Tracking multiple stages of serpentinization processes of the Yarlung Zangbo Suture Zone peridotites in southern Tibet: Implications for the tectonic evolution of the Neotethyan oceanic lithosphere

Abstract

Serpentinites are key repositories of fluid-mobile elements (FMEs) in subduction zones and record significant information about the origin and geodynamic evolution of oceanic lithosphere. Here, we report on the structural textures and mineralogical compositions of different types of serpentinites collected from the central segment of the Yarlung Zangbo Suture Zone in southern Tibet and present their bulk-rock and mineral chemistry, and Sr isotopic compositions. The main textures include massive, scaly, and gouge serpentinites exposed in the Ngamring and Sangsang ophiolites. Bulk-rock Al2O3/SiO2 and spinel Cr# values suggest that the Ngamring serpentinites originally formed in an abyssal setting, whereas the Sangsang serpentinites developed initially in a forearc mantle. Both serpentinite assemblages were subsequently incorporated into a subduction plate interface as subducted serpentinites. Massive serpentinites preserve the geochemical fingerprint of original serpentinized fluids in mid-oceanic ridge to forearc settings, whereas sheared serpentinites with scaly and gouge textures are reset in their Sr isotopic compositions and FME ratios (i.e., Cs/U, Li/U, and Rb/U) due to their reactions with slab-derived fluids. Scaly and gouge types of the Ngamring serpentinites have lower 87Sr/86Sr values (87Sr/86Sr = 0.7081−0.7082) and higher alkali element−U ratios (i.e., Cs/U, Li/U, and Rb/U) than those of the massive serpentinite types (87Sr/86Sr = 0.7091−0.7096), which indicates that they interacted with fluids at a slab interface after their initial seafloor serpentinization. In contrast, the massive Sangsang serpentinites display lower 87Sr/86Sr values (87Sr/86Sr = 0.7041−0.7043, similar to those of the Yarlung Zangbo ophiolites) and higher alkali element−U ratios than those of the sheared serpentinites (87Sr/86Sr = 0.7063−0.7087). These findings point to the significant role of the increased influx of subducted sediment-derived fluids within subduction shear zones in further affecting the serpentinization fingerprint. This study demonstrates that serpentinites with different textural, geochemical, and isotopic features within the same suture zone may represent the serpentinization products in different tectonic environments during the seafloor spreading, subduction initiation, and subduction zone evolution of oceanic lithosphere.