Magmatic processes at and near constructive plate margins as deduced from the Troodos (Cyprus) and Semail Nappe (N Oman) ophiolites

Abstract

Summary The Troodos and Semail Nappe ophiolites, both produced during Upper Cretaceous (Cenomanian) times at Neotethyan supra-subduction zone constructive margins, are sufficiently comparable to allow a generalized model of constructive-margin magmatic processes to be presented. Such processes can be traced from (i) the partial melting of lherzolitic mantle within rising asthenospheric diapirs and the formation and intramantle fractionation of picritic magmas through (ii) the feeding of these melt batches into high-level, open-system crustal magma chambers to produce phase-layered mafic and ultramafic cumulate assemblages to (iii) the upward injection of melts to form sheeted dyke complexes and eruptive sequences. Evidence suggests that there is a series of mantle diapirs, each limited in time and space, beneath a constructive margin, each feeding its own magma chamber, sheeted dyke complex and volcanic sequence. Thus the oceanic lithosphere is seemingly the product of numerous diapiric ‘point’ sources embellished by a variety of off-axis magmatic products. The spreading axes are interrupted by transform fault zones which, during extensional phases, are the sites for magmatic products that are geochemically different from those produced at the axes.

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Affiliated Institutions

• The Open University GB

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