EVIDENCE OF ULTRAHIGH-PRESSURE METAMORPHISM IN ZIRCON FROM THE RHODOPE METAMORPHIC COMPLEX, EASTERN GREECE

Evidence of metamorphism at ultrahigh-pressure (UHP) conditions is documented by the presence of coesite, diamond, and/or majoritic garnet. Overprinting during exhumation can obscure this record, making it difficult to positively identify UHP rocks. In this study, we investigated zircon grains with complex metamorphic rims to evaluate if zircon preserves a record of UHP metamorphism. We selected garnet-kyanite schists from two UHP localities–Kimi and Xanthi–within the Rhodope Metamorphic Complex (eastern Greece). Both localities have microdiamond, thus confirming metamorphism at UHP, but the pressure-temperature (P-T) paths differ, which provides an opportunity to evaluate how P-T conditions impact the transformation and preservation of evidence of UHP metamorphism.

We used cathodoluminescence (CL) imaging to identify discrete domains within zircon and collected geochemical and geochronological data using microbeam techniques with 10 to 25 um spots. Zircon from both localities have core and rim domains separated by a characteristic transition domain. The cores range in age from 2.5 Ga to 250 Ma and exhibit a negative Eu* anomaly, a Yb/Gd of 10–100 and variable Th/U (0–1.2), consistent with magmatic growth. Rims from Kimi range from 150 Ma to 125 Ma and have a negative Eu* anomaly, Yb/Gd is 0.1–1 and Th/U is 0–0.2. Rims from Xanthi range from 150 to 124 Ma and lack a Eu* anomaly, Yb/Gd ranges from 0.1–10 and Th/U is 0–0.1. Transition domains vary between 165 and 145 Ma. The transition domain is largest in zircon from Xanthi, with Yb/Gd ranging between 0–1000 and a Th/U of 0–0.5. Because the transition domains are narrow in grains from Kimi, only a few spot analyses were possible, but the data are consistent with those from Xanthi. We interpret the distinctive CL textures and unique Yb/Gd of the transition domains as evidence of UHP. The extent and preservation of transition domains appear to be functions of the differing P-T paths. The narrow transition domains and larger granulite facies metamorphic rims in the Kimi grains are a combined product of overprinting at higher peak temperatures during exhumation. Work is ongoing to determine the specific metamorphic reactions that yield the observed Yb/Gb ratio in transition zones, but reactions involving garnet and/or zircon recrystallization offer possible explanations.