Evolution of Olivine Fabrics During Deep Subduction and Exhumation of Continental Crust: Insights From the Yinggelisayi Garnet Lherzolite, South Altyn, NW China

The different olivine fabrics in ultramafic rocks have been widely used to discuss past tectonic settings, given that the olivine fabrics vary with pressure, temperature and water content. However, there are no research related to whether and how the olivine fabrics transform at different metamorphic stages in a natural rock during the process of deep subduction and exhumation. Yinggelisayi garnet lherzolites from South Altyn have experienced deep continental subduction and exhumation. The garnet lherzolites contain well‐preserved residual protolith minerals, and near‐peak (M1), granulite‐facies retrograde (M2), and amphibolite‐facies retrograde (M3) metamorphic mineral assemblages. Olivine grains in M1 formed at P‐T conditions of 2.52–3.08 GPa, 1,095–1,136°C and low water contents (183–213 ppm H/Si), and showed [010] axes sub‐normal to the foliation and [001] axes subparallel to the lineation, which is characteristic of B‐type fabric ((010)[001]). Olivine grains in M2 formed at P‐T conditions of 1.31–1.80 GPa, 851–893°C and also low water contents (93–139 ppm H/Si), and exhibited [010] axes sub‐normal to the foliation and [100] axes subparallel to the lineation, which is characteristic of A‐type fabric ((010)[100]). These observations suggest that olivine fabrics in high pressure‐ultrahigh pressure metamorphosed ultramafic rocks are different in the near‐peak and retrograde metamorphic stages, and also that the olivine fabrics can be transformed during deep continental subduction and exhumation. Therefore, the dispersed or no clear olivine fabric is probably caused by multi‐stage deformation and metamorphism, and the distinct olivine fabrics can also be used as a clue to identify geological processes and better understand metamorphism and deformation during subduction and exhumation.

. A-and D-type olivine fabrics were commonly observed in mantle rocks (Ben-Ismail & Mainprice, 1998;Boudier & Nicolas, 1995;Cao et al., 2017;Falus et al., 2008;Sun et al., 2016). B-, C-, and E-type olivine fabrics were reported in convergent boundaries (Jung et al., 2013;Katayama et al., 2005;Mizukami et al., 2004;Michibayashi et al., 2007;Skemer et al., 2006;Tommasi et al., 2000;Q. Wang et al., 2013;Warren et al., 2008;Xu et al., 2005Xu et al., , 2006. Additionally, the high pressure-ultrahigh pressure (HP-UHP) metamorphosed ultramafic rocks typically have B-and/or C-type olivine fabrics, including samples from the Cima di Gagnone in the Central Alps of Switzerland (Frese et al., 2003;Skemer et al., 2006), Otroy in western Norway (Q. , northern Qaidam in northwest China (Jung et al., 2013), Zhimafang (Xu et al., 2005(Xu et al., , 2006 and Xugou (Y.  in the Sulu UHP terrane in China, and Songshugou in the Qinling orogen in China (Sun et al., 2019). However, olivine is a common mineral in metamorphosed ultramafic rocks of various stages, and there is seldom investigation in a natural rock that whether and how the olivine fabrics change at different metamorphic stages during deep subduction and exhumation of continental crust. The Yinggelisayi garnet lherzolites from the South Altyn HP-UHP metamorphic belt have experienced complex metamorphic processes and contain multiple mineral assemblages (Liu et al., 2002;Wang et al., 2011;Zhang et al., 2005Zhang et al., , 2014, which are ideal for investigating the evolution of olivine fabrics during deep continental subduction and exhumation. The garnet Iherzolite was identified residual phases of protolith and three metamorphic stages based on the petrography and mineral chemistry. Accordingly, we focus on the olivine fabrics at the near-peak and early retrograde metamorphic stages, and discuss its geological implication.

Geological Setting
The Altyn Tagh orogen is located along the northern margin of the Qinghai-Tibet Plateau, and is situated between the Tarim Block to the north, and the Qaidam Block and Qilian and Kunlun orogenic belts to the south (Figure 2a; Wang et al., 2011). Based on structural, geochemical, and geochronological data, the Altyn Tagh orogen can be divided into four units from north to south (Figure 2b): (a) the Archean North Altyn Terrane, which comprises mainly the granulite-facies Milan Complex and overlying Annanba Group; (b) the Hongliugou-Lapeiquan subduction-collision complex, which includes early Paleozoic ophiolites, pelagic and clastic sedimentary rocks, and HP-LT blueschists and eclogites; (c) the Milanhe-Jinyanshan block, which consists mainly of Meso-Neoproterozoic low-grade metamorphic rocks, schists, limestones, sandstones, mudstones, and thick stromatolites; and (d) the South Altyn subductioncollision complex, which can be further divided into the Jianggalesayi-Danshuiquan-Yinggelisayi HP-UHP metamorphic belt and Apa-Mangya ophiolite complex (Gai et al., 2022;Liu et al., 2002Liu et al., , 2012Liu et al., , 2015Liu et al., , 2018Wang et al., 2011;Zhang et al., 2005Zhang et al., , 2014.
The Yinggelisayi area is located in the eastern part of the South Altyn HP-UHP metamorphic belt, near the Altyn Fault ( Figure 2b). Interlayered garnet lherzolite, garnet pyroxenite, and garnet-bearing granitic gneiss (Figures 3a  and 3b) form a complex lens that is 2,500 m long (E-W) and 800 m wide (N-S) (Figure 2c). The long axis of the lens and lineation in the garnet lherzolites are consistent with the gneissosity that strikes ∼280° (Figure 3c). Previous studies have shown that the Yinggelisayi garnet lherzolites have experienced UHP metamorphism. Liu et al. (2005) observed clinopyroxene exsolution in garnet from the garnet pyroxenites and proposed that the peak metamorphic conditions were >7 GPa/∼1,000°C. Dong et al. (2019) determined the peak metamorphic pressure of the mafic granulites to be 4-7 GPa based on phase equilibria. Dong et al. (2020) identified pigeonite exsolution along the (401) plane in clinopyroxene in garnetite, which constrains the minimum metamorphic conditions to  (Couvy et al., 2004;Holtzman et al., 2003;Jung, Mo, & Green, 2009;Jung, Mo, & Chol, 2009;Jung & Karato, 2001;Katayama et al., 2004;Ohuchi et al., 2011;Michibayashi et al., 2016) shown on lower-hemisphere equal-area projections. X is parallel to the lineation; Y is normal to the lineation and parallel to the foliation; Z is normal to the foliation. The arrows and pink lines in the right-hand column represent the slip direction and slip plane, respectively. 6.5-7.0 GPa/990°C. Liu et al. (2002) and Wang et al. (2011) used thermobarometry to constrain the metamorphic P-T conditions of the garnet lherzolites to 3.8-5.1 GPa/880-970°C, and 4.2-6.0 GPa/920-990°C, respectively. The Yinggelisayi garnet lherzolites also experienced HP granulite-and amphibolite-facies retrograde metamorphism (Gai et al., 2022;Liu et al., 2012Liu et al., , 2015.
Zircon U-Pb dating of the garnet lherzolites has yielded peak metamorphic ages of ca. 501 Ma  and ca. 498 Ma (Wang et al., 2011), and for felsic granulite of ca. 493 Ma  and mafic granulite of ca. 500 Ma (Dong et al., 2018). These ages for the Yinggelisayi terrane are consistent with those of HP-UHP rocks from the Jianggalesayi and Danshuiquan areas (Gai et al., 2022;Liu et al., 2012Liu et al., , 2015.

Methods
Samples were cut normal to the foliation and parallel to the lineation, and made into doubly polished thin-sections. The mineral compositions were determined with an electron microprobe (JXA-8230) at the State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, China. The instrument was operated at an accelerating voltage of 15 kV, a beam current of 10 nA, a beam diameter of 1 μm, and with 20 s count times. The natural mineral and synthetic standards were supplied by SPI Company. Different standard minerals are used to correct for different elements: diopside-Ca, olivine-Mg, ilmenite-Fe, quartz-Si, plagioclase-Al, rutile-Ti. The trace element compositions of the olivines were determined by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) at the State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, China. A laser beam diameter of 43μm and 10 Hz repetition rate were used for the analyses. The data were processed with ICPMSDataCal software using 29 Si as the internal standard (Liu, 2011). The crystallographic preferred orientations (CPO) of olivine was measured by electron backscatter diffraction (EBSD) using a scanning electron microscope (SEM JEOL 6380) at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China. The sample surface was inclined at 70° to the incident beam and the EBSD patterns were collected on a phosphor screen. The experimental conditions were as follows: accelerating voltage = 15 kV; working distance ∼20 mm; spot size = 5.0. The EBSD data were processed using HKL Channel 5 software including noise reduction, wild spikes removed and made lower hemisphere projection pole figures contoured as one-point-per-grain.The J-index was calculated to estimate the fabric strength of the samples using uncorrelated grain pairs analyzed from the EBSD data (Bunge, 1982;Skemer et al., 2005). The olivine and enstatite water contents were measured by Fourier transform infrared (FTIR; Nicolet 6700) spectroscopy at the China University of Geosciences, Wuhan, China. Unpolarized transmitted light was used to collect the FTIR spectra. Prior to analysis, the samples were made into doubly polished sections with a thickness of 150-250μm, and heated at 120°C for 24 hr to eliminate water on the surface of the sections. The water contents were calculated using the calibration method described by Bell et al. (2003) and Hans & Joseph. (2006). Seismic velocity and anisotropy were calculated from the EBSD orientation data using the elastic constants for olivine (Abramson et al., 1997), enstatite (Chai et al., 1997), diopside (Collins & Brown., 1998), garnet (Bass, 1989), and ANISch5, VpG software programs by Mainprice (1990).

Metamorphic Stages and P-T Conditions
Based on the petrography and mineral chemistry of the garnet lherzolites, four mineral assemblages were identified.

Olivine Fabrics of Garnet Lherzolite
We have made fabric analysis on 5 garnet lherzolite samples from Yinggelisayi, three samples of which have strong serpentinization, so that the olivine grains are not enough to form an ideal fabric. Therefore, we only present two samples analysis results. The garnet lherzolites contain relict protolith minerals and three metamorphic mineral assemblages. The relict protolith olivine grains are mostly rounded and undeformed, and were insufficient to obtain an ideal olivine fabric. No recrystallized olivine associated with the retrograde amphibolite-facies metamorphism (M 3 ) was observed. Thus the olivine fabrics in these two stages were not analyzed. The olivine fabrics of the coarse-grained porphyroblasts associated with M 1 and fine-grained olivine associated with M 2 (Figures 5a and 5b) were analyzed in detail. We examined the coarse-grained porphyroblasts (M 1 ) with area of olivine grains >40,000 μm 2 and create a new subset in Channel 5 program. We examined an area of fine-grained olivine surrounding the garnet, clinopyroxene, and orthopyroxene porphyroblasts of 10,000-30,000 μm 2 and extracted the olivine size in this range in Channel 5 program for the fine-grained olivine (M 2 ) to fabric analysis. The results were that the coarse-grained olivine porphyroblasts exhibited as [010] axes sub-normal to the foliation and [001] axes subparallel to the lineation, which is characteristic of the B-type fabric; and the fine-grained olivine showed as [010] axes sub-normal to the foliation and [100] axes subparallel to the lineation, which is characteristic of the A-type fabric (Figure 9).

Olivine Water Contents
Most nominally anhydrous minerals (e.g., olivine and orthopyroxene) contain a small amount of lattice-bound water more or less, which has a significant role in the deformation of such minerals (Jung & Karato, 2001;Wang, 2010). As such, we measured the olivine water contents by FTIR to assess the effects of water on the olivine fabrics in the garnet lherzolites.
FTIR absorbance spectra of olivine from different stages (M 1 and M 2 ) are shown in Figures 10a and 10b. The wavenumbers 3,400-3,800 cm −1 are shown because this region is dominated by the stretching vibrations of O-H bonds (Paterson, 1982). The dominant absorption peaks are at 3,639, 3,635, 3,624, 3,616, 3,608, 3,574, and 3,555 cm −1 , and represent structurally bound water in olivine (Bell et al., 2003;Hans & Joseph, 2006). The water contents of the coarse-grained (M 1 ) and fine-grained (M 2 ) olivine were calculated using the method of Bell et al. (2003), which yielded values of 183-213 and 93-139 ppm H/Si, respectively. The uncertainty in the FTIR analysis is ∼20%. The coarse-and fine-grained olivine mostly have lower water contents (<200 ppm H/Si, Jung, 2017), and a few coarse-grained olivine grains have higher water contents (∼278 ppm H/Si), perhaps due to later serpentinization or the presence of amphibole inclusions. The FTIR peaks at 3,699, 3,688, and 3,683 cm −1 and 3,677, 3,663, and 3,662 cm −1 can be attributed to serpentine (Jung, Mo, & Chol, 2009;Wang et al., 2007) and amphibole (Hans & Joseph, 2006;Wang et al., 2007), respectively. Jung et al. (2013) suggested that the water contents measured in olivine cannot represent the actual water content, because olivine has a high rate of H diffusion (Kohlstedt & Mackwell, 1998), and water can be readily lost or added. Therefore, we also calculated the water contents of the orthopyroxene (Figures 10c and 10d), because orthopyroxene has a lower rate of H diffusion than olivine (Mackwell & Kohlstedt, 1990), and may more accurately record the water content of the garnet lherzolites. The coarse-and fine-grained orthopyroxene grains have water contents of 21-143 ppm H/Si, which also indicate that the garnet lherzolite cystallized under low water condition at M 1 and M 2 stages.

Metamorphic Evolution
Based on the petrography and estimated P-T conditions, residual protolith minerals (i.e., inclusions) and three metamorphic stages were recognized in the Yinggelisayi garnet lherzolites. Previous studies of the metamorphism of the garnet peridotites, garnet pyroxenites, and garnetites have identified peak pressures of up to 4-7 GPa, indicating that the Yinggelisayi terrane was subducted to a depth of ∼200 km (Dong et al., 2018(Dong et al., , 2020Liu et al., 2002Liu et al., , 2005. However, the pressures obtained from the Grt-Opx thermobarometer for the garnet lherzolites vary in different studies. Liu et al. (2002) and Wang et al. (2011) obtained pressures of 4-6 GPa, and Zhang et al. (2005) and Li et al. (2013) obtained pressures of 1.7-2.7 GPa. In the present study, we obtained pressures of 2.52-3.08 GPa. The pressures obtained by Grt-Opx thermobarometry exhibit a negative correlation with the Al 2 O 3 contents of orthopyroxene (Li et al., 2013;Wu & Zhao, 2011). The pressures of 4.2-6.0 GPa estimated for the garnet lherzolites by Wang et al. (2011) were based on Grt-Opx barometry, where the Al 2 O 3 contents of the orthopyroxene were 0.30-0.66 wt.%. In our samples, the Al 2 O 3 contents of the orthopyroxene porphyroblasts are all >2.71 wt.%, much higher than those reported by Wang et al. (2011). This may be due to differences in the sampling locations, or because our samples did not record the peak pressure, given their rapid exhumation. Therefore, the coarse-grained minerals in our samples may represent the near-peak metamorphic stage. The peak metamorphism occurred at ca. 500 Ma (Dong et al., 2018;Wang et al., 2011;Zhang et al., 2005). Subsequently, Figure 9. Lower-hemisphere equal-area projections of poles to olivine. XZ is the sample section, X is the lineation, and Z is normal to the foliation. N is the number of analyzed grains. The color coding represents the data density. A half width of 20° was used to construct the figures.
the garnet lherzolites underwent HP retrograde granulite-facies metamorphism. During this stage, the garnet and clinopyroxene retrograded into Cpx 3 +Opx 3 +Amp 2 symplectites with a vermicular texture around the coarse-grained garnet, and fine-grained olivine, garnet, clinopyroxene, and orthopyroxene recrystallized around the coarse-grained porphyroblasts (e.g., garnet and clinopyroxene). The age of this retrograde metamorphic stage is ca. 480 Ma (Dong et al., 2018;Gai et al., 2022;Zhang et al., 2005). The steep P-T path, near-isothermal decompression (Figure 11), and ∼20 Myr elapsed indicate that the garnet lherzolites underwent rapid exhumation during the near-peak to HP granulite-facies metamorphism. Finally, the garnet lherzolites underwent amphibolite-facies metamorphism and Amp 3 crystallized around the fine-grained amphibole (Amp 2 ). There are several stages of granitic intrusions in the South Altyn complex, and their ages (426-385 Ma) are interpreted to be the age of the retrograde amphibolite-facies metamorphism (Liu et al., 2015).

Olivine Fabrics in the Yinggelisayi Garnet Lherzolites During Deep Subduction and Exhumation
Experimental studies have shown that A-type olivine fabric transform to B-type fabric when the pressure increases (Couvy et al., 2004;Ohuchi et al., 2011). Jung, Mo, and Green (2009) showed that A-type olivine fabric changes to B-type fabric in the pressure range of 2.5-3.1 GPa. These experimental results suggest that the olivine fabrics varies with pressure. Although, the A-, B-, C-type olivine fabrics have been observed by Q.  and Sun et al. (2019) in the same HP-UHP terrane or belt, those different fabrics are from different rocks, such as dunite, lherzolite, which correspond to regional different metamorphic stages, respectively. However, there is no directly evidence in a natural rock to reflect the olivine fabric variation at different metamorphic stages during the deep subduction and exhumation. Figure 11. Summary of published P-T paths for the Yinggelisayi area. The metamorphic facies and conditions are from Spear (1993). The Ol-out line is based on phase equilibria modeling of garnet lherzolite using THERMOCALC (Yang & Powell, 2008).
In the studied samples, the coarse-grained olivine porphyroblasts developed B-type fabric and the fine-grained olivine exhibited A-type fabric. The B-type olivine fabric developed in HP-UHP metamorphic rocks from the Cima di Gagnone in the Central Alps of Switzerland (Skemer et al., 2006), Otroy in western Norway (Q. , and Songshugou in the Qinling orogen of China (Sun et al., 2019), as well as in some HP-UHP experimental studies (Jung, Mo, & Green, 2009;Ohuchi et al., 2011). Our samples are also HP-UHP metamorphic rocks, which experienced conditions suitable for formation of the B-type olivine fabric. Moreover, the pressure estimated for the near-peak metamorphism (2.52-3.08 GPa) is consistent with the experimental results for the development of the B-type olivine fabric (2.5-3.1 GPa) obtained by Jung, Mo, and Green (2009). Although water can also promote the formation of the B-type olivine fabric Mizukami et al., 2004;Sun et al., 2019), the lower water contents of this studied olivine grains indicate that water did not have an important role in the formation of the B-type olivine fabric. The differential stress of coarse-grained olivine was estimated to be range of 17-49MPa (σ = 40.2*d −0.81 , Mercier et al., 1977), 23-65MPa (σ = 48*d −0.79 , Ross et al., 1980). This result is far less than the differential stress (>300MPa) required for the B-type olivine fabric (Bernard et al., 2019;Jung, 2017;Jung & Karato, 2001). Therefore, excluding the influence of the water content and stress, the pressure was probably the main factor that caused the formation of the B-type olivine fabric. The A-type olivine fabric is generally formed in an extensional tectonic setting (Jung, Mo, & Chol, 2009), but also occurs occasionally with B-and/or C-type fabrics in HP-UHP terranes. Previous studies have suggested that the pressure might prevent the formation of the A-type olivine fabric (Durinck et al., 2005;Jung, Mo, & Green, 2009;Wang et al., 2007). As such, the A-type olivine fabrics observed in HP-UHP terranes have mostly been interpreted as residual fabrics that existed prior to subduction (Sun et al., 2019;Q. Wang et al., 2013). However, the A-type fabric of the fine-grained olivine in the present study is obviously not a relict fabric, but formed during exhumation of the retrograde metamorphic stage. Therefore, we conclude that the coarse-grained olivine porphyroblasts developed the B-type olivine fabric because of the HP-UHP and low-water metamorphic conditions, and the fine-grained olivine showed the A-type olivine fabric due to the decompression and also low-water conditions during retrograde metamorphism and exhumation.
Therefore, distinct olivine fabrics form under different deformation and metamorphic conditions during deep continental subduction and exhumation. As such, the different olivine fabrics developed in a sample could reflect the deformation stages and may also be used as a clue to identify the different geological stages or tectonic environment.

AG-Like Olivine Fabric
The AG-type olivine fabric is different from the other olivine fabrics, and forms via least two slip systems: (010) [100] and (010)[001] (Figure 1). AG-type olivine fabric has been reported from many localities in various tectonic settings (Bascou et al., 2008;Ben-Ismail et al., 2001;Michibayashi & Mainprice, 2004;Muramoto et al., 2011;Figure 12. Lower-hemisphere equal-area projections of the olivine fabrics in the garnet lherzolites. XZ is the sample section, X is the lineation, and Z is normal to the foliation. N is the number of analyzed olivine grains. J-index is the strength of the olivine fabric. Tommasi et al., 2008). The AG-type fabric has been explained by the co-existence of an oriented melt (Holtzman et al., 2003;Tommasi et al., 2006), simultaneous activation of [100] and [001] slip directions induced by the presence of water (Jung et al., 2014Tommasi et al., 2000), [010] axial compression (Harigane et al., 2011), and transpression (Qi et al., 2018;Tommasi et al., 1999). However, the coarse-grained (M 1 ) and fine-grained (M 2 ) olivine grains of the present study have [010] axes normal to the foliation and [100] and [001] axes as a girdle parallel to the foliation, similar to the AG-type olivine fabric ( Figure 12). This AG-like fabric developed from a mixture of B-and A-type fabrics that formed at different metamorphic stages with different P-T conditions which is quite different from the real AG-type fabric formed in one deformation stage in previous studies (Holtzman  Jung et al., 2014;Tommasi et al., 2000Tommasi et al., , 2006. Therefore, when encountering the AG-like fabric or the crystalline axes girdled distribution (like D-type fabric) or no clear fabric, one should carefully consider whether it is a mixture of different geological stages, otherwise, maybe neglect many geological processes.

Implication of Seismic Anisotropy
Olivine [001] axes in M 1 are parallel to the lineation and perpendicular to the subduction direction, while the [100] axes in M 2 are parallel to the lineation and perpendicular to the extension. The process of decompression changes the olivine lattice orientation, and also has an obvious effect on the seismic anisotropy.
Seismic anisotropy calculation results showed that the garnet lherzolite (sample AY1-2) has a bulk anisotropy of Ap (P-wave) = 2.5%, As (S-wave) = 2.43%. The Ap and As of olivine alone are higher than the bulk rock, which includes orthopyroxene, clinopyroxene, and garnet in addition to olivine. Ap and As for all olivine, only coarse-grained (M 1 ), and only fine-grained olivine (M 2 ) was calculated as Ap all = 6.4%, Ap coarse = 7.6%, Ap fine = 8.1%, As all = 4.11%, As coarse = 5.54%, As fine = 5.75%, respectively ( Figure 13). The reason why Ap all and As all of olivine both lower than that of coarse-and fine-grained olivine is because of their lattice orientation. There are V 001 (moderate) of coarse-grained olivine and V 100 (max) of fine-grained olivine parallel to lineation (V 001-coarse ∥V 100-fine ∥lineation), and V 100 (max) of coarse-grained olivine and V 001 (moderate) of fine-grained olivine normal to the lineation parallel to the foliation (V 100-coarse ∥V 001-fine ∥foliation and ⊥lineation). Thus the velocity of these two direction is equalization, so the anisotropy decreases. Therefore, the olivine that underwent multiple deformation and metamorphic stages (i.e., HP-UHP metamorphic rocks) will decrease the bulk anisotropy, which has implications for geophysical interpretations of seismic anisotropy.

Conclusions
1. The Yinggelisayi garnet lherzolites from South Altyn contain residual protolith minerals, and near-peak (M 1 ), granulite-facies (M 2 ) retrograde, and amphibolite-facies (M 3 ) retrograde metamorphic mineral assemblages. The estimated P-T conditions of M 1 and M 2 are 2.52-3.08 GPa/1,095-1,136°C, and 1.31-1.80 GPa/851-893°C, respectively. 2. The coarse-grained olivine porphyroblasts crystallized at the near-peak metamorphic stage (M 1 ) during deep subduction of the continental crust and developed the B-type olivine fabric. The fine-grained olivine around the porphyroblasts recrystallized in decompression and retrograde metamorphism (M 2 ) during exhumation, and developed the A-type olivine fabric. 3. Distinct olivine fabrics developed during different stages of metamorphism and deformation, and the study of olivine fabric combining with metamorphic stages makes us better understand the metamorphism and deformation evolution history during deep subduction and exhumation of continental crust.