Cumulate pyroxenite and pyroxenite dykes in the Troodos ophiolite, Cyprus

Abstract

The Troodos ophiolite is a type example of ophiolite and has been studied for more than 50 years. Albeit numerous findings have been derived from previous investigations, some questions about the details of its formation processes are still outstanding. One of them concerns the origin of the pyroxenites in the mantle and the lower crustal section, and this is the main theme of this thesis. Integrated field, petrographical and geochemical work was conducted in this study. On the basis of distribution, the pyroxenites can be divided into two categories, crustal pyroxenites and mantle pyroxenite dykes.

The results show that the layered or massive crustal pyroxenites can be distinguished by their magmatic features, and their formations have been controlled by a series of factors, including the melt composition, change of pressure, magma replenishment and magma mixing. The crystallization sequences of the crustal ultramafic unit samples are classified into two trends. Trend (i) olivine, orthopyroxene, clinopyroxene, plagioclase has been derived from tholeiitic-boninitic transitional melts, whereas trend (ii) olivine, clinopyroxene, plagioclase, orthopyroxene from depleted boninitic melts.

Regarding the mantle section, the pyroxenite dykes have been formed by focused flow of melt during migration towards the crust. Data show that they are products of several processes, including melt-rock reactions and fractional crystallization. On the basis of modal composition, the pyroxenite dykes are divided into clinopyroxenites and orthopyroxenites. Geochemical compositions suggest that the clinopyroxenites have been derived from island arc tholeiitic melts, whereas the orthopyroxenites from boninitic melts.

The important overlap of the tholeiitic and the boninitic series throughout the sections of the ophiolite, as well as the presence of lithologies with compositions transitional between the two series, suggests that the two magmatic suites existed together. A tectonic model of subduction initiation, during which the subducting slab rolled-back rapidly, triggering asthenospheric mantle flow into the mantle wedge, inducing partial melting at a shallow level of the mantle to generate a series of island arc tholeiitic magmas and at deeper level, a series of depleted boninitic magmas. The two magmatic series have possibly been mixed during migration in the mantle, producing transitional units of the two series.