Fabrics and water contents of peridotites in the Neotethyan Luobusa ophiolite, southern Tibet: implications for mantle recycling in supra-subduction zones

Meng Yu,Qin Wang,Jingsui Yang

doi:10.1144/jgs2018-152

Abstract

The preservation of ultra-high-pressure and super-reducing phases in the Neotethyan Luobusa ophiolite in Tibet suggests their deep origin near the mantle transition zone. Dunite and harzburgite core samples from the Luobusa Scientific Drilling Project show supra-subduction zone geochemical signatures and equilibration temperatures of c. 950–1080°C. Olivine shows A-, B-, C- and E-type fabrics, and combinations of A- and E-type or B- and E-type fabrics. Transmission electron microscopy observations show straight dislocations and the activation of multiple slip systems [100](010), [001](010), [001](100) and [100](001) in olivine. The mean water content in olivine, orthopyroxene (Opx) and clinopyroxene (Cpx) from 24 peridotite samples was 16 ± 5, 90 ± 21 and 492 ± 64 ppm, respectively, which is different from the water content of hydrated peridotites above the mantle wedge. The trace element compositions of Cpx exclude significant metasomatism after melt extraction. The high hydrogen partition coefficient between Cpx and Opx (DHCpx/Opx = 5.56 ± 0.96) implies equilibrium at high pressures and rapid exhumation. Based on deformation experiments, the B- and C-type fabrics could be formed in a subduction zone at depths >200 km, whereas the A- and E-type fabrics were produced in the shallow mantle. In a process triggered by slab rollback, the Luobusa peridotites may have been rapidly exhumed within a subduction channel and mixed with the lithospheric mantle of the forearc. Supplementary material: Major oxide contents in Opx, Cpx and spinel, trace element concentration in Cpx, micrographs and TEM images of peridotite are available at https://doi.org/10.6084/m9.figshare.c.4307828 Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

Highlights

MethodsThe major element compositions of olivine, Opx, Cpx and spinel from 23 peridotite samples were determined using a Shimadzu
Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www. lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways
Consistent with previous studies on the Luobusa ophiolite (Zhou et al 1996, 2005; Dilek & Furnes 2011, 2014; Xiong et al 2015), the studied harzburgites are abyssal peridotites subjected to 10–20% partial melting, whereas the more depleted dunites are supra-subduction zones (SSZ) peridotites that have been modified by melt–rock interactions in a mantle wedge (Fig. 5)

Summary

Methods

The major element compositions of olivine, Opx, Cpx and spinel from 23 peridotite samples were determined using a Shimadzu. After rotation of the EBSD data, Opx from eight harzburgite samples and one dunite sample show the concentration of grains with the [001] axis sub-parallel to the lineation and the (100) plane sub-parallel to the foliation (Fig. 7). This fabric pattern agrees with the typical LPO of Opx in peridotites The very high absorptions between 3720 and 3750 cm−1 occur in some olivine grains (Fig. 10a) and may be caused by nonhydrogen-bonded OH, such as inclusions of a sheet silicate (Bell et al 2003) The contribution of these extrinsic hydrogens was excluded in the baseline correction used to obtain the water content of olivine and pyroxene. If we exclude the extremely water-rich Cpx with 1273 ppm in harzburgite sample B153, the mean water content of Cpx is 492 ± 64 ppm

Discussion

Conclusions