CONSTRAINTS FROM AMPHIBOLE COMPOSITION AND 40AR/39AR DATING ON THE GEOLOGIC DEVELOPMENT OF THE MASIRAH OPHIOLITE (OMAN)

The Masirah Ophiolite in the east of Oman with an age of ~150 Ma likely represents oceanic crust formed in a MOR between Africa and Madagascar/India during the initial breakup of Gondwana. To elucidate the ophiolites tectonic history, five samples of amphibolite, taken from the “Masirah Mélange Zone” (MMZ), a zone of brecciated ultramafic, mafic, sedimentary as well as other continental material at Ras Madrakah, were analyzed using EPMA on amphiboles and Ar-Ar dating on amphiboles and biotites. All analyzed samples show high levels of a retrograde overprint with the formation of epidote. Veins of calcite and plagioclase, likely representing a post-deformation hydrothermal event, are present in most samples, often associated with large flakes of biotite. Ar-Ar data from amphibole separates partly reveal disturbed age spectra, stemming from excess argon combined with argon loss, the latter reflected in the low-temperature steps. Assuming that the amphibole ages only slightly post-date the amphibolite forming metamorphic event, metamorphism took place shortly before or between 94.2 ± 1.4 Ma to 97.0 ± 1.7 Ma as obtained from three samples. Two samples provided older ages of 106.2 ± 4.3 Ma and 135.1 ± 6.4 Ma. Biotites from the two older samples, assumed to represent the late-stage hydrothermal event, provided ages of 97.0 ± 0.9 Ma and 108.1 ± 0.8 Ma. Based on amphibole chemical compositions, the amphibolites likely protolith was primary MOR basalt from the Masirah Ophiolite. Thermobarometry from EPMA data revealed peak conditions of 630 ± 37 °C and 7.4 ± 1.2 kbar, corresponding to a depth of ~25 km and a geothermal gradient of ~25 to 30 °C/km. Based on these results and on available literature, the Masirah Mélange Zone can be interpreted as a tectonically disturbed thrust sheet along which the Masirah Ophiolite obducted onto the Arabian continent. Its provenance possibly consists of various nappes of oceanic crust. Inferred from Ar-Ar ages, major tectonic activity took place over prolonged periods of time peaking at around 95 Ma with previous events going back at least until ~135 Ma. This tectonic activity might have been in response to the movement of the Indian plate in relation to Arabia.