Fig. 11. Summary of published P–T paths for the Yinggelisayi area. The metamorphic facies and conditions are from Spear (1993). The Ol-out line is based on phase equilibria modeling of garnet lherzolite using THERMOCALC (Yang et al., 2008).
7.2 Olivine fabrics in the Yinggelisayi garnet lherzolites during deep subduction and exhumation
Experimental studies have shown that A-type olivine fabric transform to B-type fabric when the pressure increases (Couvy et al., 2004; Ohuchi et al., 2011). Jung et al. (2009a) showed that A-type olivine fabric changes to B-type fabric in the pressure range of 2.5–3.1 GPa. These experimental results suggest that the olivine fabrics varies with pressure. Although, there A-, B-, C-type olivine fabrics developed in the same HP-UHP terrane or belt (Wang et al., 2013a; Sun et al., 2019), these different fabrics are from different rocks and sampling location. And we discuss the olivine fabrics at different metamorphic stages in a natural rock, which is more directly indicating that the variation of olivine fabrics during the deep subduction and exhumation.
In the studied samples, the coarse-grained olivine porphyroblasts developed B-type fabric and the fine-grained olivine exhibited A-type fabric. The B-type olivine fabric developed in HP-UHP metamorphic rocks from the Cima di Gagnone in the Central Alps of Switzerland (Skemer et al., 2006), Otroy in western Norway (Wang et al., 2013a), and Songshugou in the Qinling orogen of China (Sun et al., 2019), as well as in some HP-UHP experimental studies (Jung et al., 2009a; Ohuchi et al., 2011). Our samples are also HP-UHP metamorphic rocks, which experienced conditions suitable for formation of the B-type olivine fabric. Moreover, the pressure estimated for the near-peak metamorphism (2.52–3.08 GPa) is consistent with the experimental results for the development of the B-type olivine fabric (2.5–3.1 GPa) obtained by Jung et al. (2009a). Although water can also promote the formation of the B-type olivine fabric (Mizukami et al., 2004; Katayama & Karato, 2006; Sun et al., 2019), the lower water contents of this studied olivine grains indicate that water did not have an important role in the formation of the B-type olivine fabric. Therefore, the pressure was probably the main factor that caused the formation of the B-type olivine fabric. The A-type olivine fabric is generally formed in an extensional tectonic setting (Jung et al., 2009b), but also occurs occasionally with B- and/or C-type fabrics in HP-UHP terranes. Previous studies have suggested that the pressure might prevent the formation of the A-type olivine fabric (Durinck et al., 2005; Wang et al., 2007; Jung et al., 2009b). As such, the A-type olivine fabrics observed in HP-UHP terranes have mostly been interpreted as residual fabrics that existed prior to subduction (Wang et al., 2013; Sun et al., 2019). However, the A-type fabric of the fine-grained olivine in the present study is obviously not a relict fabric, but formed during exhumation of the retrograde metamorphic stage. Therefore, we conclude that the coarse-grained olivine porphyroblasts developed the B-type olivine fabric because of the HP-UHP and low-water metamorphic conditions, and the fine-grained olivine showed the A-type olivine fabric due to the pressure decrease during retrograde metamorphism and exhumation.
Therefore, distinct olivine fabrics form under different deformation and metamorphic conditions during deep continental subduction and exhumation. As such, the different olivine fabrics developed in a sample could reflect the deformation stages and may also be used as a clue to identify the different geological stages or tectonic environment.
7.3 AG-like olivine fabric
The AG-type olivine fabric is different from the other olivine fabrics, and forms via least two slip systems: (010)[100] and (010)[001] (Fig. 1). Although the AG-type olivine fabric is relatively uncommon worldwide, it has been reported from many localities in various tectonic settings (Ben et al., 2001; Michibayashi & Mainprice, 2004; Bascou et al., 2008; Tommasi et al., 2008; Muramoto et al., 2011). The AG-type fabric has been explained by the co-existence of an oriented melt (Holtzman et al., 2003; Tommasi, 2006), simultaneous activation of [100] and [001] slip directions (Tommasi et al., 2000), and [010] axial compression (Harigane et al., 2011). However, the coarse-grained (M1) and fine-grained (M2) olivine grains of the present study have [010] axes normal to the foliation and [100] and [001] axes as a girdle parallel to the foliation, similar to the AG-type olivine fabric (Fig. 12). This AG-like fabric developed from a mixture of B- and A-type fabrics that formed at different metamorphic stages which is quite different from the real AG-type fabric proposed by predecessors. Therefore, when encountering the AG-like fabric or the crystalline axes girdled distribution (like D-type fabric) or there are several areas in stereographic projection (or no clear fabric), we should carefully consider whether it is a mixture of different geological stages, otherwise, maybe neglect many geological processes.