Petrographic, geochemical and structural characteristics of the Oman ophiolite lower crust: Implication for magmas genesis and magma chamber processes at an oceanic spreading centre

My work consisted in (1) a fieldwork focused on Oman ophiolite layered cumulates, particularly in several massifs wherein this unit was largely unexplored, (2) a petrographic and geochemical study (electron microprobe and LA-ICP-MS) of about 700 samples and (3) crystallisation experiments on hydrous boninitic melts. The fifty petrographic facies defined, gabbroic and ultrabasic, are essential to scrupulously describe the Oman ophiolite lower crust. This lithological diversity is indicative of the high variability of intensive (P, T°, PH2O ...) and extensive (composition) parameters during magmas crystallisation within magma chambers at an oceanic spreading centre. Early orthopyroxene in primitive cumulates (XMg > 80 %) is more common that proposed by previous works and undermines some present-day ideas concerning the origin of the Oman ophiolite. I demonstrate that this lithological diversity, the orthopyroxene abundance and other petrographic and geochemical characteristics of Oman cumulates can be explained by the mixing in various proportions of two main magmas: one with tholeiitic affinity related to the partial melting of an asthenospheric source and one with andesitic to boninitic affinity related to the hydrous partial melting of a depleted lithospheric mantle source. These two magmas migrated in mantle channels ("dykes") before feeding magma chambers. The "shaker" function of magma chambers is proved in this way. The relative contribution of these two magmas evolved during the upwelling of asthenospheric diapirs within the lithosphere. Abrupt variations of the water partial pressure are also essential to explain all the observations. I evidence that syn-accretion faults, whose role in the building of the Oman ophiolite was largely underestimated, are a main vector for the penetration of hydrothermal fluids down to the crystallizing lower crust. My study also allows to better constraint the origin, still strongly discussed, of the "wehrlitic" intrusions and of the dunitic mantle/crust transition zone.