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Volume 30 Issue 6
Dec.  2019
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Songjie Wang, Xu-Ping Li, Wenyong Duan, Fanmei Kong, Zeli Wang. Record of Early-Stage Rodingitization from the Purang Ophi-olite Complex, Western Tibet. Journal of Earth Science, 2019, 30(6): 1108-1124. doi: 10.1007/s12583-019-1244-7
Citation: Songjie Wang, Xu-Ping Li, Wenyong Duan, Fanmei Kong, Zeli Wang. Record of Early-Stage Rodingitization from the Purang Ophi-olite Complex, Western Tibet. Journal of Earth Science, 2019, 30(6): 1108-1124. doi: 10.1007/s12583-019-1244-7

Record of Early-Stage Rodingitization from the Purang Ophi-olite Complex, Western Tibet

doi: 10.1007/s12583-019-1244-7
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  • Rodingitization, commonly coupled with serpentinization of ultramafic rocks, bears significant information for fluid-rock interactions and element transfer from sea-floor to subduction zone envi-ronments. Numerous outcrops of rodingites are exposed along the Yarlung Zangbo suture zone (YZSZ) of southern Tibet, providing us an excellent opportunity to probe the petrogenetic processes, and unravel their implications for regional tectonic evolution. Several studies have been performed on rodingites from the eastern to central portions of the YZSZ, whereas limited work has ever been conducted on rodingitized rocks from the western segment of the YZSZ, precluding a comprehensive understanding of this lithological type. In this paper, we present detailed studies of petrology, mineral, whole-rock geochemistry and phase equilibrium modeling on a suite of newly recognized rodingites within the Purang ophiolite massif in the southwestern part of the YZSZ. The rodingites have a major metasomatic mineral association of chlorite, clinozoisite, amphibole and minor amounts of plagioclase, representing products of an early-stage rodingitization. They generally present compositions of low SiO2 (48.89 wt.%-53.57 wt.%), Fe2O3T (3.77 wt.%-5.56 wt.%), Na2O (1.31 wt.%-1.93 wt.%), Al2O3 (4.78 wt.%-8.84 wt.%), moderate CaO (9.69 wt.%-11.23 wt.%), and high MgO (24.11 wt.%-26.08 wt.%) concentrations with extremely high Mg# values[Mg#=100×Mg/(Mg+Fe2+) molar] of 89-92. Bulk-rock recalculation reveals that the rodingites have a protolith of mantle-derived olivine gabbro or gabbronorite. They have low rare earth element compositions (∑REE=2.4 ppm-6.5 ppm) and are characterized by flat LREE and slightly enriched HREE patterns with positive Eu anomalies; they also exhibit positive anomalies in Sr, U and Pb and negative anomalies in high-field strength elements, including Nb, P and Ti, suggesting for a subduction-zone imprinting. Phase equilibrium modeling shows that the rodingitization did take place at P < 2 kbar and T=~350-400℃, consistent with low greenschist facies conditions. Taking into account of all these petrological and geochemical features, we propose that the rodingites record evidence of early-stage fluid-rock interactions between olivine gabbroic rocks and Ca-rich fluids, which may have derived from weakly serpentinized ultramafic country rocks. Although this process may initially have occurred in a mid-ocean ridge setting, an obvious overprinting by supra-subduction zone fluids in a fore-arc environment is recognized.
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Record of Early-Stage Rodingitization from the Purang Ophi-olite Complex, Western Tibet

doi: 10.1007/s12583-019-1244-7
    Corresponding author: Xu-Ping Li

Abstract: Rodingitization, commonly coupled with serpentinization of ultramafic rocks, bears significant information for fluid-rock interactions and element transfer from sea-floor to subduction zone envi-ronments. Numerous outcrops of rodingites are exposed along the Yarlung Zangbo suture zone (YZSZ) of southern Tibet, providing us an excellent opportunity to probe the petrogenetic processes, and unravel their implications for regional tectonic evolution. Several studies have been performed on rodingites from the eastern to central portions of the YZSZ, whereas limited work has ever been conducted on rodingitized rocks from the western segment of the YZSZ, precluding a comprehensive understanding of this lithological type. In this paper, we present detailed studies of petrology, mineral, whole-rock geochemistry and phase equilibrium modeling on a suite of newly recognized rodingites within the Purang ophiolite massif in the southwestern part of the YZSZ. The rodingites have a major metasomatic mineral association of chlorite, clinozoisite, amphibole and minor amounts of plagioclase, representing products of an early-stage rodingitization. They generally present compositions of low SiO2 (48.89 wt.%-53.57 wt.%), Fe2O3T (3.77 wt.%-5.56 wt.%), Na2O (1.31 wt.%-1.93 wt.%), Al2O3 (4.78 wt.%-8.84 wt.%), moderate CaO (9.69 wt.%-11.23 wt.%), and high MgO (24.11 wt.%-26.08 wt.%) concentrations with extremely high Mg# values[Mg#=100×Mg/(Mg+Fe2+) molar] of 89-92. Bulk-rock recalculation reveals that the rodingites have a protolith of mantle-derived olivine gabbro or gabbronorite. They have low rare earth element compositions (∑REE=2.4 ppm-6.5 ppm) and are characterized by flat LREE and slightly enriched HREE patterns with positive Eu anomalies; they also exhibit positive anomalies in Sr, U and Pb and negative anomalies in high-field strength elements, including Nb, P and Ti, suggesting for a subduction-zone imprinting. Phase equilibrium modeling shows that the rodingitization did take place at P < 2 kbar and T=~350-400℃, consistent with low greenschist facies conditions. Taking into account of all these petrological and geochemical features, we propose that the rodingites record evidence of early-stage fluid-rock interactions between olivine gabbroic rocks and Ca-rich fluids, which may have derived from weakly serpentinized ultramafic country rocks. Although this process may initially have occurred in a mid-ocean ridge setting, an obvious overprinting by supra-subduction zone fluids in a fore-arc environment is recognized.

Songjie Wang, Xu-Ping Li, Wenyong Duan, Fanmei Kong, Zeli Wang. Record of Early-Stage Rodingitization from the Purang Ophi-olite Complex, Western Tibet. Journal of Earth Science, 2019, 30(6): 1108-1124. doi: 10.1007/s12583-019-1244-7
Citation: Songjie Wang, Xu-Ping Li, Wenyong Duan, Fanmei Kong, Zeli Wang. Record of Early-Stage Rodingitization from the Purang Ophi-olite Complex, Western Tibet. Journal of Earth Science, 2019, 30(6): 1108-1124. doi: 10.1007/s12583-019-1244-7
  • The Tibetan Plateau is the eastern portion of the Alpine-Himalaya Tethyan Orogen, which constitutes several amalgamated terranes separated by a series of suture zones, namely, from north to south, the A'nemaqin-Kunlun, Jinshajiang, Bangong-Nujiang and Yarlung Zangbo (Luo et al., 2019; Zhang et al., 2019; Wu et al., 2018; Pan et al., 2012; Yin and Harrison, 2000; Dewey et al., 1988). The YZSZ mainly consists of tremendous Neo-Tethyan oceanic remnants and represents the southernmost and youngest suture of the Tibetan Plateau (Fig. 1a; Cheng et al., 2018; Liu et al., 2015; Dai et al., 2012, 2011; Bézard et al., 2011; Tapponnier et al., 1981). Based on spatial distribution features, the YZSZ can be divided into the eastern (Motuo-Zedang), central (Angrang-Renbu) and western (Saga to the boundary of China against India) segments (Pan et al., 1997). Furthermore, the western segment can be subdivided into two sub-belts separated by a 60-km-wide Zhongba-Zhada microcontinent, i.e., the northern Dajiweng-Saga and the southern Daba-Xiugugabu ophiolite sub-belts (e.g., Liu et al., 2015). It is generally considered that ophiolites in the YZSZ were initially emplaced onto the Great-Indian basement and then back thrust northward on the Lhasa Block; finally they were dismembered by strike slip and E-W extension (Bédard et al., 2009; Allégre et al., 1984; Nicolas et al., 1981; Tapponnier et al., 1981). Many geochronological studies have revealed that the YZSZ ophiolites were emplaced from the Mid-Jurassic to Early Cretaceous, with most ages clustering at ca. 130–120 Ma (see Dai et al., 2012 and references therein).

    The Purang ophiolite massif is located at ~40 km to the north of the Purang County, which forms a thrust sheet of mantle peridotites with an area of > 650 km2 (Xiong et al., 2019; Yang et al., 2011; Nicolas et al., 1981). It tectonically belongs to the southern Zhada-Xiugugabu ophiolite sub-belt of the western YZSZ (Fig. 1a). The Purang ophiolite complex is dominantly composed of serpentinized harzburgite, with a small amount of lherzolite, chromite-bearing dunite and olivine pyroxenite; all these units are surrounded by dark red or greyish-green siliceous rocks, intermediate-mafic lavas and marbles with a fault contact (Li et al., 2015; Yang et al., 2011). Lherzolite, dunite and olivine pyroxenite mostly occur as irregular blocks or bands within the harzburgite, some of which have clear boundaries with the host (Figs. 2a, 2b; also see Figs. 2a, 2b in Li et al., 2015). A mélange of serpentinized harzburgite that tectonically overlies Upper Triassic–Cretaceous clastic rocks occurs along the northern margin of the Purang massif. At the southeastern part of the massif diabase and gabbro dykes cutting through variably serpentinized harzburgite and lherzolite were identified. U-Pb dating on magmatic zircons from the diabase and gabbro dykes yields ages varying from 129.3±1.9 to 119.4±5.2 Ma, which were interpreted to represent the emplacement time of the ophiolites in the western segment of the YZSZ (Chen et al., 2016; Liu et al., 2011; Li J F et al., 2008).

    Rodingite occurs as lens, veins or irregular pods within serpentinized harzburgite along fractures in the southeastern margin of the Purang ophiolite massif (Fig. 2). It is fine-grained and light green in color with sizes ranging from centimeters to decimeters across; abundant cracks can be observed in the rodingite (Figs. 2c2f). Seven rodingite samples (11PL54, 55, 56, 57, 58, 59 and 62) were collected for the petrological, geochemical and phase equilibrium modeling study. In addition, whole-rock major and trace element compositions of nine gabbro samples in the Purang massif from Chen et al. (2016) were incorporated into this study for comparison. Using the dataset obtained from these samples, we investigate the petrogenesis of the rodingites and discuss their implications for geological evolution of the ophiolite complexes in this region. Mineral names in this paper are abbreviated according to the recommendations of Whitney and Evans (2010).

  • Mineral analyses were carried out using a JXA-8800R microprobe at the Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing, under conditions of an acceleration voltage of 15 kV with a beam current of 20 nA. A series of natural and synthetic standards were used for calibration, including jadeite (Si), forsterite (Mg), hematite (Fe), albite (Na, Al), rutile (Ti), rhodonite (Mn), sanidine (K), diopside (Ca), chromite (Cr) and pentlandite (Ni). Analytical precision is estimated to be about 1% for most oxides. Mineral compositions are given in Table 1.

    Analysis minerals Ed Ed Ed Ed Ed Ed Amp Amp Amp Amp Amp Amp Amp Amp Czo Czo Czo Czo Czo Czo Czo Chl Chl Chl Chl Chl Chl Chl
    SiO2 49.19 48.85 44.19 46.70 48.41 49.02 52.19 52.61 55.58 50.44 54.27 52.18 53.02 56.94 38.01 37.79 38.08 37.98 37.94 38.02 38.06 29.09 30.95 27.99 28.71 27.09 27.16 26.95
    TiO2 0.34 0.51 0.39 0.39 1.30 0.32 0.08 b.d. 0.01 0.12 0.03 0.08 0.07 0.00 0.14 0.15 0.13 0.14 0.14 0.14 0.13 0.10 0.02 0.10 0.00 0.01 0.03 0.00
    Al2O3 8.48 8.61 12.42 9.59 7.77 8.19 5.07 5.27 2.27 6.46 3.69 5.30 4.44 1.38 29.64 29.12 29.81 29.57 29.46 29.67 29.77 20.39 17.99 20.23 20.48 21.22 22.33 23.07
    Cr2O3 1.51 1.18 0.37 1.73 0.02 1.10 1.22 0.76 0.20 0.30 0.62 0.27 0.21 0.36 0.09 0.06 0.10 0.09 0.08 0.09 0.10 0.63 0.22 0.00 0.07 0.37 0.13 0.20
    Fe2O3T 2.85 3.06 11.21 9.37 9.49 2.93 6.16 2.53 2.24 6.60 2.17 6.09 6.23 1.97 4.86 5.60 4.62 4.96 5.11 4.82 4.67 11.05 11.54 14.07 11.22 16.69 15.02 15.52
    MnO 0.06 0.08 0.28 0.20 0.16 0.03 0.18 0.03 0.06 0.14 0.03 0.20 0.15 0.11 0.01 0.02 0.01 0.01 0.02 0.01 0.01 0.26 0.21 0.18 0.23 0.21 0.20 0.43
    MgO 20.87 21.30 13.93 15.67 16.64 21.53 19.49 22.86 23.75 19.47 23.00 20.42 19.80 24.24 0.01 0.01 0.01 0.01 0.01 0.01 0.01 24.79 24.32 24.50 26.69 22.41 23.55 22.36
    CaO 12.31 12.01 12.43 11.44 11.44 12.19 11.76 12.32 12.20 11.99 12.51 12.14 11.95 12.25 23.60 23.55 23.62 23.59 23.58 23.60 23.61 0.20 1.04 0.05 0.01 0.03 0.01 0.03
    Na2O 2.31 2.43 2.65 2.08 1.85 2.22 1.32 1.67 0.92 1.50 1.24 1.25 0.99 0.65 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.09 0.12 0.00 0.01 0.01 0.00 0.22
    K2O 0.09 0.08 0.18 0.09 0.10 0.05 0.04 0.02 0.02 0.03 0.03 0.06 0.05 0.04 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.00 0.00 0.07 0.05 0.03
    NiO 0.12 0.03 0.13 0.06 0.07 0.05 0.07 0.09 0.05 0.13 0.12 0.09 0.12 0.11 0.02 0.03 0.02 0.02 0.03 0.02 0.02 0.06 0.11 0.17 0.15 0.12 0.12 0.04
    Total 98.13 98.14 98.19 97.32 97.24 97.63 97.58 98.15 97.29 97.18 97.71 98.08 97.02 98.03 96.39 96.35 96.41 96.39 96.38 96.40 96.41 86.68 86.53 87.29 87.57 88.22 88.59 88.85
    Si 6.87 6.88 6.43 6.76 6.97 6.90 7.35 7.26 7.66 7.16 7.48 7.31 7.48 7.77 2.97 2.96 2.97 2.96 2.96 2.97 2.97 2.87 3.06 2.79 2.80 2.71 2.68 2.66
    Al 1.40 1.41 2.13 1.64 1.32 1.35 0.84 0.86 0.37 1.08 0.60 0.87 0.74 0.22 2.73 2.68 2.74 2.72 2.71 2.73 2.74 2.37 2.10 2.37 2.36 2.50 2.59 2.68
    Ti 0.03 0.00 0.04 0.04 0.14 0.03 0.01 0.00 0.00 0.01 0.00 0.01 0.01 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.00 0.00 0.00 0.00
    Cr 0.17 0.13 0.04 0.20 0.00 0.12 0.14 0.08 0.02 0.03 0.07 0.03 0.02 0.04 0.01 0.00 0.01 0.01 0.00 0.01 0.01 0.05 0.02 0.00 0.01 0.03 0.01 0.02
    Fe3+ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.32 0.38 0.30 0.33 0.34 0.32 0.31 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Fe2+ 0.33 0.36 1.37 1.14 1.14 0.34 0.73 0.29 0.26 0.78 0.25 0.71 0.73 0.22 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.91 0.95 1.17 0.92 1.40 1.24 1.28
    Mg 4.35 4.43 3.02 3.38 3.57 4.49 4.09 4.70 4.88 4.12 4.73 4.26 4.16 4.93 0.00 0.00 0.00 0.00 0.00 0.00 0.00 3.64 3.59 3.64 3.89 3.34 3.46 3.29
    Mn 0.01 0.01 0.03 0.02 0.02 0.00 0.02 0.00 0.01 0.02 0.00 0.02 0.02 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.02 0.02 0.02 0.02 0.02 0.04
    Ca 1.84 1.79 1.94 1.78 1.77 1.83 1.77 1.82 1.80 1.82 1.85 1.82 1.81 1.79 1.97 1.97 1.97 1.97 1.97 1.97 1.97 0.02 0.11 0.01 0.00 0.00 0.00 0.00
    Na 0.63 0.66 0.75 0.58 0.52 0.60 0.36 0.45 0.25 0.41 0.33 0.34 0.27 0.17 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.02 0.00 0.00 0.00 0.00 0.04
    K 0.02 0.01 0.03 0.02 0.02 0.01 0.01 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00
    Ni 0.01 0.00 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.00
    SUM 15.64 15.68 15.80 15.56 15.48 15.69 15.32 15.47 15.26 15.45 15.32 15.39 15.26 15.18 8.00 8.01 8.00 8.01 8.01 8.00 8.00 9.92 9.87 9.99 9.99 10.01 10.00 10.01
    Numbers of ions are on the basis of Amp=23, Czo=12.5, Chl=14, Ttn=4.5 and Ilm=3, Cpx=6, Pl=8. Abbreviations of mineral names are after Whitney and Evans (2010).

    Table 1.  Representative mineral major element compositions (wt.%) of rodingite from Purang ophiolite complex, western Tibet

    Whole-rock major and trace elements were also conducted at the Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing. Whole-rock major oxides were measured with an ADVANT'XP+ X-ray fluorescence spectrometer. Trace elements were analyzed by inductively coupled plasma mass spectrometry using an Agilent 7500 ce/cs. Total iron was determined as Fe2O3T (wt.%), and the precision is estimated to be 1%–2% for major oxides and 1%–3% for most trace elements (Table 2).

    Rock type Rodingite Gabbro
    Sample No. 11PL054 11Pl055 11PL056 11PL057 11PL058 11PL059 11PL062 11PL003 11PL008 11PL013 11PL016 11PL079 11PL103 11PL111 11PL112 11PL122
    SiO2 48.35 47.78 47.43 46.82 49.89 52.04 48.25 46.34 49.44 49.22 51.24 48.57 44.75 48.25 50.06 47.78
    TiO2 0.14 0.13 0.07 0.16 0.07 0.04 0.05 1.45 1.43 1.39 1.26 1.03 1.51 1.53 1.54 1.42
    Al2O3 6.98 8.06 7.88 8.46 6.33 4.64 6.65 14.59 15.94 15.29 14.71 14.45 16.37 16.34 14.89 13.52
    Fe2O3T 3.92 4.76 4.52 5.33 3.67 3.79 4.38 11.07 10.76 11.00 10.62 10.45 12.15 10.43 11.79 10.44
    MnO 0.08 0.10 0.09 0.11 0.07 0.08 0.09 0.17 0.17 0.17 0.16 0.17 0.19 0.16 0.18 0.17
    MgO 25.20 23.38 23.65 23.12 23.51 23.88 24.71 8.23 7.69 8.53 7.54 8.38 7.10 7.51 7.13 9.05
    CaO 9.37 9.59 9.86 9.28 10.94 10.67 9.55 9.52 9.81 9.44 9.59 9.95 10.48 9.02 8.80 8.96
    Na2O 1.49 1.72 1.87 1.63 1.84 1.27 1.39 3.57 3.98 3.96 3.97 3.97 2.76 4.59 4.35 4.13
    K2O 0.05 0.04 0.04 0.04 0.04 0.03 0.03 0.09 0.12 0.09 0.14 0.28 0.08 0.18 0.14 0.09
    P2O5 0.03 0.03 0.01 0.04 0.01 0.01 0.00 0.19 0.23 0.20 0.20 0.16 0.25 0.25 0.26 0.21
    Cr2O3 1.44 1.49 1.25 1.40 1.30 1.22 1.53 0.03 0.02 0.02 0.03 0.02 0.02 0.04 0.02 0.04
    LOI 2.29 2.78 2.54 3.11 1.83 2.17 3.23 4.52 0.52 0.72 0.62 2.61 4.56 2.14 0.95 4.48
    Total 99.35 99.84 99.21 99.51 99.50 99.85 99.86 99.76 100.02 100.03 100.08 100.05 100.20 100.44 100.11 100.30
    Mg# 92 90 90 89 92 92 91 57 56 58 56 59 51 56 52 61
    Li 1.77 1.26 2.08 1.77 0.54 0.24 1.69 6.30 5.14 5.07 1.70 4.66 4.59 4.45 4.36 2.08
    Be 0.10 0.09 0.11 0.10 0.07 0.06 0.09 0.35 0.40 0.38 0.39 0.29 0.42 0.42 0.43 0.35
    Sc 36 35 40 37 28 25 32 39 40 42 40 40 41 41 41 44
    V 200 209 126 202 166 147 183 256 286 283 275 255 300 304 306 314
    Cr 4 710 4 871 4 079 4 571 4 264 4 004 5 014 188 177 261 192 161 171 170 169 102
    Co 31.80 32.04 30.93 32.35 28.58 27.10 28.20 40.34 40.69 42.82 38.71 36.34 40.86 40.90 40.93 41.47
    Cu 2.76 5.24 4.17 2.53 5.19 5.17 3.50 72.41 65.26 74.13 59.53 14.57 61.93 61.07 60.50 58.33
    Ga 5.75 5.45 5.59 6.02 3.77 3.04 4.43 15.07 16.36 16.47 15.82 14.85 16.97 17.12 17.22 11.59
    Rb ≤0.01 ≤0.01 0.15 ≤0.01 ≤0.01 ≤0.01 2.24 0.75 0.67 0.21 0.58 3.57 0.63 0.62 0.61 0.35
    Sr 63.88 94.29 54.27 68.84 121.89 133.72 41.96 235.30 224.01 177.01 195.94 146.69 218.73 217.38 216.47 201.53
    Y 3.02 3.02 1.58 3.25 1.93 1.46 2.00 26.93 31.33 29.85 26.98 23.21 33.39 33.92 34.27 29.84
    Zr 6.97 5.39 4.98 8.18 4.38 2.73 2.04 77.99 84.66 88.05 77.74 65.10 87.77 88.58 89.11 54.47
    Nb 0.29 0.39 0.25 0.31 0.93 1.16 0.19 3.47 2.31 1.46 1.55 1.07 1.76 1.63 1.53 1.35
    Cs 0.01 ≤0.01 0.00 ≤0.01 ≤0.01 ≤0.01 ≤0.01 0.21 0.10 0.06 ≤0.01 0.02 0.05 0.04 0.03 0.03
    Ba 3.81 5.95 4.52 2.82 5.13 4.77 7.39 26.73 17.06 22.33 36.94 37.61 12.54 11.38 10.61 15.90
    La 0.29 0.28 0.25 0.26 0.24 0.23 0.65 2.32 2.71 2.48 2.37 1.88 2.90 2.94 2.97 1.95
    Ce 0.81 0.74 0.79 0.71 0.62 0.56 3.16 7.92 9.25 8.55 7.94 6.20 9.87 10.03 10.13 7.40
    Pr 0.13 0.12 0.12 0.12 0.09 0.08 0.20 1.40 1.67 1.53 1.40 1.13 1.79 1.82 1.84 1.42
    Nd 0.66 0.59 0.55 0.62 0.41 0.34 0.79 8.00 9.41 8.74 7.97 6.39 10.06 10.23 10.35 8.44
    Sm 0.21 0.18 0.17 0.22 0.11 0.08 0.18 2.75 3.21 2.99 2.72 2.22 3.42 3.48 3.52 2.96
    Eu 0.11 0.10 0.12 0.12 0.09 0.08 0.09 1.06 1.21 1.17 1.06 0.97 1.28 1.30 1.31 1.10
    Gd 0.34 0.32 0.31 0.36 0.20 0.14 0.24 3.85 4.49 4.23 3.80 3.19 4.79 4.87 4.92 4.21
    Tb 0.07 0.07 0.06 0.07 0.04 0.03 0.05 0.70 0.81 0.78 0.70 0.59 0.86 0.88 0.89 0.77
    Dy 0.48 0.46 0.40 0.52 0.29 0.21 0.30 4.66 5.43 5.17 4.62 3.97 5.80 5.89 5.95 5.18
    Ho 0.12 0.12 0.10 0.13 0.07 0.06 0.08 1.03 1.19 1.13 1.02 0.88 1.26 1.28 1.29 1.12
    Er 0.38 0.38 0.31 0.41 0.25 0.20 0.27 3.00 3.49 3.33 3.01 2.60 3.72 3.78 3.82 3.36
    Tm 0.07 0.07 0.06 0.07 0.05 0.04 0.05 0.45 0.52 0.50 0.45 0.39 0.56 0.57 0.57 0.51
    Yb 0.48 0.47 0.42 0.50 0.33 0.27 0.37 2.90 3.35 3.22 2.91 2.54 3.57 3.62 3.66 3.24
    Lu 0.08 0.08 0.07 0.08 0.06 0.05 0.06 0.44 0.51 0.49 0.44 0.39 0.54 0.55 0.55 0.49
    Hf 0.24 0.16 0.11 0.28 0.11 0.09 0.07 2.50 2.71 2.64 2.34 2.01 2.81 2.84 2.86 2.03
    Ta 0.07 0.20 0.10 0.07 0.32 0.37 0.08 0.45 0.37 0.26 0.37 0.56 0.32 0.31 0.31 0.34
    Tl 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.03 0.03 0.03 0.02 0.02 0.02 0.02 0.02 0.02
    Pb 2.55 6.03 4.36 2.22 6.84 7.19 3.70 5.28 4.70 2.26 2.96 4.80 4.42 4.35 4.30 6.29
    Th 0.23 0.04 0.39 0.03 0.03 0.03 0.98 0.14 0.12 0.09 0.11 0.09 0.11 0.10 0.10 0.05
    U 0.08 0.13 0.16 0.04 0.14 0.15 0.13 0.14 0.15 0.07 0.11 0.10 0.15 0.15 0.15 0.08
    ∑REE 4.23 3.97 3.73 4.20 2.85 2.37 6.49 40.47 47.26 44.30 40.40 33.34 50.42 51.24 51.78 42.14
    (La/Gd)N 0.73 0.75 0.71 0.63 1.07 1.37 2.34 0.52 0.52 0.51 0.54 0.51 0.53 0.53 0.53 0.40
    (Dy/Yb)N 0.67 0.65 0.64 0.70 0.57 0.52 0.55 1.08 1.08 1.07 1.06 1.05 1.09 1.09 1.09 1.07
    Eu/Eu* 1.29 1.31 1.53 1.28 1.76 2.19 1.36 1.00 0.98 1.01 1.00 1.11 0.97 0.97 0.96 0.95
    Major and trace element compositions of the gabbros are from Chen et al. (2016).

    Table 2.  Major element (wt.%), REE and trace element (ppm) concentrations of rodingites and gabbros from the Purang ophiolite complex, western Tibet

  • The rodingites consist mainly of clinopyroxene (~45%– 55%), orthopyroxene (~20%–25%), olivine (< 5%), amphibole (~10%–15%), clinozoisite (< 3%), chlorite (< 3%) and minor plagioclase, with accessory minerals including titanite, ilmenite, zircon and magnetite (Figs. 3a3i). The orthopyroxene and olivine are coarse-grained and euhedral in shape with grain sizes up to millimeters in diameter, suggesting that they are all primary magmatic minerals inherited from the precursors of the rodingites. The clinopyroxene occurs as euhedral to subhedral grains with a grain size of 0.3–0.7 mm, which should be also relict primary clinopyroxene. It is commonly replaced or surrounded by chlorite and amphibole and is commonly dismembered to fine-grained relicts (e.g., Figs. 3c3f). Two generations of amphibole can be identified in the rocks. An earlier generation of amphibole (Amp-G1) is euhedral and coarse-grained with grain sizes varying from 0.1 to 0.5 mm across, indicating that it is of magmatic origin (Figs. 3b, 3f and 3h). While a later generation of amphibole (Amp-G2) produced during rodingitization forms fine-grained and subhedral-anhedral grains which are commonly associated with chlorite and plagioclase (Fig. 3f). The Amp-G1 grains contain variable contents of Al2O3 (7.77 wt.%–12.42 wt.%), CaO (11.44 wt.%–12.43 wt.%), Fe2O3T (2.85 wt.%–11.21 wt.%), Na2O (1.85 wt.%–2.65 wt.%) and K2O (0.05 wt.%–0.18 wt.%) (Table 1). According to the Hawthorne et al. (2012) nomenclature, the Amp-G1 grains belong to the subgroup of calcium amphiboles and for convenience were plotted in the Leake et al. (1997) diagram, which reveals that the respective analyses plot in the edenite field (Fig. 4a). On the other hand, the Amp-G2 has variably lower concentrations of Al2O3 (1.38 wt.%–6.46 wt.%), Fe2O3T (1.97 wt.%–6.60 wt.%), Na2O (0.65 wt.%–1.67 wt.%) and K2O (0.02 wt.%–0.06 wt.%) when compared with those of Amp-G1 (Table 1). Based on the classification of Leake et al. (1997), the compositions of these late amphiboles plot in the tremolite, actinolite and magnesiohornblende fields (Fig. 4b). Chlorite is generally fine-grained aggregate (Fig. 3) and contains high contents of Fe2O3T (11.05 wt.%–16.69 wt.%) and MgO (22.36 wt.%–26.69 wt.%) (Table 1). Clinozoisite, mostly 0.1–0.5 mm in size, is the main epidote-group mineral in the studied rocks. It commonly grows around the rim of Amp-G1 (Figs. 3a and 3h) or exhibits subhedral grains in contact with clinopyroxene (Figs. 3d3e). The clinozoiste contains consistent SiO2 (37.79 wt.%–38.08 wt.%), Fe2O3T (4.62 wt.%–5.60 wt.%) and CaO (23.55 wt.%–23.62 wt.%) contents (Table 1).

    Figure 3.  Representative photomicrographs showing the mineralogy and microstructures of rodingites from the Purang ultramafic complex, western Tibet. (a), (b) Typical rodingite structures and the occurrence of newly formed euhedral clinozoisite and subhedral-anhedral chlorite in the rodingite; chlorite occurs as interstitial minerals among grains of amphibole and clinozoisite. (c)–(g) Images showing relict primary mineralogy of the rodingite precursors, where clinopyroxene, amphibole (Amp-G1), orthopyroxene and olivine are well preserved in the rodingite. Most of the relict primary minerals are euhedral in shape, while clinopyroxne and amphibole (Amp-G1) are commonly variably replaced, surrounded or dismembered by chlorite, amphibole (Amp-G2) and plagioclase. (h), (i) Images showing the occurrence of newly formed clinozoiste, chlorite and titanite in the rodingite, and titanite is replaced by ilmenite at the rim. Microphotographs of (a)–(g) are taken under cross-polarized light, while (h) and (i) are back-scattered electron images. Mineral abbreviations after Whitney and Evans (2010).

    Figure 4.  Plots of two generations of amphibole in the Leake et al. (1997) diagram. Relict primary amphibole (Amp-G1) shows edenitic compositions (a), while newly formed amphibole (Amp-G2) during rodingitization has magnesiohornblende, actinolite to tremolite compositions (b).

  • Major and trace element compositions of the rodingites and gabbros in the Purang ophiolite complex are summarized in Table 2 and presented in Figs. 5 and 6. The rodingites contain variable contents of LOI from 1.83 wt.% to 3.23 wt.%, reflecting low degrees of alteration. Before any further geochemical analyses, major element compositions of the rodingite and gabbro samples were normalized to 100% on a volatile-free basis, in a purpose to erase the effect of (1) early hydration by ocean-floor metamorphism and (2) later devolatilization by multiple metamorphic overprints (cf. Li et al., 2004a). The rodingites have moderate concentrations of SiO2 (48.89 wt.%–53.57 wt.%), but low amount of Fe2O3T (3.77 wt.%–5.56 wt.%), Na2O (1.31 wt.%– 1.93 wt.%), Al2O3 (4.78 wt.%–8.84 wt.%), moderate CaO (9.69 wt.%–11.23 wt.%) and high MgO (24.11 wt.%–26.08 wt.%) with high Mg# values [Mg#=100×Mg/(Mg+Fe2+) molar] of 89–92 (Fig. 5; Table 2). They also contain low concentrations of TiO2 (0.04 wt.%–0.15 wt.%) and K2O (0.03 wt.%–0.05 wt.%) (Table 2). The rodingites generally have a CIPW normative mineralogy of clinopyroxene (25.44 wt.%–37.70 wt.%), orthopyroxene (0.00– 25.98 wt.%), olivine (17.74 wt.%–39.78 wt.%) and plagioclase (18.50 wt.%–31.41 wt.%) with minor ilmenite (0.08 wt.%–0.28 wt.%) (Table 2). While the gabbros at the same locality possess similar contents of SiO2 (46.75 wt.%–51.48 wt.%), CaO (8.87 wt.%–10.94 wt.%), higher Al2O3 (14.10 wt.%–17.10 wt.%), TiO2 (1.06 wt.%–1.58 wt.%), Fe2O3T (10.61 wt.%–12.69 wt.%), Na2O (2.88 wt.%–4.31 wt.%), K2O (0.08 wt.%–0.29 wt.%), and markedly lower MgO (7.19 wt.%–9.44 wt.%) (Fig. 5; Table 2).

    Figure 5.  Plots of SiO2 vs. selected major oxide (a)–(e) and total rare earth element (f) compositions showing compositional variations of the rodingites and gabbros in the Purang ophiolite massif. Major and rare earth element compositions of the gabbros are from Chen et al. (2016).

    For trace element compositions, the rodingites have low total rare earth element (REE) concentrations of 2.4 ppm–6.5 ppm, significantly lower than those (∑REE=33.3 ppm–51.8 ppm) of the adjacent gabbros (Fig. 5; Table 2), but are higher than those of the host serpentinized peridotites (∑REE=0.09 ppm–0.78 ppm) reported by Zhou et al. (2015; and Table 5 therein). On a chondrite-normalized diagram (Fig. 6a), the rodingites exhibit flat LREE [(La/Gd)N=0.63-2.34] and slightly enriched HREE [(Dy/Yb)N=0.52-0.70] patterns with positive Eu anomalies (Eu/Eu*=1.29–2.19), while the gabbros contain higher REE contents and are characterized by slightly depleted LREE [(La/Gd)N=0.40-0.54] and flat HREE [(Dy/Yb)N=1.05-1.09] patterns with negligible Eu anomalies (Eu/Eu*=0.95–1.11), similar to the patterns of typical N-(normal) MORB (Fig. 6a). In a primitive mantle-normalized multi-element diagram (Fig. 6b), the rodingites are enriched in Sr, U and Pb and depleted in high-field strength elements (HFSE), including Nb, P and Ti, while the gabbros are variably enriched in U, Pb and Sr, and show no depletions in HFSE, also consistent with typical N-MORB.

    Figure 6.  Chondrite-normalized rare earth element (REE) patterns (a), and primitive mantle-normalized trace element patterns (b) of the rodingites and gabbros in the Purang ophiolite massif. Chondrite-normalized REE and primitive mantle-normalized trace element patterns of N-MORB, E-MORB and OIB are also involved for comparison. N-MORB, E-MORB and OIB compositions and normalization values are from Sun and McDonough (1989). Trace element compositions of the gabbros are from Chen et al. (2016). N-MORB. Normal mid-ocean ridge basalt; E-MORB. enriched mid-ocean ridge basalt; OIB. ocean island basalt.

  • In order to constrain the metamorphic conditions under which the rodingites originally formed, a P-T pseudosection was calculated in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O (NCFMATSHO) model system for the composition of a rodingite sample (11PL057). Due to the fact that newly formed minerals during rodingitization only occupy a limited volume (~5%–10%) with respect to the dominant relict primary minerals, bulk compositions of the sample may not be valid for elucidating the phase equilibria conditions in which the rodingitization have occurred. In this regard, the pseudosection was calculated by using effective mineral compositions (40 vol.% Amp+10 vol.% Chl+45 vol.% Ep+2 vol.% Ab+3 vol.% Ttn) based on petrological observations, in order to estimate much more accurate P-T conditions. Calculations were undertaken using Perple_X 6.8.6 (Connolly, 1990) and the internally consistent dataset of Holland and Powell (1998, revised version of 2004). Activity-composition models used for the related phases are as follows: amphibole (Diener et al., 2007), pyroxene (Green et al., 2007), garnet (White et al., 2007), ilmenite (White et al., 2000), epidote (Holland and Powell, 1998), chlorite (Holland et al., 1998), pumpellyite (Massonne and Willner, 2008). Rutile, titanite, quartz, vesuvianite, calcite and H2O phases are pure end members and the CORK (Compensated-Redlich-Kwong) equation of state for H2O-CO2 fluids (Holland and Powell, 1998, 1991) was used.

    The P-T pseudosection within a P-T range of 0.5–3.5 kbar and 250–500℃ is presented in Fig. 7. From the diagram it is evident that the rodingite assemblages are generally not sensitive to pressure, but are sensitive to temperature. The field appropriate to the observed mineral associations in the studied rodingites is indicated by two heavy red lines (labelled assemblage in red, Fig. 7). The stable field contains epidote, actinolite, hornblende, chlorite and albite; towards high-T side, actinolite is not stable. Therefore, the observed petrographic features combined with the modeled pseudosection constrain the development of these metasomatic minerals at P < 2 kbar and T=~350–400℃.

    Figure 7.  Phase equilibrium modeling results in the NCFMASHTO model system based on effective bulk rock compositions of a rodingite sample (11PL057). The effective bulk rock compositions calculated based on volume contents of newly formed minerals during rodingitization are shown. The field appropriate to the development of newly formed minerals is enclosed within two bold lines. Mineral abbreviations follow recommendations of Whitney and Evans (2010).

  • Determination of the protolith of a rodingite is prerequisite for understanding its petrogenesis and unraveling the implications for fluid-rock interaction and element transfer in different geological settings (e.g., Li et al., 2017; Shen et al., 2016, 2012). In the past two decades, abundant studies have revealed that the protoliths of rodingites in the central to eastern segments of the YZSZ (e.g., Jiding, Luqu, Baimarang, Bairang, Baigang, Zedang and Luobusa) are mainly gabbros and diabases (e.g., Li et al., 2017; Zhang L L et al., 2016; Huot et al., 2002). It has been well proven that the protolith of rodingite generally gains CaO and loses Na2O, K2O and SiO2 during rodingitization (e.g., Li et al., 2017, 2004a; Shen et al., 2016, 2012; Hatzipanagiotou and Tsikouras, 2001; O'Hanley, 1996; Honnorez and Kirst, 1975; Coleman, 1963); MgO, Fe2O3T and REEs, especially M-(middle) and HREE, retain unchanged or are slightly increased during this process (cf. Tsikouras et al., 2013, 2009; Li et al., 2004a). In addition, Al2O3 is generally considered as an immobile component during such low-grade metasomatic processes as rodingitization (e.g., Li et al., 2017, 2007; Walther, 1997; Ragnarsdóttir and Walther, 1985; Pearce, 1976; Gresens, 1967). These geochemical characteristics are important for constraining the protolith nature of rodingites. In this study, the rodingite is also in close spatial associations with gabbros/diabases within serpentinized peridotite at the Purang massif (Fig. 2), which suggests a possible genetic relationship among these lithological types. However, the rodingites preserve euhedral grains of clinopyroxene, orthopyroxene, olivine and hornblende (Fig. 3), significantly different from those of the gabbros/diabases in the Purang area and also in other localities of the western YZSZ (cf. Chen et al., 2016; Xiong et al., 2015; Liu et al., 2011; Li J F et al., 2008). Moreover, the rodingites have similar SiO2, CaO, obviously lower Al2O3, Fe2O3T and REE, as well as higher MgO and Cr2O3 contents than those of the adjacent gabbros (Fig. 5; Table 2; also see Table 2 in Liu et al., 2011). They also have markedly higher Mg# values (89–92) than those of the gabbros (51–61) (Fig. 5; Table 2). These petrological and geochemical features indicate that gabbros or diabases cannot be served as the protolith of the rodingites in this case; instead, the rodingites should originate from a mantle-derived rock source. Bulk-rock recalculation reveals that they have a CIPW mineralogy of plagioclase, clinopyroxene, orthopyroxene and olivine (Table 2), indicating that they were metamorphosed from olivine gabbro or olivine gabbronorite of mantle origin. This inference is consistent with the observations that abundant clinopyroxene, orthopyroxene and olivine grains exist in the rodingites, which should be relict phases that are inherited from their precursors due to incomplete degrees of rodingitization.

  • Most previous studies have shown that the degree of rodinitization is closely related with serpentinization of mantle peridotites (e.g., Li et al., 2017, 2004a; Dubińska, 1995; Schandl et al., 1989), although this relationship may not always be straightforward as argued by several studies (e.g., O'Hanley, 1996; O'Hanley et al., 1992). The Purang ultramafic rocks are fresh and have experienced weak to moderate degrees of serpentinization (Li et al., 2015), which corresponds well with the fact that rodingites only record evidence of early-stage rodingitization. The rodingites mainly have a metasomatic mineral assemblage of chlorite, clinozoisite, amphibole and albite, whereas representative mineral phases in severely rodingitized rocks, such as prehnite, vesuvianite and garnet (e.g., Li et al., 2017, 2008a, 2007), are absent (Fig. 3). Such a mineral association has been documented as representative products for the early-stage rodingitization (cf. Schandl et al., 1989), and is comparable to that of clinozoisite rodingite identified by Li et al. (2017) in the Xigaze ophiolite of the central YZSZ. This inference is fully supported by the observed petrographic features.

    Yet in what P-T conditions and tectonic environment have the metasomatic event occurred? Answers to these questions are essential to decipher the rodingitization process and its geological implications for the evolution of enormous ophiolites in the western segment of the YZSZ. Among various methods for modeling metamorphic P-T paths, quantitative calculation of metamorphic phase diagram is a well-accepted approach for elucidating phase relations in metamorphic rocks, and has been widely used in different metamorphic terranes (e.g., Liu et al., 2019; Li et al., 2018; Wei, 2018, 2011; Xia et al., 2018; Zhang et al., 2018; Wang et al., 2017; Wei et al., 2013; Wei and Wang, 2007; White et al., 2001; Carson et al., 1999). However, this approach has rarely been used to determine the P-T conditions during rodingitization, especially for partially rodingtized rocks (Li et al., 2008a). Here we use effective bulk rock compositions from an initially rodingtized rock and model the P-T conditions in the NCFMATSHO model system, which shows that the rodingitization occurred at P < 2 kbar and T= ~350–400 ℃, corresponding to low greenschist facies conditions. Under these metamorphic conditions, it is viable to infer that the rodingitization may have occurred under ocean floor metamorphism. However, the rodingites are enriched in U, Pb and Sr, and are variably depleted in Nb, P and Ti, suggesting for a subduction-related affinity (e.g., Wang et al., 2019a, b; Wang J P et al., 2019; Zheng, 2019; McCulloch and Gamble, 1991). In the diagram of V vs. Ti for modern volcanic rock associations and ophiolite lavas, the rodingites show similar features to boninite, while the gabbros plot in MORB and back-arc basin basalt fields (Fig. 8a), also indicating that the former may have derived from a SSZ environment (e.g., Polat et al., 2002; Taylor et al., 1994; Murton, 1989; Bloomer and Hawkins, 1987). In the diagrams of Zr/Y vs. Zr and La/Nb vs. Y for basaltic rocks, the rodingites plot in or close to fore-arc basin basalt field (Figs. 8b, 8c). Therefore, these geochemical characteristics all indicate that the protolith of the rodingites may have been affected by SSZ fluids in a fore-arc setting.

    Figure 8.  Discrimination diagrams of (a) V vs. Ti (after Pearce, 2014; Shervais, 1982), (b) Zr/Y vs. Zr with partial melting and fractionation trends after Pearce and Norry (1979), and (c) La/Nb vs. Y for the rodingites and gabbros from the Purang ophiolite complex, western Tibet (after Rollinson, 1993). MORB. Mid-oceanic ridge basalt; BABB. back-arc basin basalt; FABB. fore-arc basin basalt; IAT. island arc tholeiite; VAB. volcanic-arc basalt; WPB. within-plate basalt; PM. primitive mantle.

    Based on geochemical features of mantle peridotites in the Purang ophiolite massif, most studies suggested that the ophiolite complex was firstly formed in a MOR setting and then modified by SSZ-liberated fluids/melts (e.g., Guo et al., 2015; Li et al., 2015; Xiong et al., 2013; Wang et al., 2012; Liu Z et al., 2011; Xu et al., 2011a; Liu C Z et al., 2010). However, whether SSZ fluids/melts have ever affected the ophiolites has been challenged by some researchers (e.g., Zhou et al., 2015; Liu et al., 2014; Yang et al., 2011; Miller et al., 2003). Although there is limited evidence in this study to constrain the MOR processes for the formation of the ophiolites, it is documented that the protolith of the rodingites as well as the host mantle peroditites from the Purang massif have been modified in a SSZ environment during their emplacement.

  • The current study on newly recognized rodingite from the Purang area advances our knowledge on the origin of rodingitized rocks and their implications for the evolution of ophiolitic complexes along the western segment of the YZSZ. Main findings of this study are summarized as follows.

    (1) The rodingites have a major metasomatic mineral assemblage of chlorite, clinozoisite, amphibole and minor plagioclase, reflecting an early-stage rodingitization.

    (2) They were metamorphosed from mantle-derived olivine gabbros or olivine gabbronorites at P < 2 kbar and T < 350–400 ℃.

    (3) Geochemical characteristics of the rodingites indicate that their protolith has been modified by SSZ-liberated fluids/ melts in a fore-arc setting.

    (4) Further geochronological (e.g., Zircon and titanite U-Pb dating) and isotopic geochemical (e.g., Nd, Hf and Re-Os isotopes) studies are required for a comprehensive understanding on the timing, source and petrogenesis of the rodingites.

  • We acknowledge financial supports for this research from the Natural Science Foundation of Shandong Province (No. ZR2018BD019), the National Natural Science Foundation of China (Nos. 41572044, 41230960, 41803031), and the Project funded by China Postdoctoral Science Foundation (No. 2017M622232). We appreciate constructive reviews from two anonymous reviewers and the editors. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1244-7.

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