T199 Metamorphism and Orogenesis at Convergent Plate Margins (Posters)

Tuesday, 24 September 2019
9:00 AM - 6:30 PM
Hall AB, North Building

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

Paper number: T199-11
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-338777

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