Evolution of mantle peridotites from the Luobusa ophiolite in the Tibetan Plateau: Sr-Nd-Hf-Os isotope constraints

Abstract

The Luobusa ophiolite cropping out in the Yarlung-Tsangpo Suture (YTS) is well known for the recovery of super-reducing and ultra-high pressure mineral assemblages. Formation of the Luobusa ophiolite has been linked to the supra-subduction zone (SSZ) settings. This study presents the comprehensive geochemical data of refractory harzburgites, the majority of the Luobusa ophiolite. Both whole rock major elements and spinel Cr# values indicate that the Luobusa harzburgites have experienced 10–20% degrees of fractional melting from a depleted MORB mantle (DMM) source. Clinopyroxene trace elements suggest that they have experienced variable degrees (as high as 8%) of garnet-facies melting in a dry condition. On the other hand, clinopyroxenes in the Luobusa harzburgites have higher contents of light rare earth elements (LREE) than the values in mantle residues after the degrees of partial melting corresponding to their spinel Cr#. Their clinopyroxenes also show enrichment in some LILE, but no anomalies in HFSE, suggesting they were not metasomatized by subduction-related agents. The Luobusa harzburgites show consistent highly siderophile element (HSE) patterns distinctly different from the forearc peridotites. Whole rock ReOs isotopes yield Os model ages of 0.61–1.94 Ga that are much older than the formation age of the Luobusa ophiolite (i.e., 128–134 Ma), indicating that they were stemmed from ancient mantle domains within the asthenosphere. Our results argue against that the Luobusa ophiolite was originated from a supra-subduction zone (SSZ) setting. The Luobusa peridotites might have experienced evolutionary histories decoupled from their hosting chromitites.