Tectonized Neotethyan lithosphere in southeastern Tibet: Results of the Luobusa ophiolite drilling

The Luobusa ophiolite in the southeastern Tibet holds the largest chromitite resource in China. These rocks present an ideal opportunity to study the origin and evolution of ophiolite-related chromitite ores. The Luobusa Ophiolite Scientific Drilling (LSD) program resulted in two boreholes (LSD-1 to a depth of 1478 m with a core recovery of 94% and LSD-2 to a depth of 1854 m with a core recovery of 97%). The profile of both pilot-boreholes includes three main lithological units, i.e., mantle tectonites (harzburgite and lherzolite with lenses of residual dunite), mantle-crust transition (mainly cumulative dunite), and crustal cumulates (i.e., gabbros). The crustal cumulates occur at the bottom of the borehole, whereas the mantle tectonites form the upper part of the section. This stratigraphic reversal reflects a complex tectonic evolution of the Luobusa ophiolite. A large dataset of whole-rock geochemical compositions integrated with microanalytical data of olivine, pyroxene, and Cr-spinel suggests formation of the Luobusa ophiolites by variable degrees of mantle partial melting (5–40%) and evolution from the MOR to SSZ setting. The mantle-normalized PGE patterns of mantle tectonites and transitional zone dunites are distinctive, with different IPGE and PPGE anomalies coupled with variable Pd/Ir ratios, suggestive of complex origins. Similarly, the heterogeneous ¹⁸⁷Os/¹⁸⁸Os ratios of the Luobusa peridotites, with γOs values overlapping those of depleted mantle, primitive mantle, and enriched mantle sources, imply a polygenetic or multi-stage model of formation. Most of the mantle tectonites have ¹⁸⁷Os/¹⁸⁸Os ratios lower than the present-day primitive mantle value, and overlap with the chondritic values. Cumulative dunite, on the contrary, has ¹⁸⁷Os/¹⁸⁸Os ratios close to the primitive mantle value. Negative γOs values calculated for some samples may indicate disturbance of the Re-Os isotope system by cryptic metasomatism and/or serpentinization. The Re-Os isotopic characteristics of Luobusa ophiolites and the olivine and orthopyroxene concentrates are suggestive of a residual phase origin. The higher ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios in the crustal cumulate rocks can be explained as a manifestation of inauguration of melt phases in a slow spreading ridge setting, where interplay of melt extraction and melt-peridotite interaction is typical.