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Alasdair Skelton - One of the best experts on this subject based on the ideXlab platform.
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an eocene oligocene blueschist Greenschist Facies p t loop from the cycladic blueschist unit on naxos island greece deformation related re equilibration vs thermal relaxation
Journal of Metamorphic Geology, 2017Co-Authors: Alexandre Peillod, Uwe Ring, Johannes Glodny, Alasdair SkeltonAbstract:Geothermobarometric and geochronological work indicates a complete Eocene/early Oligocene blueschist-/Greenschist-Facies metamorphic cycle of the Cycladic Blueschist Unit on Naxos Island in the Aegean Sea region. Using the average pressure-temperature (P–T) method of THERMOCALC coupled with detailed textural work, we separate an early blueschist-Facies event at 576 ± 16 to 619 ± 32 °C and 15.5 ± 0.5 to 16.3 ± 0.9 kbar from a subsequent Greenschist-Facies overprint at 384 ± 30 °C and 3.8 ± 1.1 kbar. Multi-mineral Rb-Sr isochron dating yields crystallization ages for near peak-pressure blueschist-Facies assemblages between 40.5 ± 1.0 and 38.3 ± 0.5 Ma. The Greenschist-Facies overprint commonly did not result in complete resetting of age signatures. Maximum ages for the end of Greenschist-Facies reworking, obtained from disequilibrium patterns, cluster near c. 32 Ma, with one sample showing rejuvenation at c. 27 Ma. We conclude that the high-pressure rocks from south Naxos were exhumed to upper mid-crustal levels in the late Eocene and early Oligocene at rates of 7.4 ± 4.6 km Ma-1, completing a full blueschist-/Greenschist-Facies metamorphic cycle soon after subduction within c. 8 Ma. The Greenschist-Facies overprint of the blueschist-Facies rocks from south Naxos resulted from rapid exhumation and associated deformation/fluid-controlled metamorphic reequilibration, and is unrelated to the strong high-temperature metamorphism associated with the Miocene formation of the Naxos migmatite dome. It follows that the Miocene thermal overprint had no impact on rock textures or Sr-isotopic signatures, and that the rocks of south Naxos underwent three metamorphic events, one more than hitherto envisaged. This article is protected by copyright. All rights reserved.
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post peak metamorphic kyanite stabilisation in Greenschist Facies metasedimentary rocks on the isle of islay sw scottish highlands
2017Co-Authors: Alexander Lewerentz, Alasdair Skelton, Colin M Graham, Curt Broman, Elisabeth DackerAbstract:Post-peak metamorphic kyanite stabilisation in Greenschist Facies metasedimentary rocks on the Isle of Islay, SW Scottish Highlands
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flux rates for water and carbon during Greenschist Facies metamorphism
Geology, 2011Co-Authors: Alasdair SkeltonAbstract:The time-averaged flux rate for a CO2-bearing hydrous fluid during Greenschist Facies regional metamorphism was estimated to be 10–10.2 ± 0.4 m3 m−2 s−1 by combining (1) Peclet numbers obtained by ...
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Regional mapping of pre-metamorphic spilitization and associated chemical mobility in Greenschist-Facies metabasalts of the SW Scottish Highlands
Journal of the Geological Society, 2010Co-Authors: Alasdair Skelton, Fredrik Arghe, Iain K. PitcairnAbstract:Abstract: Both spilitic and non-spilitic metabasaltic sills are hosted by Greenschist-Facies metasediments in the SW Scottish Highlands. Spilitization is mainly characterized by enrichment in Na 2 O, elevated modal plagioclase and epidote segregations. Mapping of the spatial distribution of spilitic metabasalts reveals an ancient sub-sea-floor fluid cell centred on the extrusive Tayvallich Volcanics. Fluid circulation was most extensive at shallow levels where most sills were spilitized. We attribute this to pervasive flow of saline fluid, which was thermally driven by the cooling suite of lava flows and sills. Spilitization below this lithostratigraphic depth was restricted to only a few sills. Their spilitization is largely unrelated to specific properties of these sills (e.g. width, chemistry or host lithology). We conclude that fluid channelling was an intrinsic property of sub-sea-floor fluid flow either at deeper levels or earlier during fluid circulation. By profiling of the size distributions of relic phenocrysts in a partly spilitized sill, we conclude that spilitization proceeds with the symmetric propagation of a spilitization front from the sill margins towards the sill interior. Based on chemical profiling across the margin of an epidote segregation, we conclude that spilitization is associated with chemical transport on scales ranging from 0.1 to 10 m.
Valerie Magnin - One of the best experts on this subject based on the ideXlab platform.
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corona formation around monazite and xenotime during Greenschist Facies metamorphism and deformation
European Journal of Mineralogy, 2020Co-Authors: Felix Hentschel, C. A. Trepmann, E. Janots, Valerie Magnin, Pierre LanariAbstract:Abstract. Epidote/allanite–fluorapatite coronae around monazite and xenotime are investigated in Permian pegmatites deformed under Greenschist-Facies conditions during Alpine tectonometamorphism in the Austroalpine basement, Eastern Alps. The aim was to evaluate the replacement reactions involved in the formation of a corona microstructure, its age and relation to deformation. In the corona core, monazite and xenotime single crystals show domains with different composition and age. Monazite (Mnz1) and xenotime (Xen1) dating by electron microprobe (EPM) reveals an age of 250–287 Ma, consistent with the Permian magmatic age of the pegmatites. These are partly replaced by secondary monazite (Mnz2) and xenotime (Xen2) compositions yielding younger Mesozoic (170–210 Ma) and Alpine (30–120 Ma) ages. The same crystallographic orientation of the primary and secondary monazite and xenotime indicates interface-coupled dissolution–precipitation reactions. Allanite U–Th–Pb dating by laser ablation inductively coupled mass spectrometry in the corona revealed an age of 60±6 Ma, interpreted as the age of corona formation. The coronae around monazite consist of an inner zone of equant fluorapatite grains surrounded by prismatic allanite, which are surrounded by epidote enriched in heavy rare earth elements (HREEs) and REE-poor epidote grains. Compared to coronae around monazite, fluorapatite has higher REE contents and no allanite occurs in the coronae surrounding the xenotime. General reactions for monazite and xenotime breakdown can be written as follows: Mnz1 + ( Si , Ca , Al , Fe , F ) fluid → Mnz2 + LREE-Ap + Aln + HREE-Ep + Ep + ( Th , U ) O 2 + ( Th , U ) SiO 4 , Xen1 + ( Si , Ca , Al , Fe , F ) fluid → Xen2 + HREE-Ap + HREE-Ep + Ep + ( Th,U ) O 2 . The amount of replacement (judged by the relative proportions of monazite and fluorapatite) is low for monazite included in tourmaline but high within the mylonitic foliation. This dependence on the degree of replacement on the local surrounding microfabric indicates that fluid availability along grain boundaries in the matrix and cracks controlled reaction advancement, allowing the elementary mass transfer required for corona formation (e.g. input of Ca, Al, Si, Fe, F). The oblate shape of the coronae aligned within the foliation of the pegmatites and the deflected foliation around the coronae, without an outer rim of prismatic epidote showing signs of deformation, indicate that the main stage of corona formation took place during deformation and reactions were still ongoing after the main stage of deformation. The corona microstructure documents replacement reactions of a single reactant into multiple distinct mineral growth zones by dissolution and precipitation processes at non-isostatic, Greenschist-Facies conditions, which prevailed in the area to the north of the Defereggen–Antholz–Vals shear zone between the middle Cretaceous and the Oligocene. These reactions ceased before being completed, and REE gradients within single grains within the corona and on the thin-section scale are preserved, which suggests restricted and/or episodic transport of REE in the fluid phase and/or availability of fluid.
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pre orogenic upper crustal softening by lower Greenschist Facies metamorphic reactions in granites of the central pyrenees
Journal of Metamorphic Geology, 2020Co-Authors: Laura Airaghi, E. Janots, Nicolas Bellahsen, Benoit Dubacq, David Chew, Claudio Rosenberg, Maxime Waldner, Valerie MagninAbstract:Pre‐kinematic Greenschist Facies metamorphism is often observed in granites and basement units of mountain belts, but rarely dated and accounted for in orogenic cycle reconstructions. Studying pre‐kinematic alteration is challenging because of its usual obliteration by subsequent syn‐kinematic metamorphism often occurring at conditions typical of the brittle–ductile transition. It is, however, to be expected that pre‐kinematic alteration has major implications for the rheology of the upper crust. In the 305 Ma‐old Variscan basement of the Bielsa massif (located in the Axial Zone of the Pyrenees), successive fluid–rock interaction events are recorded in granites below 350°C. Combined microstructural and petrographic analysis, low‐T thermobarometry and in situ U–Th/Pb dating of anatase, titanite and monazite show extensive pre‐orogenic (pre‐Alpine) and pre‐kinematic alteration related to feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270–350°C at 230–300 Ma. This event is followed by a second fluid–rock interaction stage marked by new crystallization of phyllosilicates at 200–280°C and is associated with the formation of mylonitic shear zones and fractures parallel to the shear planes. U–Pb anatase and monazite ages as well as the microtextural relationships of accessory minerals suggest an age for this event at 40–70 Ma, consistent with independent regional geology constraints. The Variscan basement was therefore softened at late to post‐Variscan time, at least 150–200 Ma before the main Alpine shortening while Alpine‐age compression (c. 35–50 Ma) leads to the formation of a dense net of mylonites. The associated deformation, both distributed at the scale of the Bielsa massif and localized at decametric scale in mylonitic corridors, precedes the strain localization along the major thrusts of the Axial Zone. The Bielsa massif is a good example where inherited, pre‐orogenic upper crustal softening controls the deformation patterns in granitic basement units through low‐grade metamorphic reactions.
Robert A. Cliff - One of the best experts on this subject based on the ideXlab platform.
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Geochronological challenges posed by continuously developing tectonometamorphic systems; insights from Rb–Sr mica ages from the Cycladic Blueschist Belt, Syros (Greece)
Journal of Metamorphic Geology, 2016Co-Authors: Robert A. Cliff, Clare E. Bond, R. W. H. Butler, John E. DixonAbstract:Is metamorphism and its causative tectonics best viewed as a series of punctuated events or as a continuum? This question is addressed through examination of the timing of exhumation of the Cycladic Blueschist Belt. The cause of scatter beyond analytical error in Rb–Sr geochronology was investigated using a suite of 39 phengite samples. Rb–Sr ages have been measured on phengite microsamples drilled from specific microstructures in thin sections of calcschists and metabasites from the Cycladic Blueschist Belt on Syros. The majority are from samples that have well-preserved blueschist Facies mineral assemblages with limited Greenschist Facies overprint. The peak metamorphic temperatures involved are below the closure temperature for white mica so that crystallization ages are expected to be preserved. This is supported by the coexistence of different ages in microstructures of different relative age; in one sample phengite from the dominant extensional blueschist Facies fabric preserves an age of 35 Ma while post-tectonic mica, millimetres away, has an age of 26 Ma. The results suggest that micro-sampling techniques linked to detailed microstructural analysis are critical to understanding the timing and duration of deformation in tectono-metamorphic systems. North of the Serpentinite Belt in northern Syros, phengite Rb–Sr ages are generally between 53 and 46 Ma, comparable to previous dates from this area. South of the Serpentinite Belt phengite in blueschist Facies assemblages associated with extensional fabrics linked to exhumation have ages that range from 42 Ma down to c. 30 Ma indicating that extensional deformation while still under blueschist Facies conditions continued until 30 Ma. No age measurements on samples with unambiguous evidence of deformation under Greenschist Facies conditions were made; two rocks with Greenschist Facies assemblages gave phengite ages that overlap with the younger blueschist samples, suggesting blueschist Facies phengite is preserved in these rocks. Two samples yielded ages below 27 Ma; one is from a post-tectonic microstructure, the other from a Greenschist in which the fabric developed during earlier blueschist Facies conditions. These ages are consistent with previous evidence of Greenschist Facies conditions from c. 25 Ma onwards. The data are consistent with a model of deformation that is continuous on a regional scale. This article is protected by copyright. All rights reserved.
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Dating deformation using Rb‐Sr in white mica: Greenschist Facies deformation ages from the Entrelor shear zone, Italian Alps
Tectonics, 1997Co-Authors: S. R. Freeman, Simon Inger, Robert W. H. Butler, Robert A. CliffAbstract:Ages of deformation have been obtained by Rb-Sr analysis of white micas whose microstructural and chemical characteristics indicate that they crystallized or recrystallized during shear fabric formation. Since white micas commonly define deformation fabrics in medium-grade metamorphic rocks, these ages can be directly related to structural geometries with regional context. This direct method contrasts with estimates of midcrustal deformation ages derived from cooling histories because it does not rely on assumptions about the thermal structure of the crust. It does require that the dated minerals attained isotopic equilibrium with the dominant Sr reservoir at temperatures lower than the closure temperature. This resetting was apparently achieved during dynamic recrystallization of white micas in Greenschist-Facies metasediments and metagranitoid units in the western Alps. The results suggest that the Sr isotopic composition of the new mica is buffered by the coexisting high-Sr phases (calcite, feldspar or epidote) via the grain boundary network. High-strain rocks from the Entrelor shear zone system of the western Alps have yielded indistinguishable white mica Rb-Sr ages along 30 km of individual and kinematically linked shear zones. The age of the back-thrusting event is constrained at 34±1Ma, the age yielded by the younger generation of synkinematically crystallized white micas. This event was short-lived, involving at least 20 km of shortening in ∼1 m.y. or less. An earlier, variably overprinted component, dating from 38 to 37 Ma, has been identified in the mica fabric, but its kinematic significance is uncertain. This method of dating strain fabrics offers a powerful tool for tectonic studies, since isotopic resetting can be directly linked to structural geometries, microstructural textures, and PT conditions. It allows testing of kinematic models in orogens and can provide important information on the rates of geological processes in the crust.
Robert P Wintsch - One of the best experts on this subject based on the ideXlab platform.
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reaction softening by dissolution precipitation creep in a retrograde Greenschist Facies ductile shear zone new hampshire usa
Journal of Metamorphic Geology, 2017Co-Authors: Ryan J Mcaleer, David L Bish, Michael J Kunk, Karri R Sicard, Peter M Valley, Gregory J Walsh, Bryan Wathen, Robert P WintschAbstract:We describe strain localization by a mixed process of reaction and microstructural softening in a lower Greenschist Facies ductile fault zone that transposes and replaces middle to upper amphibolite Facies fabrics and mineral assemblages in the host Littleton Schist near Claremont, New Hampshire. Here, Na-poor muscovite and chlorite progressively replace first staurolite, then garnet, and finally biotite porphyroblasts as the core of the fault zone in approached. Across the transect, higher-grade fabric-forming Na-rich muscovite is also progressively replaced by fabric forming Na-poor muscovite. The mineralogy of the new phyllonitic fault-rock produced is dominated by Na-poor muscovite and chlorite together with late albite porphyroblasts. The replacement of the amphibolite Facies porphyroblasts by muscovite and chlorite is pseudomorphic in some samples and shows that the chemical metastability of the porphyroblasts is sufficient to drive replacement. In contrast, element mapping shows that fabric-forming Na-rich muscovite is selectively replaced at high-strain microstructural sites, indicating that strain energy played an important role in activating the dissolution of the compositionally metastable muscovite. The replacement of strong, high-grade porphyroblasts by weaker Na-poor muscovite and chlorite constitutes reaction softening. The crystallization of parallel and contiguous mica in the retrograde foliation at the expense of the earlier and locally crenulated Na-rich muscovite-defined foliation destroys not only the metastable high-grade mineralogy, but also its stronger geometry. This process constitutes both reaction and microstructural softening. The deformation mechanism here was thus one of dissolution-precipitation creep, activated at considerably lower stresses than might be predicted in quartzo-feldspathic rocks at the same lower Greenschist Facies conditions. This article is protected by copyright. All rights reserved.
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40ar 39ar dating of silurian and late devonian cleavages in lower Greenschist Facies rocks in the westminster terrane maryland usa
Geological Society of America Bulletin, 2010Co-Authors: Michael J Kunk, Robert P Wintsch, Bridget K Mulvey, C S SouthworthAbstract:40 Ar/ 39 Ar dating of muscovite, biotite, and K-feldspar combined with microstructural analysis of lower Greenschist-Facies, polymetamorphic, phyllitic rocks, and marbles were successfully used to decipher the thermal and tectonic histories of the Westminster and adjacent terranes in western Maryland. The presence of unreset detrital muscovite in some samples demonstrates that temperatures in these rocks never exceeded the closure temperature for argon diffusion in muscovite, ∼350 ± 50 °C. Minor biotite in some arkoses constrains the minimum metamorphic temperatures to ≥∼320 °C. These data show an Early Silurian (ca. 430 Ma) cleavage in the western part of the Westminster terrane and a Late Devonian event (ca. 370 Ma) in the eastern Westminster and adjacent Potomac terranes. These two cleavage domains are separated by the NE-trending, newly identified Parrs Ridge fault zone. We propose that the sinistral transpressive collision of the Carolina terrane with Laurentia emplaced the western portion of the Westminster terrane in the Pennsylvania embayment along the Martic fault where it was folded and cleaved at ca. 430 Ma but otherwise largely sheltered from later deformation. The later Late Devonian dextral transpressive accretion of the outboard Potomac terrane thrust rocks of the eastern Westminster and Potomac terranes to the west, causing Late Devonian (360–370 Ma) S 2 cleavage in these rocks, but only minimal discrete overprinting S 3 cleavages in rocks farther west. Final juxtaposition and thermal convergence of these terranes occurred along reactivated dextral strike-slip faults in the Alleghanian at ca. 300 Ma.
Michael Brocker - One of the best experts on this subject based on the ideXlab platform.
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the geological significance of 40ar 39ar and rb sr white mica ages from syros and sifnos greece a record of continuous re crystallization during exhumation
Journal of Metamorphic Geology, 2013Co-Authors: Michael Brocker, Suzanne L Baldwin, R ArkudasAbstract:The Attic-Cycladic crystalline belt in the central Aegean region records a complex structural and metamorphic evolution that documents Cenozoic subduction zone processes and exhumation. A prerequisite to develop an improved tectono-metamorphic understanding of this area is dating of distinct P–T–D stages. To evaluate the geological significance of phengite ages of variably overprinted rocks, 40Ar/39Ar and Rb–Sr analyses were undertaken on transitional blueschist–Greenschist and Greenschist Facies samples from the islands of Syros and Sifnos. White mica geochronology indicates a large age variability (40Ar/39Ar: 41–27 Ma; Rb–Sr: 34–20 Ma). Petrologically similar samples have either experienced Greenschist Facies overprinting at different times or variations in ages record variable degrees of Greenschist Facies retrogression and incomplete resetting of isotopic systematics. The 40Ar/39Ar and Rb–Sr data for metamorphic rocks from both islands record only minor, localized evidence for Miocene ages (c. 21 Ma) that are well documented elsewhere in the Cyclades and interpreted to result from retrogression of high-pressure mineral assemblages during lower pressure metamorphism. Field and textural evidence suggests that heterogeneous overprinting may be due to a lack of permeability and/or limited availability of fluids in some bulk compositions and that retrogression was more or less parallel to lithological layering and/or foliation as a result of, possibly deformation-enhanced, channelized fluid ingress. Published and new 40Ar/39Ar and Rb–Sr data for both islands indicate apparent age variations that can be broadly linked to mineral assemblages documenting transitional blueschist-to-Greenschist- and/or Greenschist Facies metamorphism. The data do not record the timing of peak HP metamorphism, but may accurately record continuous (partial) resetting of isotopic systematics and/or (re)crystallization of white mica during exhumation and Greenschist Facies retrogression. The form of 40Ar/39Ar phengite age spectra are complex with the lowest temperature steps yielding Middle to Late Miocene ages. The youngest Rb–Sr ages suggest maximum ages of 20.6 ± 0.8 Ma (Syros) and 22.5 ± 0.6 Ma (Sifnos) for the timing of Greenschist Facies overprinting. The results of this study further accentuate the challenges of interpreting isotopic data for white mica from polymetamorphic terranes, particularly when mixing of populations and/or incomplete resetting of isotopic systematics occurs during exhumation. These data capture the full range of isotopic age variations in retrogressed HP rocks documented in previous isotopic studies, and can be interpreted in terms of the geodynamic evolution of the Aegean.
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the base of the cycladic blueschist unit on tinos island greece re visited field relationships phengite chemistry and rb sr geochronology
Neues Jahrbuch Fur Mineralogie-abhandlungen, 2005Co-Authors: Michael Brocker, L FranzAbstract:The Cyclades archipelago in the Aegean Sea is an important study area for subduction-related metamorphism and the exhumation of high-pressure/low-temperature rocks. Of special importance for interpretation of the general tectonic development in the central Aegean region are tectonic windows that expose the rock sequences below the Cycladic Blueschist Unit (CBU). Previous work suggested that the lowermost dolomite-phyllite-quartzite sequence on Tinos Island represents such a tectonic sub- unit with a metamorphic and deformational history that is different to the overlying blueschist- and Greenschist-Facies rocks. The tectonic contact was interpreted as a thrust fault. A re-evaluation of the arguments used to support this interpretation suggests that this conclusion is questionable. Previous studies inferred that the basal sequences only underwent Greenschist-Facies metamor- phism and were not affected by a high-pressure event. However, mineral assemblage and phengite composition in the basal phyl- lites cannot be distinguished from those of overlying rock sequences, which undoubtedly have experienced high-pressure met- amorphism and a pervasive Greenschist-Facies overprint. Rb - Sr geochronology of phyllites and quartzites (phengite - whole rock pairs), previously interpreted to belong to the lower plate, yielded dates that are indistinguishable from values obtained for strongly overprinted rocks collected at higher lithostratigraphic levels. It can also be shown that sedimentary structures are pre- served in many places within the CBU. The presumed absence of such features was originally interpreted as a major contrast to the fossil-bearing basal sequences, indicative for different deformational styles. We postulate that the para-autochthonous basal unit beneath the CBU is not exposed on Tinos Island. Field observations, petrological and geochronological data of the Panormos area are fully compatible with the interpretation that the dolomite-phyllite-quartzite succession is an integral part of the CBU, as originally suggested by Melidonis (1980).
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high si phengite records the time of Greenschist Facies overprinting implications for models suggesting mega detachments in the aegean sea
Journal of Metamorphic Geology, 2004Co-Authors: Michael Brocker, D Bieling, Bradley R Hacker, Phillip B GansAbstract: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 high- pressure/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. 24Ma. 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.
