The stratigraphy, geochemistry and petrogenesis of the Troodos extrusive sequency, Cyprus

Taylor, Rex Neil (1987) The stratigraphy, geochemistry and petrogenesis of the Troodos extrusive sequency, Cyprus. University of Southampton, Doctoral Thesis.

Abstract

Detailed examination of the field relationships within the Troodos extrusive sequence has provided an insight into the structure stratigraphy and volcanology of mafic crust generated at a spreading centre. Geochemical data including major, trace and rare earth elements are presented for whole rock and glass separates from the Troodos lavas. These data, along with microprobe analysis of fresh volcanic glass and included minerals, are used to elucidate the nature of the source material and the processes operating on both mantle and melts. The most primitive (high-MgO) lithologies within the volcanics are glassy olivine basalts and picrites, which contain forsteritic olivine, magnesiochrome-spinel, and occasionally ortho- or clinopyroxene phenocrysts. These extrusives are associated with sparsely olivine-phyric and aphyric lavas, and are generally located in the upper levels of the volcanic stratigraphy. A second, compositionally discrete, low-Mg suite is frequently encountered in the lower stratigraphic levels, but in some areas the two lava suites are juxtapositioned or inter-layered. Within the primitive extrusive sections a cyclical sequence of flows is developed, which represent the products of a periodically replenished open-system magma chamber. On the basis of high-field-strength element (HFS) concentrations, each unit within such sequences can be attributed to a separate batch of liquid entering the chamber. In addition, successively higher flows represent the influx of liquids with a greater depletion in these elements. Differences between the HFS abundances within the primitive lava group can be attributed to variable extents of mantle fusion, or a heterogeneously depleted source. However, the concentrations of low-field-strength elements and light rare-earth elements (LREE) vary independently of the HFS, and must relate to the introduction of LREE and low-field-strength element enriched component(s) into the source region. Correlations between the amount of LREE enrichment and the degree of melting or source depletion, indicate that the addition of such components promoted a higher degree of fusion or induced re-melting of refractory mantle. The enrichment in LREE elements is found to be spatially heterogeneous, with the western, older ophiolite crust containing the greater enrichment.

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