White mica ⁴⁰Ar/³⁹Ar age spectra and the timing of multiple episodes of high‐P metamorphic mineral growth in the Cycladic eclogite–blueschist belt, Syros, Aegean Sea, Greece

Abstract

New geochronology from Syros in the Cycladic eclogite-blueschist belt, Aegean Sea, Greece, shows that 40 Ar/ 39 Ar geochronology consistently dates microstructural events in metamorphic rocks. We demonstrate that the age spectra depend on microstructure in a predictable and systematic way. Ages can be inferred by applying the method of asymptotes and limits to data from the step-heating experiments. The results are consistent with previously published estimates for the timing of a sequence of distinct and discrete episodes of high-P metamorphic mineral growth observed regionally across this belt. Arrhenius plots from these experiments imply that phengitic white mica is highly retentive of argon, and therefore (if these data can be extrapolated to the natural environment) the ages can be interpreted as recording the timing of episodic deformation and metamorphism. Porphyroblastic growth begins: (i) for omphacite-jadeite-eclogite facies parageneses at c. 53 Ma; and (ii) for garnetglaucophane facies parageneses at c. 47 Ma. The Kini Shear Zone started as an extensional postepidote-albite-transitional-blueschist facies shear zone that had completed operation by c. 31 Ma. The scatter in ages is due to the effect of deformation, recrystallization and multiple growth events in shear zones that continued operating for 3-6 million years from the start of each episode.

Exhumation of eclogite and blueschist (Cyclades, Greece): Pressure–temperature evolution determined by thermobarometry and garnet equilibrium modelling

“…Such shape of the P–T path for early exhumation following peak P–T conditions is also retrieved on Sifnos Island (Groppo et al., ). Moreover, this retrograde evolution of the CBU on Syros is close to that proposed by Lister and Forster (), except for their unconstrained retrograde P–T loops (Figures and ). After this first step of exhumation, a last garnet growth event was recorded at lower P–T conditions (Figures and ).…”

“…The first part of the retrograde path constrains the P–T conditions of a syn‐orogenic phase of exhumation from eclogite to blueschist facies conditions. On Syros, the timing of this first exhumation step, accommodated within the subduction channel while the overall regime was still under compression, is constrained between 50 and 35 Ma by 40 Ar/ 39 Ar and Rb/Sr ages (Cliff et al., ; Laurent et al., ; Lister & Forster, ). Syn‐orogenic exhumation kinematic indicators are well exposed on Syros, Sifnos and Tinos, showing a clear top‐to‐the E/NE sense of shear observed in preserved HP rocks (Gautier & Brun, ; Jolivet & Patriat, ; Laurent et al., ; Philippon et al., ; Roche et al., ; Trotet, Jolivet, et al., ).…”

“…The Cycladic Archipelago is located in the centre of the Aegean domain and corresponds to the deepest exhumed parts of the Hellenides–Taurides belt (Figure ). In this archipelago, the CBU, belonging to the Pindos oceanic domain (Bonneau, ; Bonneau & Kienast, ), reached peak‐pressure conditions during the formation of the Hellenides at 53–48 Ma (Figure ; Lagos et al., ; Laurent et al., ; Lister & Forster, ; Tomaschek et al., ; Uunk, Brouwer, ter Voorde, & Wijbrans, ). This HP–LT metamorphic unit was exhumed during the Eocene within the subduction channel between a top‐to‐the south thrust at the base and top‐to‐the east/northeast syn‐orogenic detachment at the top, the Vari Detachment (Figure ; Augier, Jolivet, Gadenne, Lahfid, & Driussi, ; Brun & Faccenna, ; Huet, Labrousse, & Jolivet, ; Jolivet, Faccenna, Goffé, Burov, & Agard, ; Laurent et al., ; Ring et al., , ).…”

“…(), (b) Schumacher et al. (), (c) Lister and Forster (). Mineral abbreviations are after Whitney and Evans ().…”

“…Consequently, this island has been the focus of petrological, structural and geochronological studies aimed at constraining the tectonometamorphic evolution of the CBU subduction complex (e.g. Cliff, Bond, Butler, & Dixon, ; Keiter, Ballhaus, & Tomaschek, ; Keiter, Piepjohn, Ballhaus, Lagos, & Bode, ; Lagos et al., ; Laurent et al., , ; Lister & Forster, ; Philippon, Brun, & Gueydan, ; Schumacher, Brady, Cheney, & Tonnsen, ; Soukis & Stockli, ; Tomaschek, Kennedy, Villa, Lagos, & Ballhaus, ; Trotet, Jolivet, & Vidal, ; Trotet, Vidal, & Jolivet, ). However, after a decade of investigations, the tectonometamorphic evolution of the CBU of Syros is still actively debated, as attested by the different shapes of P–T paths proposed in the literature from burial to exhumation (Figure ).…”