Petrology and origin of Masirha ophioilite and Masirah ophiolitic Melange

Recently the ophiolite model for ocean crust formation has been challenged for it has been shown that most ophiolitic basalts do not have the geochemical characteristics of basalts formed in an ocean ridge setting. New studies show that ophiolites can form in a variety of geological settings but most commonly in a subduction setting, not an ocean ridge as previously thought. This proposal is to examine the Masirah ophiolite in eastern Oman as one of the few true ocean ridge ophiolites that have been preserved. Recent ideas show that this area has become hugely important and there must be a new field expedition to better determine the stratigraphy of the ophiolitic sequence.

Recently the ophiolite model for ocean crust formation has been challenged for it has been shown that most ophiolitic basalts do not have the geochemical characteristics of basalts formed in an ocean ridge setting. New studies show that ophiolites can form in a variety of geological settings but most commonly in a subduction setting, not an ocean ridge as previously thought. This proposal is to examine the Masirah ophiolite in eastern Oman as one of the few true ocean ridge ophiolites that have been preserved. Recent ideas show that this area has become hugely important and there must be a new field expedition to better determine the stratigraphy of the ophiolitic sequence.

The Oman ophiolite is widely cited as a classic example of an ophiolite formed at a mid-ocean ridge (Boudier et al., 1997). However, now new data strongly suggest that the Oman ophiolite is subduction related (Rollinson and Adetunji, 2013; 2016; MacLeod et al. 2013; Rollinson, 2014; 2015). A new model for ophiolite formation and its link to ocean floor creation has recently come from the work of Whattam and Stern (2011) on subduction initiation. These authors showed that localised sea-floor spreading can take place in a subduction environment. Dilek and Furness (2014) proposed three possible candidates for ophiolite formed at mid-ocean ridge? the Masirah ophiolite in south-eastern Oman, Mcquarie Island in the southwest Pacific, and the incomplete Taito ophiolite located in a remote part of Chile. The background geology of Masirha ophiolite is well known. Over 700 km2 of basalts, gabbros and mantle peridotites were mapped on the Island (Moseley and Abbotts, 1979; Moseley, 1990) and in the nearby headland of Ras Madrakah on the mainland, which are thought to be part of the same sequence (Shackleton and Reis, 1990). A more thorough study was made by a group from Bern University in the 1990s under the leadership of Tjerk Peters (Peters, 2000; Peters and Mercolli, 1997: 1998). The rocks of the Masirah ophiolite are also geochemically distinct from those of the Oman ophiolite. Comparing data from Masirah (Peters and Mercolli, 1998) with those from the lower (ocean ridge like) lavas in the Oman ophiolite (McLeod et al, 2013) and with lavas from the Atlantic Bank region of the southern Indian ocean floor (Coogan et al., 2004) it is possible to show that lavas and dykes from Masirah have a narrower range of MgO and SiO2, higher CaO, Al2O3 and Na2O and are enriched in light REE, similar to lavas and dykes from the Indian ocean floor, but very different from lavas in the Oman ophiolite (Godard, 2006). References Arai, S., Miura, M., 2015. Podiform chromitites do form beneath mid-ocean ridges. Lithos, 232, 143-149. Arculus, R.J., 29 others, 2015. A record of spontaneous subduction initiation in the Izu- Bonin- Mariana arc. Nature Geoscience, doi: 10.1038/NGEO2515 Boudier, F., Nicolas, A., Ildefonse, B., Jousselin, D., 1997. EPR microplates, a model for the Oman Ophiolite, Terra Nova, 9, 79?82, Dilek, Y., Furness, H. 2014. Ophiolites and their origin. Elements, 10, 93-100. Coogan L.A., Thompson, G.M., MacLeod, C.J., Dick, H.J.B., Edwards, S.J., Hosford Scheirer, A., Barry, T.L., 2004. A combined basalt and peridotite perspective on 14 million years of melt generation at the Atlantis Bank segment of the Southwest Indian Ridge: evidence for temporal changes in mantle dynamics? Chemical Geology, 207, 13?30. Godard, M., Bosch, D., Einaudi, F., 2006. A MORB source for low-Ti magmatism in the Semail ophiolite, Chemical Geology 234, 58?78 Larsen, H.C., Cannat, M., Ceuleneer, G., Fruh-Green, G., Kodaira, S., MacLeod, C., Miller, J., Seama, N., Tatsumi, Y., Toomey, D., 2009. Oceanic crustal structure and formation. IODP thematic review, report series 2. 65 pp. MacLeod, C.J., Lissenberg, C.J., Bibby, L.E., 2013. ?Moist MORB? axial magmatism in the Oman ophiolite: the evidence against a mid ocean ridge origin. Geology, doi: 10.1130/G33904.1 Moseley, F., 1990. The structure of Masirah Island, Oman. In: Robertson, A. H. F., Searle, M. P. and Ries, A. C. (eds), The Geology and Tectonics of the Oman Region. Geological Society, London, Special Publications 1990, 49, 665-671 Moseley, F. and Abotts, I. L. 1979. The ophiolite melange of Masirah, Oman. Journal of the Geological Society of London, 136, 713-724. Moseley, F. and Abotts, I. L 1984. A geological map of the Masirah ophiolite complex, Oman. Overseas Geology and Mineral Resources, 62, HMSO Pearce, J.A., Alabaster, T., Shelton, A.W., Searle, M.P., 1981. The Oman ophiolite as a Cretaceous arc-basin complex. Phil. Trans. R. Soc. London, A, 300, 299-317. Peters, Tj., 2000. Formation and evolution of the western Indian Ocean as evidenced by the Masirah ophiolite: a review. Geol. Soc. Amer. Sp. Pap., 349, 525-536. Peters, Tj. et al., 1998. 1:50,000 Geological map of Masirah Island North and South (K-768). Directorate of Minerals, Ministry of Petroleum and Minerals, Sultanate of Oman. Peters, Tj., Mercolli, I., 1997. Formation and evolution of the Masirah ophiolitye (Sultanate of Oman). Ofioliti, 22, 15-34. Peters, Tj., Mercolli, I., 1998. Extremely thin ocean crust in the proto-Indian ocean: evidence from the Masirah ophiolite, sultanate of Oman. J. Geophys. Res., 103B, 677-689. Rioux, M., Bowring, S., Kelemen, P., Gordon, S., Dud?s,F., Miller. R., 2012, Rapid crustal accretion and magma assimilation in the Oman-U.A.E. ophiolite: High precision U-Pb zircon geochronology of the gabbroic crust. J. Geophys. Res. 117, B07201, doi:10.1029/2012JB009273. Rollinson, H.R., Adetunji, J. 2013. Mantle podiform chromitites do not form beneath ocean ridges: a case study from the Moho transition zone of the Oman ophiolite. Lithos doi 10.1016/j.lithos.2013.07.004 Rollinson, H.R., Adetunji, J. 2016. Comment on ?Podiform chromitites do form beneath mid-ocean ridges? by Arai, S. and Miura, M., Lithos, 254-255, 131-133. Rollinson, H.R. 2014. Plagiogranites from the mantle section of the Oman ophiolite: models for early crustal evolution. In (eds) Rollinson, HR, Searle, M, Abassi, I, Al-Lazki, A. and Al-Kindi, M. Tectonic Evolution of the Oman Mountains. Spec. Publ. Geol. Soc. London. p 247-261. Rollinson, H.R., 2015. Slab and sediment melting during subduction initiation: granitoid dykes from the mantle section of the Oman ophiolite. Contributions to Mineralogy and Petrology, Volume 170, Issue 3 doi: 10.1007/s00410-015-1177-9 Shackleton, R.M., Ries, A.C., 1990. Tectonics of the Masirah Fault Zone and eastern Oman. In: Robertson, A. H. F., Searle, M. P. and Ries, A. C. (eds), The Geology and Tectonics of the Oman Region. Geological Society, London, Special Publications 1990, 49, 715-724 Whattam, S. A., Stern, R.J., 2011. The ?subduction initiation rule?: a key for linking ophiolites, intra-oceanic forearcs and subduction initiation. Contributions to Mineralogy and Petrology 162, 1031-1045