Role of mantle dynamics and geochemical diversity in chromitite formation: insights from the Luobusa ophiolite, SW Tibet

The Luobusa massif within the Yarlung Zangbo Suture Zone (YZSZ) in SW Tibet hosts distinct chromitite types: low-grade, intermediate-grade, and high-grade (massive) ores, each reflecting unique mantle processes and formation conditions. Variations in Cr# and Mg# values in chromian spinels suggest a dynamic magmatic environment influenced by melt-peridotite interactions, gravitational settling, and metasomatic processes, with boninitic melts characteristic of forearc settings playing a significant role in chromitite formation. Isotopic analyses, including δ²⁶Mg, δ⁵⁶Fe, and PGE alloys, suggest high P-T conditions during chromitite genesis, indicating multiple phases of magmatic differentiation and metasomatism. The Re-depletion (T_RD) ages of 1.0–2.2 Ga for the YZSZ peridotites indicate the presence of ancient subcontinental lithospheric mantle, representing the earliest component in the mantle history of the region. These ancient mantle domains were subsequently modified through interaction with convective mantle sources, likely during the breakup of Gondwana. The chromitites, with an age of ~ 340 Ma, align with subduction-related tectonics and mark an early Paleozoic subduction episode. The peridotites, with formation ages spanning 305–376 Ma, reflect a protracted and non-linear mantle evolution influenced by mantle heterogeneity, episodic melt infiltration, and metasomatic alteration. The transition from MORB- to arc-type basaltic magmatism further suggests subduction initiation and slab rollback during this time. However, the Luobusa peridotites appear to have formed during a distinct, younger tectonic episode, possibly linked to an earlier phase of subduction or mantle plume activity. This is supported by the published Sm–Nd isochron age of 177 ± 31 Ma for Luobusa gabbro, pointing to a Mesozoic magmatic event. Re–Os isotopic data from both YZSZ and Luobusa peridotites reveal interactions between S-saturated basaltic melts and the lithospheric mantle, emphasizing the importance of melt-rock interaction and metasomatism during Neo-Tethyan subduction. Collectively, these observations support a multi-stage tectonic model for the evolution of the Neo-Tethys, involving early subduction, plume-related processes, and the recycling of ancient lithospheric materials.