Unusual bulk-rock compositions in eclogite-facies rocks from Syros and Tinos (Cyclades, Greece): implications for U–Pb zircon geochronology

Abstract

Low-temperature eclogite-facies rocks from Syros and Tinos (Cyclades, Greece) include meta-ophiolitic blocks of unclear origin (meta-olistostrome or tectonic mélange). These blocks occur in a matrix of altered serpentinite and/or semipelitic to tuffitic schists. Some samples are characterized by very high trace-element concentrations, for example, eclogites from Tinos contain up to 4950 ppm Zr and up to 480 ppm Y. Understanding of the geochemical controls on the petrogenesis of these rocks has important consequences for interpretation of geochronological results. The emphasis of this study is on the behaviour of Zr under eclogite-facies P– T conditions. Due to remarkably high modal amounts of zircon, some samples from the meta-ophiolite suite are suitable for U–Pb zircon chronology. The geological significance of zircon ages can only be correctly interpreted by assessing whether zircon crystallized from a melt or formed during subsequent hydrothermal or metamorphic processes. The results of this study lead us to conclude that infiltration of trace-element enriched fluids during eclogite-facies metamorphism caused local-scale compositional changes. Textural observations document the presence of a fracture network favouring fluid infiltration under high-pressure conditions. Such a system of microfractures might have provided infiltration paths for trace-element enriched fluids during earlier stages of high-pressure metamorphism. In most cases, subsequent recrystallization erased any evidence of the early infiltration channels, but inclusion of high-pressure phases in zircon provide evidence for this model, suggesting metasomatic alteration in a subduction-zone environment. The original fluid source and the conditions (e.g., high-F environment, highly saline brines?) under which commonly immobile elements were mobilized remain unclear. It is speculated that eclogitization of closely associated metagabbros may have released Zr and other trace elements. The results of this contribution suggest that new zircon formed during high-pressure metamorphism. Cretaceous U–Pb zircon ages are interpreted to date subduction metamorphism and not magmatic crystallization.