Olivine Petrofabric Analysis from the Leka Ophiolite Complex
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thesis / dissertation description
Peridotites, gabbros and basaltic volcanic rocks on the island of Leka, Norway in the western Scandinavian Caledonides compose a nearly complete ophiolite sequence. I examined samples from the harzburgite to dunite transition within the ultramafic tectonites of the Leka Ophiolite Complex. This rock sequence probably corresponds to the petrologic Moho, which is the true base of the oceanic crust and lies below the seismic Moho. I used universal stage techniques to determine the crystallographic preferred orientations (CPO) of olivine grains in both dunites from the base of the oceanic crust and residual harzburgites in the uppermost upper mantle. Olivine CPOs in other ophiolites are inferred to reflect patterns of asthenospheric flow at the spreading center and are used to determine the sense of shear, the relative orientation of the paleo-spreading center and the rate of spreading. The olivine CPOs in dunites and harzburgites from Leka do not match those measured in other, less deformed ophiolites like the Semail, Troodos, or Bay of Islands massifs. Therefore, I am able only to identify tentatively the orientation of the ridge at which these rocks formed. I was not able to determine a sense of shear in my samples due to inconsistent data. Several factors may account for the inconsistencies between the CPO in Leka peridotites and those from other well-known ophiolites: deformation of peridotites by shearing and/or folding, a shear sense reversal, complicated patterns at transform faults, and insufficiencies in the petro fabric methodology.