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.
Bibliographical noteFunding Information:
This study was supported by the National Natural Science Foundation of China (41504053, 41520104003, 41873032, and 41374094), the Fundamental Research Funds for the Central Universities (CUG180620), and the China Scholarship Council (201806415015). Mike Jackson at the IRM and Tao Yang at the CUG-Wuhan offered help with the magnetic measurements. We thank the Associate Editor Mark Dekkers and two reviewers who greatly improved the manuscript. This is IRM Contribution 2001. The IRM is supported by the Instruments and Facilities Program of the NSF Division of Earth Science.
- Yarlung-Zangbo suture
- Zedang ophiolite
- magnetite formation
- rock magnetism