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Discovery of eclogite in the Bangong Co–Nujiang ophiolitic mélange, central Tibet, and tectonic implications
Gondwana Research 35 (2016) 115–123
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Discovery of eclogite in the Bangong Co–Nujiang ophiolitic mélange, central Tibet, and tectonic implications Ya-Lin Dong a,b, Bao-Di Wang c, Wen-Xia Zhao b,⁎, Tian-Nan Yang d, Ji-Feng Xu e,f,⁎ a
School of Earth Science and Geological Engineering, Sun Yat_Sen University, Guangzhou, 510275, PR China Instrumentation Analysis & Research Center, Sun Yat_Sen University, Guangzhou, 510275, PR China Chengdu Institute of Geology and Mineral Resources, Chengdu, 610081, PR China d Institute of Geology, Chinese Academy of Geological Science, Beijing, 100037, PR China e State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, PR China f CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing, PR China b c
a r t i c l e
i n f o
Article history: Received 20 July 2015 Received in revised form 13 March 2016 Accepted 19 March 2016 Available online 30 April 2016 Handling Editor: Z.M. Zhang Keywords: Eclogite Ophiolite Tectonic evolution Bangong Co–Nujiang suture zone Tibetan Plateau
a b s t r a c t An eclogite has been recently identified within ophiolitic mélange in the western segment of the Bangong Co– Nujiang suture zone, at Shemalagou in the Gaize area of central Tibet. The eclogite consists of garnet, omphacite, phengite, rutile, quartz, diopside, and amphibole. The omphacite, which has not been recognized in the suture zone until this study, occurs as rare relics within diopside grains in the eclogite. Phase equilibria modeling shows that the eclogite formed under P–T conditions of 22–28 kbar and 600–650 °C with a low geothermal gradient of ca. 8 °C/km, suggesting that it formed during the subduction of oceanic crust. The protoliths of the eclogite and coexisting garnet amphibolites have geochemical characteristics similar to those of normal mid-ocean ridge basalt (N-MORB), confirming that the eclogites formed from oceanic crust. The presence of high-pressure (HP) eclogite indicates that the ophiolitic mélange in the Bangong Co–Nujiang suture zone underwent oceanic subduction and was subsequently exhumed. We conclude that this ophiolitic belt represents a newly identified HP metamorphic belt in the Tibetan Plateau, adding to the previously recognized Songduo and Longmucuo–Shuanghu eclogite belts. This discovery will result in an improved understanding of the tectonic evolution of the Bangong Co–Nujiang suture zone and the Tibetan Plateau as a whole. © 2016 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.
1. Introduction In recent years, two eclogite belts have been identified on the Tibetan Plateau, namely the Songduo eclogite belt in the Lhasa Block (e.g., Li et al., 2009a, 2009b; Yang et al., 2009; Zeng et al., 2009; Zhang et al., 2014b), and the Longmucuo–Shuanghu eclogite belt in the Qiangtang Block (e.g., Li et al., 2006; Li et al., 2009a, 2009b; Zhang et al., 2006; Zhai et al., 2009; Zhai et al., 2011a, 2011b; Zhu et al., 2013). In addition, outcrops of mafic granulite and garnet amphibolite have been reported in the Bangong Co–Nujiang suture zone (Xia et al., 2013; Wang et al., 2015; Zhang et al., 2015), and at two sites the rocks have been identifiedas “eclogite” (Xia et al., 2013; Zhang et al., 2015), although omphacite was not observed. Therefore, it remains debated whether the Bangong Co–Nujiang suture zone contains high-pressure or ultrahigh-pressure (UHP) metamorphic rocks. To better understand the tectonic evolution of the Bangong Co–Nujiang suture zone, it is ⁎ Corresponding authors at: School of Earth Science and Geological Engineering, Sun Yat_Sen University, Guangzhou, 510275, China. Tel.: +86 20 84115809, + 86 20 85290282. E-mail addresses: [email protected] (W.-X. Zhao), [email protected] (J.-F. Xu).
Fig. 1. (a) Simplified tectonic map in the Tibetan Plateau (modified from Wang et al., 2008). BNSN: Bangong Co–Nujiang Suture Zone. YZSZ: Yarlung Zangbo River Suture Zone. (b) Geological map of ophiolitic mélange in the Dong-Co area (modified from Wang et al., 2015).
http://dx.doi.org/10.1016/j.gr.2016.03.010 1342-937X/© 2016 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.
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Fig. 2. Photomicrographs of the retrograde eclogite. (a and b) Garnet with amphibole + plagioclase symplectites. (c and d) Relics of omphacite within diopside, which has rims of secondary amphibole.
necessary to further investigate the metamorphic history of the mafic granulite and garnet amphibolite in this zone. This study is the first to identify eclogite from the mafic granulite reported by Wang et al. (2015) at Shemalagou (32°19′41″N, 84°44′15″E), north of Dong-Co lake, in the western segment of the Bangong Co–Nujiang belt, indicating that HP metamorphism occurred in the Bangong Co–Nujiang suture
zone. We also discuss the tectonic significance of the eclogite in terms of the evolution of the Bangong Co–Nujiang Tethys. The mineral abbreviations used in this study are as follows: Ab = Albite, Ac = Acmite, Ae = Aegirine, Alm = Almandine, Amp = Amphibole, An = Anorthite, And = Andradite, Ap = Apatite, Bt = Biotite, Chl = Chlorite, Coe = Coesite, Cpx = Clinopyroxene, Di = Diopside, En = Enstatite, Ep =
Fig. 3. Backscattered electron images of the retrograde eclogite. (a) Diopside occurs as inclusions within garnet. (b) Garnets partly replaced by worm-like symplectites. (c and d) Minor omphacite relics within diopside.
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Table 1 Representative compositions of minerals in the studied eclogite. Mineral
Grt
SiO2 TiO2 FeO Al2O3 CaO MnO MgO K2O Cr2O3 Na2O Total O Si Ti Fe3+ Fe2+ Al Ca Mn Mg K Cr Na Sum of cations Alm (WEF) Prp (Jd) Grs (Ae) Sps And
39.68 0.08 23.31 20.12 9.24 1.00 6.26 0.02 0.01 0.01 99.73 12.00 3.06 0.00 0.12 1.38 1.83 0.77 0.07 0.74 0.00 0.00 0.00 7.97 47.11 24.84 19.77 2.23 6.33
Mineral
Di
SiO2 TiO2 FeO Al2O3 CaO MnO MgO K2O Cr2O3 Na2O Total O Si Ti Fe3+ Fe2+ Al Ca Mn Mg K Cr Na Sum of cations Wo En Fs Ac
53.79 0.15 8.87 2.93 22.52 0.21 10.88 0.03 0.02 1.12 100.52 6.00 2.00 0.00 0.00 0.27 0.12 0.88 0.01 0.59 0.00 0.00 0.08 3.95 48.06 32.31 15.30 4.33
Mineral
Ab
SiO2 TiO2 FeO Al2O3 CaO MnO MgO K2O Cr2O3 Na2O P2O5 F Total
67.42 0.01 0.26 18.94 0.33 0.00 0.01 0.09 0.01 11.51 0.00 0.00 98.58
Omp 39.69 0.07 22.83 19.65 9.08 0.99 6.38 0.00 0.06 0.02 98.77 12.00 3.09 0.00 0.14 1.37 1.80 0.76 0.08 0.75 0.00 0.00 0.00 7.99 46.36 25.42 18.82 2.23 7.00
39.67 0.17 24.76 19.50 10.52 0.84 4.61 0.00 0.02 0.05 100.14 12.00 3.08 0.01 0.16 1.45 1.78 0.87 0.06 0.56 0.00 0.00 0.00 7.97 44.64 25.70 19.10 2.01 8.55
38.42 0.22 24.37 19.28 9.84 4.39 2.36 0.00 0.01 0.05 98.94 12.00 3.07 0.01 0.13 1.50 1.81 0.84 0.30 0.31 0.00 0.00 0.00 7.97 51.33 9.64 22.13 10.18 6.68
38.26 0.25 24.00 19.18 10.23 3.58 2.89 0.00 0.01 0.00 98.40 12.00 3.06 0.02 0.15 1.46 1.81 0.88 0.24 0.34 0.00 0.00 0.00 7.96 49.97 11.78 22.60 8.30 7.33
38.64 0.16 23.91 19.39 10.13 2.67 3.40 0.00 0.00 0.01 98.31 12.00 3.08 0.00 0.12 1.47 1.82 0.86 0.18 0.45 0.00 0.00 0.00 7.98 50.34 13.85 23.25 6.17 6.39
56.11 0.00 5.42 11.09 15.60 0.10 7.17 0.10 0.04 4.36 99.99 6.00 2.00 0.00 0.00 0.18 0.48 0.61 0.00 0.38 0.00 0.00 0.31 3.96 29.80 51.90 18.30
Opx 54.73 0.12 8.93 3.06 22.41 0.20 11.11 0.01 0.04 1.33 101.93 6.00 2.00 0.00 0.00 0.27 0.13 0.87 0.01 0.59 0.00 0.00 0.09 3.97 47.22 32.56 15.16 5.06
51.72 0.38 9.65 4.29 22.60 0.17 11.36 0.00 0.17 0.46 100.80 6.00 1.91 0.09 0.00 0.30 0.10 0.90 0.01 0.63 0.00 0.00 0.03 3.97 48.16 33.68 16.38 1.78
51.28 0.35 9.20 4.41 23.32 0.14 12.00 0.00 0.15 0.38 101.23 6.00 1.91 0.09 0.00 0.28 0.10 0.91 0.00 0.65 0.00 0.00 0.03 3.97 48.08 34.77 15.31 1.44
Pl 68.18 0.00 0.37 19.25 0.68 0.00 0.06 0.04 0.03 10.69 0.00 0.00 99.30
49.31 0.00 0.31 32.28 16.05 0.00 0.03 0.00 0.02 2.44 0.00 0.00 100.44
54.12 0.00 28.74 0.66 1.79 1.08 12.30 0.01 0.01 0.12 98.82 6.00 2.10 0.00 0.00 0.96 0.04 0.07 0.04 0.71 0.00 0.00 0.01 3.93 4.16 39.77 55.58 0.48
40.03 0.05 2.73 26.24 23.92 0.09 1.93 0.01 0.01 0.19 0.00 0.00 95.20
54.62 0.26 8.24 6.21 19.06 0.28 9.13 0.00 0.00 3.11 100.91 6.00 1.99 0.01 0.00 0.25 0.27 0.74 0.01 0.50 0.00 0.00 0.22 3.99 69.78 26.51 3.71
54.91 0.20 9.36 6.85 17.50 0.32 9.82 0.02 0.03 2.44 101.45 6.00 1.98 0.02 0.00 0.29 0.29 0.68 0.01 0.53 0.00 0.00 0.17 3.97 54.47 32.33 12.20
55.92 0.17 8.24 6.11 17.55 0.28 8.81 0.00 0.00 2.98 100.46 6.00 2.00 0.01 0.00 0.25 0.26 0.72 0.01 0.49 0.00 0.00 0.21 3.95 66.40 28.10 5.50
54.63 0.15 8.36 6.28 18.50 0.30 8.89 0.02 0.01 3.33 100.50 6.00 1.98 0.00 0.00 0.26 0.27 0.73 0.01 0.50 0.00 0.00 0.23 3.98 68.80 27.70 3.50
Amp 54.26 0.00 29.28 0.87 1.82 1.07 12.25 0.01 0.00 0.15 99.71 6.00 2.09 0.00 0.00 0.97 0.04 0.08 0.04 0.70 0.00 0.00 0.01 3.93 4.19 39.23 55.96 0.62
Zo 49.86 0.00 0.27 31.80 16.57 0.03 0.02 0.00 0.07 2.61 0.00 0.00 101.23
55.80 0.04 5.15 13.00 14.45 0.10 6.41 0.02 0.05 4.92 99.94 6.00 1.99 0.01 0.00 0.16 0.54 0.56 0.00 0.35 0.00 0.00 0.34 3.95 19.70 59.20 21.10
54.09 0.01 25.48 1.17 1.80 0.94 14.72 0.00 0.06 0.13 98.40 6.00 2.07 0.00 0.00 0.87 0.05 0.07 0.03 0.84 0.00 0.00 0.01 3.94 4.14 46.93 48.40 0.53
48.24 1.00 14.46 7.66 12.91 0.13 12.00 0.08 0.00 0.88 97.36 23.00 7.09 0.11 0.74 1.04 1.33 2.03 0.02 2.63 0.01 0.00 0.25 15.25
Phen 38.94 0.05 2.11 25.66 23.94 0.04 2.30 0.02 0.00 0.13 0.00 0.00 93.19
55.60 0.00 3.49 25.09 0.05 0.01 3.03 8.90 0.02 0.04 0.00 0.00 96.23
45.39 1.02 14.88 10.49 12.60 0.11 10.87 0.10 0.00 1.83 97.29 23.00 6.74 0.11 0.56 1.21 1.83 2.00 0.01 2.41 0.02 0.00 0.53 15.42
Qz 55.52 0.00 3.09 25.34 0.05 0.02 3.32 9.20 0.09 0.08 0.00 0.00 96.71
100.36 0.04 0.05 0.00 0.00 0.03 0.00 0.01 0.00 0.00 0.00 0.15 100.64
44.30 0.25 20.53 11.29 10.99 0.48 8.13 0.10 0.00 2.01 98.08 23.00 6.69 0.03 0.41 2.18 2.00 1.78 0.06 1.83 0.02 0.00 0.59 15.59
Ap 0.02 0.00 0.16 0.03 54.14 0.05 0.00 0.00 0.48 0.00 44.10 2.40 101.38
44.50 1.76 14.31 11.30 12.22 0.12 11.12 0.13 0.00 1.83 97.29 23.00 6.59 0.57 0.50 1.27 1.97 1.94 0.02 2.45 0.02 0.00 0.52 15.85
Ttn 29.85 38.93 0.55 0.27 28.71 0.05 0.01 0.00 0.07 0.01 0.00 0.15 98.60
44.24 0.44 21.57 11.88 11.12 0.54 7.17 0.08 0.00 2.14 99.18 23.00 6.57 0.05 0.35 2.48 2.10 1.79 0.07 1.61 0.01 0.00 0.62 15.65
Ilm 0.02 53.27 44.69 0.04 0.09 2.41 0.01 0.00 0.09 0.00 0.00 0.00 100.61
(continued on next page) (continued on next page)
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Table 1 (continued) Mineral
Ab
O Si Ti Fe3+ Fe2+ Al Ca Mn Mg K Cr Na P F Sum of cations An Ab Or
8.00 3.00 0.00 0.00 0.00 0.99 0.02 0.00 0.00 0.01 0.00 0.99 0.00 0.00 5.01 1.55 97.94 0.50
Pl 8.00 3.01 0.00 0.00 0.00 1.00 0.03 0.00 0.00 0.01 0.00 0.91 0.00 0.00 4.96 3.39 96.37 0.24
8.00 2.25 0.00 0.00 0.00 1.74 0.79 0.00 0.00 0.00 0.00 0.22 0.00 0.00 5.00 78.43 21.57 0.00
Zo 8.00 2.26 0.00 0.00 0.00 1.70 0.81 0.00 0.00 0.00 0.00 0.23 0.00 0.00 5.00 77.82 22.18 0.00
24.00 6.09 0.01 0.00 0.35 4.71 3.90 0.01 0.44 0.00 0.00 0.05 0.00 0.00 15.51
Phen 24.00 6.05 0.01 0.00 0.27 4.70 3.99 0.01 0.54 0.01 0.00 0.04 0.00 0.00 15.62
12.00 3.61 0.00 0.19 0.00 1.95 0.00 0.00 0.29 0.72 0.00 0.01 0.00 0.00 6.77
12.00 3.55 0.00 0.17 0.00 1.98 0.00 0.00 0.32 0.75 0.00 0.01 0.00 0.00 6.78
Qz
Ap
24.00 11.96 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.06 12.02
24.00 0.00 0.00 0.00 0.02 0.01 8.71 0.01 0.00 0.00 0.06 0.00 5.60 1.14 15.55
Ttn 24.00 4.76 4.67 0.00 0.07 0.05 4.90 0.01 0.00 0.00 0.01 0.00 0.00 0.08 14.55
Ilm 24.00 0.00 7.96 0.00 7.43 0.01 0.02 0.57 0.00 0.00 0.01 0.00 0.00 0.00 16.00
The mineral abbreviations in this table are as same as those in the text. Mineral compositions were analyzed by JXA-8800R microprobe.
Epidote, Fs = Ferrosilite, Grs = Grossular, Grt = Garnet, Ilm = Ilmentite, Jd = Jadeite, Lws = Lawsonite, Ms. = Muscovite, Omp = Omphacite, Opx = Orthopyroxene, Or = Orthoclase, Phen = Phengite, Pl = Plagioclase, Prp = Pyrope, Qz = Quartz, Rt = Rutile, Sps = Spessartine, Tlc = Talc, Ttn = Titanite, Wo = Wollastonite, WEF = Wollastonite + Enstatite + Ferrosilite and Zo = Zoisite. 2. Geological setting The Tibetan Plateau formed as a result of collision between the Eurasian and Indian plates. From north to south, it comprises the Songpan–Ganzi, Qiangtang, Lhasa, and Himalaya terrains, separated by the Jinsha River, Bangong Co–Nujiang, and Yarlung Zangbo suture zones, respectively (Pan et al., 2012; Zhang and Santosh, 2012; Zhu et al., 2013; Wang et al., 2015). The Bangong Co–Nujiang suture zone lies in central Tibet and separates the Lhasa and Qiangtang terrains (Fig. 1a). It contains numerous ophiolitic blocks and tectonic mélanges and is divided into eastern, central, and western segments (Qiu et al., 2004). The Dong-Co ophiolitic mélange is located in the western segment of the Bangong Co–Nujiang suture zone, and mainly crops out south of the Qushenla area, to the north of Dong-Co lake. The outcrop trends NWW and extends for ~100 km. The ophiolite blocks occur within the Jurassic Namag Kangri Group and have irregular and lenticular shapes (Wang et al., 2015). The Dong-Co ophiolite does not display a complete ophiolitic sequence. Ophiolitic blocks, limestone, sandstone, and volcanic rocks occur within the mélange zone (Fig. 1b). The eclogite occurs as lenses or blocks in the amphibolites (or metamorphosed gabbro) at Shemalagou (Fig. 1b). The dark eclogite blocks trend E–W and crop out intermittently over an area with an east–west extent of ~ 30 km and a width of 0.2–0.5 km. It is in tectonic contact with surrounding rocks including gabbro, ophiolitic metamorphosed mantle peridotite, marine turbidite-facies flysch of the Jurassic Namag Kangri Group, and terrigenous clastic rocks of the Late Jurassic–Early Cretaceous Samut Luo Group. The eclogite blocks are unconformably overlain by Cretaceous Jingzhushan Group molasse (Wang et al., 2015).
Geochemistry, Chinese Academy of Science (GIGCAS), Guangzhou, China. The samples were sawn into chips and ultrasonically cleaned, initially with distilled water containing b5% HNO3 and subsequently with distilled water alone (Chen et al., 2012). Following this, ~6 g of each sample was powdered in an agate ring mill to b 200 mesh and the resulting powders were analyzed for major and trace elements. The detailed analytical procedures for XRF and ICP–MS analysis of major and trace elements are described by Goto and Tatsumi (1996) and Chen et al. (2010), respectively. A preignition procedure was used to determine the loss on ignition (LOI) prior to analyses of major elements. Analytical uncertainties for major elements were b 5% based on analyses of Chinese standard reference materials GSR-1, GSR-2, and GSR-3 (Chen et al., 2010). According to repeat analyses of United States Geological Survey (USGS) standards and Chinese standard reference materials, the precision of rare earth element (REE) and high field strength element (HFSE) analyses is estimated to be 5% (Chen et al., 2010).
4. Petrographic characteristics of the eclogite The eclogite exhibits a fine- to medium-grained texture and massive structure. It consists of garnet (~15%), zoisite (~8%), diopside (~25%), amphibole (~30%), and symplectite (~15%) along with minor omphacite, phengite, rutile, and quartz, and rare ilmenite, apatite, titanite, and zircon as accessory minerals (Figs. 2 and 3; Table 1).
3. Analytical techniques Mineral compositions were obtained using a JXA-8800R microprobe at the Instrumentation Analysis & Research Center, Sun Yat_Sen University, Guangzhou, China, with an accelerating voltage of 15 keV, a beam current of 2 × 10−8 A, and a beam diameter of 1 μm using silicate oxide standards. The mineral compositions were calculated using the Geokit program. The bulk chemical compositions of the samples were also analyzed by X-ray fluorescence spectrometry (XRF) and inductively coupled plasma–mass spectrometry (ICP–MS) at the Guangzhou Institute of
Fig. 4. Compositions of garnets from the retrograde eclogite (after Coleman et al., 1965).
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Fig. 5. Compositional profiles of garnets from the eclogite.
The euhedral–subhedral garnets occur both as coarse-grained porphyroblast and fine-grained matrix. The garnets have end-member components of Alm44–54, Prp23–26, Grs15–24, and Sps2–8 (porphyroblast), and Alm47–51, Prp9–16, Grs21–24, and Sps5–10 (matrix), respectively, and they all fall in the type-C eclogite field (Fig. 4). The cores of the
porphyroblastic garnets have a uniform composition; however, the rims have lower pyrope and grossular contents, and higher spessartine and almandine contents than the cores (Fig. 5a). The porphyroblastic garnets have been partly replaced by symplectitic coronas of amphibole + plagioclase, indicating that the eclogite has undergone decompression-
Fig. 6. X-ray mapping of a matrix garnet from the eclogite. CP is composition image; others are content mapping of each element.
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Y.-L. Dong et al. / Gondwana Research 35 (2016) 115–123 Table 2 Major and trace element compositions of the eclogite and coexisting garnet amphibolites.
Fig. 7. Compositions of omphacites in the eclogite (after Morimoto, 1988).
related retrograde metamorphism (Figs. 2a, 2b and 3b). Pyrope contents decrease, and spessartine contents increase from the core to rim of matrix garnets (Figs. 5b and 6). Most omphacites have been replaced by amphibole and diopside, and minor omphacite relics have only been identified within diopside grains in one retrograde eclogite (sample SML–7). This omphacite contains 2.44–4.92 wt.% Na2O and 27%–59% jadeite component (Figs. 3c, 3d and 7; Table 1). The phengites occur as small crystals with SiO2 contents of 53.22%–56.60%. From an analysis of 12 oxygen atoms, we calculated that the Si cation of phengite varies from 3.42 to 3.61, whereas Mg and Fe cations range from 0.22 to 0.35 and 0.10 to 0.24, respectively. Therefore, they are typical of phengites that formed under HP metamorphic conditions (Grimmer et al., 2003). Diopside and amphibole are common in the eclogite. The diopsides can be divided in two types: one occurs as inclusions within garnet and is rich in Na2O, while the other is porphyroblastic and poor in Na2O, and has exsolutions of potassium feldspar, quartz, albite, and amphibole, suggesting that the primary diopsides were rich in SiO2 and Na2O. Some diopsides have been replaced by secondary amphibole and zoisite, leading to the formation of a rim consisting of amphibole and a symplectite of zoisite + plagioclase (Figs. 2c, 2d, and 3d). Most amphiboles were produced by the breakdown of garnet and clinopyroxene. Compositionally, the amphiboles are calcium-rich and have relatively high TiO2 contents, indicating that they formed under relatively high temperatures (Genshaft and Mironova, 1995; Guo et al., 2012). From the observations described above, we conclude that the studied retrograde eclogite records three stages of metamorphism: the eclogite‐facies peak-metamorphic mineral assemblage is garnet + omphacite + rutile + phengite + quartz; the granulite-facies retrograde assemblage is fine-grained garnet + diopside + plagioclase + quartz; and the amphibolite-facies retrograde assemblage is amphibole + plagioclase + zoisite + quartz. 5. Geochemical characteristics of the eclogite Major and trace elements of five samples of the eclogite and retrograde garnet amphibolites from the Shemalagou area were analyzed at GIGCAS (Table 2). Their major element chemistry is as follows: SiO2: 41.20–48.41 wt.%, TiO2: 0.93–1.80 wt.%, FeOT: 10.75–16.75 wt.%, Al2O3: 14.68–15.89 wt.%, CaO: 11.06–13.19 wt.%, MgO: 7.57–8.06 wt.%, K2O: 0.13–0.53 wt.%, and Na2O: 1.60–2.50 wt.%. Since the rocks have been metamorphosed, we used the immobile elements Zr, Ti, Nb, and Y to classify the rocks and infer the tectonic settings in which they formed (Rameshwar and Hakim, 2006). The samples plot within the sub-alkaline basalt field of the Zr/TiO2–Nb/Y diagram (Fig. 8a), and
Sample
SML-1
SML-2
SML-4
SML-6
SML-7
SiO2 TiO2 FeOT Al2O3 CaO MnO MgO K2O Na2O P2O5 LOI Total Sc Ti V Cr Co Ni Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U LREE/HREE (La/Yb)N δEu δCe ∑REE
41.20 1.80 16.75 15.89 12.26 0.25 7.77 0.17 1.60 0.13 1.54 99.36 55.5 11,405 422 129 45.6 46.0 1.55 112 41.4 79.4 5.39 0.19 22.3 5.10 14.2 2.38 12.8 4.19 1.57 0.52 1.09 7.26 1.63 4.60 0.68 4.39 0.67 2.49 0.40 1.99 0.42 0.23 1.56 0.78 1.00 0.95 66.05
46.52 1.16 12.09 14.78 13.19 0.22 7.92 0.21 2.08 0.09 1.64 99.90 45.0 6692 331 223 43.8 73.0 2.71 150 24.3 37.8 3.26 0.92 19.1 3.17 9.16 1.51 7.95 2.54 0.99 3.34 0.67 4.24 0.94 2.62 0.38 2.42 0.37 1.27 0.22 0.76 0.19 0.06 1.69 0.88 1.03 0.97 40.28
48.02 1.22 12.18 14.68 11.06 0.22 7.63 0.53 2.50 0.10 1.74 99.89 46.2 7522 321 191 46.7 74.4 10.5 132 24.8 55.3 3.82 1.40 64.5 3.42 9.25 1.54 8.17 2.64 0.96 3.39 0.66 4.38 0.96 2.72 0.40 2.51 0.39 1.72 0.25 2.08 0.31 0.10 1.68 0.92 0.99 0.94 41.40
47.41 1.32 13.22 14.75 11.20 0.24 8.06 0.20 2.33 0.11 1.21 100.05 45.6 8117 329 113 49.6 62.1 1.97 130 27.8 57.4 4.82 0.76 18.0 3.56 10.29 1.72 9.10 2.95 1.08 3.78 0.74 4.86 1.08 3.06 0.45 2.87 0.45 1.80 0.31 1.36 0.30 0.10 1.66 0.84 0.99 0.97 45.98
48.41 0.93 10.75 14.99 13.10 0.19 7.57 0.13 2.27 0.09 1.64 100.06 43.2 6185 282 227 42.6 75.5 0.92 98.3 21.0 42.9 3.16 0.42 13.9 2.96 8.16 1.35 7.11 2.29 0.87 2.97 0.58 3.70 0.83 2.29 0.33 2.14 0.32 1.39 0.22 1.37 0.22 0.07 1.73 0.94 1.02 0.95 35.91
SML-7 is eclogite; SML-1, SML-2, SML-4, and SML-6 are garnet amphibolites. Major element, wt%; trace element, ppm (10—6).
also plot within the mid-ocean ridge basalt (MORB) field of the P2O5– TiO2, 2Nb–Zr/4–Y, and Ti/100–Zr–Sr/2 tectonic discrimination diagrams (Fig. 8b–d). The five samples have low total concentrations of rare earth elements (REEs) of 35.91–66.05 ppm. The ratios of light rare earth elements (LREE) to heavy rare earth elements (HREE) are 1.32–1.73, showing no obvious fractionation between LREE and HREE. (La/Yb)N ratios range from 0.55 to 0.94, and there are no obvious Eu or Ce anomalies (δEu = 0.99–1.03; δCe = 0.94–0.97). The rocks have flat chondritenormalized REE patterns that are consistent with those of N-MORB (Fig. 9a), indicating geochemical characteristics similar to those of NMORB. The samples studied here contain 3.16–5.39 ppm Nb, 37.84– 79.37 ppm Zr, and 13.91–22.33 ppm Ba, which are consistent with MORB (1–5 ppm Nb; 15–150 ppm Zr; ≤ 50 ppm Ba). The samples have flat patterns on a spider diagram (Fig. 9b) that are geochemically
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Fig. 8. (a) Zr/TiO2–Nb/Y diagram for the eclogite and amphibolite (from Winchester and Floyd, 1977). (b, c, and d) P2O5–TiO2, 2Nb–Zr/4–Y, and Ti/100–Zr–Sr/2 tectonic discrimination diagrams of the eclogite and amphibolite (from Bass et al., 1973; Meschede, 1986; Pearce and Cann, 1973). In (b), ORB = Ocean Ridge Basalt; OIB = Oceanic Island Basalt; ALB = Alkaline Basalt. In (c), AI + AII = Within-plate alkaline basalt; AII + C = Within-plate tholeiite basalt; B = P-type mid-ocean ridge basalt; D = N-type mid-ocean ridge basalt; C + D = Volcanic arc basalt. In (d), IAB = Island arc tholeiite basalt; OFB = Ocean ridge tholeiite basalt; CAB = Calc-alkaline basalt.
similar to N-MORB, suggesting that the protoliths of the eclogite were derived from a depleted mantle source. In summary, the geochemical characteristics of the eclogite and coexisting garnet amphibolites suggest that their protoliths were NMORB from ancient oceanic crust, possibly indicating that the oceanic crust experienced subduction and HP metamorphism. 6. Metamorphic P–T conditions of the eclogite We quantitatively estimated the metamorphic conditions under which the eclogite formed from P–T (pressure–temperature) pseudosections and mineral isopleth thermobarometry. The pseudosections were calculated with the Perple_X package (Connolly, 2005, version from August 2012) and the internally consistent thermodynamic data set of Holland and Powell (1998, updated 2002). The activity–solution models used in the pseudosection calculation were clino- and orthoamphibole (Diener et al., 2007), clinopyroxene (Green et al., 2007), garnet (White et al., 2007), cordierite (Holland and Powell, 1998), biotite (Tajčmanová et al., 2009), white mica (Coggon and Holland, 2002), and plagioclase (Fuhrman and Lindsley, 1988). The P–T pseudosection for sample SML–7 was calculated in the system MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2
(MnNCKFMASHT) with P–T ranges of 15–30 kbar and 500–900 °C (Fig. 10). The sample's chemical composition, including H2O content, was determined by the bulk rock analysis and described above. The resulting pseudosection shows that garnet and clinopyroxene are stable throughout the P–T space, amphibole is stable at T b 730 °C and P b 28 kbar, and at T N 700 °C and P b 23 kbar muscovite disappears and biotite appears. The observed mineral assemblage of garnet + clinopyroxene + muscovite + quartz + rutileis stable in a wide P–T field of 610–900 °C and 16.5–29.5 kbar (Fig. 10a). The XCa = 0.26–0.28 and XMg = 0.22–0.24 isopleths from the core of a garnet porphyroblast intersect at ca. 600–640 °C and 22–24 kbar (marked by a white-filled circle with a black dashed outline in Fig. 10b), which is close to the stability field of the observed mineral assemblage. In addition, the garnet XMg = 0.22–0.24 isopleths and phengite XSi = 3.4–3.6 isopleths intersect at ca. 600–650 °C and 26–28 kbar (marked by a yellow-filled circle with a red solid outline in Fig. 10b), which is close to the low-T side of the stability field of the eclogite mineral assemblage. Therefore, the modeling results indicate that the Shemalagou eclogite underwent low-T (600–650 °C) and high-P (22–28 kbar) metamorphism under a low geothermal gradient of ca. 8 °C/km, which would correspond to the subduction of oceanic crust.
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Fig. 9. (a) Chondrite-normalized REE patterns (normalizating values are from Boynton, 1984). (b) Primitive-mantle-normalized spider diagram (data sources as in Table 2) (normalizating values are from Hofmann, 1988).
7. Tectonic significance An eclogite generally forms in subduction zones or continental collision zones and can record important information about tectonic processes during the subduction and exhumation of oceanic and continental crust (Zhang et al., 2009; Meng et al., 2013; Sajeev et al., 2013). It has been debated whether oceanic crust subduction and/or continental collision processes took place in the Bangong Co–Nujiang suture zone. For example, Wang et al. (2008) argued that the western and middle sectors of the zone are remnants of a short-lived back-arc basin that partially opened and closed in the Middle Jurassic. However, Kang et al. (2010); Xia et al. (2013); Liu et al. (2015), and Wang et al. (2015) suggested that the zone recorded continental collision and oceanic crust subduction. Our study indicates that the eclogite at Shemalagou formed under HP condition at depths of ≥ 60 km. Therefore, the eclogitehosting amphibolite blocks (and possibly other blocks) in the Bangong Co–Nujiang suture zone have been subjected to HP metamorphism during the subduction of oceanic crust. In addition, our geochemical data indicate that the protoliths of the eclogite at Shemalagou were oceanic rocks with an N-MORB signature, and that the eclogite formed by the HP metamorphism of subducted oceanic crust. In turn, this suggests that the Bangong Co–Nujiang Tethyan Ocean existed prior to the subduction of oceanic crust. Subsequently, when the Tethyan Ocean closed due to subduction, the Lhasa and Qiangtang blocks on either side of the ocean would have collided. Therefore, the discovery of the eclogite reported in this study supports the view that the Bangong Co–Nujiang ophiolitic mélange belt in central Tibet is a paleo-subduction and/or collision zone, not the remnants of a paleo back-arc basin. We conclude that the Bangong Co–Nujiang suture zone is a newly identified HP metamorphic belt that experienced subduction and/or collision, adding
Fig. 10. P–T pseudosection of the eclogite. (b) Red lines are garnet XMg isopleths, blue lines are garnet XCa isopleths, and green lines are phengite XSi isopleths, where XMg = Mg/(Mg + Fe + Ca + Mn), XCa = Ca/(Mg + Fe + Ca + Mn), and XSi = Si cations per formula unit based on 12 oxygens (the original diagram is cited from Fig. 4 of Lei et al., 2014). For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.
to the previously recognized eclogite belts in the Tibetan Plateau at Songduo (Yang et al., 2009) and Longmucuo–Shuanghu (Zhai et al., 2009, 2011a, 2011b). Since the timing of HP metamorphism has not yet been determined in present study, the tectonic processes that resulted in the HP metamorphism in the Bangong Co–Nujiang suture zone require further investigation. 8. Conclusions (1) Eclogite was discovered at the Shemalagou ophiolitic mélange in the western segment of the Bangong Co–Nujiang suture zone, indicating that the mélange experienced HP metamorphism during the closure of the Bangong Co–Nujiang oceanic basin.
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(2) The eclogite formed under moderate-temperature (600–650 °C) and high-pressure (22–28 kbar) conditions with a low geothermal gradient of ca. 8 °C/km, suggesting that they formed during subduction of oceanic crust. (3) The eclogite has similar geochemical characteristics to N-MORB, confirming that the HP metamorphic rocks are an oceaniccrust-type eclogite. Acknowledgments Two reviewers are thanked for their helpful comments. This research was supported by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB03010300), the Major State Basic Research Program of the People's Republic of China, (2015CB452602), the Natural Science Foundation of China (41273039 and 41303028), and project GIGCAS 135 of Ji-Feng Xu (Y234021002). References Bass, M.N., Moberly, R., Rhodes, J.M., 1973. Volcanic rocks cored in the central Pacific. Leg 17. Internal Reporting of Deep Sea Drilling Project 17, 429–503. Boynton, W.V., 1984. Cosmochemistry of the rare earth elements: meteorite studies. In Rare Earth Element Geochemistry 2, 63–114. Chen, J.L., Xu, J.F., Kang, Z.Q., Wang, B.D., 2010. Origin of Cenozoic alkaline potassic volcanic rocks at Konglongxiang, Lhasa terrane, Tibetan plateau: products of partial melting of a mafic lower-crustal source? Chemical Geology 273, 286–299. Chen, J.L., Zhao, W.X., Xu, J.F., Wang, B.D., Kang, Z.Q., 2012. Geochemistry of Miocene trachytes in Bugasi, Lhasa block, Tibetan Plateau: mixing products between mantleand crust-derived melts? Gondwana Research 21, 112–122. Coggon, R., Holland, T.J.B., 2002. Mixing properties of phengitic micas and revised garnetphengite thermobarometers. Journal of Metamorphic Geology 20, 683–696. Coleman, R., Lee, D., Beatty, L.B., Brannock, W.W., 1965. Eclogites and eclogites: their differences and similarities. Geological Society of America Bulletin 76, 483–508. Connolly, J.A.D., 2005. Computation of phase equilibria by linear programming: a tool for geodynamic modeling and its application to subduction zone decarbonation. Earth and Planetary Science Letters 236, 524–541. Diener, J.F.A., Powell, R., White, R.W., Holland, T.J.B., 2007. A new thermodynamic model for clino- and orthoamphiboles in the system Na2O–CaO–FeO–MgO–Al2O3–SiO2– H2O–O. Journal of Metamorphic Geology 25, 631–656. Fuhrman, M.L., Lindsley, D.H., 1988. Ternary-feldspar modeling and thermometry. American Mineralogist 73, 201–215. Genshaft, Y.S., Mironova, N.A., 1995. Magnetopetrological study of formation conditions of the crustal interior of continents: a case study of kimberlite xenoliths from Yakutia, Siberia. Geomagnetism and Aeronomy 31, 210–229. Goto, A., Tatsumi, Y., 1996. Quantitative analyses of rock samples by an X-ray fluorescence spectrometer (II). Rigaku Journal 13, 20–39. Green, E., Holland, T., Powell, R., 2007. An order–disorder model for omphacitic pyroxenes in the system jadeite-diopside-hedenbergite-acmite, with applications to eclogitic rocks. American Mineralogist 92, 1181–1189. Grimmer, J.C., Ratschbacher, L., Williams, M., 2003. When did the ultrahigh-pressure rocks reach the surface? A 207Pb/206Pb zircon, 40Ar/39Ar while mica, Si-in-white mica, single-grain provenance study of Dabie Shan synorogenic foreland sediments. Chemical Geology 197, 87–100. Guo, L., Zhang, H.F., Pan, F.B., 2012. The high temperature metamorphism and its geological significance of late Cretaceous in Southeastern Lhasa terrane: the evidence of rutile exsolution in quartz and zircon U–Pb dating. Earth Science Frontiers 19, 228–239 (in Chinese with English abstract). Hofmann, A.W., 1988. Chemical differentiation of the earth: The relationship between mantle, continental crust and oceanic crust. Earth and Planetary Science Letters 90, 297–314. Holland, T.J.B., Powell, R., 1998. An internally consistent thermodynamic data set for phases of petrological interest. Journal of Metamorphic Geology 16, 309–343. Kang, Z.Q., Xu, J.F., Wang, B.D., Chen, J.L., 2010. Qushenla Formation volcanic rocks in north Lhasa block: products of Bangong Co–Nujiang Tethy's southward subduction. Acta Petrologica Sinica 26, 3106–3116 (in Chinese with English abstract). Lei, H.C., Xiang, H., Zhang, Z.M., Qi, M., Dong, X., Lin, Y.H., 2014. The Paleoproterozoic ultrahigh-temperature granulites of Sulu orogenic belt, and its tectonic significances. Acta Petrologica Sinica 30, 2435–2445 (in Chinese with English abstract). Li, C., Zhai, Q.G., Dong, Y.S., Huang, X.P., 2006. Discovery of the eclogite and it's geological significance in Qiangtang area, central Tibet. Chinese Science Bulletin 51, 1095–1100. Li, Z.L., Yang, J.S., Xu, Z.Q., Li, T.F., Xu, X.Z., Ren, Y.F., Robinson, P.T., 2009a. Geochemistry and Sm–Nd and Rb–Sr isotopic in the Lhasa Terrane, Tibet, and its geological significance. Lithos 109, 240–247.
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Discovery of eclogite in the Bangong Co–Nujiang ophiolitic mélange, central Tibet, and tectonic implications Ya-Lin Dong a,b, Bao-Di Wang c, Wen-Xia Zhao b,⁎, Tian-Nan Yang d, Ji-Feng Xu e,f,⁎ a
School of Earth Science and Geological Engineering, Sun Yat_Sen University, Guangzhou, 510275, PR China Instrumentation Analysis & Research Center, Sun Yat_Sen University, Guangzhou, 510275, PR China Chengdu Institute of Geology and Mineral Resources, Chengdu, 610081, PR China d Institute of Geology, Chinese Academy of Geological Science, Beijing, 100037, PR China e State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, PR China f CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing, PR China b c
a r t i c l e
i n f o
Article history: Received 20 July 2015 Received in revised form 13 March 2016 Accepted 19 March 2016 Available online 30 April 2016 Handling Editor: Z.M. Zhang Keywords: Eclogite Ophiolite Tectonic evolution Bangong Co–Nujiang suture zone Tibetan Plateau
a b s t r a c t An eclogite has been recently identified within ophiolitic mélange in the western segment of the Bangong Co– Nujiang suture zone, at Shemalagou in the Gaize area of central Tibet. The eclogite consists of garnet, omphacite, phengite, rutile, quartz, diopside, and amphibole. The omphacite, which has not been recognized in the suture zone until this study, occurs as rare relics within diopside grains in the eclogite. Phase equilibria modeling shows that the eclogite formed under P–T conditions of 22–28 kbar and 600–650 °C with a low geothermal gradient of ca. 8 °C/km, suggesting that it formed during the subduction of oceanic crust. The protoliths of the eclogite and coexisting garnet amphibolites have geochemical characteristics similar to those of normal mid-ocean ridge basalt (N-MORB), confirming that the eclogites formed from oceanic crust. The presence of high-pressure (HP) eclogite indicates that the ophiolitic mélange in the Bangong Co–Nujiang suture zone underwent oceanic subduction and was subsequently exhumed. We conclude that this ophiolitic belt represents a newly identified HP metamorphic belt in the Tibetan Plateau, adding to the previously recognized Songduo and Longmucuo–Shuanghu eclogite belts. This discovery will result in an improved understanding of the tectonic evolution of the Bangong Co–Nujiang suture zone and the Tibetan Plateau as a whole. © 2016 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.
1. Introduction In recent years, two eclogite belts have been identified on the Tibetan Plateau, namely the Songduo eclogite belt in the Lhasa Block (e.g., Li et al., 2009a, 2009b; Yang et al., 2009; Zeng et al., 2009; Zhang et al., 2014b), and the Longmucuo–Shuanghu eclogite belt in the Qiangtang Block (e.g., Li et al., 2006; Li et al., 2009a, 2009b; Zhang et al., 2006; Zhai et al., 2009; Zhai et al., 2011a, 2011b; Zhu et al., 2013). In addition, outcrops of mafic granulite and garnet amphibolite have been reported in the Bangong Co–Nujiang suture zone (Xia et al., 2013; Wang et al., 2015; Zhang et al., 2015), and at two sites the rocks have been identifiedas “eclogite” (Xia et al., 2013; Zhang et al., 2015), although omphacite was not observed. Therefore, it remains debated whether the Bangong Co–Nujiang suture zone contains high-pressure or ultrahigh-pressure (UHP) metamorphic rocks. To better understand the tectonic evolution of the Bangong Co–Nujiang suture zone, it is ⁎ Corresponding authors at: School of Earth Science and Geological Engineering, Sun Yat_Sen University, Guangzhou, 510275, China. Tel.: +86 20 84115809, + 86 20 85290282. E-mail addresses: [email protected] (W.-X. Zhao), [email protected] (J.-F. Xu).
Fig. 1. (a) Simplified tectonic map in the Tibetan Plateau (modified from Wang et al., 2008). BNSN: Bangong Co–Nujiang Suture Zone. YZSZ: Yarlung Zangbo River Suture Zone. (b) Geological map of ophiolitic mélange in the Dong-Co area (modified from Wang et al., 2015).
http://dx.doi.org/10.1016/j.gr.2016.03.010 1342-937X/© 2016 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.
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Fig. 2. Photomicrographs of the retrograde eclogite. (a and b) Garnet with amphibole + plagioclase symplectites. (c and d) Relics of omphacite within diopside, which has rims of secondary amphibole.
necessary to further investigate the metamorphic history of the mafic granulite and garnet amphibolite in this zone. This study is the first to identify eclogite from the mafic granulite reported by Wang et al. (2015) at Shemalagou (32°19′41″N, 84°44′15″E), north of Dong-Co lake, in the western segment of the Bangong Co–Nujiang belt, indicating that HP metamorphism occurred in the Bangong Co–Nujiang suture
zone. We also discuss the tectonic significance of the eclogite in terms of the evolution of the Bangong Co–Nujiang Tethys. The mineral abbreviations used in this study are as follows: Ab = Albite, Ac = Acmite, Ae = Aegirine, Alm = Almandine, Amp = Amphibole, An = Anorthite, And = Andradite, Ap = Apatite, Bt = Biotite, Chl = Chlorite, Coe = Coesite, Cpx = Clinopyroxene, Di = Diopside, En = Enstatite, Ep =
Fig. 3. Backscattered electron images of the retrograde eclogite. (a) Diopside occurs as inclusions within garnet. (b) Garnets partly replaced by worm-like symplectites. (c and d) Minor omphacite relics within diopside.
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Table 1 Representative compositions of minerals in the studied eclogite. Mineral
Grt
SiO2 TiO2 FeO Al2O3 CaO MnO MgO K2O Cr2O3 Na2O Total O Si Ti Fe3+ Fe2+ Al Ca Mn Mg K Cr Na Sum of cations Alm (WEF) Prp (Jd) Grs (Ae) Sps And
39.68 0.08 23.31 20.12 9.24 1.00 6.26 0.02 0.01 0.01 99.73 12.00 3.06 0.00 0.12 1.38 1.83 0.77 0.07 0.74 0.00 0.00 0.00 7.97 47.11 24.84 19.77 2.23 6.33
Mineral
Di
SiO2 TiO2 FeO Al2O3 CaO MnO MgO K2O Cr2O3 Na2O Total O Si Ti Fe3+ Fe2+ Al Ca Mn Mg K Cr Na Sum of cations Wo En Fs Ac
53.79 0.15 8.87 2.93 22.52 0.21 10.88 0.03 0.02 1.12 100.52 6.00 2.00 0.00 0.00 0.27 0.12 0.88 0.01 0.59 0.00 0.00 0.08 3.95 48.06 32.31 15.30 4.33
Mineral
Ab
SiO2 TiO2 FeO Al2O3 CaO MnO MgO K2O Cr2O3 Na2O P2O5 F Total
67.42 0.01 0.26 18.94 0.33 0.00 0.01 0.09 0.01 11.51 0.00 0.00 98.58
Omp 39.69 0.07 22.83 19.65 9.08 0.99 6.38 0.00 0.06 0.02 98.77 12.00 3.09 0.00 0.14 1.37 1.80 0.76 0.08 0.75 0.00 0.00 0.00 7.99 46.36 25.42 18.82 2.23 7.00
39.67 0.17 24.76 19.50 10.52 0.84 4.61 0.00 0.02 0.05 100.14 12.00 3.08 0.01 0.16 1.45 1.78 0.87 0.06 0.56 0.00 0.00 0.00 7.97 44.64 25.70 19.10 2.01 8.55
38.42 0.22 24.37 19.28 9.84 4.39 2.36 0.00 0.01 0.05 98.94 12.00 3.07 0.01 0.13 1.50 1.81 0.84 0.30 0.31 0.00 0.00 0.00 7.97 51.33 9.64 22.13 10.18 6.68
38.26 0.25 24.00 19.18 10.23 3.58 2.89 0.00 0.01 0.00 98.40 12.00 3.06 0.02 0.15 1.46 1.81 0.88 0.24 0.34 0.00 0.00 0.00 7.96 49.97 11.78 22.60 8.30 7.33
38.64 0.16 23.91 19.39 10.13 2.67 3.40 0.00 0.00 0.01 98.31 12.00 3.08 0.00 0.12 1.47 1.82 0.86 0.18 0.45 0.00 0.00 0.00 7.98 50.34 13.85 23.25 6.17 6.39
56.11 0.00 5.42 11.09 15.60 0.10 7.17 0.10 0.04 4.36 99.99 6.00 2.00 0.00 0.00 0.18 0.48 0.61 0.00 0.38 0.00 0.00 0.31 3.96 29.80 51.90 18.30
Opx 54.73 0.12 8.93 3.06 22.41 0.20 11.11 0.01 0.04 1.33 101.93 6.00 2.00 0.00 0.00 0.27 0.13 0.87 0.01 0.59 0.00 0.00 0.09 3.97 47.22 32.56 15.16 5.06
51.72 0.38 9.65 4.29 22.60 0.17 11.36 0.00 0.17 0.46 100.80 6.00 1.91 0.09 0.00 0.30 0.10 0.90 0.01 0.63 0.00 0.00 0.03 3.97 48.16 33.68 16.38 1.78
51.28 0.35 9.20 4.41 23.32 0.14 12.00 0.00 0.15 0.38 101.23 6.00 1.91 0.09 0.00 0.28 0.10 0.91 0.00 0.65 0.00 0.00 0.03 3.97 48.08 34.77 15.31 1.44
Pl 68.18 0.00 0.37 19.25 0.68 0.00 0.06 0.04 0.03 10.69 0.00 0.00 99.30
49.31 0.00 0.31 32.28 16.05 0.00 0.03 0.00 0.02 2.44 0.00 0.00 100.44
54.12 0.00 28.74 0.66 1.79 1.08 12.30 0.01 0.01 0.12 98.82 6.00 2.10 0.00 0.00 0.96 0.04 0.07 0.04 0.71 0.00 0.00 0.01 3.93 4.16 39.77 55.58 0.48
40.03 0.05 2.73 26.24 23.92 0.09 1.93 0.01 0.01 0.19 0.00 0.00 95.20
54.62 0.26 8.24 6.21 19.06 0.28 9.13 0.00 0.00 3.11 100.91 6.00 1.99 0.01 0.00 0.25 0.27 0.74 0.01 0.50 0.00 0.00 0.22 3.99 69.78 26.51 3.71
54.91 0.20 9.36 6.85 17.50 0.32 9.82 0.02 0.03 2.44 101.45 6.00 1.98 0.02 0.00 0.29 0.29 0.68 0.01 0.53 0.00 0.00 0.17 3.97 54.47 32.33 12.20
55.92 0.17 8.24 6.11 17.55 0.28 8.81 0.00 0.00 2.98 100.46 6.00 2.00 0.01 0.00 0.25 0.26 0.72 0.01 0.49 0.00 0.00 0.21 3.95 66.40 28.10 5.50
54.63 0.15 8.36 6.28 18.50 0.30 8.89 0.02 0.01 3.33 100.50 6.00 1.98 0.00 0.00 0.26 0.27 0.73 0.01 0.50 0.00 0.00 0.23 3.98 68.80 27.70 3.50
Amp 54.26 0.00 29.28 0.87 1.82 1.07 12.25 0.01 0.00 0.15 99.71 6.00 2.09 0.00 0.00 0.97 0.04 0.08 0.04 0.70 0.00 0.00 0.01 3.93 4.19 39.23 55.96 0.62
Zo 49.86 0.00 0.27 31.80 16.57 0.03 0.02 0.00 0.07 2.61 0.00 0.00 101.23
55.80 0.04 5.15 13.00 14.45 0.10 6.41 0.02 0.05 4.92 99.94 6.00 1.99 0.01 0.00 0.16 0.54 0.56 0.00 0.35 0.00 0.00 0.34 3.95 19.70 59.20 21.10
54.09 0.01 25.48 1.17 1.80 0.94 14.72 0.00 0.06 0.13 98.40 6.00 2.07 0.00 0.00 0.87 0.05 0.07 0.03 0.84 0.00 0.00 0.01 3.94 4.14 46.93 48.40 0.53
48.24 1.00 14.46 7.66 12.91 0.13 12.00 0.08 0.00 0.88 97.36 23.00 7.09 0.11 0.74 1.04 1.33 2.03 0.02 2.63 0.01 0.00 0.25 15.25
Phen 38.94 0.05 2.11 25.66 23.94 0.04 2.30 0.02 0.00 0.13 0.00 0.00 93.19
55.60 0.00 3.49 25.09 0.05 0.01 3.03 8.90 0.02 0.04 0.00 0.00 96.23
45.39 1.02 14.88 10.49 12.60 0.11 10.87 0.10 0.00 1.83 97.29 23.00 6.74 0.11 0.56 1.21 1.83 2.00 0.01 2.41 0.02 0.00 0.53 15.42
Qz 55.52 0.00 3.09 25.34 0.05 0.02 3.32 9.20 0.09 0.08 0.00 0.00 96.71
100.36 0.04 0.05 0.00 0.00 0.03 0.00 0.01 0.00 0.00 0.00 0.15 100.64
44.30 0.25 20.53 11.29 10.99 0.48 8.13 0.10 0.00 2.01 98.08 23.00 6.69 0.03 0.41 2.18 2.00 1.78 0.06 1.83 0.02 0.00 0.59 15.59
Ap 0.02 0.00 0.16 0.03 54.14 0.05 0.00 0.00 0.48 0.00 44.10 2.40 101.38
44.50 1.76 14.31 11.30 12.22 0.12 11.12 0.13 0.00 1.83 97.29 23.00 6.59 0.57 0.50 1.27 1.97 1.94 0.02 2.45 0.02 0.00 0.52 15.85
Ttn 29.85 38.93 0.55 0.27 28.71 0.05 0.01 0.00 0.07 0.01 0.00 0.15 98.60
44.24 0.44 21.57 11.88 11.12 0.54 7.17 0.08 0.00 2.14 99.18 23.00 6.57 0.05 0.35 2.48 2.10 1.79 0.07 1.61 0.01 0.00 0.62 15.65
Ilm 0.02 53.27 44.69 0.04 0.09 2.41 0.01 0.00 0.09 0.00 0.00 0.00 100.61
(continued on next page) (continued on next page)
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Table 1 (continued) Mineral
Ab
O Si Ti Fe3+ Fe2+ Al Ca Mn Mg K Cr Na P F Sum of cations An Ab Or
8.00 3.00 0.00 0.00 0.00 0.99 0.02 0.00 0.00 0.01 0.00 0.99 0.00 0.00 5.01 1.55 97.94 0.50
Pl 8.00 3.01 0.00 0.00 0.00 1.00 0.03 0.00 0.00 0.01 0.00 0.91 0.00 0.00 4.96 3.39 96.37 0.24
8.00 2.25 0.00 0.00 0.00 1.74 0.79 0.00 0.00 0.00 0.00 0.22 0.00 0.00 5.00 78.43 21.57 0.00
Zo 8.00 2.26 0.00 0.00 0.00 1.70 0.81 0.00 0.00 0.00 0.00 0.23 0.00 0.00 5.00 77.82 22.18 0.00
24.00 6.09 0.01 0.00 0.35 4.71 3.90 0.01 0.44 0.00 0.00 0.05 0.00 0.00 15.51
Phen 24.00 6.05 0.01 0.00 0.27 4.70 3.99 0.01 0.54 0.01 0.00 0.04 0.00 0.00 15.62
12.00 3.61 0.00 0.19 0.00 1.95 0.00 0.00 0.29 0.72 0.00 0.01 0.00 0.00 6.77
12.00 3.55 0.00 0.17 0.00 1.98 0.00 0.00 0.32 0.75 0.00 0.01 0.00 0.00 6.78
Qz
Ap
24.00 11.96 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.06 12.02
24.00 0.00 0.00 0.00 0.02 0.01 8.71 0.01 0.00 0.00 0.06 0.00 5.60 1.14 15.55
Ttn 24.00 4.76 4.67 0.00 0.07 0.05 4.90 0.01 0.00 0.00 0.01 0.00 0.00 0.08 14.55
Ilm 24.00 0.00 7.96 0.00 7.43 0.01 0.02 0.57 0.00 0.00 0.01 0.00 0.00 0.00 16.00
The mineral abbreviations in this table are as same as those in the text. Mineral compositions were analyzed by JXA-8800R microprobe.
Epidote, Fs = Ferrosilite, Grs = Grossular, Grt = Garnet, Ilm = Ilmentite, Jd = Jadeite, Lws = Lawsonite, Ms. = Muscovite, Omp = Omphacite, Opx = Orthopyroxene, Or = Orthoclase, Phen = Phengite, Pl = Plagioclase, Prp = Pyrope, Qz = Quartz, Rt = Rutile, Sps = Spessartine, Tlc = Talc, Ttn = Titanite, Wo = Wollastonite, WEF = Wollastonite + Enstatite + Ferrosilite and Zo = Zoisite. 2. Geological setting The Tibetan Plateau formed as a result of collision between the Eurasian and Indian plates. From north to south, it comprises the Songpan–Ganzi, Qiangtang, Lhasa, and Himalaya terrains, separated by the Jinsha River, Bangong Co–Nujiang, and Yarlung Zangbo suture zones, respectively (Pan et al., 2012; Zhang and Santosh, 2012; Zhu et al., 2013; Wang et al., 2015). The Bangong Co–Nujiang suture zone lies in central Tibet and separates the Lhasa and Qiangtang terrains (Fig. 1a). It contains numerous ophiolitic blocks and tectonic mélanges and is divided into eastern, central, and western segments (Qiu et al., 2004). The Dong-Co ophiolitic mélange is located in the western segment of the Bangong Co–Nujiang suture zone, and mainly crops out south of the Qushenla area, to the north of Dong-Co lake. The outcrop trends NWW and extends for ~100 km. The ophiolite blocks occur within the Jurassic Namag Kangri Group and have irregular and lenticular shapes (Wang et al., 2015). The Dong-Co ophiolite does not display a complete ophiolitic sequence. Ophiolitic blocks, limestone, sandstone, and volcanic rocks occur within the mélange zone (Fig. 1b). The eclogite occurs as lenses or blocks in the amphibolites (or metamorphosed gabbro) at Shemalagou (Fig. 1b). The dark eclogite blocks trend E–W and crop out intermittently over an area with an east–west extent of ~ 30 km and a width of 0.2–0.5 km. It is in tectonic contact with surrounding rocks including gabbro, ophiolitic metamorphosed mantle peridotite, marine turbidite-facies flysch of the Jurassic Namag Kangri Group, and terrigenous clastic rocks of the Late Jurassic–Early Cretaceous Samut Luo Group. The eclogite blocks are unconformably overlain by Cretaceous Jingzhushan Group molasse (Wang et al., 2015).
Geochemistry, Chinese Academy of Science (GIGCAS), Guangzhou, China. The samples were sawn into chips and ultrasonically cleaned, initially with distilled water containing b5% HNO3 and subsequently with distilled water alone (Chen et al., 2012). Following this, ~6 g of each sample was powdered in an agate ring mill to b 200 mesh and the resulting powders were analyzed for major and trace elements. The detailed analytical procedures for XRF and ICP–MS analysis of major and trace elements are described by Goto and Tatsumi (1996) and Chen et al. (2010), respectively. A preignition procedure was used to determine the loss on ignition (LOI) prior to analyses of major elements. Analytical uncertainties for major elements were b 5% based on analyses of Chinese standard reference materials GSR-1, GSR-2, and GSR-3 (Chen et al., 2010). According to repeat analyses of United States Geological Survey (USGS) standards and Chinese standard reference materials, the precision of rare earth element (REE) and high field strength element (HFSE) analyses is estimated to be 5% (Chen et al., 2010).
4. Petrographic characteristics of the eclogite The eclogite exhibits a fine- to medium-grained texture and massive structure. It consists of garnet (~15%), zoisite (~8%), diopside (~25%), amphibole (~30%), and symplectite (~15%) along with minor omphacite, phengite, rutile, and quartz, and rare ilmenite, apatite, titanite, and zircon as accessory minerals (Figs. 2 and 3; Table 1).
3. Analytical techniques Mineral compositions were obtained using a JXA-8800R microprobe at the Instrumentation Analysis & Research Center, Sun Yat_Sen University, Guangzhou, China, with an accelerating voltage of 15 keV, a beam current of 2 × 10−8 A, and a beam diameter of 1 μm using silicate oxide standards. The mineral compositions were calculated using the Geokit program. The bulk chemical compositions of the samples were also analyzed by X-ray fluorescence spectrometry (XRF) and inductively coupled plasma–mass spectrometry (ICP–MS) at the Guangzhou Institute of
Fig. 4. Compositions of garnets from the retrograde eclogite (after Coleman et al., 1965).
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Fig. 5. Compositional profiles of garnets from the eclogite.
The euhedral–subhedral garnets occur both as coarse-grained porphyroblast and fine-grained matrix. The garnets have end-member components of Alm44–54, Prp23–26, Grs15–24, and Sps2–8 (porphyroblast), and Alm47–51, Prp9–16, Grs21–24, and Sps5–10 (matrix), respectively, and they all fall in the type-C eclogite field (Fig. 4). The cores of the
porphyroblastic garnets have a uniform composition; however, the rims have lower pyrope and grossular contents, and higher spessartine and almandine contents than the cores (Fig. 5a). The porphyroblastic garnets have been partly replaced by symplectitic coronas of amphibole + plagioclase, indicating that the eclogite has undergone decompression-
Fig. 6. X-ray mapping of a matrix garnet from the eclogite. CP is composition image; others are content mapping of each element.
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Y.-L. Dong et al. / Gondwana Research 35 (2016) 115–123 Table 2 Major and trace element compositions of the eclogite and coexisting garnet amphibolites.
Fig. 7. Compositions of omphacites in the eclogite (after Morimoto, 1988).
related retrograde metamorphism (Figs. 2a, 2b and 3b). Pyrope contents decrease, and spessartine contents increase from the core to rim of matrix garnets (Figs. 5b and 6). Most omphacites have been replaced by amphibole and diopside, and minor omphacite relics have only been identified within diopside grains in one retrograde eclogite (sample SML–7). This omphacite contains 2.44–4.92 wt.% Na2O and 27%–59% jadeite component (Figs. 3c, 3d and 7; Table 1). The phengites occur as small crystals with SiO2 contents of 53.22%–56.60%. From an analysis of 12 oxygen atoms, we calculated that the Si cation of phengite varies from 3.42 to 3.61, whereas Mg and Fe cations range from 0.22 to 0.35 and 0.10 to 0.24, respectively. Therefore, they are typical of phengites that formed under HP metamorphic conditions (Grimmer et al., 2003). Diopside and amphibole are common in the eclogite. The diopsides can be divided in two types: one occurs as inclusions within garnet and is rich in Na2O, while the other is porphyroblastic and poor in Na2O, and has exsolutions of potassium feldspar, quartz, albite, and amphibole, suggesting that the primary diopsides were rich in SiO2 and Na2O. Some diopsides have been replaced by secondary amphibole and zoisite, leading to the formation of a rim consisting of amphibole and a symplectite of zoisite + plagioclase (Figs. 2c, 2d, and 3d). Most amphiboles were produced by the breakdown of garnet and clinopyroxene. Compositionally, the amphiboles are calcium-rich and have relatively high TiO2 contents, indicating that they formed under relatively high temperatures (Genshaft and Mironova, 1995; Guo et al., 2012). From the observations described above, we conclude that the studied retrograde eclogite records three stages of metamorphism: the eclogite‐facies peak-metamorphic mineral assemblage is garnet + omphacite + rutile + phengite + quartz; the granulite-facies retrograde assemblage is fine-grained garnet + diopside + plagioclase + quartz; and the amphibolite-facies retrograde assemblage is amphibole + plagioclase + zoisite + quartz. 5. Geochemical characteristics of the eclogite Major and trace elements of five samples of the eclogite and retrograde garnet amphibolites from the Shemalagou area were analyzed at GIGCAS (Table 2). Their major element chemistry is as follows: SiO2: 41.20–48.41 wt.%, TiO2: 0.93–1.80 wt.%, FeOT: 10.75–16.75 wt.%, Al2O3: 14.68–15.89 wt.%, CaO: 11.06–13.19 wt.%, MgO: 7.57–8.06 wt.%, K2O: 0.13–0.53 wt.%, and Na2O: 1.60–2.50 wt.%. Since the rocks have been metamorphosed, we used the immobile elements Zr, Ti, Nb, and Y to classify the rocks and infer the tectonic settings in which they formed (Rameshwar and Hakim, 2006). The samples plot within the sub-alkaline basalt field of the Zr/TiO2–Nb/Y diagram (Fig. 8a), and
Sample
SML-1
SML-2
SML-4
SML-6
SML-7
SiO2 TiO2 FeOT Al2O3 CaO MnO MgO K2O Na2O P2O5 LOI Total Sc Ti V Cr Co Ni Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U LREE/HREE (La/Yb)N δEu δCe ∑REE
41.20 1.80 16.75 15.89 12.26 0.25 7.77 0.17 1.60 0.13 1.54 99.36 55.5 11,405 422 129 45.6 46.0 1.55 112 41.4 79.4 5.39 0.19 22.3 5.10 14.2 2.38 12.8 4.19 1.57 0.52 1.09 7.26 1.63 4.60 0.68 4.39 0.67 2.49 0.40 1.99 0.42 0.23 1.56 0.78 1.00 0.95 66.05
46.52 1.16 12.09 14.78 13.19 0.22 7.92 0.21 2.08 0.09 1.64 99.90 45.0 6692 331 223 43.8 73.0 2.71 150 24.3 37.8 3.26 0.92 19.1 3.17 9.16 1.51 7.95 2.54 0.99 3.34 0.67 4.24 0.94 2.62 0.38 2.42 0.37 1.27 0.22 0.76 0.19 0.06 1.69 0.88 1.03 0.97 40.28
48.02 1.22 12.18 14.68 11.06 0.22 7.63 0.53 2.50 0.10 1.74 99.89 46.2 7522 321 191 46.7 74.4 10.5 132 24.8 55.3 3.82 1.40 64.5 3.42 9.25 1.54 8.17 2.64 0.96 3.39 0.66 4.38 0.96 2.72 0.40 2.51 0.39 1.72 0.25 2.08 0.31 0.10 1.68 0.92 0.99 0.94 41.40
47.41 1.32 13.22 14.75 11.20 0.24 8.06 0.20 2.33 0.11 1.21 100.05 45.6 8117 329 113 49.6 62.1 1.97 130 27.8 57.4 4.82 0.76 18.0 3.56 10.29 1.72 9.10 2.95 1.08 3.78 0.74 4.86 1.08 3.06 0.45 2.87 0.45 1.80 0.31 1.36 0.30 0.10 1.66 0.84 0.99 0.97 45.98
48.41 0.93 10.75 14.99 13.10 0.19 7.57 0.13 2.27 0.09 1.64 100.06 43.2 6185 282 227 42.6 75.5 0.92 98.3 21.0 42.9 3.16 0.42 13.9 2.96 8.16 1.35 7.11 2.29 0.87 2.97 0.58 3.70 0.83 2.29 0.33 2.14 0.32 1.39 0.22 1.37 0.22 0.07 1.73 0.94 1.02 0.95 35.91
SML-7 is eclogite; SML-1, SML-2, SML-4, and SML-6 are garnet amphibolites. Major element, wt%; trace element, ppm (10—6).
also plot within the mid-ocean ridge basalt (MORB) field of the P2O5– TiO2, 2Nb–Zr/4–Y, and Ti/100–Zr–Sr/2 tectonic discrimination diagrams (Fig. 8b–d). The five samples have low total concentrations of rare earth elements (REEs) of 35.91–66.05 ppm. The ratios of light rare earth elements (LREE) to heavy rare earth elements (HREE) are 1.32–1.73, showing no obvious fractionation between LREE and HREE. (La/Yb)N ratios range from 0.55 to 0.94, and there are no obvious Eu or Ce anomalies (δEu = 0.99–1.03; δCe = 0.94–0.97). The rocks have flat chondritenormalized REE patterns that are consistent with those of N-MORB (Fig. 9a), indicating geochemical characteristics similar to those of NMORB. The samples studied here contain 3.16–5.39 ppm Nb, 37.84– 79.37 ppm Zr, and 13.91–22.33 ppm Ba, which are consistent with MORB (1–5 ppm Nb; 15–150 ppm Zr; ≤ 50 ppm Ba). The samples have flat patterns on a spider diagram (Fig. 9b) that are geochemically
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Fig. 8. (a) Zr/TiO2–Nb/Y diagram for the eclogite and amphibolite (from Winchester and Floyd, 1977). (b, c, and d) P2O5–TiO2, 2Nb–Zr/4–Y, and Ti/100–Zr–Sr/2 tectonic discrimination diagrams of the eclogite and amphibolite (from Bass et al., 1973; Meschede, 1986; Pearce and Cann, 1973). In (b), ORB = Ocean Ridge Basalt; OIB = Oceanic Island Basalt; ALB = Alkaline Basalt. In (c), AI + AII = Within-plate alkaline basalt; AII + C = Within-plate tholeiite basalt; B = P-type mid-ocean ridge basalt; D = N-type mid-ocean ridge basalt; C + D = Volcanic arc basalt. In (d), IAB = Island arc tholeiite basalt; OFB = Ocean ridge tholeiite basalt; CAB = Calc-alkaline basalt.
similar to N-MORB, suggesting that the protoliths of the eclogite were derived from a depleted mantle source. In summary, the geochemical characteristics of the eclogite and coexisting garnet amphibolites suggest that their protoliths were NMORB from ancient oceanic crust, possibly indicating that the oceanic crust experienced subduction and HP metamorphism. 6. Metamorphic P–T conditions of the eclogite We quantitatively estimated the metamorphic conditions under which the eclogite formed from P–T (pressure–temperature) pseudosections and mineral isopleth thermobarometry. The pseudosections were calculated with the Perple_X package (Connolly, 2005, version from August 2012) and the internally consistent thermodynamic data set of Holland and Powell (1998, updated 2002). The activity–solution models used in the pseudosection calculation were clino- and orthoamphibole (Diener et al., 2007), clinopyroxene (Green et al., 2007), garnet (White et al., 2007), cordierite (Holland and Powell, 1998), biotite (Tajčmanová et al., 2009), white mica (Coggon and Holland, 2002), and plagioclase (Fuhrman and Lindsley, 1988). The P–T pseudosection for sample SML–7 was calculated in the system MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2
(MnNCKFMASHT) with P–T ranges of 15–30 kbar and 500–900 °C (Fig. 10). The sample's chemical composition, including H2O content, was determined by the bulk rock analysis and described above. The resulting pseudosection shows that garnet and clinopyroxene are stable throughout the P–T space, amphibole is stable at T b 730 °C and P b 28 kbar, and at T N 700 °C and P b 23 kbar muscovite disappears and biotite appears. The observed mineral assemblage of garnet + clinopyroxene + muscovite + quartz + rutileis stable in a wide P–T field of 610–900 °C and 16.5–29.5 kbar (Fig. 10a). The XCa = 0.26–0.28 and XMg = 0.22–0.24 isopleths from the core of a garnet porphyroblast intersect at ca. 600–640 °C and 22–24 kbar (marked by a white-filled circle with a black dashed outline in Fig. 10b), which is close to the stability field of the observed mineral assemblage. In addition, the garnet XMg = 0.22–0.24 isopleths and phengite XSi = 3.4–3.6 isopleths intersect at ca. 600–650 °C and 26–28 kbar (marked by a yellow-filled circle with a red solid outline in Fig. 10b), which is close to the low-T side of the stability field of the eclogite mineral assemblage. Therefore, the modeling results indicate that the Shemalagou eclogite underwent low-T (600–650 °C) and high-P (22–28 kbar) metamorphism under a low geothermal gradient of ca. 8 °C/km, which would correspond to the subduction of oceanic crust.
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Fig. 9. (a) Chondrite-normalized REE patterns (normalizating values are from Boynton, 1984). (b) Primitive-mantle-normalized spider diagram (data sources as in Table 2) (normalizating values are from Hofmann, 1988).
7. Tectonic significance An eclogite generally forms in subduction zones or continental collision zones and can record important information about tectonic processes during the subduction and exhumation of oceanic and continental crust (Zhang et al., 2009; Meng et al., 2013; Sajeev et al., 2013). It has been debated whether oceanic crust subduction and/or continental collision processes took place in the Bangong Co–Nujiang suture zone. For example, Wang et al. (2008) argued that the western and middle sectors of the zone are remnants of a short-lived back-arc basin that partially opened and closed in the Middle Jurassic. However, Kang et al. (2010); Xia et al. (2013); Liu et al. (2015), and Wang et al. (2015) suggested that the zone recorded continental collision and oceanic crust subduction. Our study indicates that the eclogite at Shemalagou formed under HP condition at depths of ≥ 60 km. Therefore, the eclogitehosting amphibolite blocks (and possibly other blocks) in the Bangong Co–Nujiang suture zone have been subjected to HP metamorphism during the subduction of oceanic crust. In addition, our geochemical data indicate that the protoliths of the eclogite at Shemalagou were oceanic rocks with an N-MORB signature, and that the eclogite formed by the HP metamorphism of subducted oceanic crust. In turn, this suggests that the Bangong Co–Nujiang Tethyan Ocean existed prior to the subduction of oceanic crust. Subsequently, when the Tethyan Ocean closed due to subduction, the Lhasa and Qiangtang blocks on either side of the ocean would have collided. Therefore, the discovery of the eclogite reported in this study supports the view that the Bangong Co–Nujiang ophiolitic mélange belt in central Tibet is a paleo-subduction and/or collision zone, not the remnants of a paleo back-arc basin. We conclude that the Bangong Co–Nujiang suture zone is a newly identified HP metamorphic belt that experienced subduction and/or collision, adding
Fig. 10. P–T pseudosection of the eclogite. (b) Red lines are garnet XMg isopleths, blue lines are garnet XCa isopleths, and green lines are phengite XSi isopleths, where XMg = Mg/(Mg + Fe + Ca + Mn), XCa = Ca/(Mg + Fe + Ca + Mn), and XSi = Si cations per formula unit based on 12 oxygens (the original diagram is cited from Fig. 4 of Lei et al., 2014). For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.
to the previously recognized eclogite belts in the Tibetan Plateau at Songduo (Yang et al., 2009) and Longmucuo–Shuanghu (Zhai et al., 2009, 2011a, 2011b). Since the timing of HP metamorphism has not yet been determined in present study, the tectonic processes that resulted in the HP metamorphism in the Bangong Co–Nujiang suture zone require further investigation. 8. Conclusions (1) Eclogite was discovered at the Shemalagou ophiolitic mélange in the western segment of the Bangong Co–Nujiang suture zone, indicating that the mélange experienced HP metamorphism during the closure of the Bangong Co–Nujiang oceanic basin.
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