasible rock by which to constrain dating the surrounding serpentinization (e.g., Tsujimori et al., 2005), and that mineral inclusions in zircon are correspondingly critical to such interpretations (e.g., Shi et al., 2008). Combined with our SHRIMP zircon U–Pb dating (Shi et al., 2008), the large variations in U/Th ratios and U–Th abundances for Group-I zircons, coupled with the consistent Hf isotope compositions, this study shows that Group-I zircons in the Myanmar jadeitite with the depleted Hf isotope signatures were derived either from an igneous (formation of the oceanic crust) or from hydrothermal (serpentinization and/or rodingitization) events. The tightly clustered but slightly scattered U–Pb ages of Group-I zircons (Shi et al., 2008) also suggest that serpentinization was either initialized at an early stage of the ocean crust evolution (e.g., Charlou et al., 1998; Beard and Hopkinson, 2000; Mitchell et al., 2000; Alt and Shanks, 2003) or continued at slightly later stages of the oceanic crust history (e.g., Barnes and O'Neil, 1969; Coleman, 1977). It has been suggested that the serpentinites in mélanges containing jadeitite are not ocean-floor basement, but rather the altered hanging wall of the mantle wedge (Harlow and Sorensen, 2005; Morishita et al., 2007). However, according to a numerical study of the interaction between the mantle wedge, subducting slab, and the overriding plate (Eberle et al., 2002), it is unlikely that the altered hanging wall of the mantle wedge proposed by Morishita et al. (2007) would have had experienced high-pressure and low-temperature conditions without the contamination of inherited old zircons and continental Hf. Low temperature field of the mantle wedge could be produced only when the overriding plate moves laterally (free-slip) and the subducting slab is thick, in which case, the mantle wedge might drag down material from the overriding plate, leading to contamination of possible continental Hf and older zircons. On the other hand, if the surface of the overriding plate is fixed, the subducting slab is thin and the mantle wedge impinges upon the overriding plate forming a high-temperature nose between the overriding plate and the subducting lithosphere, the P–T condition there is not suitable for jadeite formation. Nevertheless, a long-term intra-oceanic subduction-zone channel, including a mixture of subducted deep-sea sediment, oceanic crust and serpentinized mantle wedge as proposed by Krebs et al. (2008), is one possible source for the serpentinites in mélanges containing jadeitite. 5.2. Mesozoic intra-oceanic subduction within the Indo-Burman Range? Zircons having the Hf isotope feature of the depleted mantle in the Myanmar jadeitite allow for assumption of the presence of Mesozoic intra-oceanic subduction within the Indo-Burman Range. It has been considered that the jadeitite is a very uncommon rock type that decorates sutures in association with subduction and is found as isolated bodies within serpentinite-matrix mélanges (e.g., Shi et al., 2001; Harlow and Sorensen, 2005; Tsujimori et al., 2005; Sorensen et al., 2006). The inference about jadeitite sourcing in serpentinites is consistent with the conclusion of Garrido et al. (2005) concerning HFSE mobility during subduction of oceanic crust. No Hf isotope signature of the continental crust is detected in the zircons from the Myanmar jadeitite, which rules out the contribution from the

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subduction-related continental crust and, in turn, suggests a type of intra-oceanic subduction. Metabasalts in blueschist facies, which surround the serpentinites, show distinct depleted REE patterns (Shi et al., 2001). The U-shaped REE patterns with strong positive Eu anomalies of the Myanmar jadeitites (Shi et al., 2008) are similar to these depleted features, a possible reflection of a host that once crystallized plagioclase, such as MORB. The intra-oceanic subduction was inferred to have occurred within the Indo-Burman Range along the eastern Indian Plate (Similar to Fig. 13 in Krebs et al. (2008)). Geographically, the jade mines area is not precisely within the Indo-Burman Range. However, Mitchell et al. (2004) suggested that high-pressure rocks, together with flysch and ophiolites in the jade mine uplift connected with the Indo-Burman Range. The scrutiny of tectonics of northwest Myanmar also supports that the Hpakan ultramafic body containing the Myanmar jade was more likely composed of the ophiolite fragments belonging to the ophiolite of the eastern Indo-Burman Ranges (Mitchell, 1989, 2004), and was separated by later tectonic events. The subduction with which the Myanmar jadeitite is involved occurred at the Late Jurassic (Shi et al., 2008) and belongs to the Neotethys Ocean tectonic regime (e.g., Stampfli and Borel, 2002), which is slightly later than or coeval with the appearance of highpressure rocks, such as blueschist (Shi et al., 2001), in the jade mine uplift. The high-pressure rocks were deduced to have been generated by the Jurassic collision event (cf., Mitchell et al., 2004), indicating an occurrence earlier than the Early Cretaceous intra-oceanic subduction system within the Yarlung–Zangbo suture in south Tibet (e.g., Aitchison et al., 2000; Ali and Aitchison, 2008), the same Tethys Ocean tectonic regime. 5.3. Implications for petrogenesis of the jadeitite Several lines of evidence clearly indicates the presence of a jadeitesaturated vein fluid, such as trace-element compositions of jadeitites (Sorensen et al., 2006; Morishita et al., 2007; Shi et al., 2008), textures and compositions of related metosomatic amphiboles, kosmochlors and jadeitilized omphacites (Shi et al., 2003, 2005a; Yi et al., 2006). Fluid inclusions and stable isotope studies of the Guatemala and Myanmar jadeitites further unambiguously indicate that jadeite grains were formed from the fluids (Harlow, 1986; Johnson and Harlow, 1999; Shi et al., 2000, 2005b). However, the generation of the jadeite-saturated vein fluids is still not fully documented. In our study, the depleted Hf isotope features in the zircons from the Myanmar jadeitite suggest their derivation from prompt reworking of very juvenile crust during the Myanmar jadeitite crystallizing from the jadeite-saturated vein fluids. Since a number of hydrous minerals are stable in ultramafic bulk compositions at temperatures b700 °C (Hyndman and Peacock, 2003), it is unlikely that the fluids were generated from dehydration of adjacent serpentine minerals around the jadeitite, since temperature conditions for formation of the jadeitite is b400 °C (e.g., Harlow, 1994; Shi et al., 2003). However, dehydration of serpentine minerals at greater depths during the subduction possibly generates HFSE-rich fluid (Garrido et al., 2005). Alternatively, interaction between seawater and the subducted oceanic crust could have also produced HFSE-rich fluid by dissolving HFSE-rich minerals. This happens when seawater reacts with serpentinized rocks. Correlations between the fluids and seawater or seafloor sediments are revealed by the isotope features of fluid inclusions in jadeitite, Ba-bearing minerals in jadeitites, and their bulk geochemical features (e.g., Mével and Kiénast, 1986; Harlow, 1995; Johnson and Harlow, 1999; Morishita, 2005; Shi et al., 2005b; Sorensen et al., 2006; Shi et al., 2008). Such results suggest that the fluids are derived likely from seawater that was reacted with the subducted slab, with minor addition from dehydration of serpentine minerals at greater depths. They were possibly expelled by compaction of subducted sediments and collapse of porosity in the upper

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oceanic crust at shallow depths (e.g., Hyndman and Peacock, 2003). The subducted oceanic slab served as the primary source for the depleted Hf isotope composition. Acknowledgements We are indebted to R.X. Zhu, L.C. Chen, and Q.S. Liu for their kind supports during the field trip and subsequent research, as well as L.W. Xie for his help with LA-MC-ICPMS analyses. Reviews by Y.-F. Zheng and G.E. Harlow, and comments of I. Buick (Editor) greatly helped to improve the quality of the manuscript. This study was supported by the National Basic Research Program of China (2009CB421008), the National Science Foundation of China (40672046), and the Program for the New Century Excellent Talents in China (NCET-07-0771). References Aitchison, J.C., Zhu, B.D., Davis, A.M., Liu, J.B., Luo, H., Malpas, J.G., McDermid, I.R.C., Wu, H.Y., Ziabrev, S.V., Zhou, M.F., 2000. Remnants of a Cretaceous intra-oceanic subduction system within the Yarlung–Zangbo suture (southern Tibet). 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