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Garnet amphibolite is one of the common metabasic rocks exposed in collisional orogenic belt, the metamorphic evolution of which is associated closely with orogenic processes. The Yaganbuyang garnet amphibolites occur as blocks hosted by massive granitic gneiss, and consist mainly of hornblende, garnet, clinopyroxene, plagioclase, biotite, quartz with minor rutile/ilmenite and phengitic muscovite. These garnet amphibolites were interpreted to have experienced decompression-dominated evolution that can be divided into three generations (M1, M2, M3), based on the petrographic observations and phase equilibria modeling calculated by THERMOCALC. The assemblage of the first generation (M1) is inferred to possibly be dominated by garnet + omphacite + rutile + phengite + quartz, which is modeled to be roughly stable at P > 25 kbar and T > 800 °C. The second generation (M2) is characterized by the local symplectites of clinopyroxene + plagioclase produced from omphacite, indicating a near-isothermal decompression from ∼23.8 kbar/875 °C to ∼10 kbar/852 °C. The third generation (M3) is marked by the kelyphitic rims of plagioclase + hornblende around garnet and of hornblende + ilmenite around clinopyroxene, involving the late-stage retrogression from ∼9.8 kbar/848 °C to ∼5.8 kbar/645 °C. Zircon U-Pb dating yielded one group metamorphic age of c. 500 Ma that is interpreted to represent the timing of the peak eclogite-facies metamorphism. A combination of petrography observation, phase modeling results and geochronology data suggests that the Yaganbuyang garnet amphibolites have once undergone eclogite-facies metamorphism by continental subduction rather than crustal thickening. Therefore, the Yaganbuyang area is an eastward extension part of the South Altyn HP-UHP metamorphic belt. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: According to the previous research that different olivine fabrics developed under various temprerature, pressure, stress and water content, we try to discuss the transformation of olivine fabric at different metamorphic stages with different P‐T conditions in metamorphic ultramafic rocks. The Yinggelisayi garnet lherzolites from South Altyn, NW China contain residual protolith minerals, and three stages of metamorphic minerals which include the near‐peak (M1), granulite‐facies (M2) retrograde, and amphibolite‐facies (M3) retrograde metamorphic minerals base on the petrography and mineral composition. And then, we analyzed olivine fabrics in the near‐peak (M1) and early retrograde metamorphic stages (M2). Results showed that the olivine in M1 during deep subduction of the continental crust developed the B‐type olivine fabric and the olivine in M2 during exhumation developed the A‐type olivine fabric. Key Points: Yinggelisayi garnet lherzolites contain residual protolith inclusions and three stages of metamorphic mineralsOlivine in the garnet lherzolites exhibited different fabrics during deep subduction and exhumation of the continental crustOlivine fabrics will be dispersed at multi‐stage deformation and metamorphism [ABSTRACT FROM AUTHOR]

A lot of previous experimental studies on ultramafic rocks (SiO2 unsaturated system) (Ringwood and Major, 1971; Irifune et al., 1986; Gasparik, 1989; Ono and Yasuda, 1996) have demonstrated that characteristics of Si‐rich and Al‐deficient in garnet are resulted from coupled substitution of SiVI+MVI=AlVI+AlVI and SiVI+NaVII=AlVI+MVII (M=Mg, Fe, Ca) at ultrahigh pressures (UHP) (>5 GPa). The degree of substitution will be enhanced by increasing pressure which has a positive correlation with the content of SiVI, but a negative correlation with the content of AlVI in supersilic garnet. These experimental results established a theoretical foundation for further understanding the formation mechanism of the exsolution of pyroxene in garnet observed in deep mantle xenoliths and some ultrahigh pressure rocks, and also for estimating the pressure conditions of the formation of supersilic garnet before exsolution (Haggerty and Sautter, 1990; Sautter et al., 1991; van Roermund et al., 1998; Ye et al., 2000). Although some experimental studies on SiO2 saturated system have been reported (Irifune et al., 1994; Ono., 1998; Dobrazhinetskya and Green., 2007; Wu et al., 2009), the stability conditions of supersilic garnet are still lack of unified understanding. Therefore, HP‐HT experiments were carried out on felsic rocks under conditions of 6–12GPa and 1000°C–1400°C. Combined with previous experimental data, we try to figure out the minimum stable pressure and geological significants of supersilic garnet in SiO2 saturated system. Our experimental results from SiO2 saturated system show the minimum stable pressure of supersilic garnet should be ≥10 GP of stishovite stability field. These results are similar as that from experiments using starting composition similar to average upper continental crust reported by Irifune et al (1994) who yielded that garnet gradually became supersilic and Al‐deficient as pressures increased above 10 GPa, especially in a pressure interval between 13 and 18 GPa. Moreover, experiments with different starting materials (Ono, 1998; Dobrazhinetskya and Green, 2007; Wu et al. 2009) also indicate the stable pressure condition of supersilic garnet is mainly ≥9 –10 GPa in SiO2 saturated system if data of small‐size grains at low temperature are ignored due to measuring errors. Thus, it can be concluded that the minimum stable pressure of supersilic garnet in SiO2 saturated system is distinctly different from that in SiO2 unsaturated ultramafic rock system. The minimum pressure of the former is ≥9–10 GPa of stishovite stability field, while that of the latter is >5 GPa. Therefore, whether independent SiO2 phase exist or rock system is SiO2 saturated must be taken into considered when estimating the peak pressure of exsolutions in supersilic garnet in UHP rocks. Furthermore, pressure of >5 GPa directly estimated by supersilic garnet based on conclusion from SiO2 unsaturation system rather than SiO2 saturation in previous sdudies may have been underestimated and need to be re‐estimated. Supersilic garnets have been recognized by interior exsolutions of clinopyroxene in garnet pyroxene from Yinggelisayi South Altyn (Liu et al., 2005), and exsolutions of rodlike quartz+rutile in felsic gneiss from Songshugou North Qinling (Liu et al., 2003). According to the experimental results from SiO2 unsaturated system, the peak metamorphic pressure of the both SiO2 saturated rocks have been estimated to be >7Gpa and >5Gpa, respectively. However, combined with the new experimental results above, we re‐estimated that the peak metamorphic pressure of these SiO2 saturated rocks should be≥9 –10 GPa at least, implying an ultra‐deep subduction to mantle depth of stishovite stability field. This research, together with previous findings (Liu et al., 2007, 2018), shows that continental subduction to mantle depth (300km) of stishovite stability field and then exhumation to the surface is obviously more common than previously thought, and the rock types are also diverse. At the same time, it provides a new indicator and thought for recognizing the subduction to the mantle depth of stishovite stability field in UHP metamorphic belt. [ABSTRACT FROM AUTHOR]

[Display omitted] • Clockwise P–T evolution is recorded by the South Altyn Tagh felsic gneiss. • The felsic gneiss was exhumed from eclogite-facies to granulite-facies within 20 Myr. • HP–HT eclogite-facies records were preserved in the weakly deformed felsic gneiss. • The HP mineralogical records were further eliminated by the strong deformation. Felsic gneisses are usually the dominant rock types in most high-pressure (HP) and ultrahigh-pressure (UHP) terranes, although their peak metamorphic conditions and complete P–T paths are rarely known. This lack of knowledge is attributed to the high variance of mineral assemblages at HP/UHP conditions of the felsic gneisses, or to the effects of retrograde-related overprinting. Thus, it is difficult to distinguish whether felsic gneisses underwent HP/UHP metamorphism. In this paper, we focus on the felsic gneiss that has been previously documented as having formed under HP granulite-facies metamorphism with peak P–T of > 1.15 GPa and > 860 °C from the Danshuiquan locality in the South Altyn HP – UHP belt. The Danshuiquan felsic gneiss is strongly deformed and is spatially associated with weakly deformed garnet-rich felsic gneiss. On the basis of detailed microstructure observations on mineral assemblages and phase equilibria modelling, three stages of metamorphic evolution for the weakly deformed garnet-rich felsic gneiss are identified. The first stage is represented by garnet cores, revealing a prograde P–T path from 1.5 GPa/750 °C to 2.1 GPa/900 °C. The second stage is characterized by garnet mantles with multiphase inclusions, defining HP eclogite-facies metamorphic conditions of 2.2–2.6 GPa/950–1100 °C. The third stage is represented by the composition of garnet rims, biotite, and plagioclase, suggesting an HP granulite to amphibolite facies decompression cooling process to 1.1–1.3 GPa/710–770 °C. In contrast, the strongly deformed felsic gneiss just records a deformation-related decompression process from 1.1 to 1.2 GPa/750–760 °C to 0.87–0.98 GPa/760–770 °C, and finally to 0.83 GPa/740 °C. Zircon U–Pb dating yields c. 920 Ma protolith ages, c. 500 Ma eclogite-facies ages, and c. 480 Ma granulite to amphibolite facies retrograde ages for the Danshuiquan felsic gneisses. Our results show that the Danshuiquan felsic gneisses underwent a similar metamorphic evolution process to that of Yinggelisayi HP–UHP rocks in the eastern segment of the South Altyn Tagh. Most HP mineralogical records tend to be eliminated by the strong deformation at crustal depths (∼30 km), meaning that the peak metamorphic conditions of the studied felsic rocks in UHP metamorphic terranes are previously underestimated. [ABSTRACT FROM AUTHOR]