Magmatic processes recorded in the shallow plutonics of the Oman ophiolite (fast spreading oceanic centre): Implications for crustal accretion models

Fast-spreading oceanic ridges are characterized by magmatic systems with a lower crustal magma reservoir containing predominantly mush (i.e. a crystal-rich magma), punctuated by melt-rich lenses and overlain by a shallow Axial Magmatic Lens (AML). This mush-melt system plays a central role in oceanic crustal accretion, melt migration, and magmatic differentiation. After solidification away from the ridge axis, the lower crust shows a vertical layered structure from bottom to top consisting of layered gabbro (several km thick), foliated gabbro (1-2 km) and varitextured gabbro (tens to several hundreds of meters). Two end-member models have been suggested for the formation of the lower crust: the gabbro-glacier model, involving the subsidence of crystals from the AML, and the sheeted-sill model, requiring in situ crystallization of injected melt sills and ascending melts. The foliated gabbro unit, which remains relatively understudied, plays a key role in magma transfer and percolation between the different lower crustal units, as it is located at an intermediate stratigraphic position between the layer gabbro and varitextured gabbro. To better constrain accretionary processes, we selected key samples from the foliated gabbro unit of the Oman ophiolite (Wadi Tayin massif, and ICDP OmanDP Hole GT2) that represents one of the best natural analogues of fast-spreading oceanic ridges. In this study, we take advantage of the outcropping of entire crustal section and follow an integrated approach combining petrographical characterization of rocks textures and crystal morphologies with major and trace element chemical maps and spot measurements.

The results reveal the heterogeneity of the unit in terms of both textures and chemistry. Distinct differentiation paths can be identified in the thin sections. We identify a background mush composed of relatively evolved clinopyroxene generally displaying normal or inverse zoning. The associated plagioclases are overall homogeneous. This background mush is overprinted by less evolved melts. The zones that most clearly record these less evolved melts signatures are characterized by plagioclases recording cyclic zoning, whereas clinopyroxenes commonly display resorbed cores similar to the background one and inverse or more complex zoning patterns. These features are frequently associated with strongly poikilitic textures. In addition, we observe in some places inherited plagioclase cores with very low An (Anorthite) contents closely associated with accessory mineral phases that are typical of the greenschist facies.  In the uppermost foliated gabbro, skeletal cores are commonly observed in plagioclase, and clinopyroxenes display cyclic zoning or sector zoning.

Our results highlight that foliated gabbros record repeated episodes of recharge of magma reservoir by less evolved melts. Recharge melts then either interacted locally with previously hydrothermally altered crustal material or evolved within a mush through processes combining magma mixing and reactive porous flow. Plagioclase zoning indicates that fast-growth crystal morphologies are restricted to shallow levels and does not support the transfer of shallow crystals to deeper crustal levels. These observations provide new constraints on accretion models and support a significant role for melt percolation through the lower crust rather than crystal subsidence.