EVIDENCE FOR UHP METAMORPHISM IN STRONGLY OVERPRINTED ROCKS, RHODOPE METAMORPHIC COMPLEX, GREECE

Evidence of ultrahigh-pressure (UHP) metamorphism has now been documented in nearly every Phanerozoic collisional orogen. In most cases, the observation of coesite ± diamond positively confirms UHP metamorphism. Although evidence of UHP metamorphism in silicate phases, assemblages, and textures/fabrics can be much more obscure, subtle evidence of UHP mineral reactions may persist. To explore the factors that favor preservation of evidence of UHP metamorphism (e.g., temperature, composition, deformation), we examined three localities in the Greek Rhodope Metamorphic Complex in which coesite ± diamond were reported. The localities differ in the maximum temperatures attained (700, 750 and 800C) during peak/retrograde metamorphism and the garnet-kyanite schists and gneisses exhibit minor variations in composition.

Geochemical modeling results from several spatial scales show that 95% of garnet is modeled to grow at conditions <600C, but each of the localities preserve significantly higher temperatures. Garnet composition maps are consistent with diffusional re-equilibration, though some trace element zoning persists. These results are compatible with near complete recrystallization, which likely occurred when the rocks stalled at elevated temperature during exhumation. Zircon results support these interpretations and provide a wealth of additional information. Systematic variations in zircon textures and compositions among the three localities correlate with temperature and local composition. Zircon from the 700C locality commonly preserve three domains: (1) relict cores surrounded by (2) a thin CL-dark + complex CL-bright domain, which is itself surrounded by (3) variable thickness rims. Zircon within garnet often lack domain 3, but domain 2 is ubiquitous and thought to have formed at UHP conditions. At the 750C locality, zircon preserve all three domains in the phengite-rich matrix, but not in the plagioclase-rich matrix; zircon within garnet are nearly identical to those at the 700C locality. At the 800C locality, relict zircon cores are rare, but fragments of domain 2 are common. Nearly all of the zircon is domain 3 and consistent with high-T growth. These systematic and subtle variations preserve a robust history—even in samples that have been almost entirely overprinted during exhumation.