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Mete Çetİnkaplan - One of the best experts on this subject based on the ideXlab platform.

  • Epidote – lawsonite coexistence in Blueschist-Facies block from the Tavşanlı Zone – Turkey: petrological implications
    TURKISH JOURNAL OF EARTH SCIENCES, 2020
    Co-Authors: Mete Çetİnkaplan
    Abstract:

    Metamorphic evolution of an epidote–lawsonite Blueschist sample characterized by the coexistence of lawsonite and epidote from Sivrihisar area (Tavşanlı Zone) was studied herein in terms of petrology and mineral equilibria. Based on the textural evidence and phase composition, 2 prograde stages, defined by assemblage-I and -II, and 1 retrograde stage were recognized. Assemblage-I indicates epidote-Blueschist Facies conditions (12 ± 1 kbar / 485 ± 10 °C). Assemblage-II is characterized by the coexistence of epidote and lawsonite (17 ± 1 kbar / 515 ± 10 °C) corresponding to the interface of lawsonite Blueschist and epidote Blueschist Facies. Phase diagram calculations and mineral compositions revealed that along this interface, an equilibrium field with lawsonite and epidote is stable. This closed-equilibrium field is controlled by high aH2O and an elevated Fe3+/Al ratio of minerals. Pressure-temperature (P–T) estimates and textural observations indicated a counter-clockwise path during the subduction and exhumation history. The preservation of lawsonite and epidote during the retrograde stage pointed to the fact that the path followed the stability field of lawsonite and epidote during exhumation.

  • P–T–t evolution of the Cycladic Blueschist Unit in Western Anatolia/Turkey: Geodynamic implications for the Aegean region
    Journal of Metamorphic Geology, 2020
    Co-Authors: Mete Çetİnkaplan, Osman Candan, Roland Oberhänsli, Masafumi Sudo, Bénédicte Cenki-tok
    Abstract:

    Eclogite and Blueschist Facies rocks occurring as a tectonic unit between the underlying Menderes Massif and the overlying Afyon Zone / Lycian Nappes and the Bornova Flysch Zone in western Anatolia represent the eastward continuation of the Cycladic Blueschist Unit in Turkey. This high-P unit is attributed to the closure of the Pindos Ocean and consists of (i) a Triassic to Upper Cretaceous coherent series derived from passive continental margin sediments and (ii) the tectonically overlying Upper Cretaceous Selcuk melange with eclogite blocks embedded in a pelitic epidote-Blueschist matrix. The coherent series has experienced epidote-Blueschist Facies metamorphism (490 ± 25°C / 11.5 ± 1.5 kbar; 38 km depth). 40 Ar/ 39 Ar white mica and 206 Pb/ 238 U monazite dating of quartz metaconglomerate from coherent series yielded middle Eocene ages of 44 ± 0.3 and 40.1 ± 3.1 Ma for epidote-Blueschist Facies metamorphism, respectively. The epidote-Blueschist Facies metamorphism of the matrix of the Selcuk melange culminates at 520 ± 15°C / 13 ± 1.5 kbar, 43 km depth, and is dated 57.5 ± 0.3-54.5 ± 0.1 Ma (40 Ar/ 39 Ar phengite). Eclogite Facies metamorphism of the blocks (570 ± 30°C / 18 ± 2 kbar, 60 km depth) is early Eocene and dated at 56.2 ± 1.5 Ma by 206 Pb/ 238 U zircon. Eclogites experienced a nearly isothermal retrogression (490 ± 40°C / ~ 6-7 kbar) during their incorporation into the Selcuk melange. The retrograde overprints of the coherent series (410 ± 15°C / 7 ± 1.5 kbar from Dilek Peninsula and 485 ± 33°C / ~ 6-7 kbar from Selcuk-Tire area) and the Selcuk melange (510 ± 15°C / 6 ± 1 kbar) are dated at 35.8 ± 0.5-34.3 ± 0.1 Ma by 40 Ar/ 39 Ar white mica and 31.6 ± 6.6 Ma by 206 Pb/ 238 U allanite dating methods, respectively. Regional geological constrains reveal that the contact between the Menderes Massif and the Cycladic Blueschist Unit originally formed a lithosphere-scale transform fault zone. 40 Ar/ 39 Ar white mica age from the contact indicates that the Cycladic Blueschist Unit and the Menderes Massif were tectonically juxtaposed under greenschist Facies conditions during late Eocene, 35.1 ± 0.3 Ma.

  • P‐T‐t evolution of the Cycladic Blueschist unit in Western Anatolia / Turkey: Geodynamic implications for the Aegean region
    Journal of Metamorphic Geology, 2020
    Co-Authors: Mete Çetİnkaplan, Osman Candan, Roland Oberhänsli, Masafumi Sudo, Bénédicte Cenki‐tok
    Abstract:

    Eclogite and Blueschist Facies rocks occurring as a tectonic unit between the underlying Menderes Massif and the overlying Afyon Zone / Lycian Nappes and the Bornova Flysch Zone in western Anatolia represent the eastward continuation of the Cycladic Blueschist Unit in Turkey. This high-P unit is attributed to the closure of the Pindos Ocean and consists of (i) a Triassic to Upper Cretaceous coherent series derived from passive continental margin sediments and (ii) the tectonically overlying Upper Cretaceous Selçuk mélange with eclogite blocks embedded in a pelitic epidote-Blueschist matrix. The coherent series has experienced epidote-Blueschist Facies metamorphism (490 ± 25°C / 11.5 ± 1.5 kbar; 38 km depth). 40 Ar/ 39 Ar white mica and 206 Pb/ 238 U monazite dating of quartz metaconglomerate from coherent series yielded middle Eocene ages of 44 ± 0.3 and 40.1 ± 3.1 Ma for epidote-Blueschist Facies metamorphism, respectively. The epidote-Blueschist Facies metamorphism of the matrix of the Selçuk mélange culminates at 520 ± 15°C / 13 ± 1.5 kbar, 43 km depth, and is dated 57.5 ± 0.3-54.5 ± 0.1 Ma (40 Ar/ 39 Ar phengite). Eclogite Facies metamorphism of the blocks (570 ± 30°C / 18 ± 2 kbar, 60 km depth) is early Eocene and dated at 56.2 ± 1.5 Ma by 206 Pb/ 238 U zircon. Eclogites experienced a nearly isothermal retrogression (490 ± 40°C / ~ 6-7 kbar) during their incorporation into the Selçuk mélange. The retrograde overprints of the coherent series (410 ± 15°C / 7 ± 1.5 kbar from Dilek Peninsula and 485 ± 33°C / ~ 6-7 kbar from Selçuk-Tire area) and the Selçuk mélange (510 ± 15°C / 6 ± 1 kbar) are dated at 35.8 ± 0.5-34.3 ± 0.1 Ma by 40 Ar/ 39 Ar white mica and 31.6 ± 6.6 Ma by 206 Pb/ 238 U allanite dating methods, respectively. Regional geological constrains reveal that the contact between the Menderes Massif and the Cycladic Blueschist Unit originally formed a lithosphere-scale transform fault zone. 40 Ar/ 39 Ar white mica age from the contact indicates that the Cycladic Blueschist Unit and the Menderes Massif were tectonically juxtaposed under greenschist Facies conditions during late Eocene, 35.1 ± 0.3 Ma.

  • p t t evolution of eclogite Blueschist Facies metamorphism in alanya massif time and space relations with hp event in bitlis massif turkey
    International Journal of Earth Sciences, 2016
    Co-Authors: Mete Çetİnkaplan, Aral I. Okay, Amaury Pourteau, Osman Candan, Roland Oberhänsli, Fukun Chen, Huseyin Kozlu, Ersin O Koralay, Fırat Şengün
    Abstract:

    The Alanya Massif, which is located to the south of central Taurides in Turkey, presents a typical nappe pile consisting of thrust sheets with contrasting metamorphic histories. In two thrust sheets, Sugozu and Gundogmus nappes, HP metamorphism under eclogite (550–567 °C/14–18 kbar) and Blueschist Facies (435–480 °C/11–13 kbar) conditions have been recognized, respectively. Whereas the rest of the Massif underwent MP metamorphism under greenschist to amphibolite Facies (525–555 °C/6.5–7.5 kbar) conditions. Eclogite Facies metamorphism in Sugozu nappe, which consists of homogeneous garnet–glaucophane–phengite schists with eclogite lenses is dated at 84.8 ± 0.8, 84.7 ± 1.5 and 82 ± 3 Ma (Santonian–Campanian) by 40Ar/39Ar phengite, U/Pb zircon and rutile dating methods, respectively. Similarly, phengites in Gundogmus nappe representing an accretionary complex yield 82–80 Ma (Campanian) ages for Blueschist Facies metamorphism. During the exhumation, the retrograde overprint of the HP units under greenschist–amphibolite Facies conditions and tectonic juxtaposition with the Barrovian units occurred during Campanian (75–78 Ma). Petrological and geochronological data clearly indicate a similar Late Cretaceous tectonometamorphic evolution for both Alanya (84–75 Ma) and Bitlis (84–72 Ma) Massifs. They form part of a single continental sliver (Alanya–Bitlis microcontinent), which was rifted from the southern part of the Anatolide–Tauride platform. The P–T–t coherence between two Massifs suggests that both Massifs have been derived from the closure of the same ocean (Alanya–Bitlis Ocean) located to the south of the Anatolide–Tauride block by a northward subduction. The boundary separating the autochthonous Tauride platform to the north from both the Alanya and Bitlis Massifs to the south represents a suture zone, the Pamphylian–Alanya–Bitlis suture.

  • P–T–t evolution of eclogite/Blueschist Facies metamorphism in Alanya Massif: time and space relations with HP event in Bitlis Massif, Turkey
    International Journal of Earth Sciences, 2014
    Co-Authors: Mete Çetİnkaplan, Aral I. Okay, Amaury Pourteau, Osman Candan, O. Ersin Koralay, Roland Oberhänsli, Fukun Chen, Huseyin Kozlu, Fırat Şengün
    Abstract:

    The Alanya Massif, which is located to the south of central Taurides in Turkey, presents a typical nappe pile consisting of thrust sheets with contrasting metamorphic histories. In two thrust sheets, Sugozu and Gundogmus nappes, HP metamorphism under eclogite (550–567 °C/14–18 kbar) and Blueschist Facies (435–480 °C/11–13 kbar) conditions have been recognized, respectively. Whereas the rest of the Massif underwent MP metamorphism under greenschist to amphibolite Facies (525–555 °C/6.5–7.5 kbar) conditions. Eclogite Facies metamorphism in Sugozu nappe, which consists of homogeneous garnet–glaucophane–phengite schists with eclogite lenses is dated at 84.8 ± 0.8, 84.7 ± 1.5 and 82 ± 3 Ma (Santonian–Campanian) by 40Ar/39Ar phengite, U/Pb zircon and rutile dating methods, respectively. Similarly, phengites in Gundogmus nappe representing an accretionary complex yield 82–80 Ma (Campanian) ages for Blueschist Facies metamorphism. During the exhumation, the retrograde overprint of the HP units under greenschist–amphibolite Facies conditions and tectonic juxtaposition with the Barrovian units occurred during Campanian (75–78 Ma). Petrological and geochronological data clearly indicate a similar Late Cretaceous tectonometamorphic evolution for both Alanya (84–75 Ma) and Bitlis (84–72 Ma) Massifs. They form part of a single continental sliver (Alanya–Bitlis microcontinent), which was rifted from the southern part of the Anatolide–Tauride platform. The P–T–t coherence between two Massifs suggests that both Massifs have been derived from the closure of the same ocean (Alanya–Bitlis Ocean) located to the south of the Anatolide–Tauride block by a northward subduction. The boundary separating the autochthonous Tauride platform to the north from both the Alanya and Bitlis Massifs to the south represents a suture zone, the Pamphylian–Alanya–Bitlis suture.

Shah Wali Faryad - One of the best experts on this subject based on the ideXlab platform.

  • New evidence of Blueschist Facies rocks and their geotectonic implication for Variscan suture(s) in the Bohemian Massif
    Journal of Metamorphic Geology, 2012
    Co-Authors: Shah Wali Faryad, Václav Kachlík
    Abstract:

    Blueschist Facies rocks, exposed within consolidated continental blocks, provide some of the best evidence for the existence of previous suture(s). They usually occur as lenses or layers embedded within greenschist or amphibolite Facies rocks and indicate reequilibration at medium- to low-pressure conditions. In the Bohemian Massif, a few occurrences of Blueschists have been reported, and here, new evidence of high-pressure (HP) metamorphism in various lithologies is presented that suggests a larger extent of Blueschist Facies rocks along the northern border of this Massif. An earlier Blueschist Facies metamorphism is documented by inclusions of glaucophane in garnet, epidote and titanite from metabasites along with zoned white mica having a phengitic core and a muscovite rim in metapelites and orthogneisses. The estimated P–T conditions, obtained using pseudosections and mineral isopleths, correspond to Blueschist and low-temperature eclogite Facies conditions (1.1–2.0 GPa at 350–550 °C). Together with medium-temperature eclogites from different units in the Bohemian Massif they indicate a geothermal gradient of 8–10 °C km−1, which is typical for cool subduction. Radiometric dating on phengite from metapelites confirms an early Palaeozoic cooling age of c. 360 Ma for this HP metamorphic event. The presence of Blueschist Facies rocks, their P–T relations and age constraint together with those from eclogite Facies rocks allows us to locate the Variscan suture, which straddles the SE margin of Saxothuringian Zone from Erzgebirge to Sudetes, and its possible continuation to the Moldanubian Zone, where eclogite Facies and UHPM rocks are abundant.

  • Textural and age relations of polymetamorphic rocks in the HP Meliata Unit (Western Carpathians)
    Journal of Asian Earth Sciences, 2011
    Co-Authors: Shah Wali Faryad, Wolfgang Frank
    Abstract:

    Abstract Slices and tectonic blocks of amphibolite Facies crustal rocks overprinted by Blueschist Facies metamorphism occur in the Middle-Jurassic Meliata high-pressure unit of the Western Carpathians. The Early Paleozoic age for relic muscovite is known from some micaschists with Blueschist Facies overpint in the eastern sector of the Meliata unit. Based on lithology, some amphibolite Facies rocks are comparable with the Early Paleozoic basement rocks in the Gemeric unit. In this work, we present results of petrology and Ar–Ar and Rb–Sr geochronology from Blueschist Facies rocks with relicts of hornblende, muscovite, garnet, and rutile. The older amphibolite Facies metamorphism was followed by retrogression prior to the Blueschist Facies overprint. The Blueschist Facies metamorphic minerals are Ca-rich garnet, glaucophane, phengite, and albite. The coarse-grained relict muscovite indicates an Early Paleozoic age using the Ar–Ar method and a Middle-Jurassic age using the Rb–Sr technique. The presence of amphibolite Facies rocks within the Meliata unit is interpreted as involvment of Variscan/Pre-Variscan basement-unit rocks in the Middle-Jurassic subduction of the Meliata-Hallstatt oceanic basin.

  • Very low-grade metamorphism of sedimentary rocks of the Meliata unit, Western Carpathians, Slovakia: implications of phyllosilicate characteristics
    International Journal of Earth Sciences, 2003
    Co-Authors: Péter Árkai, Shah Wali Faryad, Olivier Vidal, Kadosa Balogh
    Abstract:

    The Meliata unit represents a melange-like accretionary wedge, containing Blueschist Facies tectonic blocks and slices in a Triassic and Jurassic sedimentary matrix. The Blueschist Facies rocks are tectonic remnants of the subducted parts of the Meliata-Hallstatt branch of the Tethys. The phyllosilicate assemblages in very low-grade metapelites represent metastable disequilibrium stages which the assemblages have reached during reaction progress. Therefore, temperature and pressure values of low-T metamorphism of the sedimentary series and the late stages of decompressional cooling of Blueschist Facies rocks, obtained by phyllosilicate "crystallinity", chlorite thermometric and white K-mica geobarometric methods, can be regarded as semiquantitative estimates. However, results of chlorite–white mica thermobarometry suggest that local equilibrium was approached at a microscopic scale. For deciphering the age relations of prograde and retrograde events, K–Ar isotope geochronological methods were applied. The sedimentary series and related basalts of the Meliata unit experienced high-T anchizonal prograde regional metamorphism, the temperature and pressure of which can vary between ca. 280 and 350 °C and ca. 2.5 and 5 kbar. White K-mica b geobarometry suggests possible minimal pressures of ca. 1.5 to 3 kbar. The mylonitic retrogression of Blueschist Facies phyllites is characterised by 340 °C and 4 kbar (minimal P). The low-T prograde metamorphism was synchronous with the retrograde metamorphism of the Blueschists. The ages of these two events may be between ca. 150 and 120 Ma, culminating most probably at around 140–145 Ma. Thus, the Upper Jurassic (lowermost Cretaceous) very low-grade metamorphism of the Meliata unit is younger than the subduction-related, 160–155 Ma Blueschist Facies event, and definitely older than the Cretaceous (100–90 Ma) metamorphism of the footwall Gemer Palaeozoic.

J. Abati - One of the best experts on this subject based on the ideXlab platform.

  • High-pressure rocks in the Variscan belt of Western Europe: the Malpica-Tui Complex (NW Iberian Massif)
    2015
    Co-Authors: Alicia López-carmona, Pavel Pitra, J. Abati, Gabriel Gutiérrez-alonso, James K. W. Lee, Javier Fernández-suárez
    Abstract:

    Pods and relicts of high-pressure (HP) rocks are the best record of the subduction of the north Gondwana margin at the onset of the Variscan collision. Across Western Europe, Blueschist-Facies terranes are restricted to scarce and relatively small areas, whereas eclogite-Facies terranes are more abundant. The Malpica-Tui complex (MTC) is the westernmost exposure of HP rocks in the NW Iberian Massif, and in the Variscan belt of Western Europe. It comprises two tectonically juxtaposed units separated by an extensional detachment: (i) an upper unit consisting of rocks in the Blueschist-Facies, and (ii) a lower unit in the medium temperature eclogite-Facies conditions. Assuming a northwest-directed component of subduction, in present day coordinates, the characteristics of each sequence suggest that the upper unit would occupy an oceanward position compared to the lower unit before the Variscan collision. Thus, the lower unit is interpreted as a slice of continental crust, whereas the upper unit may represent a transitional to oceanic crust of the same continental margin. The MTC preserves evidence of late Devonian HP metamorphism varying from eclogite (P~26 kbar and T~650 °C) to Blueschist-Facies conditions (19-22 kbar and 460-560 °C). Petrological analysis involving P-T-X pseudosections in the (Mn)NCKFMASHTO chemical system on the HP rocks reveals a P-T evolution characterised by a subisothermal decompression to ~10 kbar, 480 °C in the Blueschist-Facies rocks and 650 °C in the eclogites, followed by cooling to ~5 kbar at 380 °C and 500 °C, respectively. New 40Ar/39Ar data indicate a minimum age of ~370 Ma for the subduction-related HP metamorphism. Subsequent decompression to pressures of about 10 kbar started at ~360 Ma and was contemporaneous with thrust-and-fold nappe tectonics and intrusion of early Variscan granodiorites dated at ~350- 340 Ma. Final, “post-nappe”, exhumation is interpreted to have taken place from ~345-335 Ma to 320 ± 5 Ma, which is the age of the syntectonic igneous rocks emplaced in the autochthon of the MTC. These ages support the equivalence of the HP rocks from NW Iberia and their counterparts in the southern Armorican Massif. From a methodological point of view, modelling calculations of H2O and Fe2O3 on the metamorphic evolution of Blueschist-Facies rocks reveals trends that may have general applications in the investigation of rocks with similar composition: (i) subduction-zone metamorphism may occur in H2O-undersaturated conditions induced by the crystallization of a significant modal amount of lawsonite, although the transition from lawsonite Blueschist Facies to amphibolite-greenschist Facies may involve significant hydration, principally as a result of lawsonite breakdown. (ii) The analysed values of Fe2O3 may not reflect the oxidation state during the main metamorphic evolutionary stage and are probably easily modified by superficial alteration, even in apparently fresh samples

  • Blueschist-Facies metapelites from the Malpica-Tui Unit (NW Iberian Massif): phase equilibria modelling and H2O and Fe2O3 influence in high-pressure assemblages
    Journal of Metamorphic Geology, 2013
    Co-Authors: A. Lopez-carmona, Pavel Pitra, J. Abati
    Abstract:

    The Malpica-Tui Unit (Galicia, NW Spain) records eclogite- and Blueschist-Facies metamorphism during the onset of the Variscan orogeny in Europe. Petrological analysis involving pseudosections calculated using THERMOCALC shows that the Upper Sheet of this unit, the Cea' n Schists, recorded a threestage metamorphic evolution involving (i) Early subduction-related medium-pressure ⁄ low-temperature metamorphism (M1) constrained at 350-380 C, 12-14 kbar, which is only recorded in the basal part (lower metapelites, LM) of the Cea'n Schists. (ii) Subduction-related Blueschist Facies prograde metamorphism (M2) going from 19 kbar, 420 C to 21 kbar, 460 C in the LM, and from 16 kbar 430 C to 21-22 kbar, 520 C in the structurally upper metapelites (UM). (iii) Exhumation-related metamorphism (M3) is characterized by a decompression to 8-10 kbar, 470-490 C in the LM. This decompression is also recorded in the UM, but it was not possible to estimate precise P-T conditions. The calculations indicate that (i) the prograde evolution in subduction zones may occur in fluidundersaturated conditions due to the crystallization of lawsonite, even in metapelitic rocks. This significantly influences phase equilibria and hence the P-T estimates. (ii) The proportion of ferric iron also has a strong influence on phase equilibria, even in metapelites. However, the analysed values of Fe2O3 may not reflect the oxidation state during the main metamorphic evolution and are probably easily modified by superficial alteration even in apparently fresh samples. The use of P-T-X(Fe2O3) pseudosections together with petrographic observations is then necessary to estimate the real oxidation state of the rocks and correctly evaluate the P-T conditions.

  • Blueschist‐Facies metapelites from the Malpica–Tui Unit (NW Iberian Massif): phase equilibria modelling and H2O and Fe2O3 influence in high‐pressure assemblages
    Journal of Metamorphic Geology, 2012
    Co-Authors: A. Lopez-carmona, Pavel Pitra, J. Abati
    Abstract:

    The Malpica–Tui Unit (Galicia, NW Spain) records eclogite- and Blueschist-Facies metamorphism during the onset of the Variscan orogeny in Europe. Petrological analysis involving pseudosections calculated using THERMOCALC shows that the Upper Sheet of this unit, the Cean Schists, recorded a threestage metamorphic evolution involving (i) Early subduction-related medium-pressure ⁄ low-temperature metamorphism (M1) constrained at ~o350–380 oC, 12–14 kbar, which is only recorded in the basal part (lower metapelites, LM) of the Cea´n Schists. (ii) Subduction-related Blueschist Facies prograde metamorphism (M2) going from 19 kbar, 420 oC to 21 kbar, 460 oC in the LM, and from 16 kbar 430 oC to 21–22 kbar, 520 oC in the structurally upper metapelites (UM). (iii) Exhumation-related metamorphism (M3) is characterized by a decompression to 8–10 kbar, 470–490 oC in the LM. This decompression is also recorded in the UM, but it was not possible to estimate precise P–T conditions. The calculations indicate that (i) the prograde evolution in subduction zones may occur in fluidundersaturated conditions due to the crystallization of lawsonite, even in metapelitic rocks. This significantly influences phase equilibria and hence the P–T estimates. (ii) The proportion of ferric iron also has a strong influence on phase equilibria, even in metapelites. However, the analysed values of Fe2O3 may not reflect the oxidation state during the main metamorphic evolution and are probably easily modified by superficial alteration even in apparently fresh samples. The use of P–T–X(Fe2O3) pseudosections together with petrographic observations is then necessary to estimate the real oxidation state of the rocks and correctly evaluate the P–T conditions.

Michel Ballèvre - One of the best experts on this subject based on the ideXlab platform.

  • The Growth of Sodic Amphibole at the Greenschist- to Blueschist-Facies Transition (Dent Blanche, Western Alps): Bulk-rock Chemical Control and Thermodynamic Modelling
    Journal of Petrology, 2020
    Co-Authors: Paola Manzotti, Michel Ballèvre, Pavel Pitra, Benita Putlitz, Martin Robyr, Othmar Müntener
    Abstract:

    Abstract The sodic amphibole glaucophane is generally considered as indicative of Blueschist-Facies metamorphism. However, sodic amphiboles display a large range in chemical compositions, owing principally to the Fe2+Mg–1 and Fe3+Al–1 substitutions. Therefore, the whole-rock composition (namely its Na2O and FeO* content, and the Fe2+–Fe3+ ratio), strongly controls the stability field of the sodic amphiboles at the transition from greenschist- to Blueschist-Facies conditions. Neglecting these variables can lead to erroneous estimates of the metamorphic conditions and consequently the tectonic framework of the rocks. This paper explores the mechanisms that control the development of sodic amphibole and sodic pyroxene within the basement of the Dent Blanche Tectonic System (Western Alps), as a result of the Alpine metamorphic history. Field, petrographic and geochemical data indicate that sodic amphibole and sodic pyroxene form in different rock types: (1) in undeformed pods of ultramafic cumulates (hornblendite), sodic amphibole (magnesioriebeckite) forms coronas around magmatic pargasite; (2) metatonalite displays patches of radiating sodic (magnesioriebeckite) and calcic (actinolite) amphiboles; (3) sodic amphibole (magnesioriebeckite–glaucophane) occurs with high-Si potassic white mica (phengitic muscovite) in fine-grained (blue) schists; (4) in mylonitized granitoids (amphibole-gneiss) metasomatized along the contact with ultramafic cumulates, sodic amphibole (magnesioriebeckite–winchite) mainly forms rosettes or sheaves, generally without a shape-preferred orientation. Only locally are the needles aligned parallel to the stretching lineation. Pale green aegirine–augite is dispersed in an albite–quartz matrix or forms layers of fine-grained fibrous aggregates. The bulk-rock chemical composition of the different lithologies indicates that sodic amphibole and sodic pyroxene developed in Na- and Fe-rich systems or in a system with high Fe3+/Fe*. Thermodynamic modelling performed for different rock types (taking into account the measured Fe2O3 contents) reveals that sodic amphibole appears at ∼8 ± 1 kbar and 400–450 °C (i.e. at the transition between the greenschist- and Blueschist-Facies conditions) about 5 kbar lower than previous estimates. To test the robustness of our conclusion, we performed a review of sodic amphibole compositions from a variety of terranes and P–T conditions. This shows (1) systematic variations of composition with P–T conditions and bulk-rock chemistry, and (2) that the amphibole compositions reported from the studied area are consistent with those reported from other greenschist- to Blueschist-Facies transitions.

  • Greenschist or Blueschist Facies: the key role of redox conditions for the growth of sodic amphibole in the Dent Blanche Tectonic System (Western Alps)
    2019
    Co-Authors: Michel Ballèvre, Paola Manzotti, Pavel Pitra, Benita Putlitz, Othmar Müntener, Martin Robyr
    Abstract:

    The sodic amphibole glaucophane, commonly coloured in blue, is generally considered as the mineral indicative of Blueschist-Facies metamorphism (i.e. occurring in former subduction zones). However, sodic amphiboles display a large range of chemical compositions, due principally to the Fe2+Mg-1 and Fe3+Al-1 substitutions. Therefore, the whole-rock composition, and especially the oxidation state of a rock, strongly controls the stability field of the sodic amphibole at the transition from greenschist- to Blueschist-Facies. Under evaluating this point can lead to the incorrect account of the metamorphic conditions, resulting in a misinterpretation of the tectonic framework of the rocks. This work explores the mechanisms that can explain the scarcity of sodic amphibole and sodic pyroxene within the basement of the Dent Blanche Tectonic System (Western Alps), as the result of the Alpine metamorphic history. Field, petrographic and geochemical data indicate that sodic amphiboles crystallize in three different rock types. Firstly, in undeformed pods of ultramafic cumulates (hornblendite), sodic amphibole (magnesioriebeckite) forms coronas around magmatic calcic amphibole. Secondly, in mylonitized granitoids metasomatized along the contact with ultramafic cumulates (amphibole-gneiss and albitite), sodic amphibole (magnesioriebeckite-winchite) mainly forms rosettes or sheaves. Only locally the amphibole needles are aligned parallel to the mylonitic foliation and the stretching lineation. Pale green, patchy zoned aegirineaugite is dispersed in an albite-quartz matrix or forms layers of fine-grained fibrous aggregates. Thirdly, sodic amphibole (magnesioriebeckite-glaucophane) occurs with muscovite-epidote-quartz in fine-grained volcanoclastic schists. Bulk rock chemistry of the different lithologies indicates that sodic amphibole and sodic pyroxene developed in Fetotal rich system or in system with a high Fe3+ / (Fe2++Fe3+). Thermodynamic modelling has been performed for different rock types, taking into account the measured Fe2O3 contents. This allows exploring the effect of varying the oxidation state ratio and the water content. Results of these numerical models highlight the role of Fe2O3 on stabilizing sodic amphibole and sodic pyroxene and suggests that sodic amphibole is stable at ~ 8-10 kbar and 400-450°C, i.e. at the transition between the greenschist- and Blueschist-Facies. Our models suggest lower pressures compared to other estimates based on Si content in muscovite and provide better constraints on the Alpine metamorphic evolution of the Dent Blanche Tectonic System.

  • Detrital zircon geochronology in Blueschist‑Facies meta‑conglomerates from the Western Alps: implications for the late Carboniferous to early Permian palaeogeography
    International Journal of Earth Sciences, 2015
    Co-Authors: Paola Manzotti, Marc Poujol, Michel Ballèvre
    Abstract:

    In the Western Alps, the Money Complex of the Gran Paradiso Massif, metamorphosed under Blueschist Facies during the Alpine cycle, is considered to be Permo- Carboniferous in age, but no palaeontological or radiometric data constrain this interpretation. A revision of the lithostratigraphy of the Money Complex allows recognizing a polygenic (graphite-rich) and a monogenic (graphitepoor) meta-sedimentary formation. Detrital zircon U–Pb geochronology in both meta-sedimentary formations shows that (i) the main population is Cambrian and Ordovician in age, (ii) the youngest grains are Silurian and Lower Devonian, and (iii) Carboniferous zircon grains are lacking. A careful study of the age distributions in the Alps suggests that potential source for the detrital material in the Money Complex is the Briançonnais basement. Late Carboniferous magmatism is widespread in the Helvetic Zone of the Alps. Permian magmatism is dominant in the Briançonnais, the Austroalpine and the Southalpine basements. The lack of Carboniferous zircons in the Money Complex suggests that the detritus was not shed from the Helvetic zone, which was separated from the Money basin by the Zone Houillère basin, where the main drainage pattern was developed from south to north and where the depocenters migrated northwards from the Upper Missisippian to Upper Pennsylvanian. We suggest that the Money Complex may had been located to the east of the main river drainage inside the Zone Houillère basin or alternatively may represent a small basin, located on the east of the Zone Houillère.

  • Detrital zircon geochronology in Blueschist-Facies meta-conglomerates from the Western Alps: implications for the late Carboniferous to early Permian palaeogeography
    International Journal of Earth Sciences, 2014
    Co-Authors: Paola Manzotti, Marc Poujol, Michel Ballèvre
    Abstract:

    In the Western Alps, the Money Complex of the Gran Paradiso Massif, metamorphosed under Blueschist Facies during the Alpine cycle, is considered to be Permo-Carboniferous in age, but no palaeontological or radiometric data constrain this interpretation. A revision of the lithostratigraphy of the Money Complex allows recognizing a polygenic (graphite-rich) and a monogenic (graphite-poor) meta-sedimentary formation. Detrital zircon U–Pb geochronology in both meta-sedimentary formations shows that (i) the main population is Cambrian and Ordovician in age, (ii) the youngest grains are Silurian and Lower Devonian, and (iii) Carboniferous zircon grains are lacking. A careful study of the age distributions in the Alps suggests that potential source for the detrital material in the Money Complex is the Brianconnais basement. Late Carboniferous magmatism is widespread in the Helvetic Zone of the Alps. Permian magmatism is dominant in the Brianconnais, the Austroalpine and the Southalpine basements. The lack of Carboniferous zircons in the Money Complex suggests that the detritus was not shed from the Helvetic zone, which was separated from the Money basin by the Zone Houillere basin, where the main drainage pattern was developed from south to north and where the depocenters migrated northwards from the Upper Missisippian to Upper Pennsylvanian. We suggest that the Money Complex may had been located to the east of the main river drainage inside the Zone Houillere basin or alternatively may represent a small basin, located on the east of the Zone Houillere.

  • Detrital zircons in Blueschist-Facies meta-conglomerates : Implications for the Early Permian palaeo-topography of the Western Alps
    2014
    Co-Authors: Paola Manzotti, Marc Poujol, Michel Ballèvre
    Abstract:

    In theWestern Alps, the Money Unit is a monocyclic unit, metamorphosed at Blueschist Facies during the Alpine cycle. It crops out as a tectonic window below the overthrusted polycyclic, eclogite-bearing unit of the Gran Paradiso and its present position is the result of the Alpine tectonics. The Money Unit is considered to be Permo-Carboniferous in age, but no palaeontological or radiometric data constrain this interpretation. The Money unit exposes a sequence of clastic sediments and volcanics (Money Complex) that are intruded by a granitoid body (Erfaulet metagranite). In the Money Complex a polygenic (graphite-rich) and a monogenic (graphite-poor) meta-sedimentary formation have been recognized. The two sequences are separated by fine-grained biotite-amphibole gneisses, and by albite-bearing gneisses and amphibolites. Detrital zircon U-Pb geochronology in both meta-sedimentary formations shows that (i) the main population is Cambrian and Ordovician in age, (ii) the youngest grains are Silurian and Early Devonian, and (iii) Carboniferous zircons are lacking. A study of the age distributions in the Alps suggests that potential source for the detrital material in the Money Complex is the Briançonnais basement. Late Carboniferous magmatism is widespread in the Helvetic Zone of the Alps. Permian magmatism is dominant in the Briançonnais, in the Austroalpine, and in the Southern Alps. The lack of Carboniferous zircons in the Money Complex suggests that the detrital material did not derive from the erosion of the Helvetic Zone. The Helvetic Zone was separated from the Money basin by the Zone Houillère basin, where the main drainage pattern was developed from south to north and where the depocenters migrated northwards from the Namurian to the Stephanian.We suggest that the Money Complex may had been located to the east of the main river drainage inside the Zone Houillère basin or alternatively may represent a small basin, located on the east of the Zone Houillère.

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  • The Growth of Sodic Amphibole at the Greenschist- to Blueschist-Facies Transition (Dent Blanche, Western Alps): Bulk-rock Chemical Control and Thermodynamic Modelling
    Journal of Petrology, 2020
    Co-Authors: Paola Manzotti, Michel Ballèvre, Pavel Pitra, Benita Putlitz, Martin Robyr, Othmar Müntener
    Abstract:

    Abstract The sodic amphibole glaucophane is generally considered as indicative of Blueschist-Facies metamorphism. However, sodic amphiboles display a large range in chemical compositions, owing principally to the Fe2+Mg–1 and Fe3+Al–1 substitutions. Therefore, the whole-rock composition (namely its Na2O and FeO* content, and the Fe2+–Fe3+ ratio), strongly controls the stability field of the sodic amphiboles at the transition from greenschist- to Blueschist-Facies conditions. Neglecting these variables can lead to erroneous estimates of the metamorphic conditions and consequently the tectonic framework of the rocks. This paper explores the mechanisms that control the development of sodic amphibole and sodic pyroxene within the basement of the Dent Blanche Tectonic System (Western Alps), as a result of the Alpine metamorphic history. Field, petrographic and geochemical data indicate that sodic amphibole and sodic pyroxene form in different rock types: (1) in undeformed pods of ultramafic cumulates (hornblendite), sodic amphibole (magnesioriebeckite) forms coronas around magmatic pargasite; (2) metatonalite displays patches of radiating sodic (magnesioriebeckite) and calcic (actinolite) amphiboles; (3) sodic amphibole (magnesioriebeckite–glaucophane) occurs with high-Si potassic white mica (phengitic muscovite) in fine-grained (blue) schists; (4) in mylonitized granitoids (amphibole-gneiss) metasomatized along the contact with ultramafic cumulates, sodic amphibole (magnesioriebeckite–winchite) mainly forms rosettes or sheaves, generally without a shape-preferred orientation. Only locally are the needles aligned parallel to the stretching lineation. Pale green aegirine–augite is dispersed in an albite–quartz matrix or forms layers of fine-grained fibrous aggregates. The bulk-rock chemical composition of the different lithologies indicates that sodic amphibole and sodic pyroxene developed in Na- and Fe-rich systems or in a system with high Fe3+/Fe*. Thermodynamic modelling performed for different rock types (taking into account the measured Fe2O3 contents) reveals that sodic amphibole appears at ∼8 ± 1 kbar and 400–450 °C (i.e. at the transition between the greenschist- and Blueschist-Facies conditions) about 5 kbar lower than previous estimates. To test the robustness of our conclusion, we performed a review of sodic amphibole compositions from a variety of terranes and P–T conditions. This shows (1) systematic variations of composition with P–T conditions and bulk-rock chemistry, and (2) that the amphibole compositions reported from the studied area are consistent with those reported from other greenschist- to Blueschist-Facies transitions.

  • Greenschist or Blueschist Facies: the key role of redox conditions for the growth of sodic amphibole in the Dent Blanche Tectonic System (Western Alps)
    2019
    Co-Authors: Michel Ballèvre, Paola Manzotti, Pavel Pitra, Benita Putlitz, Othmar Müntener, Martin Robyr
    Abstract:

    The sodic amphibole glaucophane, commonly coloured in blue, is generally considered as the mineral indicative of Blueschist-Facies metamorphism (i.e. occurring in former subduction zones). However, sodic amphiboles display a large range of chemical compositions, due principally to the Fe2+Mg-1 and Fe3+Al-1 substitutions. Therefore, the whole-rock composition, and especially the oxidation state of a rock, strongly controls the stability field of the sodic amphibole at the transition from greenschist- to Blueschist-Facies. Under evaluating this point can lead to the incorrect account of the metamorphic conditions, resulting in a misinterpretation of the tectonic framework of the rocks. This work explores the mechanisms that can explain the scarcity of sodic amphibole and sodic pyroxene within the basement of the Dent Blanche Tectonic System (Western Alps), as the result of the Alpine metamorphic history. Field, petrographic and geochemical data indicate that sodic amphiboles crystallize in three different rock types. Firstly, in undeformed pods of ultramafic cumulates (hornblendite), sodic amphibole (magnesioriebeckite) forms coronas around magmatic calcic amphibole. Secondly, in mylonitized granitoids metasomatized along the contact with ultramafic cumulates (amphibole-gneiss and albitite), sodic amphibole (magnesioriebeckite-winchite) mainly forms rosettes or sheaves. Only locally the amphibole needles are aligned parallel to the mylonitic foliation and the stretching lineation. Pale green, patchy zoned aegirineaugite is dispersed in an albite-quartz matrix or forms layers of fine-grained fibrous aggregates. Thirdly, sodic amphibole (magnesioriebeckite-glaucophane) occurs with muscovite-epidote-quartz in fine-grained volcanoclastic schists. Bulk rock chemistry of the different lithologies indicates that sodic amphibole and sodic pyroxene developed in Fetotal rich system or in system with a high Fe3+ / (Fe2++Fe3+). Thermodynamic modelling has been performed for different rock types, taking into account the measured Fe2O3 contents. This allows exploring the effect of varying the oxidation state ratio and the water content. Results of these numerical models highlight the role of Fe2O3 on stabilizing sodic amphibole and sodic pyroxene and suggests that sodic amphibole is stable at ~ 8-10 kbar and 400-450°C, i.e. at the transition between the greenschist- and Blueschist-Facies. Our models suggest lower pressures compared to other estimates based on Si content in muscovite and provide better constraints on the Alpine metamorphic evolution of the Dent Blanche Tectonic System.

  • High-pressure rocks in the Variscan belt of Western Europe: the Malpica-Tui Complex (NW Iberian Massif)
    2015
    Co-Authors: Alicia López-carmona, Pavel Pitra, J. Abati, Gabriel Gutiérrez-alonso, James K. W. Lee, Javier Fernández-suárez
    Abstract:

    Pods and relicts of high-pressure (HP) rocks are the best record of the subduction of the north Gondwana margin at the onset of the Variscan collision. Across Western Europe, Blueschist-Facies terranes are restricted to scarce and relatively small areas, whereas eclogite-Facies terranes are more abundant. The Malpica-Tui complex (MTC) is the westernmost exposure of HP rocks in the NW Iberian Massif, and in the Variscan belt of Western Europe. It comprises two tectonically juxtaposed units separated by an extensional detachment: (i) an upper unit consisting of rocks in the Blueschist-Facies, and (ii) a lower unit in the medium temperature eclogite-Facies conditions. Assuming a northwest-directed component of subduction, in present day coordinates, the characteristics of each sequence suggest that the upper unit would occupy an oceanward position compared to the lower unit before the Variscan collision. Thus, the lower unit is interpreted as a slice of continental crust, whereas the upper unit may represent a transitional to oceanic crust of the same continental margin. The MTC preserves evidence of late Devonian HP metamorphism varying from eclogite (P~26 kbar and T~650 °C) to Blueschist-Facies conditions (19-22 kbar and 460-560 °C). Petrological analysis involving P-T-X pseudosections in the (Mn)NCKFMASHTO chemical system on the HP rocks reveals a P-T evolution characterised by a subisothermal decompression to ~10 kbar, 480 °C in the Blueschist-Facies rocks and 650 °C in the eclogites, followed by cooling to ~5 kbar at 380 °C and 500 °C, respectively. New 40Ar/39Ar data indicate a minimum age of ~370 Ma for the subduction-related HP metamorphism. Subsequent decompression to pressures of about 10 kbar started at ~360 Ma and was contemporaneous with thrust-and-fold nappe tectonics and intrusion of early Variscan granodiorites dated at ~350- 340 Ma. Final, “post-nappe”, exhumation is interpreted to have taken place from ~345-335 Ma to 320 ± 5 Ma, which is the age of the syntectonic igneous rocks emplaced in the autochthon of the MTC. These ages support the equivalence of the HP rocks from NW Iberia and their counterparts in the southern Armorican Massif. From a methodological point of view, modelling calculations of H2O and Fe2O3 on the metamorphic evolution of Blueschist-Facies rocks reveals trends that may have general applications in the investigation of rocks with similar composition: (i) subduction-zone metamorphism may occur in H2O-undersaturated conditions induced by the crystallization of a significant modal amount of lawsonite, although the transition from lawsonite Blueschist Facies to amphibolite-greenschist Facies may involve significant hydration, principally as a result of lawsonite breakdown. (ii) The analysed values of Fe2O3 may not reflect the oxidation state during the main metamorphic evolutionary stage and are probably easily modified by superficial alteration, even in apparently fresh samples

  • Blueschist-Facies metapelites from the Malpica-Tui Unit (NW Iberian Massif): phase equilibria modelling and H2O and Fe2O3 influence in high-pressure assemblages
    Journal of Metamorphic Geology, 2013
    Co-Authors: A. Lopez-carmona, Pavel Pitra, J. Abati
    Abstract:

    The Malpica-Tui Unit (Galicia, NW Spain) records eclogite- and Blueschist-Facies metamorphism during the onset of the Variscan orogeny in Europe. Petrological analysis involving pseudosections calculated using THERMOCALC shows that the Upper Sheet of this unit, the Cea' n Schists, recorded a threestage metamorphic evolution involving (i) Early subduction-related medium-pressure ⁄ low-temperature metamorphism (M1) constrained at 350-380 C, 12-14 kbar, which is only recorded in the basal part (lower metapelites, LM) of the Cea'n Schists. (ii) Subduction-related Blueschist Facies prograde metamorphism (M2) going from 19 kbar, 420 C to 21 kbar, 460 C in the LM, and from 16 kbar 430 C to 21-22 kbar, 520 C in the structurally upper metapelites (UM). (iii) Exhumation-related metamorphism (M3) is characterized by a decompression to 8-10 kbar, 470-490 C in the LM. This decompression is also recorded in the UM, but it was not possible to estimate precise P-T conditions. The calculations indicate that (i) the prograde evolution in subduction zones may occur in fluidundersaturated conditions due to the crystallization of lawsonite, even in metapelitic rocks. This significantly influences phase equilibria and hence the P-T estimates. (ii) The proportion of ferric iron also has a strong influence on phase equilibria, even in metapelites. However, the analysed values of Fe2O3 may not reflect the oxidation state during the main metamorphic evolution and are probably easily modified by superficial alteration even in apparently fresh samples. The use of P-T-X(Fe2O3) pseudosections together with petrographic observations is then necessary to estimate the real oxidation state of the rocks and correctly evaluate the P-T conditions.

  • Blueschist‐Facies metapelites from the Malpica–Tui Unit (NW Iberian Massif): phase equilibria modelling and H2O and Fe2O3 influence in high‐pressure assemblages
    Journal of Metamorphic Geology, 2012
    Co-Authors: A. Lopez-carmona, Pavel Pitra, J. Abati
    Abstract:

    The Malpica–Tui Unit (Galicia, NW Spain) records eclogite- and Blueschist-Facies metamorphism during the onset of the Variscan orogeny in Europe. Petrological analysis involving pseudosections calculated using THERMOCALC shows that the Upper Sheet of this unit, the Cean Schists, recorded a threestage metamorphic evolution involving (i) Early subduction-related medium-pressure ⁄ low-temperature metamorphism (M1) constrained at ~o350–380 oC, 12–14 kbar, which is only recorded in the basal part (lower metapelites, LM) of the Cea´n Schists. (ii) Subduction-related Blueschist Facies prograde metamorphism (M2) going from 19 kbar, 420 oC to 21 kbar, 460 oC in the LM, and from 16 kbar 430 oC to 21–22 kbar, 520 oC in the structurally upper metapelites (UM). (iii) Exhumation-related metamorphism (M3) is characterized by a decompression to 8–10 kbar, 470–490 oC in the LM. This decompression is also recorded in the UM, but it was not possible to estimate precise P–T conditions. The calculations indicate that (i) the prograde evolution in subduction zones may occur in fluidundersaturated conditions due to the crystallization of lawsonite, even in metapelitic rocks. This significantly influences phase equilibria and hence the P–T estimates. (ii) The proportion of ferric iron also has a strong influence on phase equilibria, even in metapelites. However, the analysed values of Fe2O3 may not reflect the oxidation state during the main metamorphic evolution and are probably easily modified by superficial alteration even in apparently fresh samples. The use of P–T–X(Fe2O3) pseudosections together with petrographic observations is then necessary to estimate the real oxidation state of the rocks and correctly evaluate the P–T conditions.