Fig. 8. Plots of MgO versus (a) FeO and (b) Al2O3 for orthopyroxene.
The amphibole comprises three types. The first type (Amp1) occurs as inclusions in coarse-grained garnet (Fig. 4d), and has MgO = 17.05 wt.%, FeO = 8.24 wt.%, and CaO = 9.86 wt.%. The second type (Amp2) is mostly fine-grained amphibole (Fig. 4h–i, k, and m) and symplectites with clinopyroxene and orthopyroxene that surround the garnet porphyroblasts. Amp2 has MgO = 14.34–16.76 wt.%, FeO = 6.17–9.22 wt.%, and CaO = 11.18–12.22 wt.%. The third type (Amp3) surrounds Amp2 (Fig. 4m) and was formed late.
Spinel is only present in matrix, in association with fine-grained olivine, garnet, clinopyroxene, and orthopyroxene (Fig. 4l). The spinel has Al2O3 = 60.03–62.30 wt.%, MgO = 14.04–15.65 wt.%, FeO = 18.50–20.67 wt.%, and Cr2O3 = 0.54–0.94 wt.%.
4.2 Metamorphic stages and P-T conditions
Based on the petrography and mineral chemistry of the garnet lherzolites, four mineral assemblages were identified.
(1) The assemblage Ol1+Opx1+Cpx1+Amp1is preserved as inclusions in coarse-grained garnet or clinopyroxene porphyroblasts. These minerals probably represent relicts of the protolith. Given that the protolith was a low-pressure ultramafic–mafic igneous rock, the pressure should be <0.8 GPa (Liu et al., 2002). The temperature estimated by Opx–Cpx thermometry (Taylor, 1998) was 1226–1306°C at P = 0.8 GPa.
(2) The assemblage Grt1+Ol2+Opx2+Cpx2(M1) occurs as coarse-grained porphyroblasts that are in equilibrium with each other. We estimated the P–T conditions by Grt–Cpx barometry (Simakov & Taylor, 2000), Grt–Opx barometry (Nickel & Green, 1985; Taylor, 1998), and Opx–Cpx thermometry (Taylor, 1998; Brey & Kohler, 1990), which yielded P = 2.52–3.08 GPa and T = 1095–1136°C (Table 1).
Table 1. Pressure–temperature estimates for representative samples of garnet lherzolite.