TSK 11 Göttingen 2006 Rambousek et al. The Serifos Metamorphic Core Complex (Greece) — kinematic investigations of the southern detachment mylonites Vortrag Christian Rambousek1 Bernhard Grasemann1 Konstantin Petrakakis1 Michael A. Edwards1 Christoph Iglseder1 András Zámolyi1 The island of Serifos is situated about 100 km SSE of Athens in the Aegean Sea and belongs to the Attic-Cycladic massif. The geology of Serifos is largely characterized by a shallow hornblende- biotite granodiorite pluton that in- truded in the late Miocene into a pre- viously deformed (under blueschist con- ditions) sequence mainly consisting of ortho- and paragneisses, calc-silicate marbles, amphibolites and schists. The pluton has a dome-shaped body occu- pying the central and southern parts of the island (Salemink 1985). The Serifos MCC is the very western continuation of a zone of syn- to post tectonic intru- sions younging from the East (Naxos, Paros ∼12Ma) to the West (Serifos 9– 8 Ma). Whereas the older intrusions in the East show a top to the North ge- ometry, the Serifos MCC has developed a South-directed low-angle detachment fault. The northern contact of the Serifos plu- ton is intrusive. In the SE, towards structurally higher levels, a core be- comes foliated with increasing intensity and, under greenschist facies conditions, transformed into S-dipping low temper- ature mylonitic to ultramylonitic rocks with abundant SSW-directed kinematic 1 Department of Geodynamics and Sedimen- tology, Structural Processes Group, University of Vienna, Austria indicators (SCC′ fabrics, sigma and delta clasts, flanking structures and mica fish). This zone forms the main greenschist facies to brittle/ductile de- tachment of the Serifos MCC. The lin- eation in these rocks has a remarkably consistent NNE-SSW direction. The fo- liation however varies and follows the dome shaped structure caused by the exhumation and unloading of the MCC. Quantitative kinematic indicators and micro structures with monoclinic sym- metry have been investigated in order to characterize the flow within the main detachment zone. It is well known that in mylonitic zones, an increase in in- tensity of deformation is normally ex- pressed by a decrease in grain size, ac- companied by recrystallisation (Berthé et al. 1979) as it can be observed in the ultramylonitic granodiorites of the SE detachment zone. Here the unde- formed core becomes foliated towards the S and, with increasing intensity, turns into a ultramylonite with quartz- feldspar- and biotite-porphyroblasts in a very fine grained matrix. Rigid objects in rocks undergoing pene- trative ductile non-coaxial flow will tend to rotate with respect to the kinematic frame of the bulk flow, and disturb the developing foliation pattern at a small adjacent domain. To investigate the ro- tational behaviour of porphyroblasts in aspect to their shape, thin sections of the ultramylonitic Hbl–Bt granodiorite of the southern detachment were ana- lyzed with the image analysis program Scion Image. Qtz, feldspar and biotite were separately plotted in aspect to their orientation (θ) and the normalized length-width ratio (B∗) of their ideal strain ellipsoid. In contrast to the plas- tically deforming quartz, the feldspar shows brittle deformation which sug- 1 Rambousek et al. TSK 11 Göttingen 2006 gests maximum deformation tempera- tures of about 350°C. Our microstruc- tural investigation reveals that feldspar grains with variable aspect ratio record no stabilization forming-clast geome- tries. In contrast Qtz clasts show both: stabilization of grains with high aspect ration inclined against the shear direc- tion and grains with low aspect ratios with no stable position. Bt always forms textbook examples of mica fish type clast with stable position inclined against the shear direction. These ob- servations from natural mylonites con- firm results from analogue and numer- ical models (e.g. Ceriani et al. 2003, Marques et al. 2005; Schmid & Pod- ladchikov 2005), which suggest a strong dependence of the shape and the clast matrix coupling on the rotational be- haviour and stable position of clasts. References Ceriani S, Mancktelow NS & Pennacchioni G (2003) Analogue modelling of the influence of shape and particle/matrix interface lubri- cation on the rotational behaviour of rigid particles in simple shear. Journal of Struc- tural Geology, 25(12), 2005–2021 Berthé D, Choukroune P & Jegouzo P (1979) Orthogneiss, mylonite and non coaxial De- formazion of granites: The example of the South Armorican Shear Zone. Journal of Structural Geology 1, 31–42 Grasemann B, Petrakakis K, Iglseder C, Ram- bousek C, Zamolyi A & Draganits E (2004) The Serifos Metamorphic Core Complex (Western Cyclades, Greece). 5thInterna- tional Symposium on Eastern Mediterranean Geology, Thessaloniki, Greece. 14-20th April 2004 Marques FO, Taborda, R & Antunes J (2005) Influence of a low-viscosity layer between rigid inclusion and viscous matrix on inclu- sion rotation and matrix flow: A numerical study. Tectonophysics, 407(1-2), 101 Salemink J (1985) Skarn and ore formation at Serifos, Greece, as a consequence of gran- odiorite intrusion. Unpublished PhD Thesis, University of Utrecht Schmid DW & Podladchikov YY (2005) Man- tled porphyroclast gauges. Journal of Struc- tural Geology, 27(3), 571 2