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

Low-temperature eclogite-facies rocks from Syros and Tinos (Cyclades, Greec e) include meta-ophiolitic blocks of unclear origin (meta-olistostrome or t ectonic melange), These blocks occur in a matrix of altered serpentinite an d/or semipelitic to tuffitic schists. Some samples are characterized by ver y high trace-element concentrations, for example, eclogites from Tinos cont ain 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 remarka bly high modal amounts of zircon, some samples from the meta-ophiolite suit e are suitable for U-Pb zircon chronology. The geological significance of z ircon ages can only be correctly interpreted by assessing whether zircon cr ystallized from a melt or formed during subsequent hydrothermal or metamorp hic processes, The results of this study lead us to conclude that infiltration of trace-el ement enriched fluids during eclogite-facies metamorphism caused local-scal e compositional changes. Textural observations document the presence of a f racture network favouring fluid infiltration under high-pressure conditions . Such a system of microfractures might hac e provided infiltration paths f or trace-element enriched fluids during earlier stages of high-pressure met amorphism. In most cases, subsequent recrystallization erased any evidence of the early infiltration channels, but inclusion of high-pressure phases i n zircon provide evidence for this model, suggesting metasomatic alteration in a subduction-zone environment. The original fluid source and the condit ions (e.g., high-F environment, highly saline brines?) under which commonly immobile elements were mobilized remain unclear. It is speculated that ecl ogitization of closely associated metagabbros may have released Zr and othe r trace elements. The results of this contribution suggest that new zircon formed during high-pressure metamorphism. Cretaceous U-Pb zircon ages are i nterpreted to date subduction metamorphism and not magmatic crystallization . Q 2001 Elsevier Science B.V. All rights reserved.