Abstract

Field and petrological data, combined with major, trace and rare earth element analysis of mafic rocks that structurally underlie the Semail ophiolite in Saih Hatat, have led to their subdivision into five groups: (a) crossite-bearing mafic schists that represent metamorphosed within-plate alkali basalts, erupted in a Proterozoic continental rift or ocean basin setting; (b) greenschists, blueschists, garnet-blueschists, clinopyroxene-rich layers and eclogites, representing metamorphosed Permian basaltic flows and tuffs with enriched-MORB affinities; (c) crossite-bearing metavolcanics, representing alkali basalts, trachyandesites and dacites erupted on oceanic islands in a small ocean basin or through rifted continental crust during the late Permian; (d) lawsonite- and pumpellyite-bearing metavolcanic blocks incorporated in a Cretaceous tectonic melange, and originally erupted as alkali basalts on oceanic islands in the Hawasina basin; and (e) unmetamorphosed tholeiites formed at a spreading centre in the Hawasina basin and preserved in the Hawasina thrust sheets.

A reconstruction of the tectonic history of Saih Hatat based on these data suggests that continental rifting took place in the Permian, developing two distinct basins separated by rifted continental material. Some of the basaltic blocks that formed in the outboard Hawasina basin were tectonically dislocated and incorporated into a tectonic melange formed in a foredeep during the early stages of obduction. Emplacement of the Semail ophiolite onto the continental margin resulted in high-pressure, low-temperature metamorphism of this melange and the underlying units.

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