ALBERT

Description: 40Ar/39Ar age spectrum analysis of phengite separates from Naxos, part of the Attic Cycladic Metamorphic Belt in Greece, indicates that cooling following high-pressure, low- to medium-temperature metamorphism, M1, occurred about 50 Ma ago. Phengite has 40Ar* gradients that suggest that part of the scatter observed in conventional K–Ar ages was caused by diffusion of radiogenic argon from the minerals during a younger metamorphism, M2. In central Naxos, this metamorphism (M2) has overprinted the original mineral assemblages completely, and is associated with development of a thermal dome. Excellent 40Ar/39Ar plateaus at 15.0 ± 0.1 Ma, 11.8 ± 0.1 Ma, and 11.4 ± 0.1 Ma, obtained on hornblende, muscovite and biotite, respectively, from the migmatite zone, indicate that relatively rapid cooling followed the M2 event, and that no significant thermal overprinting occurred subsequent to M2. Toward lower M2 metamorphic grade, 40Ar/39Ar plateau ages of hornblendes increase to 19.8 ± 0.1 Ma; concomitantly the proportion of excess 40Ar in the spectra increases as well. We propose that the peak of M2 metamorphism occurred beween 15.0 and 19.8 Ma ago. K–Ar ages of biotites from a granodiorite on the west coast are indistinguishable from those found in the metamorphic complex, and hornblende K–Ar ages from the same samples are in the range 12.1–13.6 Ma. As the latter ages are somewhat younger than most ages obtained from the metamorphic complex, intrusion of the granodiorite most likely followed the peak of the M2 metamorphism. The metamorphic evolution of Naxos is consistent with rapid crustal thickening during the Cretaceous or early Tertiary, causing conditions at which supracrustal rocks experienced pressures in the range 900–1500 MPa. Transition to normal crustal thicknesses ended the M1 metamorphism about 50 Ma ago. The M2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust, possibly related to the migration of the Hellenic volcanic ar°C in a southerly direction through the area.