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Volume 30 Issue 6
Dec.  2019
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Reconstruction the Process of Partial Melting of the Retrograde Eclogite from the North Qaidam, Western China: Constraints from Titanite U-Pb Dating and Mineral Chemistry

  • Retrograde eclogite and garnet amphibolite of the Lüliangshan unit of the Shenglikou area, North Qaidam, were studied with emphasis on rutile and titanite. A special focus is on the formation of rutile and its corona of titanite (Ttn1) in retrograde eclogite and on coarse-grained titanite (Ttn2) from the garnet amphibolite. Using zirconium (Zr)-in-rutile and Zr-in-titanite thermometers, the temperatures estimated for the formation of an early generation of rutile are 823-884 ℃ at 2.5-2.8 GPa, while 812-894 ℃ at 1.3-1.5 GPa are derived for the formation of coronitic Ttn1 in the retrograde eclogite. Therefore, isothermal decompression must have occurred during exhumation, which also has triggered the partial melting of the retrograde eclogite. Ttn2 of the garnet amphibolite has high REE contents and high Th/U ratios, indicating that it is newly grown from a Ti, Ca, and LREE enriched anatectic melt derived from the partial melting of retrograde eclogite. LA-ICP MS U-Pb dating yields a lower intercept age of 423±4 Ma for Ttn2, which is consistent with the granulite-facies metamorphic age of the retrograde eclogite. Moreover, a temperature of 781-823℃ at 1.0-1.2 GPa is obtained for Ttn2, which fits the P-T conditions of the HP granulite-facies metamorphic stage (P=1.07-1.24 GPa and T=774-814℃), and documents that the crystallization of the melt occurred at the granulite-facies stage at 423 Ma. The high amount of REE of the garnet amphibolite is a consequence of the formation of Ttn2 from the melt. The contents and ratios of Zr and Hf in rutile and Ttn2 differ from those in the garnet amphibolite, and the whole rock Zr/Hf ratios of retrograde eclogite and garnet amphibolite are both higher than the respective ratios in rutile and Ttn2, suggesting that rutile and titanite cannot be the major carriers of Zr and Hf accounting for the high whole rock Zr/Hf ratios.
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Reconstruction the Process of Partial Melting of the Retrograde Eclogite from the North Qaidam, Western China: Constraints from Titanite U-Pb Dating and Mineral Chemistry

    Corresponding author: Yuting Cao, 619ting@163.com
  • 1. Shandong Key Laboratory of Depositional Mineralization and Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266590, China
  • 2. Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
  • 3. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China
  • 4. Xi'an Center of Geological Survey, China Geological Survey, Xi'an 710054, China

Abstract: Retrograde eclogite and garnet amphibolite of the Lüliangshan unit of the Shenglikou area, North Qaidam, were studied with emphasis on rutile and titanite. A special focus is on the formation of rutile and its corona of titanite (Ttn1) in retrograde eclogite and on coarse-grained titanite (Ttn2) from the garnet amphibolite. Using zirconium (Zr)-in-rutile and Zr-in-titanite thermometers, the temperatures estimated for the formation of an early generation of rutile are 823-884 ℃ at 2.5-2.8 GPa, while 812-894 ℃ at 1.3-1.5 GPa are derived for the formation of coronitic Ttn1 in the retrograde eclogite. Therefore, isothermal decompression must have occurred during exhumation, which also has triggered the partial melting of the retrograde eclogite. Ttn2 of the garnet amphibolite has high REE contents and high Th/U ratios, indicating that it is newly grown from a Ti, Ca, and LREE enriched anatectic melt derived from the partial melting of retrograde eclogite. LA-ICP MS U-Pb dating yields a lower intercept age of 423±4 Ma for Ttn2, which is consistent with the granulite-facies metamorphic age of the retrograde eclogite. Moreover, a temperature of 781-823℃ at 1.0-1.2 GPa is obtained for Ttn2, which fits the P-T conditions of the HP granulite-facies metamorphic stage (P=1.07-1.24 GPa and T=774-814℃), and documents that the crystallization of the melt occurred at the granulite-facies stage at 423 Ma. The high amount of REE of the garnet amphibolite is a consequence of the formation of Ttn2 from the melt. The contents and ratios of Zr and Hf in rutile and Ttn2 differ from those in the garnet amphibolite, and the whole rock Zr/Hf ratios of retrograde eclogite and garnet amphibolite are both higher than the respective ratios in rutile and Ttn2, suggesting that rutile and titanite cannot be the major carriers of Zr and Hf accounting for the high whole rock Zr/Hf ratios.

0.   INTRODUCTION
  • Partial melting is widely recognized during the exhumation process of the subducted slabs (Cao et al., 2019a, 2017; Yu et al., 2019; Liu et al., 2015; Chen et al., 2013a, b; Gao et al., 2012a; Zong et al., 2010). The main mechanisms triggering the partial melting are: (1) heating processes during hot exhumation or under a thickened crust (Song et al., 2014; Nahodilová et al., 2011; Stowell et al., 2010); (2) decompression associated with rapid exhumation (Yu et al., 2014); (3) the addition of fluid by dehydration of hydrous minerals (Zheng et al., 2011; Liebscher et al., 2007; Auzanneau et al., 2006; Schmidt et al., 2004; Hermann, 2002; Skjerlie and Patiño Douce, 2002). Fluid or melt derived from partial melting can significantly influence elemental mobility, mineral assemblage, the preservation of high pressure (HP) or ultrahigh-pressure (UHP) rocks, and the rheology of the subducted crust (Zheng et al., 2011, 2007; Ragozin et al., 2009; Zhao et al., 2007; Hermann et al., 2006; Wallis et al., 2005; Zheng, 2004). Thus fluid or melt plays a critical role in accelerating the exhumation of UHP slabs (Chen and Zheng, 2015; Labrousse et al., 2002; Hermann et al., 2001). During the processes of exhumation fluid influx and partial melting, the composition of the peak mineral assemblage can be partially or completely obliterated. Therefore, a precise P-T estimate for the peak stage can be challenging.

    Rutile is a dominant carrier of high field strength elements (HFSE) (Gao et al., 2014; Schmidt et al., 2009; Zack et al., 2002) and also a primary accessory mineral in HP and UHP eclogite-, amphibolite- and granulite-facies over a wide range of P-T conditions. During the retrogression, rutile usually transforms into ilmenite or titanite. In zircon-buffered assemblages, Zr incorporation in rutile is demonstrated to be temperature dependent (Zack et al., 2004, 2002). Therefore, Zr-in-rutile thermometer is a widely used thermometer for high-grade metamorphic rocks (Li X L et al., 2017; Ferry and Watson, 2007; Tomkins et al., 2007; Watson et al., 2006; Zack et al., 2004).

    Titanite, as an accessory mineral, is widely stable in rocks of various metamorphic grades, especially in HP/UHP eclogite- facies rocks and (HP) granulite-facies rocks (Zhang L J et al., 2018; Gao et al., 2012b; Kylander-Clark et al., 2008). Titanite has been explored extensively for the geochronology due to its relatively high closure temperature in the U-Pb isotopic system (T > 700 ℃). Hayden et al. (2008) has documented that the Zr-in-titanite thermometry is pressure dependent, and Zr concentrations in titanite can be used as a powerful geothermometer when titanite is in equilibrium with zircon, rutile and quartz.

    Moreover, rutile and titanite are commonly considered to incorporate significant amounts of Nb and Ta, Zr and Hf (Lucassen et al., 2010; John et al., 2008; Storkey et al., 2005). Bea et al. (2006) has found that rutile, titanite and garnet, such as zircon, also contain a certain amount of Zr and Hf. Therefore, in subduction zone related metamorphic rocks, the distribution of Zr, Hf, Nb and Ta is generally controlled by the accessory minerals zircon, rutile and titanite (Foley et al., 2002; Rudnick et al., 2000). However, partial melting of crustal rocks can efficiently fractionate trace elements due to the elevated temperature (Wang et al., 2019; Xiong et al., 2019; Chen and Zheng, 2015; Stepanov and Hermann, 2013; Kessel et al., 2005).

    Accessory minerals of this study are rutile from the retrograde eclogite that preserved high-pressure assemblages and titanite from the garnet amphibolite that remained as a residue after partial melting of the retrograde eclogite (Cao et al., 2017). The partial melting of the retrograde eclogite was identified to take place during the "hot" exhumation from the peak eclogite- facies to the granulite-facies stage by the dehydration of zoisite and sodic amphibole (Cao et al., 2017). Since the retrograde overprinting the entire peak mineral assemblage of the retrograde eclogite could not be determined, the estimated P-T conditions of the eclogite-facies event is not precisely defined, from which only a part of the exhumation path could be derived. In addition, lack of the information on element migration during partial melting impedes a proper understanding of the melt/fluid activity in the subduction/exhumation zone. In order to address this problem, we herein introduce mineral trace element and titanite U-Pb age studies for the retrograde eclogite and the garnet amphibolite using LA-ICP-MS and microprobe. The titantite dating results of our study could provide a tighter constraint for the timing of melt crystallization. Zr-in-rutile and Zr-in-titanite thermometric calculations are also conducted on rutile and titanite from the retrograde eclogite and the garnet amphibolite, respectively. These results, combined with previous zircon U-Pb ages, re-constrain the P-T-t history and help to uncover the partial melting mechanism for the retrograde eclogite from the Lüliangshan UHP unit.

1.   GEOLOGICAL BACKGROUND AND SAMPLES
  • The North Qaidam HP/UHP terrane is located at the northeastern part of the Tibet Plateau, and extends ~400 km (Fig. 1a). There are four HP/UHP metamorphic units from the southeast to northwest: the Dulan, the Xitieshan, the Lüliangshan and the Yuka units. Previous geochronological data for the HP/UHP rocks yielded peak metamorphic ages of 420–460 Ma (Zhang C et al., 2017; Ren et al., 2016), and retrograde metamorphic ages of 400–425 Ma (Chen et al., 2009; Zhang J X et al., 2009a, b, 2008; Song et al., 2006, 2005a). The identification of coesite and microdiamond in zircon of pelitic gneiss and eclogite provided direct evidence that continental crust was subjected to UHP metamorphism (Zhang J X et al., 2010; Zhang G B et al., 2009a, b, 2008; Song et al., 2005a; Yang et al., 2001). Combined previous zircon U-Pb ages documented that the North Qaidam continental slab was subducted to ~120 km (or > 200 km, Song et al., 2004) at 450–425 Ma, and subsequently exhumed to mid-crust levels at ~ 420 Ma; and then returned to the surface at ~400 Ma (Xiong et al., 2011). Recently, partial melting of the HP-UHP rocks (eclogites and gneisses) have been widely studied in this zone (Yu et al., 2019; Cao et al., 2017; Zhang G B et al., 2015; Liu et al., 2014; Song et al., 2014; Chen et al., 2012). The time of the partial melting was estimated ranging from 410 to 438 Ma. This age range is identical to the retrograde ages (400–432 Ma) of the HP-UHP rocks and correspond to the first exhumation-related granulitic stage of the HP-UHP rocks (Yu et al., 2014; Chen et al., 2009; Zhang J X et al., 2008; Song et al., 2006, 2005a).

    Figure 1.  Geological and tectonic map of North Qaidam (a), geological map of the Lüliangshan (Shenglikou) area (b) and field occurrences of retrograde eclogite and garnet amphibolite (c) (modified from Cao et al., 2017).

    The studied area is located in the Lüliangshan unit of the Shenglikou area (Fig. 1a). The unit mainly consists of granitic gneiss, paragneiss and ultramafic rocks (Fig. 1b). The ultramafic rocks consist of garnet pyroxenite, dunite, garnet lherzolite and mafic granulite (Zhang J X et al., 2008; Song et al., 2005a, b, 2004), and occur as irregular lenses wrapped in country gneisses. The garnet peridotite from the Lüliangshan unit has experienced UHP metamorphism and was subducted to mantle depths > 200 km, according to the interpretation of exsolution textures (see details in Song et al., 2005b, 2004). Recently, small amounts of omphacite inclusions within garnet were identified, pointing to a "fresh" eclogite as a precursor of the studied retrograde eclogite (Cao et al., 2017; Zhang J X et al., 2007).

    Retrograde eclogite and garnet amphibolite (Fig. 1c) investigated were collected at the same location as reported by Cao et al. (2017). The garnet amphibolite is interpreted to represent an anatectic residue of the retrograde eclogite (Fig. 1c). Previous studies on the retrograde eclogite have documented a peak eclogite-facies metamorphism at ~440 Ma, a granulite-facies metamorphism at ~420 Ma, and an amphibolite-facies overprint later than 420 Ma (Cao et al., 2017). Partial melting due to the dehydration of zoisite and sodic amphibole was proposed to occur during the "hot" exhumation from eclogite-facies to granulite-facies stages (Cao et al., 2017).

2.   PETROGRAPHY AND MINERALOGY
  • The retrograde eclogite exhibits a porphyroblastic structure (Fig. 2a) and is mainly composed of garnet (Grt), amphibole (Amp), omphacite inclusion in garnet (Omp-in), diopsidic clinopyroxene (Di), plagioclase (Pl), quartz (Qz) and accessary minerals of zircon (Zrn), rutile (Rt), and ilmenite (Ilm). Multi-stage metamorphism had been recognized for the retrograde eclogite by Cao et al. (2017): (1) peak eclogite-facies stage with the mineral assemblage of Grt+Omp-in+Rt±Qz and (2) HP granulite-facies stage with the development of Grt+Di+Pl1+Ilm+Qz defined by P-T conditions of 1.07–1.24 GPa and 774–814 ℃; (3) late amphibolite-facies stage with Grt+Amp+Pl2+Qz±Ilm, yielding P=0.55–0.68 GPa and T=619–694 ℃. The retrograde eclogite shows several unequivocal microtextures of partial melting; for details see Cao et al. (2017). Rutile occurs as micron-sized grains in the matrix (Fig. 2b). Some rutile grains are surrounded by rims of ilmenite (Figs. 2c and 2d); some other rutile grains are surrounded by coronas of titanite (Ttn1) (Figs. 2e and 2f).

    Figure 2.  Microphotographs showing the occurrences of rutile and titanite in the retrograde eclogite from the Lüliangshan area, North Qaidam. (a) Mineral assemblage of retrograde eclogite (modified from Cao et al., 2017); (b) micron-sized rutile in the matrix; (c), (d) rutile relics surrounded by ilmenite; (e), (f) rutile grains are surrounded by titanite coronas (Ttn1).

    The garnet amphibolite contains fine-grained garnet, coarse-grained amphibole, plagioclase, quartz and titanite, and lacks clinopyroxene and rutile. Titanite (Ttn2) occurs as euhedral or subhedral grains in areas where coarse-grained amphiboles accumulated, or in the matrix, and its size ranges between 100 and 400 μm (Figs. 3a and 3b). The banded Qz+Pl veins along the intergranular spaces between amphibole grains (Fig. 3c) and Kfs-bearing veins cross-cutting garnet and amphibole grains (Fig. 3d) imply that portions of the crystallized melt remained trapped within the garnet amphibolite residue (Cao et al., 2017).

    Figure 3.  Microphotographs showing the occurrence of titanite from the retrograde eclogite from the Lüliangshan area, North Qaidam. (a), (b) Coarse-grained titanite occurs as euhedral to subhedral grains distributed in the coarse-grained amphibole; (c) banded Qz+Pl veins along the intergranular spaces of the amphibole grains; (d) Kfs-veins crossing garnet and amphibole grains. (c), (d) Modified from Cao et al. (2017).

3.   ANALYTICAL METHODS
  • All the analyses were performed at the State Key Laboratory of Continental Dynamics in Northwest University, Xiʼan, China. Mineral analyses were conducted on a JXA-8230 microprobe (same conditions as in Cao et al., 2017). Zr and Hf contents of micron-sized rutiles and corona Ttn1 in the retrograde eclogite were also analyzed with JXA-8230 microprobe. The instrument was operated at an accelerating voltage of 15 kV, 50 nA probe current and a beam diameter of 2 μm. Microprobe standards used were: calcium, calcite; silicium, quartz; zirconium, ZrO2; titanium, rutile; iron, magnetite.

    Mineral trace element and titanite U-Pb isotope analyses were both performed on thin sections by in-situ LA-ICP-MS method using a Agilient 7500a LA-ICP-MS coupled with a ComPex102 Excimer 193 nm ArF laser and MicroLas GeoLas 200M optics. Laser ablation spot sizes were set to 44 µm width for mineral trace element analyses, and 32 μm width for titanite U-Pb isotope analyses. Standard materials, Nist610, GSE-1G, and BCR-2G were used for multiple-standard calibration. Nist610 was used as the external calibration standard and 29Si as the internal standard. The standard titanite BLR-1 was used as the reference and repeated analyses gave a weighted mean of U-Pb ages at 1 049±7 Ma (2σ, n=12). 207Pb/206Pb, 207Pb/235U, 206Pb/238U and 208Pb/232Th ratios were calculated using GLITTER 4.0 program. The concordia diagrams were made using ISOPLOT (version 4.0) (Ludwig, 2003).

    Mineral abbreviations are after Whitney and Evans (2010).

4.   RESULTS
  • Results for rutile and titanite analyses from the retrograde eclogite and the garnet amphibolite, respectively, are listed in Tables 1, 2 and S1.

    Element Rutile Rutile Rutile Rutile Rutile Rutile Rutile Rutile Ttn1 Ttn1 Ttn1 Ttn1 Ttn2 Ttn2 Ttn2
    SiO2 0.04 0.02 0.01 0.01 0.02 0.02 0.01 0.03 30.34 29.87 29.67 29.85 30.18 29.61 30.23
    TiO2 99.28 97.98 99.11 98.31 99.88 98.65 99.32 100.05 36.33 36.12 35.97 37.11 35.85 36.83 36.81
    Al2O3 0.04 0.00 0.03 0.03 0.02 0.02 0.03 0.04 1.12 1.39 1.13 1.87 1.41 0.98 1.04
    FeO 0.16 0.11 0.17 0.13 0.17 0.17 0.18 0.19 0.42 0.45 0.34 0.20 0.37 0.47 0.41
    CaO 0.01 0.05 0.06 0.02 0.07 0.10 0.05 0.13 27.89 28.38 28.22 27.90 28.56 27.65 28.16
    ZrO2 0.18 0.14 0.13 0.18 0.14 0.14 0.16 0.19 0.07 0.04 0.03 0.05 - - -
    HfO2 0.08 0.09 0.09 0.08 0.09 0.11 0.07 0.06 bdl bdl bdl bdl - - -
    Total 99.90 98.52 99.64 98.85 100.44 99.45 100.11 100.94 96.18 96.33 95.38 96.99 96.36 95.55 96.65
    Si 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.03 1.01 1.02 1.00 1.02 1.01 1.02
    Ti 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.93 0.92 0.93 0.94 0.91 0.94 0.93
    Al 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.04 0.06 0.05 0.07 0.06 0.04 0.04
    aFe3+ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.00 0.01 0.01 0.01
    Ca 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.01 1.03 1.03 1.00 1.03 1.01 1.02
    Total 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 3.02 3.03 3.03 3.02 3.03 3.02 3.02
    Zr (ppm) 1 355 1 036 940 1 296 999 1 051 1 177 1 421 481 259 185 370 - - -
    Hf (ppm) 661 780 721 678 797 924 551 526 0.00 0.00 0.00 0.00 - - -
    Zr/Hf 2.05 1.33 1.30 1.91 1.25 1.14 2.14 2.70 - - - - - - -
    bXAl - - - - - - - - 0.05 0.06 0.05 0.07 0.06 0.04 0.04
    Ttn1. Corona titanite surrounding the rutile in the retrograde eclogite; Ttn2. coarse-grained titanite in the garnet amphibolite; a. All Fe as Fe3+; b. XAl=Al/[Al+Ti+Fe3+]; bdl. below limit of detection.

    Table 1.  Major (wt.%) and trace element compositions of rutile and titanite from the retrograde eclogite and garnet amphibolite

    Mineral Zr (ppm) T (℃, Tomkins et al., 2007)
    2.5 GPa 2.8 GPa
    Rutile 1 355 864 878
    Rutile 1 036 834 848
    Rutile 940 823 838
    Rutile 1 296 858 873
    Rutile 999 830 844
    Rutile 1 051 835 850
    Rutile 1 177 848 862
    Rutile 1 421 869 884
    Mineral Zr (ppm) T (℃, Hayden et al., 2008)
    1.3 GPa 1.5 GPa
    Ttn1 481 870 894
    Ttn1 259 832 855
    Ttn1 185 812 835
    Ttn1 370 853 877

    Table 2.  Temperatures for rutile and titanite in the retrograde eclogite by Zr-in-rutile and Zr-in-titanite

  • The contents of Zr and Hf of rutile from the retrograde eclogite vary from 940 ppm to 1 421 ppm and from 526 ppm to 924 ppm, respectively (Table 1). Their Zr/Hf ratio lies between 1.14 and 2.70. Ttn1 from the retrograde eclogite has a SiO2 content of 29.85 wt.%–30.34 wt.%, CaO content of 27.89 wt.%–28.38 wt.%, Al2O3 content of 1.12 wt.%–1.87 wt.% and FeO content of 0.20 wt.%–0.45 wt.% (Table 1). The XAl [=Al/(Al+Ti+Fe3+)] values are 0.05–0.07, which documents that the titanite belongs to the low-Al metamorphic type (Oberti et al., 1991). Ttn1 contains Zr varying from 185 ppm to 481 ppm. The Nb, Ta and Hf concentrations are below detection limit of the microprobe.

  • Ttn2 from garnet amphibolite has an identical major element composition as Ttn1 from retrograde eclogite. The major element composition of Ttn2 is relatively uniform with 29.61 wt.%–30.23 wt.% for SiO2, 27.65 wt.%–28.56 wt.% for CaO, 0.98 wt.%–1.41 wt.% for Al2O3 and 0.37 wt.%–0.41 wt.% for FeO (Table 1). The XAl values are 0.04–0.06, likewise indicating a low-Al metamorphic type (Oberti et al., 1991). Ttn2 has high ΣREE 5 968 ppm–10 153 ppm, high HFSE contents (Nb 425 ppm–998 ppm; Ta 32.19 ppm–70.95 ppm; Zr 198 ppm–273 ppm; Hf 24.32 ppm–38.82 ppm), high contents of Th (143 ppm to 245 ppm) and U (311 ppm to 568 ppm) with Th/U ratios of 0.36 to 0.56 (Table S1). It is commonly enriched in light (L-) REE and depleted in HREE, and exhibits obvious LREE-HREE fractionation and no obvious Eu anomalies in the chondrite- normalized REE diagram (Fig. 4).

    Figure 4.  Chondrite-normalized REE pattern of Ttn2 of the garnet amphibolite. Normalization after Sun and McDonough (1989).

  • The peak pressure mineral assemblage of Grt+Omp-in+ Rt±Qz implies that the retrograde eclogite had experienced eclogite-facies metamorphism. Therefore, temperatures were calculated for pressures between 2.5 and 2.8 GPa (quartz stable eclogite-facies field) for the micron-sized rutile, using Zr-in-rutile geothermometer of Tomkins et al. (2007). Zirconium contents of 940 ppm to 1 421 ppm measured correspond to Zr-in-rutile temperatures of 823–869 ℃ at 2.5 GPa and 838–884 ℃ at 2.8 GPa (Table 2).

    In order to calculate the retrograde temperature, the Zr-in- titanite geothermometer of Hayden et al. (2008) was applied. As described above, Ttn1 occurs as a corona around rutile, indicating that Ttn1 was formed during the transition from eclogite-facies to granulite-facies metamorphism. Therefore, a corresponding pressure formation interval of 1.3 to 1.5 GPa was chosen to calculate the temperature. The measured Zr contents of 185 ppm– 481 ppm (Table 1) correspond to temperatures of 835–894 ℃ at 1.5 GPa and 812–870 ℃ at 1.3 GPa (Table 2).

    The Zr-in-titanite geothermometer of Hayden et al. (2008) was also used to calculate the formation temperature of coarse- grained Ttn2 in the garnet amphibolite at selected pressures between 1.0 to 1.2 GPa (granulite-facies). Quartz and ilmenite are present in the assemblage, and unit activities of both TiO2 and SiO2 are assumed. Accordingly, measured Zr contents of 198 ppm–273 ppm (Table S1) correspond to temperatures of 781–799 ℃ at 1.0 GPa and 804–823 ℃ at 1.2 GPa (Table S1).

  • Thirty LA-ICP-MS analyses of the coarse-grained titanites were performed on thin sections of the garnet amphibolite. Titanite U-Pb dating results are listed in Table S2. All the data spots on the Tera-Wasserburg plot give a lower intercept at 423±4 Ma (MSWD=2.7).

5.   DISCUSSION
  • In the retrograde eclogite, the idenfication of omphacite inclusions within garnet implies that the rock has experienced a peak eclogite-facies metamorphism with the mineral assemblages of Grt+Omp-in+Rt±Qz. Subsequently, during the retrogression process from eclogite-facies to granulite-facies metamorphism, partial melting of the eclogite occurred (Cao et al., 2017). Petrographic study shows that Ttn1 occurs as thin corona around Rt (Figs. 2e and 2f). Combined with the information related to the anatectic textures addressed in Cao et al. (2017) and mineral chemical compositions, Ttn1 is likely to be the product of a peritectic reaction between rutile and Ca-rich phases. This observation implies that Ttn1 is a replacement product of rutile, formed during partial melting. However, Rt and Ttn1 are too small to be dated using LA-ICP-MS, and the accurate time of anataxis can't be obtained from the present study. Melt produced by the sodic-amphibole and/or zoisite dehydration and break down would also release Ca and contemporaneous breakdown of rutile would result in the growth of titanite at the expense of rutile. Petrographical study shows that the coarse grained Ttn2 from the garnet amphibolite is subhedral to euhedral (Figs. 3c and 3d); it does not occur in retrograde eclogite and felsic vein. This titanite has high ΣREE contents and high Th/U ratios (0.36–0.56), and exhibits LREE enriched patterns without obvious Eu anomaly (Fig. 4). Previous studies show that magmatic titanites have variable but higher Th/U ratios and higher REE contents than metamorphic titanites (Spencer et al., 2013; Gao et al., 2012b; Storey et al., 2007). According to the occurrence and element compositions (Cao et al., 2017), Ttn2 should be newly formed from a Ti, Ca and LREE enriched anatectic melt. Thus, the dating result of Ttn2 can represent the age of crystallization from melt. U-Pb dating for the Ttn2 grains yields a lower intercept at 423±4 Ma (Fig. 5). That age would indicate that the partial melt derived from the eclogite started to crystallize at 423 Ma. This titanite U-Pb age is consistent with the granulite-facies metamorphic overprint (420 Ma) of the retrograde eclogite and the formation age (422 Ma) of the felsic vein (Fig. 6) which was investigated by zircon U-Pb dating, suggesting that Ttn2 formed simultaneously with the crystallization of the melt that forms the felsic veins (Cao et al., 2017) at HP granulite facies conditions. Moreover, numerous studies also suggested that in North Qaidam, felsic leucosomes derived from retrograde eclogite and gneiss recorded anatectic ages of 433–422 Ma (Yu et al., 2019 and references therein). These anatectic ages and the formation age of Ttn2 are younger than the UHP metamorphic ages, but both correspond to the granulite-facies metamorphic ages (Cao et al., 2017), indicating that partial melting of the subducted slab occurred during the exhumation from levels which reflect the peak UHP metamorphic stage to levels where the granulite-facies overprint took place (between 420–440 Ma).

    Figure 5.  U-Pb Tera-Wasserburg concordia diagram for Ttn2 in the garnet amphibolite.

    Figure 6.  Diagram showing the distribution of ages for the retrograde eclogite from Cao et al. (2017).

  • In numerous subduction zones, UHP metamorphic rocks became overprinted by various stages of retrogression (mostly granulite-facies and amphibolite-facies) during the exhumation stage (Cao et al., 2019a, b; Li Y et al., 2019; Zhang C et al., 2019, 2018, 2017, 2012; Wang L et al., 2018; Li P et al., 2017; Liao et al., 2016; Wang C et al., 2014; Zhu et al., 2014; Liu et al., 2012; Zhao et al., 2010). Cao et al. (2017) reported that the retrograde eclogite in this study has experienced peak eclogite-facies metamorphism at 440 Ma, granulite-facies retrogression at 420 Ma (Fig. 6), and amphibolite-facies overprint later than 420 Ma. These ages are consistent with peak metamorphic ages of 420–454 Ma and retrograde ages of 400–427 Ma for the HP/UHP rocks from the Lüliangshan unit (see Table S6 in Ren et al., 2016). In addition, Song et al.(2005b, 2004) identified evidence of UHP- metamorphism, suggesting deep continental subduction to mantle depths of more than 200 km. Combined results of previous zircon U-Pb ages indicate that the Lüliangshan unit has been subducted to > 200 km at 420–454 Ma, subsequently was exhumed to crustal depths (30–40 km) at 400–427 Ma before it finally returned to the earth surface. Therefore, an exhumation rate of 0.85–1.06 cm/yr was calculated for the UHP Lüliangshan unit, which is identical to that of the Xitieshan unit (1–2 cm/yr) in North Qaidam (Yu et al., 2019, 2014), the Dabie-Sulu UHP terrane (Liu and Liou, 2011) and other UHP terranes (Cao et al., 2019b; Liu et al., 2018; Kylander-Clark et al., 2008).

    Numerical modeling demonstrates that fast exhumation rates of UHP rocks to crustal levels (for instance in the range of 2–4.5 cm/yr) trigger the melting process (Sizova et al., 2012; Ellis et al., 2011). Previous studies have documented that if peak metamorphic temperature is high and the rocks follow a nearly isothermal exhumation, partial melting is expected to occur inside deeply subducted crustal slices during their exhumation (Chen et al., 2013b; Labrousse et al., 2011; Zheng et al., 2011; Hermann, 2002). Therefore, the decompression associated with rapid exhumation plays an important role for partial melting. Cao et al. (2017) studied a retrograde eclogite from the Lüliangshan Unit in North Qaidam which has experienced a "hot exhumation" overprint at HP granulite-facies conditions, indicating a heating and decompression-related melting process (Fig. 7 between the two red circles on dashed black exhumation path). However, since the minerals of the pre- and peak-HP/UHP metamorphic stages have been nearly totally transformed by the retrogression, it was not possible to determine the peak pressure conditions precisely. However, a partial melting process during the "hot exhumation stage" was suspected. In this study, coronitic titanite (Ttn1) is identified enveloping micron-sized rutile in the retrograde eclogite (Figs. 2e and 2f), indicating that Ttn1 is formed due to the breakdown of rutile during the process of partial melting. The temperature for the formation of rutile is estimated to be 823–884 ℃ at 2.5–2.8 GPa using Zr-in-rutile thermometer, while the growth of the Ttn1 envelope occurred at temperatures of 812–894 ℃ and pressures of 1.3–1.5 GPa (using Zr-in-titanite thermometer (Table 2)). The two temperatures are highly consistent, and both lie upon the wet melting curve of basalt (red line in Fig. 7). Combined with the information on the existence of UHP metamorphism in the Lüliangshan unit, an isothermal decompression path is inferred (Fig. 7, yellow arrow). In essence, the North Qaidam UHP terrane has experienced an exhumation rate of 0.85–1.06 cm/yr for the Lüliangshan unit and 1–2 cm/yr for the Xitieshan unit (Yu et al., 2014), which is similar to the exhumation rates of melt-bearing rocks in other UHP terranes. Therefore, the retrograde eclogite has experienced a fast exhumation with an isothermal decompression after the peak UHP metamorphic stage, primarily accounting for the partial melting. According to paragenesis, chemical composition and U-Pb dating results, coarse-grained titanite (Ttn2) in the garnet amphibolite is considered to be a newly crystallized phase from the anatectic melt that was formed simultaneously with the felsic veining (422 Ma) and HP granulite facies metamorphic event which the eclogite experienced (420 Ma) (Figs. 3a, 3b and 4). Temperatures of 781–823 ℃ at 1.0–1.2 GPa (Table S1) are calculated for the formation of Ttn2 using Zr-in-titanite thermometer, which represents the temperature of crystallization from melt. This temperature is identical to the temperature that prevailed during P-T conditions of the HP granulite-facies event (P=1.07–1.24 GPa, T=774–814 ℃). Therefore, as shown in Fig. 7, from Ttn1 to Ttn2, both decompressing and cooling occurred; the HP granulite-facies metamorphism and the felsic veining took place at the same time.

    Figure 7.  Re-modeling the metamorphic process, illustrating the partial melting of retrograde eclogite during decompression.

    Phase relations and metamorphic facies illustrating the stability of Ti-minerals in water-saturated MORB are after Peacock and Wang (1999) and John et al. (2011). The red line for the wet melting curve for basaltic systems are after Peacock et al. (1994).

  • In this study, numerous coarse-grained, euhedral Ttn2 are identified in the garnet amphibolite, rather than in the retrograde eclogite and the felsic vein. According to the data presented in Section 5.2 (Figs. 3a and 3b, Table 1), Ttn2 (as well as rutile) was newly formed from a Ti, Ca and LREE enriched anatectic melt during decompression, while zoisite was dehydrating. The newly formed Ttn2 suggests that the anatectic melt started to crystallize inside the garnet amphibolite before migrating and forming the felsic veins. During the process of the crystallization, abundant amounts of REE (LREE) were incorporated in Ttn2, resulting in an increase of the whole rock REE in the garnet amphibolite (see Figs. 5c and 5d in Cao et al., 2017) and a decrease of REE in the amphibole of the garnet amphibolite (see Fig. 7d in Cao et al., 2017). Therefore, the formation of Ttn2 plays an important role as to the whole rock REE content of the garnet amphibolite.

    Chemical composition shows that rutile from the retrograde eclogite contains abundant amounts of Zr (940 ppm–1 421 ppm) and Hf (526 ppm–924 ppm) (Table 1), which are higher than the whole rock Zr and Hf contents of the retrograde eclogite (Figs. 8a and 8c). Breakdown of rutile could liberate Zr and Hf to become accommodated by the melt. Zr and Hf contents of Ttn2 from the garnet amphibolite are obviously lower than those of rutile from the retrograde eclogite, but higher than the whole rock Zr and Hf of the garnet amphibolite (Figs. 8a and 8c). However, the whole rock Zr and Hf of garnet amphibolite are approximately equivalent with that of retrograde eclogite (Fig. 8c). In addition, Ttn2 shows higher Zr/Hf ratios than rutile (Fig. 8b), indicating that Ttn2 prefers to incorporate Zr over Hf. However, the whole rock Zr/Hf ratios of retrograde eclogite and garnet amphibolite are both higher than that of rutile and Ttn2 (Figs. 8b and 8d), respectively, suggesting that rutile and titanite cannot be the significant phases controlling the concentration of the Zr and Hf of the whole rock. Other phases, such as zircon and garnet, might contribute to the high whole rock Zr/Hf ratios.

    Figure 8.  Correlations of trace elements of Zr, Hf and their ratios for rutile and titanite from the retrograde eclogite and the garnet amphibolite, respectively.

6.   CONCLUSIONS
  • (1) The formation of rutile in eclogite occurred at temperatures of 823–884 ℃ at 2.5–2.8 GPa, while P-T conditions of 1.3–1.5 GPa and 812–894 ℃ were derived for the formation of titanite coronas (Ttn1) at the expense of rutile, using the Zr-in-rutile and Zr-in-titanite thermometers. The current results and earlier geochronological and petrological data indicate an exhumation rate of 0.85–1.06 cm/yr and an UHP nature of the Lüliangshan unit. The results on the retrograde eclogite from the North Qaidam document a fast isothermal decompression path from a peak pressure eclogite-facies to a granulite-facies stage. The fast exhumation and the breakdown of hydrous phases such as zoisite trigger the partial melting.

    (2) Coarse-grained titanites (Ttn2) in the garnet amphibolite were formed at 423±4 Ma, an age which is identical to the granulite-facies metamorphic age the retrograde eclogite experienced. The formation conditions of Ttn2 (781–823 ℃ at 1.0–1.2 GPa) are also consistent with the HP granulite-facies metamorphic conditions (P=1.07–1.24 GPa and T=774–814 ℃). During the decompression, at the granulite-facies stage, partial melts were crystallized at 423 Ma.

    (3) Due to the formation of REE-enriched Ttn2, the whole rock REE contents of the garnet amphibolite increased compared to the retrograde eclogite. Zr and Hf contents of rutile and Ttn2 are not proportional to the whole rock Zr and Hf contents of the garnet amphibolite, respectively, and the whole rock Zr/Hf ratios of retrograde eclogite and garnet amphibolite are both higher than the respective ratios of rutile and Ttn2, suggesting that rutile and titanite cannot be the only carriers of Zr and Hf accounting for the high whole rock Zr/Hf ratios.

ACKNOWLEDGMENTS
  • This study was supported by the National Natural Science Foundation of China (No. 41872053), the Natural Science Foundation of Shandong Province (No. ZR2019BD046), the Opening Foundation of the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources (No. J1901-16) and the State Key Laboratory of Continental Dynamics, Northwest University (No. 17LCD07), and China Geological Survey (No. DD20190376). We are grateful to Drs. Xiaoming Liu and Huadong Gong for their help with chemical and isotopic analyses at Northwest University, Xiʼan, China. Thanks are due to the editors and the two anonymous reviewers for their constructive comments that greatly helped to improve the manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1253-6.

    Electronic Supplementary Materials: Supplementary materials (Tables S1–S2) are available in the online version of this article at https://doi.org/10.1007/s12583-019-1253-6.

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