Zheng-bin Gou, Bao-di Wang, Dong-bing Wang, Zhi-min Peng

Chengdu Center, China Geological Survey, Ministry of Natural Resources, Chengdu 610081, China

Keywords:Garnet amphibolites Eclogite-facies Palaeo-Tethys Ophiolitic mélange Triassic Changning-Menglian suture zone Southeast Tibetan Plateau Geological survey engineering

ABSTRACT The garnet amphibolites from the newly identified Wanhe ophiolitic mélange in the Changning-Menglian suture zone (CMSZ) provide a probe to elucidate the evolution of the Triassic Palaeo-Tethys. An integrated petrologic, phase equilibria modeling and geochronological study of the garnet amphibolites,southeast Tibetan Plateau, shows that the garnet amphibolites have a peak mineral assemblage of garnet,glaucophane, lawsonite, chlorite, rutile, phengite and quartz, and a clockwise P-T path with a prograde segment from blueschist-facies to eclogite-facies with a peak-metamorphic P-T conditions of 2000-2100 MPa and 495-515°C, indicating a cold geothermal gradient of about 240-260°C/GPa. Theretrograde metamorphic P-T path is characterized by nearly isothermal decompression to lower amphibolite-facies and subsequent cooling to greenschist-facies. The metamorphic zircons have fractionated HREE patterns and significant negative Eu anomalies, and therefore the obtained zircon U-Pb age of 231 ± 1.5 Ma is interpreted to be the timing of the amphibolite facies metamorphism occurrence. The present study probably indicates that the garnet amphibolites in the Wanhe ophiolitic mélange was the retrograded highpressure eclogite-facies blueschist, instead of the previously proposed eclogites, and the garnet amphibolites recorded the subduction and exhumation process of the Palaeo-Tethys Oceanic crust in the Triassic.

1. Introduction

Ophiolites represent relicts of ancient oceanic crust and upper mantle that were incorporated into continental margins during collisions, ridge-trench interactions, and/or subductionaccretion events (Dewey JF and Bird JM, 1971; Dilek Y and Furnes H, 2011, 2014). They generally occur along suture zones either as large well-preserved ophiolite bodies, such as the Semail ophiolite that preserves a complete section from upper mantle to the uppermost pillow basalts (Goodenough KM et al., 2014; Searle M and Cox J, 1999), or as smaller fragments within accretionary complexes, such as the Franciscan Complex (Wakabayashi J, 2017a, 2017b). In later case, the ophiolite fragments are commonly associated with accretionary mélanges and high-pressure (HP) metamorphic rocks, i.e. blueschists and eclogites, which are most likely to have been scraped off from the downgoing plates (Cawood PA et al., 2009; Ernst WG, 2005; Wakabayashi J, 1999).Recovery of the metamorphic evolution for these HP rocks then becomes critical to enhance our knowledge of the associated suture zone and convergent plate margin processes(Agard P et al., 2009; Bhowmik SK and Ao A, 2016; Ernst WG, 2005; Wakabayashi J and Dilek Y, 2011). However,such HP rocks are commonly found as lenses or blocks in ophiolitic mélanges, which may have exhumed from different depths and indicate variableP-Tevolutions (Davis PB and Whitney DL, 2006; Klemd R et al., 2011). Thus,comprehensive studies are necessary to explore the HP metamorphism recorded in the ophiolitic mélanges. Another key problem is the lack of robust approach to determine the peakP-Tconditions of the widely distributed garnet blueschists and garnet amphibolites, which may result in the neglect or underestimation of their potential eclogite-facies metamorphism (Tian ZL and Wei CJ, 2014; Vitale Brovarone A et al., 2011; Wei CJ et al., 2010).

The Changning-Menglian suture zone (CMSZ) in the southeast Tibetan Plateau was the main branch of the eastern Palaeo-Tethys Ocean, together with the Longmu Co-Shuanghu suture zone (LSSZ) to northwest and the Inthanon-Bentong-Raub suture zone (IBRS) to south (Fang N et al.,1994; Li C et al., 2006; Metcalfe I, 1992, 1996, 2002, 2006,2013; Şengör AMC et al., 1984; Wu H et al., 1995). The CMSZ comprises typical lithological assemblage of a subduction-accretionary complex, including ophiolitic mélanges, shallow to deep marine sedimentary rocks,metamorphosed crustal complexes (e.g. Lancang group), arc volcanic rocks, and low-temperature and high-pressure (LTHP) metamorphic rocks (Fan W et al., 2015; Jian P et al.,2009a, 2009b; Metcalfe I, 2011; Peng T et al., 2008; Wang BD et al., 2013; Wang F et al., 2019a; Wang HN et al., 2019b,2020). It provides a rare opportunity to assess the HP metamorphism in the ophiolitic mélanges and its relation with the associated suture zones. In the CMSZ, blueschists and lawsonite-bearing eclogites have been reported as lenses or blocks in ophiolitic mélanges and/or the Lancang group (Chen GY et al., 2017; Fan W et al., 2015; Li J et al., 2017; Sun ZB et al., 2017a; Wang F et al., 2016, 2019a; Xu GX et al., 2016).The blueschists have been determined to yield the peakP-Tconditions of 400-950 MPa and 300-450°C, indicating a shallow underthrusting (Fan W et al., 2015; Wang F et al.,2016). A recent study has recovered a clockwiseP-Tpath from the lawsonite-bearing eclogites, with the peakP-Tconditions of 2400-2600 MPa and 520-530°C, which is inferred to form part of a cold oceanic subduction system(Wang HN et al., 2019b). By contrast, less attention has been paid on the widely distributed garnet amphibolites, which were interpreted as the retrograde products of the lawsonitebearing eclogites (Li J et al., 2015) and underwent extensive retrogression (Chen GY et al., 2017; Wang HN et al., 2019b;Xu GX et al., 2016). In this case, both garnet amphibolites and retrograde eclogites, occur as lenses or blocks in ophiolitic mélange, may have shared the sameP-Tevolutions.However, for those amphibolites without omphacite, the authors cannot confirm whether they have experienced eclogite-facies or amphibolite-facies metamorphism, which is critical to determine the scale of the high-pressure metamorphism in the studied area. Thus, further detailed work is necessary to assess this interpretation based on the investigation of the metamorphic evolution of the garnet amphibolites. Therefore, this paper combines petrology,mineral compositions, phase equilibria modeling, and geochronology to evaluate the metamorphicP-Tpath and time of the garnet amphibolites in the CMSZ and discusses the tectonic implications for the evolution of the Palaeo-Tethys Ocean, which could enhance our understanding on metamorphism recorded in ophiolitic mélange in CMSZ.

2. Geological setting

The CMSZ, extending southwardly to northwestern Laos and northwardly to the central Qiangtang, separates the Baoshan-Tengchong Block (BTB) to the west from the Lanping-Simao Block (LSB) to the east (Li C et al., 2006;Metcalfe I, 1996, 2002, 2011). The BTB comprises the Proterozoic low-grade to amphibolite-facies metamorphic basements and the overlying Paleozoic and Mesozoic sequences (Zhong DL, 1998; Jian P et al., 2009a; Metcalfe I,1996, 2002). The LSB is composed of the Proterozoic metamorphosed volcano-sedimentary rocks and the Upper Palaeozoic and Mesozoic sequences (Zhong DL, 1998). In this block, the Palaeozoic sedimentary sequence is represented by limestones and siliciclastic rocks, which shows similar lithology to the Yangtze Block (Fang N et al., 1998;Zhong DL, 1998). The Permian-Triassic volcano-plutonic complexes (basalts, andesites, dacites and rhyolites) along the western margin of the LSB (Fig. 1a) have the typical subduction-related arc volcanic geochemical affinity (Peng T et al., 2008). Their eruption or emplacement were dated at 286-249 Ma based on the zircon U-Pb dating of the andesite,basalt, gabbro and tonalite (Hennig D et al., 2009; Jian P et al., 2009a, 2009b; Peng T et al., 2008). These magmatic rocks probably formed during the subduction of the Paleo-Tethyan Ocean (Hennig D et al., 2009; Peng T et al., 2008). To the west of the volcano-plutonic complexes, the Lincang granite batholiths with the emplacement ages of 240-200 Ma intruded into the Proterozoic basements of the LSB (Fig. 1a),and have been proposed to form in post-collisional (Dong GC et al., 2013; Hennig D et al., 2009; Peng T et al., 2006, 2013;Wang F et al., 2014) or syn-collisional settings (Jian P et al.,2003; Peng T et al., 2008).

The CMSZ is consists of ophiolitic mélanges, shallow marine carbonates, radiolarian siliceous rocks,metamorphosed crustal complexes (Lancang group), arc volcanics, amphibolites, blueschists, and eclogites (Fan W et al., 2015; Jian P et al., 2009a, 2009b; Li J et al., 2015;Metcalfe I, 2011; Peng T et al., 2008; Wang BD et al., 2013;Xu GX et al., 2016). These lithological fragments constitute a typical subduction-accretionary complex (Cawood PA et al.,2009; Wakabayashi J, 1999), which is believed to represent the suture zone marking the closure of Paleo-Tethyan Ocean(Wang FN et al., 2019b). The blueschists in the CMSZ are commonly found as lenses or layers in mica schists (Lancang Group, Fig. 1a), exhibiting OIB-like geochemical affinity(Fan W et al., 2015; Wang F et al., 2016, 2019a), which are interpreted to be derived from a basaltic seamount (Fan W et al., 2015). The peak-metamorphicP-Tconditions of the blueschists were estimated at 500-900 MPa and 300-450°C(Fan W et al., 2015), followed by the late greenschist-facies retrograde metamorphism (Wang F et al., 2016). A magmatic protolith age of the blueschists was dated at 260 ± 4 Ma from the zircons with weakly oscillatory zoning and high Th/U ratios (Fan W et al., 2015). The blueschist-facies metamorphism was dated at 243-225 Ma through U-Pb dating of zircon rims and40Ar/39Ar plateau ages of phengite and glaucophane (Fan W et al., 2015; Wang F et al., 2019a).

Three major ophiolitic mélanges occur in the CMSZ,including the Niujingshan, Tongchangjie and Wanhe ophiolitic mélanges (Jian P et al., 2009a, 2009b; Liu GC et al.,2017; Sun ZB et al., 2017b; Wang BD et al., 2013). The Niujingshan ophiolitic mélange in the vicinity of Shuangjiang County (Fig. 1a) comprises mainly the dismembered harzburgites, pyroxenites, gabbros, diabase dikes and basalts in a strong sheared matrix of sandstone, siltstone and shale,with possible geochemical features of an SSZ-type ophiolite(Jian P et al., 2009a). A zircon U-Pb age of 267.1 ± 3.1 Ma was obtained from one metagabbro sample, which is interpreted as the time of the Permian sea-floor spreading above a subduction zone (Jian P et al., 2009a, 2009b). The Tongchangjie ophiolitic mélange (or known as Nantinghe ophiolite; Fig. 1b) are dominated by the dismembered amphibolites, actinolitites, schists, serpentinized olivinepyroxenites, cumulative gabbros, meta-gabbros and metabasalts (Wang BD et al., 2013). They are proposed to represent the Ordovician oceanic crust based on the zircon UPb ages of 473-439 Ma from the cumulate gabbros (Wang BD et al., 2013) and 449.3 ± 8.4 Ma from one OIB-like basalt(Sun ZB et al., 2017b). The recent identified Wanhe ophiolitic mélange (Fig. 1b) is composed of phengite-bearing quartz schists, greenschists, tonalites, plagioclasites, cumulate gabbros, amphibolites, and minor retrograde eclogites (Liu GC et al., 2017). One zircon U-Pb age of 470.8 ± 5.3 Ma has been obtained from one cumulate plagioclasite, indicating a record of the Ordovician oceanic crust (Liu GC et al., 2017).Some scattered Ordovician-Silurian subduction-related arc volcanic rocks (basalt, andesite, and dacite) were reported in the CMSZ, which are proposed to form during the subduction of Proto-Tethyan Ocean (Nie X et al., 2015; Wang DB et al.,2016). The reported lawsonite-bearing eclogites occur as lenses or blocks in mica schists in the Wanhe ophiolitic mélanges (Fig. 1b), and record a clockwise metamorphicP-Ttpath (Wang HN et al., 2019b). Their protolith and metamorphic ages have been determined to be 451 ± 3 Ma and 246-245 Ma based on the U-Pb dating of magmatic and metamorphic zircons, respectively (Wang HN et al., 2019b).The igneous precursors of these eclogites show similar geochemical features to the OIBs and E-MORBs (Sun ZB et al., 2017a; Wang HN et al., 2019b), which are inferred to be originated from a basaltic seamount in an intra-oceanic tectonic setting (Wang HN et al., 2019b).

Fig. 1. a-Geological map of the Sanjiang area in the southeastern Tibetan Plateau, with an inset map showing tectonic framework of southeastern Asia (modified from Wang HN et al, 2019b). N. Q.-North Qiangtang terrane; S. Q.-South Qiangtang terrane; CMSZ-Changning-Menglian suture zone; LSSZ-Longmu Co-Shuanghu suture zone; IBRS- Inthanon-Bentong-Raub suture zone; JASS-Jinshajiang-Ailaoshan-Song Ma suture. b-Sketch geological map of the Changning-Menglian orogen showing the location of the lawsonite-bearing eclogites and the studied samples (modified from Sun ZB et al, 2017a). 1-Quaternary sediments; 2-Jurassic sequences; 3-Upper Triassic sediments; 4-western Late Paleozoic metasedimentary rocks; 5-eastern Late Paleozoic metasedimentary rocks; 6-Early Paleozoic metamorphic complex (Lancang group);7-Precambrian metamorphic basement; 8-Eocene syengranite; 9-Middle Triassic biotite monzonitic granite; 10-Permian granodiorite;11-Tongchangjie ophiolitic mélange; 12-Wanhe ophiolitic mélange; 13-lithologic boundary; 14-unconformable boundary; 15-fault;16-sample locations (red stars: this study; yellow stars: lawsonite-bearing eclogites).

3. Methodology

3.1. Analytical methods

3.1.1. Bulk rock and mineral compositions

About 500 g of rock for each sample was crushed and powdered for chemical analysis using standard methods.Bulk-rock compositions were analyzed at the Southwest China Supervision and Inspection Center of Mineral Resources, Ministry of Land and Resources. Major oxides were determined by X-ray fluorescence (XRF; AXIOS-X) for which the analytical precision is generally better than 1%.Loss on ignition (LOI) was determined by placing 1 g sample in the furnace at 1000°C for several hours before cooling in a desiccator and re-weighing.

Mineral compositions were analyzed using a JEOL JXA 8230 electron microprobe (EMP) at the School Earth Sciences, China University of Geosciences, Wuhan. The operating conditions were 15 kV accelerating voltage, a 20 nA beam current and count time of 10 s for peak and background.The beam diameter was set to 5 µm. Natural or synthetic standards were used with ZAF corrections.

3.1.2. Zircon isotope and trace element analysis

Zircon was separated from sample 15SJ-25 using conventional techniques. U-Pb isotope and trace element of zircon were conducted synchronously by LA-ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan.Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as those in Liu YS et al. (2008, 2010a, 2010b). Ablation was performed using an Excimer 193 nm GeoLas 2005 Laser System with spot sizes of 32 µm. An Agilent 7500a ICP-MS instrument was used to acquire ion-signal intensities. Helium was used as a carrier gas and argon as the make-up gas, which was mixed with the carrier gas via a T-connector before entering the plasma. Nitrogen was added to the central gas flow (Ar + He) of the Ar plasma to decrease the detection limit and improve precision (Hu ZC et al., 2008). Each analysis incorporated a background acquisition of approximately 20-30 s (gas blank) followed by 50 s of data acquisition from each sample, using the Agilent Chemstation.Off-line selection and integration of background and analytic signals, and time-drift correction and quantitative calibration for trace element analyses and U-Pb isotope data were performed by ICPMSDataCal (Liu YS et al., 2008, 2010a).

Zircon 91500 was used as an external standard for U-Pb isotope analysis and was analyzed twice every 8 analyses.Time-dependent drifts of U-Th-Pb isotope ratios were corrected using a linear interpolation (with time) for every eight analyses according to the variations of 91500 (i.e., 2 zircon 91500 + 8 samples + 2 zircon 91500; Liu et al., 2010a).Preferred U-Th-Pb isotopic ratios used for 91500 are those from Wiedenbeck M et al. (1995). Uncertainties on the preferred values for the external standard 91500 were propagated through to the results. Concordia diagrams and weighted mean age calculations were made using a python script UPbplot.py (Noda A, 2017), in which the weighted mean algorithm of McLean NM et al. (2011) is applied. Dates are considered concordant if the analysis is no more than 10%discordant (Spencer CJ et al., 2016). Trace element compositions of zircon were calibrated against the external standard (NIST610) combined with internal standardization(Liu YS et al., 2010a).

3.2. Phase equilibria modeling

P-Tpseudosections were calculated in the MNCKFMASHTO (MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O) system. The calculations were performed using the software GeoPs 2.1 (Xiang H, 2020) with the internally consistent thermodynamic dataset, ds62, of Holland TJB and Powell R (2011). The algorithms employed in this software are similar to Perple_X (Connolly J and Kerrick D,1987; Connolly J and Petrini K, 2002; Connolly JAD, 1990,2005), in which the predicted assemblages is strictly based on Gibbs free energy minimization. The authors adopt the most recent available activity-composition models of White RW et al. (2014b) for garnet, chlorite and biotite, Green ECR et al.(2016) for clinopyroxene and clinoamphibole, White RW et al. (2014a) for white micas (muscovite and paragonite),Holland TJB and Powell R (2011) for epidote, Holland TJB and Powell R (2003) for plagioclase, and Holland TJB and Powell R (1998) for talc. The fluid phase is considered to be pure H2O and set in excess. Albite, coesite, kyanite,lawsonite, quartz, rutile, and titanite are regarded as pure endmember phases.

The bulk-rock compositions obtained by XRF analysis for calculating the pseudosections were normalized in the MnNCKFMASHTO system where the O [Fe3+/(Fe3++ Fe2+)]value is estimated by adding up the Fe3+content calculated based on charge balance for each constituent mineral (Tian ZL and Wei CJ, 2013, 2014). P2O5was disregarded as it mostly enters accessory apatite, and CaO accounting for apatite was subtracted from the bulk-rock compositions. MnO is considered in calculation because of its critical role in stabilizing garnet under lower pressures and temperatures(Gaidies F et al., 2008; Marmo BA et al., 2002). However, as garnet is zoned, and its growth may lead to fractionation of the bulk-rock compositions (Evans TP, 2004; Marmo BA et al., 2002; Tinkham DK and Ghent ED, 2005), the analyzed compositions by XRF may not be appropriate for modeling the metamorphic stages related to garnet rim growth. Thus, an effective bulk composition was generated following the method of Evans TP (2004) by subtracting garnet core compositions from the analyzed bulk-rock composition,which is applied to model the peak and post-peak metamorphism.

4. Bulk rock and mineral compositions

The garnet amphibolites are collected from the Wanhe ophiolitic mélange in the CMSZ (Fig. 1b). They occur as elongated lenses or blocks within metapelites (Figs. 2a, b).The garnet amphibolites have SiO2of 48.44%-54.78%, Al2O3of 13.87%-14.98%, MgO of 4.05%-6.69%, Na2O of 2.72%-3.17% and K2O of 0.54%-1.17%, belonging to basalt or andesibasalts according to Total Alkali-Silica (TAS)diagram (Supplementary Table S1; Le Bas MJ et al., 1986).Among them, one sample 15SJ-25 with higher SiO2(54.78%)and lower MgO (4.05%) was selected for zircon U-Pb dating.

Fig. 2. Outcrop photographs and photomicrographs of the garnet amphibolites and metapelites from the Changning-Menglian suture zone.a-Elongated garnet amphibolites enclosed in metapelites. b-Blocks of garnet amphibolites. c-Garnet porphyroblast and matrix minerals of amphibole, plagioclase and titanite in garnet amphibolites; the porphyroblastic garnet is partially replaced by chlorite along its rim; sample 15SJ-28. d-Garnet porphyroblast with inclusions of quartz, lawsonite, epidote, amphibole and rutile in garnet amphibolites. The yellow line marked in garnet grain shows the location of zoning profile in Fig. 3b; sample 15SJ-28. (e) Medium-grained amphibole with core-rim structure and fine-grained amphibole-plagioclase symplectites in garnet amphibolites; sample 15SJ-28. f-Garnet porphyroblast in the matrix consisting of amphibole, plagioclase and titanite in garnet amphibolites; sample 15SJ-25. The subscripts “c” “r” “m” and “s” refer to the textural position“core” “rim” “matrix” and “symplectite”. The classification of amphiboles can be found in Figs. 4a, b. Mineral abbreviations are after Whitney DL and Evans BW (2010): Ab-albite; Act-actinolite; Amp-amphibole; Bt-biotite; Chl-chlorite; Coe-coesite; Cpx-clinopyroxene; Di- diopside; Ep-epidote; Gln-glaucophane; Grt-garnet; Hbl-hornblende; Ky-kyanite; Lws-lawsonite; Ms-muscovite; Omp-omphacite; Pl-plagioclase; Pg-paragonite; Qz-quartz; Rt-rutile; Ttn-titanite.

The garnet amphibolites are mainly composed of garnet(5 vol.%-15 vol.%), amphibole (45 vol.%-55 vol.%),plagioclase (25 vol.%-30 vol.%), titanite (3 vol.%-5 vol.%),with minor chlorite (1 vol.%-3 vol.%), rutile (about 1 vol.%)and quartz (about 1 vol.%). Garnets generally occur as porphyroblasts in the matrix of amphibole, plagioclase and titanite (Fig. 2c), and are partly replaced by chlorite along their rims (Figs. 2c, d). The porphyroblastic garnet contains individual inclusions of amphibole, lawsonite, epidote,chlorite, rutile and quartz, and multiple-phase inclusions of amphibole + quartz and lawsonite + epidote (Fig. 2d). The compositional ranges of garnet areXFe= 0.36-0.70,XCa=0.21-0.29,XMg= 0.03-0.09 andXMn= 0.00-0.37 [XFe= Fe2+/(Fe2++ Mn + Mg + Ca),XCa,XMgandXMndefined similarly](Table S2). X-ray mapping images and microprobe traverse show that the garnet porphyroblast exhibits significant compositional zoning (Fig. 3a). TheXMndecreases sharply from core (0.37-0.22) to rim (0.07-0.00) (Fig. 3b), forming a“bell-shaped” profile, typical of garnet with growth zoning as described by Spear FS (1993). By comparison, the garnet has a slight increase inXMg(0.03 to 0.09) and an abrupt increase inXFe(0.36 to 0.70) from core to rim; a slight decrease inXFe(0.70 to 0.62) is observed in the garnet rim (Fig. 3b). By contrast, theXCashows no distinct change from core to rim(Fig. 3b).

Fig. 3. a-X-ray mappings of garnet porphyroblast in the garnet amphibolites; b-compositional profiles of garnet porphyroblast from the garnet amphibolites. XFe = Fe2+/ (Fe2+ + Mn + Mg + Ca), XCa, XMg and XMn defined accordingly.

Amphibole occurs in two different texture domains, with medium- to coarse-grained amphibole (Ampc-r) in the matrix(Fig. 2e), and fine-grained amphibole (Amps) in symplectite(Figs. 2d-e). The matrix amphiboles are commonly subhedral to anhedral and exhibit core-rim structure in BSE images (Fig.2e), while the amphiboles in symplectites are anhedral and intergrow with plagioclase (Figs. 2c, e). As shown in Fig. 4a,the matrix amphibole cores (Ampc) are rich in Si and Mg#[Mg/(Mg + Fe2+)], belonging to actinolite according to nomenclature of Leake BE et al. (1997). By comparison, the matrix amphibole rims (Ampr) have lower Si and Mg#, can be classified as magnesio-hornblende, ferro-hornblende, ferrotschermakite, pargasite and ferro-pargasite (Figs. 4a, b; Table S2). The compositions of amphibole in symplectites overlap with those in the matrix, including actinolite, magnesiohornblende and ferro-pargasite (Fig. 4a; Table S2).Plagioclase also occurs in two texture domains, with mediumgrained plagioclases (Plm) in the matrix (Figs. 2c-e), and finegrained one (Pls) in amphibole-plagioclase symplectites(Fig. 2e). They contain comparable compositional ranges with anorthite components of 1%-35%, including andesine,oligoclase, and albite (Fig. 4c; Table S2). The matrix plagioclase is dominated by albite. Minor phengite occurs in the matrix and has high Si of 3.27-3.58 pfu (Fig. 4d; Table S2).

5. P-T pseudosections

TheP-Tpseudosection modeled for sample 15SJ-28 using the analyzed bulk-rock composition is dominated by quadriand quini-variant fields with a few tri- and hexa-variant fields(Figs. 5a, b). The pseudosection is contoured forXMgandXCaisopleths of garnet and Si of phengite (Fig. 5b). In the “lawin” fields of lawsonite-bearing mineral assemblage, theXMgisopleths show steep slopes withXMgincreasing with the increase of temperature, and theXCaisopleths have flat slopes withXCadecreasing with the increase of pressure. By comparison, in the epidote-bearing assemblage fields, theXCaandXMgisopleths have similar slopes withXCadecreasing andXMgincreasing with increase of pressure and temperature. The Si isopleths of phengite have moderate positive slopes, and increase with increasing pressure.

Fig. 4. a-Si (p.f.u.) vs. Mg/(Mg + Fe2+) and b-(Na + K)A vs. Mg/(Mg + Fe2+) diagrams of amphibole in the garnet amphibolite (after Leake BE et al., 1997). c-Ab-An-Or diagram of plagioclase in the garnet amphibolites. d-Mg + Fe2+ vs. Si (p.f.u.) diagram of phengite in the garnet amphibolites. The subscript “c” “r” and “s” refer to core, rim and symplectite.

The mineral assemblage in sample 15SJ-28, containing Grt + Amp + Chl + Ep + Lws + Qz + Rt + Qz + Ms, is stable underP-Tconditions at about 1300-2100 MPa and 430-540°C. But, theXCaandXMgisopleths of the garnet rim intersect at a higher pressure field with a lawsonite-bearing and epidote-absent mineral assemblage of Chl + Gln + Grt +Lws + Ms + Rt + Qz + Ms (white-filled circles in Fig. 5b).TheXCaandXMgisopleths of the garnet core intersect at lower temperature and pressure conditions (red-filled circles in Fig. 5b) than those of the garnet rim. These intersection spots from both the garnet core and rim form a nearly isobaric heatingP-Tpath from about 440°C at about 1800 MPa to 470°C at 1900 MPa (Fig. 5b).

Fig. 5. a, b-P-T pseudosection for the garnet amphibolite sample 15SJ-28 calculated in the system MnNCKFMASHTO using the bulk-rock composition normalized based on XRF analysis: SiO2 = 56.70 (mol%), Al2O3 = 9.70, CaO = 5.31, MgO = 10.49, FeO = 10.25, K2O = 0.80,Na2O = 3.32, TiO2 = 1.75, MnO = 0.30, O = 1.38; c, d-P-T pseudosection calculated with an effective bulk composition generated by subtracting garnet core compositions from the above analyzed bulk-rock composition: SiO2 = 57.57 (mol%), Al2O3 = 9.47, CaO = 5.02, MgO = 10.88,FeO = 9.55, K2O = 0.84, Na2O = 3.51, TiO2 = 1.85, MnO = 0.01, O = 1.30. Tri-, quini-, hexa- and septi-variant fields are increasing heavily shaded. The pseudosections are contoured with isopleths of XMg and XCa in garnet and Si in phengite. The red- and white-filled circles represent intersections of XMg and XCa isopleths of garnet core and rim, respectively. The yellow lines with arrow represent the inferred prograde P-T paths. The mineral symbols are shown in Fig. 2

TheP-Tpseudosection modeled with the effective bulk composition of sample 15SJ-28 is also dominated by quadriand quini-variant fields with a few tri- and hexa-variant fields(Figs. 5c, d). Compared with the pseudosection of Fig. 5a, the topologies are not changed significantly except for the garnetbearing assemblage stability fields with relatively high pressure and temperature conditions (Fig. 5c). The positions ofXMgandXCaisopleths of garnet are approximately the same in the two sets of pseudosection (Figs. 5b, d). The Si isopleths of phengite are nearly the same in both the pseudosections(Figs. 5b, d). The metamorphic conditions and progradeP-Tpath were reconstructed using the intersections of isopleths in the pyrope (XMg= 0.03-0.09) and grossular (XCa= 0.21-0.28)contents of the garnet.XCaisopleths in the field of Chl + Gln +Grt + Lws + Ms + Rt + Qz + Ms show a flat positive slope,withXCadecreasing as pressure increases. The isopleths ofXMgshow consistent moderate negative slope and a positive correlation with temperature. The garnet rim composition with lowestXCa(0.21) yields the highestPcondition of about 2100 MPa at 495°C, while the composition of the garnet rim with the highestXMg(0.09) yields the highestTcondition of about 515°C at 2000 MPa. These twoP-Tconditions are consistent within uncertainties of the isopleths (about 100-200 MPa and 8-10°C). Thus, the peak-metamorphicP-Tcondition of sample 15SJ-28 is determined to be 2000-2100 MPa and 495-515°C (Figs. 5b, d), which is roughly consistent with that constrained by the Si (3.3-3.6) of phengite (Figs. 5b,d; Table S2).

As shown in Fig. 5c, plagioclase is stable at low-P(＜950-1100 MPa) and high-T(＞460°C) region, albite is stable at low-P (＜800-950 MPa) and low-T(＜475 °C) region.These two phases are present in both matrix and symplectite in the garnet amphibolites. Considering the high peakmetamorphic temperature of 495-515°C, the plagioclase may have formed at early stage, and albite at late stage. In addition, biotite with stability conditions of ＜ 700-900 MPa and 400-520°C is absent in the studied samples (Fig. 5c).Thus, the authors infer that, after the peak metamorphism, the garnet amphibolites experienced a nearly isothermal decompression to the stability fields of plagioclase-bearing mineral assemblages and a subsequent cooling to the fields of albite-bearing and biotite-absent assemblages (Fig. 5d).

6. Zircon geochronology

Zircon U-Pb isotope and trace element compositions of sample 15SJ-25 are presented in Table S3. Zircons (80-100 µm)display anhedral to subhedral morphology generally with rounded shapes. Most grains show core-rim structure in cathodoluminescence (CL) images (Fig. 6a). The cores show dark luminescence without zoning or with weak spongy zoning (Fig. 6a), indicating a metamorphic recrystallization(Corfu F et al., 2003; Wu Y and Zheng Y, 2004). The rims,which show brighter luminescence, probably reflect lead loss resulting from an interaction with fluids after the metamorphic recrystallization (Liati A et al., 2002), but they are too narrow (＜20 µm) to analyze. The cores have relatively high Th/U ratios (0.38-2.84; Table S3), variable REE contents (197×10-6-677×10-6, av. 466×10-6), and fractionated HREE patterns with significant negative Eu anomalies (δEu=0.072-0.638; Fig. 6b).

Fifteen spots were analyzed on the cores of zircons. The four analytical spots with discordant U-Pb ages are excluded from further consideration (blue dashed circles in Fig. 6c). Of the eleven concordant analyses, ten spots form a single population with a weighted mean206Pb/238U age of 231.0 ±1.5 Ma (MSWD = 3.7; Fig. 6d), while the remaining one yields a youngest206Pb/238U date of 220.5 ± 2.2 Ma which lies outside of analytical uncertainty of the main population(Fig. 6d).

7. Discussion

7.1. Metamorphic P-T-t path of the garnet amphibolites

Based on the observed petrographic characteristics and phase equilibria modeling, the metamorphic evolution process of the garnet amphibolites can be divided into three stages:(I) prograde to peak stage, (II) post-peak nearly isothermal decompression stage, (III) late cooling stage (Fig. 7).

The prograde to peak stage that was constrained based on the growth zoning of the garnet porphyroblast by phase equilibria modeling recorded aP-Tpath with the increase of both pressure and temperature from blueschist facies to eclogite facies (Fig. 7). The peakP-Tcondition was at 2000-2100 MPa and 495-515°C based on the highestXMgand/or lowestXCavalues of garnet rim. The predicated peakmetamorphic mineral assemblage is Chl + Gln + Grt + Lws +Qz + Rt + Ms. However, the actinolite and epidote inclusions observed in the garnet porphyroblasts are inconsistent with the predicted mineral assemblage. The actinolite is probably the retrograde product of the former glaucophane, as previously reported in the Dabieshan low-Teclogite (Lou YX et al., 2009; Wei CJ et al., 2010). In addition, epidote may also be the retrograde product of the former lawsonite inclusion (Fig. 2d), which has widely reported in the lawsonite-bearing blueschists and eclogites (Tsujimori T et al., 2006; Wang HN et al., 2019b; Wei CJ and Clarke GL,2011). The estimated peakP-Tcondition and progradeP-Tpath are consistent with a cold apparent thermal gradient of 240-260°C/GPa (Fig. 7), similar to the geotherm for subduction of cold oceanic lithosphere (Peacock SM, 2003).A similar progradeP-Tpath has been reported in the lawsonite-bearing eclogites in the adjacent areas of the CMSZ(Figs. 1b, 7; Wang HN et al., 2019b).

Fig. 6. a-Cathodoluminescence images of zircon from the garnet amphibolite sample 15SJ-25, showing locations of analyzed spots and relevant ages (in Ma). b-chondrite-normalized REE patterns of zircons (data of chondrite after Sun SS and McDonough WF, 1989). c-U-Pb concordia diagram of zircons. d-diagram of zircon 206Pb/238U ages. The red and blue circles/lines refer to concordant and discordant dates, respectively. The orange circle/line indicates the date excluded from the weighted mean age calculation.

The post-peak metamorphism is characterized by nearly isothermal decompression based on the appearance of plagioclase-bearing and biotite-absent mineral assemblage and it stabilityP-Tconditions of about 900-1000 MPa and 490-540°C. The decompressionP-Tpath involves three substages: (1) The early decompression path passes the narrow fields containing both lawsonite and epidote, and induces a dehydration reaction Grt + Lws = Chl + Gln + Ep +H2O, which is consistent with the replacement of lawsonite inclusions by epidote in the garnet (Fig. 2d) and the partial consumption of garnet rim by chlorite (Fig. 2d). Breakdown of lawsonite to form epidote has been widely reported in LT eclogites and blueschists (Vitale Brovarone A and Beyssac O,2014; Wei CJ and Clarke GL, 2011), including the lawsonitebearing eclogites in the CMSZ (Wang HN et al., 2019b).(2) Decompression proceeds into Ca-rich amphibole-bearing fields at about 1100 MPa, probably resulting in retrogression of glaucophane to Ca-rich amphibole (e.g. actinolite), which is in accordance with the occurrence of moderate-grained actinolite (Ampc) in the matrix (Fig. 2e). (3) Further decompression leads to the appearance of plagioclase that was observed in the matrix (Figs. 2c, e). The symplectites of amphibole-plagioclase in the matrix are also predicted to occur during this substage.

The late cooling stage is characterized by the occurrence of albite-bearing and biotite-absent assemblage, which are modeled to stable atP-Tconditions of about 800-900 MPa and ＜470°C, indicating an overprint of greenschist facies metamorphism (Fig. 7).

The normalized REE patterns of zircon cores are characterized by enriched HREE with significant negative Eu anomalies (Fig. 6b), which are generally interpreted as zircon growth associated with garnet resorption in the presence of feldspar (Kohn MJ and Kelly NM, 2018; Rubatto D, 2017).According to phase equilibria modeling result (Fig. 5c),coincidence of garnet consumption and plagioclase growth is consistent with the decompression stage at about 900-1000 MPa and 490-540°C. Thus, the authors interpret the weighted mean206Pb/238U age of 231.0 ± 1.5 Ma (MSWD = 3.7; Fig. 6d)obtained from zircon cores as the timing of lower amphibolite facies metamorphism during retrogression (Fig. 7). It is noted that the zircons contain unusual high Th/U ratios (0.38-2.84).A similar case has been observed in the metamorphic zircons(Th/U = 0.84-2.26) from the LT-HP eclogites in the LSSZ(Zhai QG et al., 2017). An important control on the zircon Th/U ratios is the coeval growth of Th-rich phases such as monazite and allanite which are the dominant reservoir of Th.The formation of these minerals result in low Th/U ratios of zircon in eclogites, amphibolites and granulite-facies rocks(Rubatto D, 2017; Yakymchuk C et al., 2018). However,monazite is not a common mineral in the HP rocks (Rubatto D, 2017), and is absent in the studied garnet amphibolites. In addition, allanite and/or epidote are not observed in both the garnet amphibolites in this study and the LT-HP eclogites in the LSSZ (Zhai QG et al., 2017), although they are present in many metamorphic rocks with low zircon Th/U ratios (＜0.1)(Rubatto D, 2017). Furthermore, the epidote is modeled to be absent in the lower amphibolite-facies conditions of about 900-1000 MPa and 490-540°C (Fig. 5c). Thus, the high Th/U ratios of zircon are result of the lack of Th-rich minerals in the studied garnet amphibolites. The lower amphibolite-facies metamorphic age (231.0 ± 1.5 Ma) is younger than the proposed peak eclogite facies metamorphic ages of 245 ± 2 Ma and 246 ± 2 Ma that were obtained by zircon U-Pb dating in lawsonite-bearing eclogite in the adjacent areas of CMSZ(Wang HN et al., 2019b). Therefore, the authors infer that the exhumation of the garnet amphibolites may last at least 15 Ma,indicating an apparent exhumation velocity of about 3.5 mm/a.

Fig. 7. Summary of the metamorphic P-T-t path of the studied garnet amphibolites, the eclogites and blueschists from the Changning-Menglian and Longmu Co-Shuanghu suture zones. The major phase stability fields and boundaries of metamorphic facies follow Brown M (2009). The transition line of lawsonite and epidote is after Wei CJ and Clarke GL (2011), which is calculated based on the phase equilibria modeling of typical MORB composition. A-amphibolite facies; AE-amphibole eclogite facies; AEE-amphibole-epidote eclogite facies; ALE-amphibolelawsonite eclogite facies; BS-blueschist; E-HPG-medium-temperature eclogite-high-pressure granulite metamorphism; G-granulite facies;GS-greenschist facies; LM-low grade metamorphism; D09, Dong YS and Li C, 2009; F15, Fan W et al., 2015; W16, Wang F et al., 2016;W19b, Wang HN et al., 2019b; Z06, Zhang KJ et al. (2006); Z09, Zhai QG et al., 2009; Z11, Zhai QG et al., 2011b; Z18, Zhang YX et al.,2018.

The garnet amphibolites were previously interpreted as the retrograde products of the eclogites in the same ophiolitic mélange (Chen GX et al., 2017; Li J et al., 2017; Wang HN et al., 2019b). Although omphacite has not been identified in the studied samples, lawsonite is indeed found as inclusions in garnet (Figs. 2d, 3a), which indicates that the hosting rocks have experienced the HP metamorphism (Tsujimori T et al.,2006; Whitney DL and Davis PB, 2006). As summarized above, the garnet amphibolites probably experienced a progradeP-Tevolution from blueschist-facies to eclogitefacies, followed by a clockwise retrogradeP-Tpath through the amphibolite-facies to greenschist-facies (Fig. 7). ThisP-Tpath has a similar shape to those of the lawsonite-bearing eclogites but runs at lower pressure and temperature conditions than those of the retrograde eclogites reported by Wang HN. (2019b) (Fig. 7). The peak assemblage of the garnet amphibolites is modeled to be Chl + Gln + Grt + Lws +Qz + Rt + Ms, which is likely a typical blueschist assemblage.Moreover, several studies reported that the garnet blueschists could share the same peak eclogite-facies metamorphic conditions with the coexisting eclogites (Gómez-Pugnaire MT et al., 1997; Li JL et al., 2012; Schliestedt M, 1986; Tian ZL and Wei CJ, 2014; Vitale Brovarone A et al., 2011). Thus, the authors interpret that the garnet amphibolites in the Wanhe ophiolitic mélange were the retrograde products of eclogitefacies garnet blueschists rather than eclogites, which is in good accordance with the absence of omphacite in most garnet amphibolites. This conclusion indicates that the lenses or blocks in ophiolitic mélange are most likely to be exhumed from different depth, although they have experienced eclogite-facies metamorphism.

7.2. Tectonic implication

The CMSZ, located in the southeast Tibetan Plateau, is an important suture zone that marks the tectonic boundary between the Sibumasu continent and the Yangtze Plate (Fang N et al., 1998; Jian P et al., 2009a; Li C et al., 2006; Metcalfe I, 1996, 2002, 2011; Zhong DL, 1998). The CMSZ contains the Palaeozoic ophiolitic mélanges, shallow-deep water sedimentary rocks, meta-supracrustal rocks, subductionrelated arc-like volcanic rocks and LT-HP metamorphic rocks(Fan N et al., 2015; Jian P et al., 2009a, 2009b; Metcalfe I,2011; Peng T et al., 2008; Wang F et al., 2013), which constitute a typical subduction-accretionary complex(Cawood PA et al., 2009; Wakabayashi J, 1999). In the newly identified Wanhe ophiolitic mélange (Liu GC et al., 2017), the garnet amphibolites occur as lenses or blocks, and are consist mainly of garnet, amphibolite, and plagioclase. The present study shows that the garnet amphibolites have experienced eclogite-facies metamorphism with a cold apparent thermal gradient of 240-260°C/GPa (Fig. 7). The zircon U-Pb ages and REE patterns indicate that a retrograde lower amphibolite-facies metamorphism of the amphibolites occurred at 231 ± 1.5 Ma. The lawsonite-bearing eclogites in the same ophiolitic mélange yield a similar clockwiseP-Tpath and cold apparent thermal gradient of 210-220°C/GPa(Fig. 7), with an eclogite-facies metamorphic age of 246-245 Ma (Wang HN et al., 2019b). The protolith age (451 ±3 Ma) of the lawsonite-bearing eclogites (Wang HN et al.,2019b) is broadly consistent with the emplacement age of the cumulate plagioclasite (470.8 ± 5.3 Ma) in the Wanhe ophiolitic mélange (Liu GC et al., 2017) and those of cumulate gabbros (473-439 Ma) and OIB-like basalt (449.3 ±8.4 Ma) in the nearby Tongchangjie ophiolitic mélange (Sun ZB et al., 2017b; Wang F et al., 2013). In these cases, the authors propose that the garnet amphibolites and the lawsonite-bearing eclogites, represent the former Ordovician oceanic crusts, have experienced synchronous subduction in a cold thermal regime but reached to the different depths (about 60 km and 75 km) during the middle Triassic (246-245 Ma),which were followed by the relatively fast exhumation (about 3.5 mm/a) and the subsequent cooling during the late Triassic(231 Ma). This interpretation is consistent with the occurrence of the subduction-related Permian-Triassic volcano-plutonic complexes (286-249 Ma; Hennig D et al., 2009; Jian P et al.,2009a, 2009b; Peng T et al., 2008) and the syn/postcollisional Triassic Lincang granite batholiths (240-200 Ma;Dong YS et al., 2013; Hennig D et al., 2009; Jian P et al.,2003; Peng T et al., 2006, 2008, 2013; Wang F et al., 2014)along the western margin of the LSB (Fig. 1a).

The Triassic LT-HP eclogites occur as lenses or blocks within schists and marbles in the Longmu Co-Shuanghu suture zone (LSSZ) that is located in the western-central Qiangtang Block (Fig. 1a; Dong YS and Li C, 2009; Pullen A et al., 2008; Zhai QG et al., 2009, 2011a, 2011b; Zhang KJ et al., 2006). The eclogites have a peak mineral assemblages of Grt +Omp + Rt + Ms ± Lws ± Gln, and a clockwiseP-T-tpath with the peakP-Tconditions of 2000-2500 MPa and 425-520°C(Fig. 7; Dong YS and Li C, 2009; Zhai QG et al., 2009,2011a, 2011b; Zhang KJ et al., 2006), and are roughly consistent with that of the eclogite-facies rocks in the CMSZ(Fig. 7). The eclogite-facies metamorphic ages have been dated at 244-233 Ma by the garnet Lu-Hf isochron dating(Pullen A et al., 2008) and 237-232 Ma by the U-Pb dating of zircons that contain micro-inclusions of garnet, omphacite,phengite and rutile (Zhai QG et al., 2017), although Dan W et al. (2018) argued the zircon U-Pb dates represented the protolith age of the eclogites. This age range (244-232 Ma) is consistent with that (246-231 Ma) of the eclogite-facies rocks in the CMOB (this study; Wang HN et al., 2019b). Moreover,both the CMSZ and LSSZ eclogite-facies rocks exhibit EMORB- and OIB-type geochemical affinity (Dan W et al.,2018; Sun ZB et al., 2017a; Wang HN et al., 2019b; Zhai QG et al., 2011a). An ophiolitic mélange in the LSSZ is contact in fault with the country rocks of the eclogites, which comprises the serpentinites, gabbros, basalts and minor plagiogranites(Zhai QG et al., 2016). In this mélange, the zircon U-Pb dating of gabbros and plagiogranites yields the magmatic ages between 501 Ma and 437 Ma, which is overlapped with that of the fragments in the CMSZ ophiolitic mélanges that contain eclogite-facies rocks (473-439 Ma; Liu GC et al.,2017; Sun ZB et al., 2017b; Wang F et al., 2013). In these cases, the ophiolitic mélanges and the associated eclogitefacies rocks are comparable in the CMOB and LSSZ in field occurrence, geochemical features and formation ages. Thus,the authors suggest that these two suture zones represent the main convergent boundary in the Palaeo-Tethys domain between the Sibumasu terrane derived from Gondwana and the IndoChina-Yangtze Block derived from “Asiatic Hunic superterrane” (Metcalfe I, 1992, 1996, 2002, 2006, 2011,2013; Şengör AMC et al., 1984; Wu H et al., 1995).

Along the eastern extension of the LSSZ near Baqing County, the Triassic MT-HP eclogites with a peak assemblage of Grt + Omp + Rt + Ms + Qz were recently reported to yield a minimum pressure of 2500 ± 100 MPa at 730 ± 60°C(Zhang YX et al., 2018). However, the peakP-Tconditions of the two eclogite samples are estimated based on the thermobarometry of one single Grt-Omp-Ms pair for each sample (Zhang YX et al., 2018). More calculations based on multiple Grt-Omp-Ms pairs from each sample are recommended to get a more reliable peakP-Tcondition,although the final results may not be significantly changed.The U-Pb dating on zircons with micro-inclusions of garnet,omphacite, rutile and phengite yields the MT-HP eclogitefacies metamorphic age of 227-221 Ma (Jin X et al., 2019;Zhang YX et al., 2018), which is younger than the LT-HP eclogite-facies metamorphic ages from the CMSZ (246-245 Ma;Wang HN et al., 2019b) and the LSSZ (244-232 Ma; Pullen A et al., 2008; Zhai QG et al., 2017). This indicates that the Baqing MT-HP eclogites may be derived from the continental subduction which was following the oceanic subduction represented by the LT-HP eclogites in the CMSZ and LSSZ.This interpretation is consistent with the continental arc basalt characteristics of the protoliths of the Baqing eclogites (Jin X et al., 2019; Zhang YX et al., 2018).

8. Conclusions

(i) The garnet amphibolites in the CMSZ have a peak mineral assemblage of garnet, glaucophane, lawsonite,chlorite, rutile, phengite and quartz.

(ii) Phase equilibria modeling shows that the garnet amphibolites underwent peak-metamorphism under conditions of 2000-2100 MPa and 495-515°C and a clockwiseP-Tpath with a retrograde process of nearly isothermal decompression to lower amphibolite-facies and subsequent cooling to greenschist-facies.

(iii) Zircon U-Pb dating results indicate that the amphibolite facies metamorphism occurred at 231 ± 1.5 Ma.

(iv) This study shows that the garnet amphibolites in the Wanhe ophiolitic mélange was the retrograded high-pressure eclogite-facies blueschist, instead of the previously proposed eclogites, and providing a new insight into subduction and exhumation process of the Palaeo-Tethys Oceanic crust in the Triassic.

CRediT authorship contribution statement

Zheng-bin Gou carried out the experiment and wrote the manuscript with support from Bao-di Wang. Dong-bing Wang and Zhi-min Peng fabricated the samples. Bao-di Wang conceived the original idea and helped supervise the project.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgment

This work was financially supported by the National Natural Science Foundation of China (41802071, 41773026 and 41303028) and the Geological Survey of China-Nepal railway (202008000000180117). The authors thank two anonymous reviewers for constructive and critical reviews that significantly helped to improve the manuscript.

Supplementary dataset

Supplementary dataset(Table S1, Table S2, and Table S3)to this article can be found online at doi: 10.31035/cg2021010.