David G. Bailey

The extrusive rocks of the Troodos Ophiolite in the Margi area of Cyprus are highly variable in form and mineralogy, consisting of ten distinct cooling unit lithologies. The volcanic sequence can be divided into three major lithostratigraphic units, primarily on the basis of phenocryst mineralogy. The lowermost unit, Unit I, consists of aphyric to plagioclase and clinopyroxene microphyric volcanics. Unit II is approximately 175 m thick and consists largely of olivine phyric to picritic pillows and flows. At the top of the volcanic pile lies Unit III, which is an approximately 50 m thick sequence of aphyric to sparsely olivine phyric pillows and thin flows.

The boundary between Units I and II coincides with the previously mapped boundary between the classical "Lower" and "Upper Pillow Lava" sequences, and is often marked by a thin layer of umberiferous sediment. This boundary also separates rocks which belong to the two major chemical suites which are currently recognized within the massif, the "arc-tholeiite" and "boninite" suites, respectively.

The major element compositional variation of the extrusive rocks within Units II and III (the "boninite" suite), can be largely explained by simple closed-system fractionation of olivine, clinopyroxene, and minor chrome spinel. Incompatible trace element concentrations of the rocks within these units are consistent with derivation from a strongly depleted mantle source and suggest periodic additions of parental melt fractions to a shallow level magma chamber.

The "ultrabasic lavas" of the Margi area all contain 45 to 65 modal percent phenocrystic olivine. The majority of these rocks formed by post-extrusion gravitative settling of olivine phenocrysts from moderately phyric lavas. A small proportion were extruded as "picritic lavas", but these represent samples of magma chamber cumulates that were mixed with small amounts of melt and extruded as a crystal liquid mush during periods of advanced tapping.

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Pages: 229
Supervisor: Paul Robinson