Titanite-Bearing Omphacitite from the Jade Tract, Myanmar: Interpretation from Mineral and Trace Element Compositions ⇑ Yi-Nok Ng A, Guang-Hai Shi A, , M

Journal of Asian Earth Sciences 117 (2016) 1–12

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Journal of Asian Earth Sciences

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Titanite-bearing omphacitite from the Jade Tract, Myanmar: Interpretation from mineral and trace element compositions ⇑ Yi-Nok Ng a, Guang-Hai Shi a, , M. Santosh a,b a State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China b Centre for Tectonics, Resources and Exploration, Department of Earth Sciences, University of Adelaide, Adelaide 5005, Australia article info abstract

Article history: Jadeitite is a rare rock composed predominantly of jadeite, which typically occurs in association with tec- Received 6 July 2015 tonic blocks of high-pressure/low-temperature metabasaltic rocks such as eclogite or blueschist, often as Received in revised form 4 December 2015 a matrix in exhumed serpentinite mélanges. Omphacitite are far less common occurring together with Accepted 9 December 2015 jadeitite, such as those in the ‘‘Jade Tract” of Hpakan area in Myanmar. The omphacitite in this locality Available online 9 December 2015 is mostly composed of omphacite and jadeite, with minor titanite, ilmenite, epidote and zircon. The jadeite formed after omphacite shows a lower Jd-content than that in the neighboring white jadeitite. Keywords: The omphacite shows signiﬁcant variation in Jd-content and is associated with aegirine augite. Both rocks Myanmar jadeitite show relatively linear and upslope pattern from LREE to HREE, and a slight enrichment of Ba, Th, U, Zr and Omphacitite Titanite Hf relative to chondrites, in the absence of Eu-anomaly. The titanite occurs in two groups: one as discrete Ilmenite islands replacing ilmenite, and the other as precipitation within jadeite veinlet. Titanite grains show con- Replacement vex patterns from LREE to HREE, and depletion of Sr, Zr and Hf with no Eu-anomaly. Chemical characteristics of the titanite and ilmenite alteration around titanite suggest that the omphacitite is of secondary origin, likely derived from pyroxenite through the replacement of pyroxene by jadeite. Based on the previous ﬁndings of jadeitization of chromitite, serpentine, and rodingite, it is suggested that protoliths such as plagiogranite or gabbro trapped within serpentinite mélange might have undergone jadeitization. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction blocks within serpentinite (Coleman, 1961; Dobretsov, 1963)or through pressure solution and re-deposition in fractures (Harlow, Jadeitite is a rare rock that is composed mainly of jadeite, with 1994). In later studies, jadeitites were interpreted as the result of minor amounts of omphacite, diopside, kosmochlor, and other vein precipitation from fluid flowing through serpentinite (Shi clinopyroxene components. It is usually found in serpentinite et al., 2000, 2003, 2005b, 2012; Harlow and Sorensen, 2005; mélange formed at high-pressure low-temperature (HP/LT) condi- Sorensen et al., 2006; Harlow et al., 2007, 2012a, 2014, 2015; tions and records fluid transport and water–rock reaction in the Cárdenas-Párraga et al., 2012). In a recent review by Tsujimori subduction factory (e.g., Harlow and Olds, 1987; Shi et al., 2003, and Harlow (2012), the global occurrence of jadeitite was catego- 2005a; Harlow et al., 2014, 2015; Sorensen et al., 2006; Tsujimori rized into two types: fluid precipitates from a Na–Al–Si-rich fluid and Harlow, 2012). Jadeitites also record the transport of large (P-type), and metasomatic replacement of plagiogranite, metagab- ion lithophile elements, such as Li, Ba, Sr, and Pb, as well as ele- bro, etc. (R-type) by a similar fluid. This classification establishes a ments generally considered more refractory, such as U, Th, Zr, linkage among jadeitite and its associated rocks, and their petroge- and Hf (e.g., Shi et al., 2008a,b, 2009, 2010; Simons et al., 2010; netic relationship is particularly important in evaluating the tecton- Yui et al., 2013). The rock finds utility as a popular gemstone and ics of the associated HP–LT rocks. Among nineteen or more jadeitite as carved materials, from antiquity in the form of tools, adorn- localities reported worldwide, the most important and well-studied ments, and symbols of prestige to contemporary jewelry (e.g., ones include those in Myanmar (e.g., Chhibber, 1934a,b; Harlow Harlow et al., 2015; Lucas et al., 2015). and Olds, 1987; Harlow and Sorensen, 2001, 2005; Shi et al., The petrogenesis of jadeitite has been a subject of debate. Early 2001, 2003, 2005a), Guatemala (e.g., Harlow et al., 2004, 2006; theories proposed a metasomatic replacement process of tectonic Tsujimori et al., 2005; Flores et al., 2013), Japan (e.g., Kobayashi et al., 1986; Tsujimori, 2002) and Cuba (e.g., García-Casco et al., 2009; Cárdenas-Párraga et al., 2012). The other localities are ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (G.-H. Shi). described in recent reviews by Shi et al. (2012), Tsujimori and http://dx.doi.org/10.1016/j.jseaes.2015.12.011 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved. 2 Y.-N. Ng et al. / Journal of Asian Earth Sciences 117 (2016) 1–12

Harlow (2012) and Harlow et al. (2014, 2015). Among these occur- Recently omphacitites bearing titanite were found in the rences, attention has been focused on jadeitite, and associated rocks Hpakan Jade Mine Tract in Myanmar (Fig. 1). In this study we such as kosmochlor aggregates, jadeitized rodingite, and albite focus on these omphacitites, and report the occurrence, texture rocks (Ouyang, 1984; Wang et al., 2013). Another rare and less and chemical composition of titanite and titanite-rimmed ilmenite investigated association is omphacitite (Yi et al., 2006), which is in the omphacitites in an attempt to evaluate the origin of composed of more complex mineral components. this rock.

Fig. 1. (a) Simplified tectonic map of northern Myanmar (modified after Morley, 2004), and (b) geological sketch map of the Myanmar jadeitite area (modified after Bender, 1983), jadeitite outcrops have been listed previously by him. Y.-N. Ng et al. / Journal of Asian Earth Sciences 117 (2016) 1–12 3

2. Geological setting and petrography indistinct prismatic shapes are cut by jadeite, suggesting replacement of omphacite by jadeite. Jadeite occurs as veins infiltrating The jadeite uplift (Fig. 1) containing the Myanmar jadeite and crosscutting omphacite matrix. In a wider vein (Fig. 2a), along deposit and high-pressure rocks in Hpakan area of Kachin State the omphacite matrix is a thin layer of microcrystalline jadeite, straddles the northwestern parts of the Sagaing fault belt whereas larger, needle-like jadeite crystals radiate in fan sprays (Bertrand et al., 1999; Bertrand and Rangin, 2003). The region perpendicular to the thin layer (Fig. 2c). Two-phase (liquid–gas) forms part of the Indo-Burma Range (Mitchell et al., 2004), the inclusions, similar to those described by Shi et al. (2005b), are eastern boundary of which is generally defined by a discontinuous found in the larger jadeite grains. In smaller veins, only microcrys- zone of serpentinite mélange. The Sagaing Fault is a major right- talline jadeites occur. lateral strike-slip fault that roughly runs north–south and contin- The titanite can be categorized into two groups. Group-A ues into the Andaman Sea. It is argued that the uplift, the titanite, which ranges in size from 50 to 500 lm, is found Tagaung-Myitkyina Belt and the Indo-Burma Range were once a within omphacite as isolated grains or as local clusters (Fig. 2b). contiguous belt, which has been separated by the Sagaing Fault, Occasionally titanite carries relict ilmenite (Fig. 2e), indicating that leaving the jadeite uplift straddling along the fault between the titanite is a replacement at the expense of ilmenite. Group-B Belt and the Range (Shi et al., 2014). Previous studies proposed that titanite, which also ranges from 50 to 500 lm in size, is found this belt was once in an intra-oceanic subduction setting, associ- in the infiltrating jadeite veins (Fig. 2d). It lacks relict ilmenite ated with the Woyla Intra-Oceanic Arc or the Incertus Arc during inclusion (Fig. 2f), which suggests local precipitation rather than the Mesozoic (e.g., Shi et al., 2009, 2014). However, there is still replacement. some controversy over the timing of the jadeitization event(s). Among the accessory minerals, zircon is usually less than Shi et al. (2008a,b) reported three zircon U–Pb ages of circa 163, 100 lm long and is found in omphacite matrix. Epidote occurs only 147 and 122 Ma, which represent three episodes (magmatic and/ in omphacite matrix, whereas apatite is found only in small jadeite or hydrothermal). Yui et al. (2012) and Yui (2014), however, argued veins. that the Jurassic age is inherited from a protolith, with younger age of circa 77 Ma is recorded in their zircon rims. Either way, collision 3. Analytical methods between the Greater Indian Plate and Eurasia (circa 55–50 Ma) occurred after the jadeitite formation. Back-Scattered Electron (BSE) images and EPMA (Electron Probe The Jade Tract is characterized by serpentinized peridotite Micro-Analysis) data were obtained at the Geological Lab Center, and dunite bodies sandwiched by phengite-bearing blueschist, China University of Geosciences, Beijing, using a JXA-8100 Electron garnet-bearing amphibolite, diopside-bearing marble, and quart- Microprobe Analyzer operated at 15 kV accelerating voltage, 20 nA zite (Chhibber, 1934a,b; Shi et al., 2001). Within the serpentinite beam current and <10 lm spot size. The EPMA standards include bodies, some massive jadeitite veins (termed ‘‘dikes” by the following minerals: andradite for Si and Ca, rutile for Ti, corun- Chhibber, 1934b) or blocks of more than 10 m wide occur dum for Al, hematite for Fe, eskolaite for Cr, rhodonite for Mn, bun- (Harlow et al., 2014). The jadeitites are often surrounded by senite for Ni, periclase for Mg, albite for Na, K-feldspar for K, and amphibole-rich margins of up to 1 m thickness, based on which barite for Ba. Mineral formulae of titanite were recalculated by some workers termed these rocks as amphibolite, comprising MINPET 2.02 software (Richard, 1988–1997) on basis of 3 cations six types of amphiboles: nyböite, eckermannite, katophorite, and 5 oxygens. Formulae of clinopyroxene were recalculated by glaucophane, richterite, and winchite (Bleeck, 1907; Chhibber, PX-NOM, a spreadsheet program created by Sturm (2002), with 1934b; Shi et al., 2003; Nyunt et al., 2009; Harlow et al., Fe2+/Fe3+ estimation using the equation of F =12(1 4/S) from 2014). Other related rocks such as kosmochlor occur typically Droop (1987) (in F =2X (1 T/S), following the method of in the proximity of chromite (e.g., Harlow and Olds, 1987; Shi Okamoto and Maruyama (2004), where T represents the ideal et al., 2005a). Cr-omphacitite with Ba-minerals (Shi et al., number of cations per formula unit and S describes the observed 2010) and jadeitized rodingites (e.g., Wang et al., 2012)also cation total per X oxygens assuming all iron is Fe2+). occur. Veins from later stage(s) contain albite (Wang et al., Trace element compositions were analyzed on polished thick 2013), zeolite, and pectolite (e.g., Shi et al., 2012). Other rare sections (50 lm thick) by in-situ LA-ICP-MS (Laser Ablation minerals such as trinephline and fabriesite (Ferraris et al., Inductively-Coupled Mass Spectrometry) at the Geological Lab 2014) have also been reported in the Jade Tract. Small blocks Center, China University of Geosciences, Beijing. The laser ablation of omphacitite ranging in color from dark green to black typi- system is a New Wave UP193SS, which is equipped with a 193 nm cally occur as boulders (Fig. 2c in Shi et al., 2012) distributed ArF excimer laser. An Agilent 7500A ICP-MS instrument was used around the Hpakan city (Fig. 1). to acquire ion-signal intensities. Laser ablation spots were set to The omphacitite sample (locally called ‘‘black jade” – black in be 35 lm in diameter, with the laser energy of 60 mJ, a frequency reflect light, but green in transmitted light) was collected near of 5 Hz and e beam energy of 12 J/cm2. Hpakan in the Jade Tract. Four samples of omphacitite for this Standard materials for off-line selection and integration of investigation were purchased from a local miner who excavated background signals, and time-drift correction and quantitative these from the Uru boulder conglomerate west of Hpakan town calibration were performed using workstation data-processing (Fig. 1). These are roughly round in shape, compact, dark green program included with the 7500A operating system. The NIST in color, and fine grained (similar to the sample in Fig. 2c in Shi SRM 610 was used as a reference material for calibration, and NIST et al., 2012). Thin sections were prepared for petrographic study SRM 612 as a quality control material (QCM). The concentration of and for microprobe analysis. various elements (V, Cr, Mn, Co, Ni, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, The major minerals in the samples investigated are omphacite Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Th, U) were (>85 vol.% of the rock) and jadeite (10 vol.%), with minor amounts acquired using Ca as internal standard. Analytical uncertainties of titanite, zircon, ilmenite, epidote and apatite. The jadeite vein for most elements are less than 10%, except for Cr, Ta and Cd which shows sharp boundary with the omphacite matrix (Fig. 2a). are less than 15%. To ensure highest reliability of data, the time- Omphacite occurs as prismatic, subhedral or granular crystals up resolved spectrum of every element for each sample was carefully to 1 mm in size. In the jadeite domain, omphacite occurs as examined. Data for which fluctuations in the spectrum were isolated grains with irregular shapes (Fig. 2b), and some with observed were discarded. 4 Y.-N. Ng et al. / Journal of Asian Earth Sciences 117 (2016) 1–12

Fig. 2. Photomicrograph and BSE images illustrating petrography of the studied samples: (a) A wide jadeite vein against the omphacite aggregate (crossed-polars). (b) Omphacite aggregate with titanite (Group-A) crosscut by jadeite veins. (c) Jadeite crystals, microcrystalline jadeite layer in a wider jadeite vein (crossed-polars). (d) Group-B titanite in jadeite vein. (e) BSE image of a Group-A titanite containing relict ilmenite. (f) BSE images of the framed area of (d); Ttn(A) and Ttn(B) refer to Group-A and -B titanite, respectively.

4. Mineral chemistry elements (Fig. 4c), jadeite and omphacite display a general upslope pattern from left-hand-side (i.e. elements of lower compatibility) EPMA spot analyses of jadeite veins (‘‘Jd veins” in Table 1) show toward right-hand-side (i.e. elements of higher compatibility), Jd-content varying from 54.0 to 84.2 mol%, Ae-content from 11.6 to with enrichment of U, Zr and Hf. Eu-anomaly is mild positive ⁄ 27.0 mol% and Quad-content from 2.2 to 19.9 mol%. Most of the (Eu/Eu = 1.04 for jadeite, 1.16 for omphacite). analyses confirm with the Jadeite Zone of Morimoto (1988) Despite the textural differences, the chemical composition of (Fig. 3) with the exception of spot no. 1 in sample YX-1 (a spot titanite (CaO, TiO2, and SiO2) does not display any significant varia- close to jadeite vein border) which is compositionally omphacite. tion between the two groups (Table 2). The Al2O3 content is low Omphacite (‘‘Omp matrix” in Table 1) shows Jd-content between (max. XAl [=Al/(Ti + Al)] = 0.059), showing that Ti is only slightly 21.3 and 55.6 mol%, Ae-content from 0 to 38.9 mol% and Quad- replaced by Al. Although measurement of F is lacking due to analyt- content from 20.3 to 46.1 mol.%. Some aegirine-augite (no. 11 of ical limitation, the low Al-content suggests a low (F, OH)-content YX-2, no. 16 and 23 of YN-1) is also observed within omphacite. since their substitutions are paired by [Ti + O] M [Al + (F, OH)] Since the thin sections are made on jadeite infiltrated area, the (e.g., Enami et al., 1993). The total content of trace elements in above values may suggest overlapped compositions and conflicting titanite is low (Table 3), with a convex pattern characterized by ‘‘forbidden zones” for clinopyroxenes as suggested by Green et al. depletion in LREE, a somewhat flat-top in the MREE, and a slight (2007). drop in HREE (Fig. 4b). No pronounced Eu-anomaly is observed; The LA-ICP-MS spot measurements on a large jadeite crystal however a negative Gd-anomaly occurs, the cause of which remains (from the center of a thick jadeite vein) and from the central part unknown. Elements of lower compatibility are more depleted, and of an omphacite aggregate show similar REE and trace elements elements of higher compatibility are flat-topped; depletions of Th, (Table 3). In trace element variation diagrams (Fig. 4a), both show La, Sr, Zr and Hf are observed, whereas enrichment of other ele- linear increase from LREE to HREE. In a spidergram plot of trace ments (e.g., Ba, Nb, and Nd) are not strong (Fig. 4d). Table 1 Chemical compositions of clinopyroxenes in omphacitite from the Jade Mine Tract, Myanmar, as determined by EPMA.

Sample YX-1 YX-2 YN-1 YN-3 Point Vein Mx Mx Vein Mx Mx Mx Mx Mx Mx Vein Mx Vein Mx Mx Vein Vein Mx Vein Vein Mx Mx Mx Vein No. 1 2 3 4 5 6 7 8 9 10 11 21 22 1 2 3 4 11 12 13 14 16 23 13

SiO2 56.80 55.62 55.49 57.62 55.88 55.56 57.13 55.33 55.99 56.24 56.52 55.48 57.69 56.47 57.68 56.38 56.45 54.54 56.35 57.55 54.80 53.20 53.24 54.61 TiO2 0.43 0.29 0.17 0.28 n.d. 0.26 0.33 0.26 0.22 0.04 0.14 0.11 0.61 0.10 n.d. 0.72 1.25 0.27 1.32 0.81 0.10 0.06 0.11 1.07 Al2O3 13.64 10.35 10.67 18.23 9.36 9.56 10.11 10.60 9.60 7.40 13.10 8.52 20.85 11.43 11.91 15.66 15.41 5.97 15.90 20.54 8.37 7.59 7.52 15.91 1–12 (2016) 117 Sciences Earth Asian of Journal / al. et Ng Y.-N. Fe2O3 7.75 5.21 5.64 7.05 11.82 10.95 9.23 10.80 4.67 9.30 8.92 9.21 3.73 2.45 0.00 7.27 6.00 9.17 6.28 4.78 7.53 13.46 12.90 6.78 FeO 0.00 2.42 3.62 0.31 0.00 1.27 0.00 0.00 2.88 0.50 0.00 0.00 0.00 0.00 2.66 0.00 0.51 0.00 0.00 0.00 0.00 0.60 0.00 0.00 MnO 0.10 n.d. n.d. 0.08 0.15 0.14 0.26 0.45 0.13 0.40 0.12 0.15 0.11 0.02 0.18 n.d. 0.11 0.30 0.19 0.10 0.24 0.34 0.36 0.12 MgO 3.76 5.75 5.13 0.62 4.20 4.82 4.85 4.23 6.28 6.57 3.47 7.54 0.10 8.18 8.16 2.53 2.61 8.83 2.40 0.19 8.22 4.87 5.21 2.18 CaO 5.93 10.30 9.37 2.55 7.50 6.91 7.38 7.01 9.79 9.40 5.19 9.51 1.11 11.74 11.33 5.19 4.71 13.68 3.05 1.38 11.69 12.35 11.63 4.11 Na2O 11.81 8.82 9.00 13.81 11.00 10.29 11.14 10.67 8.70 9.19 11.86 9.79 15.47 8.53 7.99 12.66 12.36 7.17 14.28 15.55 8.08 8.24 8.74 14.22 K2O n.d. n.d. n.d. 0.11 n.d. 0.03 n.d. n.d. 0.02 n.d. n.d. n.d. 0.01 n.d. n.d. 0.02 n.d. 0.03 n.d. n.d. 0.08 0.01 n.d. 0.08 Cr2O3 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.02 n.d. 0.05 0.14 n.d. 0.15 n.d. Total 100.22 98.76 99.09 100.66 99.91 99.79 100.43 99.35 98.28 99.04 99.32 100.31 99.68 98.92 99.91 100.43 99.41 99.98 99.77 100.95 99.25 100.72 99.86 99.08 Si 1.98 2.00 2.00 1.99 1.99 1.99 2.01 1.98 2.02 2.02 1.99 1.95 1.97 1.99 2.02 1.96 1.98 1.96 1.94 1.94 1.96 1.92 1.93 1.90 Ti 0.01 0.01 0.01 0.01 – 0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.02 0.00 – 0.02 0.03 0.01 0.03 0.02 0.00 0.00 0.00 0.03 Al (T) 0.02 – 0.00 0.01 0.01 0.01 – 0.02 – – 0.01 0.05 0.03 0.01 – 0.04 0.02 0.04 0.06 0.06 0.04 0.08 0.07 0.07 Al (M1) 0.54 0.44 0.45 0.73 0.38 0.39 0.42 0.42 0.41 0.31 0.54 0.30 0.80 0.47 0.49 0.61 0.62 0.21 0.59 0.76 0.31 0.25 0.25 0.58 3+ Fe (M1) 0.23 0.16 0.17 0.20 0.36 0.33 0.28 0.32 0.14 0.28 0.27 0.28 0.11 0.07 – 0.22 0.18 0.28 0.19 0.14 0.23 0.41 0.41 0.21 Fe2+ – 0.07 0.11 0.01 0.00 0.04 – – 0.08 0.02 – – – – 0.08 – 0.02 – – – – 0.02 – – Mn 0.00 – – 0.00 0.01 0.00 0.01 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.01 – 0.00 0.01 0.01 0.00 0.01 0.01 0.01 0.00 Mg 0.20 0.31 0.27 0.03 0.22 0.26 0.25 0.23 0.34 0.35 0.18 0.40 0.01 0.43 0.43 0.13 0.14 0.47 0.12 0.01 0.44 0.26 0.28 0.11 Ca 0.22 0.40 0.36 0.09 0.29 0.26 0.28 0.27 0.38 0.36 0.20 0.36 0.04 0.45 0.43 0.19 0.18 0.53 0.11 0.05 0.45 0.48 0.44 0.14 Na 0.80 0.61 0.63 0.92 0.76 0.71 0.76 0.74 0.61 0.64 0.81 0.67 1.02 0.58 0.54 0.85 0.84 0.50 0.96 1.02 0.56 0.57 0.61 0.96 K – – – 0.0 0.00 0.00 – – 0.00 – – – – – – 0.00 – 0.00 – – 0.00 – – 0.00 Cr – – – – 0.00 – – – – – – – – – – – – 0.00 0.00 0.00 – 0.00 – Total 4.00 4.00 4.00 4.00 4.00 4.00 4.01 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 Jd 55.63 45.14 45.60 72.82 38.20 38.97 44.47 42.45 44.78 33.70 54.07 33.16 86.22 49.45 53.87 61.68 65.12 21.27 67.58 82.25 32.09 22.69 23.71 63.87 Ae 23.66 16.11 17.15 20.34 36.70 32.84 29.58 32.51 15.47 30.01 27.00 30.76 11.60 7.73 0.00 22.35 18.53 28.71 21.42 14.91 23.75 37.68 38.91 23.95 Quad 20.71 38.75 37.25 6.84 25.10 28.18 25.95 25.03 39.75 36.29 18.92 36.08 2.18 42.82 46.13 15.97 16.35 50.02 11.00 2.84 44.16 39.63 37.38 12.19 a Mineral Omp Omp Omp Jd Omp Omp Omp Omp Omp Omp Jd Omp Jd Omp Omp Jd Jd Ae-Aug Jd Jd Omp Ae-Aug Ae-Aug Jd a Nomenclature follows Morimoto (1988). Note: n.d. = not detected, Mx: matrix. Formulae calculated on the basis of 6 Oxygen. 5 6 Y.-N. Ng et al. / Journal of Asian Earth Sciences 117 (2016) 1–12

Ilmenite occurs as patches in titanite (Fig. 2e). Due to the small size, only one EPMA spot measurement with reliable accuracy is acquired, and the results show Mn-rich nature (6.23 wt.%; Table 4).

5. Discussion and conclusions

5.1. Characteristics of trace elements in titanite and omphacite

Titanite in our study shows Zr/Hf = 14 –37, Nb/Ta = 5 –14 and Th/U = 0.1 –1.6 (Fig. 5), which is different from the values in magmatic titanite which has Zr/Hf (27 –66), Nb/Ta (8 –71) and Th/U (2–8) by Marks et al. (2008). Titanite from both plutonic rocks (Marks et al., 2008) and alkaline intrusions (Vuorinen and Hålenius, 2005) have patterns of enriched LREE and depleted HREE (i.e. a downslope pattern), which are different from the features of titanite in this study. Also, the Ta-, Nb-, Y- and/or Zr-enriched titanites in pegmatite, carbonatitic rock, syenite and Ae-enriching jadeitic pyroxene described by Cˇerny´ et al. (1995), Chakhmouradian et al. (2003), Liferovich and Mitchell (2005) and Fig. 3. Compositional plot for pyroxenes after Morimoto (1988). Harlow et al. (2012b) are in contrast to the features observed in this study. The chemical features of titanite in our samples are remarkably similar to those of metamorphic titanites (in both REE and incompatible elements) such as those reported by Gao

Fig. 4. (a) Chondrite-normalized REE patterns of the omphacite and jadeite. (b) Chondrite-normalized REE patterns of titanite. (c) Spidergram of trace elements of omphacite and jadeite. (d) Spidergram of trace elements of titanite. Chondrite data refer to Sun and McDonough (1989). Table 2 Chemical compositions of titanite in omphacitite from the Jade Mine Tract, Myanmar as determined by EPMA.

YX-1 YX-2 YN-1 YN-3

No.1234567111213212223121112131415162122232426111415182111

SiO2 30.07 30.59 30.11 30.50 30.60 30.62 30.50 31.54 31.78 32.17 30.24 30.29 30.23 30.27 30.00 31.07 31.81 31.38 31.65 30.98 32.27 30.06 30.14 30.53 30.56 30.68 30.60 30.89 30.29 30.63 30.96 30.78

TiO2 40.23 40.26 40.73 40.31 39.73 40.49 40.44 39.16 39.08 39.76 41.49 40.41 40.91 40.54 40.96 39.26 38.84 40.33 39.01 38.75 39.09 41.50 41.94 41.19 40.72 40.71 39.96 39.78 39.54 39.96 40.90 40.10 .N ge l ora fAinErhSine 1 21)1–12 (2016) 117 Sciences Earth Asian of Journal / al. et Ng Y.-N. Al2O3 0.93 0.85 0.83 0.64 0.65 0.60 0.53 1.02 0.71 0.65 0.63 0.97 0.83 0.63 0.45 0.98 0.99 0.75 1.18 1.34 1.41 0.73 0.47 0.99 1.05 0.91 1.50 1.53 1.57 1.15 1.06 1.01 FeOT 0.57 n.d. 0.37 0.40 0.57 0.46 0.12 0.52 0.45 0.39 0.32 0.16 n.d. 0.30 n.d. 0.60 0.53 0.20 0.36 0.14 0.31 0.05 0.07 0.19 0.14 0.24 0.25 0.41 0.35 0.21 0.32 0.40 MnO n.d. 0.36 n.d. n.d. n.d. n.d. n.d. n.d. 0.28 0.16 0.05 n.d. 0.01 0.22 n.d. n.d. n.d. 0.22 n.d. n.d. n.d. n.d. n.d. 0.16 n.d. n.d. 0.28 0.04 0.04 0.04 0.10 n.d. MgO n.d. n.d. n.d. 0.01 n.d. n.d. 0.19 n.d. n.d. n.d. n.d. 0.08 0.08 0.17 n.d. 0.09 n.d. 0.03 0.07 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.08 n.d. n.d. n.d. CaO 26.78 27.13 26.93 26.59 26.70 26.87 26.57 27.08 26.49 26.90 26.74 27.01 27.07 27.46 26.81 27.25 26.71 26.66 27.36 28.15 27.28 26.52 27.03 27.16 27.01 27.07 27.11 26.94 26.92 27.12 27.48 26.89

Na2O n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.32 0.24 0.08 0.11 n.d. n.d. n.d. n.d. n.d. 0.09 0.09 0.09 0.17 0.17 0.19 0.12 0.18 0.13 0.07 0.19 0.21 0.16 0.16 0.26

Cr2O3 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.06 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.12 n.d. n.d. n.d. 0.22 n.d. n.d. n.d. 0.14 0.14 0.05

P2O5 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.05 n.d. 0.08 n.d. 0.07 0.07

V2O5 0.64 0.38 n.d. 0.44 0.84 n.d. 0.36 n.d. n.d. n.d. n.d. n.d. n.d. 0.83 0.62 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. Total 99.22 99.57 98.97 98.90 99.09 99.04 98.71 99.32 99.10 100.28 99.56 99.05 99.18 100.41 98.85 99.24 98.88 99.64 99.71 99.44 100.53 99.16 99.87 100.33 99.66 99.96 99.82 99.78 99.08 99.41 101.19 99.56

Si 0.99 1.00 0.99 1.00 1.00 1.01 1.00 1.03 1.04 1.04 0.99 0.99 0.99 0.98 0.99 1.02 1.04 1.02 1.03 1.01 1.04 0.99 0.98 0.99 1.00 1.00 1.00 1.01 0.99 1.00 1.00 1.00 Al 0.04 0.03 0.03 0.03 0.03 0.02 0.02 0.04 0.03 0.03 0.02 0.04 0.03 0.02 0.02 0.04 0.04 0.03 0.05 0.05 0.05 0.03 0.02 0.04 0.04 0.04 0.06 0.06 0.06 0.04 0.04 0.04 Ti 0.99 0.99 1.01 1.00 0.98 1.00 1.00 0.96 0.96 0.97 1.02 1.00 1.01 0.99 1.01 0.97 0.96 0.99 0.95 0.95 0.95 1.02 1.03 1.00 1.00 1.00 0.98 0.97 0.98 0.98 0.99 0.98 2+ Fe 0.02 0.00 0.01 0.01 0.02 0.01 0.00 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.00 0.02 0.02 0.01 0.01 0.00 0.01 0.00 0.00 0.01 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Mg – – – 0.00 – – 0.01 – – – – 0.00 0.00 0.01 – 0.01 – 0.00 0.00 – – – – – – – – – 0.00 – – – Mn – 0.01 – – ––––0.01 0.01 0.00 – 0.00 0.01 – – – 0.01 – – – – – 0.01 – – 0.01 0.00 0.00 0.00 0.00 – Na––––––––0.02 0.02 0.01 0.01 – – – – – 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 Ca 0.94 0.95 0.95 0.94 0.94 0.95 0.94 0.95 0.93 0.93 0.94 0.95 0.95 0.96 0.95 0.96 0.94 0.93 0.95 0.99 0.94 0.93 0.94 0.94 0.94 0.94 0.95 0.94 0.95 0.95 0.95 0.94 Cr–––––––––– ––0.00 – ––––––– 0.00 – – – 0.01 – – – 0.00 0.00 0.00 Rcats 2.98 2.98 2.99 2.97 2.97 2.98 2.97 2.99 3.00 2.99 2.98 2.99 2.99 2.97 2.97 3.00 2.99 2.98 3.00 3.01 3.00 2.98 2.99 2.99 2.99 2.99 3.00 3.00 3.00 3.00 3.00 3.00

XAl 0.04 0.03 0.03 0.02 0.02 0.02 0.02 0.04 0.03 0.03 0.02 0.04 0.03 0.02 0.02 0.04 0.04 0.03 0.05 0.05 0.05 0.03 0.02 0.04 0.04 0.03 0.06 0.06 0.06 0.04 0.04 0.04

Note: n.d. = not detected. Calculated on basis of 5 Oxygen. 7 8

Table 3 Trace element compositions of titanite, jadeite and omphacite in omphacitite from the Jade Mine Tract, Myanmar as determined by LA-ICP-MS.

Sample YX-1(Titanite) YX-2(Titanite) YX-1 YX-1 No.a 3(1) 3(2) 4(1) 4(2) 6 7 22 2(1) 2(2) 12(1) 12(2) 15 21 5 (Omp) 11 (Jd) V 842.7 479.4 226.6 232.7 219.8 966.1 94.8 25.1 102.3 150.4 33.9 312.8 294.9 99.13 43.41 Cr 9.2 2.6 1.5 1.1 4.6 11.9 6.4 10.6 10.7 0.7 4.2 0.7 0.7 2.63 60.20 Mn 1689.0 414.5 264.0 251.8 224.9 1379.2 380.0 130.2 178.9 179.8 112.2 204.8 201.6 902.87 371.36 Co 2.1 0.3 0.5 0.3 0.4 5.1 3.2 0.5 0.5 0.1 0.1 0.1 0.1 5.82 10.78 Ni 25.0 3.4 3.9 4.1 0.9 97.0 28.4 7.8 9.3 1.5 1.6 1.3 1.3 60.93 40.03 Rb 3.5 0.5 0.6 0.7 0.3 4.7 1.4 0.6 0.5 0.5 1.1 0.3 0.3 0.19 0.23 1–12 (2016) 117 Sciences Earth Asian of Journal / al. et Ng Y.-N. Sr 890.2 424.2 319.8 139.9 169.7 2074.3 397.0 351.4 344.5 130.2 268.2 277.4 312.7 14.72 5.45 Y 7226.3 1012.6 1017.2 1042.9 494.2 9242.8 2945.5 1523.6 1085.6 1091.9 2536.7 629.9 657.7 10.96 9.55 Zr 167.7 112.3 97.2 44.9 722.8 516.2 52.5 70.6 103.2 1856.8 26.4 42.0 466.7 57.23 64.35 Nb 428.9 108.8 72.7 64.2 64.3 476.4 110.9 81.6 54.1 55.0 138.0 50.2 60.8 0.45 0.41 Cs 0.1 0.0 0.1 0.1 0.0 0.5 0.0 0.0 0.0 0.0 0.0 0.2 0.0 0.02 0.02 Ba 40.1 11.6 14.3 11.5 7.6 95.0 13.5 34.0 28.8 6.6 24.6 42.5 48.5 5.46 4.16 La 126.0 9.0 14.9 13.2 14.7 120.5 46.7 57.2 17.4 22.1 95.2 15.3 13.5 0.20 0.13 Ce 817.8 71.3 111.1 94.3 105.8 888.5 352.4 449.3 143.5 178.5 737.4 128.6 106.8 0.89 0.46 Pr 236.2 20.7 28.7 22.3 24.0 266.0 106.2 112.1 38.4 45.2 174.6 31.8 26.7 0.18 0.10 Nd 1767.0 160.6 203.2 143.8 145.5 1937.0 754.8 677.0 250.3 295.0 1072.5 210.8 175.8 1.04 0.55 Sm 951.4 101.8 114.9 71.6 64.2 1099.3 423.8 301.3 129.2 138.1 489.9 98.2 83.8 0.50 0.40 Eu 384.9 47.0 51.2 29.4 26.0 516.7 184.0 113.6 57.4 55.0 191.3 39.2 35.3 0.23 0.14 Gd 1610.8 164.3 169.1 90.1 70.6 1512.2 508.2 242.4 126.1 129.3 445.0 90.0 86.0 0.74 0.42 Tb 249.4 33.1 35.0 24.9 16.8 363.2 128.6 65.3 40.3 38.3 112.6 23.1 22.4 0.20 0.16 Dy 1668.4 232.1 227.7 191.3 104.9 2230.2 750.5 379.0 261.3 249.6 666.5 146.7 152.1 1.63 1.50 Ho 334.3 47.5 45.3 44.0 20.8 459.0 145.5 69.0 52.1 50.3 119.6 28.6 31.1 0.42 0.41 Er 868.4 126.3 118.7 121.3 54.0 1126.5 332.3 151.8 124.2 128.8 269.4 72.2 80.8 1.20 1.20 Tm 120.6 18.4 17.5 18.1 8.1 153.0 43.1 18.2 16.9 18.2 33.9 10.2 11.9 0.22 0.21 Yb 862.5 134.8 120.0 115.1 52.3 1012.1 260.5 94.7 94.2 108.3 185.2 65.0 81.1 1.78 1.74 Lu 59.1 11.2 9.9 10.7 5.4 76.1 18.2 6.5 7.5 9.1 12.4 5.8 7.8 0.35 0.24 Hf 9.9 7.4 5.2 2.4 19.4 31.5 3.7 3.4 4.5 50.9 1.6 1.4 13.6 1.73 1.90 Ta 37.6 10.0 6.7 5.2 4.5 51.1 21.7 6.5 6.3 7.1 17.6 5.5 5.9 0.04 0.04 Th 1.0 0.1 0.2 0.1 0.1 1.0 0.4 0.4 0.2 0.2 1.0 0.1 0.2 0.09 0.09 U 1.8 0.7 1.0 0.6 0.3 9.1 1.4 0.9 1.3 0.3 0.6 0.6 0.6 0.10 0.06 Zr/Hf 16.9364 15.1334 18.6130 18.8529 37.3540 16.3760 14.0831 20.5320 23.1798 36.5017 16.5912 29.5845 34.2170 33.0809 33.8684 Nb/Ta 11.4041 10.8671 10.8921 12.3385 14.2031 9.3213 5.1083 12.6347 8.6252 7.7181 7.8298 9.0906 10.3754 10.7143 9.2325 Th/U 0.5519 0.1459 0.1699 0.2460 0.4636 0.1084 0.2867 0.4237 0.1695 0.5452 1.6295 0.2065 0.3419 0.8500 1.5464 ⁄ Lu/Hf0.9506 5.9707 1.10971.5054 1.12241.9042 1.11884.4748 1.18070.2801 1.22522.4150 1.21234.8794 1.28481.8983 1.37591.6944 1.25760.1787 1.25267.8239 1.27434.1056 1.27250.5682 1.15600.2023 1.03810.1242 Eu/Eu(Gd/Lu)N 3.3682 1.8183 2.1031 1.0457 1.6098 2.4555 3.4513 4.5878 2.0665 1.7576 4.4212 1.9089 1.3714 0.2613 0.2200 p a ⁄ Sample and No. are same as these in Tables 1 and 3. Eu/Eu =EuN/ (SmN ⁄ GdN), where N denotes normalization to chondrite values after Sun and McDonough (1989). Unit in ppm. Y.-N. Ng et al. / Journal of Asian Earth Sciences 117 (2016) 1–12 9

Table 4 Chemical compositions of other accessory minerals in omphacitite from the Jade Mine Tract, Myanmar as determined by EPMA.

Sample YX-1 YX-2 No. 12 14 15 18 5 6 7

SiO2 0.95 67.28 n.d. 38.75 n.d. 37.81 37.68

Al2O3 0.09 19.32 n.d. 17.93 n.d. 17.10 18.30

TiO2 59.40 0.08 0.15 3.18 0.19 6.24 2.16

Na2O 0.16 12.24 n.d. 1.55 n.d. 1.10 1.17 CaO 1.91 0.08 53.39 32.00 53.85 28.80 31.75 FeOT 31.84 0.10 n.d. 4.64 n.d. 4.47 4.51 MgO n.d. n.d. n.d. n.d. n.d. 0.38 0.44 MnO 6.23 0.11 0.06 n.d. n.d. n.d. 0.26

P2O5 n.d. n.d. 43.57 n.d. 43.18 n.d. n.d. Total 100.56 99.23 97.17 98.05 97.22 95.91 96.28 Mineral Ilmenite Albite Apatite Epidote Apatite Epidote Epidote a n.d. = not detected.

Fig. 6. Discrimination plot for the titanites (after Kowallis et al., 1997; Mohammad and Maekawa, 2008). Diamonds are from this investigation, and ‘‘K” stands for Kowallis.

enrichment of Zr and Hf is observed, which is similar to the adjacent jadeitite (Shi et al., 2008a,b), but without a positive Eu-anomaly. In most cases, the Eu-anomalies are related to feldspar fractionation. In this case, the lack of negative Eu-anomaly would imply that omphacitite (or its protolith) did not experience feldspar fractionation. The lack of positive Eu-anomaly would suggest that omphacitite was not a replacement product from a feldspar-rich protolith.

5.2. Origin of omphacitite

Studies over the past 20 years have interpreted jadeitite either as the direct precipitate from hydrous fluids released in subduction channel through dewatering into the overlying mantle wedge, or Fig. 5. Comparison of ratios of Zr/Hf, Nb/Ta and Th/U of the titanites (Marks from as the fluid-induced metasomatic replacement of oceanic pla- Marks et al. (2008); Gao (Mag): magmatic titanite from Gao et al. (2012); Gao giogranite, graywacke, or metabasite along the channel margin (Meta): metamorphic titanite from Gao et al. (2012)). (e.g., Harlow et al., 2015). However, the petrogenesis of jadeitite- related omphacitites are poorly understood (Yi et al., 2006; et al. (2012) (Fig. 4b and d). Furthermore, comparison with the Shigeno et al., 2012). The possibilities can be considered for the results in Kowallis et al. (1997) (who distinguished igneous and omphacitite occurrence: (1) fluid precipitation (i.e., no protolith); metamorphic titanite from Fe- vs. Al-content) and Mohammad (2) jadeitite as a protolith was metasomatically transformed into and Maekawa (2008) (who utilized the methodology), confirms a omphacitite; and (3) another precursor rock (other than jadeitite) metamorphic origin for the titanites of the present study (Fig. 6). was transformed into omphacitite. The occurrence of titanite and Omphacite and jadeite in this study have depleted LREE and its mineral chemistry suggest derivation from another precursor enriched HREE, and the M/HREE concentrations are ten times more rock (other than jadeitite) that was transformed into omphacitite. than that of the white/colorless neighbor jadeitite (which is highly Petrological observations preclude the formation of omphacitite homogeneous, Jd > 98 vol.%; Shi et al., 2008a,b). When compared to through direct precipitation from fluids. Formation of titanite can the Ba-minerals-bearing Cr-omphacitic rock of Shi et al. (2010), the occur during metamorphism (closed system), metasomatism omphacite in this study is not enriched in Ba and Cr. However, (open system), or by direct precipitation, whereas the occurrence 10 Y.-N. Ng et al. / Journal of Asian Earth Sciences 117 (2016) 1–12 of ilmenite is commonly related to upper mantle conditions. It is Myanmar (Shi et al., 2012; Wang et al., 2012), leading to the sugges- possible that the assemblage ilmenite ± titanite is produced by tion of a Ti-rich basic protolith. For some basic rocks, their bulk TiO2 fluid precipitation and/or metasomatism (e.g., Ionov et al., 1999; can reach up to more than 2.8 wt.% (e.g., Shi et al., 2009). Therefore, Putnis and Austrheim, 2010 and references therein). The occur- pyroxenite would be the most possible candidate as the protolith of rence of titanite-rimmed ilmenite represents at least one stage of the omphacitite, excluding the possibility of feldspar-bearing gab- phase alteration, which in turn implies at least one stage of bro. The process of omphacitization is then interpreted as the replacement for omphacitite formation, excluding direct fluid replacement of pyroxene in pyroxenite by jadeitic material, into precipitation. an omphacite–jadeite system. Eventually, the omphacitite with It is also unlikely that the jadeitite served as protolith of the jadeite vein and minor titanite were formed. omphacitite. The occurrence of jadeite as a later product relative to omphacite clearly excludes the possibility of a jadeitite protolith. 5.3. Undiscovered rock types replaced by jadeitic fluids? In addition, no major occurrence of titanite has been reported in homogeneous jadeitites from Myanmar. The Ti-content is low in Recent studies have revealed extensive jadeitization in the serpentinite, and Ti-mobility is suggested to be low in metasomatic Myanmar Jade Tract. More than thirty mineral species have been

fluid because of its extremely low solubility in H2O(Tropper and identified so far in jadeitite and related rocks, including jadeite, Manning, 2005). If nucleation of titanite (Group-B) occurred, it chromite, kosmochlor, Cr-jadeite (Ouyang, 1984; Harlow and would be a local phenomenon compared to the wide-spread jadeiti- Olds, 1987; Shi et al., 2008a,b); nyböite, eckermannite, katophorite, zation. In addition, albitization is a significant post-jadeitization glaucophane, richterite, winchite, kaolinite, magnesio-arfvedsonite event in Myanmar (Wang et al., 2013). As both jadeitization and (Shi et al., 2003; Oberti et al., 2014a,b, 2015); vesuvianite albitization require a Na–Al–Si fluid, if extensive jadeitite-to- (e.g., Nyunt et al., 2009); Cr-free omphacite, titanite (Yi et al., omphacitite transformation occurred, an additional Ca–Mg/ 2006); celsian, hyalophane, cymrite, Ba-zeolite (Shi et al., 2010); Fe-rich fluid would be required, which is in conflict with the fluids grossular, Mg-omphacite (Wang et al., 2012); albite (e.g., Wang associated with these two processes. et al., 2013); banalsite, analcime, natrolite, Ca-thomsonite, pecto- Several lines of evidence lead to the suggestion of another pre- lite, uvarovite, allanite, phlogopite, zircon, graphite, quartz, dias- cursor rock (other than jadeitite) for the omphacitite, and the pre- pore, pyrite, and galena (Bleeck, 1908; Mével and Kiénast, 1986; cursor rock is more likely a mafic, pyroxene-bearing rock. Some Shi et al., 2012; Tsujimori and Harlow, 2012 and references omphacites with residual diopside core from the Myanmar Tract therein). Among these, kosmochlor and Cr-jadeite were reported (e.g., Shi et al., 2012; Wang et al., 2013) clearly show that the as replacement minerals after chromite by jadeitic fluids (Ouyang, omphacite was replaced at the expense of pre-existing diopside. 1984; Harlow and Olds, 1987; Shi et al., 2005a). Amphiboles like As omphacite is the intermediate member in jadeite–omphacite– nyböite, eckermannite, katophorite, glaucophane, richterite, win- diopside or jadeite–aegirine–diopside systems, and petrographic chite, kaolinite, magnesio-arfvedsonite were found as replacement observations show that jadeite formed earlier than its neighbor of serpentinized peridotite by jadeitic fluids (Shi et al., 2003; Oberti omphacite in omphacite-bearing rocks such as jadeitized rodingite et al., 2014a,b, 2015). The same situation occurs in the omphacite, (Wang et al., 2012), the potential precursor rock for the omphaci- which was formed by jadeitization (e.g., Yi et al., 2006; Shi et al., tite in the present case is inferred to be diopside-bearing rocks. 2010, 2012; Wang et al., 2012) It is known that diopside is a common mineral constituent in Kosmochlor rock, amphibole rock, jadeitized rodingite and mafic rocks within serpentinite mélange (e.g., Tsujimori, 1997; omphacitite are the best examples for jadeitization in the Myan- Miyazoe et al., 2009; Marchesi et al., 2013). Among the mafic rocks mar Jade Tract, and their protoliths (chromite rock, serpentinite, in the mélange – such as gabbro, blueschist, eclogite, or pyroxenite, rodingite, pyroxenite) form part of the serpentine mélange ilmenite occurs either in (meta-) gabbro (e.g., amphibolite facies (e.g., Shi et al., 2003, 2005a; Yi et al., 2006; Wang et al., 2012). metagabbro; Liou et al., 2009; Ding et al., 2013), (meta-) basalt, All these rocks are R-type ones as classified by Tsujimori and (meta-) rodingite (e.g., Mohammad and Maekawa, 2008), pyroxen- Harlow (2012). Since all the rock types of the protoliths are not ite (Zhang et al., 2009) or other rocks. However, the known P–T represented by the mélange, it is possible that some of the rock conditions for formation of the jadeitite (e.g., Shi et al., 2012) types generated by the extensive jadeitization in the Myanmar exclude metabasites in eclogite facies, and the absence of garnet jadeitite area have not been identified. in this study does not support garnet-bearing rodingite (Wang Among the typical lithologies found in serpentinite mélanges, et al., 2012) as protolith. Thus, the potential candidates for the pro- basalts and pillow lavas, plagiogranites, gabbros, pyroxenite, and toliths of the omphacitite are pyroxenite and gabbro. mantle peridotites, together with metamorphic rocks such as In titanite, the lack of Eu-anomaly suggests a pyroxene- metagabbro, metabasite, rodingite, and listvenite also occur dominated protolith which has nil or very minor plagioclase. In (e.g., Shirdashtzadeh et al., 2010 and references therein). When the REE patterns of titanite, those in pyroxenite (Marks et al., these igneous and metamorphic rocks are infiltrated by sodic fluids 2008) do not show any Eu-anomaly, whereas those associated with with high chemical activity under high-P and low-T conditions in intermediate to acidic (meta-) volcanic, feldspar-rich rocks (Storey subduction zone, several new rock types would form in relation to et al., 2007) and metagranite (Gao et al., 2012) show prominent neg- jadeitite. Among these, jadeitized metabasite (such as metagabbro ative Eu-anomaly. These features suggest a fractionation mecha- or metabasalt) or jadeitized plagiogranite would be highly probable. nism of Eu between titanite and surrounding minerals. It has also Therefore, we speculate that these rocks are likely to be identified as been shown (Gao et al., 2012) that metamorphism would weaken the new jadeitized rock types in the Myanmar jadeitite area in the negative Eu-anomaly. Titanite reported here resembles that of future studies. metamorphic titanite (Gao et al., 2012), but has no Eu-anomaly. As most Fe in omphacite is Fe3+ (Table 1), the lack of Eu-anomaly Acknowledgements in our samples is not caused by low oxygen fugacity (e.g., Sverjensky, 1984). This in turn implies that the protolith of We are indebted to W.Y. Cui and R.X. Zhu for their kind support omphacitite has no or very minor feldspar. In addition, the jadeite during the field work and subsequent research. We thanks J.W. Yin and omphacite in this study contain higher content of Ti (max. and Q. Mao for their help with EMPA, and L. Su with LA-ICP-MS.

TiO2 = 1.25 wt.%) than those in other jadeite and omphacite (mostly The ﬁrst author would like to thank G.E. Harlow for the warm <0.3 wt.%) in jadeitite, or even in the jadeitized rodingite from hospitality and in-depth discussions during a trip to American Y.-N. Ng et al. / Journal of Asian Earth Sciences 117 (2016) 1–12 11

Museum of Natural History (AMNH). Thoughtful and constructive Harlow, G.E., Sorensen, S.S., 2005. Jade (Nephrite and Jadeitite) and serpentinite: comments by G.E. Harlow, K. Okamoto and A. Garcia-Casco do metasomatic connections. Int. Geol. Rev. 47, 113–146. http://dx.doi.org/ 10.2747/0020-6814.47.2.113. great helps for improving this manuscript and are gratefully Harlow, G.E., Hemming, S.R., Avé Lallemant, H.G., Sisson, V.B., Sorensen, S.S., 2004. appreciated. This research was supported by the National Two high-pressure-low-temperature serpentinite-matrix mélange belts, Natural Science Foundation of China (No. 41373055), Specialized Motagua fault zone, Guatemala: a record of Aptian and Maastrichtian collisions. Geology 32, 17–20. http://dx.doi.org/10.1130/G19990.1. Research Fund for the Doctoral Program of Higher Education (No. Harlow, G.E., Sorensen, S.S., Sisson, V.B., Cleary, J., 2006. Jadeite jade from 20120022110004). Guatemala: distinctions among multiple deposits. 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