Petromagnetic Characteristics of Serpentinization and Magnetite Formation at the Zedang Ophiolite in Southern Tibet

Variably serpentinized peridotites from the Zedang ophiolite in southern Tibet were magnetically and petrologically examined to understand the serpentinization process and evaluate the origin of magnetic anomalies in ultramafic-hosted tectonic settings. Magnetite occurs in the serpentine and brucite veins and is identified as the dominant magnetic carrier by thermomagnetic and petrological analyses. The magnetic susceptibility increases rapidly from <0.001 to ~0.02 SI for the <50% serpentinized samples followed by nearly constant values of 0.02–0.03 SI above 50% serpentinization. This transition corresponds with the formation of Fe-poor serpentine mesh (2–3 wt% FeO) and magnetite in the early stages and the replacement of mesh center olivine by Fe-rich serpentine (4–5 wt% FeO) without magnetite in the late stages. Brucite veins occur in the 50–70% serpentinized samples and indicate serpentinization temperatures from ~250 to <100°C. The serpentinization may initiate at an oceanic spreading ridge center under high temperatures (>250–300°C) to produce magnetite and subsequently continue at lower temperatures (<200–250°C) in near-seafloor settings and limit the magnetite formation, possibly associated with ophiolite emplacement. These serpentinized peridotites have higher magnetization intensities (average 2.26 Am⁻¹) than dolerite dykes and basaltic volcanics (mostly <1 A m⁻¹) in the area and should be the major source of aeromagnetic highs in the south Tibetan ophiolite belt.