Petrology and phase equilibria of HP/LT eclogite at Gaoqiao, western Dabie and implications for lawsonite development in continental subduction zones

Lawsonite is of great significance for understanding fluid activity, element migration and crust–mantle interactions in subduction zones. Though studies have predicted lawsonite to be present under the P–T regime during continental subduction, no lawsonite has been documented from natural (U) HP rocks in continental orogenic belt. In this study, we work on HP–LT eclogite (GQ–1 and GQ–2) at Gaoqiao, western Dabie to explore lawsonite formation and preservation during continental subduction and exhumation. Both samples have ubiquitous polymineralic aggregates of epidote/clinozoisite + paragonite/albite ± other minerals showing distinct rectangular or rhombic shapes developed as inclusions in garnet or in the matrix. Combined with recalculated bulk compositions similar to that of ideal lawsonite, we interpret these polymineralic aggregates to be pseudomorphs after lawsonite. Phase equilibrium modelling combined with compositional isopleth thermobarometry have constrained a segment of the prograde to peak stages to evolve from 19.0 to 19.5 kbar, ∼470°C to ∼20.0 kbar, 500C–505°C, then to ∼25 kbar, 530C–555°C in lawsonite stability fields. The prograde P–T path shows a two-stage P–T evolution, with the first stage following a geothermal gradient of ∼7°C/km and the second stage decreasing to ∼6°C/km. Initial exhumation was inferred to follow an isothermal decompression process leading to lawsonite breakdown to form epidote/clinozosite ± paragonite via the reaction lawsonite + omphacite→ epidote + glaucophane ± paragonite + H2O at ∼19 kbar, 550°C. Modeled P/T–X pseudosections calculated at T = 550°C and p = 25 kbar show that, when H2O content in bulk composition is more than 1.1 wt%, a certain amount of lawsonite (>13 mode%) should be present in eclogite. On the other hand, in the compositional range of natural intracontinental plate basalts, variations on O (Fe3+), X MgO [MgO/(MgO+FeO)], X CaO [CaO/(CaO+MgO+FeO+MnO+Na2O)], X Na2O [(Na2O/(CaO+Na2O)] and X Al2O3 [Al2O3/(Al2O3+CaO+Na2O)] in bulk compositions have little influence on lawsonite development. In combination with previous studies, we conclude that during continent subduction along low geothermal gradient (<8°C/km), lawsonite could be formed under H2O present conditions. The absence of lawsonite in natural eclogite might be ascribed to retrograde overprint during exhumation.

The Sulu-Dabie mountain is a classic continental subduction-collisional orogenic belt formed by the Triassic collision between the south China Block and the north China Block (Zheng, 2008;Wu et al., 2009;Wu and Zheng, 2013). In the western Dabie, well-preserved HP/UHP metamorphic rocks have peak P-T conditions of 26-31 kbar and 520°C-670°C (Liu et al., 2004a;Liu et al., 2006;Wei et al., 2010;Xia et al., 2022a;, locating in lawsonite-eclogite facies fields. Besides, ubiquitous veins in eclogites and their host rocks manifest presence of fluid during eclogite facies metamorphism (Liu et al., 2004a;Cheng et al., 2009;Wei et al., 2010), beneficial for the formation of lawsonite. In this study, we show eclogites at Gaoqiao, western Dabie have abundant composite mineral aggregates of epidote/ clinozoisite + paragonite/albite ± other minerals with distinct rectangular or rhombic shapes in the matrix or as inclusions in garnet. These polymineralic aggregates were demonstrated to be pseudomorphs after lawsonite. To establish the P-T evolution and explore factors influencing the development and decomposition of lawsonite during continental subduction and exhumation, we performed detailed petrography, mineral chemistry and phase equilibrium modeling on eclogites at Gaoqiao, western Dabie. Mineral abbreviations in this study follow those used by the thermodynamic dataset of .
A larger number of geochronological studies on blueschist, eclogite and the host granitic gneisses and metasedimentary rocks have conformed the consistency on eclogite facies metamorphism in western Dabie and the eastern Dabie and Sulu belts, which constitute a huge continuous Triassic orogenic belt. For instance, in western Dabie, the prograde stage metamorphism was constrained to be 239-226 Ma, the peak UHP metamorphism to be~226 Ma, the early retrograde eclogite facies metamorphism to be 216-213 Ma and the later retrograde amphibolite facies metamorphism to be~212 Ma (Wu and Zheng, 2013 and references therein). However, in Huwan shear zone, late Carboniferous eclogite facies metamorphism ages of 310 Ma have been reported for eclogite and its country rock gneisses (Wu et al., 2009;Liu et al., 2011). In combination with some eclogites showing oceanic crust geochemical signatures, the eclogite facies metamorphism was interpreted to represent an early oceanic subduction (Wu and Zheng, 2013;Zhou et al., 2015).

Sampling
In this study, eclogite samples were collected at Gaoqiao in the Hong'an HP eclogite unit, western Dabie ( Figure 1B). Eclogites at Gaoqiao occur either as blocks (4-8 m in diameter) or as intercalated layers (0.5-4 m in width) in the host garnet micaschist ( Figure 1C). Various types of veins, including phengite-quartz veins and quartz-epidote-calcite veins developed in eclogites ( Figure 1D). At some places, elongated eclogite fragments are found in micaschist, with quartz veins roughly along schistosity in the host rock ( Figures 1E, F). To quantify P-T evolution for eclogite at Gaoqiao, we select two eclogite samples (GQ-1 and GQ-2)  Liu et al., 2004a). In this study, eclogite was sampled at Gaoqiao in the Hong'an HP eclogite unit, western Dabie. (C) Cross-section of eclogite and country rock at Gaoqiao, western Dabie. Eclogite occurs as blocks (4-8 m in diameter) or intercalated layers (0.5-4 m in width) in the country rock garnet micaschist. (D) Quartz-muscovite veins developed in layered eclogite. (E) Elongated eclogite fragments and quartz veins roughly along schistosity in micaschist. (F) Sketch of field features for (E).

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frontiersin.org 03 from the intercalated layers for petrology and phase equilibrium modeling.

Analytical methods
The major-element compositions of the minerals were analyzed using a JEOL JXA-8100 electron probe microanalyzer (EPMA) at the Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, Ministry of Natural Administration, and the JEOL JXA-8230 EPMA at the Center for Global Tectonics, School of Earth Sciences, China University of Geosciences (Wuhan), respectively. During the experimental operation, the acceleration voltage is set to 15 kV, the current is 20 nA, and the beam spot diameter is 1 or 5 μm. Dwell times were 10 s on element peaks and half that on background locations adjacent to peaks. Raw X-ray intensities were corrected using a ZAF (atomic number, absorption, fluorescence) correction procedure. A series of natural and synthetic SPI standards were utilized and changed based on the analyzing minerals. The following standards were used: sanidine (K), pyrope garnet (Fe, Al), diopside (Ca, Mg), jadeite (Na), rhodonite (Mn), olivine (Si), rutile (Ti) and apatite (P). Representative results are given in Tables 1, 2. Elemental mapping was performed using a JEOL JXA-8230 EPMA at the Center for Global Tectonics, School of Earth Sciences, China University of Geosciences (Wuhan). 15 kV accelerating voltage, 100 nA probe current, 1 μm×1 μm pixel size and stage scan model have been utilized during EPMA mapping. The EPMA stage mapping is performed at relatively high probe current to compensate for reduced pixel dwell time, and acquire the higher spatial resolution. All mapping time is spent at the WDS X-ray peak positions. Figure 3 shows the results.
The backscattered electron (BSE) images and energy spectrometer (EDS) analysis were performed at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan). The equipment used was a FEI Quanta200 scanning electron microscope (SEM) equipped with an EDAX EDS system. The operating conditions have an accelerating voltage of 20 kV, a set spot size of 200-400 nm and an emission current of~100 μA. The working distance between the BSE probe and the thin section is 11-12 mm.
The whole rock major element analyses were completed in the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan). The samples were analyzed for the major elements using an X-ray fluorescence spectrometer (model ShiMadzuXRF-1800). The X-ray target material is Rh, the power of the test is 2800 W, the analysis voltage is 40 kV, the current is 70 mA, and the raster diameter is 30 mm. Calibration curves used for quantification were produced by bivariate regression of data from~63 reference materials encompassing a wide range of silicate compositions. The measurement procedure and data quality were monitored by repeated samples (one in eight samples), USGS standard AGV-2 and Chinese National standards GSR-1 and GRS-7. The results are given in Table 3.

Omphacite
Omphacite mainly occurs as fine rock-forming minerals in the matrix or as inclusions in garnet. At places, omphacites in the matrix were replaced by symplectite of amphibole and albite along rims ( Figure 2F). Omphacite in garnet occurs as individual grain ( Figure 2C), or together with phengite, paragonite, zoisite and quartz constitute composite mineral aggregates ( Figure 2C; Figures 5I-L). In both samples, omphacite has similar compositions. In the matrix, omphacite has slightly higher X Jd [=Na/(Na+Ca)] (0.39-0.53) than the inclusion omphacite (0.32-0.51; Figure 4A).
Frontiers in Earth Science frontiersin.org The above petrographic observations and mineral chemistry show that four metamorphic stages can be inferred for eclogite at Gaoqiao, western Dabie. The prograde stage is evidenced by garnet core and its inclusions of o + gl + ph + q + ru + law (pseudomorphs) ± ep. The peak stage is evidenced by garnet rim, its inclusions of o + ph + gl + q + law (pseudomorphs) + ru and rock-forming minerals of o + ph + gl +q + law (pseudomorphs) in the matrix. The early retrograde stage is evidenced by the breakdown of lawsonite to form polymineralic aggregates, glaucophane to form amphibole, garnet to form amphibole, and/or epidote and omphacite to form symplectite. Therefore, the mineral assemblages are inferred to be cpx + amp + pl + ph + q + ru + ep ± pa. The late retrograde stage is evidenced by coarse amphibole, epidote and albite in the matrix and albite replacing paragonite and/or phengite in polymineralic aggregates in garnet. Mineral assemblage for this stage was inferred to be amp + ep + ab + q + ilm/sph.

Methods
To constrain P-T evolution and explore factors influencing the formation and breakdown of lawsonite for eclogite at Gaoqiao, western Dabie, phase equilibrium modeling was performed. We use Thermocalc 3.33 software  and the associated ds55 database updated November 2003) in for modeling. A-x relationships used are as follows: garnet (White et al., 2005), clinopyroxene (Green et al., 2007), amphibole (Diener and Powell, 2012), epidote and talc , chlorite , muscovite (Coggon and Holland, 2002) and plagioclase (Holland and Powell, 2003). Lawsonite, kyanite, quartz/coesite, and H 2 O are considered as   (Liu et al., 2004a;Cheng et al., 2010). Whole rock compositions obtained by XRF were used for modeling after CaO, FeO T and MgO correction for CO 2 content in calcite and P 2 O 5 content in apatite (Table 3). Considering garnet has distinct growth zoning (Figure 3), the growth of garnet may lead to fractionation of the bulk rock composition (Evans, 2004). In this study, we used the corrected XRF bulk rock composition to model P-T conditions for garnet core growth. For modeling garnet rim growth, we get the effective bulk rock composition using the principle of Rayleigh fractionation to deduct garnet core composition from the corrected XRF bulk rock composition following the method recommended by Evans (2004). The results are shown in Table 3. In addition, considering that different oxides in the bulk composition

P-T pseudosections using the corrected XRF bulk rock composition
For eclogites GQ-1 and GQ-2, phase diagrams constraining P-T conditions for garnet core and mantle growth were calculated under the NCKMnFMASHO system using the bulk rock compositions in Table 4 for given P-T ranges of 400°C-600°C and 10-30 kbar (Figures 6A, B; Figures 7A,B). The results show that, the P-T pseudosections are dominated by di-and trivariant fields, with few quadri-and quinivariant fields. Lawsonite-bearing phase assemblages are present at high P fields of 11-30 kbar at 400°C-600°C and epidote-bearing phase assemblages are present at low P fields of <20 kbar and 400°C-600°C. Because of slightly different bulk rock composition, the phase diagram for GQ-1 ( Figure 6A) has larger di-bearing but smaller q-absent fields than that for GQ-2 ( Figure 7A).
Compositional isopleths for X gr and X py in garnet and Si content in phengite have been calculated for the concerned P-T range ( Figure 6B; Figure 7B). In the lawsonite-bearing phase assemblage fields, isopleths for X gr in garnet have a flat slope and decrease with pressure, while isopleths for X py have a negative steep to vertical slope and increase with temperature ( Figure 6B; Figure 7B. On the other hand, in the epidote-bearing phase assemblage fields, both X gr and X py isopleths have negative moderate slopes, X gr decreases while X py increases with temperature ( Figure 6B; Figure 7B). Isopleths for Si content in phengite have negative moderate slope and increases with pressure. Besides, the calculated H 2 O mode isopleths have a steep slope and roughly decrease with temperature ( Figure 6B; Figure 7B).
For both eclogites GQ-1 and GQ-2, the observed inclusions of omphacite, glaucophane, amphibole, lawsonite pseudomorphs of ep + pa, phengite, chlorite and quartz in garnet core to mantle correspond to the modeled phase assemblages of g + o + gl + law ± chl ± ep + q + mu + H 2 O ( Figure 6A; Figure 7A). In these phase assemblage fields, the measured X spss in garnet inner core (0.150-0.035 for both GQ-1 and GQ-2) and X gr (from 0.347 to 0.313 for GQ-1 and from 0.324 to 0.308 for GQ-2) and X py (from 0.049 to 0.030 for GQ-1 and from 0.053 to 0.034 for GQ-2) from garnet core to mantle have constrained a segment of the prograde P-T evolution from 19 to 20 kbar, 470°C-475°C to 20-21 kbar, 510°C ( Figure 6B; Figure 7B). However, in the phase assemblage fields of g + o + gl + ep + chl ± law + q + mu + H 2 O the measured Si content (3.22-3.34 p.f.u for GQ-1 and 3.33-3.39 p.f.u for GQ-2) in phengite from garnet core corresponds to pressure of 10-18 kbar at 400°C-510°C. The slightly low pressure may either be disequilibrium between garnet core and the included phengite, or phengite composition being reset during retrogression considering paragonite/albite replacing phengite inclusions at some places.
In GQ-1, the highest Si of 3.42 p.f.u. was measured for phengite in the garnet rim. At a peak temperature of 555°C, the Si isopleths of phengite is limited to 22.5 kbar in the peak mineral combination g+o+gl+law+q+mu+H 2 O, which is about 2 kbar lower than the pressure limited by X gr (Figures 6B, D). In GQ-2, the highest Si of 3.53 p.f.u. was measured for phengite in the garnet rim. At a peak temperature of 555°C, the Si isopleths of phengite is limited to 23.5 kbar in the peak mineral combination g+o+gl+law+q+mu+H 2 O, which is about 1.5 kbar lower than the pressure limited by X gr (Figures 7B,  D). Based on the isopleths of saturated water content of the relevant mineral assemblage ( Figure 6B; Figure 7B), the P-T path of the metamorphism process crosses the isopleths of the decrease of saturated water content and evolves in the direction of decreasing water content. It shows that H 2 O exists in the process of metamorphism and the evolution of minerals is dominated by dehydration (Guiraud et al., 2001).

P-T pseudosections using an effective bulk rock composition
For eclogites GQ-1 and GQ-2, phase diagrams constraining P-T conditions for garnet rim0. growth were calculated under the NCKMnFMASHO system using the effective bulk rock compositions in Table 3 for the given P-T ranges of 400°C-600°C and 10-30 kbar (Figures 6C, D; Figures 7C, D). The results show that the topology and phase relations in Figure6C; Figure 7C are similar to those of Figure 6A; Figure 7A, except that garnet is absent in the fields p <18 kbar and T <485°C. Besides, in Figure 7D, actinolite is present in the modeled phase assemblage fields of 12-28 kbar and 400°C-485°C.
In the phase assemblage field of g + o + gl + law + mu + q + H 2 O, P-T conditions constrained by isopleths of the measured X py (0.051-0.132 for GQ-1 and 0.052-0.127 for GQ-2) and X gr (0.316-0.234 for GQ-1 and 0.331-0.230 for GQ-2) in garnet rim are~25 kbar and 530C-555°C. Those values are interpreted to represent P max -T conditions for eclogite at Gaoqiao, western Dabie. The maximum Si content of 3.53 p.f.u. In phengite included in garnet rim from eclogite GQ-2 corresponds to P of 22-24 kbar at 530°C-555°C, slightly lower than that constrained by garnet rim compositions. Lower Si content of 3.27-3.39 p.f.u. for phengite in the matrix ( Figure 4E) may indicate compositional reset of phengite during exhumation after the Pmax stage (Xia et al., 2020).
The observed mineral assemblage of g + o + amp + ep + q + ph + pa in the matrix corresponds to the modeled phase assemblage of g + o + hb + ep + q + ph + pa + H 2 O with P-T regime of 10-16 kbar, 520°C-555°C ( Figure 6D; Figure 7D). Combined with lawsonite breakdown to form composite inclusions of ep/czo + pa/ab + ph, low Si contents of 3.27-3.39 p.f.u. In phengites in the matrix or constituting lawsonite pseudomorphs in garnet, we infer the early retrograde stage may follow an isothermal decompression process during subsequent exhumation from the P max stage. In the phase assemblage field of g + o + gl + law + q + mu + H 2 O, the retrograde P-T path may firstly be tangential to the calculated isopleths for H 2 O mode content in relevant hydrous phases. Then, the P-T path crossed lawsoniteepidote transition fields at 19 kbar at 550C-560°C. The breakdown of lawsonite via the reaction law + o→ ep + gl ± pa + H 2 O would release a significant amount of H 2 O and the P-T path crossed the H 2 O decrease contours with H 2 O content in the Frontiers in Earth Science frontiersin.org bulk rapidly decreasing from 1.00 wt% to 0.52 wt%. When the P-T path entered the fields of g + o + gl/hb + ep ± pa + q + H 2 O with p <13 kbar, amphibole developed at the expense of glaucophane, and caused overprint of epidote-amphibolite facies metamorphism. Note that following this P-T path, H 2 O contours increased, implying additional external fluid may enter the rock system. Further decompression to 11 kbar led to the formation of albitic plagioclase after paragonite.

P/T-X pseudosections modeling the influence of various oxides in bulk composition on the stability of lawsonite
To evaluate influence of the contents of H 2 O, O (Fe 3+ ) and other oxides (MgO, CaO, Na 2 O and Al 2 O 3 ) in the bulk rock composition on the stability of lawsonite during metamorphic evolution, we calculated a series of P/T-X phase diagrams for eclogite GQ-1 (Supplementary FIGURE 6 P-T pseudosections for the eclogite GQ-1 using primary bulk rock composition (A,B) and effective bulk rock composition (C,D) in Table 4 gr represents calculated isopleth for grossular in garnet, py represents calculated isopleth for pyrope in garnet, m represents calculated isopleths for Si in phengite and h represents calculated isopleths for H 2 O modal content saturated for relevant phase assemblages. Yellow arrows represent inferred P-T evolution from the prograde to peak, then to retrograde stages. C, M, R1 and R2 represent compositions of garnet core, mantle and rim (inner rim and outer rim), respectively. In figure (D), the calculated geothermal gradient during the prograde process is 6°C-7°C/km (assuming 2.8 km/1 kbar, after Wei et al., 2010). In (A), the number represents: 1. g o gl law chl ta -q, 2. g o law ta, 3. g o gl law di -q, 4. g o gl law ky, 5. g o glaw ky, 6. g o gl ky, 7. g o gl ep ky, 8.g o gl pa ky, 9. g o gl ep act -q, 10. g o gl ep hb, 11. g o hb pa, 12. g o di ep hb pl, 13. g hb pL di. In (C), the number represents: 1. g o law, 2. g o gl law di -q, 3. g o gl law ky, 4. g o gl ky, 5. g o gl ep ky, 6. g o gl pa ky, 7. g o gl law chl ep, 8. o gl chl ep act, 9. g o gl ep hb, 10. g o gl hb pa, 11. g o hb pa, 12. g o di ep hb pl, 13. g di hb pl.

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frontiersin.org Figures S1-S6). T-X (at p = 25 kbar) and P-X (at T = 550°C) pseudosections were calculated under the NCKMnFMASHO system using the compositions in Supplementary Table S1 for the given T ranges of 400°C-600°C or P ranges of 10-30 kbar, respectively.

P/T-H 2 O pseudosections
Supplementary Figure S1 shows

P/T-X MgO pseudosections
The calculated T-X MgO phase diagram at p = 25 kbar (Supplementary Figure S3A) shows that at X MgO <0.39, the stability field for lawsonite-bearing phase assemblages reduces when X MgO decreases. At X MgO >0.39, lawsonite stability field changes little when X MgO increases. Similarly, in P-X MgO phase diagram at 550°C (Supplementary Figure S3B), lawsonite stability field reduces when X MgO decreases at X MgO <0.26, but changes little when X MgO increases at X MgO >0.26.

P/T-X CaO pseudosections
The calculated T-X CaO phase diagram at p = 25 kbar (Supplementary Figure S4A) shows that lawsonite stability field increases when X CaO increases and at X CaO >0.36, lawsonite is stable in the T range of 400°C-650°C. In Supplementary Figure S4B, the calculated P-X CaO phase diagram at T = 550°C shows that lawsonitebearing phase assemblages are stable at p >19 kbar and the variation of X CaO has little influence on the stability of lawsonite.

P/T-X Na2O pseudosections
Supplementary Figure S5A shows that in the T-X Na2O phase diagram calculated at 25 kbar, lawsonite stability field decreases distinctly when X Na2O increases from 0.1 to 0.5. On the other hand, in the T-X Na2O phase diagram calculated at 550°C, lawsonite stability field shows little change at p <20 kbar when X Na2O changes from 0.1 to 0.44. However, at X Na2O >0.44, lawsonite is absent in the pressure range of 10-30 kbar.

P/T-X Al2O3 pseudosections
In Supplementary Figure S6A for T-X Al2O3 phase diagram calculated at p = 25 kbar, lawsonite is stable at T <620°C and its stability field increases when X Al2O3 increases from 0.3 to 0.61. At X Al2O3 >0.61, lawsonite stability field changes little to X Al2O3 . In Supplementary Figure S6B for P-X Al2O3 phase diagram calculated at T = 550°C, lawsonite is absent at X Al2O3 <0.50. At X Al2O3 >0.50, lawsonite is stable at p >19.5 kbar and its stability field changes little when X Al2O3 increases.
Thirdly, combined with the volume ratios, densities and compositions of the minerals in the composite inclusions, the bulk composition of the pseudomorphs was estimated using the method of Orozbaev et al. (2015). The restricted lawsonite compositions are shown in Table 4. The results show that, SiO 2 , Al 2 O 3 and CaO contents in the reconstructed lawsonite composition are very similar to that of ideal lawsonite while FeO and Na 2 O contents are slightly higher (Table 4). Reasons could be uncertain in estimating volume ratio of minerals in the composite inclusions, or the introduce of Na + , Fe and Mg due to H 2 O migration during lawsonite breakdown. Tiny fractures developed around the composite inclusions ( Figure 5) may have been acted as pathways for fluid ingress or egress (Lü et al., 2019). Nonetheless, the proportions of SiO 2 , Al 2 O 3 and CaO are very similar to that of ideal lawsonite (Table 4).

P-T evolution
Based on the above petrographic observations, mineral chemistry and phase equilibrium modelling, a complete P-T path comprising the later prograde, P max , initial decompression and late retrograde stages have been inferred for eclogite at Gaoqiao, western Dabie.

Prograde to peak stages
A segment of the late prograde stage metamorphic evolution was inferred based on inclusions in garnet core and garnet core composition showing distinct growth zoning. Phase equilibrium modeling (Figures 6, 7) shows that, in the modeled phase assemblage fields g + o + gl + law ± chl ± ep ± q± di + mu + H 2 O, garnet inner core composition (X gr =0.347-0.308, X py = 0.053-0.030 and X spss =0.150-0.035) corresponds to a P-T regime of 19-20 kbar, Frontiers in Earth Science frontiersin.org 470C-475°C ( Figures 6B; Figures 7B), interpreted to represent the initial growth of garnet. The decease of X gr and increase of X py from garnet outer core to rim further correspond to a P-T evolution tõ 20 kbar, 500C-505°C and then to~25 kbar, 530C-555°C. Notably, for both eclogites GQ-1 and GQ-2, maximum pressures constrained by the maximum Si content in phengite (22.5 kbar at 510°C-560°C for GQ-1; 23.5 kbar at 520°C-550°C for GQ-2) are slightly lower than those constrained by the garnet outer rim compositions (Figures 6B, C; Figures 7B, C). Possible reasons could be: (1) systematic uncertainties propagated from each endmember enthalpy in the dataset (±0.4 kbar within two sigma error); (2) random uncertainties propagated from the analytical uncertainties for EPMA of phengite and garnet (within~2% relative); (3) not found phengite with higher Si content during petrographic observations; or (4) possible re-equilibration of Si in phengite during retrogression (Liu et al., 2004a;Jahn et al., 2005;Liu et al., 2006;Wei et al., 2010).
The prograde to peak metamorphism shows a two-stage P-T evolution defined by the slope of the thermal gradient of the P-T path. The first stage P-T path with a gentle positive slope is dominated by heating and slight increase in pressure, following a geothermal gradient of~7°C/km ( Figure 6D; Figure 7D). On the other hand, the second P-T path shows a moderate positive slope with geothermal gradient decreasing to~6°C/km. Such a P-T path pattern has been reported by Wei et al. (2010) for eclogite in the Xinxian UHP unit in western Dabie and was interpreted to reflect a change of subduction rate from slow to fast.

Retrograde stages
During initial exhumation, lawsonite broke down to form composite mineral aggregates of ep/czo + pa/ab + other minerals in the matrix or as inclusions in garnet. Phase equilibrium modeling shows that the transition of lawsonite to ep ± pa occurred at 19 kbar and 550°C-560°C. This pressure is consistent to P constrained by isopleths of Si in phengites from the composite inclusions (3.33-3.41 p.f.u.) or from the aggregates in the matrix (3.28-3.39 p.f.u.), the former gives P of~20 kbar at 555°C and the later of~19 kbar at 555°C. Therefore, we estimate the early decompression stage subsequentially passed through the lawsonitepresent field of g + o + gl + law + mu + q + H 2 O, lawsonite-epidote coexistent fields of g + o + gl + law + ep + mu + q + H 2 O ± pa and lawsonite-absent field of g + o + gl + ep + mu + q + H 2 O ± pa. Whether or not the inferred P-T path crossed the paragonite fields is unknown, because effective bulk rock composition may have been changed during the retrogression. Anyhow, we suggest the initial retrograde P-T evolution to follow an isothermal decompression process, similar to that proposed by Wei et al. (2010) for eclogite at Xinxian. Combined lawsonite-out boundary with isopleths of Si in phengite from the matrix and from composite inclusions in garnet, we interpret the P-T regime of 19-20 kbar and 550C-560°C to represent the likely P-T conditions for the early retrograde stage. During initial exhumation, in the phase assemblage field of g + o + gl + law + mu + q + H 2 O, H 2 O content in the bulk composition saturating relevant minerals changed little from 25 to 20 kbar ( Figure 8B). On the other hand, when passing through the field g + o + gl + law + ep + mu + H 2 O+ q, lawsonite transformed to epidote via the reaction law + o → ep + gl ± pa + H 2 O and released a large                 Figure 8B). Later retrogression led to the replacement of glaucophane by amphibole, omphacite by symplectitic amp + pl ± cpx, garnet by amp + pl corona, phengite by paragonite and paragonite by albite, indicating epidote-amphibolite facies overprint. In the modeled phase diagrams (Figure6D; Figure 7D), the later retrograde P-T path successively passed through the phase assemblage fields of g + o + gl + ep + mu + q + H 2 O ± pa, g + o + ep + hb + mu + q + pa + H 2 O and g + o + ep + hb + mu + q + pl + H 2 O. However, this process roughly followed a H 2 O increase tendency ( Figure 8B). Traditionally viewpoint would expect a fluid-deficient circumstance for the rock system to preserve the mineral assemblage with minimum H 2 O content (Guiraud et al., 2001). Therefore, widespread epidoteamphibolite facies overprint in the matrix minerals may imply external fluid infiltration to rock system during late-stage retrogression.

Factors affecting lawsonite development in continental subduction zones
Studies on natural lawsonite-bearing blueschist and eclogite show that formation of lawsonite requires cold subduction into mantle depths (Tsujimori et al., 2006;Tsujimori and Ernst, 2014). Experimental studies and thermodynamic modeling using a representative MORB composition have shown that under H 2 O-saturated conditions, subduction along a geothermal gradient of <8°C/km (which is common in many classic continental subduction zones) would form (U)HP/LT metamorphic rocks in lawsonite stability field (e.g., Okamoto and Maruyama, 1999;Wei, 2011;Tsujimori and Ernst, 2014). Therefore, we can conclude that lawsonite would be expected to form during continental subduction under the assumption of cold and fluidpresent conditions. Such a conclusion has been evidenced by pseudomorphs interpreted to be after lawsonite and geochemistry studies (Li et al., 2005;Wei et al., 2010;Guo et al., 2013). However, the problem is, although compiled P-T estimates for natural (U)HP metamorphic rocks in continental subduction/collisional orogenic belts fall in lawsonite stability field (Penniston-Dorland et al., 2015), no lawsonite grain has been reported in these rocks. Then, what factors affect the formation and stability of lawsonite during continental subduction and exhumation?

Influence of whole rock composition
The first thing to be considered is fluid (here refers to water) availability during continental subduction since lawsonite is a simple water-rich mineral. A traditional view is that compared to oceanic subduction zones, continental subduction zones are relatively old, dry and cold, with high viscosity, low water availability and low density, thus leading to weak fluid activity during subduction Ernst et al., 2007). In recent years, however, numerous studies have found that a significant amount of H 2 O (or hydroxyl) can be stored in various hydrous minerals (e.g., amphibole, phengite, lawsonite, epidote and talc) or nominally anhydrous minerals (e.g., garnet, omphacite, quartz/coesite and kyanite) in continental subduction zones (e.g., Kessel et al., 2005;Hermann et al., 2006;Zheng et al., 2011). In many classic continental subduction-collisional orogenic belt, such as the Alpine, the Himalaya and the Sulu-Dabie, studies on fluid activity during continental subduction include but not limited to dominant and recessive water-bearing components in HP-UHP rocks, rich veins in HP-UHP terrains, abundant water-bearing minerals formed by eclogite retrogression, and polyphase solid inclusions representing the presence of melt/fluid in eclogite minerals (Liou et al., 1998;2009;Xiao et al., 2000;Franz et al., 2001;Fu et al., 2002;2003;Spandler and Hermann, 2006;Zhang Z. et al., 2007;2008;Zheng and Hermann, 2014). Therefore, in continental subduction zones, it may be really lack fluid activity, but might be rich in some conditions (Chopin, 2003;Bebout, 2007;Beaumont et al., 2009;Zheng, 2009;Zhao et al., 2015;2021).
In this study, for the eclogites at Gaoqiao, abundant hydrous minerals were identified as inclusions in garnet showing distinct growth zoning, implying water presence during prograde metamorphic evolution. Besides, as an equivalent to the eclogites, low-grade blueschist and greenschist in the Mulanshan unit suggest water present conditions in shallow subduction zones. These rocks would release free water via dehydration reactions to form eclogite when subduction continuously proceeded. To quantify the influence of water content on the formation of lawsonite, we have calculated P/ T-H 2 O phase diagrams (Supplementary Figure S1). Our modeling shows that, in the phase assemblage fields with H 2 O > 0.036 wt%, lawsonite is stable under >400°C at 25 kbar or >19 kbar at 550°C. Under H 2 O unsaturation conditions (<1.152 wt% at 25 kbar, 550°C; Supplementary Figure S1), the amount of lawsonite is correlated to H 2 O content in the bulk rock composition. For instances, lawsonite decreases from 13 to 0 mode% when H 2 O content decreases from 1.152 to 0.036 wt% (Supplementary Figure S1). Previous studies have shown that, in (U)HP eclogite, a significant amount of water could be incorporated as both structural OH and molecular H 2 O in nominally anhydrous minerals (e.g., Kessel et al., 2005;Hermann et al., 2006;Zheng et al., 2011). For instance, for eclogite in Sulu-Dabie, the total H 2 O content including fluid inclusions, hydrous mineral inclusions and structural hydroxyl could be up to 1170-20745 ppm in omphacite and 522-1584 ppm in garnet (Chen et al., 2007). In combination with H 2 O retained in some hydrous minerals (e.g., glaucophane, phengite, talc, epidote), we can conclude that for eclogite in western Dabie formed at P-T conditions of 25 kbar, 550°C, water content should be sufficient for the formation of lawsonite.
For O and other oxides (MgO, CaO, Na 2 O and Al 2 O 3 ) in the bulk rock composition, their influence factors on the formation of lawsonite have also been discussed using P/T-X pseudosections (Supplementary Figures S2-S6). The results show that, lawsonite stability field changes little relative to O and X MgO variations (Supplementary Figures S2, 3). On the other hand, X Al2O3 , X Na2O and X CaO in the bulk composition may control lawsonite stability field to some extent. For instance, in the T-X CaO phase diagram at 25 kbar, lawsonite stability field increases when X CaO increases at X CaO <0.36 (Supplementary Figure S4A); in the T-X Na2O phase diagram calculated at 25 kbar, lawsonite stability field varies distinctly at X N2O = 0.1-0.5 and in the P-X Na2O phase diagram calculated at 550°C, lawsonite is absent at X Na2O >0.44 (Supplementary Figure S4B); in the T-X Al2O3 phase diagram calculated at 25 kbar, lawsonite varies distinctly at X Al2O3 = 0.3-0.61 and in the P-X Al2O3 phase diagram calculated at 550°C,  Figure S8).

Influence of P-T evolution
Experimental studies show that lawsonite could be stable at T <830°C with maximum P of 80-90 kbar (Okamoto and Maruyama, 1999), similar to that (T < 760°C and p < 80 kbar) calculated using phase equilibrium modeling using whole-rock composition of the eclogite GQ-2 at Gaoqiao. Therefore, the preservation of lawsonite requires peak temperature during metamorphic evolution to not exceed its thermal stability. When T exceeds the up-temperature of lawsonite stability field, lawsonite may break down to form kyanite via the reaction law → ky + czo/ep + q + H 2 O or to form ep/czo via the reaction law + jd → o + g + czo/ ep + H 2 O (Wei et al., 2010;Tsuchiya and Hirajima, 2013;Tsujimori and Ernst, 2014;Ren et al., 2018).
Another crucial factor influencing the preservation of lawsonite is retrograde evolution process during exhumation (Wei, 2011;Fedele et al., 2018). In general, two patterns of exhumation P-T path may facilitate lawsonite preservation. One is refrigerated exhumation characterized by either hairpin-like (e.g., Ernst, 1988;Vitale Brovarone et al., 2011) or counterclockwise (e.g., Tsujimori and Ernst, 2014;Hunziker et al., 2017) P-T path. Substantial cooling during retrogression allows the retrograde evolution across lawsonite stability field and allows the preservation of lawsonite both in the matrix or as inclusions in porphyroblastic minerals . Another is isothermal decompression characterized by fast exhumation that lawsonite could be preserved only as inclusions in refractory minerals (e.g., garnet, pyrite; Li et al., 2013;Xia et al., 2020), or in a specific condition, in the matrix as has been reported for lawsonite-bearing eclogite xenoliths in kimberlitic pipes from Colorado Plateau (Usui et al., 2006). Isothermal decompression may lead to retrograde P-T path across lawsonite-epidote transition curve and thus to breakdown of lawsonite. The breakdown of lawsonite could release a significant amount of H 2 O, causing retrogression and forming polymineralic aggregates showing pseudomorphs of lawsonite (Wei, 2011;Tian and Wei, 2013;Xia et al., 2020). In this study, by combining petrographic studies and phase equilibrium modeling, we show the retrograde P-T path for the eclogites at Gaoqiao to follow an isothermal decompression process. Therefore, the breakdown of lawsonite may be due to the isothermal exhumation.

luid behavior during lawsonite breakdown
As mentioned above, previous studies have proved intense fluid activity in continental subduction zones. During subduction, our phase equilibrium modelling for the eclogites at Gaoqiao (Figure 6B; Figure 7B) shows that the modal contents of hydrous minerals (e.g., chlorite, lawsonite and glaucophane) gradually decreased to form anhydrous minerals (e.g., garnet and omphacite) along the progressive P-T evolution ( Figure 8A). The

FIGURE 8
Modal variations for different phases (mode%) and corresponding H 2 O content (wt%) saturated for relevant assemblages from g + o + gl + chl + law + q + mu (A) to g + o + ep + hb + pa + pl + q + mu (B) along the inferred P-T path in Figure 6.
Frontiers in Earth Science frontiersin.org modeled reactions are o + chl → g + gl + law + H 2 O at low temperature (<501°C) and gl + law → g + o + H 2 O at high temperature (>501°C). Figure 8A shows that, during this process, water content in the bulk composition to be saturated relevant hydrous minerals decreased from 2.30 to 1.02 wt% (Point 1-10). Subsequent ITD exhumation led the P-T path to cross lawsonite-epidote transition fields ( Figure 6D; Figure 7D). However, detailed mechanism for lawsonite breakdown is still unknown. On one hand, Hamelin et al. (2018) proposed that lawsonite breakdown must be in part driven by reaction of lawsonite with the celadonite content of phengitic mica in the matrix, to form celadonite-poor phengite in the pseudomorphs. Phengite should become less rich in celadonite with an increase in temperature to drive lawsonite breakdown. On the other hand, the breakdown of lawsonite may only be driven by pressure decrease to cross the H 2 O-decrease isopleths during exhumation (Wei, 2011). Under this assumption, phengite with lower celadonite in pseudomorphs could be formed from K-bearing fluid at relatively lower pressure when lawsonite transformed to epidote.
Our phase diagrams show that, the breakdown of lawsonite released almost~50% of H 2 O in the bulk rock composition (from 1.02 to 0.52 wt%) (Point 15 to 17 in Figure 8B). Based on petrographic observation and mineral chemistry, we infer a range of elements migrated with fluid to form various types of polyphase pseudomorphs after lawsonite. For type 1 pseudomorphs with mineral aggregates of czo/ep + pa/ab ± q ( Figures 5A-D), the balanced reaction 1 implies additional Na + and/or Si 4+ to the original lawsonite. For type 2 pseudomorphs with amp-bearing mineral aggregates of czo + pa/ab + amp ( Figures 5E-H), except for added Na + and lost water, Fe/Mg should be introduced by fluid (reactions two and 3). Omphacite may participate in this process. For type 3 pseudomorphs with largely phengite, paragonite/albie and rare czo/ep ( Figures 5I,K), K + and Na + should be introduced by fluid ingress and Ca 2+ must have been lost to form amphibole or epidote in the matrix by fluid egress. For type 4 pseudomorphs dominated by quartz and/or phengite, Ca 2+ and/or Al 3+ must be lost while Si and/or K should be introduced by fluid ingress (Figures 5J, L). Tiny fractures around these polymineralic inclusions are interpreted to provide pathways for fluid activity.
Accordingly, the mechanism causing lawsonite breakdown during continental subduction and exhumation may be due to the change of P-T conditions and fluid environments, which promote the lawsonite of lawsonite by reaction with surrounding minerals. In addition, phase equilibrium modelling suggests that during continental plate subduction, lawsonite can absorb the water that was released from water-bearing minerals during the initial stages of subduction (e.g., chlorite, chlorite, hornblende, etc.) and carry it into the deeper parts of the subduction zone. In cold geothermal gradient subduction zones, it can even carry water into the mantle to release, causing partial melting and facilitating crust-mantle interactions (Zheng, 2009;Zheng et al., 2011). During the exhumation process, the lawsonite breakdown and releases abundant water due to the change of P-T conditions, resulting in the formation of metamorphic veins in (U)HP eclogites and the growth of new minerals, and causing local element migration or metasomatism (Li et al., 2005;Herman et al., 2006;Chen et al., 2007;Guo et al., 2013;Zheng and Hermann, 2014). And the fluid released during the deep subduction exhumation process can accumulate locally, and may even cause the overlying rock to dehydrate and melt to form syn-exhumation magma (Miller et al., 2002;Zheng, 2009;Zhao et al., 2015).

Conclusion
(1) Abundant polymineralic aggregates of epidote/clinozoisite + paragonite/albite ± other minerals showing distinct rectangular or rhombic shapes developed as inclusions in garnet or in the matrix from the eclogites at Gaoqiao, western Dabie. Recalculated bulk composition of the composite pseudomorphs is similar to that of ideal lawsonite. Therefore, we interpret the composite mineral aggregates as pseudomorphs after lawsonite.
(2) Based on petrology and phase equilibrium modelling, a convex P-T path has been inferred for the eclogites at Gaoqiao, western Dabie.

Data availability statement
The original contributions presented in the study are included in the article/Supplementary Material. further inquiries can be directed to the corresponding authors.