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).