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

Cretaceous (Cenomanian) times at Neotethyan supra-subduction zone constructive mar-gins, 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 umulate assemblages to (iii) the upward injection of melts to form sheeted yke 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 yke complex and volcanic sequence. Thus the oceanic lithosphere is seemingly the product of numerous diapiric 'point ' sources embel-lished 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. Evidence that ophiolites are fragments o