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Department of Earth Sciences, Open University, Milton Keynes, Bucks MK7 6AA, UK
Boninites are rare high-MgO and compatible-element-rich basalts that are depleted in incompatible trace elements. Whilst their high magnesium number indicates equilibrium with upper-mantle peridotite, their depleted trace-element geochemistry indicates a highly refractory and depleted source. Many models account for boninite magmatism in destructive margin environments but none has so far explained their erratic spatial and temporal occurrence. Recent studies along the southern Troodos margin have revealed a history of boninite magmatism within a complex supra-subduction zone transform fault. The transform fault boninites form a suite that cross-cuts a less trace-element-depleted magmatic sequence forming the Troodos oceanic crust that was generated at a constructive plate margin.
Melting models demonstrate that the transform fault boninites were probably produced by a 10% melt of the upper mantle which had already been depleted by the extraction of the Troodos crust and mid-ocean ridge basalts (MORB). In contrast with the Troodos crustal sequence, however, the boninites are also enriched in two separate components, selected high field strength elements and large-ion lithophile elements. Such incompatible-trace-element enrichments are unlikely to have survived the earlier source depletion events. Therefore the enrichment of the transform boninites must have been added to their source immediately prior to or during their genesis within the supra-subduction transform environment.
The identification of rapid dilation and transtensional transform displacement along the southern Troodos margin suggests important tectonic factors in the formation and emplacement of boninite magmas. The requirement of an extensional regime and rifting of trace-element-depleted oceanic crust formed above a subduction zone (eg extension and rifting of an extended fore-arc region) explains the erratic and rare nature of boninite magmatism.
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