Résumé
About 20 years ago, two competing models were proposed for the formation of the lower, gabbroic crust at fast-spreading ridges. The lower crust is either formed by downward flow of mushy material from the shallow axial melt lens (gabbro glacier), or by sill intrusions (sheeted sills). To further test these end-member models, we characterized the vertical distribution of Crystallographic Preferred Orientations (CPO) in Wadi Gideah gabbro section (Sumail ophiolite, Sultanate Oman), using the Electron Backscattered Diffraction (EBSD) technique. CPO were measured on 67 gabbro samples, documenting a 5 km thick section, with an average interval of 80 m between samples. EBSD data sets were processed using MTEX, a free Matlab toolbox. Average misorientation in grains (angle between each pixel orientation and mean orientation of the grain) is very low ( approximately 0.25 degrees ). This is consistent with magmatic flow in these rocks, and the paucity of crystal-plastic overprint. The strength (J index) of plagioclase CPO increases down-section, with a more pronounced variability in the layered gabbros. For clinopyroxene, the difference between upper (foliated) and lower (layered) gabbros is stronger, with low J in upper gabbros, and higher and more variable J in lower gabbros. In upper gabbros the symmetry of plagioclase and clinopyroxene CPO progressively evolves downward to progressively more oblate. Continuing down-section, the trend reverses, with progressively more prolate CPO in lower gabbros. The crystallographic fabric variability in the lower crust section calls for distinct formation mechanisms in the upper and lower gabbros. It is consistent with a hybrid model for crustal formation (Boudier et al., 1996, doi:10.1016/0012-821X(96)00167-7). The genesis of the upper foliated gabbro can be at least partly explained by the gabbro glacier model, while the continuous emplacement of sheeted sills at various depths is a more plausible model for the lower layered gabbro section.