Masirah-the other Oman ophiolite:A better analogue for mid-ocean ridge processes?

Oman has two ophiolites-the better known late Cretaceous northern Oman(or Semail)ophiolite and the lesser known and smaller,Jurassic Masirah ophiolite located on the eastern coast of the country adjacent to the Indian Ocean.A number of geological,geochronological and geochemical lines of evidence strongly suggest that the northern Oman ophiolite did not form at a mid-ocean ridge but rather in a supra-subduction zone setting by fast spreading during subduction initiation.In contrast the Masirah ophiolite is structurally part of a series of ophiolite nappes which are rooted in the Indian Ocean floor.There are significant geochemical differences between the Masirah and northern Oman ophiolites and none of the supra-subduction features typical of the northern Oman ophiolite are found at Masirah.Geochemically Masirah is MORB,although in detail it contains both enriched and depleted MORB reflecting a complex source for the lavas and dykes.The enrichment of this source predates the formation of the ophiolite.The condensed crustal section on Masirah(ca.2 km)contains a very thin gabbro sequence and is thought to reflect its genesis from a cool mantle source associated with the early stages of sea-floor spreading during the early separation of eastern and western Gondwana.These data suggest that the Masirah ophiolite is a suitable analogue for an ophiolite created at a mid-ocean ridge,whereas the northern Oman ophiolite is not.The stratigraphic history of the Masirah ophiolite shows that it remained a part of the oceanic crust for ca.80 Ma.The chemical variability and enrichment of the Masirah lavas is similar to that found elsewhere in Indian Ocean basalts and may simply reflect a similar provenance rather than a feature fundamental to the formation of the ophiolite.

SSZ Semail Ophiolite vs MORB Masirah Ophiolite: A Perspective from Podiform Chromitites

One of the major topics of debate in ophiolite geology is the original tectonic setting of ophiolites. New studies show that most ophiolites are formed more frequently in a suprasubduction zone(SSZ) environment and that only a very small number of ophiolites have formed in an oceanic range(MOR). The Masirah ophiolite is one of the few oceanic ridge ophiolites that have been preserved, and the evidence that was formed in a subduction environment is missing(Moseley and Abbotts 1979, Dilek and Furnes, 2011;Rollinson, 2017). Masirah Island, the Batain and Ras Madrah areas of eastern Oman are almost entirely composed of a well-developed ophiolite, known as the Masirah ophiolite(Fig. 1), which is, however, completely unrelated to the nearby Semail Ophiolite in the northern Oman Mountains(Fig. 2). The Masirah ophiolite is Jurassic in age and represents oceanic lithosphere derived from the Indian Ocean, but is about 15–20 Myr later than emplacement of midCretaceous Semail ophiolite in northern Oman. The presence of basaltic to rhyolitic lavas of calc-alkaline affinity and boninites in the lava sequence of the Semail ophiolite led several researchers to propose a back-arc basin model for this ophiolite(e.g. Tamura and Arai, 2006;Godard et al., 2008;Rollinson and Adetunji, 2015). The Masirah Ophiolite shows close affinities with MORB peridotites in general. Most of the olivine from the Masirah harzburgites show Fo contents that are similar to those of olivine from MORB. Both pyroxenes in these harzburgites have similar Mg# values, Al2O3 and Cr2O3 contents to those of pyroxenes from MORB peridotites. The observed primitive mantlenormalized REE patterns showing enrichment in LREEs indicate that the Masirah peridotites have been modified by fluids or melts enriched in LREEs in a MORB environment. Podiform chromitites housed in ophiolites today interpreted as magmatic deposits formed during the reaction of molten rock in environments spike in the middle of the ocean(MOR) or suprasubduccion zone(SSZ)(Arai and Matsukage, 1998;Rollinson and Adetunji, 2015). The Masirah chromitites has a mineral chemistry similar to the mineral chemistry of chromite crystallized from MOR magmas. The Cr# values of chromite in the Masirah chromatite are similar to those of MOR peridotites. These findings suggest that the ultramafic and mafic cumulate rock assemblages overlying the upper mantle peridotites in the Masirah ophiolite represent the products of magma evolution in a MOR initiation stage within the proto Indian Ocean. Coexisting high-and low-Cr# associations of chromitite and dunite have been found in the Semail ophiolite, which illustrates the common situation of ophiolites having both SSZ and MOR geochemical signatures. Cr# varies from 40–60 for shallow chromite bodies, and over the range 70–80 for the deep locations. This diversity of chromitite types suggests two stages of magmatic activity were responsible for the chromitite genesis, in response to a switch of tectonic setting. The first is residual from lower degree, partial melting of peridotite, which produced lowCr# chromitites at the Moho transition zone, possibly in a midocean-ridge setting. The second chromitite-forming event involves higher degree partial melting, which produced high-Cr# discordant chromitite in the upper mantle, possibly in a suprasubduction zone setting. Assemblages of mono-and poly-phase silicate inclusions(including olivine, orthopyroxene, clinopyroxene, amphibole, phlogopite, serpentine, native Fe, FeO, alloy, sulfide, calcite, laurite, celestine and halite) within chromite have been observed in the low Cr# podiform chromitites from the Semail and Masirah ophiolites. The existence of hydrous silicate inclusions in the chromite calls for a role of hydration during chromite genesis. High-T bright green hornblende–edenite included in the chromites is evidence of the introduction of water in the magma at the end of the chromite crystallization. Such paragenesis points to the presence of hydrous fluids during the activity of the shear bands.