# Concordant pulse in Mn, Y and HREEs concentrations during UHP eclogitic garnet growth: Transient rock dynamics along a cold subduction plate interface

Zhou Tan, Philippe Agard, Jun Gao, Tao Hong, Bo Wan

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Zhou Tan, Philippe Agard, Jun Gao, Tao Hong, Bo Wan. Concordant pulse in Mn, Y and HREEs concentrations during UHP eclogitic garnet growth: Transient rock dynamics along a cold subduction plate interface. Earth and Planetary Science Letters, 2020, 530, p.501-507. 10.1016/j.epsl.2019.115908. insu-03594508

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1 **Concordant pulse in Mn, Y and HREEs concentrations**

2 **during UHP eclogitic garnet growth: transient rock**

3 **dynamics along a cold subduction plate interface**

4

5 **Key words:** UHP oceanic eclogite; garnet zoning; Zr-in-rutile thermometry;

6 diffusion modeling; cold subduction regime; Chinese SW-Tianshan;

7

8 **Authors:** Zhou Tan<sup>a,b, *</sup>, Philippe Agard<sup>b</sup>, Jun Gao<sup>c,d,*</sup>, Tao Hong<sup>c,d</sup>,

9 **Bo Wan<sup>a,d</sup>**

10 a. State Key Laboratory of Lithospheric Evolution, Institute of Geology and

11 Geophysics, Chinese Academy of Sciences, 100029 Beijing, China

12 b. Sorbonne Université, CNRS-INSU, Institut des Sciences de la Terre de Paris,

13 ISTeP UMR 7193, F-75005 Paris, France

14 c. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics,

15 Chinese Academy of Sciences, 100029 Beijing, China

16 d. College of Earth and Planetary Sciences, University of Chinese Academy of

17 Sciences, 100049 Beijing, China

18 e. Institute of Earth and Environmental Sciences, University Freiburg, 79104

19 Freiburg, Germany

20 *Corresponding author:

21 **Zhou Tan**, tanzhou@mail.iggcas.ac.cn; **Jun Gao**, gaojun@mail.iggcas.ac.cn;
22 **Abstract**

We herein report strikingly concordant Mn, Y, and HREE garnet zoning in

ultrahigh-pressure low-temperature oceanic eclogites found in the fossil

Devonian-Carboniferous subduction plate interface in SW-Tianshan.

Zr-in-rutile thermometry, thermodynamic modelling and diffusion-controlled

garnet growth modeling show that this type of zoning can be explained by a

short-lived transient, most likely a ~80 °C thermal pulse (from 490 to 570 °C).

This event could be as transient as ~300 Ka, assuming a prograde to peak

burial duration of 5 Ma. Such garnet zoning is observed for grains sampled

meters apart (Fig. 1d) and was detected in other eclogites in the region. This

transient event could reflect the short-lived movement of UHP oceanic slices,

once detached, towards the hotter mantle wedge, and/or deformation-induced

remobilization of fluids enhancing diffusion-controlled garnet growth.
35 **1. Introduction**

36 Garnet plays a vital role in disclosing mechanical and thermal processes
37 at plate boundaries, by constraining complex pressure-temperature-time
38 histories (e.g., Baxter et al., 2017; Caddick and Kohn, 2013, and references
39 therein) and kinetics of metamorphic processes (e.g., Angiboust et al., 2012;
40 Berg et al., 2013; King et al., 2004; Moore et al., 2013; Prior et al., 2002; Skora
41 et al., 2006; Yang and Rivers, 2003). Garnet also yields unique information
42 about nucleation and growth (Carlson, 2011), fluid-infiltration events (King et
43 al., 2004; Li et al., 2013) and timescales of peak metamorphic heating (Ague
44 and Baxter., 2007), burial/exhumation and retrogression (Caddick et al., 2010;
45 Skora et al., 2009).

46 In particular, crucial information can be retrieved by studying special
47 garnet compositional zoning, e.g. (i) oscillatory/rhythmic zoning of major
48 elements recording fluid infiltration or subtle P-T path perturbations (Garcia -
49 Casco et al., 2002; Li et al., 2013; Schumacher et al., 1999), (ii) healed fracture
50 zoning in major elements related to subduction zone seismicity and
51 eclogite-facies brittle deformation (Angiboust et al., 2012; Broadwell et al.,
52 2019), (iii) Y+REEs secondary peak zoning revealing major or accessory
53 mineral breakdown (King et al., 2004; Pyle and Spear, 1999; Yang and Rivers,
54 2003) or diffusion-controlled dynamic growth (Skora et al., 2006).
This study focuses on the evolution of deeply subducted, coesite-bearing
oceanic eclogites from the SW-Tianshan UHP metamorphic complex, whose
P-T-time evolution was recently reappraised (Tan et al., 2017). An unusual
garnet zoning with concordant, annular enrichment in Mn, HREEs and Y (Figs.
2a-c), but not in other major (Fe, Ca, Mg) or key trace elements (L/MREEs, Ti,
Sr, Zr, Sc, Rb and P; Fig. 3), is observed in some rocks. Based on Zr-in-rutile
thermometry and detailed petrology, geochemical diffusion and
thermodynamic modeling, we suggest that a short-lived thermal pulse of
~80°C best accounts for this joint Mn, HREEs & Y enrichment, possibly with
some help from fluid remobilization. We then discuss implications for element
mobility and transient rock dynamics along a deep cold subduction plate
interface.

67

## 2. Geological setting

The Chinese Southwestern Tianshan HP/UHP-LT metamorphic complex
extends for about 200 km along the Southwestern Central Tianshan Suture
Zone (SCTSZ; Figs. 1a-b). It is correlated with the Atbashi metamorphic
complex in the Southwestern Tianshan Accretionary Complex (Hegner et al.,
2010) and the Fan-Karategin metamorphic belt (Volkova and Budanov, 1999).
The SCTSZ bounds to the north the Chinese section of the HP-UHP/LT
75 metamorphic complex, known as the Akeyazi metamorphic complex (AMC). To
76 the north lies a LP-HT Palaeozoic active continental margin (Gao et al., 1998),
77 mainly made of amphibolite- and granulite-facies rocks, along with Late
78 Silurian and Early Carboniferous continental arc-type volcanic and
79 volcaniclastic rocks and granitoids (Gao et al., 2009). To the south the AMC is
80 overlain by unmetamorphosed Palaeozoic sedimentary strata, which also
81 represent the northern, passive continental margin of the Tarim plate (Carroll
82 et al., 1995).

83 The AMC is predominantly composed of strongly schistose
84 metavolcanoclastics hosting mafic metavolcanics, marble horizons (Bayet et
85 al., 2018) and rare ultramafic rocks as pods, lenses, boudins, thin layers or
86 massive blocks (Gao and Klemd, 2003). The AMC was mainly interpreted as a
87 tectonic mélange and thought to have formed in a subduction accretionary
88 wedge during norward subduction of the Southwestern Tianshan Ocean (Gao
89 et al., 1999; Klemd et al., 2014). Whole-rock geochemical data for the mafic
90 metavolcanics suggest oceanic basalt or arc-related affinities (Gao and Klemd,
91 2003; John et al., 2008).

92 A compilation of regional P-T-time constraints (Tan et al., 2017) shows
93 that most peak metamorphic estimates for eclogites and prograde blueschists
94 lie in the range 480-580°C and 1.5-3.0 GPa. A range of P-T conditions of
95 450-570°C at 2.7-3.3 GPa was obtained for eclogite-facies micaschists.
96 Evidence for UHP metamorphism comes from both relict coesite inclusions in
97 garnet from several localities (stars in Fig. 1b) and thermodynamic calculation.
98 The timing of peak metamorphism falls at 315 ± 5 Ma (garnet growth related
99 zircon U-Pb ages), i.e. in a very restricted range with respect to a ~100
100 Ma-long subduction (Tan et al., 2017; their figures 13, 16). Metavolcanoclastic
101 rocks from SW-Tianshan preserve P-T conditions slightly higher than most
102 exhumed HP/UHP oceanic units (Agard et al., 2009; Plunder et al., 2015; Fig.
103 1c).
104

105 **3. Sample description**

106 Three fresh samples of coesite-bearing UHP eclogite (A, B, C; Figs. 1d,
107 S2, S4) were collected from a hm-scale eclogitic mafic slice (Fig. 1b), along
108 adjacent meter thick fresh eclogite layers (Fig. 1d; N42°31′57″, E81°12′37″;
109 Tan et al., 2017). They mainly consist of garnet, omphacite, glaucophane,
110 phengite/paragonite, epidote/clinozoiste and rutile/titanite in variable amounts
111 (Figs. S2, S4). Their N-MORB type bulk-rock compositions, together with
112 analysed εNd(t) ~+8.5 (t=312 Ma) and modeled bulk-rock δ¹⁸O ~8.47‰,
113 suggest that the protolith was altered oceanic crust (Tan et al., 2017). Peak
114 burial conditions, in part recorded by garnet compositions, is in the range
115 470-550 °C, 2.7-3.2 GPa, similar to other reported UHP slices in the region
116 (Tan et al., 2017; their figure 13b).

117 Garnet occurs as idioblastic porphyroblasts with radius ~0.12-1.58 mm
118 (Figs. S2, S4) in the fine-grained omphacite matrix. Garnet size distribution
119 was systematically surveyed for each sample (Fig. S4; see Appendix A for
details). Garnet cores are generally inclusion-rich and host tiny crystals (~5 to
120 50 µm in diameter, Fig. 4c) of omphacite, epidote, chlorite, paragonite, quartz
121 and glaucophane/barroisite (as exemplified by garnet A1; Figs. 4c, S2a).
122

123 Box-shaped inclusions of aggregates of paragonite+epidote (±chlorite) are
124 interpreted as pseudomorphs after lawsonite (Tan et al., 2017, 2018). Coesite
125 in the mantle of garnet A1 (Figs. 2a, 4c) is located immediately outside the
126 annular, concordant Mn-HREE-Y pulse (Figs. 2a-c). Omphacite occurs as
127 subhedral-anhedral fine grains (~0.05 to 0.1 mm across), either as the main
128 matrix phase or as inclusions in garnet (Fig. 4c). Blue-green amphibole is
129 found as glaucophane inclusions in garnet (Fig. 4c) and barroisite with
130 glaucophane cores in the matrix. Rutile (Figs. 2d, S1a) is present both in
131 garnet porphyroblasts as armored relics and in the matrix, where partly
132 replaced by titanite. Chlorite is included in or cuts across garnet (Figs. 4c,
133 S2a).
134

134
135 **4. Mineral geochemistry**

136 Systematic major and trace element analyses were performed for garnet
137 of all sizes, for relevant inclusions hosted in garnet (i.e., A1) and for matrix
138 minerals. Representative data are shown in Table S2.

139 **4.1. Garnet compositional profile and mapping**

140 Microprobe major element composition profiles and mapping were
141 conducted for eight garnet porphyroblasts representative of the size
142 distribution in the studied eclogites (Figs. S2, S3, S4). Additional
143 high-resolution mapping for key elements (Mn, Y, Ti and P; Figs. 2a, 2b, S1)
144 and LA-ICP-MS trace element profiles were performed on one coesite-bearing
145 garnet grain (i.e., A1, Fig. 2).

146 **4.1.1. Concordant, annular zoning in Mn, Y and HREEs**

147 Garnet growth zoning reveals a concordant pulse in Mn, Y and HREEs
148 concentrations, as shown for garnet porphyroblast A1 in figures 2, S2, S4. This
149 similarity between Mn, Y and HREEs compositional profiles (Figs. 2c, 3) is not
150 observed, however, for other major and trace elements (Figs. 3a, e-h).

151 In detail, Mn, Ho and Dy present relatively wide central peaks (~1/3 of
152 garnet diameter, Figs. 3b-d; Mn 6072 ± 1269 ppm; Ho 3.38 ± 0.788 ppm; Dy
153 13.9 ± 2.74 ppm; Table S2) compared to the rest of HREEs (i.e., Tm, Er, Lu
and Yb) and Y (~1/9 of the diameter; Figs. 3b-d; Tm 1.56 ± 4.55 ppm; Er 10.0 ± 2.64 ppm; Lu 1.54 ± 0.535 ppm; Yb 11.2 ± 3.58 ppm, Table S2). A secondary peak is observed at a rather constant core<sub>peak</sub>-secondary<sub>peak</sub> distance (0.77 ± 0.06 mm for garnet A1, considering uncertainties from LA-ICP-MS analyses spots interval ca. 0.12 mm; Fig. 2c) before the outermost rim is depleted again to approximately 1/5 to 1/3 of the absolute concentrations with respect to their core peaks (Fig. 3, Table S2). Note that only Y concentrations determined by LA-ICP-MS analyses are shown in figure 2b since concentrations of ~40 to 110 ppm are too low to be detected by microprobe analyses.

In contrast, neither central nor secondary peaks can be observed on the profiles and mapping of other garnet major components (i.e., Fe, Mg and Ca; Figs. 3a, S1c, S1d). The X<sub>grs</sub> (mol % of grossular) gradually decreases from 0.18-0.20 to 0.15-0.16 then increases to 0.22-0.24 in the outermost rim, while the X<sub>prp</sub> monotonously increases from 0.09-0.10 to 0.22-0.23 but shows minor oscillations at the outermost rim (Fig. 3). The X<sub>alm</sub> evolution is quite simple with almost constant values from core to mantle (i.e., 0.67-0.7), then gradually decreases to 0.54-0.57 at the outermost rim (with a small near-edge plateau; Fig. 3).

Compositional profiles of MREEs and key trace element (most LREEs analyses are below detect limits; Table S2) also largely differ from the Mn-HREE-Y zoning (Figs. 3e-h). None of the central peaks can be identified
175 (e.g., Sm and Eu are even depleted in the core; Fig. 3e). Secondary peaks can
176 be noticed among the profiles of some elements (e.g., Sc, Sm), but their
177 position varies for each element. Ti and P mapping (Figs. S1a-b) suggests joint
178 growth of garnet with Ti- and P-bearing phases.

179 **4.1.2. Concordant Mn pulse among diverse size garnets**

180 Eight representative garnet grains from samples A-B-C were selected for
181 detailed compositional investigations (garnets A1-2-3, B1-2-3-4, C1, ranging
182 from 0.35 to 1.58 mm in radius), based on an extensive garnet size survey
183 (Figs. 4g, S4). Mn profiles vary with respect to garnet radius (Figs. 4a-b): (i)
184 large garnet grains (A1 ~1.22mm; A2 ~1.11mm; B1: ~1.58mm; B2: ~0.90mm;
185 C1: ~1.12mm) show marked central peaks and symmetrical secondary peaks;
186 (ii) middle-size garnet grains (A3: ~0.42mm; B3: ~0.51mm) show depleted
187 cores with moderate secondary peaks yet Mn concentrations comparable to
188 the large ones (cMn ~0.66 to 0.72 wt%); (iii) the small garnet B4 (R ~0.35mm)
189 shows a central peak with Mn concentration comparable to secondary peaks
190 of larger garnets (cMn ~0.75 wt%). Corepeak-secondarypeak distances are
191 remarkably similar among large garnets at 0.77 ± 0.04 mm (n=7, 1σ). Although
192 not all garnets were obviously cut through their cores, aligning secondary
193 peaks of the large garnets (Fig. 4a) reveals that their cores are almost aligned,
194 suggesting nucleation started simultaneously. The smaller ones, showing no or
limited Mn pulse (Fig. 4b) could either represent garnet crystals not cut through their center and/or garnets that started growing later.

These relationships between garnet size and Mn zoning suggest that large garnets nucleated almost simultaneously, before middle and then small garnets, explaining a different record of yet similar Mn-HREE-Y enrichment.

The fact that this is observed for samples meters apart (Fig. 1d) suggests the existence of an external control (at outcrop- or km-scale) on diffusion-controlled nucleation and growth mechanism.

## 4.2. Major/trace elements composition of associated minerals

In order to assess the potential mass contribution of Mn, Y and HREEs from associated minerals to this unusual garnet zonation, major and trace element analyses were performed for minerals either included in garnet A1 (i.e., omphacite, amphibole, chlorite, rutile and box-shape paragonite + epidote; Fig. 4c) or in the adjacent matrix. Representative compositions are listed in Table S2.

Chlorite inclusions show the highest average MnO concentrations, ~0.68 wt% (n=8, Fig. 4d, Table S2), i.e. ~60% of garnet core values. Other minerals show variable but insignificant MnO amounts <0.08 wt% (Fig. 4d, Table S2).

Since omphacite and amphibole inclusions are commonly too small for
LA-ICP-MS analyses (~5 to max. 50 µm in diameter, Fig. 4c), trace element compositions considered here are those of matrix minerals. Due to the lack of fresh lawsonite in our samples (fresh lawsonite in the region appears as inclusions, e.g., in quartz and pyrite; Du et al., 2011; Du et al., 2014; Li et al., 2013), literature data are used here (Spandler et al., 2003; Vitale Brovarone et al., 2014. Although the later are mainly lawsonites from metasomatites at lithologic contacts).

Lawsonite and the different garnet domains show similar HREEs enrichments patterns, with a notable plateau ((Lu/Tb)N ~0.32-1.35; Table S2) forty to eighty times higher than that of chondrite (ΣHREEs+Y ~69.5-123, Fig. 4e, Table S2). Lawsonite, however, has much higher concentrations of LREEs, and to a lesser extent MREEs, than garnet. Omphacite and amphibole display flat REEs patterns quite comparable to chondrite in HREEs and Y (Fig. 4e).

## 4.3. Trace element composition of rutile and Zr-in-rutile thermometry

Rutile is either found as anhedral inclusions in garnet (ca. 5 to 30 µm in diameter) or armored by omphacite in the matrix (Figs. 2d, S1a, S5).
Concentrations of Zr, V, Cr, Nb, Si, Fe and Al were measured by EPMA following the procedure of Zack et al. (2004) for rutile grains of all samples,
either included in garnets or in the matrix, avoiding potential relicts of pre-existing Zr-rich phases along grain margins (e.g., ilmenite, Austrheim et al., 2008) and contamination from host minerals (i.e., Si concentrations >200 ppm were discarded). Results are listed in Table S3. Average concentrations of V, Cr and Nb in rutile grains show no correlation with garnet zoning and fall broadly within the range reported for eclogites of the same region (i.e., 1000-2200, 250-1000 and 30-110 ppm; Chen et al., 2012; Li et al., 2013; Zhang et al., 2016).

By contrast, concentrations of Zr in rutile show a trend with respect to garnet zoning (Figs. 2d-e, S5). Concentrations close to the core and rim of garnets A1/B1/C1 (from different samples A/B/C) are approximately the same ~21.1 ± 7.04 ppm (n=100), while inclusions within (or close to) the annular domain of Mn-HREE-Y enrichment yield almost three-times higher values ~63.9 ± 7.10 ppm (n=50). Importantly, this gap in Zr concentration in rutile is also observed for the grains dispersed in the omphacite matrix (Figs. 2d-e, S5), although in this case no clear spatial relationship can be found with any mineral. Additional trace element analysis by LA-ICP-MS for rutile in garnet and matrix, on grains large enough for a ~60 µm laser beam, give results concordant with those obtained through EPMA (Fig. S6; Table S3). This consistency ensures that the very low concentration of Zr-in-rutile by EPMA, even close to the detection limit of ~20 ppm (Zack et al., 2004), is reliable.
Three commonly applied calibrations (Tomkins et al., 2007; Watson et al., 2006; Zack et al., 2004) were used to estimate temperatures from Zr concentrations in rutile (Table S3). We considered a pressure value of 27 kbar (for the calibration of Tomkins et al., 2007) in accordance with P-T estimates for the sample and with the presence of coesite inclusions next to the mantle garnet A1 (Fig. 2a; Tan et al., 2017). Considering pressure variations (i.e., from 2.4 to 2.8 GPa; Tan et al., 2017) and silica polymorph transition (α-quartz to coesite) result in only minor changes in estimated ΔT, around 15°C. Temperature estimates (Fig. 2e) for core, rim and matrix range from ca. 380 to 540 °C, while rutile grains adjacent to the annular Mn-HREE-Y pulse and some in the matrix yield ~80 °C higher temperature estimates, whatever the calibrations (Fig. 2e).

## 5. Thermodynamic modeling

Thermodynamic modelling was used to predict the evolution of mineral modes during garnet growth (notably chlorite and lawsonite; Figs. 4d-e) and evaluate their impact as a source and/or local buffer on the Mn, Y and HREEs budget of the rock. In order to account for element fractionation during garnet growth, three pseudosections were calculated based on evolving effective bulk rock compositions (EBC0-1-2; Fig. 4f, Table S1). The evolution of relevant
mineral modes (Fig. 4f) was calculated along a pressure-constant heating path at 2.75 GPa, as supported by coesite inclusions within the Mn-HREE-Y ring (Fig. 2a) and previous P-T estimates for the various growth domains (M1 to M5, Tan et al., 2017; their figures 7d, 12b). Heating from 450 to 490 °C is followed by a transient 80 °C T pulse (to 570 °C), broadly consistent with regional $T_{max}$ ~500-600 °C by Zr-in-rutile and RSCM thermometers (Bayet et al., 2018; Chen et al., 2012; Meyer et al., 2016). Details on effective bulk rock compositions (EBCs) and $K_D^E$ estimates are given in Appendix A (see also Table S1).

Calculations were performed for different water activity scenarios. Under water-saturated conditions (i.e., $\alpha_{H_2O}$ = 1), moderate amounts of chlorite and lawsonite (~ca. 4-5 vol %) break down during the T pulse (for similar eclogite-facies dehydration see Groppo and Castelli, 2010; Martin et al., 2014; Tsujimori and Ernst, 2013; Vitale Brovarone et al., 2014). For a lower water activity of 0.8, chlorite and lawsonite breakdown increases slightly to ca. 6-7 vol %. Additional chlorite and lawsonite could break down before the T pulse for a water activity down to ~0.6. In the scenario where the T pulse coincides with a change in $\alpha_{H_2O}$ (i.e., from 0.6 to 1.0 on heating, and 1.0 to 0.6 on cooling), the extent of chlorite and lawsonite breakdown is limited to 1-1.5 vol % (Fig. 4f).

295
297 Diffusion modeling (Figs. 5, 6) is herein used to evaluate the conditions
298 required to produce a concordant Mn-HREE-Y enrichment during a ~80°C T
299 pulse, as inferred from petrological data. Parameters in diffusion modeling are
300 briefly introduced in Table 1 (check Appendix B for the detail).

301 Skora et al. (2006) explained the existence of central or annular (but
302 spatially discordant) maxima for both Y and HREEs by thermally accelerated
303 diffusion and garnet REE uptake from the surrounding matrix, assuming a
304 constant garnet growth rate and a non-linear T increase. Similar crystal
305 diffusion modelling has been performed with a prescribed interface
306 velocity/growth rate, instead of calculated as part of the solution (Alexander,
307 2011; Ketcham and Carlson, 2012; Watson and Müller, 2009). However,
308 porphyroblast growth rate depends on the exact growth mechanism (Ague and
309 Carlson, 2013; Carlson, 1989) and on the competition between crystals
310 interacting during diffusion-controlled growth, suggesting time-dependent
311 growth rates with an important stochastic component (Carlson, 2011; Hesse,
312 2012). A time-dependent growth rate is therefore used here for diffusion
313 modeling, except for the parametric study performed with a linear T-t path and
314 growth rate in the next paragraph.
315 **6.1. Influence of key parameters for a linear T-t path**

316 For the sake of comparison, the system size (r), garnet radius (R_f),
317 pre-exponential factor (D_0m) of diffusion coefficient in matrix and activation
318 energy (Q_m) were first tested along a linear T-time path (450 to 550 °C in 5 Ma:
319 black line in Fig. 5b) to evaluate their impact on the existence and location of
320 trace element peaks, using Y as a reference (Figs. 5g-j; Table S4).

321 Increasing system size (r) from 0.2 to 0.5 cm (Fig. 5g) can lead to an
322 outward shift of the annular Y peak of ~0.46 mm (i.e. ~2/5 of fixed garnet
323 radius, Table S4). Increasing garnet radius (R_f) from 0.72 to 1.44 mm (Fig. 5h)
324 will also produce an outward shift of ca. 0.28 mm. However, changing the
325 pre-exponential factor (D_0m) and activation energy (Q_m) results in larger shifts
326 around 0.60-0.68 mm (~1/2 of fixed garnet radius, Table S4), for D_0m ∈
327 [4.0×10¹¹ to 4.0×10¹⁴ cm²/a] or Q_m ∈ [320 to 280 kJ/mol].

328 Most importantly, these diagrams illustrate how the shifts in peak location
329 predicted from linearly T~time paths depend on modeled system/crystal scales
330 (i.e., r and R_f) and intrinsic differences of D_0m and Q_m thermodynamic
331 properties among elements (e.g. Mn, HREEs+Y). Chances of producing
332 spatially concordant pulses in Mn, Y and HREEs among diverse size garnets
333 and distinct samples are therefore very limited (Figs. 2, 3).
334 **6.2. Modeling garnet zoning with a transient thermal pulse**

335 Three distinct T-t trajectories with fixed $T_0$ (at 450°C; Figs. 5b-f) and
336 variable $T_f$ associated with a 80°C T pulse of 1 Ma were then tested to
337 evaluate the impact of different $T_f$ (500 to 550°C) and pre-exponential
338 diffusion factors in matrix $D_{0m}$ (8×10$^{10}$ to 8×10$^{13}$ cm$^2$/a). Assumptions on
339 $r$, $R_f$, $D_{0m}$, $Q_m$ and the boundary conditions used in the following are given in
340 Appendix B (and Table S4). The Mn profile of garnet A1 is taken as reference
341 (Figs. 5c-f; modeling Runs 16 to 27, Table S4). The linear T-t path within 5 Ma
342 from 450 to 550°C is used for comparison.

343 With a relatively fast $D_{0m} \sim 8 \times 10^{13}$ cm$^2$/a (Fig. 5c; i.e., a value comparable
344 to experimental constraints at ca. 1000°C for Mn diffusivity in natural diopside,
345 converted to $Q_m = 300$kJ/mol; Dimanov and Sautter, 2000; Dimanov and
346 Wiedenbeck, 2006), no secondary peak is produced, whatever the T-t path
347 (and with or without T pulse). All profiles resemble a classical “Rayleigh
348 fractionation” pattern (e.g., Hollister, 1966; Otamendi et al., 2002). Decreasing
349 $D_{0m}$ by one to three log$_{10}$ units in magnitude (Figs. 5d, 5e, 5f) reduces the gap
350 between modeled and observed profiles, especially for the Mn secondary peak
351 (Figs. 5e, 5f). However, depending on $D_{0m}$, the early part of the modeled Mn
352 profiles, before the secondary peak, may largely deviate from the observed
353 profile. The best fit is observed for $D_{0m}$ of $8 \times 10^{11}$ cm$^2$/a. Further tests explore
354 the duration of garnet growth ($t_f$) versus that of the T pulse ($t_p$; Figs. 5k-l;
355 Runs 28 to 33, Table S4). At fixed $D_{0m} \sim 8 \times 10^{11}$ cm$^2$/a, a transient $t_p$ of 0.3 Ma
356 (with $t_f$ set as 5 Ma) fits measured profiles much better than a longer lived $t_p$
357 of 1 Ma ($t_f$ set as 2 Ma; Fig. 5k).

358 Overall, the best-fit (red profile in Fig. 5l) is obtained with conditions of $T_f$
359 = 500°C, $D_{0m} = 8 \times 10^{11}$ cm$^2$/a and ~5 Ma $t_f$ accompanied by transient (~ca.
360 1/20 of $t_f$) ~80°C T pulse (Run 30; Table S4).

361 **6.3. Fitting garnet profiles with a T pulse**

362 Further tests were performed to try and reproduce 1) annular Mn zonation
363 among diverse size garnets and 2) annular joint Mn-HREE-Y pulse in garnet
364 A1 under the same scenario.

365 In order to account for differences in radius and grain distribution in the
366 different samples (i.e., garnets A1-2, B1, C1, Fig. 6a), some minor adjustment
367 of the $t_f$ and $D_{0m}$ (for Mn) parameters is needed with respect to those used
368 to best fit Mn zoning in garnet A1 (Fig. 5), allowing $D_{0m}$ to vary by less than
369 one order of magnitude only. Best-fit results and fitting parameters are
370 presented in figures 6a, b. The similar distance to the annular Mn pulse in the
371 different large garnets (i.e., $R_p \sim 0.77$ mm; Figs. 4a, 6a') and timing (i.e., $\approx$
372 4.65 Ma, based on garnet A1 radius and a 5 Ma growth duration) logically
373 implies slightly different growth durations of diverse size garnets (Fig. 6a';
374 Table S4).
Potential differences in diffusion rate of HREEs and Y in the matrix need to be considered to account for the concordant Mn-HREE-Y zoning, since experiments show that differences in ionic radius amongst the HREES impact diffusivity in diopside (then cause slightly offset for the spatially position of secondary pulse. Fig. 5i. Skora et al., 2006), from Dy to Lu, by ~1 log₁₀ unit (converted to Q = 300kJ/mol, Van Orman et al., 2001), but still falls within the uncertainty of our LA-ICP-MS spatial analyzing resolution (Fig. 2b). Additional tests, increasing diffusion rates by about one log₁₀ unit with decreasing ionic radius (Dy to Lu), were therefore applied to obtain best-fit value of D₀m for each HREE and Y profile (see figures 6c-6i and Table S4; Run 37 to 43).

## 7. Discussion

### 7.1. Interpretation of key petrological observations

#### 7.1.1 Unusual garnet zoning in Mn, HREEs and Y

No such garnet zoning, with a concordant annular zoning in Mn, HREEs & Y (Figs. 2, 3) and consistent Mn zoning amongst diverse sized garnets (Figs. 4, S3), was reported so far. Previous reports of secondary peak garnet zoning in Mn and/or HREEs (± Y), all different in some way, include: 1) partial correlation (with asymmetric peaks) of Mn, Ca, Sc, Zr, Y and M/HREEs attributed to Mn-rich epidote breakdown and/or resorption of garnet
395 (W-Labrador metapelite; Yang and Rivers, 2003); 2) correlated Y, HREEs and
396 Hf peaks in Cignana UHP eclogite attributed to clinozoisite + titanite
397 breakdown (King et al., 2004); 3) positive or negative Ca, Fe and Mg
398 anomalies coincident with (disordered) secondary peaks of MREEs, attributed
399 to element sequestration into garnet cores combined with changes in major
400 garnet-forming reactions (UHP eclogites, WGR Norway; Konrad-Schmolke et
401 al., 2008); 4) oscillatory zoning of Mn decoupled from Fe due to slight
402 fluctuations in decompression rate and P-T path (Schumacher et al., 1999); 5)
403 Mn, Y and REEs multi-secondary peaks possibly linked to externally derived
404 fluids (Franciscan blueschist; Moore et al., 2013); 6) Ca spike in the mantle of
405 almandine-rich garnets in amphibolites interpreted as a disequilibrium feature
406 (New Mexico, USA; Chernoff and Carlson, 1997). Skora et al. (2006)
407 interpreted their discordant MREE/HREEs mantle maxima garnet zoning in
408 Zermatt-Sass Fee eclogites as reflecting the interplay between growth and
409 intergranular diffusion rates, hence departing from Rayleigh fractionation.

410 Whether garnet zoning reported here results from a change in source
411 (fluid-mediated or not) and/or in P-T conditions (i.e., a change in diffusion or
412 incorporation properties) is evaluated below.

413

414 **7.1.2 Bimodal Zr concentrations in rutile: evidence for a thermal pulse?**

415 Zr concentrations found in rutile grains (Figs. 2d-e, S5, Table S3) match
those reported in nearby eclogites (Chen et al., 2012; Li et al., 2013; Zhang et al., 2016). Given the relatively moderate regional $T_{max}$ (<500-600°C), these Zr signatures can probably be considered as pristine and not affected by later intracrystalline diffusive resetting (Cherniak et al., 2007; Kohn et al., 2016; Penniston-Dorland et al., 2018), therefore providing a faithful record of the temperature evolution. The length-scale for Zr intracrystalline diffusion in rutile at about 550°C is indeed ~4-5 µm in 1 Ma (using the diffusion coefficient of Cherniak et al., 2007), hence unlikely to reset most rutile grains >20 µm (diffusion from all sides would reset a ~8-10 µm grain). Rates of resetting could nevertheless be faster if interface-coupled dissolution-reprecipitation was involved (J. Ague, pers. comm.). Yet no dependence of Zr concentrations with grain size was observed.

Several alternative interpretations to a thermal pulse, or cautionary statements, can be mentioned:

— matrix rutile grains with ~20 ppm Zr (Figs. 2d-e, S5, Table S3) may correspond to retrograde reequilibration and exsolution of high Zr-phases to produce lower-Zr rutile during cooling (Ague et al., 2013; Penniston-Dorland et al., 2018), contrary to rutile hosted in garnet or matrix omphacite. No Zr-bearing phase was found rimming matrix rutile grains (Figs. S1e-f), however, strengthening the existence of a ~80°C temperature pulse. There remains the possibility that the low Zr content of lower-T rutiles prevented the formation of
437 Zr-rich phases and promoted Zr dispersion in the matrix, perhaps to adjacent
438 but not rimming zircons (see Ague et al., 2013).

439 — zircon may have been undersaturated in the growth medium during
440 the formation of both garnet core and rim (inducing low Zr-in-rutile inclusions),
441 but saturated during the growth of the annular area due to enhanced diffusion
442 and/or fluid availability. The presence of zircon (e.g., Fig. S7) indeed does not
443 mean that the growing garnet and rutile are saturated in the zircon component,
444 in the case of low concentrations and/or slow transport of Zr in the growth
445 medium. The fact that Zr concentration in garnet does not seem to vary much
446 across the garnet profile (Fig. 3g) nevertheless suggests that the zircon
447 saturation state was relatively constant during garnet growth (and/or implies
448 that Zr solubility in garnet is largely insensitive to a T excursion).

449 — changes in fluid composition may have considerable effects on Zr
450 concentrations in rutile. Ingression of CO2-rich fluids, in particular, which limit
451 the solubility of quartz (e.g., Manning., 1994) and depress aSiO2, would lead to
452 a Zr-in-rutile increase without significant change in temperature (see Tomkins
453 et al., 2007). Moreover, changes in fluid composition could enhance or retard
454 intergranular elemental solubility for select major and trace elements (e.g., a
455 small change in CO2 or Cl-F composition of the fluid may increase the solubility
456 of Y+HREE and HFSE respectively; Carlson et al., 2014; Galvez et al., 2015).
457 Evidence for infiltration by exotic fluids can probably be ruled out in the light of
zircon oxygen isotope data and zircon-garnet-omphacite-coesite equilibrium on these samples (Tan et al., 2017), which advocate for an isotopically closed system at the m-dm scale (for oxygen at least). Small fluid amounts would however not be able to alter the internal buffering of oxygen isotopes significantly. Moreover, ingression of not so exotic fluids from nearby eclogites cannot be ruled out. Finally, the relatively low/trace concentration of Mn-HREE-Y in garnet indicates that considerable fluid activities may not have been necessary to affect garnet chemistry.

466

### 7.1.3 Mineral breakdown and potential element sources

The source of Mn/Y/HREEs could be provided by the breakdown and/or differential resorption of Mn- and/or REE-rich major minerals such as epidote, lawsonite, chlorite, apatite and Ti-rich phases. The main carriers of L/MREEs (± HREEs), Sc, and Sr in blueschists/eclogites (i.e., within the “subduction factory”) are epidote group minerals and lawsonite. Apatite, ilmenite, titanite, rutile and zircon, respectively, serve as important carriers of P + Mn + HREEs, Ti + Mn + REEs, Ti + HREEs and Zr + HREEs (Spandler et al., 2003; Spandler and Pirard, 2013). The lack of corresponding secondary peaks of MREEs and key trace elements (Ti, Sr, Zr, Sc, Rb, P; Figs. 3e-h) essentially allows to discard breakdown of these minerals as a major source. No breakdown of apatite and titanite/rutile is observed together with garnet growth, as shown by
479 Ti and P mapping (Figs. S1a-b). Alternatively, this exact suite of elements (i.e., 480 Mn-HREE-Y) might be simultaneously enriched during fluid-mediated garnet 481 growth while HFSE like Ti and Zr would not due to their limited solubilities in 482 many geologic fluids (Ague., 2017 and references therein).

483 Predicted amounts of chlorite and lawsonite breakdown accompanying 484 garnet growth along a temperature pulse such as supported by Zr-in-rutile data 485 (Fig. 2d) lie in the range 1.5-7.0 vol % (lws) and 1.0-6.0 vol % (chl), whatever 486 the aH₂O value (Fig. 4f). Their breakdown predicts a joint supply of major and 487 trace elements with a 40/35/30 wt% increase in L/M/HREEs and 21% in CaO 488 (for lawsonite) and a 11/3/22 wt% increase in MnO, MgO and FeO for chlorite. 489 These joint enrichments conflict, again, with the decoupled trends observed for 490 Fe, Mg, Ca and Mn, HREEs, Y in the various garnet profiles/maps (Figs. 2, 3, 491 S3). In contrast, the best candidates seem to be early grown, low-T and 492 essentially Mn-rich garnets, whose significant resorption, e.g. through Ostwald 493 ripening, would jointly supply Mn/Y/HREEs and lesser amounts of Ca/Fe/Mg.

494 Overall, the joint supply of Mn/Y/HREEs could have been provided by 495 fluids, dissolution of small crystals of early grown garnets, or through a 496 differential response to diffusion. The latter possibility is explored in the next 497 section. We note however, that heating, deformation and fluid activities can be 498 tightly coupled. Heating could give the Zr-in-rutile T variations and speed up 499 intergranular diffusion while deformation might have opened up some porosity,
increasing permeability and fluid accessibility with also the enhancement of the
rates of transport to growing garnet. In other words, even if the pulse relates to
diffusion, it does not have to be purely thermal.

503

504 **7.2. Can garnet zoning be modelled by a transient thermal pulse?**

505 *Implications for thermodynamic modeling*

Control on Y+REEs concentrations in garnet by diffusion through the
intergranular medium was first proposed by Skora et al. (2006) and
reappraised by Hesse (2012) and Moore et al. (2013). This predicts, however,
that the location of Y+REEs (mantle) peaks will differ for each element (Skora
et al., 2006; this study; Figs 5g-j) owing to the significant dependence of
uptake on competing rates of crystal growth (R(t), Eq. (7)), intergranular
diffusion (Dm(T), Eq. (8), Fig. 5i), changes in system size (r, Fig. 5g), crystal
radius (Rf, Fig. 5h) and activation energy (Q, Fig. 5j).

We herein show that a joint Mn/Y/HREEs zoning can nevertheless
develop if a specific T-time pulse modifies the rate of intergranular diffusion, as
illustrated by figure 7. T-time paths with a transient ΔT of ~80°C (Run 16 to 27,
Table S4) indeed allow to fit Mn profiles (color lines in Figs. 5c-f) at moderate
D0m (8.0 × 10¹¹cm²/a). Tests were performed for different values of grain size
(Rf), growth (tf) and T pulse durations (tp) with constant concentration pulse
520 position ($R_p$=0.77mm), T range ($T_f$) and pre-exponential diffusion factors ($D_{0m}$).
521 Tests evaluating the interplay of $t_f$ and $t_p$ (Figs. 5k, 5l) argue for the
522 necessity of a transient T pulse with duration $t_p$ ~300 Ka (~1/20 of $t_f$ set as 5
523 Ma) to best-fit the observed Mn profile of garnet A1 (Run 30, Table S4; red-line,
524 Fig. 5l). By contrast, no secondary peaks can be retrieved, even for small $D_{0m}$
525 (Fig. 5f), by applying a linear T-time path with identical growth rate derived
526 from fixed $R_p$ (Eq. (7b)).

527 The zoning patterns described here reveal an increase in Mn
528 concentration coincident with a temperature increase, which apparently
529 contradicts usual thermobarometric inferences (e.g., Baxter et al., 2017;
530 Caddick and Kohn, 2013). Diffusion-controlled garnet growth is generally not
531 considered when applying thermobarometry, and this study suggests that care
532 should be taken in directly relating major elements (especially Mn) zoning with
533 T and P evolutions. The lack of significant zoning in Ca, Fe and Mg associated
534 with the thermal pulse (whilst Fe and Mg compositional excursions are
535 predicted via P-T modeling: Fig. S8) may indicate that the thermal pulse was
536 too short to effectively modify the diffusion of these cations in the system, or
537 rather that, due to the low abundance of Mn in the matrix with respect to
538 Fe/Mg/Ca, garnet ability to incorporate it is mainly controlled by matrix diffusion,
539 especially the difference of intergranular diffusivities between various
540 divalent/trivalent cations. Large differences between Ca&Mg and Mn&Y&Dy
541 intergranular diffusivities (~4-9 log10 units, converted to Q= 300KJ/mol, T =
542 1000°C) are indeed reported for natural diopside by recent petrological
543 experiments (Dimanov et al., 2010; Van Orman et al., 2001; Zhang et al.,
544 2010). A secondary compositional pulse could also be buried if applying a
545 relatively fast D<sub>0m</sub> (Figs. 5c-f, S8).
546

547
548
**7.3. Geodynamic significance: transient thermal pulse and/or fluid mobility?**

549
550
551
552
553
554
555
556
557
558
559
560
561
The data collected here demonstrate the existence of a transient event, down to ~90 km depth 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), with the following characteristics: (i) T most likely transiently increased by ~80°C over a small fraction of the entire garnet growth interval, as shown by variations in Zr concentration in rutile grains; (ii) diffusion-limited Mn, HREEs & Y experienced significant increases in diffusivity; (iii) there is no evidence that trace elements other than REE+Y responded to the transient (Fig. 3), nor other major elements nominally in thermodynamic equilibrium in garnet (Ca, Fe, Mg); (iv) there is no evidence for systematic mineral breakdown in the rock in response to the transient; (v) resorption of early grown garnets is the most likely source of Mn-HREE-Y; (vi) modeling of the specific annular Mn-HREE-Y zoning
through a T-time pulse can explain all zoning patterns, except point iv
(however, if sufficient early grown garnets are dissolved then EBC changes,
and thus Fe-Mg and Ca concentrations in the annular pulse cannot be
expected to be large); (vii) massive influx of exotic fluids can be ruled out, but
not cryptic fluid ingression from nearby rocks of broadly similar composition.

Variations in Zr in rutile concentrations are observed for garnet grains
from samples meters apart (Fig. 1d), calling for an outcrop-scale process at
least. Its spatial extension is in fact probably much larger, since a similar
concordant Mn-HREE-Y zoning can be observed in eclogites and mafic schists
in two adjacent valleys ~10 km apart (Klemd et al., 2011: samples FTS9-2 and
FTS5-15, Figs. 2a-c, 2k-i; Meyer et al., 2016: samples 11-06-5 and 11-06-6,
Figs. 4d, 4g). Further evidence can probably be gathered from the STMB, yet
preservation/formation of this type of zoning requires an almost closed system
behaviour since final patterns of garnet Mn profiles are extremely sensitive to
the variation of D₀m (Figs. 5c-f). This is probably not the case for most
samples across the region, given the number of blueschist/eclogite veins
arguing for fluid circulation (e.g., Gao et al., 2007; John et al., 2008; Li et al.,
2013). Any external supply of Mn, HREEs and Y could similarly boost
intergranular diffusivities, thereby hindering the formation of a pulse and
producing a monotonic profile (i.e., Rayleigh Fractionation. Hollister, 1966;
Otamendi et al., 2002) or oscillatory zoning (e.g., Li et al., 2013; Moore et al.,
583 2013). Overall, we therefore view the existence of a thermal pulse, possibly in
584 combination with deformation-induced remobilization of interstitial fluids, as the
585 most likely explanation for the concordant pulse in Mn, Y and HREEs
586 concentrations during UHP eclogitic garnet growth.

587 Several processes can be envisioned for such a thermal transient during
588 deep cold oceanic subduction (~5°C/km, peak T <500-600°C, Bayet et al.,
589 2018; Tan et al., 2017): (i) heat advection from syn-metamorphic mafic
590 magmas together with potential fluid flow (Ague and Baxter, 2007; Baxter et al.,
591 2002), but magmas are rare during the demise of the SW-Tianshan ocean
592 from ca. 320 to 300 Ma (Gao et al., 2009; Gao et al., 2006; Long et al., 2011;
593 Tan et al., 2019), and/or radioactive decay, but this is likely to be <30°C along
594 the subduction interface (Penniston-Dorland et al., 2015; Royden, 1993); (ii)
595 shear heating, which is proportional to velocity times rock strength, could in
596 principle modify the thermal budget along the subduction thrust by up to
597 ~200°C (Mako and Caddick, 2017; see discussion in Penniston-Dorland et al.,
598 2015) but will be small in fluid-rich environments or buffered by endothermic
599 dehydration reactions predicted across the 490-570°C temperature range (i.e.,
600 lawsonite and chlorite). Furthermore, no mylonites/ultramylonites advocating
601 for high strain are observed (fine-grained omphacite is only found at the
602 margin of the studied UHP eclogitic slices; Tan et al., 2017) and lattice prefered
603 orientations of omphacite in strained eclogites from exposures ~1 km away
604 (Soldner et al., 2016) advocate for maximum strain around peak to early
605 exhumation, hence rather after peak garnet growth. The duration of shear
606 heating would also probably exceed that of the modeled thermal pulse (i.e.,
607 ~300 ka across 5 Ma); (iii) translation of UHP slices nearer to a relatively hotter
608 mantle wedge, for example following detachment from the slab (Agard et al.,
609 2018; Fig. 8). Tectonic slices characterize the STMB (Bayet et al., 2018),
610 rather than mélange formation and/or convective motion of subducted material
611 (e.g., Gerya et al., 2002; Blanco-Quintero et al., 2011). The maximum
612 thickness of the subduction interface is constrained by the slicing/underplating
613 of UHP-LT units in Tianshan, which supports a transient thickness of a few km
614 (Bayet et al., 2018), whereas ~300 m is a more reasonable thickness for the
615 subduction interface in the absence of slicing/underplating (Agard et al., 2018).
616 Assuming a thickness between 0.5 and 5 km (in the broad range of
617 geophysical imaging, i.e. 2-5 km; Abers, 2005), a short-lived ~80°C thermal
618 excursion lasting ~0.15 Ma would correspond to displacements on the order of
619 ~3 mm to 3 cm/a. So far most estimates of the duration of growth for cm-scale
620 garnets point to the Ma scale: ~1 Ma (Sifnos blueschists, Greece; Dragovic et
621 al., 2012); ~8 Ma, but through a rapid pulse lasting just a few 100 ka at most
622 (Dragovic et al., 2015); 3-5 Ma (Townshend Dam, Vermont, USA; Gatewood et
623 al. 2015); ~7.5 Ma (Austrian Alps; Pollington and Baxter, 2010); ~7 Ma (Dabie
624 orogen, Cheng et al., 2008; SW-Tianshan, Soldner et al., 2016). A transient
625 0.3-3 cm/a movement of UHP eclogitic slices is in any case compatible within
626 the range of typical exhumation rates worldwide, either for (U)HP oceanic or
627 continental rocks (~1-5 mm/a, or ≥cm/a, respectively; Agard et al., 2009).

628

629 **8. Conclusion**

630 Detailed petrological investigations on major and trace element
631 geochemistry of garnet highlight a concordant annular Mn, Y and HREEs
632 zoning, visible at the outcrop- and probably km-scale, in a representative UHP
633 oceanic eclogitic slice in SW-Tianshan. This pattern reveals the existence of a
634 transient event which, according to Zr-in-rutile thermometry, likely coincided
635 with a ~80 °C temperature increase. Mineral compositions together with
636 thermodynamic modeling indicate that mineral breakdown alone cannot
637 account for this unusual garnet zonation. Successful modeling of Mn-HREE-Y
638 enrichment by diffusion-controlled garnet growth further advocates for a
639 short-lived thermal excursion (i.e., ~300 ka if the overall garnet growth was 5
640 Ma), possibly in combination with deformation-induced remobilization of
641 interstitial fluids. Such a transient thermal excursion is tentatively tied to the
642 fast movement of detached UHP oceanic slices towards the hotter mantle
643 wedge. This study also highlights the necessity of considering
644 diffusion-controlled garnet growth whenever assigning major element
645 (especially Mn) zoning to specific P-T conditions.

646

647 Acknowledgements

648 This study was funded through projects of the National Key R & D

649 Program of China (No. 2018YFC0606004), China Geological Survey (No.

650 DD20190370) and the National Natural Science Foundation of China (No.

651 41390445, 41025008) to Prof. Jun Gao and by the International Training

652 Network ZIP (Marie Curie Actions, Zooming In-between Plates, REA grant

653 agreement No. 604713) to Prof. Philippe Agard. Additional supports were

654 provided by programs of the Chinese Academy of Sciences of China (No.

655 GJHZ1776, 2013047) to Prof. Bo Wan and by the project of National Natural

656 Science Foundation of China (No. 41802093) to Dr. Tao Hong. We would like

657 to thank the editor Prof. Rajdeep Dasgupta, Prof. Jay J. Ague, Prof. Ethan

658 Baxter and two anonymous reviewers for their constructive comments that

659 greatly helped in improving the article. We greatly benefitted from discussions

660 with C. Chopin, L. Labrousse, Y. Chen, J-J. Yang, J-L. Li, and dudes from the

661 ISteP “boy's band” (M. Locatelli, G. Bonnet and M. Soret). We further thank

662 Prof. Yi Chen for the support of microprobe analysis in IGGCAS and M. Fialin

663 & N. Rividi for the help of rutile trace element microprobe analyses in ISteP.

664
665 **Figure and Table Captions**

666 **Fig. 1:** Geological background of the SW-Tianshan LT-HP/UHP metamorphic
667 complex. (a) Location of the research area with respect to other key UHP
668 reports in China; (b) Close-up view showing the regional geological setting and
669 sampling location: previous UHP reports based on coesite inclusions in garnet
670 are highlighted by red stars (modified after Tan et al., 2017). SCTSZ: South
671 Central Tianshan Suture Zone; (c) Comparison of the P-T conditions
672 experienced by SW-Tianshan metavolcanics with those of exhumed oceanic
673 rocks worldwide (modified after Agard et al., 2009; Plunder et al., 2015)
674 highlighting the provenance of these rocks from a deep cold subduction zone
675 (i.e., ~100 km, ~5 °C/km); (d) Location of samples on the outcrop.

676 **Fig. 2:** Key features of the specific garnet zoning investigated here,
677 exemplified by garnet A1: (a) Manganese mapping reveals a euhedral annular
678 Mn-rich zone in the mantle of garnet A1, which hosts coesite in its outer rim
679 (Tan et al., 2017); (b) Yttrium laser-ICP-MS points and profile (superimposed
680 on Y mapping, fogged by microprobe detect limitation); (c) Superimposed
681 microprobe and laser-ICP-MS profiles of garnet A1 reveal concordant
682 concentration pulses in Mn, Y and HREEs; (d) Spatial variations of Zr content
683 in rutile grains located within different garnet domains (i.e., inclusions in the
684 core, annular pulse, rim or in the matrix). Skeleton map exported from matrix
685 data of Mn, Y, Ti and P mapping via XMapTools (Lanari et al., 2014). (e)
Results of Zr-in-rutile thermometry (Table S3) using three different calibrations, pointing to a ~80 °C thermal pulse, and histograms (results below the empirical detect limit (EDL) are fainted) of Zr concentration of rutile grains (whatever as garnet inclusion or in matrix) in all samples.

**Fig. 3:** Variations in the concentrations of (a) major divalent cations, (b-d) HREEs and Yttrium, (e-f) MREEs and (g-h) other key trace elements for garnet A1, along the profile shown in Fig. 2b. These variations outline the existence of a concordant Mn-HREE-Y garnet zoning. Grey vertical overlays mark the location of the secondary peak in Mn concentration (taking into account uncertainties on spot location, ~0.12 mm, during LA-ICP-MS measurement).

**Fig. 4:** (a-b) Mn concentration profiles for diverse sized garnets. Large garnet grains display a variably high Mn core peak and an annular secondary peak (except B2), with similar core<sub>peak</sub>-annular<sub>peak</sub> distance ($R_p$~0.77 ± 0.04 mm; n=7, $1\sigma$). Middle size garnet grains (A3, B3) display an annular peak with depleted core, and small garnet grains (e.g., B4) only a core peak. These patterns are more readily observed in element maps (Fig. S3); (c) Map of inclusions hosted in garnet A1; (d-e) MnO concentrations and REEs patterns of key mineral inclusions in garnet A1, showing that chlorite and lawsonite represent potential budgets of Mn and HREEs. Major and trace elements composition of lawsonite were cited from literature (Spandler et al., 2003; Vitale Brovarone et al., 2014); (f) Evolution of mineral modes predicted by
Perple_X thermodynamic modeling on stepwise fractionated effective bulk rock compositions (EBCs, Fig. S9, Table S1) along different activity and/or temperature paths. Modelling shows that garnet growth coincides with a certain amount of chlorite and/or lawsonite breakdown. (g) Systematic garnet size survey (with corresponding histogram) on each sample. The three main groups of garnet with different colors are following the criteria in figure S4.

**Fig. 5:** Influence of key parameters on diffusion-controlled garnet growth. (a) Sketch depicting the spherical geometry, finite system size and diffusion in both the crystal and matrix considered by Hesse (2012), after which modelling is performed (see App. B for details). (b) Linear (black) or pulse-like (color) T-time paths with variable $T_{final}$. (c-f) Tests exploring the range of plausible pre-exponential $D_{0m}$ values needed to match the Mn garnet concentration profile. (g-j) Influence of key parameters on the possibility to produce secondary concentration pulse (Y is here taken as example), along a linear T-time path (black line in Fig. 6b). (k-l) Modelled Mn concentration profiles for different T-time paths and durations for fixed $D_{0m}$ (~$10^{12}$ cm$^2$/a). The best-fit is obtained for a transient thermal excursion of ~0.3 Ma (out of 5 Ma; red-line). Parameters and details are given in Table S4.

**Fig. 6:** Modelling of the concordant Mn-HREE-Y garnet zoning for diverse size garnets and in garnet A1 (Runs 30, 34-36 for a-b; Runs 37-43 for c-l; Table S4).

See text for details.
**Fig. 7:** Simplified sketches depicting the impact of diffusion-controlled garnet growth (here during a transient thermal pulse). Left column: growth of diverse size garnets at thin-section scale. The concordant Mn-HREE-Y pulse is highlighted as the red-zone. Grey overlays indicate the respective sizes of diffusion halos. Right column: evolution of concentrations as a function of time (or garnet radius).

**Fig. 8:** Tentative interpretation of the thermal excursion in terms of rock dynamics along a deep cold subduction plate interface (modified after Guillot et al. (2009) and Tan et al. (2019)). The red arrows mark the rock trajectory while the circular blue arrows indicate potential local fluid mobilization.

**Table 1:** Brief introduction of parameters in diffusion modeling.

**Appendix A. Thermodynamic modeling for the estimate of mineral modes evolution and effective partition coefficient**

Garnet size distribution (Fig. S4) was investigated to derive effective bulk rock compositions (EBCs, Fig. S9, Table S1), i.e. taking into account progressive garnet fractionation from the whole-rock composition, and to evaluate the evolution of mineral mode (Fig. 4f) under a specific P-T loop (i.e., T pulse, Fig. 4f). This allows estimating dynamic effective partition coefficient
748 $K_D^E$ of specific elements during the modeling of diffusion-controlled garnet growth (App. B.).

750 Garnet size distribution (Fig. S4) was estimated using Photoshop manual coloring (for each garnet grain) and assuming euhedral garnet ranges from 751 pentagon- to octagon-shape to derive radius (and corresponding uncertainties). 752 Modal proportions of each garnet group (large, middle and small; see main text) 753 for grouping strategy and Figs. 4a, 4b, S4) are used to infer approximate 754 volume proportions for EBC calculation.

756 Using major elements profiles (Figs. 4a, 4b) and mapping (Fig. S3), 757 estimates of effective bulk rock compositions are first conducted by defining:

758 $V_g = aR_g$

759 (1a)

760

761 $a = \frac{V_{grt-x}^{measured}}{R_{grt-x}^{max}}$

762 (1b)

763 where $V_g$ represents the volume % of garnet during growth, $R_g$ is the radius 764 of garnet (the same as defined in App. B.), while $a$ is the corresponding ratio 765 constant for different garnet groups and will be respectively calculated by $a = 766 V_{grt-x}^{measured}/R_{grt-x}^{max}$ ($grt-x$ subscript refers to the three garnet groups, 767 $V_{grt-x}^{measured}$ is the measured approximate values mentioned above and $R_{grt-x}^{max}$ 768 is the maximum radius of each garnet group; Figs. 4, S4). Element 769 concentrations along garnet profiles (i.e., microprobe and laser-ICP-MS results)
770 are converted to volume concentrations and described via polynomial fitting

771 (Fig. S9):

$772 \quad C_g = f(V_g)$

773 (2)

774 where $f(V_g)$ represent the fitted polynomial equations for given element

775 concentration profile as a function of $V_g$. The fractionated mass $C_g$ from the

776 matrix, as a result of garnet growth, can be calculated by integrating $f(V_g)$

777 within the relevant range $V_g \in [0, V_g]$:

$778 \quad C_g = \sum_{V_{grt-x}}^{x=L,M,S} \int_{0}^{V_{grt-x}} f(V_g) dV$

779 (3)

780 Successive fractions were thus subtracted XRF bulk rock composition to

781 account for the growth of garnet core, mantle and rim and estimate EBCs used

782 for thermodynamic and diffusion modelling. Calculated EBCs are shown in

783 Table S1.

784 Thermodynamic modelling (MnNCKFMASHTO system) was performed

785 with Perple_X (Connolly, 1990, 2005) using successively the unfractionated

786 bulk-rock composition (i.e., EBC0, Table S1) and garnet-fractionated effective

787 bulk rock compositions (EBC1-2, Table S1) to estimate minerals modes along

788 the near-peak prograde path. Pressure is fixed at 2.75 GPa for simplification,

789 and temperature increases from 450 to 500 °C, with a short-lived ~80 °C T

790 excursion. The database and solid solutions are the same as in Tan et al.
791 (2017). Different values of αH2O (Fig. 4f), as well as a temperature-dependent
792 change in αH2O, were applied via T-αH2O pseudosections to test its influence on
793 the predicted mineral modes.

794 Generally, garnet central peak concentrations (of given element) of the
795 profiles were compared to the bulk rock composition to estimate the bulk
796 distribution coefficient K<sub>D</sub> (Hollister, 1966), assuming that the effective matrix
797 concentration during garnet growth equals that of the bulk rock composition
798 (Skora et al., 2006). K<sub>D</sub> is treated as constant at a given temperature and
799 pressure, for a sufficiently dilute trace component between two phases (e.g.,
800 Hesse, 2012; Skora et al., 2006). Two values of effective K<sub>D</sub><sup>E</sup>, before and after
801 the concordant pulse in concentrations, were considered (Fig. 7):

$$
\begin{equation}
802 \quad K_D^E = \left( \frac{c_{g-c}^i}{c_{EBC0}^i} ; \frac{c_{g-ml}^i}{c_{EBC1}^i} \right) \left( \frac{v_g^i}{v_{EBC0,1}^i} \right) \tag{4}
803 \end{equation}
$$

804 Where $c_{EBC0}^i$, $c_{EBC1}^i$ and $c_{g-c}^i$, $c_{g-ml}^i$ are the concentration of element $i$, 805 respectively, in effective bulk rock composition and corresponding position 806 from garnet profiles with respect to the representative Mn, Y and HREEs 807 annular zoning of garnet A1 (Figs. 4, S9). For simplification, the variable of 808 ($\frac{v_g^i}{v_{EBC0,1}^i}$), which describe the ratio between P-T dependent activity 809 coefficients $v_g^i$ and $v_{EBC0,1}^i$, is treated to approach unity. Calculations of initial 810 $K_D$ (i.e., $\frac{c_{g-c}^i}{c_{EBC0}^i}$) will be particular sensitive to uncertainties in the location of the 811 central cuts with respect to garnet cores. Potentially missed central peaks of
some garnets were estimated in proportion of the sharp central peak pattern of
the largest garnet B1 (for Mn) and relative smaller garnet A1 (for Y, Tm, Lu, Yb
and Er). Values of $K_D^E$ for each element are listed in Table S4.

815

## 816 Appendix B. Finite volume diffusion modeling

The finite volume method from Hesse (2012), with spherical geometry in a
uniform matrix, is used to compute the diffusion and partitioning of trace
elements during porphyroblast growth (Fig. 5a). Diffusion on both sides of the
moving crystal–matrix interface are explicitly considered in this study (i.e.,
intragranular and matrix diffusion), whereas most previous studies only
consider diffusion in the matrix (e.g., Castro et al., 2008; Skora et al., 2006;
Skora et al., 2008; Watkins et al., 2008) or inside the growing porphyroblast
(e.g., Alexander, 2011). The motion of the crystal–matrix interface and the
thermal evolution are prescribed as time functions. The appropriate conditions
for the diffusion flux across the crystal–matrix interface are derived from
discrete mass conservation.

The evolution of the concentration profiles is controlled by:

$$ \frac{\partial(r^2 c_g)}{\partial t} + \frac{\partial(r^2 F_g(c_g))}{\partial r} = 0, \text{ on } \Omega \in [r_{min}, R] \times [0, t_f] \quad (1a) $$

$$ \frac{\partial(r^2 c_m)}{\partial t} + \frac{\partial(r^2 F_m(c_m))}{\partial r} = 0, \text{ on } \Omega \in [R, r_{max}] \times [0, t_f] \quad (1b) $$

Where $r^2 c_k$ and $F_k$ represent the accumulation and the diffusive flux of the
trace element in the k-phase. The phase subscript *g* refers to garnet and *m* to the matrix. The exponential factor 2 on *r* reflects the radial dependence of volume and area in spherical coordinates. The Diffusive flux *F*<sub>*k*</sub> in k-phase is defined by:

$$F_k = -D_k \frac{\partial c_k}{\partial r} \qquad (2)$$

Where $D_k$ is the diffusion coefficient in k-phase, and initial conditions in each spatial domain are given by:

$$c_g(r, 0) = c_g^0, \text{ on } \Omega \in [r_{min}, R]$$

(3a)

$$c_m(r, 0) = c_m^0, \text{ on } \Omega \in [R, r_{max}]$$

(3b)

Four boundary conditions are required. Two are at the internal interface *R* (i.e., garnet radius, Fig. 5a) and the other two are at the outer boundaries of the domain *r* (i.e., system size, Fig. 5a). Hesse (2012) assumed that concentration profiles are symmetric across the center of the porphyroblast and that the matrix surrounding it acts as a closed system, so that:

$$F_g(r_{min}) = F_m(r_{max}) = 0$$

(4)

At the moving internal boundary during modeling, the diffusive flux $F_l$ across the porphyroblast interface is continuous while the concentration profile is discontinuous because of the partitioning of trace elements, controlled by the
effective partition coefficient $c_g = K_D^E c_m$ (also see App. A. for the definition and estimates of $K_D^E$). The conditions at the interface are therefore described as:

$$ \lim_{r \to R^{-}} F_{g}(r) = \lim_{r \to R^{+}} F_{m}(r) = F_{l}(R) \qquad (5a) $$

$$ \lim_{r \to R^{-}} c_{g}(r) = K_{D}^{E} \lim_{r \to R^{+}} c_{m}(r) \qquad (5b) $$

While $T(t)$ and $R(t)$ are described by power-laws functions with preset exponential factors by Hesse (2012), these were modified here to test the impact of a thermal pulse on Mn-HREE-Y garnet zoning. $T(t)$ is therefore computed as:

$$ = \frac{(T_f - T_0)t}{t_n + t_p + t_{n'}} + T_0, t \in [0, t_n] \qquad (6a) $$

$$ = \left( \frac{T_f - T_0}{t_n + t_p + t_{n'}} + \frac{2\Delta T_p}{t_p} \right) t + T_0 - \frac{2\Delta T_p t_n}{t_p}, t \in [t_n, t_n + \frac{1}{2} t_p] \qquad (6b) $$

$$ = -\left(\frac{T_f - T_0}{t_n + t_p + t_{n'}} + \frac{2\Delta T_p}{t_p}\right) t + T_0 + \frac{2(T_f - T_0)(t_n - t_p)}{t_n + t_p + t_{n'}} + \frac{2\Delta T_p(t_n + t_p)}{t_p}, t \in [t_n + \frac{1}{2} t_p, t_n + t_p] \qquad (6c) $$

$$ = \frac{(T_f - T_0)t}{t_n + t_p + t_{n'}} + T_0, t \in [t_n + t_p, t_n + t_p + t_{n'}] \qquad (6d) $$

Where $T_0$ and $T_f$ represent the initial and final value at $t = 0$ and $t = t_n + t_p + t_{n'}$, respectively. $\Delta T_p$ represents the absolute change of temperature during the thermal pulse and is fixed at a constant 80°C following estimates of Zr-in-rutile thermometry (Figs. 2, S1, Tables S3, S4). $t_n$, $t_p$ and $t_{n'}$ are the variable on different modeling runs for constraining the time duration of processes before ($t_{normal}$), during ($t_{pulse}$) and after ($t_{normal'}$) the T pulse. To
account for the concordant Mn zoning among diverse size garnets (i.e. at $R_p$ = 874 0.77 ± 0.04 mm; Fig. 4), the garnet growth rate $R(t)$ is computed using a 875 polynomial power-law equation with variable exponent $n$: 876

$$R(t) = R_0 + (R_f - R_0)(\frac{t}{t_n+t_p+t_{n'}})^n, t \in [0, t_n+t_p+t_{n'}]$$ 877 (7a)

$$n = \log_{\left(\frac{t_n+0.5t_p}{t_n+t_p+t_{n'}}\right)}\left(\frac{R_p-R_0}{R_f-R_0}\right) \quad (7b)$$

Where $R_0=0$, and $R_f$ are 1.22, 1.10, 1.58 and 1.12 mm respectively (for A1-2, 880 B1 and C1 garnets; Figs. 4, S2). $R_p$, in contrast, was set constant at 0.77 mm 881 (Fig. 4a), except for test calculations performed along an ordinary T-t path 882 (Figs. 5b, 5g-j) for which $R(t)$ follows a linear evolution. Since all garnet growth 883 rates are the same and set to follow that of garnet A1 (i.e., in Eq (7a); n=6.34), 884 slight to moderate differences are predicted for their total growth duration. 885

The diffusion coefficient of divalent/trivalent cations in matrix and growing 886 porphyroblast is expressed as a function of temperature $T$ by Arrhenius 887 equation: 888

$$D_k = D_{0k}\exp\left(\frac{-Q_k}{UT}\right) \quad (8)$$

Where $D_{0k}$ is the pre-exponential factor, $Q_k$ is the constant activation energy, 890 $U$ is the universal gas constant, and $k$ is the phase subscript (herein, 891 represent garnet porphyroblast and matrix respectively). In this study, the 892 constant activation energy $Q_k$ will be fixed at 300 kJ/mol (except for 893 benchmarks tests for varying $Q_k$, Table S4) for simplification since the finite 894
volume model from Hesse (2012) is mostly sensitive to variations on $Q_k$ (Fig. 5j). The interested reader is referred to Hesse (2012) for further modelling details. The pressure and oxygen fugacity dependences of diffusion coefficient are neglected in this study, since the variations, when other factors are held constant, are relatively limited within our P-T-fO₂ window: experiments (e.g., Carlson, 2006; Chu and Ague. 2015) demonstrate that diffusion coefficients will vary over ~1.0 log₁₀ unit across the pressure range from 2.4 to 2.8GPa; over ~1.5 log₁₀ unit across the range of oxygen fugacity from the iron-wüstite buffer to the magnetite-hematite buffer.

Assumptions on key parameters ($r$, $R_f$, $D_{0m}$, $Q_m$) and boundary conditions used for diffusion-controlled garnet growth modeling are the following:

- garnet radius ($R_f$) strictly follows the measurements of the four large garnets (A1, A2, B1, C1) preserving the whole Mn-HREE-Y pulse.

- system size ($r$) controls the overall element mass balance. Estimating it is difficult because it requires 3D investigation of the nucleation and growth history of the nearest neighbors (Carlson, 2011; Skora et al., 2006). Element accessibility for the earliest-grown (and largest) garnets suggests an initially large system size, decreasing with time as new (relatively smaller) garnets nucleate in the vicinity of pre-existing porphyroblasts (Skora et al., 2006). According to our garnet survey (in 2D), the distance between the nearest
916 neighbors and the large garnet B1 is ca. 0.52 cm (red line in Fig. S4),
917 suggesting that the half-distance from center to center to the nearest neighbor
918 is a reasonable estimate for its system size (~ca. 0.26 cm); a similar
919 assumption was made for all garnets.

920 — the garnet growth rate used here is expressed as a function of the final
garnet radius ($R_f$) and time duration of the various steps of T-t path ($t_n, t_p$ and
$t_{n'}$). It is neither a linear nor square root function of time, contrary to previous
921
922 studies (e.g., Baumgartner et al., 2005; Skora et al., 2006; Weare et al., 1976).
923
This assumption allows to reproduce core-peak to secondary-peak distance and to
924
925 fit the joint annular Mn-HREE-Y pulse observed in any given garnet and the
926 similar Mn zoning among diverse size garnets (taking place at $R_p$= 0.077 ±
927 0.004 cm, Fig. 4).
928

928 — initial element concentrations in the matrix are those of the
929 unfractionated bulk rock composition (i.e., EBC0, calculated by normalized
930 XRF+ICP-MS data, Table S1).

931 — the activation energy ($Q_k$) and pre-exponential diffusion factor $D_{0k}$ are
932 theoretically covariant for each element in both the porphyroblast ($Q_g, D_{0g}$,
933 garnet) and matrix ($Q_m, D_{0m}$, mainly clinopyroxene) but generally poorly
934 constrained (Brady and Cherniak, 2010) or missing (e.g., in the P-T window
935 considered here, UHP and $T_{max}<500-600°C$). For the sake of simplicity, the
936 activation energy is here fixed at ~300 kJ/mol ($Q_g = Q_m$), following
experiments for Mn, HREEs & Y diffusivities in garnet/diopside (at ~ca.
937
1000 °C) giving Q~ca.~ 220 to 480 kJ/mol (Carlson, 2006; Carlson, 2012; Chu
938
and Ague, 2015; Dimanov and Sautter, 2000; Dimanov and Wiedenbeck, 2006;
939
Van Orman et al., 2001; Van Orman et al., 2002). Similarly, D~ is set at a
940
fixed 1×10⁶ cm²/a for Mn, HREEs and Y diffusivities in garnet (yet, at ~ ca.
941
1000 °C, experiments yield estimates one order of magnitude smaller; Carlson,
942
2006; Carlson, 2012; Chu and Ague, 2015; Van Orman et al., 2002).
943

— by contrast, we vary the pre-exponential diffusion factor $D_{0m}$ in the
944
matrix to fit the observed garnet zonation profiles. The fitted $D_{0m}$ hence
945
represents the bulk diffusive response of the matrix to a “sink” or source term
946
for a specific divalent/trivalent cation, and serves as a macroscopic effective
947
property of each sample (i.e., it includes potential effects of the structure of
948
grain boundaries, of porosity, as well as of the conceivable contribution of the
949
trace elements’ accumulation on the grain boundaries and intra-grain diffusion;
950
e.g., Dohmen and Chakraborty, 2003; Ganguly, 2010; Hiraga et al., 2004).
951

— the duration of investigated garnet A1 growth is set at 5 Ma, broadly
952
consistent with the estimated duration of garnet growth ($t_f$) of 7.6 ± 6.8 Ma,
953
inferred from the gap between Lu-Hf and Sm-Nd ages of regional blueschists
954
(Soldner et al., 2016). Relevant boundary conditions for each modeling round
955
are listed in Table S4.
956

957
958 **Appendix C. Analytical methods**

959 **Major elements electron microprobe analyses and mapping**

960 Most major element compositions of garnet, mineral inclusions and matrix
961 minerals, as well as relevant profiles and mapping analyses, were obtained by
962 electron microprobe analyses at IGGCAS with JEOL JXA 8100 under
963 presettings of wavelength dispersive spectrometers with an acceleration
964 voltage of 15 kV, a beam current of 15 nA, at 1 to 3 µm beam size and 30 s
965 counting time. Additional high resolution Mn, Y, P and Ti mapping for key
966 garnet grain (A1, Figs. 2a, 2b, S1) with 15 kV acceleration voltages and 200
967 nA beam current under 60 ms dwell time was conducted on CAMECA SX Five
968 FE at IGGCAS. Natural minerals and synthetic oxides were used as standards,
969 and a program based on the ZAF procedure was used for data correction.
970 Average compositions for mineral chemistry of studied UHP eclogite are
971 presented in Table S2.

972 **Laser-ablation ICP-MS trace element analyses**

973 In situ trace element point and profile (Figs. 2b, 2c, 3) analyses of zoned
garnet, associated matrix mineral and rutile were performed by LA-ICP-MS at
974 IGGCAS with a single collector quadrupole Agilent 7500a ICP-MS, equipped
975 with an UP193Fx argon fluoride New Wave Research Excimer laser ablation
976
system. Glass reference material NIST SRM 610 was used as standards for external calibration while NIST SRM 612 was used for quality control monitoring. LA-ICP-MS measurements were conducted using a spot size diameter of 60 µm with a laser frequency of 15 Hz and 0.63 GW/cm2 and a fluence of 3.32 J/cm2. Acquisition time was 20 s for the background and 120 s for the mineral analyses. The Ca-content of garnet, omphacite and amphibole and the Ti-content of rutile determined by EPMA was used as internal standard.
Reproducibility and accuracy, which were determined for NIST SRM 610 and NIST SRM 612, are usually <8% and <6%. Trace element concentrations were then calculated using GLITTER Version 3 (Van Achterbergh et al., 1999).
Average trace element composition of the relevant garnet domains, associated minerals and rutile are given in Table S2, following their microprobe data.

## Zirconium contents of rutile electron microprobe analyses

Concentrations of Zirconium in rutile were determined by electron microprobe following the method outlined by Zack et al. (2004), since the size of most rutile euhedral inclusions in garnet A1 (Fig. 2d) were too small (ca. 5 to 30 µm in dia., Figs. 2d, S1) to be analysed by LA-ICP-MS. All analyses were conducted with a CAMECA SX Five in ISTeP, Sorbonne Université, France.
EPMA conditions were optimised so that detect limits on Zr could be improved by four times (20 ppm compared with 80 ppm). Acceleration voltage was set at
997 20 kV with 120 nA beam current and 5 µm beam diameters. A H-type
998 spectrometer was chosen for measuring Zr-Lα because of its highest
999 sensitivity for this element. Detailed settings for spectrometers followed the
1000 Table 2 of Zack et al. (2004). Every 10 analyses of unknown rutile were
1001 bracketed by 2 analyses of synthetic rutile standard for true zero-concentration
count rates on the peaks and to exclude any machine drift. Summary of rutile
1002
1003 measurements with respect to their location is presented in Table S3.

1004

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1388
<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>

a

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

b

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

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

Zirconium in rutile
(as grtA1-in VS in matrix)

Matrix (+n)
Garnet
Apatite
Titanite
Rutile

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

e

rutile inclusions
in garnet (n=150)

rutile in matrix
(n=139)
Garnet endmembers

<figure><img src="image_11.png" /></figure>

Middle rare earth elements

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

Heavy rare earth elements + Yttrium

Other key trace elements
Concordant annular Mn zoning among diverse size garnet (grt-l)

<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>

Homogeneous Mn
distribution in matrix
during annular <sub>peak</sub> growth
~ 11 %, Stdev/Av

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

Mineral inclusions in garnet A1

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

Key mineral composition

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

<table><caption>HP/UHP related minerals</caption><thead><tr><th>This study</th><th>From literature</th></tr></thead><tbody><tr><td><strong>Garnet</strong></td><td><strong>Lawsonite</strong><br>(S. 2003. v. B. et al., 2014)</td></tr><tr><td><strong>Core, n=8</strong></td><td><strong>Gln/Brs in matrix, n=11</strong></td></tr><tr><td><strong>M-p inner border, n=9</strong></td><td><strong>Omp in matrix, n=23</strong></td></tr><tr><td><strong>M-pulse, n=8</strong></td><td><strong>Lws as inferred</strong></td></tr><tr><td><strong>Rim, n=10</strong></td><td><strong>exist around peak, n=101</strong></td></tr></tbody></table>

Mineral modes evolution from thermodynamic modeling

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

Spherical geometry

A finite volume diffusion modeling

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

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

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

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

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

<figure><img src="image_33.png" /><figcaption>Y (µg/g)</figcaption></figure>

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

<figure><img src="image_35.png" /></figure>
<figure><img src="image_36.png" /></figure>

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

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

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

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

<figure><img src="image_41.png" /></figure>
**Stage 0 (initial)**

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

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

**Stage 1 (tn)**

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

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

**Stage 2 (tn+0.5tp)**

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

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

**Stage 3 (tn+tp+tn')**

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

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

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

Volcanic arc

Paleo South
Tianshan Ocean

S

N

PKC

Accretionary prism

600°C

900°C

1200°C

Mantle
wedge

crust

oceanic

Subducting

Lithosphere
mantle

600°C

km

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

(Serpentinized)
peridotite

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

Sediment

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

Basalt

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

Gabbro

exhumation

fluids

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

ca. ± 80 °C T excursion
(within ~ 300 Ka ?)

transient
underplating

Mantle wedge

90 km

Moho

Decoupling zone

100 km

Cold subduction
plate interface

110 km
<table><thead><tr><th>Parameters</th><th>Description</th></tr></thead><tbody><tr><td>R<sub>f</sub></td><td>Final size (radius) of investigated porphyroblast</td></tr><tr><td>R<sub>0</sub></td><td>Initial size of investigated porphyroblast</td></tr><tr><td>R<sub>p</sub></td><td>Position (Dist. to core) at which the Mn-Y-HREE secondary pulse occurred</td></tr><tr><td>r</td><td>Modeled system size</td></tr><tr><td>D<sub>0m</sub></td><td>The pre-exponential diffusion factor of intergranular diffusivity</td></tr><tr><td>D<sub>0g</sub></td><td>The pre-exponential diffusion factor of intragranular diffusivity</td></tr><tr><td>Q<sub>0m</sub></td><td>The activation energy of intergranular diffusivity</td></tr><tr><td>Q<sub>0g</sub></td><td>The activation energy of intragranular diffusivity</td></tr><tr><td>T<sub>0</sub></td><td>Initial temperature</td></tr><tr><td>ΔT</td><td>The magnitude of thermal pulse</td></tr><tr><td>T<sub>f</sub></td><td>Final temperature</td></tr><tr><td>c<sub>0m</sub></td><td>Initial concentration in modeled system for giving element</td></tr><tr><td>E<sub>KD</sub></td><td>The effective partition coefficient for giving element</td></tr><tr><td>t<sub>f</sub></td><td>Final time duration for each round of modeling</td></tr><tr><td>t<sub>n</sub></td><td>Interval time duration before the thermal pulse happened</td></tr><tr><td>t<sub>p</sub></td><td>Interval time duration of the thermal pulse</td></tr><tr><td>t<sub>n'</sub></td><td>Interval time duration after the thermal pulse happened</td></tr><tr><td>n</td><td>Exponent of R(t) power-law function</td></tr></tbody></table>

Note: Check App. B. for the detail of Finite volume diffusion modeling