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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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rapid fluid flow along fractures at Greenschist facies conditions on syros greece
American Journal of Science, 2016Co-Authors: Barbara Irene Kleine, Zhihong Zhao, Alasdair SkeltonAbstract:This thesis aims to provide a better understanding of the role of mountain building in the carbon cycle. The amount of CO2 released into the atmosphere due to metamorphic processes is largely unknown. To constrain the quantity of CO2 released, fluid-driven reactions in metamorphic rocks can be studied by tracking fluid-rock interactions along ancient fluid flow pathways. The thesis is divided into two parts: 1) modeling of fluid flow rates and durations within shear zones and fractures during Greenschist- and blueschist-facies metamorphism and 2) the assessment of possible mechanisms of fluid infiltration into rocks during Greenschist- to epidote-amphibolite-facies metamorphism and controlling chemical and mineralogical factors of reaction front propagation.On the island Syros, Greece, fluid-rock interaction was examined along a shear zone and within brittle fractures to calculate fluid flux rates, flow velocities and durations. Petrological, geochemical and thermodynamic evidence show that the flux of CO2-bearing fluids along the shear zone was 100-2000 times larger than the fluid flux in the surrounding rocks. The time-averaged fluid flow velocity and flow duration along brittle fractures was calculated by using a governing equation for one-dimensional transport (advection and diffusion) and field-based parameterization. This study shows that fluid flow along fractures on Syros was rapid and short lived.Mechanisms and controlling factors of fluid infiltration were studied in Greenschist- to epidote-amphibolite-facies metabasalts in SW Scotland. Fluid infiltration into metabasaltic sills was unassisted by deformation and occurred along grain boundaries of hydrous minerals (e.g. amphibole) while other minerals (e.g. quartz) prevent fluid infiltration. Petrological, mineralogical and chemical studies of the sills show that the availability of reactant minerals and mechanical factors, e.g. volume change in epidote, are primary controls of reaction front propagation.
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Preservation of Blueschist-facies Minerals along a Shear Zone by Coupled Metasomatism and Fast-flowing CO2-bearing Fluids
Journal of Petrology, 2014Co-Authors: Barbara Irene Kleine, Alasdair Skelton, Benjamin Huet, Iain K. PitcairnAbstract:This thesis aims to provide a better understanding of the role of mountain building in the carbon cycle. The amount of CO2 released into the atmosphere due to metamorphic processes is largely unknown. To constrain the quantity of CO2 released, fluid-driven reactions in metamorphic rocks can be studied by tracking fluid-rock interactions along ancient fluid flow pathways. The thesis is divided into two parts: 1) modeling of fluid flow rates and durations within shear zones and fractures during Greenschist- and blueschist-facies metamorphism and 2) the assessment of possible mechanisms of fluid infiltration into rocks during Greenschist- to epidote-amphibolite-facies metamorphism and controlling chemical and mineralogical factors of reaction front propagation.On the island Syros, Greece, fluid-rock interaction was examined along a shear zone and within brittle fractures to calculate fluid flux rates, flow velocities and durations. Petrological, geochemical and thermodynamic evidence show that the flux of CO2-bearing fluids along the shear zone was 100-2000 times larger than the fluid flux in the surrounding rocks. The time-averaged fluid flow velocity and flow duration along brittle fractures was calculated by using a governing equation for one-dimensional transport (advection and diffusion) and field-based parameterization. This study shows that fluid flow along fractures on Syros was rapid and short lived.Mechanisms and controlling factors of fluid infiltration were studied in Greenschist- to epidote-amphibolite-facies metabasalts in SW Scotland. Fluid infiltration into metabasaltic sills was unassisted by deformation and occurred along grain boundaries of hydrous minerals (e.g. amphibole) while other minerals (e.g. quartz) prevent fluid infiltration. Petrological, mineralogical and chemical studies of the sills show that the availability of reactant minerals and mechanical factors, e.g. volume change in epidote, are primary controls of reaction front propagation.
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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 ...
Gernold Zulauf - One of the best experts on this subject based on the ideXlab platform.
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the tectonometamorphic and magmatic evolution of the uppermost unit in central crete melambes area constraints on a late cretaceous magmatic arc in the internal hellenides greece
Gondwana Research, 2017Co-Authors: Silviu O Martha, Wolfgang Dorr, Axel Gerdes, Jochen Krahl, Jolien Linckens, Gernold ZulaufAbstract:Abstract Crete consists of a nappe pile that formed during Alpine subduction and collision. The lower nappes belong to the External Hellenides, whereas the uppermost nappe is ascribed to the Pelagonian Zone of the Internal Hellenides. The Uppermost Unit consists of several subunits including the Asterousia Crystalline Complex (ACC), which comprises metasedimentary rocks, (meta)granitoids and serpentinite, the protolith age and the tectonometamorphic evolution of which are largely unknown. In the present study, we present new structural, microfabric and geochronological data from the Uppermost Unit in the Melambes area (central Crete). 206Pb/238U zircon ages (LA-ICP-MS and ID-TIMS) indicate granitic and dioritic intrusions between 71.9 ± 0.6 and 76.9 ± 0.3 Ma. Identical ages have previously been obtained from comparable intrusions in eastern Crete and on Anafi. The composition and geochemical signature suggest an extended magmatic arc along the southern active margin of the Pelagonian-Lycian Block. Post-intrusive shearing transformed granite into orthogneiss, whereas diorite remained free from foliation, because of the lower amount of mechanically weak phases. Deformation microfabrics suggest top-to-the SE shearing under amphibolite facies conditions of the ACC and at Greenschist facies conditions of rocks at the base of the ACC referred to as Akoumianos Greenschist. The Akoumianos Greenschist is considered as the northern part of the Pindos realm that was subducted underneath the Pelagonian-Lycian active margin. Based on our new and on published data, the following orogenic stages are suggested to have contributed to the evolution of the Hellenides during the Late Cretaceous to Eocene: (1) pre-middle Campanian collision and subduction of the Pindos lithosphere underneath the southern margin of the Pelagonian-Lycian terrane led to obduction and offscraping of serpentinized ocean floor and stacking of the ACC during amphibolite facies top-to-the SE thrusting, (2) formation of a Campanian magmatic arc along the Pelagonian-Lycian active margin; (3) Maastrichtian collision and stacking of the magmatic arc during top-to-the SE mylonitic shearing; (4) Palaeocene top-to-the SE Greenschist-facies shearing of the ACC on top of the Akoumianos Greenschist; (5) Late Eocene thrusting of the Uppermost Unit on top of the Arvi and Pindos units. Thus, top-to the SE was the dominant shear sense in the southern Aegean from at least the mid-Late Cretaceous until the Eocene.
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new structural and u pb zircon data from anafi crystalline basement cyclades greece constraints on the evolution of a late cretaceous magmatic arc in the internal hellenides
International Journal of Earth Sciences, 2016Co-Authors: Silviu O Martha, Wolfgang Dorr, Axel Gerdes, Rainer Petschick, Janina Schastok, P Xypolias, Gernold ZulaufAbstract:The Asterousia Crystalline Complex consists of Late Cretaceous amphibolite facies metamorphic rocks and associated granitoids, which can be found in exposures on Crete and the Cyclades (Greece). It is attributed to the Uppermost Unit and therefore to the Pelagonian domain of the Internal Hellenides. The tectonometamorphic evolution of this unit is still a matter of debate. We present new structural and petrological data of Asterousia-type rocks and Greenschist facies metamorphic rocks from the island of Anafi in the southern Aegean Sea as well as U–Pb zircon ages of granitoids from Anafi. The crystalline sequence of Anafi rests on top of Eocene flysch and comprises from bottom to top: (a) Anafi Greenschist; (b) Anafi Amphibolite Group (orthoamphibolite with intercalations of metasedimentary rocks at the base); and (c) Chalepa Group (amphibolite facies metasediments with slices of serpentinite and granitoids). LA-ICP-MS and ID-TIMS 206Pb/238U zircon ages of granodiorite from the Chalepa Group reveal several similar zircon populations suggesting continuous emplacement of granitoids inside a magmatic arc from ca. 72.5 to 79 Ma. The minimum emplacement age of granodioritic magma, deduced from the 206Pb/238U median age of the youngest zircon population, is 72.6 +0.1/−0.2 Ma. Deformation (micro)fabrics of granodiorite result from low strain obtained at T > 600 °C. This along with the U–Pb ages and published K–Ar ages indicates intrusion of the plutonic rocks at deep structural levels followed by very slow cooling. Monzogranitic dykes cutting through granodiorite in north-eastern Anafi are undeformed and yielded a 206Pb/238U median age of 69.9 +0.7/−0.7 Ma. Based on the new and published data, the following implications for the tectonometamorphic evolution on Anafi can be made: (1) obduction and accretion of mantle slices (serpentinite) to the Asterousia-type rocks were prior to amphibolite facies metamorphism; (2) intrusion of granitoids during the middle to late Campanian within a magmatic-arc setting and coeval shift of the magmatic arc towards the south; (3) Maastrichtian intrusion of dykes; (4) Palaeocene Greenschist facies metamorphism and coeval ductile top-to-the SE thrusting of the Anafi Amphibolite Group on top of the Anafi Greenschist; (5) post-Eocene brittle top-to-the SE thrusting of the Anafi Greenschist and the Anafi Amphibolite Group on top of flysch sediments; (6) clockwise rotation of the system and (Early) Oligocene brittle top-to-the SE thrusting of the Chalepa Group on top of all units mentioned above; and (7) ongoing clockwise rotation of the Aegean block during the Oligocene to Early Miocene and change in stress field (NE–SW compression).
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structural style deformation mechanisms and paleodifferential stress along an exposed crustal section constraints on the rheology of quartzofeldspathic rocks at supra and infrastructural levels bohemian massif
Tectonophysics, 2001Co-Authors: Gernold ZulaufAbstract:Abstract Investigations of Variscan shortening of quartzofeldspathic rocks have been carried out along an exposed crustal section located in the western part of the Tepla Barrandian unit (Bohemian Massif). The study focuses on determining differential stress, deformation mechanisms and geometry of Cadomian metagreywackes and Cambrian granitoids which show different grade of Variscan Barrovian-type metamorphism ranging from the Greenschist-/subGreenschist-facies boundary (semibrittle deformation regime) to the high-grade amphibolite facies. The semibrittle deformation regime (T=ca. 300°C) is characterized by large-scale thrusting within a folding environment and by similar intensities of cataclasis, diffusive mass transfer, and crystal plasticity of quartz, the latter including low-temperature migration recrystallization. Within Cadomian quartz veins of the Greenschist-facies part, dislocation creep of quartz operated at differential stresses ranging from ca. 70 MPa in the lowermost Greenschist-facies to ca. 20 MPa in the uppermost Greenschist-facies. These values have been derived by paleopiezometry using the grain size of recrystallized quartz. They are interpreted as upper bounds for the bulk stress because of intense diffusive mass transfer within the adjacent metagreywackes. A marking increase in recrystallized grain size of quartz occurs within the lower Greenschist-facies, where low-temperature grain boundary migration is replaced by subgrain rotation. Thus, causion is needed when applying the recrystallized grain size paleopiezometer to this crustal level. Pressure solution of quartz and development of discrete crenulation cleavage was common in fine-grained metagreywackes of the lower Greenschist-facies, whereas the upper Greenschist-facies (T=ca. 500–570°C) is characterized by increasing intensity of metamorphic reactions, grain coarsening and development of a schistosity. The entire Greenschist-facies level forms a suprastructure where viscosity contrasts between mica- and quartz-rich layers were sufficiently high for buckle folding. Strain-protected intercalated metagranitoids, on the other hand, do not show pervasive deformation fabrics. The amphibolite-facies part forms an infrastructure characterized by pervasive mylonitic shearing of both Cambrian granitoids and metagreywackes. It is suggested that the combination of the following processes has contributed to weaken the amphibolite-facies level making nonlocalized flow possible: (1) increased recrystallization of feldspar; (2) crystal-plastic softening of quartz due to high-temperature grain boundary migration and activity of prism slip; and (3) enhanced diffusive mass transfer in the form of metamorphic reactions. The structural breaks at the semibrittle crustal level and at the Greenschist-/amphibolite-facies boundary supports the view that rheological boundaries are prone to form major detachment horizons in the continental crust.
W G Ernst - One of the best experts on this subject based on the ideXlab platform.
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diverse mineral compositions textures and metamorphic p t conditions of the glaucophane bearing rocks in the tamayen melange yuli belt eastern taiwan
Journal of Asian Earth Sciences, 2013Co-Authors: Chinho Tsai, Yoshiyuki Iizuka, W G ErnstAbstract:Abstract This paper presents new petrologic data for high-pressure, low-temperature (HP–LT) metamorphic rocks at Juisui. We reinterpret the so-called “Tamayen block” ( Yang and Wang, 1985 ) or “Juisui block” ( Liou, 1981; Beyssac et al., 2008 ) as a tectonic melange. It is not a coherent sheet but rather a mixture dominated by Greenschist and pelitic schist with pods of serpentinite, epidote amphibolite, and rare blueschist. Four types of glaucophane-bearing rocks are newly recognized in this melange. Type I is in contact with Greenschist lacking glaucophane and garnet. Glaucophane is present only as rare inclusions within pargasite. This type records metamorphic evolution from epidote blueschists-, epidote amphibolite-, to Greenschist-facies. Type II contains characteristic zoned amphiboles from barroisite core to Mg-katophorite mantle and glaucophane rim, implying an epidote amphibolite-facies stage overprinted by an epidote blueschists-facies one. Type III includes winchite and indicates P – T conditions of about 6–8 kbar, approaching 400 °C. Type IV contains paragonite but lacks garnet; amphibole shows a Na–Ca core surrounded by a glaucophane rim. This type shows a high-pressure (?) epidote amphibolite-facies stage overprinted by an epidote blueschists-facies one. Amphibole zoning trends and mineral assemblages imply contradictory P – T paths for the four types of glaucophane-bearing rocks—consistent with the nature of a tectonic melange. The new P – T constraints and petrologic findings differ from previous studies ( Liou et al., 1975; Beyssac et al., 2008 ).
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high pressure low temperature metamorphism in northern hubei province central china
Journal of Metamorphic Geology, 1993Co-Authors: G Zhou, J G Liou, Y J Liu, Elizabeth A Eide, W G ErnstAbstract:The Qinling-Dabie accretionary fold belt in east-central China represents the E-W trending suture zone between the Sino-Korean and Yangtze cratons. A portion of the accretionary complex exposed in northern Hubei Province contains a high-pressure / low-temperature metamorphic sequence progressively metamorphosed from the blueschist through Greenschist to epidote-amphibolite / eclogite facies. The Hongan metamorphic belt can be divided into three metamorphic zones,based on progressive changes in mineral assemblages: Zone Ⅰ,in the south,is characterized by transitional blueschist-Greenschist facies; ZoneⅡis characterized by Greenschist facies; ZoneⅢ,in the northern most portion of the belt,is characterized by eclogite and epidote-amphibolite facies sequences. Changes in amphibole compositions from south to north as well as the appearance of increasingly higher pressure mineral assemblages toward the north document differences in metamorphic P-T conditions during formation of this belt. Preliminary P-T estimates for Zone I metamorphism are 5 ~ 7 kbar,350 ~ 450℃; estimates for Zone III eclogites are 10 ~ 22 kbar,500 ±50℃. The petrographic,chemical and structural characteristics of this metamorphic belt indicate its evolutionin a northward-dipping subduction zone and subsequent uplift prior to and during the final collisionbetween the Sino-Korean and Yangtze cratons.
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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40ar 39ar and oxygen isotope studies of polymetamorphism from tinos island cycladic blueschist belt greece
Journal of Metamorphic Geology, 1993Co-Authors: Michael Brocker, Alan Matthews, Hans Kreuzer, Martin OkruschAbstract:Petrological, oxygen isotope and 40Ar/39Ar studies were used to constrain the Tertiary metamorphic evolution of the lower tectonic unit of the Cyclades on Tinos. Polyphase high-pressure metamorphism reached pressures in excess of 15 kbar, based on measurements of the Si content in potassic white mica. Temperatures of 450–500° C at the thermal peak of high-pressure metamorphism were estimated from critical metamorphic assemblages, the validity of which is confirmed by a quartz–magnetite oxygen isotope temperature of 470° C. Some 40Ar/39Ar spectra of white mica give plateau ages of 44–40 Ma that are considered to represent dynamic recrystallization under peak or slightly post-peak high-pressure metamorphic conditions. Early stages in the prograde high-pressure evolution may be documented by older apparent ages in the high-temperature steps of some spectra. Eclogite to epidote blueschist facies mineralogies were partially or totally replaced by retrograde Greenschist facies assemblages during exhumation. Oxygen isotope thermometry of four quartz–magnetite pairs from Greenschist samples gives temperatures of 440–470° C which cannot be distinguished from those deduced for the high-pressure event. The exhumation and overprint is documented by decreasing ages of 32–28 Ma in some Greenschists and late-stage blueschist rocks, and ages of 30–20 Ma in the lower temperature steps of the Ar release patterns of blueschist micas. Almost flat parts of Ar–Ar release spectra of some Greenschist micas gave ages of 23–21 Ma which are assumed to represent incomplete resetting caused by a renewed prograde phase of Greenschist metamorphism. Oxygen isotope compositions of blueschist and Greenschist facies minerals show no evidence for the infiltration of a δ18O-enriched fluid. Rather, the compositions indicate that fluid to rock ratios were very low, the isotopic compositions being primarily controlled by those of the protolith rocks. We assume that the fundamental control catalysing the transformation of blueschists into Greenschists and the associated resetting of their isotopic systems was the selective infiltration of metamorphic fluid. A quartz–magnetite sample from a contact metamorphic skarn, taken near the Miocene monzogranite of Tinos, gave an oxygen isotope temperature of 555° C and calculated water composition of 9.1%. The value of δ18O obtained from this water is consistent with a primary magmatic fluid, but is lower than that of fluids associated with the Greenschist overprint, which indicates that the latter event cannot be directly related to the monozogranite intrusion.
Silviu O Martha - One of the best experts on this subject based on the ideXlab platform.
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the tectonometamorphic and magmatic evolution of the uppermost unit in central crete melambes area constraints on a late cretaceous magmatic arc in the internal hellenides greece
Gondwana Research, 2017Co-Authors: Silviu O Martha, Wolfgang Dorr, Axel Gerdes, Jochen Krahl, Jolien Linckens, Gernold ZulaufAbstract:Abstract Crete consists of a nappe pile that formed during Alpine subduction and collision. The lower nappes belong to the External Hellenides, whereas the uppermost nappe is ascribed to the Pelagonian Zone of the Internal Hellenides. The Uppermost Unit consists of several subunits including the Asterousia Crystalline Complex (ACC), which comprises metasedimentary rocks, (meta)granitoids and serpentinite, the protolith age and the tectonometamorphic evolution of which are largely unknown. In the present study, we present new structural, microfabric and geochronological data from the Uppermost Unit in the Melambes area (central Crete). 206Pb/238U zircon ages (LA-ICP-MS and ID-TIMS) indicate granitic and dioritic intrusions between 71.9 ± 0.6 and 76.9 ± 0.3 Ma. Identical ages have previously been obtained from comparable intrusions in eastern Crete and on Anafi. The composition and geochemical signature suggest an extended magmatic arc along the southern active margin of the Pelagonian-Lycian Block. Post-intrusive shearing transformed granite into orthogneiss, whereas diorite remained free from foliation, because of the lower amount of mechanically weak phases. Deformation microfabrics suggest top-to-the SE shearing under amphibolite facies conditions of the ACC and at Greenschist facies conditions of rocks at the base of the ACC referred to as Akoumianos Greenschist. The Akoumianos Greenschist is considered as the northern part of the Pindos realm that was subducted underneath the Pelagonian-Lycian active margin. Based on our new and on published data, the following orogenic stages are suggested to have contributed to the evolution of the Hellenides during the Late Cretaceous to Eocene: (1) pre-middle Campanian collision and subduction of the Pindos lithosphere underneath the southern margin of the Pelagonian-Lycian terrane led to obduction and offscraping of serpentinized ocean floor and stacking of the ACC during amphibolite facies top-to-the SE thrusting, (2) formation of a Campanian magmatic arc along the Pelagonian-Lycian active margin; (3) Maastrichtian collision and stacking of the magmatic arc during top-to-the SE mylonitic shearing; (4) Palaeocene top-to-the SE Greenschist-facies shearing of the ACC on top of the Akoumianos Greenschist; (5) Late Eocene thrusting of the Uppermost Unit on top of the Arvi and Pindos units. Thus, top-to the SE was the dominant shear sense in the southern Aegean from at least the mid-Late Cretaceous until the Eocene.
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new structural and u pb zircon data from anafi crystalline basement cyclades greece constraints on the evolution of a late cretaceous magmatic arc in the internal hellenides
International Journal of Earth Sciences, 2016Co-Authors: Silviu O Martha, Wolfgang Dorr, Axel Gerdes, Rainer Petschick, Janina Schastok, P Xypolias, Gernold ZulaufAbstract:The Asterousia Crystalline Complex consists of Late Cretaceous amphibolite facies metamorphic rocks and associated granitoids, which can be found in exposures on Crete and the Cyclades (Greece). It is attributed to the Uppermost Unit and therefore to the Pelagonian domain of the Internal Hellenides. The tectonometamorphic evolution of this unit is still a matter of debate. We present new structural and petrological data of Asterousia-type rocks and Greenschist facies metamorphic rocks from the island of Anafi in the southern Aegean Sea as well as U–Pb zircon ages of granitoids from Anafi. The crystalline sequence of Anafi rests on top of Eocene flysch and comprises from bottom to top: (a) Anafi Greenschist; (b) Anafi Amphibolite Group (orthoamphibolite with intercalations of metasedimentary rocks at the base); and (c) Chalepa Group (amphibolite facies metasediments with slices of serpentinite and granitoids). LA-ICP-MS and ID-TIMS 206Pb/238U zircon ages of granodiorite from the Chalepa Group reveal several similar zircon populations suggesting continuous emplacement of granitoids inside a magmatic arc from ca. 72.5 to 79 Ma. The minimum emplacement age of granodioritic magma, deduced from the 206Pb/238U median age of the youngest zircon population, is 72.6 +0.1/−0.2 Ma. Deformation (micro)fabrics of granodiorite result from low strain obtained at T > 600 °C. This along with the U–Pb ages and published K–Ar ages indicates intrusion of the plutonic rocks at deep structural levels followed by very slow cooling. Monzogranitic dykes cutting through granodiorite in north-eastern Anafi are undeformed and yielded a 206Pb/238U median age of 69.9 +0.7/−0.7 Ma. Based on the new and published data, the following implications for the tectonometamorphic evolution on Anafi can be made: (1) obduction and accretion of mantle slices (serpentinite) to the Asterousia-type rocks were prior to amphibolite facies metamorphism; (2) intrusion of granitoids during the middle to late Campanian within a magmatic-arc setting and coeval shift of the magmatic arc towards the south; (3) Maastrichtian intrusion of dykes; (4) Palaeocene Greenschist facies metamorphism and coeval ductile top-to-the SE thrusting of the Anafi Amphibolite Group on top of the Anafi Greenschist; (5) post-Eocene brittle top-to-the SE thrusting of the Anafi Greenschist and the Anafi Amphibolite Group on top of flysch sediments; (6) clockwise rotation of the system and (Early) Oligocene brittle top-to-the SE thrusting of the Chalepa Group on top of all units mentioned above; and (7) ongoing clockwise rotation of the Aegean block during the Oligocene to Early Miocene and change in stress field (NE–SW compression).
