extensional Messaria shear zone and associated brittle detachment faults, Aegean Sea, Greece, The

Journal of the Geological Society, Jul 2005 by Kumerics, Christine, Ring, Uwe, Brichau, Stéphanie, Glodny, Johannes, Monié, Patrick

^sup 40^Ar/^sup 39^Ar data

Mylonites. White mica grains from the mylonitic metapegmatite (IK02-4) and the metasediment (IK02-8) sample were dated with the spot-ablation technique. Both samples are intensely foliated and the foliation is made up by quartz, opaque minerals and phengite, which is sheared and recrystallized in distinct foliationparallel shear zones. The sections for ^sup 40^Ar/^sup 39^Ar dating are from the most thoroughly sheared and recrystallized parts of the samples.

In each section 6-10 spots were analysed. The ages of IK02-4 range from 13.8 ±4.8 to 5.4 ± 1.8Ma and have a weighted mean age of 10.8 ± 1.1 Ma (1σ errors). The ages from IK02-8 range from 11.1 ±0.3 to 9.5 ± 0.5 Ma and have a weighted mean age of 10.5 ± 2.4 Ma (Table 2). We interpret the ages to date phengite recrystallization during ductile deformation in the Messaria shear zone.

Altherr et al. (1982) reported K/Ar and Rb/Sr white mica ages of 11-10 Ma from metasediments in the footwall of the Messaria extensional fault system. Our detailed thin-section work on samples from localities where Altherr et ai (1982) collected their samples suggests that white mica completely recrystallized during mylonitization. Therefore, we argue that mylonitization and recrystallization caused complete isotopic re-equilibration and that the K/Ar ages date mylonitization-related mineral growth. Altherr et al.'s and our ages young in a northward direction.

Granites. Muscovite and biotite crystals from granite samples IKl, IK2 and IK7 were analysed with the step-heating laserprobe technique. IK1 and IK2 are from the I-type granite in the western part of Ikaria and IK7 from the S-type granite in the south. Biotite from sample IK2 yields a plateau age of 17.89 ± 0.47 Ma (1σ errors) for c. 90% of released argon (Fig. 15a). Biotite from sample IK7 gives a plateau age of 10.66 ±0.30 Ma for c. 81% of released argon, and muscovite from the same sample yields a slightly younger but within error similar plateau age of 10.18 ± 0.46 Ma for c. 100% of released argon (Table 3, Fig. 15b and c). Biotite from sample IKl gives an age of 9.26 ± 0.90 Ma (Table 3); however, because the biotite was too small to provide a complete age spectrum this age is based on a single heating step only. The first heating steps for each sample show a large amount of atmospheric argon probably released from the surface or structural defects of the mineral at the beginning of degassing.

The 17.9Ma biotite ^sup 40^Ar/^sup 39^Ar age from sample IK2 of the I-type granite is much older than all other ages from this granite and is poorly understood. We note that the zircon fission-track age from IK2 (see below) is also distinctly older than the zircon fission-track ages from the other samples from the I-type granite. The only possible reason we can envisage for the relatively old age is that IK2 is from an older granitic xenolith in the I-type granite that was not (completely) reset or that several bodies of I-type granites were emplaced on Ikaria.