Geochemistry and Petrology of Ultramafic Rocks from the Troodos Ophiolite, Cyprus: New Perspectives on the Evolution of Boninitic Melts

The Troodos Ophiolite Complex in Cyprus consists of Neotethyan oceanic crust that formed above a subduction zone in a so-called suprasubduction zone setting (SSZ). In this geological environment, fluids from the subducting slab induce melting and metasomatism within the mantle wedge. Such processes are required for the formation of characteristic lithologies like boninites that are often associated with back-arc lavas.Because of the low incompatible element abundances of SSZ mantle rocks and the associated analytical difficulties, previous geochemical analyses mainly focused on igneous rock suites of these ophiolites. Here, we used quadrupole ICP-MS to provide a complete trace element dataset for mantle harzburgites and pyroxenites from the western part of the Troodos mantle section. It confirms that the harzburgites are highly depleted up to 30 times more depleted than the primitive mantle. Both harzburgites and pyroxenites display U-shaped trace element patterns typical for SSZ settings, caused by the refertilization of depleted mantle with fluid mobile elements. Our data confirm previous assumptions about the highly refractory nature of the above-mentioned mantle region, which likely represents the mantle source for the Troodos boninite melts. Additionally, mineral microanalyses by EPMA reveal a link between ultramafic cumulates that precipitated from boninitic melts (Shen et al. (2020), Lithos) and pyroxenite veins in the sampled mantle section. We therefore propose a multi-stage model for the boninitic melts that includes 1) reactions of the percolating melt with the mantle peridotite, 2) the formation of ultramafic cumulates near the MOHO and 3) the eruption of the boninitic lavas.