MAGNETIC FABRIC AND PROPERTY CHARACTERIZATION OF UPPERMOST FOLIATED GABBROS FROM THE OMAN OPHIOLITE

Although oceanic crust covers the majority of Earth’s surface, the magmatic processes operating underneath spreading centers that form middle to lower crust are still not fully understood. Emplacement processes of igneous rocks can be understood by measuring oriented samples of oceanic crust, either by examining silicate fabrics or by measuring anisotropy of magnetic susceptibility (AMS), which has been shown to commonly correlate with existing fabrics. However, retrieving samples directly from the seafloor for this purpose is a difficult and expensive process. Instead, many attempts to understand these processes have focused on ophiolites as a proxy for less accessible oceanic crust.

This study focuses on 18 sites sampled over about 0.5 km in upper foliated gabbros in the Samail Massif in the Oman Ophiolite. Varying degrees of foliation were observed in the field, so an average of 10 samples were taken from each site both across and along certain foliated zones. Measurements of the magnetic fabric, paleomagnetic remanence, and other rock magnetic properties were then taken from the samples.

Our results show that the magnetic fabrics measured with AMS generally agree with the steep SW-dipping foliation observed in the field. The K_min axes match the poles to this foliation with a NE trend and subhorizontal plunge. Although no macroscopic lineation was observed in the field, the K_max axes are consistent within each site and were shallowly to moderately plunging to the NW. The degree of anisotropy (P_j) values range from 1.02 to 1.53, with an average of 1.15. Paleomagnetic remanence directions generally have northward declinations and shallow, negative inclinations, which is consistent with previous studies for the region. Other rock magnetic measurements indicate the presence of nearly pure magnetite in most samples. These data agree with bulk susceptibility values of 2.5 x 10^-4 to 2.6 x 10^-2 SI, suggesting that the AMS signal is dominated by preferred orientation of magnetite. This is an ongoing project and attempts to characterize the magnetite, its origin, and relationship to the silicate fabric are underway.