Distribution and origin of high magnetic anomalies at Luobusa Ophiolite in Southern Tibet

Abstract

The Luobusa Ophiolite, Southern Tibet, lies in the eastern portion of Indus–Yarlung Zangbo suture zone that separates Eurasia from the Indian continent. An aeromagnetic reconnaissance survey has revealed an EW-trending Yarlung Zangbo River aeromagnetic anomaly zone, and it is considered to be caused mainly by the Indus–Yarlung Zangbo Ophiolite. The Luobusa Ophiolite reflects the eastern portion of the Yarlung Zangbo River aeromagnetic anomaly zone. Conventionally, the ultramafic rock in the Luobusa Ophiolite is considered as the origin of the high magnetic anomalies. However, results from the surface magnetic survey and the magnetic susceptibility measurements from drill cores indicate that the high magnetic anomalies are distributed inhomogeneously in the Luobusa Ophiolite. In some cases, the susceptibility exhibits more than 30 times difference between two sides of the same sample. A fact emerged that the susceptibility of dunite with serpentinization is higher than that of fresh dunite, harzburgite and chromite when we analyzed the measurement results. In order to understand the origin of the high magnetic anomalies, we measured the density and susceptibility of 17 samples, microscopic and electron probe analyses have been performed as well. The result indicates the presence of dunite with serpentinization containing an abundant of micro-fissures filled with magnetite. Olivine has a susceptibility of about 2.7–351 (×10−5 SI), pyroxene about 16–320, and chromite about 200–800. All these units feature relatively low susceptibility in ultramafic rock, and only the magnetite is characterized by a high susceptibility of about 200,000 (×10−5 SI). Based on these observations, we conclude that the precipitation of magnetite in the process of serpentinization of the olivine caused by the geological process in the Luobusa Ophiolite is the origin of high magnetic anomalies.