Magnetic properties of serpentinized peridotites from the Dongbo ophiolite, SW Tibet: implications for suture-zone magnetic anomalies

Magnetic properties of a suite of variably serpentinized peridotites from the Dongbo ophiolite, SW Tibet (China), have been investigated to determine the magnetic signatures of suture zones. The degree of serpentinization (S) for these peridotites is mainly in the range of S < 60%. Petrography, mineral chemistry, and thermomagnetic analyses reveal that magnetite occurring in the interior of various serpentine veins is the predominant magnetic phase. Magnetic hysteresis and first-order reversal curve diagrams suggest that the magnetite is mixture of interacting single-domain, and pseudo-single-domain and/or multidomain particles. Superparamagnetic magnetite occurs in the S = 40–60% serpentinized but weakly magnetic dunites. Overall, the magnetite content and magnetic susceptibility increase consistently from ~0 to 20% of serpentinization and then decrease from S = 30 to 60%. Peak magnetite abundance is at S~25% (density, ~3.1 ± 0.05 g cm^–3) and may suggest localized enrichment of fluids. The low magnetite abundance in dunite results from low-temperature serpentinization. Finally, strong intensities of magnetization (1–10 Am^–1) reside in the S > 60% and S = 20–30% serpentinized peridotites, indicating that peridotites with such degrees of serpentinization contribute to the aeromagnetic anomalies within the Yarlung-Zangbo suture zone in south Tibet.