Petrofabric Investigation of Gabbros from the Oman Ophiolite: Comparison between AMS and Rock Fabric


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Abstract

The anisotropy of magnetic susceptibility was measured on 42 gabbros sampled across a complete plutonic sequence from the Oman ophiolite. The rock fabrics, investigated in the field and through plagioclase crystallographic fabric measurements, were compared to the magnetic fabrics. This comparative study reveals that from the paleo-Moho to the top of the foliated gabbros level, 73% of the rocks display a good correspondence in orientation, between the magnetic and rock fabric orientation. In these rocks, the AMS is controlled by secondary magnetites located in the fracture network of the olivines, and probably, but to a lesser extent, by secondary magnetites located in the exsolution lamellae of the clinopyroxenes. The high correlation between the AMS ellipsoid orientation and the rock fabric orientation is explained by the fact that the magnetic foliation is essentially constrained by the orientation of the olivine fracture planes, which is in turn constrained by the orientation of the overall magmatic rock fabric. In contrast to the primary mineral phases, the orientation of magnetite crystals in these gabbros is not due to their alignment in a flowing magma, so their preferred orientation, although usually mimicking that of the rock fabric, does have not the same origin. Furthermore, given that the preferred orientation of the anisometric secondary magnetites is much less perfect than the preferred orientation of the plagioclases, no correlation between the shape and magnitude of the AMS and plagioclase fabrics can be established. In the uppermost levels of the sequence there is no correspondence between the magnetic and rock fabric orientation. The magnetism of these rocks is mainly carried by primary magnetite and ilmenite grains. These minerals occur as small and scattered interstitial grains that exhibit neither alignment nor parallelism with the pre-existing rock fabric. Hence, the anisotropy, shape and orientation of the AMS ellipsoid are independent of the rock fabric ellipsoid. Although in the Wadi Al Abyad gabbros, just like in other magnetite bearing rocks (Rochette et al., 1992; Archanjo et al., 1995), the AMS cannot be used to evaluate the shape and strength of the finite strain ellipsoid, it can be reliably used to get the orientation of the rock fabric ellipsoid when the AMS is controlled by secondary magnetites.

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