The origin of the Dazhuka ophiolitic peridotites in the Yarlung Zangbo Suture, South Tibet

Abstract:

Abstract:

The Dazhuka ophiolite in the central Yarlung Zangbo Suture (South Tibet) is one of the best exposures of Neo-Tethyan oceanic lithosphere. The ophiolite includes a well-preserved section from mantle peridotite, lower crustal gabbro and dolerite, to upper crustal basaltic rocks. The crust is remarkably thin (~1-2 km) relative to the thick mantle. Although its crustal origin and evolution have been well addressed, the mantle origin is still poorly understood. In this study, we have carefully examined the harzburgties, the main body from the uppermost mantle of the Dazhuka ophiolite, and analyzed their whole-rock and mineral major- and trace-element compositions to constrain the mantle origin.

The harzburgites contain rare clinopyroxene (<3%), and show a porphyroblastic texture with plastic deformation. Small (<200 μm) grains of clinopyroxene are enclosed by porphyroblastic (~2-3 mm) orthopyroxene or occur within the matrix, sometimes inter-grown with spinel and/or amphibole. The whole-rock compositions of the harzburgites show low Al₂O₃ (0.57-1.42 wt%) ，CaO (0.14-1.43 wt%), TiO₂ (~0.01%), and high MgO (42.75-44.91 wt%), Cr₂O₃(0.31-0.51), NiO (0.28-0.30), as well as Mg# (Mg²⁺/(Mg²⁺+Fe²⁺)) (0.90-0.91). The olivine Mg# (0.90-0.92), spinel Cr# (Cr³⁺/(Cr³⁺+Al³⁺)) (0.26-0.67), orthopyroxene Mg# (0.90-0.92), Cr# (0.06-0.26), and clinopyroxene Mg# (0.92-0.95), Cr# (0.11-0.29) suggest that the Dazhuka harzburgites have experienced 10–19% degrees of partial melting, forming a moderately refractory oceanic lithospheric mantle. Most clinopyroxenes in the harzburgites have depleted LREE patterns, consistent with a moderately refractory mantle. Some clinopyroxenes display LREE-enriched patterns, with elevated Pb, Sr and Zr contents, implying modal metasomatism by addition of hydrous mafic melts. Combined with evidence from previous studies (Girardeau et al. 1985a, b; Liu et al. 2019; Xia et al.2003), we proposed that the Dazhuka ophiolitic mantle was moderately melting residues generated in a slow- or ultraslow-spreading mid-ocean ridge. The rige was rapidly transformed as an infancy subduction zone during the ophiolite obduction in early Cretaceous (~130-120 Ma).