Evolution of olivine fabrics during deep subduction and exhumation of continental crust: Insights from the Yinggelisayi garnet lherzolite, South Altyn, NW China
Guojian Geng1, Liang Liu1*, Haijun Xu2, Wenqiang Yang1, Chao Wang1, Yongsheng Gai1, Tuo Ma1, Xin Li1, Xiaoying Liao1, Tong Li1
1State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an, 710069. China;
2School of Earth Sciences, China University of Geosciences, Wuhan, 430074, China.
Corresponding author: L. Liu (liuliang@nwu.edu.cn)
Key Points:
Olivine in the garnet lherzolites exhibited different fabrics during deep subduction and exhumation of the continental crust.
Olivine fabrics will be dispersed at multi-stage deformation and metamorphism.
Abstract
The different olivine fabrics in ultramafic rocks have been widely used to discuss past tectonic settings, given that the olivine fabrics vary with pressure, temperature and water content. However, there are no researches that whether and how the olivine fabrics transform at different metamorphic stages in a natural rock during the process of deep subduction and exhumation. Yinggelisayi garnet lherzolites from South Altyn have experienced deep continental subduction and exhumation. The garnet lherzolites contain well-preserved residual protolith minerals, and near-peak (M1), granulite-facies retrograde (M2), and amphibolite-facies retrograde (M3) metamorphic mineral assemblages. Olivine grains in M1 formed at P–T conditions of 2.52–3.08 GPa, 1095–1136°C and low water contents (183–213 ppm H/Si), and showed [010] axes sub-normal to the foliation and [001] axes subparallel to the lineation, which is characteristic of B-type fabric ((010)[001]). Olivine grains in M2 formed atP–T conditions of 1.31–1.80 GPa, 851–893°C and also low water contents (93–139 ppm H/Si), and exhibited [010] axes sub-normal to the foliation and [100] axes subparallel to the lineation, which is characteristic of A-type fabric ((010)[100]). These observations suggest that olivine fabrics in HP-UHP metamorphosed ultramafic rocks are different in the near-peak and retrograde metamorphic stages, and also that the olivine fabrics can be transformed during deep continental subduction and exhumation. Therefore, the dispersed or no clear olivine fabric probably caused by multi-stage deformation and metamorphism, and the distinct olivine fabrics can also be used as a clue to identify geological processes and better understand metamorphism and deformation during subduction and exhumation.
Plain Language Summary
The Yinggelisayi garnet lherzolites from South Altyn, NW China contain residual protolith minerals, and near-peak (M1), granulite-facies (M2) retrograde, and amphibolite-facies (M3) retrograde metamorphic stages. The olivine porphyroblasts at the near-peak metamorphic stage (M1) during deep subduction of the continental crust developed the B-type olivine fabric and the fine-grained olivine at retrograde metamorphism (M2) during exhumation developed the A-type olivine fabric. The distinct olivine fabrics are likely as a clue to indentify the olivine formed at different metamorphic stages.
1 Introduction
Olivine fabrics are described in terms of their relationship to the crystallographic framework (x, y, and z axes), and have an important role in controlling deformational micro-structures (Michibayashi et al., 2016). Based on natural and experimental studies, olivine fabrics are commonly categorized into A-, B-, C-, D-, E-, and AG-type, and have dominant slip systems of (010)[100], (010)[001], (100)[001], {0kl}[100], (001)[100], and (010)[h0l], respectively (Fig. 1; Jung and Karato, 2001; Holtzman et al., 2003; Couvy et al., 2004; Katayama et al., 2004, 2008; Jung et al., 2009a, 2009b, 2013; Ohuchi et al., 2011). Each type of olivine fabric has different formation conditions and geological settings (Xu et al., 2005, 2006; Katayama and Karato, 2006; Jung et al., 2009b; Harigane et al., 2011; Wang et al., 2013a, 2013b; Park and Jung, 2015; Michibayashi et al., 2016). The HP-UHP metamorphosed ultramafic rocks typically have B- and/or C-type olivine fabrics, including samples from the Cima di Gagnone in the Central Alps of Switzerland (Frese et al., 2003; Skemer et al., 2006), Otroy in western Norway (Wang et al., 2013a), northern Qaidam in northwest China (Jung et al., 2013), Zhimafang (Xu et al., 2005, 2006) and Xugou (Wang et al., 2013b) in the Sulu UHP terrane in China, and Songshugou in the Qinling orogen in China (Sun et al., 2019). However, olivine is a common mineral in metamorphosed ultramafic rocks of various stages, and there is seldom of investigation in a natural rock that whether and how the olivine fabrics change at different metamorphic stages during deep subduction and exhumation of continental crust. The Yinggelisayi garnet lherzolites from the South Altyn HP-UHP metamorphic belt have experienced complex metamorphic processes and contain multiple mineral assemblages (Liu et al., 2002; Zhang et al., 2005, 2014; Wang et al., 2011), which are ideal for investigating the evolution of olivine fabrics during deep continental subduction and exhumation. We divide garnet lherzolite into the residual protolith minerals and three metamorphic stages by petrography and geochemistry firstly, and focus on the olivine fabrics at the near-peak and early retrograde metamorphic stages and discuss its geological implication.