# Architecture and P-T-deformation-time evolution of the Chinese SW-Tianshan HP/UHP complex: Implications for subduction dynamics

Zhou Tan, Philippe Agard, Patrick Monie, Jun Gao, Timm John, Léa Bayet, Tuo Jiang, Xin-Shui Wang, Tao Hong, Bo Wan, et al.

► To cite this version:

Zhou Tan, Philippe Agard, Patrick Monie, Jun Gao, Timm John, et al. Architecture and P-T-deformation-time evolution of the Chinese SW-Tianshan HP/UHP complex: Implications for subduction dynamics. Earth-Science Reviews, 2019, 197, pp.102894. 10.1016/j.earscirev.2019.102894 hal-02445527

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**Architecture and P-T-deformation-time evolution of the**

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**Chinese SW-Tianshan HP/UHP complex: implications for**
**subduction dynamics**

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**Key words:** UHP/HP metamorphism; P-T-deformation-time histories; $^{40}$Ar-$^{39}$Ar in situ laser ablation analyse; Juxtaposition; Cold subduction regime; Subduction dynamics; Chinese SW-Tianshan;

**Authors:** Tan, Zhou<sup>a,b,c,*</sup>, Agard, Philippe<sup>b,e,*</sup>, Monié, Patrick<sup>f</sup>, Gao, Jun<sup>c,d</sup>, John, Timm<sup>g</sup>, Bayet, Léa<sup>g</sup>, Jiang, Tuo<sup>h</sup>, Wang, Xin-Shui<sup>c</sup>, Hong, Tao<sup>c</sup>, Wan, Bo<sup>a,d</sup>, Caron, Benoît<sup>b</sup>

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a. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, 100029 Beijing, China

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b. Sorbonne Université, CNRS-INSU, Institut des Sciences de la Terre de Paris, ISTE P UMR 7193, F-75005 Paris, France

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c. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, 100029 Beijing, China

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d. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

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e. Insitut Universitaire de France, F-75005 Paris, France

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f. Geoscinces Montpellier, UMR5243, Université Montpellier 2, CNRS, Place E. Bataillon, F-34095 Montpellier CEDEX 05, France

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g. Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstr. 74-100, D-12449 Berlin, Germany

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h. Laboratory of Isotope Geochemistry, Wuhan Centre of China Geological Survey, 430205 Wuhan, China

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*Corresponding author:

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**Tan, Zhou**, tanzhou@mail.iggcas.ac.cn; **Agard, Philippe**, philippe.agard@upmc.fr;

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30 **Abstract**

We present the first comprehensive P-T-deformation-time and kinematic constraints for HP/UHP eclogite, blueschist and greenschist-facies metavolcano-sedimentary rocks cropping out in the Chinese SW-Tianshan metamorphic complex (within the ~30 km wide N-S Akeyazi-Kebuerte area). We reappraise this HP/UHP “mélange”, which should be divided into three main tectonic units, from north to south, according to their discrepant lithologies and P-T-time-deformation histories.

These three units crop out in a tectonic window beneath greenschist-facies metavolcanics. P-T estimates point to 1) UHP-LT conditions around 2.6-2.9 GPa at ~520 °C for the metavolcano-sedimentary rocks of the northern HP/UHP unit, 2) HP-LT conditions of 1.8-2.1 GPa and ~500 °C for the central blueschist horizon and 3) lower-blueschist facies conditions of 1.0-1.5 GPa at ~485 °C for the southern ultramafic-mafic unit. In situ laser probe Ar-Ar age constraints (with textural control) on recrystallized phengites from the HP/UHP unit cluster within 315 to 325 Ma. Phengite ages from the central blueschist horizon are 10-15 Ma older, at ~325-345 Ma. Laser-ICP-MS U-Pb dating on zircon from the south ultramafic-mafic unit yield ages around ~360 Ma for metamorphic overgrowths. In contrast, step-heating Ar-Ar phengite age constraints for the greenschist-facies metavolcano-sediments fall within 280-300 Ma. These new field and P-T-time data disclose an episodic exhumation of three main tectonic slices, respectively from ~85 km, ~65 km and ~45 km depths, and a progressive change of metamorphic gradients from ~12 to ~6-7 °C/km, which could reflect a cooling of the subduction system with time. Final juxtaposition at ~20 km was probably achieved around 300 Ma, prior to collision.

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# 1. Introduction

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Processes responsible for the occasional recovery of ocean-derived high- to ultra-high pressure low temperature rocks (HP-UHP/LT; Agard et al., 2009) are essential to constrain subduction dynamics and gain insights on the meter- to hectometer-scale constitution of the plate boundary (Agard et al., 2018; Bachmann et al., 2009; Doglioni et al., 1998, 2007; Stöckhert, 2002 and references therein). The diversity of subducted material, either from the downgoing plate (generally the uppermost section, i.e. pelagic sediments with basaltic oceanic crust and/or seamounts) or the upper plate (i.e., ashes from arc-volcanism, detrital sediments from the accretionary wedge and/or basement material, if basal erosion occurs), is likely to influence viscosity, density and thus buoyancy, and to control chemical and mechanical behaviors within the plate interface (Bayet et al., 2018; Bebout, 2007, 2018; John et al., 2010; Yamato et al., 2007). Regardless of its origin, this subducted material is indeed stripped from the slab as mélanges (e.g., Blanco-Quintero et al., 2011; Wakabayashi, 2011, 2015) or tectonic slices (e.g., Angiboust et al., 2013; Bayet et al., 2018; Bonnet et al., 2018; John et al., 2010; Locatelli et al., 2018, 2019) and later juxtaposed during exhumation.

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To document processes operating in the 80-100 km depth range, where a significant change in large-scale mechanical coupling between the plates is suspected (e.g., Wada and Wang, 2009; Syracuse et al., 2010; van Keken et al., 2011; Agard et al., 2018), the Chinese South Tianshan Metamorphic Belt (STMB) is a potentially important example. It is characterized by an unusually thick (~5 km) pile of well-preserved HP/UHP metavolcanoclastics (Bayet et al., 2018) wrapping oceanic eclogite boudins/slices with numerous coesite findings (e.g., Bayet et al., 2018; Lü & Zhang, 2012; Tan et al., 2017).
However, despite numerous studies providing detailed petrological, geochemical evolutions or geochronology data (e.g., Gao et al., 2007; John et al., 2008; Klemd et al., 2011; Tan et al., 2017, 2018; van der Straaten et al., 2012 and references therein), the internal structures and tectonic evolution of the STMB remain poorly constrained. Why is it so thick so deep, and what can we learn from this exceptional example across a depth range critical for mechanical coupling (Agard et al., 2018; Bayet et al., 2018)? Whereas Bayet et al. (2018, 2019) describes in a comprehensive study the details of the lithological and structural-petrological evolution of the internal coesite-bearing UHP domain, the present study provides for the first time a detailed, fully coupled tectono-metamorphic-radiometric study across the whole STMB, combining a composite ~30 km long North-South cross-section with systematic average PT estimates and in-situ Ar-Ar dating (together with zircon U-Pb and Ar-Ar step-heating measurements) for each of the individual domains to provide a picture of the evolution of the area based on radiometric constraints tightly linked to specific deformation stages. Results allow to discuss the conditions of recovery, exhumation rates and implications for subduction processes along with regional geodynamics.

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## 2. Geological Setting of SW-Tianshan Accretionary Complex

The Chinese Southwestern Tianshan high- to ultrahigh-pressure low-temperature (HP/UHP-LT) accretionary complex constitutes the eastern part of the Kazakhstan collage system (Gao et al., 2017; and references therein) within the Altaids (also called the Central Asian Orogenic Belt; Fig. 1a). It extends for about 200 km along the Southwestern Central Tianshan Suture Zone (SCTSZ, Fig. 1b), and correlates with the Atbashi (Hegner et al., 2010; Loury et al., 2018; Simonov et al., 2008),
105 Chatkal (Loury et al., 2016; Mühlberg et al., 2016) and Fan-Karategin (U)HP
106 metamorphic complexes (Volkova and Budanov, 1999) in the Kyrgyzstan part of the
107 South Tianshan Orogenic Belt (STOB). The SCTSZ bounds to the north the Chinese
108 section of the HP-UHP/LT metamorphic complex, known as the Akeyazi
109 metamorphic complex (AMC, Fig. 1c). This contact, now a ~0.5 km-wide dextral
110 strike-slip shear zone, was active from the Early Permian to Early Triassic (Gao &
111 Klemd, 2003; Laurent-Charvet et al., 2003). To the north lies a LP-HT Palaeozoic
112 active continental margin (Allen et al., 1993; Gao et al., 1998; Klemd et al., 2015),
113 mainly made of amphibole- and granulite-facies rocks, along with Late Silurian and
114 Early Carboniferous continental arc-type volcanic, volcaniclastic rocks and granitoids
115 (Gao et al., 2009a; Xia et al., 2014). To the south the AMC is overlain by
116 unmetamorphosed and/or low grade Palaeozoic sedimentary strata, which also
117 represent the northern, passive continental margin of the Tarim plate (Carroll et al.,
118 1995).

119 The AMC is predominantly composed of strongly schistosed metavolcanoclastics
120 hosting mafic metavolcanics as pods, lenses, boudins, thin layers or massive blocks
121 (Gao & Klemd, 2003) and marble horizons (Bayet et al., 2018; Gao and Klemd,
122 2003), as well as very rare ultramafic rocks (<< 5 vol%; Gao et al., 1999; Li et al.,
123 2007). Whole-rock geochemical data (Fig. 3b) for the mafic metavolcanic rocks
124 suggest oceanic basalt to arc-related affinities (e.g., Gao and Klemd, 2003; John et
125 al., 2008; Liu et al., 2014; Meyer et al., 2016; Tan et al., 2017, 2018) while the
126 metasediments, often mixed with volcanoclastic material, have characteristics similar
127 to upper continental crust, proximal trench and, more rarely, distal pelagic sediments
128 (e.g., Bayet et al., 2018, 2019; Liu et al., 2014; Meyer et al., 2016; Tan et al., 2017).

129 Previous work mostly focused on individual HP/UHP rocks/outcrops for the
reconstruction of P-T-(time) histories (e.g., Li et al., 2016; Lü et al., 2012; Tan et al., 2017), fluid-rock interaction processes and/or geochemical budgets (e.g., Beinlich et al., 2010; John et al., 2012; van der Straaten et al., 2008). The compilation of regional P-T (time) constraints (Figs. 2a, 3a, c, Table S4; also see Tan et al., 2017) indicates that most peak metamorphic estimates for eclogite and prograde blueschist yield eclogite facies (EC facies) (U)HP-LT conditions within the range 480-580 °C and 1.5-3.0 GPa. A range of P-T conditions ~450-570 °C at 2.7-3.3 GPa was obtained for some EC facies metavolcanoclastics. Evidence for UHP metamorphism comes from both coesite relics in garnet (from several localities: stars in Figs. 1c, 3, 4, 5) and thermodynamic calculation. The spread of regional P-T estimates (Figs. 2, 3; also see Tan et al., 2017, their Figure 13) likely arises from contrasting assumptions for thermodynamic modeling (e.g., difficulties in determining effective bulk rock composition, different solution models and Fe³⁺ content and/or assessing H₂O activity) and/or from the complexity of metamorphic evolutions in individual HP-UHP tectonic slices (Bayet et al., 2018; Tan et al., 2017). However, for a more regional perspective Bayet et al. (2018, 2019) provide a comprehensive P-T data set for region between the Atantayi and the Akesayi rivers, which shows that the entire central (ultra-high) high-pressure unit reached pressures around 2.5 ± 0.2 GPa at temperatures clustering around 536 ± 12 °C, indicating spatial uniform PT conditions around quartz-coesite transition (Fig. 2b, Table S4; Data No. 33).

Available age data (Fig. 3d, Table S5) and corresponding Pmax estimates (Tan et al., 2017, their Figure 2) evidence a considerable spread in ages, with largely overlapping ages for eclogites and blueschists (whatever the protolith; Fig. 3d). Tan et al. (2017), based on the detailed P-T-time-isotopic history of a representative coesite-bearing eclogitic slice (Fig. 2b), suggested that the timing of peak HP/UHP
metamorphism falls within 315 ± 5 Ma, thus Late Carboniferous time. This concords
with some previous works (e.g., Gao et al., 2009a; Klemd et al., 2005, 2011; Su et
al., 2010; Yang et al., 2013) and points to a rather restricted range with respect to a
~100 Ma-long lasting subduction system (Gao et al., 2009a, 2009b). Some
controversy exists, however, to the east of SCTSZ, in the Beishan arc terrane of
South Tianshan Orogenic Belt: some workers propose that accretion may have
lasted until the Early Triassic (Tian et al., 2014), while others suggest a Permian
collision (e.g., Xiao et al., 2004). Regional ages <280 Ma (e.g., Zhang et al., 2007)
or > 325 Ma (e.g., Gao et al., 1995, 2000, 2006) were previously considered as
either resulting from intrinsic limitations of isotopic dating (e.g., excess Ar, Nd
disequilibrium in Sm-Nd system; difficulty to link zircon U-Pb ages to precise
metamorphic stages) or taken as evidence for distinct HP-UHP episodes (Tan et al.,
2017). When set back against the global compilation (e.g., Agard et al., 2009, 2018;
Guillot et al., 2009), the STMB sits somewhere, in terms of both P-T record and
lithological composition, between ocean-derived and continental-derived subduction
fragments (Fig. 2c).

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172 3. Structural Data

Structural information for the SW-Tianshan Metamorphic Belt is scarce (Gao et al., 1999), with only few recent reports for the Akeyazi Metamorphic Complex (Bayet et al., 2018, 2019; Lin et al., 2008; Soldner et al., 2017; Tan et al., 2017) and for the nearby Kekesu valley (~60km to the east, Fig. 1c. Gao et al., 1995). Based on the new investigations in this study and previous regional structural works, a revised structural map is proposed for the entire area (Figs. 4a-d).

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## 3.1 Structural organization of the Akeyazi Metamorphic Complex along the Kebuerte valley

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An approximately 30 km-long N-S cross-section was constructed along the Kebuerte valley (Figs. 4d, 5a, d). It cuts across the whole South Tianshan Orogenic Belt, namely, from north to south: the south margin of Central Tianshan continental arc, the AMC, and the unmetamorphosed to low grade Palaeozoic sedimentary strata bounded by the North Haerkeshan Peak Fault (NHF). A recently published cross-section through the region between the Atantayi and Akesayi valleys (Bayet et al., 2018) is shown for comparison (Fig. 5e).

In the study area (i.e., Kebuerte valley), the AMC shows a metamorphic dome structure (Fig. 5d) that can be further divided into six sub-units bounded by hm-/km-scale contacts (Figs. 6, S1) on the basis of petrological, lithological and structural characteristics (e.g., Figs. 5-7):

1) Unit GS1: this ~3 km-wide GS facies unit (Figs. 5a, d, 6a, S1a) separates the CTB continental arc from the HP/UHP exposures. It consists of strongly schistosed chl-phengite-biotite-quartz schist (Figs. S2d, S3a) and actinolite-epidote-chlorite-quartz/albite schist (Figs. 8g, S3b), with few garnet-bearing, significantly retrogressed metapelites (Fig. 9h), m-/hm-scale marble horizons and low-grade metamorphosed and deformed magmatic intrusions (~420 Ma, Pu et al., 2011). Mylonitic horizons are found in places;

2) Unit UH: It represents an example of deeply buried, uppermost oceanic crust covered by a km-thick coesite-bearing sequence dominated by trench infill metasediments with variations of volcanoclastic and pelitic components (Bayet et al., 2018, 2019). It is predominantly composed of strongly schistosed meta-pelitic/-psammitic volcanoclastics (Figs. 8a-b, 9a, S3c) hosting mafic metavolcanics as
pods, lenses, boudins, thin layers or massive blocks (e.g., Fig. 8a; Gao et al. ; Bayet et al 2018; Soldner et al., 2017; Tan et al., 2017) as well as some marble horizons (Fig. 5e; Bayet et al., 2018). It corresponds to the so-called HP/UHP metamorphic 'mélange' (e.g., Klemd et al., 2011; Li et al., 2016. Fig. 4b), host to most previous P-T-time(-fluid) works (Figs. 3c-d; Tables S4-S5) and coesite reports (Figs. 1c, 4a, d; Bayet et al., 2018; Lü et al., 2009; Lü & Zhang, 2012; Tan et al., 2017; Yang et al., 2013). This unit crops out along this North-South profile for ~9 km (Figs. 4a, d, 5a, d) and is here separated by two normal shear sense faults (Figs. 5, 6a-b, S1a) from the GS1 unit to the north and an EC/BS facies blueschist unit to the south. Noteworthily, Bayet et al. (2018, 2019) identified several internal, km-scale individual tectonic slices in this unit (Figs. 4a, d, 5e).

3) Unit EB: this ~4 km-large EC/BS facies unit (Figs. 4a, d, 5a, d-e) is located in the central and lowermost portion of the AMC on this transect. It lies structurally below the UH unit to the north (Fig. 6b), and is overthrust on its southern side by a meta-mafic/-ultramafic unit (MU, Fig. 6c) through a GS facies contact zone. It is characterized by relatively homogeneous, moderately schistosed blueschist horizons (Figs. 8c-d, 9b-c) hosting lenses and/or pillow-shaped eclogite/blueschist blocks (Fig. 8c).

4) Unit MU: this unit comprises metamorphosed mafic and ultramafic rocks (Fig. 6c), which is the first report of meta-ultramafic rocks within the AMC. Similar lithologies have so far been reported only in the Changawuzi-Muzaerte valley (~40 km to the west, Fig. 1c; Gao et al., 1999; Li et al., 2007). To the north, the meta-ultramafic rocks are closely associated with a GS facies, weakly schistosed phengite-chlorite-quartz/albite schist shear zone (Figs. S2e, S3d) and overthrust by a ~1 km-wide metabasic slice to the south (Figs. 6c, 8e, 9e-g). The mutual
relationships between the MU and UH units (both overlie the EB unit) cannot be ascertained due to glaciar coverage (Figs. 5a, 7a, c), but the attitude of schistosity and aerial views suggest that MU overlies UH.

5) Unit GT: this ~5km-wide GS facies transitional zone (Figs. 5a, d, 6d, S1d) is located at the southern side of the MU. The boundary between GT and MU could be more complex than the simple contact drawn on Figure 5d. GT is overthrust by a strongly retrogressed meta-mafic slice (Fig. S3g), which could represent the southern margin of the MU or a local small sub-unit. The GT unit is bounded by a km-scale normal shear zone to the south (Figs. 6d, S1b-c). The GT mainly consists of moderately to strongly schistosed GS facies chlorite-phengite-quartz schist ± garnet relics or pseudomorphs (Figs. 8f, 9i, S2g-h, S3g-i), with few blueschist blocks previously reported close to its northern margin (Fig. 3, Gao et al., 2000);

6) Unit GS2: this unit shows lithologies and structural patterns very similar to those of the GS1 unit, with mainly strongly deformed phengite-chlorite-albite/quartz (Figs. 8h, S1c, S3j) and phengite-biotite quartz schists (Figs. 5a, d, 6d). It is bounded to the south by a major EW-trending fault, the North Haerkeshan Peak Fault (NHF; Figs. 5a, d, 6d, S1b, d).

## 3.2 Deformation stages and deformation patterns

Deformation stages are herein essentially defined based on textures and mineralogy. The main schistosity (noted as $S_2$ in the north of the study area by Soldner et al., (2017) and as $S_n$ in adjacent valley by Bayet et al. (2019)) most commonly corresponds to a blue amphibole bearing, blueschist facies schistosity ($S_1$) is only seldom preserved in eclogitic or eclogite to blueschist facies boudins, in UH or EB units for example, Fig. S2a; Soldner et al., 2017; Tan et al., 2017). This
schistosity is in fact composite, depending on the extent of retrogression and
exhumation-related deformation, and may encompass a range of P-T conditions,
from late eclogite to incipient blueschist and even down to a transitional blueschist to
greenschist facies. A detailed mapping of regional schistosity by facies should in
principle be feasible but was not performed here, in part because eclogite and/or
blueschist facies schistosities may correspond to diachronous events in the different
sub-units. Crenulation of the dominant schistosity is also evidenced in several
exposures (as in Bayet et al., 2018) and more strongly developed in the GS1 and
GS2 units. For the sake of comparison with thin-sections and radiometric constraints,
deformations are hereafter termed D<sub>e</sub> (eclogite-), D<sub>b</sub> (blueschist-) and D<sub>g</sub>
(greenschist-facies). White micas analyzed in situ by laser ablation (for Ar-Ar dating)
were denoted S if following the schistosity and C along shear planes.

The dominant schistosity and various stretching lineations are reported in Figures
1c, 5a-c, and 7. The schistosity varies from place to place but dominantly strikes
NW-SE (Figs. 5a-c, 7), whatever the lithologies. Some differences are observed for
the GS1 and GS2 units, where schistosity is more E-W (Figs. 5a-c, 7).

Eclogite to blueschist facies lineations are indicated by elongated omphacite or
glaucophane-quartz and/or phengite, respectively (Figs. 8a-d, 13b, S2a-c, S4). They
cluster along a NNW-SSE strike at ~N160 (Figs. 5a-c, 7). By contrast, GS facies
stretching marked by oriented crystals of amphibole, quartz or albite (and more
rarely phyllosilicates, i.e. phengite, chlorite, or biotite; Figs. 8f-h, 9i, 13b, S2d-e, g-h,
S3a-b, d, h-j, S4) lies dominantly along a NEE-SWW trend, around N~040 (Figs. 5a-
c, 7). GS facies lineations (Figs. 8g-h, S1c, S3b, j) observed close to the north and
south boundaries of the AMC are nevertheless aligned along a more N-S trend (Figs.
5a-c, 7).
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In addition to recumbent folding and flattening (e.g., Figs. S2b-c), regional deformation is characterized by non-coaxial shear movements, as exemplified by pervasive S-C structures (Figs. 8, 9i, 13b, S2-S4) and asymmetric folding (although less conspicuous in the EB unit). Shear senses associated with EC/BS and GS facies metamorphism, whether at outcrop scale or in thin-section, are very consistent throughout the area, and respectively top to NNW and top to NEE (Fig. 5a-c).

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## 4. Sampling strategy and sample description

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Sample selection (Figs. 4a, 5) was intended at providing samples for average PT comparison between the different sub-units (rather than detailed individual trajectories by thermodynamic modelling or conventional and Raman-based thermobarometry; e.g., Tan et al., 2017), ideally for each of the metamorphic equilibration stages, seeking enough textural control on deformation and/or recrystallisation stages that in-situ Ar-Ar dating could be used to provide age constraints with deformation control. Sampling strategy, geographic coordinates and parageneses are given in Table 1. A brief sample description is given below:

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— UH unit:

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Sample #135 is a strongly schistosed pelitic metavolcanoclastic rock hosting an eclogite lens (Fig. 8a) with well-preserved eclogitic paragenesis of Grt-Gln-LwsPs-Omp-Ph-Qz-Rt (abbreviations after Table 1; Figs. 9a, 13b) and clear top to N shear sense as indicated by the orientations of glaucophane, phengite and quartz along the schistosity and the shear bands (Figs. 13b, S4). Omphacite was detected both as inclusion in garnet (Fig. 9a) and as relict in the Gln+Qz matrix. Fresh lawsonite was not found but lawsonite pseudomorphs around in garnet (made of Ep+Pg, they are referred to as LwsPs in the following; Fig. 9a). This sample represents the host

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rock of the coesite-bearing HP/UHP eclogite (Figs. 3-5; Tan et al., 2017) and is taken
here to represent D<sub>e</sub>.

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Sample #26 is a strongly schistosed felsic metavolcanoclastic rock with Grt-Ph-Qz-LwsPs-Chl-Rt (Fig. S3c) sampled close to the coesite-bearing felsic metapelite of Yang et al. (2013) (Figs. 3-5). Top to N sense of shear is indicated by stretched recrystallized quartz (Fig. 8b) and phengite oriented along the schistosity and shear band (Fig. S4). As for sample #135, no 'fresh' undecomposed lawsonite was detected but only pseudomorphs in garnet (Ca-rich Grt+Pg, Fig. S3c). It is also representative of the metavolcanoclastic matrix host rock of the UH unit.

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Retrograde overprinting is generally characterized by a GS to Lower-BS facies
Ttn-Chl±Brs±Carb mineral assemblage (Figs. 9a, S3c).

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— EB unit:

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Sample #117 is a strongly to moderately schistosed pelitic metavolcanoclastic rock
hosting some eclogite lenses (Fig. 8c) with Grt-Gln-Ph-Qz-Rt assemblage (Figs. 9b,
S4) and relatively obvious top to N shear sense (Fig. S4) as indicated by
glaucophane, and minor quartz aligned along the schistosity. It is located close to the
northern margin of the EB unit and is representative of the lithology of the blueschist
horizon.

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Sample #32 is a strongly schistosed pelitic metavolcanoclastic rock with Grt-Gln-Qz-LwsPs-Ph-Ctd-Rt assemblage (Figs. 9c, 13b, S4) and clear top to N shear sense again (Figs. 13b, S4). No fresh lawsonite was detected but only pseudomorph in garnet (Ep+Pg; Fig. 9c). It is located in the central part of the EB unit (Fig. 8d). Similar to the UH unit, retrograde overprinting is marked by the Ttn-Chl-Brs±Bi mineral assemblage (Figs. 9b-c).

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— MU unit:
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Sample #157 is a moderately to weakly schistosed felsic metavolcanoclastic rock with GS facies Ph-Chl-Ab-Qz assemblage (Figs. S3d, S4). Shear sense indicators are mostly top to NE (Fig. S2e). While located at the northern margin of the MU unit and associated with meta-ultramafic rocks, this horizon could also represent a second order tectonic contact between the EB and MU units (Fig. 6c). Both in situ and step-heating <sup>40</sup>Ar-<sup>39</sup>Ar radiometric analyses were performed for this sample.

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Sample #156-1 is a moderately to strongly schistosed metamafic/ultramafic schist with Mg-rich Chl-Ttn assemblage (likely a metasomatic chloritite, Fig. S3e). It is associated with the serpentinite slice in the MU unit as vein or thin layers (Fig. S2f).

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339

Sample #156-2 is a meta-ultramafic (rodingite?) vein with Grs-Di-Chl assemblage (Fig. S3f). It locally crosscuts the serpentinite slice reported in the MU unit (Fig. S2f).

340

341

Sample #156-3 is the main representative lithology of the serpentinite slice in the MU unit, with Atg-Di assemblage (Fig. 9d). It is strongly schistosed but no sense of shear can be observed (Fig. S2f). As for the other two former samples, only microprobe work was performed for this sample.

342

343

344

345

Sample #158 is more mafic than typical samples from the EB unit. It was sampled next to probably former pillows (Fig. 8e). It shows a complex mineral assemblage with Grt-Czo-Gln-Ph-Omp (+minor Qz, Figs. 9e-g). Elongated euhedral actinolite is wrapped by a relatively anhedral glaucophane matrix, and both are replaced by later barroisite. Phengite and omphacite are rare and only found as relicts in the matrix (Figs. 9f-g) or as inclusion in epidote. Ttn-Ab-Chl is the overprinting mineral assemblage. Zircon separation was performed for LA-ICPMS trace elements and U-Pb measurements for this sample.

346

347

348

349

350

351

352

353

— GT unit:

354

Sample #147 is a weakly schistosed, relatively mafic rock with a Grt-Ph-Pg-Chl-
355

Ab-Qz-Ep assemblage (Fig. S3g). The existence of relictual glaucophane implies it
probably experienced lower BS facies metamorphism prior to the regional GS facies
overprinting marked by Bi+Chl largely replacing garnet/phengite (Fig. S3g). No clear
shear sense could be found on the outcrop, and only $ ^{40}Ar-^{39}Ar $ step-heating was
performed for this sample.

356

357

358

359

360

Sample #148-1 is a strongly schistosed felsic metavolcanoclastic rock with Grt-Ph-Chl assemblage (Figs. S3h, S4). Top to NEE sense of shear is observed as indicated by stretched recrystallized quartz vein (Fig. S2g) and mica "fish" (Figs. S3h, S4). It is located in the central portion of the GT unit. The sample was prepared for in situ phengite $ ^{40}Ar-^{39}Ar $ radiometric, specifically testing whether the large mica "fish" (~3mm across; Figs. S3h, S4) recorded potential excess Ar.

361

362

363

364

365

366

Sample #148-2 is a strongly schistosed felsic metavolcanoclastic rock with Grt-Ph-Chl assemblage (Figs. S3i, S4), sampled close to #148-1 but with contrasting sense of shear in the GT unit. Top to N shear sense is indicated by relictual sigma-type porphyroblasts, phengite and quartz oriented along both the schistosity and shear bands (Figs. S3i, S4).

367

368

369

370

371

Sample #71 is a strongly schistosed pelitic metavolcanoclastic rock with Ph-Chl-Qz-Ab assemblage (Figs. 9i, 13b, S4) sampled close to the southern margin of the GT unit. Top to S sense of shear is indicated by stretched recrystallized quartz veins (Fig. 8f), and Ph+Chl+Qz along shear planes (Figs. 9i, S4).

372

373

374

375

— GS1 and GS2 units:

376

Sample #J2 is a strongly schistosed pelitic metavolcanoclastic rock with Ph-Bi-Chl-Qz (Fig. S3a) from the central part of the GS1 unit. Top to NEE sense of shear is indicated by stretched recrystallized quartz vein (Fig. S2d) and Ph+Qz along the schistosity and shear planes (Fig. S3a).

377

378

379
380

Sample #102 is a moderately to strongly schistosed pelitic metavolcanoclastic rock (Fig. 8g) from the south margin of GS1, close to the UH unit with Act-Ep-Chl-Ph-Qz-Ab-Ttn-Carb assemblage (Fig. S3b). Top to N sense of shear is indicated by Chl+Ph+Qz bearing S-C structures (Figs. 8g, S3b). Only microprobe analyses were performed for this sample.

381

382

383

384

385

Sample #22 is a weakly schistosed metavolcanoclastic rock located in the GS1 unit adjacent to the north margin of the UH unit, with Grt-Ph-Bi-Chl-Qz assemblage (Fig. 9h). Relictual glaucophane/barroisite hints to BS facies metamorphism prior to the regional GS facies. No clear sense of shear could be found on outcrop/sample-scale, therefore phengite grains were picked up only for <sup>40</sup>Ar-<sup>39</sup>Ar step-heating radiometric analysis.

386

387

388

389

390

391

Sample #146 is a strongly schistosed pelitic rock from the northern margin of GS2, close to the GT unit (Fig. 8h), with Chl-Ph-Qz-Ab-Ttn assemblage (Fig. S3j). Top to S sense of shear is indicated by stretched recrystallized quartz veins (Fig. 8h) and chlorite+phengite+quartz along the schistosity and/or shear planes (Fig. S3j).

392

393

394

395

Mineral habits, textural relationships and microstructural location were used to select minerals for multiequilibrium calculations. In general, the following deformation-crystallization relationships were observed:

396

397

398

— In the schistosed garnet-bearing metavolcanoclastic samples (e.g., samples #135, #26, #117, #32, #148), relative euhedral or relict garnet appears as millimeter to centimeter-sized sigma-type porphyroblasts in textural equilibrium with recrystallized blue amphibole-phengite-quartz tails (Figs. 8d, 9a-c, S3c, h-i).

399

400

401

402

— In the schistosed garnet-free metavolcanoclastic samples (e.g., samples #J2, #102, #157, #71, #146), biotite/phengite-chlorite±green amphibole±epidote appear as matrix minerals in textural equilibrium around albite/quartz porphyroblasts

403

404
405

constituting synkinematic structures with clear shear senses along millimeter-sized
shear bands (Figs. 8f-h, 9i, S2d-e, S3a-b, d, j).

406

407

— In the weakly schistosed metavolcanoclastic and metavolcanics samples (e.g., samples #22, #147, #148), although garnet is significantly replaced by chlorite-biotite/phengite and displays irregular rims, textural equilibrium with matrix phengite-quartz/albite±chlorite±relict blue amphibole is preserved in places (Figs. 9h, S3g-i).

408

409

410

411

— In the undeformed metavolcanics sample (sample, #158), a complex metamorphic history is recorded by multistage green/greenish blue/blue amphibole growth together with garnet±epidote±omphacite±phengite±albite (Figs. 9e-f).

412

413

414

Minerals involved in the same microstructural domain (e.g., S-C structures, Figs. 13b, S4) and in close contact were considered to have crystallized coevally. In parageneses involving garnet, mineral inclusions (e.g., Figs. 9a-c, e, S3c) were assumed to be in equilibrium with the corresponding garnet core/mantle compositions. Minerals recrystallized as tails around sigma-type porphyroblasts were assumed to be in equilibrium with the garnet rim compositions. Mineral assemblages used to derive P-T estimates are detailed in Table 3.

415

416

417

418

419

420

421

422

# 5. Analytical Methods

423

## 5.1 Whole-rock chemistry

424

425

426

427

428

429

Analyses were performed at the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS), using ICP-MS with a Finnigan MAT ELEMENT spectrometer and Institut des Sciences de la Terre de Paris (ISTeP), Sorbonne Université, France with Agilent 8800 Triple Quadrupole ICP-MS. Relative standard deviations (RSD) are within ±10% for most trace elements but reach ±20% for V, Cr, Co, Ni, Th and U according to analyses of rock standards. Relevant data is
430 plotted in Figure 10 and listed in Table S1.

431

432 **5.2 Mineral Chemistry**

433 Mineral analysis was performed on key minerals preserved from retrogression, as
434 inclusions within garnet and/or in the matrix, using a CAMECA SX FIVE electron
435 probe at ISTE>P, Sorbonne Université (15Kv, 10nA, WDS), France using Fe₂O₃ (Fe),
436 MnTiO₃ (Mn, Ti), diopside (Mg, Si), Cr₂O₃ (Cr), orthoclase (Al, K), anorthite (Ca) and
437 albite (Na) as standards. Data is shown in Figure 11 and representative analyses are
438 listed in Table 2.

439

440 **5.3 Thermobarometry**

441 **5.3.1 THERMOCALC average P-T**

442 The internally consistent thermodynamic dataset (tc-ds55) and the program
443 THERMOCALC v3.33 (HOLLAND and POWELL, 2004, 1990) were used for
444 multiequilibrium P-T estimates. Structural formulae and end-member activities were
445 calculated with the AX program (ftp://www.esc.cam.ac.uk/pub/minp/AX/). Only P-T
446 estimates passing the equilibrium test criteria (e.g., sigfit, hat; Holland & Powell,
447 1998) have been considered in this study (Fig. 12). Precision of P-T estimates is
448 generally on the order of 0.05-0.2 GPa for pressure and 10-30 °C for temperature.
449 End-members with a very low activity (e.g., Fe-celadonite/celadonite for some
450 phengite, Fig. 11c), yielding a large uncertainty on the results, were removed during
451 calculation (check Table 3 for detail).

452 **5.3.2 Raman Spectroscopy of Carbonaceous Material Thermometry**

453 The Raman spectroscopy of carbonaceous material (RSCM, calibrated in the
454 range 330-650 °C; Beyssac et al., 2002) is used here to obtain peak temperature
estimates independent from those of THERMOCALC. While absolute calibration is ±
50 °C, inter-sample comparison allows detecting much smaller temperature
differences, around 10-15 °C (Beyssac et al., 2004). RSCM was done on thin
sections of graphite-bearing metavolcanoclastics oriented perpendicular to the
foliation by focusing the laser beam beneath a transparent crystal. Raman spectra
were obtained with a Renishaw INVIA Reflex Raman microspectrometer at the
Laboratoire de Géologie of the Ecole Normale Supérieure, France. Spectra were
excited at room temperature with the 514.5 nm line of a 20 mW Ar Spectra Physics
laser through a LEICA 100X objective (NA 0.90). At least 13 spectra were collected
for each sample, to take into account the CM heterogeneities, and processed with
the program Peak Fit 4.0. Temperature estimates are given in Table 3.

466 **5.3.3 Titanium in Biotite Thermometry**

An empirical Ti-in-biotite geothermometer (Henry et al., 2005), calibrated in the
range 0.4-0.6 GPa and 480-800 °C, for compositions of X<sub>Mg</sub> = 0.275-1 and Ti = 0.04-
0.60 apfu, is used to derive temperature estimates for regional GS facies
metamorphism recorded by biotite-bearing metavolcanoclastics (Table 3). Precision
of the Ti-in-biotite thermometer is estimated to be ± 24°C at the lower temperature
range and improves to ± 12 °C at higher temperatures.

473

474 **5.4 Radiochronology**

Eleven samples for which P-T-kinematic constraints were collected (Section 5 and
6) were selected from different tectonic sub-units (Figs. 5d, 13a). Seven of them with
syntectonic textural features (Figs. 13b, S4) were dated by the <sup>40</sup>Ar-<sup>39</sup>Ar in situ laser
ablation technique on phengite grains from 500 µm thick rock slices already
analysed by microprobe. <sup>40</sup>Ar-<sup>39</sup>Ar step-heating analyse on phengite (Fig. S6) was
performed on four weakly to moderately schistosed samples (Figs. 9h, S3a, d, g).

Additional radiometric constraints by zircon U-Pb isotopic analysis were obtained for
one metamafic sample from the MU unit (Figs. 9e-g, S6). Age results are presented
in Figure 13 together with representative examples of $^{40}$Ar-$^{39}$Ar in situ dating. Further
details are given in Figures 13b, S4 and S6, and data are listed in Tables 4, S2 and
S3.

486 **5.4.1 **$^{40}$Ar-$^{39}$Ar In Situ and Step-Heating Analyses**

During this study, both in situ and step-heating $^{40}$Ar/$^{39}$Ar experiments were
conducted on representative samples within the three main units recognized in the
studied area. After selection of the thick sections and single micas, samples were
irradiated for 40 hours in the core of the Triga Mark II nuclear reactor of Pavia (Italy)
with several aliquots of the Taylor Creek sanidine standard (28.34 ± 0.10 Ma) as flux
monitor. Argon isotopic interferences on K and Ca were determined by irradiation of
KF and CaF$_2$ pure salts from which the following correction factors were obtained:
($^{40}$Ar/$^{39}$Ar)K = 0.00969 ± 0.00038, ($^{38}$Ar/$^{39}$Ar)K = 0.01297 ± 0.00045, ($^{39}$Ar/$^{37}$Ar)Ca =
0.0007474 ± 0.000021 and ($^{36}$Ar/$^{37}$Ar)Ca = 0.000288 ± 0.000016.

Argon analyses were performed at Géosciences Montpellier (France) with an
analytical system that consists of: (a) a UV laser for in situ experiments and a CO2
laser for the step-heating experiments, (b) different lenses for laser beam focusing,
(c) a steel sample chamber, maintained at 10⁻⁸ - 10⁻⁹ bar, with a drilled copper plate
for individual grains, (d) an inlet line for purification of gases including two Zr-Al
getters, (e) a multi-collector mass spectrometer (Argus VI from Thermo-Fisher). After
loading in the sample chamber, the samples were baked at about 120 °C for 24
hours to remove atmospheric argon from the sample surface. For each in situ
experiment, argon was extracted from a 150×300 µm surface corresponding to a
mixture of several phengite grains, given grain size (Figs. 13b, S4). The crater is a 505 30-40 µm approximate hemisphere surrounded by a circular wall made of melted 506 material. Incision of the sample did not exceed 10-20 µm deep depending on the 507 three-dimensional orientation of the phengite crystals. Step-heating experiments on 508 single grains were conducted with a duration of heating of 30 seconds at each step. 509

A custom-made software was used to control the laser intensity, the timing of 510 extraction/purification and the data acquisition. The ArArCalc software© v2.5.2 was 511 used for data reduction and plotting. Atmospheric $ ^{40}\text{Ar} $ was estimated using a value 512 of the initial $ ^{40}\text{Ar}/^{36}\text{Ar} $ ratio of 295.5. Ages are reported with one sigma uncertainty. 513

### 5.4.2 U-Pb dating and trace element analysis of zircon by LA-ICP-MS

U-Pb dating and trace element analysis of zircon were simultaneously conducted 515 by LA-ICP-MS at the Wuhan SampleSolution Analytical Technology Co., Ltd., China. 516 Detailed operating conditions for the laser ablation system and the ICP-MS 517 instrument and data reduction are the same as in Zong et al. (2017). Laser sampling 518 was performed using a GeolasPro laser ablation system that consists of a 519 COMPexPro 102 ArF excimer laser (wavelength of 193nm and maximum energy of 520 200 mJ) and a MicroLas optical system. An Agilent 7700e ICP-MS instrument was 521 used to acquire ion-signal intensities. Helium was applied as a carrier gas. Argon 522 was used as the make-up gas and mixed with the carrier gas via a T-connector 523 before entering the ICP. A “wave” signal smoothing device is included in this laser 524 ablation system (Hu et al., 2015). The spot size and frequency of the laser were set 525 to 60 µm and 6 Hz, respectively, in this study. Zircon 91500 and glass NIST610 were 526 used as external standards for U-Pb dating and trace element calibration, 527 respectively. Each analysis incorporated a background acquisition of approximately 528 20-30 s followed by 50 s of data acquisition from the sample. An Excel-based 529
software ICPMSDataCal was used to perform off-line selection and integration of
background and analyzed signals, time-drift correction and quantitative calibration for
trace element analysis and U-Pb dating (Liu et al., 2009). Concordia diagrams and
weighted mean calculations were made using Isoplot/Ex_ver3 (Ludwig, 2003).

534

535 6. Results

536 6.1 Bulk chemistry of major lithologies

537 The protoliths of the studied samples mainly correspond to metavolcanoclastics,
538 metavolcanics and meta-ultramafic rocks.

539 — The metavolcanoclastic samples locally contain mud/silty/sand components that
540 either resemble metapelite or quartz-rich metagraywacke as reflected by the main
541 parageneses, which comprise Grt-Ph-Amp-Qz-Rt±Cpx±Ep±Chl±Ttn±Ctd±Pg±Bi
542 (e.g., samples #22, #135, #117, #32; abbreviations after Table 1), Grt-Ph-Qz-Chl-
543 Rt±Pg±Ab±Ttn (e.g., sample #26) and Ph-Qz-Chl-Ab±Grt±Pg±Ttn±Amp±Carb±Bi±Ep
544 (e.g., samples #J2, #102, #157, #148, #71, #146). They uniformly show significant
545 Rb-Ba-Th-U-Pb enrichments and notable Nb-Ta depletions in primitive mantle-
546 normalized (PM) trace element pattern and a steep negative slope ((La/Yb)N = 1.69-
547 32.2, Table S1) in chondrite-normalized REEs pattern (Fig. 10), similar to that of the
548 average upper continental crust (UCC, Rudnick & Gao, 2003) or trench sediment (Li
549 & Schoonmaker, 2003; Plank & Langmuir, 1998). These lithologies are found in all
550 sub-units, without significant differences, except in MU.

551 — The metavolcanics are dominantly mafic, as reflected by Grt-Amp-Ph-Ep-Rt-
552 Ttn±Cpx±Pg±Qz±Bi±Chl±Ab±Carb parageneses (e.g., samples #158, #147). They
553 were here only sampled in units MU and GT. Their moderate Rb-Ba-Th-U-Pb
554 enrichments and lack of detectable Nb-Ta depletions in primitive mantle-normalized
trace element pattern, and a moderately negative slope ((La/Yb)N = 5.51-5.95, Table S1) in chondrite-normalized REEs pattern (Fig. 10) fall into the range between that of ocean island basalt and enriched mid-ocean ridge basalt (OIB, E-MORB, Sun & McDonough, 1989).

— The meta-ultramafic rocks are serpentinized and/or metasomatized with parageneses consisting of MgChl-Ttn (sample #156-1), Grs-Cpx-Chl (sample #156-2), Atg-Cpx (sample #156-3). They present Rb-Ba-Th-U-Pb concentrations comparable to PM and a slightly positive slope ((La/Yb)N = 0.27-1.53, Table S1) in chondrite-normalized REEs pattern (Fig. 10).

## 6.2 Mineral compositions

Mineral compositions analyzed by microprobe are listed in Table 2 with structural formulae and Fe³⁺ content (via charge balance) at given oxygen atoms number, calculated by the AX program (ftp://www.esc.cam.ac.uk/pub/minp/AX/). Classification schemes for amphibole and pyroxene are respectively after Hawthorne & Oberti (2006) and Morimoto (1988). Phengite classification is after the following scheme: X<sub>Cel</sub> = Fe²⁺ + Mg, X<sub>Ms</sub> = K - X<sub>Cel</sub>, X<sub>Pg</sub> = Na, X<sub>Prl</sub> = 1 - X<sub>Cel</sub> - X<sub>Ms</sub> - X<sub>Pg</sub>. Chlorite classification follows the scheme: X<sub>Clin+Daph</sub> = Fe²⁺/5, X<sub>Sud</sub> = Fe³⁺, X<sub>Ames</sub> = 1 - X<sub>Clin+Daph</sub> - X<sub>Sud</sub>. Garnet is classified according the scheme: X<sub>Grs</sub> = Ca/(Ca + Fe²⁺ + Mg + Mn), X<sub>Prp</sub> = Mg/(Ca + Fe²⁺ + Mg + Mn), X<sub>Alm+Sps</sub> = 1 - X<sub>Grs</sub> - X<sub>Prp</sub>.

### 6.2.1 Phengite

White mica compositions are mostly phengitic with variations along the tschermak Si-(Fe, Mg) = Al-Al exchange vector between muscovite and celadonite, as shown by Figure 11c. The deviation from the purely phengitic substitution trend (Fig. 11c) is observed for all samples, which suggests that a certain amount of ferric iron is
present. Phengite is the main matrix mineral in most metavolcanoclastic samples (Figs. 9, 13b, S3-S4), showing several distinct generations with respect to porphyroblasts and/or C-S structures (see Section 4). Microprobe images reveal complex multistage growth structures (e.g., Fig. S7) and local biotite overgrowths (Figs. 9h, S3g). The Si content of samples from the UH unit (3.38-3.66 apfu) is higher than that from the EB (3.34-3.50), MU (3.35-3.45), GS1/2 (3.07-3.45) and GT units (3.05-3.38), especially with a few crystals with high Si content in sample #26 (Si = 3.66). Larger variations of compositions (Fig. 11c) are observed in samples #148 and #71 (GT), #135 (UH) and #117 (EB), and partial clusters in #157 (MU) and #148 (GT). These variations are compared to age constraints, in particular those given by in situ $^{40}$Ar/$^{39}$Ar analyses (Fig. 13c).

### 6.2.4 Amphibole

Blue or blue-green amphiboles were detected either as inclusion or matrix mineral in most garnet-bearing samples (samples #22, #135, #117, #32, #158, #147, Figs. 9a-c, e-h, S3g) with compositions varying largely and ranging from glaucophane to barroisite to actinolite (Fig. 11a). Barroisite replaces glaucophane in many of the high grade metavolcanoclastics (samples #135, #117, #32, Figs. 9a-c), as a consequence of retrogression or decompression, but the opposite evolution is documented in sample #158, with euhedral actinolite rimmed by an anhedral glaucophane matrix (Figs. 9e-g).

### 6.2.3 Clinopyroxene

Omphacitic clinopyroxene (Fig. 11b) is common in metavolcanic rocks and true eclogites of the UH unit. In the selected metavolcanoclastic samples, it was only found as small crystals in garnet and/or in the matrix of samples #135 and #158 (~100 µm, Figs. 9a, e-f, S7a), with average compositions between ~48% jadeite and
605 ~6% acmite end-members (and ~46% quadrilateral component, Q, which include
606 diopside, hedenbergite, enstatite and ferrosilite) for the former and between ~30%
607 jadeite and ~5% acmite end-members (and ~65% Q) for the latter, which is out of the
608 common area of omphacite from regional eclogites (unpublished compilation).
609 Diopside (Fig. 11b) appears as a stable mineral wrapped by antigorite matrix or vein
610 crosscutting grossular matrix in metaultramafic rocks (samples, #156-1 to -3, Figs.
611 9d, S3e-f).

612 **6.2.4 Chlorite**

613 Chlorite compositions are mainly ferrous and range between clinochlore/daphnite
614 and amesite along the tschermak exchange vector (Fig. 11d), with X_Mg ~0.16-0.50. It
615 either appears as the main matrix mineral in the low to intermediate grade
616 metavolcanoclastics (Figs. 9i, S3a-b, d, j) or retrograde mineral rimming/cutting
617 porphyroblasts (Figs. 9a-c, h, S3g-i). Significantly magnesium-rich chlorite, with X_Mg
618 ~0.55-0.88, is found only as prograde mineral inclusion in garnet (Figs. 9b, S3c) or
619 matrix mineral in metaultramafic samples (Figs. S3e-f). Deviation toward sudoite is
620 very limited.

621 **6.2.5 Garnet**

622 Garnet is dominantly ferrous (FeO ~23.7-34.8 wt%), with X_Mg values ranging from
623 0.04 to 0.24 (Fig. 11e). The evolution of garnet zoning is characterized by a Mn
624 decrease with a Ca, Fe and Mg fluctuation toward rim in samples from the UH and
625 EB sub-units (Fig. 11e), with limited reequilibration on the garnet rims. In detail,
626 diverse patterns (Fig. 11e) are encountered for metavolcanoclastic (e.g., samples
627 #135, #32) and metavolcanic rocks (e.g., samples #158, #147) from the different
628 sub-units.
629 **6.2.6 Other minerals**

630 No 'fresh' lawsonite was found but only box-shaped pseudomorphs included in
631 garnet, that potentially formed after lawsonite (i.e., with epidote +
632 albite/paragonite±chlorite assemblages; samples #135, #32, #26; Figs. 9a, c, S3c).

633 Coesite was assumed to appear at or near the metamorphic peak of UH samples
634 since i) sampling locations (i.e., samples #135, #26, Figs. 4a, 5a) are very close to
635 previous UHP reports of coesite, ii) the entire UH unit was confirmed as a UHP unit
636 with ubiquitous coesite findings and a determined maximum burial pressure of at
637 least 2.5. ± 0.2 GPa (Figs. 1c, 4a, d; Bayet et al., 2018). Epidote appears as matrix
638 mineral (Figs. 9e-g, S4b, g) and inclusion (occasionally within the box-shaped
639 lawsonite pseudomorphs; Figs. 9a, c), with a pistacite composition varying between

640 0.12 and 0.58 but broadly constant in each sample. Paragonite occurs as inclusions
641 in garnet (mainly as part of lawsonite pseudomorphs, Figs. 9a, c, S3c) or
642 occasionally as porphyroblasts in the matrix (i.e., samples #117, #32, Figs. 9b-c)
643 parallel or subparralel to the schistosity, which are interpreted to result from
644 retrograde metamorphism (e.g., Tan et al., 2017). Chloritoid was only found as
645 inclusion in the garnet of sample #32 (Fig. 9c) with X<sub>Mg</sub> of ~0.26. Biotite either
646 appears as the main mineral making up the schistosity in sample #J2 (Fig. S3a) or
647 replaces garnet/phengite (Figs. 9h, S3g) in samples #22, #32 and #147. Its
648 composition lies along the tschermak substitution trend between eastonite and
649 phlogopite/annite with Si ~2.75-2.91, X<sub>Mg</sub> ~0.34-0.58 and Ti ~0.06-0.10 apfu.
650 Plagioclase is pure albite in composition in all samples.

651

652 **6.3 Thermobarometric results**

653 P-T estimates were obtained with THERMOCALC for 13 samples (Fig. 12a; see
Table 3 for details on calculations and Table 2 for corresponding analyses), except for three GS facies samples whose mineral assemblage are too high-variant to ensure a satisfactory fit. Metavolcanoclastics from the UH sub-unit, with Grt<sub>core/mantle/rim</sub>-Qz(Coe)-Lws(Pseudomorph)±Ph±Gln±Omp±Tlc assemblages (samples #135, #26, Figs. 9a, S3c) yield UHP peak burial around 470-565 °C and 2.75 GPa (±0.15) on average. The Grt<sub>mantle</sub>-Qz-Brs-Lws(Pseudomorph)±Ph±Ctd±Chl and Grt<sub>rim</sub>-Qz-Gln-Lws(Pseudomorph)±Ph±Ctd±Chl assemblages of samples #117 and #32 from the EC/BS EB unit (Figs. 9b-c) record a prograde increase in pressure and temperature from 1.8 GPa (±0.10), 490 °C (±15), to peak burial ~2.1 GPa (±0.15), 505 °C (±15). The P-T history of the meta-mafic/-ultramafic unit (MU) is constrained by sample #158, Figs. 9e-g), which experienced a much lower pressure. Three stages are recognized, respectively constrained by Grt<sub>core</sub>-Act-Ep±Qz, Grt<sub>rim</sub>-Gln-Omp-Ph-Ep±Qz±Pg and Grt<sub>out-rim</sub>(i.e., re-equilibrated during decompression)-Brs-Ep-Chl±Qz±Pg±Ab assemblages, from prograde ~0.95 GPa (±0.18) and 464 °C (±28) to peak burial ~1.46 GPa (±0.16), 484 °C (±29) and post-peak ~0.93 GPa (±0.06), 556 °C (±13). P-T estimates for samples located to the north and south of the AMC (GS1 and GS2 sub-units) and from the transitional GT sub-unit (samples #J2, #102, #22, #147, #148, Figs. 9h, S3a-b, g-i) reveal broadly comparable GS to lower-BS facies P-T histories, with peak burial around 0.7-1.0 GPa, 470-520 °C and post-peak conditions of ~0.4-0.6 GPa, 520-570 °C for Chl-Qz-Ph-Ab±Grt<sub>relict</sub>±Gln/Brs±Bi±Pg assemblages. Similar GS to lower-BS facies conditions around 0.95-1.20 GPa, 495-530 °C were obtained for Chl-Qz-Ph-Ab±Grt<sub>out-rim</sub>±Act±Bi±Pg assemblages formed as retrograde overprint on metavolcanoclastics from the UH (samples, #135, #26) and EB units (sample, #32).

RSCM temperature estimates for metavolcanoclastics from the UH unit range
679 between 506 ± 15 °C (n=29) and 523 ± 20 °C (n=19) with an average value of 515 ±
680 9 °C. Similar temperatures are found for the EB unit, between 497 ± 19 °C (n=26)
681 and 527 ± 19 °C (n=24) with an average value of 512 ± 15 °C. RSCM estimates for
682 rocks from the GS1, GS2 and GT units cluster between 521 ± 36 (n=15, sample
683 #J2), 518 ± 34 (n=17, #102), 547 ± 29 (n=21, #22), 501 ± 35 (n=25, #157), 528 ± 33
684 (n=19, #147), 522 ± 27 (n=22, #148), 500 ± 20 (n=13, #71) and 535 ± 15 (n=17,
685 #146), with an average value of 522 ± 16 °C. Those RSCM temperatures are
686 somewhat lower/higher than T estimated with THERMOCALC for some individual
687 samples, but the discrepancy remains within the bounds of the RSCM calibration (±
688 50 °C, Beyssac et al., 2002). Overall, good fits exist in samples for which both the
689 RSCM temperature and the peak burial estimates are available (samples, #102, #22,
690 #26, #117, #32, #148, Fig. 12a). The RSCM temperature estimates in this study are
691 also consistent with previous works which propose 536 ± 11 °C maximum peak
692 temperature for the AMC also based on RSCM and Zr-in-rutile thermometry (Bayet
693 et al., 2018, 2019; Meyer et al., 2016).

694 Titanium-in-biotite thermometry on biotite-bearing samples #J2, #22, #147 (GS1
695 and GT units) and #32 (EB unit) yields estimates between 588 ± 19 (n=10), 588 ± 10
696 (n=12), 585 ± 21 °C (n=11) and 502 ± 26 (n=7), with a slightly higher average value
697 than THERMOCALC and RSCM estimates (i.e., 566 ± 43 °C; Fig. 12a). This
698 estimate is nevertheless in agreement with the recent Zr-in-titanite temperature
699 constraints at ~569-608 °C for retrogressed eclogite in the AMC (Zhang et al., 2018).

700

## 6.4 Radiochronological results

701 Radiometric ages from in situ phengite <sup>40</sup>Ar-<sup>39</sup>Ar dating range between 269.0 and
702 366.2 Ma (Fig. 13a, Table 4). Individual samples (Fig. S5) span ranges of 270-341
and 304-351 Ma for the UH unit, 326-368 and 294-366 Ma for the EB unit, and 267-328, 291-333 and 269-306 Ma for the GT units. Despite the age scatter, even within individual samples, none of them shows significant co-variations in ³⁷Ar/³⁹Ar and ³⁸Ar/³⁹Ar ratios, or in atmospheric contribution, that could account for age dispersion (Table 4).

The ages or age clusters amongst the various samples therefore either reflect 'real' tectonic episodes or the presence of excess argon, as documented in some HP settings (e.g., Klemd et al., 2005; Laurent et al., 2017; McDonald et al., 2018; Warren et al., 2012) but not all (e.g., Agard et al., 2002; Augier et al., 2005). In detail, most radiometric ages agree with the chronological relationship inferred from corresponding mineral textures, i.e. phengite in shear bands cutting across the schistosity are effectively younger. This is the case for samples #135, #32 and #71 (Fig. 13b). Sample #135 shows that highly phengitic mica grains (Si⁴⁺ ~3.50-3.60 apfu) following the schistosity yield ages between 319 and 326 Ma, whereas younger ages on average ca. 314-317 Ma are obtained along shear bands with lower Si content phengite (Si⁴⁺ ~3.40-3.50). Younger ages ~271-310 Ma in small disoriented microdomains adjacent to the shear bands could relate to latest stage phengite recrystallization. One older age (~334 Ma) was obtained from an inclusion within garnet with very high Si content (Si⁴⁺ ~3.64): this could represent the real age of an early grown (UHP) phengite and/or some excess argon sequestered in garnet (as is likely in sample #117; Fig. S4), or both. Similar age patterns with deformation-controlled phengite recrystallization can be observed for the rest of the metavolcanoclastics dated by in situ ⁴⁰Ar-³⁹Ar (Figs. 13b, S4), except for sample #157 whose rare phengite grains along a weak schistosity yield ages ~272-282 Ma. Hence, the mean age value of each sample is calculated by those absolute ages
729

(marked by red circles in Fig. S5) probed from phengite grains following the schistosity and along shear bands (i.e., S-C structure), which is structurally tied to post-peak deformation and metamorphism (Figs. 13b, S4).

730

731

732

Weighted plateau or integrated ages of phengite in samples #J2, #22, #157 and #147, from regional GS facies units (GS1, GS2 and GT, Figs. 5a, d, 13a), are 276.6 ± 0.9, 299.9 ± 1.9, 326.1 ± 1.3 and 304.1 ± 1.0 Ma respectively (Fig. S6, Table S2).

733

734

735

Dated phengites are moderately phengitic mica in all samples ($Si^{4+}$: 3.05-3.15 apfu for #J2, 3.25-3.35 for #22, 3.10-3.30 for #157 and 3.30-3.40 for #147, Fig. 11c, Table 2), with stronger retrogressive effects for samples #22 and #147 (Figs. 9h, S3g). The

736

737

738

plateau age ~277 Ma of phengite in sample #J2, which exhibits clear GS facies shear sense indicators (Figs. S2d, S3a), may relate to dextral strike-slip shearing along the western part of SCTSZ (Figs. 1c, 4a). The plateau age ~326 Ma of phengite in weakly schistosed sample #157 is significantly older than ages obtained by in situ measurements (~276 Ma, Fig. S4), which is a bit puzzling but may reflect

738

739

740

741

742

743

local recrystallization and/or fluid circulation along micro-cracks and/or some contamination by minor amounts of chlorite (e.g., De Jong et al., 2001; Di Vincenzo et al., 2016).

744

745

746

Zircon grains of the metamafic sample #158 from the MU unit present core-rim texture in CL image (Fig. S6) with apparent $^{206}$Pb/$^{238}$U ages varying from 351 to 2075 Ma (Th/U ~0.09-0.99, Table S3). Dark/grey oscillatory zircon cores (Fig. S6) were characterized by typically Ce positive and Pr & Eu negative anomalies, steep positive slope from LREEs to HREEs ((Lu/La)$_{N}$ ~1984-248266), significant enrichment in HREEs ((Lu/Sm)$_{N}$ ~50.9-147.4) and inclined HREEs pattern ((Lu/Tb)$_{N}$ ~9.2-14.4, Table S3) in chondrite-normalized REEs diagram, with Th/U ~0.52-0.99 and a weighted mean age (if exclude discordant, older inherited core) of 448.5 ± 4.7

747

748

749

750

751

752

753
Ma (MSWD=0.56, n=9). These observations support a magmatic origin (e.g., Corfu et al., 2003; Schaltegger & Davies, 2017). The bright zircon rims yield a weighted mean age of 359.2 ± 3.6 Ma (MSWD=1.14, n=8), with relatively gentle slope from LREEs to HREEs ((Lu/La)N ~13.7-141.0) and obvious HREEs plateau ((Lu/Tb)N ~1.3-2.2, Table S3) but no detectable Ce-Pr-Eu anomalies in chondrite-normalized REEs diagram. The REEs pattern of bright zircon rims and their smaller Th/U ~0.09-0.26 suggest a metamorphic origin equilibrated with the growth of relatively LREEs- and MREEs-poor & HREEs-rich phase (e.g., garnet; see Rubatto, 2017).

762

## 7. Discussion

### 7.1 Distinct metamorphic sub-units?

Our north-south section across the entire Akeyazi metamorphic complex reveals the existence of major contacts separating domains with different lithologies and/or tectonic patterns (as illustrated on the cross-section of Figure 5d), that we regard as distinct km-scale tectonic sub-units. Overall, asymmetric shearing is more pronounced in UH, while upright symmetrical folding is typical of EB. Mafic boudins are also more numerous in UH. The lateral extension of these sub-units, limited by the presence of glaciers to the east, is partly inferred from Bayet et al. (2018, 2019; Figs 4a, 4d, 7b). For example, the more mafic EB unit likely corresponds to the pillow unit described by Bayet et al. (2018, 2019).

Our observations (Figs. 5-7) highlight the following deformation patterns:

- BS facies asymmetric shear senses with a northward component prevail for all lithologies and sub-units (except in the MU unit, where kinematic indicators are rare and not statistically significant). This points to the importance of simple shear during exhumation (also see Bayet et al., 2018, 2019). No indication of dominant syn-
779 eclogite pure shear deformation was found at the outcrop scale along our transect,
780 and is possibly only preserved in boudins (e.g., LPO measurements of Soldner et al.,
781 2017) and/or slices (Tan et al., 2017; their Figure 4).

782 — top to NEE and a few top to N/S GS facies shear senses are restricted to the
783 GS1, GS2, GT units and the north margin of the MU.

784 We propose that the top to NNW shear senses (mostly BS facies, and more rarely
785 transitional EC/BS or BS/GS) correspond to exhumation dynamics following
786 northward subduction (e.g., Scheltens et al., 2015), while regional dextral strike-slip
787 deformation is marked by the dominance of top to NEE GS facies shear sense.

788 P-T estimates inferred from average P-T calculations suggest contrasting peak
789 burial for the sub-units. Care in selecting potentially co-stable minerals/analyses and
790 statistical fits obtained make these estimates relatively robust. In the UHP/HP
791 complex (UH, Fig. 12a), maximum burial P-T estimates cluster around 470-565 °C,
792 2.75 GPa (±0.15), similar to most recent estimates (e.g., Bayet et al., 2018; Lü &
793 Bucher, 2018), and consistent with the range of 480-580 °C, 2.2-3.0 GPa obtained
794 by earlier studies (e.g., Du et al., 2014; Lü & Zhang, 2016; Tan et al., 2017; Yang et
795 al., 2013; Zhang et al., 2018) and the widespread occurrence of coesite (Figs. 1c,
796 4a, d; Bayet et al., 2018; Lü et al., 2009; Lü & Zhang, 2012; Tan et al., 2017; Yang et
797 al., 2013). Overall, it agrees well with the 531 ± 11 °C and 2.5 ± 0.2 GPa that is
798 determined for the entire coesite-bearing UH unit (Bayet et al., 2018; Figs. 1, 2, 4). In
799 contrast, somewhat lower peak pressure of ~2.1 GPa (±0.15), 505 °C (±15) were
800 obtained for the EB unit, with moderately to highly phengitic mica and in general
801 lesser amounts of omphacite than in UH for similar bulk compositions (Fig. 12a). We
802 regard the possibility of having 'missed' UHP samples and/or that retrogression was
803 so pervasive that all hints of UHP metamorphism was removed as fairly unlikely but
still possible. This unit resembles the pillow-basalt bearing unit (Figs. 4d, 5e) of Bayet et al. (2018), whose P-T conditions were unfortunately not determined. Different conditions were also obtained for the MU ultramafic/mafic unit equilibrated at ~1.46 GPa (±0.16), 484 °C (±29). Rocks from the northern- and southernmost units (GS1, GS2 and GT, Fig. 12a) yield GS to lower-BS facies P-T conditions around 0.4-1.0 GPa, 470-570 °C. The GT unit shows evidence for an earlier, higher pressure blueschist facies episode whose P-T conditions could not be determined. Similar GS to lower-BS facies conditions around 0.95-1.20 GPa, 495-530 °C (using moderately phengitic mica) were determined for the retrograde overprint of the UH and EB samples (#135, #26, #32).

Different age clusters (Fig. 14; discussed further below) also emerge for peak burial of the sub-units from the data obtained in this study, i.e., ~320 Ma for UH, ~330 Ma for EB, ~360 Ma for MU (from metamorphic zircon overgrowth). A similar age of 360 Ma was in fact obtained by Ar-Ar dating on the MU unit (Figs. 14d, 15a, c, Table S5). Similar ages around 300-310 Ma are found, in contrast, for early exhumation of these units and for GS facies metamorphism (Figs. 12b, 14).

These observations and field mapping (e.g., cross-section of Figure 5d) therefore suggest that the Akeyazi metamorphic complex, at least in the surroundings of the Kebuerte valley, is a metamorphic dome made of a stack of individual sub-units or nappes, which is consistent with a coherent pile of sub-units for the internal coesite-bearing UH domain (Bayet et al., 2019, 2018). This rules out the view that the AMC might be a subduction mélange in the sense of Cloos & Shreve (1988a, 1988b) or Meyer et al. (2016), although in places original metasedimentary horizons have been sheared, dismembered, and turned into rootless fold hinges for example (Fig. 5d). That the exposed core of the metamorphic dome (i.e., the EB unit) does not
829

correspond to the most deeply buried slice also requires thrusting at some point of
the tectonic history. The contrasting ages for the different sub-units indicate
diachronous detachment from the subducting slab.

830

831

832

833

## 7.2 P-T-t trajectories, exhumation rates and tectonic stacking

834

The successive detachment of sub-units (MU, EB then finally UH, Figs. 12b, 16b-e) reveals a change in metamorphic gradient with time, from an intermediate to cold subduction regime (i.e., from ~12 to 6-7 °C/km; Fig. 12b; also see Agard et al. (2018) and Angiboust et al. (2016)). The accuracy and significance of Ar-Ar ages (i.e., whether they are 'real' tectonic episodes or 'blended' ages with excess argon) is therefore critical and more thoroughly discussed here.

835

836

837

838

839

840

As outlined above (Section 6.4), a key observation is that most radiometric ages
respect the chronological relationship inferred from mineral textures, i.e. phengite in
shear bands cutting across the schistosity are effectively younger (e.g., as visible in
samples #135 or #32, Figs. 13b, S4). Ages obtained from highly phengitic mica in the
UH unit, at ~320 ± 4 Ma, also match the results of previous geochronological studies
(Figs. 3c-d, mostly focused on the UH unit in adjacent valleys, check Section 2), i.e.
peak metamorphic conditions at 315 ± 5 Ma (Figs. 14b, 15a, c, Table S5). Another
argument in favor of the absence of random excess argon is that the dispersion of in
situ ages increases with the greater dispersion in phengite chemistry (Fig. 13c; e.g.,
notably in samples #135, #117, #32, #148, #71).

841

842

843

844

845

846

847

848

849

850

Our observations confirm that recrystallization enhanced by deformation and fluid
circulation is commonly more effective than volume diffusion (Agard et al., 2002;
Augier et al., 2005; Laurent et al., 2017), and that the 'closure temperature' for Ar-Ar
system of phengite is not reached at temperatures < 550°C (as in Syros: Laurent et

851

852

853
al., 2017; but also depends on grain size and chemical composition), since all ages
within a given sample should be the same otherwise. We therefore conclude that i)
the radiometric ages obtained here mainly result from phengite recrystallization in
deformation sites and/or deformation-controlled reequilibrations, with only minor
effects of excess argon (and/or contamination by minerals adjacent to phengite) and
ii) that the sub-units UH and EB reached peak conditions diachronously at ~320 and
~330 Ma, respectively. The metamorphic zircon rim overgrowth is interpreted to
constrain peak burial of the MU unit at ~360 Ma (whereas the magmatic core is ~450
Ma). This evolution reflects a change in metamorphic gradient and possibly of the
subduction thermal regime with time (Fig. 12b).

Peak burial for the EB unit at ~330 Ma is reminiscent of ages ~328 ± 26 Ma
obtained on 'blueschists' from the adjacent Atantayi-Akesayi valleys (Figs. 14c, 15a,
c, Table S5) and the Kekesu valley (although located ~60km to the East; Figs. 1c,
15a, c, e), mostly with the Ar-Ar step-heating technique (Gao & Klemd, 2003; Gao et
al., 2000, 2006; Klemd et al., 2005). Although this age dispersion was previously
interpreted as reflecting some excess Ar (including by us; e.g., Tan et al., 2017), or
the prograde growth of an eclogite-facies mineral assemblage (Klemd et al., 2011),
these ages may be a further indication of the existence of a distinct metamorphic
episode around 330 Ma in the AMC (Figs. 14c, 15a, c). This could correspond to the
peak burial age ~330 Ma of the Atbashi range HP/UHP complex (Fig. 14; also see
Loury et al., 2018), further to the west along the Tianshan range (Fig. 1b).

For the MU unit, only two previous phengite-glaucophane Ar-Ar ages around
~364-370 Ma (Figs. 14d, 15a, c, Table S5) were reported from samples immediately
to the west of the MU unit (Gao et al., 2000). Some ultramafic lithologies were
reported in the Changawuzi valley (~40 km to the west, Figs. 1c, 3a, 15a-b), with an
879

age of 308 ± 5 Ma for blackwall formation (Shen et al., 2016), but no eclogite
boudins or Ti-chondrodite/clinohumite were found in the MU unit contrary to the
Changawuzi samples.

880

881

882

Rocks from the GS facies sub-units (GS1, GS2 and GT, Fig. 13a) yield ages
around ~275-318 Ma matching i) retrogression ages (Figs. 13, S4) found for the UH
and EB sub-units, ii) the ~293-316 Ma phengite Ar-Ar plateau ages of regional GS
facies rocks at the Kekesu valley (Wang et al., 2010; Xia et al., 2016), iii) the ca. 304
Ma titanite U-Pb age of retrogressed eclogite (Zhang et al., 2018) along the western
part of SCTSZ, and iv) the ~280-290 Ma mica Ar-Ar plateau ages of GS facies rocks
related to dextral strike-slip shearing along the eastern part of SCTSZ (Laurent-
Charvet et al., 2003).

883

884

885

886

887

888

889

890

The juxtaposition of the four sub-units preserving a HP-LT history (namely the UH,
EB, MU and GT) and marking the center of the AMC metamorphic dome could
therefore take place around 280-300 Ma at ~20km depth (Figs. 12b, 16e), since
ages from moderately phengitic, recrystallized mica were detected in all these
samples (Figs. 13b, 14, S4). As for prograde paths, exhumation P-T paths slightly
differ (Figs. 12b), with a trend from clearly clockwise for MU (i.e., with a marked
thermal excursion towards higher T) to near isothermal for EB and even with a slight
cooling for UH (while that of GT is clockwise but more moderately than MU). This
could be explained if, in addition to a progressive cooling of the subduction regime
from MU to EB/UH, residence time at depths was longer for EB than for UH (similar
patterns have been reported for successive stacking of units in Oman; Agard et al.,
2010).

891

892

893

894

895

896

897

898

899

900

901

902

Exhumation velocities could not be calculated for individual samples but a first-
order constraint can be inferred: assuming that ages for samples #135 and #26 are

903
representative of the average UHP to GS history (from ~320 to 280-300 Ma, Figs. 12a, 13) the decrease from ~2.75 to ~0.65 GPa in approximately 20-40 Ma yields ~1.8-3.5 mm/yr for the UH unit, consistent with our previous independent estimate for a coesite-bearing oceanic eclogite in the same sub-unit (Tan et al., 2017). The same approach yields ~0.8-1.3 mm/yr for the EB unit, showing that both units have exhumation velocities comparable to most oceanic blueschists and eclogites (i.e., ~1-5 mm/yr; Agard et al., 2009; Guillot et al., 2009).

911

## 7.3 Implications for subduction dynamics and Late Carboniferous subduction history

Importantly, our structurally controlled P-T-time data suggest the diachronous recovery of different sub-units (Figs. 12b, 16b-e), from ~45 km (MU; 360 Ma), ~65 km (EB; 330 Ma) and ~85 km depth (UH; 320 Ma). This rock record highlights discrete, transient episodes of detachment from the descending slab, in particular with respect to long-lived South Tianshan ocean subduction duration of ~120 Myr (Scheltens et al., 2015; Tan et al., 2017).

The punctuated recovery likely results from changes in mechanical coupling (Agard et al., 2018), whose origin could be (i) a change in slab geometry (e.g., slab rollback: Brun & Faccenna, 2008; slab kink: Klemd et al., 2011) (Figs. 16b-c) and/or (ii) a modification of the physical properties along the plate interface (i.e., mantle rheology, fluid regime, etc; Agard et al., 2018). Incidentally, whole-rock analyses (Fig. 10) reveal that there was in first approximation no drastic change in volcanoclastic content throughout this subduction history. In fact, Bayet et al. (2019) describes moderately lithological variations across the metavolcanoclastic-metasedimentary sequence defining the UH, from north-to-south developing from more pelitic to more
929 mafic and psammitic, which is interpreted as resulting from an increasing input of
930 proximal greywackes and volcanoclastic arc material with time into the trench.

931 Exhumation and final recovery of the South Tianshan Metamorphic Belt (SMTB)
932 could have been assisted by internal buoyancy. The existence of a serpentinite
933 channel is unlikely given the lack of significant serpentinite exposures (see Section
934 2), but matrix rocks of the STMB are buoyant enough, even at UHP conditions (i.e.,
935 <3.3 g/cm³, considering felsic:pelitic:mafic:serpentinite proportions of ~35:50:10:5;
936 this study and Bayet et al., 2018; see calculations by Angiboust and Agard, 2010). A
937 continental buoy is thus not inherently needed to explain their preservation, although
938 continental subduction as proposed in the Atbashi range further west could be an
939 additional help (according to Loury et al. (2015, 2018), the Pobeda granulite massif
940 exhumed in the Cenozoic might represent either the subducted crust or older
941 basement corresponding to the Tarim block, but its age is still missing). Importantly,
942 the age pattern reported here, the existence of EC/BS facies internal contacts and
943 thick metasedimentary slices preserving pervasive coesite findings (Bayet et al.,
944 2019) seem to rule out the possibility of 'long exhumation period (>70 Ma) of the
945 UHP terrane in the AMC (Zhang et al., 2013; Zhang et al., 2018), which would have
946 erased much of the subduction history.

947 Final tectonic stacking of the UH, EB, MU and GT units around 280-300 Ma at
948 ~20km depth also suggests, contrary to some interpretations (e.g., Gao et al., 2017;
949 Wang et al., 2018; Zhang et al., 2018), that the peak burial of the UHP does not
950 necessarily mark oceanic closure (and/or does not relate to continental collision). In
951 particular, the ~300 Ma phengite <sup>40</sup>Ar-<sup>39</sup>Ar step-heating ages (Figs. 13a, S6) from
952 both the glaucophane-bearing GT and GS1 units suggest that subduction was still
953 active around the Late Carboniferous (i.e., ~300 Ma). This inference is further
supported by the following regional magmatic indications (Fig. S8): i) the histogram
of the crystallization ages of regional igneous rocks from tectonic plates to the north
(i.e., PKC: the Paleo Kazakhstan Continent) and south (i.e., STB + Tarim Craton) of
the Paleo South Tianshan ocean show a broadly consistent peak around ~280-300
Ma (see the compilation in Wang et al., (2018), and references therein); ii) Detrital
zircon U-Pb ages of (meta-) sediments from both the north/south plates and the
accretionary complex (including the AMC and Atbashi metamorphic complex at the
western part of SCTSZ, and the Wuwamen complex at the eastern part of SCTSZ,
Fig. 1b) uniformly show either youngest detrital zircon ages of ~300 Ma or peaks
around ~280-300 Ma (check the compilation in Wang et al., (2018), and references
therein); iii) ~285 Ma strongly peraluminous, post-collisional leucogranites cut across
the HP-LT belt in the STMB (Fig. 15; Gao et al., 2011b). Overall, available magmatic
and/or metamorphic constraints (e.g., this study) tend to show that the subduction of
the Paleo South Tianshan ocean basin and relevant deposition were probably still
active during the Late Carboniferous (Fig. 16a). Final emplacement of the AMC
complex could have taken place around early Permian-late Carboniferous (i.e.,
~280-300 Ma, Fig. 16e), prior to the collision of the Paleo Kazakhstan Continent and
the Tarim Craton (+STB, Fig. 16a).

972

8. Conclusions
___

The main results of our study of the Akeyazi metamorphic complex, in the Chinese
Southwestern Tianshan metamorphic belt (i.e., a portion of the South Central
Tianshan Suture Zone, which also comprises the Atbashi, Chatkal and Fan-
Karategin (U)HP metamorphic complexes), can be summarized as follows:

• What was previously proposed as a HP/UHP “melange” should be considered
979

into several coherent, kilometre-scale, essentially metavolcanoclastic tectonic
units bearing contrasting P-T-time-D histories (and to some extent lithologies).

980

981

* The present-day geometry of the AMC is that of a metamorphic dome preserving evidence for internal nappe stacking.

982

983

* Pervasive deformation following eclogite and/or blueschist-eclogite peak burial is marked by pervasive BS facies, exhumation-related shear senses with a top to the north component.

984

985

986

* At least four of the six sub-units identified here, namely the UH (~2.75 GPa and 470-565 °C), EB (~2.1 GPa and 505°C), MU (~1.45 GPa and 485°C) and GT units (>~0.7-1.0 GPa and 470-520°C) experienced subduction and were buried to depths of ~85, 65, 45 and 30km respectively.

987

988

989

990

* In situ laser probe $^{40}$Ar-$^{39}$Ar radiometric dating of phengite and zircon U-Pb dating yield peak burial ages of 320 ± 1, 332 ± 2, 359 ± 2 Ma and ~280-310 Ma, respectively, for the UH, EB, MU and regional GS facies units (GS1, GS2 and GT), with recrystallization enhanced by fluid circulation and deformation.

991

992

993

994

* At least four short-lived episodes of detachment (~5-10 Ma) occurred in the South Tianshan ocean subduction zone, beneath the Paleo Kazakhstan Continent, at ~360, ~330, ~320 and ~300-310 Ma, with respect to long-lived subduction processes (>120 Ma).

995

996

997

998

* The tectono-metamorphic evolution records a progressive change of metamorphic gradients from ~12 to ~6-7 °C/km, which could reflect a cooling of the subduction system with time.

999

1000

1001

* Juxtaposition and exhumation of the UH, EB, MU and GT units to mid-crustal depth (~20 km) was accomplished around 290-300 Ma, at rates on the order of 1-3 mm/yr.

1002

1003
1004

* The existence of ~300 Ma glaucophane-bearing BS/GS facies
metavolcanoclastic units (i.e., the GT and part of GS1 units) suggests that the
subduction of the Paleo South Tianshan ocean basin was probably still active
during late Carboniferous.

1005

1006

1007

1008

* Final juxtaposition of the diverse sub-units making the metamorphic dome could have occurred at around ~280-290 Ma, broadly coeval with dextral strike-slip movements along the STCSZ.

1009

1010

1011

1012

Acknowledgements

1013

1014

1015

1016

1017

1018

1019

1020

1021

1022

1023

1024

1025

1026

1027

This study was funded through projects of the National Key R & D Program of
China (No. 2017YFC0601206), China Geological Survey (No. DD20190370) and the
National Natural Science Foundation of China (No. 41390445, 41025008) to Prof.
Gao Jun and by the International Training Network ZIP (Marie Curie Actions,
Zooming In-between Plates, REA grant agreement No. 604713) to Prof. Philippe
Agard. Additional supports were provided by programs of the Chinese Academy of
Sciences of China (No. GJHZ1776, 2013047) to Prof. Wan Bo and by the project of
National Natural Science Foundation of China (No. 41802093) to Dr. Hong Tao. We
greatly benefitted from discussions with Labrousse L., Yang J-J., Chen Y., Li J-L.,
Locatelli M. and Bonnet G. We also thank Fialin M. and Rividi N. (CAMPARIS),
Boudouma O. (ISTeP), Arnaud N. (Géosciences Montpellier) and Deldicque D. (ENS
Paris, France) for analytical support, Delairis E. and Devaux D. (ISTeP) for the
preparation of thin sections. We further thank the meticulous help of Prof. Klemd
Reiner and Li J-L during field work.
1028 **Figures captions**

1029 *Color should be used for all figures in print*

1030 **Figure 1:** (a) Tectonic sketch showing the middle to western segments of the Altaids, modified after Gao et al. (2017), Soldner et al. (2017) and Xiao et al. (2015). Colored stars with number mark the occurrence of high (Ultra-high) pressure/low temperature metamorphic rocks (see Gao et al. (2017) and references therein): 1. Akeyazi, ~320 Ma; 2. Atbashi, ~320 Ma; 3. Chatkal, ~301 Ma; 4. FanpKarategin, ?; 5. Beishan, ~465 Ma; 6. Chara Zone, ~445 Ma; 7. SW Mongolia, ~540 Ma; 8. Gony Altai, ~630 Ma; 9. Aktyuz, ~460-470 Ma; 10. Chu-Yili, ~490 Ma; 11. Makbal, ~475-500 Ma; 12. Barleik, ~492-504 Ma; 13. Central inner Mongolia, <235 Ma; 14. Toudaoqiao, ~490-510 Ma; 15. Yilan, ~180~210 Ma; 16. Kokchetav, ~524-537 Ma. (b) Simple sketch showing the relationship between main contacts and tectonic units in the South Tianshan Orogen and adjacent region. CTB = Central Tianshan Block, KNTB = Kyrgyzstan North Tianshan Block, NTSZ = North Tianshan Suture Zone, TNNSZ = Terskey-North Nalati Suture Zone, SCTSZ = South Central Tianshan Suture Zone, DNSZ = Djalair-Naiman Suture Zone, NTF = North Tarim Fault, TTF = Talas-Fergana Fault. (c) Geological map of the Southwestern Tianshan HP/UHP metamorphic belt (STMB). The structural data was plotted according to the measurement from this study, Bayet et al. (2018), Gao et al. (1998), Lin et al. (2008) and Soldner et al. (2017). The location of coesite reports follows the compilation of Tan et al. (2017), Bayet et al. (2018) and Lü et al. (2019). SCTSZ = South Central Tianshan Suture Zone, NHF = Northern Haerkeshan Peak Fault.

1050 **Figure 2:** (a) Regional P-T compilation of the different lithologies (numbers correspond to publications listed in Table S4). Light-blue thin lines represent calculated stability limits of lawsonite for the MORB (Tsujimori and Ernst, 2014) and
pelitic bulk-rock compositions (Wei et al., 2009). (b) Detailed P-T-time history of a representative coesite-bearing oceanic eclogitic slice (Tan et al., 2017), consistent with the result from recent (Quartz Raman elastic) pressure-(Zr-in-rutile & RSCM) temperature mapping (Bayet et al., 2018). GS, EA, AM, GR, BS and EC respectively indicate greenschist, epidote-amphibolite, amphibolite, granulite, blueschist and eclogite metamorphic facies. (c) P-T characteristics of the STMB compared to the global compilation (after Agard et al., 2009; Guillot et al., 2009). Worldwide representative fossil subduction suture zone is marked by bold text & rectangle. KO: Kokchetav; SQD: South Qinling-Dabie China; NQDS: North Qinling-Dabie-Sulu China; DM: Dora Maira Italy; ZS: Zermatt Switzerland; VO: Voltri Italy; FC: Franciscan C. California; SI: Sistan Iran; SLC: Schistes Lustrés Corsica; SLA: Schistes Lustrés Western Alps; ZA: Zagros Iran. Coesite-Quartz (Coe-Qz) transformation follows the one that Bohlen and Boettcher (1982) calibrated experimentally.

**Figure 3:** (a) Close-up views of the Akeyazi metamorphic complex (AMC) showing areas encompassed by our review of previous works. (b) Protolith types inferred from previous works. Color key: Green, eclogite; Blue, blueschist; Grey, metavolcanoclastics. Abbreviation: N/E-MORB: Normal-/Enriched-middle ocean ridge basalt; OIB: Oceanic island basalt; UCC: Upper continental crust. (c) Previous P-T estimates in various localities (corelated to Fig. 2a). Numbers on the lower-right corner of the boxes are related to publications collected in Table S4. CTB, PS and RB respectively indicate P and/or T estimates conducted via conventional thermobarometer, pseudosection modeling and Raman-based thermobarometer. (d) Available age data (also see Tan et al., 2017). Letters on the lower-left corner of the boxes refer to lithology: "M" for mafic (typical eclogite), "m" for matrix (mainly meta-
1078 volcanic sediment with abundant amphibole) and "m" for micaschist (mainly felsic).
1079 Colors refer to different isotopic radiochronology methods on contrasting mineral
1080 system with specific closure temperature. Numbers on the upper-right corner of the
1081 boxes refer to publications compiled in Table S5.

1082 **Figure 4:** (a) Close-up views of the Akeyazi metamorphic complex (AMC) showing
1083 sampling localities and reappraised tectonic sub-units & contacts. The location of
1084 coesite reports follows the compilation of Tan et al. (2017) and Bayet et al. (2018).
1085 The location of lawsonite reports were cited from Li et al. (2013), Du et al. (2011) and
1086 Du et al. (2014). Samples with black frame were studied with the electron probe
1087 microanalysis (Table 1). (b, c) Simple sketches showing the controversy about the
1088 regional geodynamic model: Tectonic mélange (e.g., Klemd et al., 2011; Li et al.,
1089 2016) versus UHP/HP sub-belts (e.g., Lü et al., 2012). (d) Reappraisal geological
1090 map of the AMC (this study).

1091 **Figure 5:** (a) Close-up view of the area along the Kebuerte valley showing sampling
1092 sites, measured shear senses and the location of field observations (Fig. 6). (b) The
1093 summary of the pattern of regional schistosity and lineation. Detail data with respect
1094 to measurement location are presented in Figure 7. (c) Simple map with the plots of
1095 structural element showing the location of field observations corresponding to those
1096 in Figures 8, S2 and S3. (d) Approximately N-S ~30km-long section constructed,
1097 along the Kebuerte valley, across the previous so-called HP/UHP mélange (e.g.,
1098 Klemd et al., 2011; Li et al., 2016) and further touch to the South margin of the STMB
1099 around the Northern Haerkeshan Fault (NHF), with detailed sample sites and
1100 analysis strategy. (e) The Akesayi section from recent work (Bayet et al., 2018) was
1101 also cited for comparison.

1102 **Figure 6:** Panoramic views of the main sub-units and contacts in the AMC, with
representative sample localities. Also check Figure S1 for the rest panoramic
pictures.

**Figure 7:** (a) The structural sketch showing the extension on E-W of the revised AMC, with (b, c) the detail data of field structural measurements for the schistosity and lineation.

**Figure 8:** Outcrop observation of representative samples in this study. The letter of sub-figures corresponds to the localities marked in Figure 5c. (a) Metavolcanoclastics (sample, #135) matrix hosting eclogite lens in the UHP/HP complex (UH unit). (b) Stretched quartz veins in metavolcanoclastics (sample, #26) from the UH unit advocating for non-coaxial shearing (top to N). (c) Blueschist matrix (sample, #117) hosting eclogite lens in the EC/BS facies blueschist horizon (EB unit). (d) σ-type garnet porphyroclast in blueschist (sample, #32) from the EB unit indicating a top to NNW shear sense. (e) Likely pillow structure in the metamafic slice (sample, #158) from the Metamafic/ultra-mafic unit (MU unit). (f) Stretched quartz veins in metavolcanoclastics (sample, #71) from the GS facies transitional unit (GT unit) indicating a top to SEE shear sense. (g) Stretched chlorite-quartz assemblage in the GS facies metavolcanoclast (sample, #102) from the North SCTSZ mylonitized shear zone (GS1 unit) advocating for a top to N shear sense. (h) Stretched chlorite-quartz assemblage in the GS facies metavolcanoclast (sample, #146) from the South SCTSZ mylonitized shear zone (GS2 unit) indicating for a top to S shear sense. Also check Figure S2 for the rest of outcrop observations.

**Figure 9:** Microphotographs of some of the samples investigated in the present study. (a, b, c) Grt-Ph-Amp-Qz-Rt±Cpx±Ep±Chl±Ttn±Ctd±Pg±Bi assemblages and relevant garnet inclusion patterns in metavolcanoclastics from the UH and EB units. (d) Serpentinized ultra-mafic rock (sample, #156-3) with relict diopside. (e) Grt-Act-
1128 Gln-Brs-Ep-Ttn(+ relict Cpx and Ph) assemblage and relevant garnet inclusion
1129 pattern in metamafic sample #158 from the MU unit. (f, g) The pattern of the
1130 appearance of amphiboles, with euhedral actinolite columnar crystals rounded by
1131 anhedral glaucophane matrix then both partly replaced by barroisite rim, in sample
1132 #158 is distinct compared with those in samples from the UH and EB units (normally
1133 Gln replaced by Brs im), further indicating contrasting P-T (-time) experiences. (h)
1134 Strongly retrogressed metavolcanoclast (sample, #22) located in the GS1 unit with
1135 relict glaucophane and garnet replaced by Chl+Bi assemblage. (i) S-C structure
1136 constructed by Chl+Ph+Qz assemblage in GS facies metavolcanoclast (sample,
1137 #71) from the GT unit indicating a top to E shear sense. Also check Table 1 for the
1138 detail of mineral assemblage of each sample and Figure S3 for the rest of
1139 microphotographs. Mineral abbreviations are Qz, quartz; Grt, garnet; Ph, phengite;
1140 Pg, paragonite; Bi, biotite; Di, diopside; Omp, Omphacite; Gln, Glaucophane; Brs,
1141 barroisite; Act, Actinolite; Ep/Czo, epidote/clinozoisite; Chl, chlorite; Ctd, chloritoid;
1142 Rt, rutile; Ttn, titanite; Fsp/Ab, feldspar/albite; Carb, carbonate; Atg, antigorite.

1143 **Figure 10:** Primitive mantle-normalized spider diagram and chondrite-normalized
1144 REE pattern for samples in this study. Reference curves for average N-MORB and
1145 OIB are from Sun and McDonough, (1989). The upper continental crust (UCC) and
1146 shale & pelagic clay are respectively after Rudnick and Gao, (2003) and Plank and
1147 Langmuir, (1998). The detail of bulk rock trace element compositions is listed in
1148 **Table S1.**

1149 **Figure 11:** Mineral plots of compositions determined with the electron probe
1150 microanalysis (check **Table 2** for detail data). X<sub>Mg</sub> is herein after taken as
1151 Mg/(Mg+Fe<sup>2+</sup>). (a) Na (site B) versus Si content (apfu) in all the AMC amphiboles
1152 analyzed in this study. Compositions mainly cluster in the glaucophane field and
around a composition intermediate between actinolite and barroisite but almost span the entire range. Terminology is after Hawthorne & Oberti (2006). (b) Composition of sodic and calcic clinopyroxenes (Morimoto, 1988). (c) Phengite composition variations, which are mostly parallel to the muscovite-celadonite (Ms-Cel) joint. The variations are dominated by the tschermak substitution exchange vector Si(Mg, Fe)-2Al as for most metavolcanoclastics and metavolcanics. Note that phengite in sample #135 and #26 from the UH unit reach exceptionally high tschermak values (Si⁴⁺>3.6, Table 2). (d) Chlorite plot mostly along the amesite-clinochlore (Ames-Clin) joint as a result of the tschermak substitution. (e) Triangular plot for garnet composition expressed as a function of their grossular, pyrope and almandine (+spessartite) content. Thick gray arrows indicate core to rim evolution of the composition observed in the garnet profiles of each sample. Garnet profile patterns of representative samples from different sub-units are also present as examples.

Mineral abbreviations, Gln: glaucophane; Wnc: winchite; Barrois./Katoph.: barroisite/katophorite; Act: actinolite; Mg-Hornbl.: Magnesio-hornblende; Quad: quadrilateral component which include diopside, hedenbergite, enstatite and ferrosilite; Jd: jadeite; Aeg: Aegirine; Wo: wollastonite; Di: diopside; Aug: augite; Pgt: pigeonite; Cen: (clino)enstatite; Cfs: (clino)ferrosilite; Ms: muscovite; Cel: celadonite; Prl: pyrophyllite; Clin+Daph: clinochlore+daphnite; Ames: amesite; Sud: sudoite; Grs: grossular; Prp: pyrope; Alm+Sps: almandine+spessartite.

**Figure 12:** P-T plot showing the results of our P-T estimates using THERMOCALC (HOLLAND and POWELL, 1990) together with estimates of metamorphic temperature by the Raman spectroscopy of carbonaceous material (RSCM, Beyssac et al., 2002; Beyssac et al., 2004) and Ti-in-biotite geothermometry (Henry et al., 2005). Ellipses indicate confidence intervals obtained with THERMOCALC. Ellipses
with nondense dash-, soild- and dense dash-lines respectively correspond to P-T
estimates for pre-peak, peak and post peak metamorphism. (a) Results without
further interpretations. Light-blue thin lines represent calculated stability limits of
lawsonite for the MORB (Tsujimori and Ernst, 2014) and pelitic bulk-rock
compositions (Wei et al., 2009). Asterisk indicates the avPT test (sample #32 as
example) calculated with epidote instead of assuming lawsonite present. (b) Results
plotted with synthetic P-T paths, radiometric indications and geodynamic implications
(e.g., exhumation velocities) for different sub-units. Colored rectangles (solid-line:
metavolcanics, dash-line: metavolcanoclastics) showing previous works, which
belong/close to the same sub-units as redefined in this study (Fig. 15c, Table S5),
with relatively precise P-T estimates tied to comparable age constraints. Numbers in
black boxes correspond those in Table S5, for the detail compilation of regional P-T-
time works. Notes that average peak P-T estimates correspond to a metamorphic P-
T gradient decrease for rocks exhumation from different depths: the UH ~6, Eb ~7,
MU ~10 and GT ~12°C/km. The maximum burial conditions of each sub-units were
reached at the timing broadly as their index minerals (i.e., phengite or zircon)
recorded and revealed by in situ Ar-Ar or U-Pb technique (Figs. 13, S4): the UH
~320, EB ~330, MU ~360 and GT ~ 310 Ma. The final juxtaposition of these sub-
units could take place around 280-300 Ma, as echoed by the dominance of ages
within this range probed from moderately phengitic mica in samples amongst all
those four sub-units (Figs. 13, S4). Check Table 2 for corresponding mineral
compositions and Table 3 for detailed results as well as mineral assemblage that
used for avPT calculations.

**Figure 13:** (a) Radiometric ages obtained for all samples, with respect to respective localities on Kebuerte section. Check Tables 4, S2 and S3 for detailed isotopic
results. (b) Examples of in situ $^{40}$Ar-$^{39}$Ar laser probe ablation result. Overlays show surface areas from which argon was extracted during measurement, and numbers correspond to ages obtained. Phengite grains following the schistosity are marked as white overlays, while light blue overlays represent those encountered along late shear bands. The in situ Ar-Ar probing ages (e.g., used in Figures 12b, 13a, 14, 15 and text) for constraining the deformation around peak metamorphism of each samples are calculated from the average of absolute ages obtained from phengite grains following the schistosity and shear bands. Orange overlays indicate phengite grains with ambiguous old age that probably results from excess Ar or imply early growth of phengite. Green overlays locate phengite grains with relative younger age which could indicate later stage recrystallization. Colored sketch, exported from the combined microprobe mapping matrix data via Xmaptools (https://www.xmaptools.com, Lanari et al., 2014), clearly show the mineral assemblage with respect to deformation. (c) Diagrams of phengite chemistry versus in situ Ar ages (and dispersion within individual samples), indicating larger dispersion of in situ ages with greater dispersion in phengite chemistry. Detail of the rest of in situ Ar-Ar probing results are illustrated in Figure S4 (Table 4).

**Figure 14:** The age histograms of each sub-unit in the AMC from this study. Black dash-line spectrum represents age pattern of previous works from adjacent areas with respect to respective sub-units. Other major geological constraints are also recalled for comparison: (i) discrete hydrothermal zircon ages obtained from regional rodingite (Li et al., 2010) probably witness the fate of SW-Tianshan ocean basin; (ii) The range of formation age of regional igneous rocks and ophiolites are cited from Jiang et al. (2014) and Wang et al. (2018). (iii) Metamorphic ages of (U)HP rocks from the Kyrgyz SW-Tianshan are cited from Loury et al. (2016), Loury et al. (2018)
1228 and Mühlberg et al. (2016). The color of bins corresponds to ages tied with some P-T
1229 estimates (after the color bar in Figure 15b)

1230 **Figure 15:** Regional compilation, from west to east, of age constraints for the major
1231 valleys (Kekesu, Akeyazi-Akesayi/-Habutengsu/-Kebuerte/-Atantayi, Muzaerte-
1232 Changawuzi; Figs. 1c). Bold letters refer to results from this study, while grey
1233 numbers correspond to previous works. Empty dashed rectangles indicate previous
1234 works lacking precise P-T constraints and/or tectonic control. The color of bars
1235 highlights ages linked to some P-T estimates. Those with black bars mark ages with
1236 metasomatic or magmatic origins (see Table S5 for details).

1237 **Figure 16:** (a) Paleogeographic reconstruction during the Gondwana assembly at
1238 ~510-470 Ma and Pangea assembly at ~300-280 Ma (inspired and modified after
1239 Domeier and Torsvik (2014), Wan et al. (2013, 2018) and Wang et al. (2018)) with
1240 emphasizing the tectonic evolution of Paleo South Tianshan Ocean. LA = Laurentia,
1241 BA = Baltica, SI = Siberia, PKC = Paleo-Kazakhstan Continent, SC = South China
1242 Block, K = Karakum Craton, T = Tarim Craton, IN = Indochina, NC = North China
1243 Craton, AM = Amuria. (b) Interpretative sketches (inspired after Angiboust et al.
1244 (2016), Locatelli et al. (2018) and Plunder et al. (2015)) relating the formation and
1245 exhumation of the SW-Tianshan HP/UHP-LT accretionary complex during the period
1246 360-280 Ma.

1247

1248 **Tables:**

1249 **Table 1:** Parageneses of the Main Samples Studied in Chinese SW-Tianshan
1250 HP/UHP-LT Accretionary Complex

1251 **Table 2:** Representative Analyses of the Main Mineral Groups Found in Samples
1252 from Different Sub-units in the Akeyazi Metamorphic Complex
1253 **Table 3:** Calculations Performed with THERMOCALC Average P-T and RSCM & Ti-
in-Biotite Thermometers

1254 **Table 4:** Results of In Situ Phengite Ar-Ar Radiometric Data

1256
1257

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<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

Previous PT estimates with/without
radiochronological constraint

UHP reports with coe finds
UHP reports by CTB/PS/RB-estimates
<figure><img src="image_11.png" /></figure>

after Klemd, Li et al

after Lü et al

this study
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>
<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>
<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>
<figure><img src="image_26.png" /></figure>

UH

a

<figure><img src="image_27.png" /></figure>

b

<figure><img src="image_28.png" /></figure>

EB

c

<figure><img src="image_29.png" /></figure>

NNW

SSE

<figure><img src="image_30.png" /></figure>

MU

e

<figure><img src="image_31.png" /></figure>

NWW

SEE

<figure><img src="image_32.png" /></figure>

GS1

g

<figure><img src="image_33.png" /></figure>

h

GT

GS2
<figure><img src="image_34.png" /></figure>

UH

a

<figure><img src="image_35.png" /></figure>

b

<figure><img src="image_36.png" /></figure>

EB

C

<figure><img src="image_37.png" /></figure>

d

<figure><img src="image_38.png" /></figure>

MU

e

<figure><img src="image_39.png" /></figure>

g

<figure><img src="image_40.png" /></figure>

GS1

h

<figure><img src="image_41.png" /></figure>

i

GT
UH

<figure><img src="image_42.png" /></figure>

EB

<figure><img src="image_43.png" /></figure>

MU

<figure><img src="image_44.png" /></figure>

GS
+
GT

<figure><img src="image_45.png" /></figure>

<figure><img src="image_46.png" /></figure>

<figure><img src="image_47.png" /></figure>

<figure><img src="image_48.png" /></figure>

<figure><img src="image_49.png" /></figure>
<figure><img src="image_50.png" /></figure>

<figure><img src="image_51.png" /></figure>

<figure><img src="image_52.png" /></figure>

<figure><img src="image_53.png" /></figure>
<figure><img src="image_54.png" /></figure>
<figure><img src="image_55.png" /></figure>

b UH

<figure><img src="image_56.png" /></figure>

C

<figure><img src="image_57.png" /></figure>

EB

<figure><img src="image_58.png" /></figure>

GT

<figure><img src="image_59.png" /></figure>
GS1+2

<figure><img src="image_60.png" /></figure>

<figure><img src="image_61.png" /></figure>

UH

<figure><img src="image_62.png" /></figure>

EB

<figure><img src="image_63.png" /></figure>

MU

<figure><img src="image_64.png" /></figure>

GT

<figure><img src="image_65.png" /></figure>
<figure><img src="image_66.png" /></figure>

leucogranite dike
~ 285 Ma
(Gao et al., 2011)
ca. 510 to 470 Ma

<figure><img src="image_67.png" /></figure>

<figure><img src="image_68.png" /></figure>

ca. 300 to 280 Ma

<figure><img src="image_69.png" /></figure>

<figure><img src="image_70.png" /></figure>

a

NNW

South Tianshan ocean

SSE

<figure><img src="image_71.png" /></figure>

b

slab rollback/kink or kinematics change?

<figure><img src="image_72.png" /></figure>

c

<figure><img src="image_73.png" /></figure>

ca. 330 Ma

slab rollback/kink or kinematics change?

<figure><img src="image_74.png" /></figure>

d

<figure><img src="image_75.png" /></figure>

GS facies detachment ?

ca. 310-320 Ma

UHP detachment

transient deep accretion
of mass sediment

<figure><img src="image_76.png" /></figure>

e

<figure><img src="image_77.png" /></figure>

exposed section

ca. 280-300 Ma

approximate scale

<figure><img src="image_78.png" /></figure>