High precision Lu–Hf geochronology of Eocene eclogite-facies rocks from Syros, Cyclades, Greece

Abstract

Garnet-bearing high pressure, low temperature (HP/LT) metamorphic rocks of the Cycladic Blueschist Unit have been investigated by Lu–Hf geochronology. Eclogites from Syros Island yield precise ages of 52.2 ± 0.3 Ma, 51.4 ± 0.4 Ma, and 50 ± 2 Ma. Preserved major- and trace element growth zoning in garnet suggests that the closure temperature of the Lu–Hf system in garnet was higher than the estimated peak metamorphic temperature of ∼ 500 °C. Hence, Lu–Hf ages most likely reflect garnet growth in the studied samples. Our new Lu–Hf Grt ages are in excellent agreement with previously published U–Pb SHRIMP ages of metamorphic zircon and the maximum reported white mica Rb–Sr and Ar–Ar ages from Syros. Garnet growth in all samples apparently took place over a narrow time interval because the Lu–Hf ages cluster tightly even though the distribution of Lu between cores and rims varies among samples. We find no evidence for garnet generations that are significantly older or younger than the given isochron ages. This, in conjunction with the agreement among the ages of metamorphic zircon, the maximum white mica ages, and the Lu–Hf ages, strongly suggests that the 52 Ma age dates a single HP event that affected these rocks.