High‐Si phengite records the time of greenschist facies overprinting: implications for models suggesting mega‐detachments in the Aegean Sea

Abstract

In the lower main unit of the Attic-Cycladic crystalline belt (Greece), white mica geochronology (Rb-Sr, K-Ar, 40 Ar-39 Ar) has established the timing of at least two metamorphic events: well-preserved highpressure/low-temperature (HP/LT) rocks yielded Eocene ages (c. 53-40 Ma) and their greenschist facies counterparts provided Oligocene-Miocene dates (c. 25-18 Ma). Marbles from Tinos Island contain high-Si phengite with Rb-Sr (phengite-calcite) and 40 Ar-39 Ar white mica ages between 41 and 24 Ma. All Ar age spectra are disturbed and 40 Ar-39 Ar total fusion ages generally are 3-6 Ma older than corresponding Rb-Sr ages. Due to the polymetamorphic history, we consider inheritance from the HP stage as the most likely cause for the complex Ar age spectra and the older 40 Ar-39 Ar dates. This concept also suggests that the Rb-Sr system is more sensitive to modification during overprinting than the Ar isotope system, because resetting of the Sr isotope system can be accomplished more quickly by Sr exchange with other Ca-rich phases, whereas lack of pervasive deformation and/or restricted availability of synmetamorphic fluids has favoured partial inheritance of the Ar isotope system. On Tinos, the lowermost part of the metamorphic succession has experienced a pervasive greenschist facies overprint. Si-rich phengite from marbles representing this lithostratigraphic level yielded Rb-Sr ages of c. 24 Ma. If the earlier metamorphic history is not taken into account, such data sets may lead to the erroneous conclusion of Miocene HP metamorphism. This study indicates that this phengite experienced pervasive rejuvenation of the Rb-Sr isotope system during overprinting, without significant changes in Si content, due to bulk-compositional constraints. This leads to the conclusion that in the absence of critical mineral assemblages the Si value of phengite is not a reliable indicator for metamorphic pressures in impure marbles. Recent studies have reported large displacements (>100 km) for detachment faults in the Aegean Sea. A critical parameter for such models is the age of HP metamorphism as deduced from white mica dating in the basal units of the Cyclades. We question the underlying idea of Miocene HP metamorphism and suggest, instead, that this age constrains the timing of the greenschist facies overprint and that the existence of mega-detachments in the study area requires further investigation.

The geological signature of a slab tear below the Aegean

“…(Altherr et al, 1982;Henjes-Kunst et al, 1988;Wijbrans and McDougall, 1988;Schliestedt et al, 1994;Keay et al, 2001;Hejl et al, 2002;Parra et al, 2002;Ring et al, 2003;Bröcker et al, 2004;Jolivet et al, 2004a;Kumerics et al, 2005;Brichau et al, 2006;Bröcker and Franz, 2006;Duchêne et al, 2006;Martin et al, 2006;Brichau et al, 2007;Skarpelis et al, 2008;Baltatzis et al, 2009;Iglseder et al, 2009;Liati et al, 2009;Seward et al, 2009;Bolhar et al, 2010;Brichau et al, 2010;Denèle et al, 2011;Ring et al, 2011;Schneider et al, 2011;Grasemann et al, 2012;Bargnesi et al, 2013;Berger et al, 2013;Bröcker et al, 2013;Soukis and Stockli, 2013;Beaudoin et al, 2014;Huet et al, 2014;Laurent et al, 2015) A C C E P T E D M A N U S C R I P T…”

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

“…~9 kbar, 550-570 °C) followed by a greenschist-facies retrograde metamorphism observed in the Andros, Tinos and Kea islands, in Lavrion (figure 6) [Altherr et al, 1982; Bröcker et al, 2004;Jolivet and Brun, 2010;Grasemann et al, 2012;Huet et al, 2014;Scheffer et al, 2015] and in the Olympos, Ossa and Pelion tectonic windows [Schermer, 1993;Lips et al, 1999;Lacassin et al, 2007]. In the lower Miocene, HT-MP metamorphism (i.e.…”

Later stages of evolution of an epithermal system: Au–Ag mineralizations at Apigania Bay, Tinos Island, Cyclades, Hellas, Greece

“…Granodiorite have intruded along syn-magmatic ductile faults (Boronkay 1995;Mastrakas and St. Seymour 2000;Zeffren et al 2005), while its contact with the country rocks served as a conduit for a boron-and fluorine-rich leucogranite, which formed peripherally to the granodiorite (Mastrakas 2007). The Tinos granodiorite was emplaced during the Mid-to Late-Miocene in a transpressional regime (Mastrakas and St. Seymour 2000;Bröcker et al 2004), and subsequently deformed under ductile to brittle conditions in a Late Miocene to Pliocene transtensional setting (Boronkay and Doutsos 1994;Mastrakas and St. Seymour 2000). K-Ar ages of ca.…”