ZR-IN-RUTILE THERMOMETRY IN BLUESCHISTS OF SYROS, GREECE

The Cycladic blueschist belt is associated with Eocene high-pressure metamorphism from Eurasia-African plate subduction and Miocene extensional overprinting. To constrain the peak metamorphic temperatures during subduction and to further our understanding of subduction dynamics, the Zr-in-rutile crystallization thermometer has been applied to blueschists and eclogites of Syros, Greece.

Samples were collected in blueschist units from across the island and include metabasalts (blueschists and eclogites) with clinopyroxene (omphacite) + garnet + glaucophane + epidote and micaschists with muscovite + glaucophane + garnet + epidote ± paragonite ± chloritoid. All samples analyzed contain rutile, quartz and zircon. Additionally, titanohematite-rutile exsolution textures were found in both a blueschist and eclogite sample.

Rutile grains were mapped by electron microprobe for Cr, Fe, Nb and Zr. Zoning is not apparent in Cr and Nb maps but Fe has minor irregular zoning. Zr maps of rutile in one sample have halos of high Zr concentration, only some of which is associated with visible (in BSE) zircon grains. Furthermore, Zr halos are larger than the zircon inclusion grain boundaries, suggesting significant secondary fluorescence. These inclusion textures emphasize the necessity of collecting X-ray maps on all samples used for thermometry.

Measured Zr concentrations range from 10 to 50 ppm with precisions of ca ±7 ppm. Rutile grains have inter-granular composition variation but lack obvious core to rim zoning. Temperatures calculated for blueschists range from 450-520 ºC and for eclogites from 500-560 ºC with uncertainties on the order of ±10 ºC (at 500 ºC). Little temperature variation is observed within suites of blueschist or eclogite from individual tectonic blocks, but temperature differences between blocks suggest that rutile thermometry may be a useful tool to constrain subduction processes.