Understanding phengite argon closure using single grain fusion age distributions in the Cycladic Blueschist Unit on Syros, Greece

Uunk, Bertram; Brouwer, Fraukje M.; Voorde, M. ter; Wijbrans, J. R.

doi:10.1016/j.epsl.2017.12.031

Cited by 46 publications

(75 citation statements)

References 52 publications

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“…This c . 340 Ma age pattern is, thus, in agreement with the partial preservation of a crystallization age from the prograde metamorphic path (e.g., Uunk, Brouwer, ter Voorde, & Wijbrans, ; Warren, Hanke, & Kelley, ). Apart from this later sample, all ages range from 328 to 319 Ma.…”

Section: Discussionsupporting

confidence: 81%

“…The spectrum of this latter c. 340 Ma age (grain 02, Figure 11a) exhibits a complex shape, which might reflect some recrystallization processes. This c. 340 Ma age pattern is, thus, in agreement with the partial preservation of a crystallization age from the prograde metamorphic path (e.g., Uunk, Brouwer, ter Voorde, & Wijbrans, 2018;Warren, Hanke, & Kelley, 2012). Apart from this later sample, all ages range from 328 to 319 Ma.…”

Section: Hp Metasedimentary Unitsupporting

confidence: 85%

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Tectonometamorphic evolution of the Atbashi high‐Punits (KyrgyzCAOB, Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

Loury

Rolland

Guillot

et al. 2018

Journal Metamorphic Geology

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The South Tien Shan (STS) belt results from the last collision event in the western Central Asian Orogenic Belt (CAOB). Understanding its formation is of prime importance in the general framework of the CAOB. The Atbashi Range preserves high‐P (HP) rocks along the STS suture, but still, its global metamorphic evolution remains poorly constrained. Several HP units have been identified: (a) a HP tectonic mélange including boudins of mafic eclogites in a sedimentary matrix, (b) a large (>100 km long) high‐P metasedimentary unit (HPMU) and (c) a lower blueschist facies accretionary prism. Raman Spectroscopy on carbonaceous material combined with phengite and chlorite multiequilibria and isochemical phase diagram modelling indicates that the HPMU recorded homogeneous P–T conditions of 23–25 kbar and 560–570°C along the whole unit. 40Ar/39Ar dating on phengite from the HPMU ranges between 328 and 319 Ma at regional scale. These ages are interpreted as (re‐) crystallization ages of phengite during Tmax conditions at a pressure range of 20–25 kbar. Thermobarometry on samples from the HP tectonic mélange provides similar metamorphic peak conditions. Thermobarometry on the blueschist to lower greenschist facies accretionary prism indicates that it underwent P–T conditions of 5–6 kbar and 290–340°C, highlighting a 17–20 kbar pressure gap between the HPMU‐tectonic mélange units and the accretionary prism. Comparison with available geochronological data suggests a very short time span between the prograde path (340 Ma), HP metamorphic peak (330 Ma), the Tmax (328–319 Ma) and the final exhumation of the HPMU (303–295 Ma). Extrusion of the HPMU, accommodated by a basal thrust and an upper detachment, was driven by buoyant forces from 70–75 km up to 60 km depth, which directly followed continental subduction and detachment of the HPMU. At crustal depths, extrusion was controlled by collisional tectonics up to shallow levels. Lithological homogeneity of the HPMU and its continental‐derived character from the North Tien Shan suggest this unit corresponds to the hyper‐extended continental margin of the Kazakh continent, subducted southward below the north continental active margin of the Tarim craton. Integration of the available geological data allows us to propose a general geodynamic scenario for Tien Shan during the Carboniferous with a combination of (a) N‐dipping subduction below the Kazakh margin of Middle Tien Shan until 390–340 Ma and (b) S‐dipping subduction of remaining Turkestan marginal basins between 340 and 320 Ma.

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Section: Discussionsupporting

confidence: 81%

Section: Hp Metasedimentary Unitsupporting

confidence: 85%

Tectonometamorphic evolution of the Atbashi high‐Punits (KyrgyzCAOB, Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

Loury

Rolland

Guillot

et al. 2018

Journal Metamorphic Geology

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“…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., , ).…”

Section: Geological Contextmentioning

confidence: 99%

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

Laurent

Lanari

Naïr

et al. 2018

Journal Metamorphic Geology

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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.

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“…Phengite v. muscovite diffusion parameters Uunk et al (2018) used muscovite parameters as determined by Harrison et al (2009) to model results for their phengite geochronology on Syros. The effect of different P-T-t paths was modelled using activation volume to correct for the effects of pressure.…”

Section: Microstructures At the Base Of The Upper Platementioning

confidence: 99%

“…The effect of different P-T-t paths was modelled using activation volume to correct for the effects of pressure. Uunk et al (2018) require cooling below greenschist facies temperatures before closure occurs. This result can be extrapolated to the eclogite-blueschist unit on Ios, in which case exhumation-related cooling would need to have occurred at least 8-10 myr earlier than the onset of the operation of the SCSZ.…”

Section: Microstructures At the Base Of The Upper Platementioning

confidence: 99%

⁴⁰ Ar/ ³⁹ Ar thermochronology in the Ios basement terrane resolves the tectonic significance of the South Cyclades Shear Zone

Forster

Koudashev

Nie

et al. 2019

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We conducted 39 Ar diffusion experiments using potassium feldspar from the South Cyclades Shear Zone on Ios, in the Cyclades, Aegean Sea, Greece. Irradiated samples were step-heated in an ultra-high-vacuum resistance furnace attached to a mass spectrometer, thus also allowing 40 Ar/ 39 Ar geochronology. Conjoint inversion of these datasets allowed estimation of the relevant diffusion parameters, which were then used to forward model the effect of arbitrary temperature-time histories. Simulations used Monte Carlo methods to improve approximations to the observed age spectra. Two periods of rapid cooling could be inferred. The South Cyclades Shear Zone commenced operation during or shortly after the Eocene-Oligocene transition. Episodes of south-directed movement continued into Early Miocene time, however, with the footwall still hot enough to cause biotite ± garnet metamorphic mineral growth at the base of the overlying, already substantially exhumed, eclogite-blueschist unit. Since its footwall continued to cool, the South Cyclades Shear Zone was an extensional shear zone during both episodes of its operation.

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Understanding phengite argon closure using single grain fusion age distributions in the Cycladic Blueschist Unit on Syros, Greece

Cited by 46 publications

References 52 publications

Tectonometamorphic evolution of the Atbashi high‐Punits (KyrgyzCAOB, Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

Tectonometamorphic evolution of the Atbashi high‐Punits (KyrgyzCAOB, Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

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

⁴⁰ Ar/ ³⁹ Ar thermochronology in the Ios basement terrane resolves the tectonic significance of the South Cyclades Shear Zone

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Understanding phengite argon closure using single grain fusion age distributions in the Cycladic Blueschist Unit on Syros, Greece

Cited by 46 publications

References 52 publications

Tectonometamorphic evolution of the Atbashi high‐Punits (KyrgyzCAOB, Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

Tectonometamorphic evolution of the Atbashi high‐Punits (KyrgyzCAOB, Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

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

40 Ar/ 39 Ar thermochronology in the Ios basement terrane resolves the tectonic significance of the South Cyclades Shear Zone

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⁴⁰ Ar/ ³⁹ Ar thermochronology in the Ios basement terrane resolves the tectonic significance of the South Cyclades Shear Zone