The petrology, geochemistry and structure of the plutonic rocks of the Oman ophiolite

Browning, Paul (1982). The petrology, geochemistry and structure of the plutonic rocks of the Oman ophiolite. PhD thesis The Open University.



The Oman Ophiolite represents a fragment of oceanic crust and upper mantle that was emplaced onto the Arabian continental margin during the late Cretaceous. An area of 3000km2, comprising the Haylayn and Rustaq Blocks, has been mapped and specific localities were investigated in order to examine the petrological and structural processes that operate during and subsequent to the formation of oceanic lithosphere at a spreading axis.

Four successive plutonic suites have been recognized :
(i) the Mantle Sequence, Cumulate Sequence and High Level Intrusives that form the lower part of the classic Penrose ophiolite stratigraphy, which has been intruded by three Later Plutonic Suites,
(ii) the Late Trondhjemite-Gabbro Complexes,
(iii) the Late Peridotite-Gabbro Complexes, and
(iv) the Late Biotite Granites.

The tectonite harzburgite of the Mantle Sequence represents the residuum formed during 177. partial melting of upward flowing fertile spinet lherzolite mantle at pressures of 20kB or more. The primary magmas generated (tholeiitic picrites, minimum MgO 14%) supplied a sub-oceanic spreading ridge magma chamber in which formed the layered mafic and ultramafic rocks of the Cumulate Sequence and the isotropic gabbros, diorites and trondhjemites of the High Level Intrusives. Structures and petrofabrics developed in the Mantle Sequence preserve lines and planes of mantle flow beneath the oceanic crust. Layering in the Cumulate Sequence formed in situ within a long-lived, dynamic, compositionally stratified, open system magma chamber.

The Later Plutonic Suites record the post-spreading igneous history of the Oman Ophiolite. The Late Trondhjemite-Gabbro Complexes and the Late Peridotite-Gabbro Complexes reflect incipient arc formation and subsequent arc-rifting on and in young oceanic crust. The Late Biotite Granites are considered to be syn-emplacement plutons derived from the underthrust Arabian continental margin.

A back-arc (marginal) basin rather than a major ocean basin is considered to be the likely tectonic setting in which these successive magmatic episodes occurred.

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