ATYPICAL DISSOLUTION-REPRECIPITATION REACTIONS IN ZIRCON FROM HP/UHP METAPELITES

Cathodoluminescence (CL) imaging and electron probe microanalysis reveal distinct textures and compositional anomalies in nearly all zircon found in phosphatic-garnet bearing metapelites from the Kimi, Xanthi, and Sidironero localities of the diamondiferous Rhodope ultrahigh-pressure terrane (Greece). Zircon grains from all three Rhodope localities share these features: oscillatory CL zoned magmatic cores with embayed outer margins; a narrow CL-dark band enriched in U, P, and Y located immediately against the magmatic core; and a rim domain with convolute CL zoning, low U, and elevated Hf. The CL-dark band is interpreted as marking a reaction front produced by dissolution-reprecipitation mechanisms that propagated into the magmatic core, similar to reaction fronts observed in laboratory experiments with xenotime (Harlov & Wirth, 2012).

Although previous studies have demonstrated dissolution-reprecipitation as a mechanism that forms metamorphic rims in natural zircon, those examples lack the characteristic narrow CL-dark band enriched in U, P, and Y, which suggests that the formation of this characteristic band and the accumulation of these minor and trace elements requires specific conditions and/or geochemistry. The CL-dark band is enriched in xenotime component, suggesting decreased miscibility between zircon and xenotime. Previous experimental work indicates that increased pressure—particularly along a cool geothermal gradient—decreases miscibility between zircon and xenotime (Tomaschek & Ballhaus, 2003), thus providing a mechanism to explain the composition, formation, and location of the CL-dark band. These observations suggest that narrow CL-dark bands enriched in U, P, and Y located immediately against unreacted zircon cores may prove useful as a new indicator of metamorphism following a subduction (HP/LT) trajectory.

Identical features are observed in nearly all zircon found in restitic, phosphatic-garnet bearing metapelites located in along the western margin of the Goshen Dome, western MA. The similar and distinctive zircon reaction textures and compositions in Rhodope and western MA suggest a common P-T trajectory and/or maximum pressure.