(3) The assemblage
Grt2+Ol3+Opx3+Cpx3+Amp2+Spl
(M2) occurs in the fine-grained matrix and surrounds or
is located in fractures in the coarse-grained minerals. Cpx+Opx+Amp
intergrowths are only developed around coarse-grained garnet, and their
compositions are almost the same as those in the fine-grained matrix. As
such, we infer the intergrown and fine-grained matrix minerals represent
the earlier retrograde metamorphic stage. The retrograde metamorphicP–T conditions were estimated to be 1.31–1.80 GPa and
851–893°C by Grt–Opx (Nickel & Green, 1985; Taylor, 1998) and
Cr-in-spinel (O’Neill, 1981) barometry, and Opx–Cpx thermometry
(Taylor, 1998; Brey & Kohler, 1990), which corresponds to the HP
retrograde granulite-facies metamorphism.
(4) The final assemblage comprises Amp3 that
crystallized around the matrix amphibole, which suggests that the garnet
lherzolites experienced late retrograde amphibolite-facies metamorphism
(M3). Olivine was not formed during this stage. Phase
equilibria modeling of garnet lherzolite using THERMOCALC (Yang et al.,
2008) showed that the olivine was not significantly affected by the
pressure, but olivine became absent at 450–500°C. Given this
temperature range, we infer the pressure was <1 GPa based on
the amphibolite facies metamorphic stage of Spear (1993).
5. Olivine fabrics of garnet lherzolite
The Yinggelisayi 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 (M3)
was observed. Thus the olivine fabrics in this two stages were not
analyzed. The olivine fabrics of the coarse-grained porphyroblasts
associated with M1 and fine-grained olivine associated
with M2 (Fig. 5a–b) were analyzed in detail. We
examined the area of olivine grains of >40,000 μm² for the
coarse-grained porphyroblasts (M1) 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 μm² and extracted the olivine size in this range in
Channel 5 program for the fine-grained olivine (M2) 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 (Fig. 9).