11:15 AM - 11:30 AM
[SMP36-03] Variation of magma source in the Oman ophiolite inferred from distribution of magmatic dykes in the mantle
Keywords:Oman ophiolite, Mantle source, Pyroxenite
The distribution map of the magmatic dykes show that troctolites and olivine gabbros crop out in two localized area, the main one is centred on the Maqsad mantle diapir. Pyroxenites are highly dominant in the mantle section of the Oman ophiolite but Opx-rich concordant and discordant dykes are abundant only in the northern massifs. On the other hand, these features are scarce or absent in the southern massifs where troctolite and olivine gabbro dykes are abundant. The dykes chemical composition show that they can be linked to two different magmatic series: troctolites and olivine-gabbros are probably issued from a typical MOR magma, but pyroxenite and Opx-rich gabbronorites are richer in Si and poor in incompatible elements, showing depleted characteristics closer to andesitic to boninitic magma. The dykes lithological nature is partly related to the nature of the parental magma and troctolites and Opx-free olivine gabbros crystallised exclusively from the MOR magma. However, pyroxenites are ubiquitous and may have crystallised from both types of magma. This is in particular reflected in Cr-spinel chemistry, which Cr# is higher and Ti contents lower, and the higher amount of Opx when the parental magma is andesitic. The pyroxenites distribution largely overlap with concordant layers over the ophiolite, suggesting that a pyroxenitic source issued from layer melting may be a significant part of the mantle source for the ophiolite.
The dyke and layer distribution all over the mantle section of the Oman ophiolite show a clear magmatic dichotomy between the southern and northern parts of the ophiolite. Classical MOR-type magma was preponderant southward to the Nakhl massif while the northern areas are dominated by Si-rich depleted and/or primitive magma. Mineral chemistry in the lower crust show characteristics intermediate between the two types of melt that circulated in the mantle, suggesting that magma mixing hardly occur in the mantle but may be an important process of ocean crust building. In the lower crust, the most MOR-like gabbros are strongly dominant in the area close to the Maqsad mantle diapir and, curiously, in the Fizh area. On the other hand, the andesitic end-member becomes dominant in the central massifs, far from the Maqsad diapir. This North-South chemical gradation shows that mixing occurs along the ridge axis, implying that magma can circulate on significant distances along the ridge axis at crustal level.