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

Abstract

High‐P rocks such as eclogite and blueschist are metamorphic markers of palaeo‐subduction zones, and their formation at high‐P and low‐T (HP–LT) conditions is relatively well understood since it has been the focus of numerous petrological investigations in the past 40 years. The tectonic mechanisms controlling their exhumation back to the surface are, however, diverse, complex and still actively debated. Although the Cycladic Blueschist Unit (CBU, Greece) is among the best worldwide examples for the preservation of eclogite and blueschist, the proposed P–T evolution followed by this unit within the Hellenic subduction zone is quite different from one study to another, hindering the comprehension of exhumation processes. In this study, we present an extensive petrological data set that permits refinement of the shape of the P–T trajectory for different subunits of the CBU on Syros. High‐resolution quantitative compositional mapping has been applied to support the thermobarometric investigations, which involve semi‐empirical thermobarometry, garnet equilibrium modelling and P–T isochemical phase diagrams. The thermodynamic models highlight the powerful use of reactive bulk compositions approximated from local bulk compositions. The results are also combined with Raman spectrometry of carbonaceous material (RSCM) to retrieve the metamorphic peak temperature distribution at the scale of the island. A major result of this study is the good agreement between all the independent thermobarometric methods, permitting reconstruction of the prograde and retrograde P–T trajectories. Garnet compositional zoning was used to retrieve prograde, peak and retrograde growth stages in line with the results of the P–T isochemical phase diagrams, RSCM temperature and peak‐pressure crystallization of the garnet–omphacite–phengite assemblage. Our results are consistent with previous thermobarometric estimates from other occurrences of CBU rocks (Tinos, Andros), suggesting a multistage exhumation process with (1) early syn‐orogenic exhumation within the subduction channel, (2) isobaric heating at mid‐crustal depths (~10–12 kbar) following thermal re‐equilibration of the lithosphere from a cold syn‐orogenic regime in the subduction zone to a warmer post‐orogenic regime in the back‐arc domain and (3) exhumation and cooling related to a post‐orogenic phase of extension following slab retreat. Expanding to the general aspects of subduction zones, we suggest that such metamorphic evolution of HP–LT units should be regarded as a characteristic feature of exhumation driven by slab rollback.

Zircon U‐Pb Chronostratigraphy and Provenance of the Cycladic Blueschist Unit and the Nature of the Contact With the Cycladic Basement on Sikinos and Ios Islands, Greece

“…These Cycladic rocks underwent at least two major Cenozoic metamorphic events, characterized by Eocene subduction‐related HP/LT blueschist to eclogite facies metamorphism and Oligocene‐Miocene greenschist to amphibolite facies retrogression during slab roll‐back and subsequent trench retreat and magmatic arc migration (Coleman et al, ; Jolivet & Brun, ; Lister et al, ; Royden, ; Schneider et al, ). Exhumation took place in two stages, the Eocene synsubduction stage and Oligo‐Miocene back‐arc extension with the exhumation of MCCs along low‐angle normal faults (e.g., Jolivet & Brun, ; Laurent et al, ; Ring et al, ). Four main detachment fault systems with divergent kinematics exhumed a series of MCCs in the Cyclades: the North Cycladic Detachment System, the Paros‐Naxos extensional fault system, the West Cycladic Detachment System, and the Santorini Detachment System (Beaudoin et al, ; Brichau et al, ; Gautier & Brun, ; Grasemann et al, ; Iglseder et al, ; Jolivet & Brun, ; Kumerics et al, ; Lister et al, ; Ring et al, ; Royden, ; Schneider et al, , Soukis & Stockli, ; Urai et al, ).…”

Slab fragmentation beneath the Aegean/Anatolia transition zone: Insights from the tectonic and metamorphic evolution of the Eastern Aegean region

“…15a) [e.g. Laurent et al, 2018]. In Pourteau et al [2010;2013], the Kurudere unit is correlated with the CBU as a whole, but the lithologies and P-T conditions are not equivalent to the typical occurrence of the CBU (Fig.…”

Tracing fluid transfers in subduction zones: an integrated thermodynamic and δ<sup>18</sup>O fractionation modelling approach

“…MnO was excluded because it overemphasizes the stability of garnet at low metamorphic conditions (T ≤ 350 °C). The conditions of the garnet-in reaction in Mn-absent systems (MORB, altered MORB) match the results obtained for garnet nucleation in natural rocks (e.g., Laurent et al, 2018). Thermodynamic modelling was performed using the internally consistent dataset of and subsequent updates (tc55, distributed with Theriak-Domino 04.02.2017, see Supplementary material S1).…”