A field, petrological and geochemical study of the Masirah Ophiolite, Oman

[摘要] A reconnaissance survey of the 1000 Km\(^2\) of Masirah Island, Oman, has revealed a fully-developed ophiolite complex which is believed to represent a fragment of Cretaceous ocean crust and upper mantle generated at a constructive plate margin. The complex consists of mantle serpentinites, plutonic rocks ranging from dunite to trondhjemite, a sheeted dyke complex and pillow lava-sediment sequences, all of which have been chemically and petrographically analysed. Several belts of serpentinite occur within the ophiolite associated with major fault-lines. The serpentinites are clearly derived from depleted harzburgitic mantle and their field relations suggest that some were emplaced in the oceanic environment. The chemistry of the plutonic rocks suggests that they are products of dominantly open-system fractional crystallisation of tholeiitic liquid(s), possibly in several discrete magma chambers. Modelling of trace and RE elements suggests that moderate degrees of mantle peridotite melting were involved in production of the magma chamber parental liquid(s). At a higher crustal level sheeted dyke-massive gabbro relationships are interpreted in a model of roof underplating, which causes a decreasing frequency of dyke injection. Metamorphism of the sheeted dykes and lavas is interpreted as sub-sea floor in origin and its effect on whole-rock chemistry is assessed. The dykes and lavas have a chemistry largely typical of present-day ocean tholeiites and the relative contributions of the processes of partial melting and fractional crystallisation to that chemistry are evaluated. Two localised volcanic groups were identified, which appear to have enriched chemistries compatible with origin at off -axis oceanic islands. A major tectonic zone cross-cuts the ophiolite units and has features reminiscent of the modern oceanic transform faults. The importance of this structure, both in the oceanic environment and during the process of ophiolite emplacement, is assessed. Intrusive into the ophiolite is a granite whose trace and RE element chemistry is alien to the oceanic environment and suggests melting of continental crust Finally, a synthesized model of the former constructive margin is produced and an attempt is made to define the type of spreading centre represented. Comparison of the Masirah Ophiolite with the Semail Ophiolite of the Oman Mountains suggests that their former correlation may be ill founded. An assessment of late Mesozoic -early Tertiary plate motions indicates an origin during Cretaceous sea-floor spreading of an early Indian Ocean. Several features may indicate a slow-spreading, much-faulted, constructive margin.

[发布机构] University:University of Birmingham;Department:Department of Geological Sciences

[关键词] G Geography. Anthropology. Recreation;GB Physical geography