Superimposed Tectono-Metamorphic Episodes of Jurassic and Eocene Age in the Jadeite Uplift, Myanmar, As Revealed by 40Ar/39Ar Dating☆

Gondwana Research 26 (2014) 464–474

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Superimposed tectono-metamorphic episodes of Jurassic and Eocene age in the jadeite uplift, Myanmar, as revealed by 40Ar/39Ar dating☆

Guanghai Shi a,b,⁎, Weiyan Lei a,HuaiyuHeb,YiNokNga, Yan Liu a, Yingxin Liu a,YeYuana, Zhijuan Kang a,GenXiea a State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China b Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China article info abstract

Article history: The Myanmar jadeite uplift forms an important link between the Indo-Burma Range and the Tagaung–Myitkyina Received 24 February 2013 Belt. Two contrasting ages of Jurassic (152.4 ± 1.5 Ma of glaucophane from blueschist) and Eocene (44.8 ± 1.1 Received in revised form 12 August 2013 and 45.0 ± 1.3 Ma of phengitic muscovites from quartz schists in the uplift) were yielded using 40Ar/39Ar dating. Accepted 16 August 2013 The Jurassic age of the glaucophane, even older than the forming age of the jadeitite from recent relevant litera- Available online 24 August 2013 tures, and for the reason of possible excess argon involved, is interpreted as the lower limit of the subduction age. Previous studies correlated this subduction with the Woyla intra-oceanic arc, or the Incertus Arc to the west. The Keywords: High-pressure low-temperature Eocene ages of phengitic muscovites are interpreted as the time of an intra-continental shearing deformation metamorphism event, against the timing of HP/LT metamorphism as previously suggested. Combined with other studies, it is 40Ar/39Ar geochronology suggested that the Tagaung–Myitkyina Belt and the Indo-Burma Range belonged to a single belt, which has Glaucophane been separated by the Sagaing Fault, leaving the jadeite uplift straddling along the fault between the Belt and Phengitic muscovite the Range. We propose a rapid exhumation model for the Myanmar jadeitite at ~45 Ma, coeval with onset of Myanmar jadeite the Sagaing Fault. © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction the Indian plate was overridden by the West Burma Block (Holt et al., 1991; Mitchell et al., 2004, 2007; Searle et al., 2007), and ophiolitic The jadeite uplift in Hpakan area of Kachin State in Myanmar, or the rocks with similar characteristics are exposed in two parallel belts: the so-called Jade Mine Tract, is the largest and most commercially impor- ‘Eastern Belt’ and the ‘Western Belt’ (Mitchell, 1993; Acharyya, 2007). tant source of jadeitite on the Earth. Unfortunately, their emplacement There is still controversy about emplacement ages of ophiolites in and formation age are not clear and, consequently, their tectonic setting these two belts: the ‘Eastern Belt’ is inferred to mark the locus of the sub- and geodynamic importance are hotly debated. Both sides of the jadeite duction zone into which the ophiolites were accreted since Mesozoic uplift lie in the Indo-Burma Range and the Myitkyina Belt, which are key (Gansser, 1980; Acharyya et al., 1990; Mitchell, 1993; Shi et al., 2008, regions to understand the tectonic evolution and geodynamics process- 2009a), whilst the ‘Western Belt’ was inferred to have been caused by es of West Burma, and to establish the linkage with Southeast Asia. It is a late Oligocene terminal collision between the Indian and the Burmese generally accepted that Southeast Asia is composed of continental continental blocks (Sengupta et al., 1989; Acharyya et al., 1990; blocks which separated from Gondwana (e.g. Allen et al., 2008; Barber Sengupta et al., 1990). and Crow, 2009; de Jong et al., 2009; Metcalfe, 2011; Hall, 2012). The The Myitkyina Belt, also called the Tagaung–Myitkyina Belt (TMB, West Burma (Fig. 1a) Block, together with the Lhasa and Woyla Blocks, Fig. 1a; Acharyya, 2007; Mitchell et al., 2007; Searle et al., 2007)or was rifted from NW Australian Gondwana in the Late Triassic to Late the Bhamo–Myitkyina Belt (Cruickshank and Ko, 2003), consists of a Jurassic and was accreted to proto-SE Asia in Cretaceous time (Metcalfe, serpentinite mélange zone, known as the Myitkyina–Mandalay 1998), and thus it is interpreted as a continuation of the West Sumatra ophiolite in the Myitkyina valley, north of Mandalay. Recently an block (Barber and Crow, 2009). In the eastward subduction zone, the eclogite boulder has been found in this Belt (Enami et al., 2012), Indo-Burma Range runs from the Himalaya to the north and to Sumatra conﬁrming that a high-pressure metamorphic event occurred. This to the south (Allen et al., 2008). It is considered that oceanic crust of Belt is located roughly along the northeast extension of the Sagaing Fault (Gururajan and Choudhuri, 2003; Vigny et al., 2003). However, relationships between the Myitkyina Belt and the Indo-Burma Range ☆ This article belongs to the Special Issue on Orogenesis and metallogenesis in the are still in dispute. Sengör et al. (1988) suggested that the two are Sanjiang Tethyan Domain. the southern continuation of the Tsangpo Suture and the Bangong- ⁎ Corresponding author at: State Key Laboratory of Geological Processes and Mineral – Resources, China University of Geosciences, Beijing 100083, China. Tel.: +86 10 82321578. co Nujiang Suture of Tibet, respectively. However, Mitchell (1993) E-mail addresses: [email protected], [email protected] (G. Shi). suggested that the Tsangpo ophiolite extended into the Indo-Burma

1342-937X/$ – see front matter © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gr.2013.08.007 G. Shi et al. / Gondwana Research 26 (2014) 464–474 465 466 G. Shi et al. / Gondwana Research 26 (2014) 464–474

Range through Tidding and Myitkyina–Mandalay as a single suture, aggregates of kosmochlor, Cr-bearing jadeite, Cr-bearing omphacite which was offset about 450 km along the dextral Sagaing transform and jadeitized rodingite (Shi et al., 2005a; Yi et al., 2006; Wang et al., fault during post-early Eocene time. 2012). Occurrences of glaucophane-schist and jadeitite reflect a HP/LT Between the Belt and the Range, the jadeite uplift (“jadeite uplift” forming condition (Shi et al., 2001, 2003, 2005a, 2012b). In addition, consists of jadeitite-bearing serpentinite mélange and other metamor- strong dynamic deformation prevailed upon jadeitite in the uplift, and phic units) occurs along the northern extension of the Sagaing Fault. hence the jadeite crystals exhibit preferred orientation, or become The uplift comprises of metamorphic rocks such as glaucophane-schist, intensely mylonitized that the stone becomes transparent (Shi et al., together with mica-schist, amphibolite, and the Hparkan–Tammaw 2009b). Furthermore, the relationship between jadeite forming serpentinite mélange hosting the Myanmar jadeitite (Chhibber, 1934; subduction-related fluids and deep-sea material has been confirmed Shi et al., 2001). The presences of high pressure rocks, like blueschist by a number of recent studies based on isotope compositions of the and jadeitite and associated rocks the presence of alkali amphibole fluids from the fluid inclusions, the findings of Ba-bearing minerals, (Shi et al., 2003, 2012b), omphacitite (Yi et al., 2006), and kosmochlor and the discovery of type I deep sea spherules in the Myanmar jadeitite aggregates (Shi et al., 2005a) and jadeitized rodingite (Wang et al., (e.g., Shi et al., 2005b; Sorensen et al., 2006; Deng et al., 2009; Shi et al., 2012)confirm local involvement of subduction process. However, the 2010; Simons et al., 2010; Sorensen et al., 2010; Deng et al., 2011; Shi published age data are very limited and somehow contradictory. Goffé et al., 2011; Deng et al., 2013). Eclogite has not been found in the et al. (2000) derived 40Ar/39Ar ages of 80 Ma–30 Ma on phengites in Myanmar jadeitite uplift so far, although some occasional occurrences the mélange tectonic blocks around the Myanmar jadeitite and sug- have been reported in jadeitite localities elsewhere (see Harlow and gested the presence of an Oligocene high-pressure metamorphic Sorensen, 2005; Tsujimori and Harlow, 2012), and also from near the event. The ages are much younger than those revealed by Late Jurassic Myanmar jadeitite uplift in the Naga Hills within the Indo-Burma zircons U–Pb ages and Mesozoic 40Ar/39Ar ages in the Myanmar jadeitite Range (Chatterjee and Ghose, 2009) and the Kumon range (Enami (ca. 163.2 Ma and 146.5 Ma by Shi et al., 2008; ca. 158 Ma by Qiu et al., et al., 2012). Three fresh samples, two of phengitic muscovite-bearing 2009; Qi et al., 2013; Yui et al., 2013). It leads to a question whether two quartz-schist (A7 and A9) from site 1 and one of glaucophane-schist subduction zones are juxtaposed around the jadeitite area. However, the (P4) from site 2 (Fig. 1b), ~5 km north of the site 1, were selected for available age data are inadequate to address this question, and to deter- microstructural studies and 40Ar/39Ar dating. These rocks are from schist mine the temporal evolution of the jadeite uplift and its tectonic rela- and amphibolites outside the Hparkan–Tammaw mélange. tionship between the Range and the Belt in vicinity. Here we report The phengitic muscovite-bearing quartz-schist (sample nos. A4 and 40Ar/39Ar dating of three samples from the jadeite uplift: glaucophane- A9) consists of quartz (~79 vol.%), phengitic muscovite (~15 vol.%), bearing quartz-schist and phengitic muscovite-bearing quartz-schists chlorite (~3 vol.%), and accessory minerals of zircon, apatite etc. that may have correlation with the jadeitite-forming event, in an at- (~3 vol.%), without glaucophane and barroisite. Quartz crystals are tempt to interpret the relations among the uplift, the Range and the strain-free and have aspect ratios of 1:2 to 1:3. The quartz grain bound- Belt. Based on the results, we discuss the implications on the preserva- aries are straight and almost at 90° with respect to basal cleavage of the tion and exhumation of the jadeitite. muscovite grains, which show penetrative foliation through the pinning effect (Vernon, 1999, 2004). Phengitic muscovites display shape pre- 2. Geological setting and petrography ferred orientation, kinking, bending, and curving similar to the features of rocks which experienced shear deformation and recrystallization. The The jadeite uplift (Fig. 1a) incorporating the Myanmar jadeite depos- micaceous rock shows broadly equilibrium texture, which is typical of it and high-pressure rocks in the Hpakan area of Kachin State straddles post-tectonic (annealing) recrystallization (Fig. 2a, b). the north-western parts of the Sagaing fault belt (Bertrand et al., 1999; In contrast, the glaucophane-bearing quartz-schist (sample no. Bertrand and Rangin, 2003). The region belongs to the Indo-Burma P4), comprising quartz (~82 vol.%), phengitic muscovite (~6 vol.%), Range (Mitchell et al., 2004), whose eastern boundary is generally de- barroisite + glaucophane (~9 vol.%) and accessory minerals of zircon fined by a discontinuous line of serpentinite mélange. The rocks in the and apatite (~3 vol.%), shows only slight deformation. Quartz grains Indo-Burma Range are progressively younger from east to west. The vary greatly in size, from several-μm up to 1 mm long. They show micro- timing of formation of the serpentinized/rodingitized of the ultramafic structural features of sub-grain, high angle serrated boundaries with rocks hosting the jadeitite is ca. 163 Ma based on zircon U–Pb dating slightly undulatory extinction. Phengitic muscovites exhibit perfect (Shi et al., 2008; Yui et al., 2013). The subduction age corresponding to shapes without conspicuous kinking, bending, and curving, and lack formation age of the Myanmar jadeite, however, is still under controver- preferred orientation both in shape and in crystallography (Fig. 2c, d). sy, from ca. 157 Ma, 147 Ma, to the lowest 77 Ma (Shi et al., 2008; Qiu Glaucophane and barroisite are euhedral without orientation; they et al., 2009; Qi et al., 2013; Yui et al., 2013). Qi et al. (2013) suggest coexist and occur mainly in association with phengitic muscovite. that the jadeitite was formed during the early Cretaceous (135 Ma) high-pressure (HP) metasomatism and then experienced a late Creta- ceous (93 Ma) HP metasomatism. A Mesozoic intra-oceanic subduction 3. Technique and results associated with the uplift is suggested by depleted mantle Hf isotope sig- natures of zircons in the jadeitite (Shi et al., 2009a). The representative chemical compositions of phengitic muscovite In the jadeite uplift, metamorphosed rocks including phengitic and glaucophane/barroisite (Table 1) were acquired at the Institute of muscovite-bearing glaucophane-schist, stilpnomelane-bearing quartz- Geology and Geophysics, Chinese Academy of Sciences (IGGCAS) using ite, phengitic muscovite-bearing quartz-schist, garnet-bearing amphib- a JXA-8100 Electron Microprobe Analyzer (EMPA) with a voltage of olite and diopside-bearing marble occur (Bender, 1983; Shi et al., 2001) 15 kV, a beam current of 10 nA and a spot size less than 10 μm. Details around the Hparkan–Tammaw serpentinite mélange (Fig. 1b). Within about EMPA standards and recalculations of mineral formulae refer to the serpentinite mélange, massive veins and blocks of primary jadeitite Shi et al. (2010). Nomenclature of the micas is referenced as Rieder occur. Related to the jadeitite are: metasomatic amphibole consisting of et al. (1999). Muscovite in the schists (samples A4, A9) is phengitic eckermannite, magnesiokatophorite, nyböite, glaucophane, richterite muscovite. Amphibole in the schist (sample P4) consists of glaucophane and/or winchite occur (Shi et al., 2003), as well as metasomatic and barroisite.

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; Searle et al., 2007). G. Shi et al. / Gondwana Research 26 (2014) 464–474 467

Fig. 2. Petrography of the studied samples. (a) Photomicrographs of sample A4 (crossed) and (b) sample A9 (crossed). The two samples are phengitic muscovite-bearing quartz schist, sharing similar microstructure features. Quartz (Qz) grains are prolonged with obvious grain boundary gliding, and phengitic muscovites (Ms) had been kinked, curved or even torn out, showing shape preferred orientation. (c) Photomicrographs of sample P4 (crossed) and (d) (polarized). Quartzes have high angle serrated boundaries and show slightly undulatory extinction. Phengitic muscovites mostly have perfect shapes without kinking, bending or curving, and are not predominantly orientated. Glaucophanes (Gln) and barroisites (Brs) are euhedral (mineral abbreviations after Whitney and Evans, 2010).

Samples were crushed and sieved between 40 and 80 mesh reported as internal error combining the analytical error and the error (380–200 μm) fractions and washed with distilled water. The phengitic on the J-value. Details of the step-heating analysis were outlined in muscovite in samples A4, A9, and glaucophane-barroisite (the coexisting He et al. (2006). The plateau and isochron ages were calculated using assemblage) in sample P4 were separated through hand-picking under a ArArCALC (Koppers, 2002). A plateau age forming an age spectrum binocular microscope. After washing with acetone in an ultrasonic bath is assigned when at least three successive incremental heating steps for several times, the grains were rinsed with distilled water and dried. yield consistent apparent ages within error limits, and covers a mini- About 5 mg grains were wrapped in aluminium foil, and irradiated to- mum of 50% of the total released 39Ar. The results of the 40Ar/39Ar exper- gether with 85G003-sanidine standards, and optical CaF2 and K-glass iments are listed in Table 2 and plotted as age spectrum and isotope monitors in position H8 of the 49-2 reactor in Beijing for 30 h with correlation diagrams in Fig. 3. 0.5 mm cadmium foil shield. To be concordant with the GTS04, all 40Ar/39Ar ages presented After the irradiation and a period of cooling, the total fusion of stan- herein are calculated to be consistent with an age of 28.34 ± dards and argon step-heating analyses of samples by furnace were 0.28 Ma for the 85G003 sanidine from the Taylor Creek rhyolite performed at the 40Ar/39Ar Geochronology Lab in the IGGCAS on an (Renne et al., 1998), and all uncertainties reported herein are in 2 MM5400 mass spectrometer operating in static mode. The total system sigma internal uncertainties. It is noted that Renne et al. (2010, blanks (1000 °C, 20 min) were in the range of 4.9–5.8 × 10−16 mol for 2011) concluded that on the basis of intercalibration with FCs, they mass 40, 0.9–1.4 × 10−18 mol for mass 39, 8.7–9.2 × 10−19 mol for recommend an age of 28.619 ± 0.034 Ma for Taylor Creek sanidine mass 37, and 1.8–2.1 × 10−18 mol for mass 36. Mass discrimination (TCs). (0.009934–0.009958 per atomic mass unit) was monitored by analysis The phengitic muscovite separated from schists A4 yields a slightly of 40Ar/36Ar air pipette aliquots each day. Ca, K correction factors were cal- disturbed age spectrum (Fig. 3). Basically it forms the top of a dome- 40 39 −4 culated from the CaF2 and K-glass monitors: ( Ar/ Ar)K =8.8×10 , shaped age spectrum. Such age spectra may be due to incorporated ex- 39 37 −4 36 37 −4 ( Ar/ Ar)Ca =7.24×10 ,and(Ar/ Ar)Ca =2.39×10 .The cess argon, or the presence of more than one generation because of re- data were corrected for system blanks, mass discriminations, interfering crystallization of the mica (e.g. Wijbrans and McDougall, 1986; de Jong, Ca, K derived argon isotopes, and the decay of 37Ar since the time of the 2003). The shape of the spectrum is considered to be the result of more irradiation. Decay constant and isotopic abundance ratios used: than one generation of recrystallization. Step 3 to step 11, which ac- 40 −10 −1 40 39 Ktot = (5.543 9 ± 0.010) ∗ 10 a ; K/K = 0.01167 at.% (Steiger count for 74.0% of the total Ar released, deﬁne a plateau age of and Jäger, 1977). The uncertainty of J-value (0.2–0.5% in this work) is 44.0 ± 1.2 Ma (MSWD = 2.7). An inverse isochron age of 44.5 ± one standard deviation of mean; this was propagated into the ﬁnal 2.9 Ma (MSWD = 3.1), calculated from 9 steps that formed the plateau, plateau and isochron ages, and contributed about 40% to the total uncer- is consistent with the top dome-shape plateau age. The 40Ar/36Ar inter- tainty in these age determinations. The uncertainties of the ages were cept of 293.3 ± 11.0 is not distinguishable from the air ratio. 468 G. Shi et al. / Gondwana Research 26 (2014) 464–474

Table 1 Representative chemical compositions of phengitic muscovite, glaucophane and barroisite in the schists from Myanmar.

A4 A9 P4

Point123123Point1234

SiO2 48.58 49.58 49.67 47.98 51.20 48.48 SiO2 56.42 57.36 51.54 49.55

TiO2 0.38 0.20 0.31 0.51 0.38 1.54 TiO2 0.00 0.00 0.33 0.00

Al2O3 30.88 28.2 27.95 36.79 32.9 28.11 Al2O3 9.67 6.57 10.48 11.40

Cr2O3 00.00 0.09 0.09 0.00 0.00 0.00 Cr2O3 0.00 0.09 0.00 0.10 FeO 3.93 3.99 4.26 1.01 1.89 4.62 FeO 15.23 18.08 17.53 18.00 MnO 0.00 0.03 0.04 0.00 0.00 0.00 MnO 0.00 0.26 0.00 0.18 MgO 1.51 1.65 1.76 0.91 1.94 3.24 MgO 8.74 8.11 8.71 8.58 CaO 0.00 0.10 0.06 0.17 0.14 0.00 CaO 1.76 0.16 4.17 4.79

Na2O 0.37 0.24 0.37 0.80 0.00 0.76 Na2O 6.02 6.79 5.43 5.51

K2O 9.62 10.02 9.51 7.16 7.52 9.22 K2O 0.51 0.00 0.22 0.35 Total 95.27 94.10 94.01 95.34 95.94 95.96 Total 98.35 97.33 98.53 98.46

Si 3.25 3.37 3.37 3.11 3.30 3.25 SiT 7.82 8.05 7.29 7.08

AlIV 0.75 0.63 0.63 0.89 0.70 0.75 AlT 0.18 0.00 0.71 0.93 3+ AlVI 1.69 1.63 1.61 1.93 1.80 1.46 Fe T 0.00 0.00 0.00 0.00

Ti 0.02 0.01 0.02 0.03 0.02 0.08 AlC 1.40 1.09 1.04 0.99 3+ Fe 0.00 0.00 0.00 0.00 0.00 0.00 MgC 1.81 1.70 1.84 1.83 2+ 2+ Fe 0.22 0.23 0.24 0.06 0.10 0.26 Fe C 1.28 1.23 1.29 1.28

Mg 0.15 0.17 0.18 0.09 0.19 0.32 MgB 0.00 0.00 0.00 0.00

Ca 0.00 0.01 0.00 0.01 0.01 0.00 CaB 0.26 0.00 0.63 0.73

Na 0.05 0.03 0.05 0.10 0.00 0.10 NaB 1.62 1.85 1.37 1.27

K 0.82 0.87 0.82 0.59 0.62 0.79 NaA 0.00 0.00 0.12 0.26 Mineral Ms Ms Ms Ms Ms Ms Gln Gln Brs Brs

Ms: phengitic muscovite, Gln: glaucophane, Brs: barroisite.

Table 2 40Ar/39Ar step heating data for phengitic muscovites (samples A4 and A9) and glaucophane (sample P4).

Temp 39Ar/40Ar ± 1 s.d. 36Ar/40Ar ± 1 s.d. 37Ar/39Ar 39Ar 40Ar* 40Ar*/39Ar Apparent age ± 2 s.d. (10−2) (10−4) Cum (%) (%) (Ma)

A4, 5.34 mg, J = 0.004612 910 7.04 ± 0.11 22.77 ± 0.27 0.05 8.77 32.71 4.64 38.24 ± 2.19 940 8.53 ± 0.20 20.18 ± 0.23 0.05 11.98 40.36 4.73 38.95 ± 2.26 960 8.03 ± 0.08 18.78 ± 0.22 0.04 14.78 44.51 5.55 45.56 ± 1.59 980 8.36 ± 0.07 19.00 ± 0.20 0.05 16.58 43.85 5.25 43.13 ± 1.40 1000 6.10 ± 0.06 22.93 ± 0.26 0.04 5.56 32.25 5.29 43.45 ± 2.24 1030 4.04 ± 0.12 26.20 ± 0.34 0.04 5.41 22.57 5.58 45.87 ± 4.89 1060 3.62 ± 0.04 27.66 ± 0.33 0.04 4.63 18.25 5.04 41.44 ± 4.44 1090 3.77 ± 0.13 27.14 ± 0.30 0.05 4.77 19.79 5.26 43.21 ± 4.97 1130 5.08 ± 0.05 25.31 ± 0.31 0.00 5.89 25.22 4.96 40.83 ± 3.04 1170 6.21 ± 0.11 21.61 ± 0.23 0.06 7.35 36.14 5.82 47.77 ± 2.54 1210 8.99 ± 0.05 17.76 ± 0.21 0.05 9.06 47.53 5.29 43.48 ± 1.37 1260 9.60 ± 0.12 18.04 ± 0.28 0.04 3.98 46.70 4.87 40.04 ± 1.90 1360 3.05 ± 0.54 28.08 ± 0.46 0.08 1.23 17.03 5.59 45.91 ± 17.83

A9, 5.16 mg, J = 0.004613 700 4.55 ± 1.63 30.70 ± 1.55 0.39 0.02 9.28 2.04 16.88 ± 21.06 800 15.39 ± 0.43 10.19 ± 0.32 0.00 0.45 69.89 4.54 37.40 ± 2.33 900 8.77 ± 0.02 18.13 ± 0.17 0.00 18.38 46.42 5.30 43.54 ± 0.96 930 17.32 ± 0.04 1.18 ± 0.01 0.00 67.36 96.52 5.57 45.79 ± 0.23 940 18.49 ± 0.07 1.04 ± 0.04 0.00 4.88 96.92 5.24 43.11 ± 0.32 960 16.42 ± 0.45 5.04 ± 0.25 0.00 0.62 85.12 5.18 42.63 ± 2.45 990 12.98 ± 0.30 7.95 ± 0.24 0.00 0.55 76.49 5.89 48.39 ± 2.41 1050 10.15 ± 0.10 10.93 ± 0.14 0.00 1.11 67.71 6.67 54.69 ± 1.25 1100 11.69 ± 0.09 12.50 ± 0.12 0.00 3.05 63.05 5.40 44.35 ± 0.83 1150 15.88 ± 0.05 4.77 ± 0.09 0.00 3.04 85.90 5.41 44.46 ± 0.41 1200 12.35 ± 0.24 9.48 ± 0.23 0.00 0.54 71.99 5.83 47.89 ± 2.05

P4, 5.36 mg, J = 0.004611 800 2.70 ± 0.03 18.86 ± 0.24 0.02 1.39 44.25 16.37 131.26 ± 5.24 850 1.70 ± 0.01 24.37 ± 0.23 0.00 2.10 28.00 16.49 132.19 ± 6.37 900 4.19 ± 0.01 7.60 ± 0.07 0.00 5.78 77.54 18.52 147.86 ± 1.08 950 5.12 ± 0.01 1.13 ± 0.01 0.00 16.13 96.66 18.88 150.57 ± 0.72 980 5.17 ± 0.01 0.40 ± 0.00 0.00 20.50 98.82 19.10 152.26 ± 0.72 1010 5.18 ± 0.01 0.35 ± 0.00 0.00 19.10 98.98 19.10 152.27 ± 0.81 1040 5.12 ± 0.01 0.66 ± 0.01 0.00 13.83 98.04 19.16 152.77 ± 0.71 1090 4.96 ± 0.01 1.11 ± 0.01 0.00 13.22 96.71 19.49 155.27 ± 0.76 1140 4.94 ± 0.01 1.32 ± 0.02 0.00 5.27 96.10 19.44 154.89 ± 0.78 1190 4.90 ± 0.02 1.72 ± 0.03 0.01 1.56 94.92 19.37 154.33 ± 1.21 1240 4.72 ± 0.04 2.55 ± 0.04 0.01 0.77 92.47 19.61 156.16 ± 2.67 1300 4.37 ± 0.56 4.30 ± 0.08 0.01 0.35 87.28 19.95 158.78 ± 38.98 G. Shi et al. / Gondwana Research 26 (2014) 464–474 469

Fig. 3. Age spectrums and inverse isochron of phengitic muscovites (samples A4, A9) and glaucophane–barroisite (sample P4).

The phengitic muscovite separated from schist A9 also yields a may reflect more than one generation of recrystallization, combining slightly disturbed age spectrum (Fig. 3). Six steps, which account for with some chemical variation and the more than one generation 97.3% of the total 39Ar released, define a nearly plateau age of 44.8 ± through recrystallization, the internally consistent plateau segments 1.1 Ma (MSWD = 39.4). An inverse isochron age of 45.0 ± 1.3 Ma between the A9 and A4 suggest that excess argon or contamination by (MSWD = 45.5), calculated from 6 steps that formed the plateau, is impurities is low. Therefore, the weighted mean age of 44.9 ± 0.2 Ma consistent with the plateau age. The 40Ar/36Ar intercept of 286.9 ± of two inverse isochron ages is taken to represent the cooling age of 34.5 is not distinguishable from the air ratio. the phengitic muscovite in A4 and A9. The meaning of this age (A9) is limited as it consists of one step Glaucophane separated from the glaucophane-schist P4 yields an releasing 67% of the gas, and the two main steps about 85%. All age dis- upward staircase age spectrum (Fig. 3). Three consecutive steps which cordance and dome-shape, as for instance shown by spectrum A4, account for 53.4% of the total 39Ar released define a weighted plateau would be destroyed in this process; there is a lot of scatter in the final age of 152.4 ± 1.5 Ma (MSWD = 0.6). An inverse isochron age of degassing. Although this age of phengitic muscovite is discordant and 151.6 ± 2.1 Ma, calculated from 3 steps that formed the plateau, is 470 G. Shi et al. / Gondwana Research 26 (2014) 464–474 consistent with the plateau age (Fig. 3). The radiogenic 40Ar of these three (2007) and Shi et al. (2008) that the subduction in the jadeite uplift steps is higher than 98%, and all these points plot closely together in the took place during Mesozoic. inverse isochron plot. In terms of excess-Ar evaluation of glaucophane, Whether recrystallization occurred or not is another key factor to in- it has been well recognized in particular that amphibole (or other terpret the 40Ar/39Ar age. The diffusion of unincorporated ions inside rocks) adjacent to ultramafic rocks can accommodate excess Ar, yielding crystals is fundamentally a function of temperature, assuming that older ages. However, the glaucophane hosted rock, consisting of the crystal is under static environment (e.g. Lo et al., 2000). However, ~82 vol.% quartz, would not accommodate excess Ar from the Si- not all minerals remain static once they formed; they might have unsaturated ultramafic rocks. Petrographic study excludes possible experienced recrystallization, which would facilitate diffusion of the in- presence of fluid inclusions in the amphiboles, which can cause corporated ions inside individual crystal or among neighbor crystals excess-Ar. It would mean that these staircase age spectra might point (e.g. Stünitz, 1998; Bestmann et al., 2008; Shi et al., 2009b)ateven to the presence of more than one phase. It is known that many lower temperature than the closure temperature. In general, multiple glaucophane crystals are zoned and/or may have submicroscopic stages of phengite formation are very common in schists (e.g. Bakker white mica, and can carry K. The EPMA (Table 1) data yielded Ca/K ra- et al., 1989; Radvanec et al., 1994; Itaya and Fujino, 1999). Regional tios of 3–20 for glaucophane, and 0.01–0.02 for mica. The 37/39 ratios shearing might have caused multiple deformation and they could be (Table 2) of the dated glaucophane and EPMA Ca/K do not match, preserved in mica texture. Without considering recrystallization factor, suggesting that the Na-amphibole was chemically recrystalised (to closure temperatures are 300–400 °C of micas (Reddy et al., 1997), white mica), implying that the age refers to included mica. This can 400–480 °C of phlogopites (Dodson, 1973), or 400–500 °C of phengites explain the much younger ages during early degassing and older (Monie and Chopin, 1991; Kelley and Wartho, 2000). On the other hand, ages at the end. This process can also change the value of the “age if deformation is overprinted, later closure of the K–Ar system records plateau” (e.g. Sisson and Onstott, 1986; de Jong and Wijbrans, 2006; de the result of retrogression and subsequent ductile deformation (Itaya Jong et al., 2009). The huge error in the 40/36 intercept (408.2 ± 200.4) and Takasugi, 1988; West and Lux, 1993; Mülch et al., 2002; Parra leaves the possibility that excess argon is involved, leading to that the et al., 2002; de Jong, 2003; Bosse et al., 2005; Yang et al., 2009, 2013). glaucophane Ar/Ar age (~152 Ma) might not be so easy and straightfor- At this rate, phengitic muscovite in the quartz-schist records later clo- ward to explain. Nevertheless, considering that glaucophane–barroite sure of the K–Ar due to ductile deformation. Thus, the weighted age coexist with the muscovite, the presence of included mica is not in con- (44.9 ± 0.2 Ma) on the phengitic muscovite in quartz-schist is flict with the suggestion that the plateau dates may coincide closely with interpreted as the time of post-tectonic recrystallization, i.e. the the time of blueschist metamorphism, we regard that the 40Ar/39Ar age overprinted ductile deformation event. (~152 Ma) represent as, or at least the lower limit age of, the formation The age of ~45 Ma point to a deformation event, which is broadly of glaucophane. coeval with the metamorphic overgrowth of zircon grains and the growth of metamorphic monazite at sillimanite grade in the adjacent 4. Discussion area (e.g. Barley et al., 2003; Searle et al., 2007), is slightly older than the apatite fission track ages of 40–36 Ma along the Mae Ping fault 4.1. Interpretations of 40Ar/39Ar dating zone (Upton, 1999; Morley, 2004), and significantly older than the 40Ar/39Ar mica ages from Mogok rocks adjacent to the Sagaing Fault of Two contrasting textures of phengitic muscovite of the studied sam- 30–18 Ma (Bertrand et al., 1999), as well as the 40Ar/39Ar deformation ples, together with two distinct 40Ar/39Ar datings and previous relevant ages of ductile shear far more south along the Sagaing Fault reported literatures (Shi et al., 2008; Qi et al., 2013; Yui et al., 2013), suggest that by Bertrand and Rangin (2003) and Bertrand et al. (2001).Accordingly, two or more metamorphic events took place in the jadeite uplift. the ages are coeval with the event induced by the India–Eurasia colli- In terms of texture, the glaucophane-schist (P4) does not show any ob- sion, and close to the timing of formation of the huge fault (e.g. Chung vious shape orientation, and the quartz grains are characterized by high et al., 1998, 2005; Yin, 2006; Ali and Aitchison, 2008). angle serrated boundaries with slight undulatory extinction. The almost- intact crystal shapes of its constituent minerals of phengitic muscovite 4.2. Tectonic implications and glaucophane (Fig. 2c and d) allow us to suggest that they were formed predominantly under static conditions, without apparent shear This study re-confirms the Jurassic HP/LT metamorphic event deformation. Contrastingly, the quartz-schist (A4, A9) records shear de- (e.g. Mitchell et al., 2007; Shi et al., 2008; Qiu et al., 2009), considered formation and post-tectonic (annealing) recrystallization: the phengitic to be coeval with the intra-oceanic subduction as suggested by mantle muscovites in this rock display shape preferred orientation, kinking, depleted magmatic zircons from the Myanmar jadeitite (Shi et al., bending and curving. 2008, 2009a). Determination of the Jurassic jadeite intra-oceanic Our 40Ar/39Ar dating (152.4 ± 1.5 Ma) of the glaucophane in subduction between the Indian Plate and the West Burma Block has im- glaucophane-schist is interpreted to represent as or the lower limit plications for understanding reconstructions of the Indonesian region age of glaucophane formation. It is known that amphiboles have variable and the Indian Ocean. According to previous studies, this subduction closure temperatures due to their chemical composition differences, as was inferred to be from the Woyla intra-oceanic arc or the Incertus variations of Fe/Mg ratio can lead to slightly differential diffusion Arc, another intra-oceanic arc west of the Woyla Arc (e.g. Hall, 2012). (Monie and Chopin, 1991). As most of the amphiboles have closure tem- The subduction of the Woyla Arc was initiated at about 160 Ma and perature of ~500 °C, the measured glaucophane formed predominantly was interpreted as a Late Jurassic–Early Cretaceous intra-oceanic arc under static metamorphism, and it coexists with intact phengitic (Wajzer et al., 1991; Barber, 2000; Barber and Crow, 2009; Shi et al., muscovite (closure temperature 400–500 °C of phengites, Monie 2012a), which is almost the same as (or slightly younger than) the and Chopin, 1991; Kelley and Wartho, 2000). We therefore consider ~163 Ma zircon SHRIMP U–Pb age reported from the Myanmar jadeitite, that the closure temperature of glaucophane is significantly higher than which corresponds to the formation time of the host ultramafic rocks of ~400 °C (e.g. Reddy et al., 1997), which is still higher than the tempera- the jadeitite (Shi et al., 2008, 2009a; Yui et al., 2013). The jadeite uplift is ture (~370 °C) proposed for the formation of the adjacent Myanmar more likely related to the Woyla Arc. The related groups include the jadeitite (e.g. Mével and Kiénast, 1986; Shi et al., 2003; Oberhänsli Spontang Arc (Pedersen et al., 2001), the Zedong Terrane of southern et al., 2007). As the 40Ar/39Ar age of the glaucophane is consistent with Tibet (which is an intra-oceanic arc formed in the Late Jurassic; (or slightly older) the SHRIMP U–Pb ages on zircons from the Aitchison et al., 2007); the Kohistan–Ladakh Arc (which may have Myanmar jadeitite (Shi et al., 2008; Qiu et al., 2009), our data re- been in an equatorial position in the Late Cretaceous and might have confirm the interpretations by Mitchell et al. (2007), Searle et al. collided with India in the Early Paleocene; Khan et al., 2009; Pedersen G. Shi et al. / Gondwana Research 26 (2014) 464–474 471 et al., 2010), as well as the Mawgyi Nappe in Burma (the Woyla Group in intra-oceanic subduction from zircons of the uplift (Shi et al., 2008, Sumatra and the Meratus ophiolite of SE Borneo, see Figs. 5–8andFig.38 2009a), the Jurassic intra-oceanic subduction for the Yarlung zone from Hall, 2012). All of above were northeast-facing intra-oceanic arcs, (Aitchison et al., 2000; Ali and Aitchison, 2008; Zhu et al., 2009; Wang emplaced as nappes onto the western margin of SE Asia in the late et al., 2010; Aitchison et al., 2011; Wang et al., 2013a; Zhu et al., Early Cretaceous (Mitchell, 1992, 1993). 2013), and recognition of a Jurassic SSZ-type ophiolite and related Cenozoic ages (~45 Ma) are identified on the phengitic muscovite in rock units in the Myitkyina region (Yang et al., 2012) implies that the quartz-schist. However, these ages do not point to HP/LT metamorphism, Indo-Burma Range, together with the jadeite uplift is the southern con- as no high-pressure mineral assemblage has been found in the schist. tinuations of the Tagaung–Myitkyina Belt, is the southeastern part of the They are interpreted as a recrystallization event, which is coeval with Yarlung zone (c.f., Mitchell et al., 2007). the onset of the Sagaing fault. Cenozoic ages have been reported in some previous studies and interpreted by Goffé et al. (2000) to repre- 4.3. Implications for exhumation mode of the jadeitite sent the time of high-pressure metamorphic events. However, these au- thors did not present mineral compositions and textures of the analyzed It is widely acknowledged that jadeitites always occur in subduction phengite. Based on mineralogical compositions and microstructures of zones in association with serpentinite mélange, HP/LT metamorphic the phengitic mica in the present investigation, we infer that their rocks like blueschist and eclogite (e.g. Harlow and Sorensen, 2005)ex- “phengite” might have been the phengitic mica, and might have experi- cept minor jadeitite xenoliths in kimberlitic pipes in the Colorado Pla- enced shear deformation. If so, the ages reported in their study would teau (c.f., Tsujimori and Harlow, 2012). The studied rocks were refer to an event not induced by high-pressure metamorphic event. collected from schists surrounding the serpentinite mélange hosting Combined with the previous data (e.g. Leloup and Kienast, 1993; the jadeitite. However, jadeitite occurrences are very rare, with only Mitchell et al., 2007; Searle et al., 2007), we suggest that there were no 19 localities reported worldwide (Tsujimori and Harlow, 2012), which prevailing Cenozoic HP/LT metamorphic events in the Indo-Burma are significantly less than those of serpentinite mélange, blueschist, Range, along the Sagaing Fault, the Mogok metamorphic belt, and in even rarer than the coesite- and/or diamond-bearing eclogite occur- the Ailao shan–Red River shear zone. rences (Liou et al., 2009). In certain jadeitite-containing HP/LT meta- In comparison to the published data of the Mogok metamorphic belt, morphic belt, jadeitite outcrops occupy only a small proportion, it is suggested that the jadeite uplift is not a segment of the Mogok estimated to be less than 1% of the whole belt. It leads to a question of metamorphic belt. The Mogok belt is a high-grade metamorphic belt, whether the jadeitite is difficult to be formed in nature, or not easy to which plays a key role in the network of deformation zones that accom- be preserved during and after the exhumation. modated strain during the northwards movement of the India plate and Evidence increasingly demonstrates that jadeitite-forming materials resulting extrusion or rotation of the Indochina block (e.g. Bertrand are derived from subducted oceanic crust. For example, the common oc- et al., 1999; Barley et al., 2003; Mitchell et al., 2007; Searle et al., currences of Ba and Sr minerals in the Guatemala, Japan and Myanmar 2007). At this point, it has the same function as the Sagaing Fault and jadeitites (Kobayashi et al., 1987; Harlow, 1995; Miyajima et al., 1999; the Red River shear zone (e.g. Bertrand et al., 1999; Zhang and Morishita, 2005; Shi et al., 2010), and the finding of type-I deep-sea Schärer, 1999; Garnier et al., 2002; Searle et al., 2007; Kyi Khin et al., spherules (Shi et al., 2011) in the Myanmar jadeitite have led to the sug- 2013). However, Jurassic HP/LT rocks have not been found in the gestion that the fluids were at least partially derived from oceanic sed- Mogok metamorphic belt. Furthermore, an Andean-type margin of iments on altered subducted oceanic crust. The required aqueous southern Eurasia, instead of an intra-oceanic subduction for the jadeitite solutions rich in Na, Al, and Si, which could be derived from dehydration (Shi et al., 2009a), is suggested from Jurassic magmatic zircons in the of the sediments, the altered oceanic crust (e.g. Shi et al., 2009a; Simons Mogok belt (Barley et al., 2003). On the other hand, an ~43 Ma age for et al., 2010; Harlow et al., 2011) and from rodingitization in association high-grade metamorphism revealed by overgrowth on Jurassic zircons with serpentinization (Wang et al., 2012), are expected to have pro- in the orthogneiss near Mandalay (e.g. Barley et al., 2003; Searle et al., duced the jadeitite. In comparison to rare occurrences of jadeitite, meta- 2007) is almost the same as the age from our 40Ar/39Ar dating, somatic albitite appears commonly in jadeitite-free serpentinite suggesting similar events related to the India–Eurasian collision. mélange and high-pressure metamorphic belt; some albitites are meta- The jadeite uplift is more likely a segment of the northern part of the somatic products of retrograded jadeitite during exhumation. This Indo-Burma Range and southern part of the Tagaung–Myitkyina Belt. means that preservation of the jadeitite in subduction zones is a key rea- The southern part of the Tagaung–Myitkyina Belt near Mandalay is ter- son for its rarity. A successful preservation would need an abrupt minated by the Sagaing Fault (Fig. 1). We assume that the current change of P–T conditions during exhumation, since jadeite would southern part is not the real termination of the original Belt, and the react with the residual fluids or be decomposed if pressure and temper- missing part might have been truncated by the fault. Geometrically, ature decreased too slowly. In the Myanmar jadeitite, Na-rich fluids (a the truncated segments would be distributed along the fault or on its chemically active fluid) still prevail after the jadeite formation, as other side. Coincidence of the spatial configuration of the Tagaung– more than ten Na-rich minerals formed at lower-pressure relative to Myitkyina Belt, the jadeite uplift and the Indo-Burma Range (Fig. 1a) the jadeite (Shi et al., 2012b; Wang et al., 2013b), suggesting that the is a positive evidence for this assumption. Suggestion by Mitchell et al. jadeite is difficult to be preserved without any change, and a rapid exhu- (2007) that both the Range and the Belt belong to Jurassic ophiolite mation is needed for the preservation. continuations of the Yarlung zone supports this assumption. In the The structural setting of strike slip fault, thrust fault or under- Indo-Burma Range, Mesozoic ophiolite and HP/LT metamorphic rocks plating structures, which could tectonically result in rapid exhuma- are overlain unconformably by Upper Albian–Lower Cenomanian lime- tion of high-pressure terranes (e.g. Michard et al., 1993; Platt, 1993; stone (e.g. Bhattacharjee, 1991; Acharyya, 2007; Mitchell et al., 2007; Agard et al., 2009) commonly occur across or adjacent to almost all Chatterjee and Ghose, 2009; Maurin and Rangin, 2009). Within the the jadeitite deposits worldwide. For example, the Guatemala jadeitite Tagaung–Myitkyina Belt serpentinized harzburgite, basalt and chert occurs in serpentinite mélange in distinct settings on both sides of the are also overlain by Albian limestone (Mitchell et al., 2007). The mid- Motagua fault, the current North American–Caribbean plate boundary Cretaceous limestone in the Indo-Burma Range (United Nations, 1979; (e.g. Harlow et al., 2007, 2011). The jadeitite from Sierra del Convento, Mitchell, 1993) is identical to those in the Tagaung–Myitkyina Belt. eastern Cuba (Garcia-Casco et al., 2009) is located close to the Orient The jadeite uplift with serpentinite mélange and high-P metamor- fault zone, which is a left lateral strike slip fault (Rodriguez and phosed rocks are likely to be the residual fragments of the former Córdoba, 2010). The Myanmar jadeitite straddles the Sagaing Fault. complete Belt, as it appears along (or straddles) the Sagaing Fault. Other examples related to the thrust fault are as follows: jadeitite from Furthermore, the coherence among our suggestion of the Jurassic the Itoigawa-Omi District, Japan, which lies close to Itoigawa–Shizuoka 472 G. Shi et al. / Gondwana Research 26 (2014) 464–474

Tectonic Line (ISTL), is a geological boundary between the pre-Tertiary proposed, explaining the rarity of jadeitite (estimated to be less unit to the west and the Neogene units (Tsujimori et al., 2006). It is an than 1% of the whole jadeitite-containing HP/LT metamorphic belt). active thrust fault between the east and the northern segment of the ISTL (Ogawa et al., 2002 and references therein). Also in Japan, the Osayama jadeitite-bearing serpentinite mélange has been developed Acknowledgments along a Paleozoic thrust fault between the Oeyama ophiolite and Renge blueschist belt (Tsujimori, 1997; Tsujimori and Itaya, 1999). The We appreciate R.X. Zhu and L.C. Chen for their kind supports during fi the field trip, Mao, Q. and Ma, Y.G. for their helps with EMP analyses and Syum-Keu ultrama c complex containing the Polar Urals jadeitite is dis- 40 39 tributed along the Polar Urals thrust-and-fold belt (Meng et al., 2011 and Sang, H.Q. for Ar/ Ar analyses. Discussions with Harlow, G.E. and references therein). Likewise, the Iran jadeitite veins lie between the Flores Reyes, K. during SGH's visiting at the AMNH are very helpful Zagros fold-and-thrust belt (representing the Arabian plate) and the and gratefully appreciated. Thoughtful comments and careful editorial metamorphic rocks and volcanic arc sequences of the Sanandaj–Sirjan handling from Journal Editor Santosh, M. constructive and thoughtful Zone (northern continental margin of Tethys) in southeastern Iran reviews by de Jong K. and Tsujimori T. are gratefully appreciated. This (Oberhänsli et al., 2007). The New Idria jadeitite-bearing serpentinite study was supported by the National Basic Research Program of China body forms core of the Coalinga antiform along the crest of the Diablo (2009CB421008), the Doctoral Program of Higher Education of China Range between the San Andreas Fault on the west and the San Joaquin (20120022110004) and the Fundamental Research Funds for the Valley on the east (Tsujimori et al., 2006), where well-preserved Central Universities (2001YXL048). underplating structure occurs (Kimura et al., 1996). The Sagaing Fault is inferred as a key structure responsible for the References rapid exhumation of the Myanmar jadeitite. The fact that the jadeitite Acharyya, S.K., 2007. Collisional emplacement history of the Naga–Andaman ophiolites is found along the Fault, which does not appear in the Indo-Burma and the position of the eastern Indian suture. Journal of Asian Earth Sciences 29, Range and the Tagaung–Myitkyina Belt, should be considered as 229–242. 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Remnants of a Cretaceous intra-oceanic subduc- to124Maor77Ma(e.g.Qi et al., 2013; Yui et al., 2013). Our 40Ar/39Ar tion system within the Yarlung-Zangbo suture (southern Tibet). Earth and Planetary Science Letters 83, 231–244. data constrain the time of the rapid exhumation, and illustrate that the Aitchison, J.C., McDermid, I.R.C., Ali, J.R., Davis, A.M., Zyabrev, S.V., 2007. Shoshonites in jadeitites formed from a period from ~147 Ma to Cretaceous; they southern Tibet record Late Jurassic rifting of a Tethyan intraoceanic island arc. Journal remained stable with transformation into albitite (e.g., Wang et al., of Geology 115, 197–218. Aitchison, J.C., Xia, X.P., Baxter, A.T., Ali, J.R., 2011. Detrital zircon U–Pb ages along the 2013b). Some parts were quickly exhumed at ~45 Ma along Sagaing Yarlung-Tsangpo suture zone, Tibet: implications for oblique convergence and Fault. collision between India and Asia. Gondwana Research 20, 691–709. 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