Fig. 10. Representative FTIR spectra of olivine and orthopyroxene. (a) Coarse-grained olivine with inclusions (sample AY1-2). (b) Fine-grained olivine without inclusions or serpentinization (sample AY1-2). (c) Coarse-grained orthopyroxene (sample AY1-2). (d) Fine-grained orthopyroxene (sample AY16-2).
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 (Fig. 10c–d), 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 M1 and M2 stages.
7. Discussion
7.1 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 (Liu et al., 2002, 2005; Dong et al., 2018, 2019, 2020). 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 Al2O3contents of orthopyroxene (Wu & Zhao, 2011; Li et al., 2018). 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 Al2O3 contents of the orthopyroxene were 0.30–0.66 wt.%. In our samples, the Al2O3 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 (Zhang et al., 2005; Wang et al., 2011; Dong et al., 2018). Subsequently, the garnet lherzolites underwent HP retrograde granulite-facies metamorphism. During this stage, the garnet and clinopyroxene retrograded into Cpx3+Opx3+Amp2symplectites 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 isca . 480 Ma (Zhang et al., 2005; Dong et al., 2018; Gai et al., 2022). The steep P–T path, near-isothermal decompression (Fig. 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 Amp3 crystallized around the fine-grained amphibole (Amp2). 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).