Geochemical mapping of a paleo-subduction zone beneath the Troodos Ophiolite

Abstract

Supra-subduction zone ophiolites such as the Cretaceous Troodos Ophiolite of Cyprus are fragments of oceanic crust formed by seafloor spreading close to subduction zones. Their exact tectonic setting of origin has been intensively debated. Although many supra-subduction zone ophiolites are thought to represent fore-arc crust, created during subduction initiation, others may have formed at a subducting ridge, or in a back-arc, ridge-trench-trench/transform triple junction or ‘plate edge’ setting. We carried out major and trace element analyses of 515 fresh volcanic glasses from 7 detailed sections through the Troodos lava sequence in order to determine the regional and temporal variation in the composition of Troodos magmatism, and hence reconstruct the distance and orientation of the Troodos spreading axis relative to the former subduction zone. Troodos glasses range from boninite through tholeiitic basalt and andesite to dacite. All glasses are enriched in fluid-mobile trace elements, and variably depleted in the high-field strength elements compared to Mid-Ocean Ridge Basalt (MORB). None of these glasses therefore have compositions identical to Izu-Bonin-Mariana fore-arc lavas that have been proposed to be the prime example of lavas formed during subduction initiation. Boninites are apparently restricted to the southern margin of the Troodos Ophiolite, and glasses from the southeast margin of the ophiolite are the most depleted and contain the strongest input of subduction zone fluid and melt signature. These geographic variations in glass composition indicate that the Troodos Ophiolite formed by NW-SE directed spreading (at 91 Ma) approximately 100–120 km above an eastward-dipping subducting plate. The orientation of the Troodos spreading axis relative to the former trench could be explained if the Troodos Ophiolite formed in a fore-arc position by subduction initiation at a transform fault. However, the lack of glasses with fore-arc basalt composition, and similarities between the trace element compositions of Troodos glasses and those from the Fonualei basin and northern Lau Basin in the southwest Pacific suggest that the Troodos Ophiolite formed in a ridge-trench-trench or ridge-trench-transform triple junction setting, at a back-arc spreading centre that propagated into arc and fore-arc crust.