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Aral I. Okay - One of the best experts on this subject based on the ideXlab platform.

  • Pre-collisional accretionary growth of the southern Laurasian active margin, Central Pontides, Turkey
    Tectonophysics, 2016
    Co-Authors: Mesut Aygül, Aral I. Okay, Roland Oberhänsli, Masafumi Sudo
    Abstract:

    Abstract Cretaceous subduction–accretionary complexes crop out over wide areas in the central part of the Pontides, northern Turkey. To the north, the wedge consists of a low-grade metaflysch sequence with blocks of marble, Na-amphibole-bearing metabasite (PT = 7–12 kbar; 400 ± 70 °C) and serpentinite. 40Ar/39Ar phengite ages from the phyllites of the metaflysch are ca. 100 Ma. The metaflysch sequence is underlain by oceanic crust-derived HP/LT metabasites and micaschists along a major detachment fault. The metabasites are epidote-Blueschists consisting of glaucophane, epidote, titanite, and phengite locally with garnet. Fresh lawsonite-Blueschists are exposed as blocks along the detachment fault. Peak metamorphic conditions of a garnet-Blueschist are constrained to 17 ± 1 kbar and 500 ± 40 °C and of a lawsonite-Blueschist to 14 ± 2 kbar and 370–440 °C. 40Ar/39Ar phengite dating on the micaschists constrains the HP/LT metamorphism as 101–92 Ma, younging southward. Middle Jurassic (ca. 160 Ma) accretionary complexes consisting of Blueschist to lower greenschist facies metabasites, marble and volcanogenic metasediment intercalations are exposed at the southern part of the Cretaceous wedge. In the studied area, the North Anatolian Fault forms the contact between Cretaceous and Middle Jurassic HP/LT metamorphic rocks. Wide distribution of Cretaceous subduction–accretionary complexes implies accretionary tectonic continental growth along the Laurasian active margin. High amount of clastic sediment flux into the trench has a major effect on enlarging the wedge during the Albian. Tectonic thickening of the oceanic HP/LT metamorphic sequence, however, was possibly achieved by propagation of the decollement along the retreating slab which can create the space necessary for progressive deep level basal underplating and extension of the wedge for subsequent syn-subduction exhumation.

  • Late Cretaceous Blueschists and ophiolites in Thrace (Turkey): geodynamic implications
    2010
    Co-Authors: Gültekin Topuz, Aral I. Okay
    Abstract:

    Best evidence for the location and periods of activity of palaeo-subduction zones are provided by the tectonic setting and age of Blueschists and eclogites, especially those associated with ophiolites. Eclogites and ultra-high pressure rocks are widely reported from the crystalline rocks of the Rhodope Massif. However, their tectonic setting and their age are controversial with radiometric ages scattered between Jurassic and Eocene. A newly discovered Blueschist sliver in the southern Turkish Thrace close to the Rhodope Massif provides robust data for a Late Cretaceous high pressure metamorphism, and hence for Late Cretaceous subduction in the region. The Blueschist facies rocks in Thrace occur as a tectonic sliver, 9 km long and 1 km wide, bounded by the strands of the North Anatolian Fault. Two types of Blueschist-facies rock assemblages occur in the sliver: (i) A serpentinite body with numerous diabase dykes showing an incipient Blueschist-facies metamorphism (ii) a wellfoliated and thoroughly recrystallized unit consisting of metabasite, marble and metachert. The Blueschists are associated with serpentinite, pelagic limestone, radiolarian chert and greywacke and are unconformably overlain by Upper Eocene shallow marine limestones. Field relations in combination with the bore hole data suggest that the tectonic sliver forms a positive flower structure within the Miocene clastic rocks in a transpressional strike-slip setting, and represents an uplifted part of the pre-Eocene basement of southern Thrace.

  • BlueschistS, ECLOGITES, OPHIOLITES AND SUTURE ZONES IN NORTHWEST TURKEY: A REVIEW AND A FIELD EXCURSION GUIDE
    Ofioliti, 2010
    Co-Authors: Aral I. Okay, Donna L. Whitney
    Abstract:

    The Tavsanli Zone in western and south-central Turkey constitutes one of the largest and best preserved high-pressure metamorphic belts in the world. Blueschists and scarce lawsonite-eclogites crop out in an east-west-rending zone, 50-60 km wide and about 250 km long, and are tectonically overlain by an oceanic accretionary complex and large ophiolite slabs. Undeformed Early to Middle Eocene granodiorites (ca. 50 Ma 40Ar/39Ar isochron ages), intrude the Blueschists and the overlying ophiolite. The Blueschists of the Tavsanli Zone represent the subducted north-facing passive continental margin of the Anatolide- Tauride block. They were partly exposed by, or were at high crustal levels during the latest Cretaceous, prior to the Paleocene continent-continent collision. The Blueschists of the Tavsanli Zone comprise a variety of metasedimentary and metamagmatic lithologies. The metasedimentary sequence, originally deposited on continental crust, comprise a schist-marble series of Mesozoic age with peak pressure and temperatures of ~24 kbar and 430-500°C, respectively, and with Late Cretaceous (ca. 80 Ma) metamorphic ages. The oceanic lithologies are made up mainly of metabasites with metacherts, phyllites, which have undergone metamorphism from incipient Blueschist facies to lawsonite-eclogite facies. The guide, prepared for a three-day field trip, provides information on the tectonic setting, tectonostratigraphy, petrology, and geological evolution of the Tavsanli Zone Blueschists and lawsonite-eclogites followed by detailed description of the field stops. It also includes data on the Triassic Blueschists and eclogites from north of the Tavsanli Zone.

  • Late Cretaceous Blueschist facies metamorphism in southern Thrace (Turkey) and its geodynamic implications
    Journal of Metamorphic Geology, 2008
    Co-Authors: Gültekin Topuz, Aral I. Okay, Rainer Altherr, Muharrem Satir, Winfried H. Schwarz
    Abstract:

    A Blueschist facies tectonic sliver, 9 km long and 1 km wide, crops out within the Miocene clastic rocks bounded by the strands of the North Anatolian Fault zone in southern Thrace, NW Turkey. Two types of Blueschist facies rock assemblages occur in the sliver: (i) A serpentinite body with numerous dykes of incipient Blueschist facies metadiabase (ii) a well-foliated and thoroughly recrystallized rock assemblage consisting of Blueschist, marble and metachert. Both are partially enveloped by an Upper Eocene wildflysch, which includes olistoliths of serpentinite-metadiabase, Upper Cretaceous and Palaeogene pelagic limestone, Upper Eocene reefal limestone, radiolarian chert, quartzite and minor greenschist. Field relations in combination with the bore core data suggest that the tectonic sliver forms a positive flower structure within the Miocene clastic rocks in a transpressional strike-slip setting, and represents an uplifted part of the pre-Eocene basement. The Blueschists are represented by lawsonite-glaucophane- bearing assemblages equilibrated at 270-310 � C and � 0.8 GPa. The metadiabase dykes in the serpentinite, on the other hand, are represented by pumpellyite-glaucophane-lawsonite-assemblages that most probably equilibrated below 290 � C and at 0.75 GPa. One metadiabase olistolith in the Upper Eocene flysch sequence contains the mineral assemblage epidote + pumpellyite + glaucophane, recording P-T conditions of 290-350 � C and 0.65-0.78 GPa, indicative of slightly lower depths and different thermal setting. Timing of the Blueschist facies metamorphism is constrained to c. 86 Ma (Coniacian ⁄ Santonian) by Rb-Sr phengite-whole rock and incremental 40 Ar- 39 Ar phengite dating on Blueschists. The activity of the strike-slip fault post-dates the Blueschist facies metamorphism and exhumation, and is only responsible for the present outcrop pattern and post-Miocene exhumation (� 2 km). The high-P ⁄ T metamorphic rocks of southern Thrace and the Biga Peninsula are located to the southeast of the Circum Rhodope Belt and indicate Late Cretaceous subduction and accretion under the northern continent, i.e. the Rhodope Massif, enveloped by the Circum Rhodope Belt. The Late Cretaceous is therefore a time of continued accretionary growth of this continental domain.

  • Triassic Blueschists and eclogites from northwest Turkey: vestiges of the Paleo-Tethyan subduction
    Lithos, 2002
    Co-Authors: Aral I. Okay, Olivier Monod, Patrick Monié
    Abstract:

    Abstract Triassic eclogite and Blueschist facies rocks occur as a thrust sheet, 25-km long and over 2-km thick, in an Eocene fold-and-thrust belt in northwest Turkey along the Izmir–Ankara suture. The thrust sheet consists mainly of metabasites with minor marble, phyllite and metachert, and rare lenses of serpentinite. The common Blueschist facies mineral assemblage in the metabasites is sodic amphibole+epidote+albite+chlorite+phengite±garnet. Sodic amphibole commonly shows replacement by barroisite, and there is continuous petrographic transition from Blueschist–metabasites to barroisite-bearing epidote–amphibolites. Eclogite with the mineral assemblage of garnet+sodic pyroxene+sodic–calcic amphibole+epidote is found only in one locality. P–T conditions of the epidote–Blueschist facies metamorphism are estimated as 450±50 °C and 11±2 kbar. The Blueschist formation was followed by a decrease in pressure and increase in temperature, leading to the development of barroisite-bearing epidote–amphibolites. Phengite, sodic amphibole and barroisite Ar/Ar ages from three metabasic rocks range between 215 and 205 Ma, and indicate Late Triassic high-pressure metamorphism. The Triassic Blueschists in northwest Turkey constitute part of a much larger allochthonous tectonic unit of Triassic mafic volcanic rocks. They probably represent the upper layers of a Triassic oceanic plateau, which was accreted to the Laurasian margin during the latest Triassic. The close spatial association of the Triassic and Cretaceous Blueschists along the Izmir–Ankara suture suggests that the suture represents a long-lived plate boundary of Late Palaeozoic to early Tertiary age.

Jun Gao - One of the best experts on this subject based on the ideXlab platform.

  • Blueschist facies rehydration of eclogites tian shan nw china implications for fluid rock interaction in the subduction channel
    Chemical Geology, 2008
    Co-Authors: Francois Van Der Straaten, Timm John, Volker Schenk, Jun Gao
    Abstract:

    Chemical changes associated with the rehydration of dry eclogites to form Blueschists were studied to obtain information about the chemistry of the fluids infiltrating during this retrograde metamorphic overprint. The studied eclogites of the Tian Shan were formed during Carboniferous subduction of pillow basalts that show typical ocean island basalt geochemical signatures. The retrograde P–T path is characterised by decompression associated with cooling, typical for fragments of a subducting slab that are ascending in the cool and hydrated subduction channel. The fluids infiltrating the eclogites are interpreted to represent dehydration fluids of the down going slab that ascended to the overlying subduction channel. The rehydration of the eclogites proceeded from the pillow margins producing two concentric shells consisting of glaucophane-dominated Blueschists (inner shell) and phengite–ankerite Blueschists (outer shell) replacing omphacite and garnet of the eclogite. Mass-balance calculations based on major and trace element compositions of eclogites and Blueschists point to high element mobility during fluid infiltration and associated metamorphic reactions. Blueschists show lower contents of REE, Y, Sr, Pb, U, P, Ca, Na, Al and Si compared to eclogites, indicating a loss of these elements, but they show higher contents of volatiles, Mg, transition metals and LILE, indicating a gain of these elements. The chemical changes point to a composition of the infiltrating fluid that has an affinity to fluids derived from serpentinites, i.e. with low concentrations in Si, Al, Ca and Na and high concentrations in Mg, Co and Ni. The gain of Cu, Mn, Zn and LILE in the Blueschists points to an addition of a sediment-derived component in the serpentinite-derived fluid. The pronounced enrichment of K2O and of Ba at constant Ba/Th ratios in the Blueschists resembles those of island arc basalts derived from a fluid-enriched source. The low element load of the fluid equilibrated with Mg-rich melange zones induces the mobility of most of the elements during the eclogite–Blueschist transformation to compensate for disequilibrium between the mafic rock and the infiltrating fluid. The associated volume loss enhances the fluid infiltration into the eclogite interior. Increasing precipitation rates and/or diminishing diffusion rates for element removal brought the eclogite–Blueschist transformation to an end, which resulted in the preservation of eclogite in the pillow cores.

  • Blueschist-facies rehydration of eclogites (Tian Shan, NW-China): Implications for fluid–rock interaction in the subduction channel
    Chemical Geology, 2008
    Co-Authors: Francois Van Der Straaten, Timm John, Volker Schenk, Jun Gao
    Abstract:

    Chemical changes associated with the rehydration of dry eclogites to form Blueschists were studied to obtain information about the chemistry of the fluids infiltrating during this retrograde metamorphic overprint. The studied eclogites of the Tian Shan were formed during Carboniferous subduction of pillow basalts that show typical ocean island basalt geochemical signatures. The retrograde P–T path is characterised by decompression associated with cooling, typical for fragments of a subducting slab that are ascending in the cool and hydrated subduction channel. The fluids infiltrating the eclogites are interpreted to represent dehydration fluids of the down going slab that ascended to the overlying subduction channel. The rehydration of the eclogites proceeded from the pillow margins producing two concentric shells consisting of glaucophane-dominated Blueschists (inner shell) and phengite–ankerite Blueschists (outer shell) replacing omphacite and garnet of the eclogite. Mass-balance calculations based on major and trace element compositions of eclogites and Blueschists point to high element mobility during fluid infiltration and associated metamorphic reactions. Blueschists show lower contents of REE, Y, Sr, Pb, U, P, Ca, Na, Al and Si compared to eclogites, indicating a loss of these elements, but they show higher contents of volatiles, Mg, transition metals and LILE, indicating a gain of these elements. The chemical changes point to a composition of the infiltrating fluid that has an affinity to fluids derived from serpentinites, i.e. with low concentrations in Si, Al, Ca and Na and high concentrations in Mg, Co and Ni. The gain of Cu, Mn, Zn and LILE in the Blueschists points to an addition of a sediment-derived component in the serpentinite-derived fluid. The pronounced enrichment of K2O and of Ba at constant Ba/Th ratios in the Blueschists resembles those of island arc basalts derived from a fluid-enriched source. The low element load of the fluid equilibrated with Mg-rich mélange zones induces the mobility of most of the elements during the eclogite–Blueschist transformation to compensate for disequilibrium between the mafic rock and the infiltrating fluid. The associated volume loss enhances the fluid infiltration into the eclogite interior. Increasing precipitation rates and/or diminishing diffusion rates for element removal brought the eclogite–Blueschist transformation to an end, which resulted in the preservation of eclogite in the pillow cores

  • New Age Constraints on the Metamorphic Evolution of the High‐Pressure/Low‐Temperature Belt in the Western Tianshan Mountains, NW China
    The Journal of Geology, 2005
    Co-Authors: Reiner Klemd, Jun Gao, Michael Bröcker, Bradley R. Hacker, Phillip B. Gans, Klaus Wemmer
    Abstract:

    Abstract The western Tianshan high‐pressure/low‐temperature orogenic belt in NW China contains eclogite‐facies metavolcanic rocks and omphacite‐bearing Blueschists. Previous Sm‐Nd (omphacite, garnet, glaucophane, whole rock) and 40Ar/39Ar (crossite) dating of eclogite‐facies rocks has suggested an age of ca. 345 Ma as the best approximation for the timing of peak metamorphic conditions. The samples described here are Blueschist‐facies rocks that formed during or after the transition from the eclogite‐facies to the epidote‐Blueschist‐facies and subsequently experienced an incipient greenschist‐facies overprint. By use of white mica geochronology (K‐Ar, 40Ar/39Ar, Rb‐Sr), an attempt is made to date postpeak metamorphic stages of the complex PT path. Rb‐Sr and 40Ar/39Ar ages range between 313 and 302 Ma and 323 and 312 Ma, respectively, but mostly cluster at ca. 310–311 Ma, indicating that the studied samples recrystallized at this time. However, the 40Ar/39Ar age spectra show complex release patterns that a...

  • primary fluids entrapped at Blueschist to eclogite transition evidence from the tianshan meta subduction complex in northwestern china
    Contributions to Mineralogy and Petrology, 2001
    Co-Authors: Jun Gao, Reiner Klemd
    Abstract:

    The Tianshan is the first locality worldwide where primary fluids at Blueschist to eclogite transition have been documented. Veins containing high-pressure minerals in massive host eclogites transitional to Blueschists, or eclogite boudins surrounded by Blueschists, indicate that a free fluid phase was present at the time of eclogitization. The high-pressure veins are predominantly composed of omphacite fibers with minor quartz or calcite. The transition from Blueschist- to eclogite-facies parageneses occurs as "dehydration" halos around these veins. Clinozoisite, paragonite, glaucophane, and omphacite inclusions preserved in garnet porphyroblasts in wall eclogites document the transformation of Blueschist to eclogite. C-axis-parallel, non-planar populations of fluid inclusions were trapped during the growth of omphacite in high-pressure veins and dehydrated wall rocks. Low salinity H2O + NaCl ± solid-bearing inclusions are preserved in omphacite fibers in veins and matrix omphacite of wall rocks. None of the isochores of these low salinity aqueous fluid inclusions intersect peak eclogite-facies metamorphic conditions, suggesting that, although the textural evidence constrains the entrapment of fluid inclusions to peak metamorphic conditions, their densities must have been modified during exhumation. The fluids are interpreted to have been derived from the host Blueschist as a result of dehydration reactions such as 13 Gln+5 Czo=9 Prp+26 Jd+12 Di+19 Qtz+15 H2O and Gln+Pg=Prp+3 Jd+2 Otz+2 H2O. The similarity of vein and wall rock mineral compositions, fluid inclusion characteristics and O-isotope data also favor an internal source for the fluids. The major element composition of veins indicate that Si, Na, and Ca-rich aqueous fluids were released during dehydration at a depth of 50±10 km within a Paleozoic subduction zone.

  • The mineralogy, petrology, metamorphic PTDt trajectory and exhumation mechanism of Blueschists, south Tianshan, northwestern China
    Tectonophysics, 1995
    Co-Authors: Jun Gao, Xu-chang Xiao, Yaoqing Tang, Jun Wang, Min Zhao
    Abstract:

    Abstract The south Tianshan Blueschist belt extends for about 200 km along the south Tianshan suture from Changawuzhi to the upper Kekesu river. The high-pressure—low-temperature (high- P /low- T ) metamorphic rocks contain sodic amphibole, garnet, phengite, albite, quartz, epidote, chlorite and stilpnomelane. These Blueschists occur as small blocks, lenses, laminae and bedded slabs or sheets in greenschist facies country rocks. A few dismembered ophiolitic nappe slices and marble lenses are interlayered in the high- P / T metamorphic belt. Metabasite Blueschists are mainly E-type MORB, less commonly N-type. The protoliths of the meta-sedimentary rocks are graywackes, which contain volcaniclastic material of E-type MORBs, and probably some continental basement materials, derived from the Tarim plate. These protoliths may represent subduction complexes or meta-melanges, which formed within an accretionary wedge on the southern side of the Yili-Central Tianshan plate (YCTP) at the end of the early Paleozoic. The mineral assemblages include sodic amphibole (mainly crossite) + garnet + phengite + epidote + chlorite + albite, estimated to have crystallized at 450°C and 9–10 kbar. These conditions correspond to the epidote-Blueschist facies and are the peak metamorphic conditions. The prograde metamorphism is represented by lawsonite-Blueschist facies and the retrograde is greenschist facies. The metamorphic PTDt trajectory of the Blueschists runs clockwise and reveals that the metamorphism revolved from laumontite facies (?), lawsonite-Blueschist facies (?), epidote-Blueschist facies (408 Ma?) to greenschist facies (345 Ma), during the creation and exhumation of the Blueschists in the south Tianshan suture. We have recognized six stages of deformation in the Blueschists. The earlier three stages were concomitant with metamorphism, the later three postdated the metamorphism. On the basis of the deformation characteristics of the Blueschists and coherent rocks, the regional geology and the metamorphic PTDt path we discuss the exhumation mechanism of the Blueschists. The post-collisional ductile thrust within the suture probably exhumed the Blueschists and coherent rocks, allowing them to escape retrograde overprinting in the greenschist facies. The influence of erosion on the exhumation of Blueschists could not be considered directly and quantitatively because of the absence of Devonian strata and the few exposures of Early Carboniferous clastic rocks, all without detritus of Blueschists.

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  • Exhumation of Blueschists along a Tethyan suture in northwest Turkey
    Tectonophysics, 1998
    Co-Authors: Aral I. Okay, Nigel Harris, Simon P. Kelley
    Abstract:

    A Blueschist belt formed during the mid-Cretaceous (∼88 Ma) high-pressure metamorphism of a passive continental margin sequence of shale, siltstone, quartzite and limestone occurs immediately south of the Tethyan suture in northwest Turkey. A transect across the Tethyan suture was mapped to constrain the exhumation of Blueschist-facies assemblages. The presence of jadeite, glaucophane and lawsonite in the Blueschists indicates P-T conditions of 20 kbar and 430°C. The Blueschists are tectonically overlain by a Cretaceous oceanic accretionary complex showing an incipient Blueschist metamorphism. A peridotite slab with subvertical layering lies along a low-angle fault contact over the Blueschists and the accretionary complex. Isolated slices of Early Cretaceous (101 ± 4 Ma) garnet-amphibolites occur at the base of the peridotite. The garnet-amphibolites, which show an incipient Blueschist-facies overprint, have probably formed during the oceanic subduction in the foot-wall of the subduction zone and later underplated the hanging-wall. Blueschists and peridotites are abruptly truncated in the north by the steeply dipping suture fault, that juxtaposes unmetamorphosed and slightly deformed Triassic to Jurassic sediments of the northern continent against the peridotite and the accretionary complex. Tectonic juxtaposition of the lower-pressure accretionary complex and peridotites over the higher-pressure Blueschists implies the removal of a 45-km-thick oceanic mantle sequence above the Blueschists. Regional geological and geometric constraints suggest that the exhumation of the Blueschists was achieved by a combination of two simultaneously acting mechanisms. The first was the detachment of the upper crustal Blueschist sequence from its basement in the subduction zone and its incorporation to the hanging-wall followed by buoyant ascent. The second mechanism is the progressive shallowing of the subducted continental lithosphere resulting in the thinning of the mantle wedge above the subducted slab. The Blueschists were partly exposed prior to the Paleocene continent-continent collision. Post-collisional Eocene (52-48 Ma) calc-alkaline plutons, that were emplaced in the Blueschists and the overlying peridotite at ∼10 km depth, are interpreted to have formed by crustal anatexis from advective heating by mantle melts. These have probably been generated during the upwelling of the asthenosphere under the shallowing continental lithosphere.

  • tectonic setting petrology and geochronology of jadeite glaucophane and chloritoid glaucophane schists from north west turkey
    Journal of Metamorphic Geology, 1994
    Co-Authors: Aral I. Okay, Simon P. Kelley
    Abstract:

    The north-west Turkish Blueschists represent a subducted passive continental margin sequence dominated by metaclastic rocks and marble. The depositional age of the Blueschist protoliths are probably Palaeozoic to Mesozoic, while the age of the high-pressure/low-temperature metamorphism is Late Cretaceous. Blueschists are tectonically overlain by a volcanosedimentary sequence made up of accreted oceanic crustal material that locally shows incipient Blueschist metamorphism and by spinel peridotite slices. The metaclastic rocks with regional jadeite and glaucophane, which comprise the lower part of the Blueschist unit, make up an over 1000-m-thick coherent sequence in the Kocasu region of north-west Turkey. Rare metabasic horizons in the upper parts of the metaclastic sequence with sodic amphibole + Iawsonite but no garnet indicate lawsonite Blueschist facies metamorphism. The Blueschist metaclastics in the Kocasu region are practically free of calcium and ferric iron and closely approximate the NFMASH system in bulk composition. Two low-variance mineral assemblages (with quartz and phengite) are jadeite + glaucophane + chlorite + paragonite and chloritoid + glaucophane + paragonite. The metaclastics comprise up to several-metres-thick layers of jadeite schist with quartz, phengite and nearly 100 mol% jadeite. Phase relations in the metaclastics show that the chloritoid + glaucophane assemblage, even in Fe2+-rich compositions, is stable in the jadeite stability field. In the NFASH system the above assemblage without the accompanying garnet has a narrow thermal stability field. Mineral equilibria in the metaclastics involving chloritoid, glaucophane, jadeite, paragonite and chlorite indicate metamorphic P-T conditions of 20 ± 2 kbar and 430 ± 30 d C, yielding geothermal gradients close to 5d C km-1, one of the lowest geotherms recorded. Blueschists in the Kocasu region, which have been buried to 70 km depth, are tectonically overlain by the volcanosedimentary sequence and by peridotite buried not deeper than 30 km. Phengites from two jadeite schists were dated by Ar/Ar laser probe; they give an age of 88.5 ± 0.5 Ma, interpreted as the age of metamorphism. Blueschists and the overlying peridotite bodies are intruded by 48-53-Ma-old granodiorite bodies that were emplaced at 10 km depth. This suggests that the exhumation of Blueschists by underplating of cold continental crust, and normal faulting at the Blueschist-peridotite, interface occurred during the Late Cretaceous to Palaeocene (88-53 Ma).

  • Tectonic setting, petrology and geochronology of jadeite + glaucophane and chloritoid + glaucophane schists from north-west Turkey
    Journal of Metamorphic Geology, 1994
    Co-Authors: Aral I. Okay, Simon P. Kelley
    Abstract:

    The north-west Turkish Blueschists represent a subducted passive continental margin sequence dominated by metaclastic rocks and marble. The depositional age of the Blueschist protoliths are probably Palaeozoic to Mesozoic, while the age of the high-pressure/low-temperature metamorphism is Late Cretaceous. Blueschists are tectonically overlain by a volcanosedimentary sequence made up of accreted oceanic crustal material that locally shows incipient Blueschist metamorphism and by spinel peridotite slices. The metaclastic rocks with regional jadeite and glaucophane, which comprise the lower part of the Blueschist unit, make up an over 1000-m-thick coherent sequence in the Kocasu region of north-west Turkey. Rare metabasic horizons in the upper parts of the metaclastic sequence with sodic amphibole + Iawsonite but no garnet indicate lawsonite Blueschist facies metamorphism. The Blueschist metaclastics in the Kocasu region are practically free of calcium and ferric iron and closely approximate the NFMASH system in bulk composition. Two low-variance mineral assemblages (with quartz and phengite) are jadeite + glaucophane + chlorite + paragonite and chloritoid + glaucophane + paragonite. The metaclastics comprise up to several-metres-thick layers of jadeite schist with quartz, phengite and nearly 100 mol% jadeite. Phase relations in the metaclastics show that the chloritoid + glaucophane assemblage, even in Fe2+-rich compositions, is stable in the jadeite stability field. In the NFASH system the above assemblage without the accompanying garnet has a narrow thermal stability field. Mineral equilibria in the metaclastics involving chloritoid, glaucophane, jadeite, paragonite and chlorite indicate metamorphic P-T conditions of 20 ± 2 kbar and 430 ± 30 d C, yielding geothermal gradients close to 5d C km-1, one of the lowest geotherms recorded. Blueschists in the Kocasu region, which have been buried to 70 km depth, are tectonically overlain by the volcanosedimentary sequence and by peridotite buried not deeper than 30 km. Phengites from two jadeite schists were dated by Ar/Ar laser probe; they give an age of 88.5 ± 0.5 Ma, interpreted as the age of metamorphism. Blueschists and the overlying peridotite bodies are intruded by 48-53-Ma-old granodiorite bodies that were emplaced at 10 km depth. This suggests that the exhumation of Blueschists by underplating of cold continental crust, and normal faulting at the Blueschist-peridotite, interface occurred during the Late Cretaceous to Palaeocene (88-53 Ma).

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  • metamorphic p t t d evolution of u hp metabasites from the south tianshan accretionary complex nw china implications for rock deformation during exhumation in a subduction channel
    Gondwana Research, 2017
    Co-Authors: Jérémie Soldner, Emilien Oliot, Karel Schulmann, Pavla Štípská, Vladimír Kusbach, Robert Anczkiewicz
    Abstract:

    The late Carboniferous accretionary system of the South Tianshan orogen (North-Western China) underwent complex structural and polymetamorphic evolution. Combined petrological, geochronological and microstructural analysis of (ultra)high-pressure (UHP) metabasites (eclogites and Blueschists) enclosed in metapelites show a relict coarse-grained eclogitic fabric S2 surrounded by a dominant fine-grained eclogite and Blueschist facies retrograde fabric S2. The S2 fabric is reworked by upright folds F3 that are responsible for a major shortening of the whole accretionary system. For both the eclogite and Blueschist, peak and retrograde P–T conditions have been thermodynamically constrained at 25–26 kbar and 425–500 °C and 10–13 kbar and 500−550 °C respectively, suggesting a shared exhumation history. The garnet-whole rock-amphibole isochron in the Blueschist yielded Lu–Hf and Sm–Nd ages of 326.0 ± 2.9 Ma and 318.4 ± 3.9 Ma respectively, interpreted to date the prograde to peak metamorphic assemblage. The retrograde path of the eclogite is characterized by heterogeneous omphacite recrystallization into a mylonitic fine-grained matrix and crystallization of blue amphibole. Microstructures in both pristine porphyroclastic and recrystallized fine-grained domains in the eclogite indicate a gradual evolution from constriction-dominated (L>S-type) to flattening-dominated (S>L-type) type of deformation, increase of fabric intensity reflected by gradually growing M-indexes and the development of lattice preferred orientation (LPO) typical for dislocation creep under slightly hydrated conditions. Recrystallization of the matrix in the Blueschist is homogeneous, which indicates a matrix dominated channel flow during exhumation. These LPOs evolutions suggest a significant mechanical coupling with the upper plate concomitant with oroclinal bending of the Kazakh orocline. Lock up of Kazakh orocline is responsible for further stress increase resulting in horizontal shortening of South Tianshan accretionary wedge and development of D3 upright folding and steepening of the whole sequence.

  • Metamorphic P–T–t–d evolution of (U)HP metabasites from the South Tianshan accretionary complex (NW China) — Implications for rock deformation during exhumation in a subduction channel
    Gondwana Research, 2017
    Co-Authors: Jérémie Soldner, Emilien Oliot, Karel Schulmann, Pavla Štípská, Vladimír Kusbach, Robert Anczkiewicz
    Abstract:

    The late Carboniferous accretionary system of the South Tianshan orogen (North-Western China) underwent complex structural and polymetamorphic evolution. Combined petrological, geochronological and microstructural analysis of (ultra)high-pressure (UHP) metabasites (eclogites and Blueschists) enclosed in metapelites show a relict coarse-grained eclogitic fabric S2 surrounded by a dominant fine-grained eclogite and Blueschist facies retrograde fabric S2. The S2 fabric is reworked by upright folds F3 that are responsible for a major shortening of the whole accretionary system. For both the eclogite and Blueschist, peak and retrograde P–T conditions have been thermodynamically constrained at 25–26 kbar and 425–500 °C and 10–13 kbar and 500−550 °C respectively, suggesting a shared exhumation history. The garnet-whole rock-amphibole isochron in the Blueschist yielded Lu–Hf and Sm–Nd ages of 326.0 ± 2.9 Ma and 318.4 ± 3.9 Ma respectively, interpreted to date the prograde to peak metamorphic assemblage. The retrograde path of the eclogite is characterized by heterogeneous omphacite recrystallization into a mylonitic fine-grained matrix and crystallization of blue amphibole. Microstructures in both pristine porphyroclastic and recrystallized fine-grained domains in the eclogite indicate a gradual evolution from constriction-dominated (L>S-type) to flattening-dominated (S>L-type) type of deformation, increase of fabric intensity reflected by gradually growing M-indexes and the development of lattice preferred orientation (LPO) typical for dislocation creep under slightly hydrated conditions. Recrystallization of the matrix in the Blueschist is homogeneous, which indicates a matrix dominated channel flow during exhumation. These LPOs evolutions suggest a significant mechanical coupling with the upper plate concomitant with oroclinal bending of the Kazakh orocline. Lock up of Kazakh orocline is responsible for further stress increase resulting in horizontal shortening of South Tianshan accretionary wedge and development of D3 upright folding and steepening of the whole sequence.

  • Late Cretaceous Blueschist metamorphism in the Indus Suture Zone, Shangla region, Pakistan Himalaya
    Tectonophysics, 2000
    Co-Authors: Robert Anczkiewicz, Jean-pierre Burg, Igor M. Villa, Martin Meier
    Abstract:

    Abstract Rb–Sr and 39Ar–40Ar phengitic muscovite dating of the transitional Blueschist–greenschist facies rocks from the Shangla region of Pakistan Himalaya resulted in a concordant ca. 80 Ma age interpreted as a time of metamorphic peak. 39Ar–40Ar age spectra of sodic amphiboles were unsuitable for precise age determination. Mass-balance calculations show that the K concentration in glaucophane/crossite is ca. 25 ppm, and the K–Ar budget is dominated by inclusions of phengitic muscovite. E–W-oriented (parallel to Kohistan arc) stretching lineation defined by preferred orientation of amphiboles was accompanied by synkinematic growth of riebeckite rims on the edges of microboudins. Evolution of the pressure conditions based on the Al2O3 content in sodic amphibole shows that this deformational stage was accompanied by a decrease in pressure from ca. 700 to ca. 400 MPa. This, together with our dating results, strongly suggests ≥10 km late Cretaceous exhumation of the Shangla Blueschists. The exhumation may have been associated with the increase of arc parallel displacement rate due to changes of arc obliquity. This led to thinning caused by arc parallel extension and to exhumation of the Shangla Blueschists. A lack of strong Eocene metamorphic overprint implies that after the peak metamorphism, Blueschists were accreted to the Kohistan arc, which helped to prevent burial and metamorphism during continental collision. The final emplacement of the Shangla Blueschists most likely occurred during the India–Kohistan arc collision and was accompanied by the weak, locally developed greenschist facies overprint.

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  • Blueschists of the amassia stepanavan suture zone armenia linking tethys subduction history from e turkey to w iran
    International Journal of Earth Sciences, 2009
    Co-Authors: Yann Rolland, Michel Corsini, Marc Sosson, Ghazar Galoyan, Sandra Billo
    Abstract:

    The Amassia–Stepanavan Blueschist-ophiolite complex of the Lesser Caucasus in NW Armenia is part of an Upper Cretaceous-Cenozoic belt, which presents similar metamorphic features as other suture zones from Turkey to Iran. The Blueschists include calcschists, metaconglomerates, quartzites, gneisses and metabasites, suggesting a tectonic melange within an accretionary prism. This Blueschist melange is tectonically overlain by a low-metamorphic grade ophiolite sequence composed of serpentinites, gabbro-norite pods, plagiogranites, basalts and radiolarites. The metabasites include high-P assemblages (glaucophane–aegirine–clinozoisite–phengite), which indicate maximal burial pressure of ∼1.2 GPa at ∼550°C. Most Blueschists show evidence of greenschist retrogression (chlorite—epidote, actinolite), but locally epidote-amphibolite conditions were attained (garnet—epidote, Ca/Na amphibole) at a pressure of ∼0.6 GPa and a temperature of ∼500°C. This LP–MT retrogression is coeval with exhumation and nappe-stacking of lower grade units over higher grade ones. 40Ar/39Ar phengite ages obtained on the high-P assemblages range between 95 and 90 Ma, while ages obtained for epidote-amphibolite retrogression assemblages range within 73.5–71 Ma. These two metamorphic phases are significant of (1) HP metamorphism during a phase of subduction in the Cenomanian–Turonian times followed by (2) exhumation in the greenschist to epidote-amphibolite facies conditions during the Upper Campanian/Maastrichtian due to the onset of continental subduction of the South Armenian block below Eurasia.

  • Blueschists of the Amassia-Stepanavan Suture Zone (Armenia): linking Tethys subduction history from E-Turkey to W-Iran
    International Journal of Earth Sciences, 2009
    Co-Authors: Yan Rolland, Michel Corsini, Sandra Billo, Ghazar Galoyan
    Abstract:

    The Amassia–Stepanavan Blueschist-ophiolite complex of the Lesser Caucasus in NW Armenia is part of an Upper Cretaceous-Cenozoic belt, which presents similar metamorphic features as other suture zones from Turkey to Iran. The Blueschists include calcschists, metaconglomerates, quartzites, gneisses and metabasites, suggesting a tectonic mélange within an accretionary prism. This Blueschist mélange is tectonically overlain by a low-metamorphic grade ophiolite sequence composed of serpentinites, gabbro-norite pods, plagiogranites, basalts and radiolarites. The metabasites include high-P assemblages (glaucophane–aegirine–clinozoisite–phengite), which indicate maximal burial pressure of ∼1.2 GPa at ∼550°C. Most Blueschists show evidence of greenschist retrogression (chlorite—epidote, actinolite), but locally epidote-amphibolite conditions were attained (garnet—epidote, Ca/Na amphibole) at a pressure of ∼0.6 GPa and a temperature of ∼500°C. This LP–MT retrogression is coeval with exhumation and nappe-stacking of lower grade units over higher grade ones. 40Ar/39Ar phengite ages obtained on the high-P assemblages range between 95 and 90 Ma, while ages obtained for epidote-amphibolite retrogression assemblages range within 73.5–71 Ma. These two metamorphic phases are significant of (1) HP metamorphism during a phase of subduction in the Cenomanian–Turonian times followed by (2) exhumation in the greenschist to epidote-amphibolite facies conditions during the Upper Campanian/Maastrichtian due to the onset of continental subduction of the South Armenian block below Eurasia.