Abstract
We established a 5,000 m thick profile through the paleo lower oceanic crust of the Samail ophiolite (Sultanate of Oman, Wadi Gideah), in order to investigate accretion processes beneath fast-spreading mid-ocean ridges. The Samail ophiolite is regarded as best on-land analog for fast-spreadoceanic crust, and Wadi Gideah allows sampling of the entire lower crust along the Wadi bed. Here, we provide microstructural constraints to lower crustal accretion beneath fast-spreading mid-ocean ridges, which reveal changing microstructures and fabrics with depth. Grain size coarsening occursfrom the foliated to the layered gabbro section. A ∼350 m thick zone of gabbroic rocks from the profile top, interpreted as frozen fillings of the axial melt lens and defined as varitextured gabbros, shows intergranular textures without crystallographic preferred orientations. Rocks from the varitextured/foliated gabbro transition (∼500 m thick) and from the upper foliated gabbros (∼600 m) are foliated and lineated. The lineation is absent or very weak in the underlying ∼800 m thick lower foliated gabbros. In layered gabbros, the fabric gradually strengthens and becomes more lineated down section with local scattering at small spatial scales. This implies distinct accretion mechanisms in the deep and shallow plutonic crust. For the layered and lower foliated gabbros, our data suggest in-situ crystallizing individual magma reservoirs. Dominant lineation in the upper foliated gabbros and the varitextured/foliated gabbro transition suggests vertically transported crystal-laden melts or mushes being consistent with subsiding crystal mushes from the axial melt lens, and/or crystallizing upward migrating melt expelled from the crystal mush beneath.