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Geological Society, London, Special Publications; 1984; v. 13; p. 55-62;
DOI: 10.1144/GSL.SP.1984.013.01.05
© 1984 Geological Society of London

Magma Chambers: Products and Processes

Melt percolation beneath a spreading ridge: evidence from the Semail peridotite, Oman

R. T. Gregory

Department of Geology, Arizona State University, Tempe, AZ85287, USA

Field mapping in the Semail ophiolite complex, southeastern Oman Mountains reveals a thick oceanic crustal section (on average >6 km thick) underlain by a 9–12 km thick upper-mantle sequence consisting of a basal harzburgite-dunite zone overlain by a foliated harzburgite containing concordant layers of dunite (cm-m scale) and olivine orthopyroxenite (cm scale). The harzburgite and the concordant layers are crosscut by discordant dunite bodies (cm-km scale) and by dykes of dunite, websterite, olivine clinopyroxenite, and gabbro (cm-m scale). The sequence and mineralogy of these features are all consistent with the interpretation that both the concordant and discordant layers represent crystallization or reaction products between ascending melts and host peridotite. Dunite is the only volumetrically significant discordant or concordant rock type crosscutting the peridotite representing ~50% of the basal 3–4 km of the peridotite section and <15% elsewhere. Addition of ~2 km of mantle dunite to the Semail average crustal composition suggests that the average melt percolating up through the Semail mantle had picritic affinities (PLAG 32, DIOP 16, OL 44, SI 08). Field evidence does not require that the harzburgite be cogenetic with the ascending melts that produced the overlying oceanic crustal section. The extremely depleted character of the Semail harzburgite results from interaction with transient melts in the regime of overall high melt/rock ratio (0.5–0.8) found beneath the fossil late-Cretaceous ridge. The melt/rock ratio is calculated assuming that the basal harzburgite-dunite represented the shallow-level, ‘off-axis’, boundary of the zone of upward magma migration beneath the ridge. The apparently thicker than normal crustal section and the depleted character of the Semail peridotites may suggest that the supply of melt to the Semail ridge system was greater than could be accommodated by spreading in the closing Hawasina ocean basin.