Metamorphic evolution of the Attic Cycladic Metamorphic Belt on Naxos (Cyclades, Greece) utilizing ⁴⁰Ar/³⁹Ar age spectrum measurements

Abstract

U'Ar/"Ar 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, M I , occurred about 50 Ma ago. Phengite has *"AT* 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 ( M 2 ) has overprinted the original mineral assemblages completely, and is associated with development of a thermal dome. Excellent "'Ar/'yAr plateaus at 15.0 2 0.1 Ma, 11.8 -C 0.1 Ma, and 11.4 2 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 Mz. Toward lower M I metamorphic grade, Q'Ar/"9Ar plateau ages of hornblendes increase to 19.8 2 0.1 Ma; concomitantly the proportion of excess '"Ar in the spectra increases as well. We propose that the peak of M 2 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 M 2 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 'Current address: Z. W.O. Laboratorium voor Isotopen Geologie, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands.M , metamorphism about 50 Ma ago. The M 2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust, possibly related to the migration of the Hellenic volcanic arc in a southerly direction through the area.

The nature and origin of the Barrovian metamorphism, Scotland: diffusion length scales in garnet and inferred thermal time scales

“…Wijbrans & McDougall (1988) and later Lister & Baldwin (1993) (Wijbrans & McDougall 1988) and Ios (Baldwin & Lister 1998) of the Greek Cyclades and the D'Entrecasteaux Islands of New Guinea (Lister & Baldwin 1993). In addition, sensitive highresolution ion microprobe U/Pb work carried out on migmatites from Naxos by Keay et al (2001) has demonstrated a number of distinct zircon overgrowth events associated with numerous brief metamorphic heating episodes (on Barrovian geotherms) spanning a period of about 4 Ma.…”

Multiphase boudinage: a case study of amphibolites in marble in the Naxos migmatite core

“…1990; Wijbrans and McDougall, 1988). Large-scale isoclinal folds and other deformation structures formed during D1 were overprinted by top-to-north shearing of D2, starting at the onset of crustal thinning and post-orogenic extension in the early Miocene (Mercier et al, 1976;Ring et al, 2010;Urai et al, 1990).…”

Kinematic reconstructions and magmatic evolution illuminating crustal and mantle dynamics of the eastern Mediterranean region since the late Cretaceous

“…The latter was partly accreted to the Pelagonian nappe contributing to the building of the Hellenides, while its buried part underwent early Eocene HP-LT metamorphism recorded in the Cycladic Blueschists unit and the Cycladic basement [Bonneau and Kienast, 1982;Bonneau, 1984;Wijbrans and McDougall, 1988;Tomaschek et al, 2003;Lagos et al, 2007]. These units were then progressively exhumed up to the end of the Eocene in the Cyclades and in the Olympos, Ossa and Pelion tectonic windows as a synorogenic extrusion wedge bounded by the Vari detachment on top (in Syros) and the SCSZ at the base (in Ios) (figure 9) Ring et al, , 2010Brun and Faccenna, 2008;Huet et al, 2009;Jolivet and Brun, 2010].…”

“…~15-20 kbar, ~500 °C) whose metamorphic peak is dated at ~50 Ma, forming the Cycladic Blueschists unit and the Cycladic basement [Bonneau and Kienast, 1982;Bonneau, 1984;Wijbrans and McDougall, 1988;Tomaschek et al, 2003;Lagos et al, 2007]. Exhumation of these metamorphic units first occurred between 45 and 35-30 Ma in a syn-orogenic context, following a cold retrograde path (figure 5) [Altherr et al, 1979;Bonneau and Kienast, 1982;Avigad and Garfunkel, 1989;Trotet et al, 2001].…”