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.