Paleomagnetism of the Samail Ophiolite, Oman: 1. the sheeted dike complex at Ibra

Abstract

We have studied the rock magnetism and paleomagnetism of 200 oriented samples from the middle Cretaceous sheeted dike sequence of the Samail ophiolite, Oman. The samples are from 28 sites spanning a cross‐strike distance of 30 km. Most of the dikes have undergone green‐schist metamorphism. The natural remanent magnetization (NRM) of the samples is about 1 A/m and Qn is also near 1. Thermal and alternating field demagnetization treatment resolves both normal (north) and reversed (south) polarities. About one third of the samples have low median destructive fields (MDF) less than 100 Oe (7.96×103 A turn/m). These low MDF samples have scattered directions and are classified as undetermined polarities (U). Hysteresis measurements show pseudo‐single domain carriers predominate in all samples, although the low MDF samples are suggested to have the largest grains in the samples studied. Thermomagnetic curves show both magnetite and minor maghemite. Thermal demagnetization and microscopic study reveals that hematite is abundant and is the carrier of the R remanence. In addition, several dikes exhibit both N and R polarities. These facts, coupled with the degree of metamorphism, indicate that part of the remanence is secondary and could be due to hydrothermal metamorphism below the seafloor. We suggest that the N polarity is primary and resides in exsolved magnetite, while the R polarity is secondary and resides primarily in metamorphic‐origin hematite. Several short R events observed in the Albian through Cenomanian magnetostratigraphic record are candidates for the R event in the ophiolite if the R polarity originated near the spreading center. Alternately, nonreproducible self‐reversal is another possible origin for the R polarity. Regardless of whether the reversal originated at the spreading center, secondary magnetization in the dikes considerably increases the directional dispersion. Another major factor affecting dispersion is viscous remanent magnetization (VRM) in the larger magnetite grains. This extreme dispersion disallows the Samail ophiolite dike layer as a significant source for marine magnetic anomalies, regardless of the strength of the NRM or the value of Q.The stable N and R directions yield a paleomagnetic pole which is far sided, suggesting northward movement of the ophiolite. However, the paleomagnetic pole is not as far sided as the contemporaneous Africa pole, which suggests a component of north‐south closure between the ophiolite and Africa prior to northward movement.