Blueschist-facies Conditions Research Articles

Deformation and pressure-temperature-time history of the External Tuscan Units in the Northern Apennines (Italy): The case of the Punta Bianca Unit

In this study we investigated through a multidisciplinary approach the still poorly known tectono-metamorphic evolution of the Punta Bianca Unit in the Northern Apennines. The Punta Bianca Unit is part of the Tuscan Metamorphic Units, a group of units derived from the Adria passive margin, metamorphosed at different conditions, and forming the backbone of the Northern Apennine belt. We combined meso- and microstructural analyses, 40Ar/39Ar white-mica geochronology and multi-equilibrium geothermobarometry from high-resolution X-ray chemical maps, to unravel the deformation and metamorphic history of this part of the belt. Meso- and microstructural data indicate that the Punta Bianca Unit recorded two main phases of ductile deformation (here referred to Dp-1 and Dp) associated with syn-kinematic growth of K-white mica, chlorite, calcite, quartz on the related tectonic foliations (Sp-1 and Sp), followed by a later ductile deformation phase (Dp+1) lacking of metamorphic blastesis. P-T estimates complemented by microstructural data suggest that peak metamorphic conditions reached ∼0.8 GPa and ∼350°C and occurred synchronously with the first deformation phase (Dp-1). Temperature values were also confirmed by Raman spectroscopy of carbonaceous material on selected samples. This stage was followed by the exhumation of the Punta Bianca Unit, as testified by decreasing pressure and temperature down to ∼0.4 GPa and ∼300°C respectively, together with the development of the main foliation (Sp). At the regional scale, the Tuscan Metamorphic Units have been mostly affected by HP-LT metamorphic gradients equilibrated under blueschist-facies conditions (up to ∼1.4 GPa). Results from the present work on the contrary, suggest that the Punta Bianca Unit never reached such HP-LT conditions, testifying that it was deformed at relatively upper structural levels, thus highlighting an important variation in the tectono-metamorphic evolution of the Tuscan Metamorphic Units along strike in the Northern Apennines. 40Ar/39Ar laserprobe data (using both the in-situ and step-heating techniques) indicate a minimum age for the onset of continental subduction of ∼20 Ma (Dp-1), which was followed in close succession by exhumation at ∼16 Ma. This approach, if applied to different tectonic units building up the nappe pile of the Northern Apennines, could be successful in better unravelling the tectonic history.

Deformation Pattern of Well-Preserved High-Pressure Rocks (SE Syros, Cyclades)

New, detailed geological/structural mapping and field-based structural analysis were carried out to investigate the deformation pattern of well-preserved high-pressure rocks of the Blueschist Unit exposed in SE Syros (Cyclades, Greece). Geological mapping revealed the occurrence of extensive alternations between different rock groups, as well as interfingering patterns in map-scale that are possibly the result of folding. The earlier ductile deformation phase recognized in the mapped area is associated with the development of a penetrative foliation, which was formed at eclogite/blueschist-facies conditions under peak metamorphism. The subsequent main deformation phase occurred under blueschist facies conditions synchronous with the early stages of exhumation of the high-pressure rocks. This phase is mainly associated with the formation of WNW-trending folds and a pervasive axial planar foliation linked with ESE-directed shearing. The main deformation ceased under blueschist-facies conditions, and exhumation of the rocks to greenschist-facies conditions took place under very weak and localized deformation. Greenschist retrogression observed in the southwestern part of the mapped area seems to be controlled by fluids, rather than by intense deformation and formation of major syn-greenschist shear zones.

The Maira-Sampeyre and Val Grana Allochthons (south Western Alps): review and new data on the tectonometamorphic evolution of the Briançonnais distal margin

Here we describe the structure, the high-pressure, low-temperature (HP-LT) metamorphism and tectonic evolution of the Briançonnais distal margin units from the south Western Alps. The studied area extends southwest of the Dora-Maira (U)HP basement units and east-southeast of the classical Briançonnais nappes. A new structural map accompanied by geological profiles shows the thrusting of the oceanic nappes (Monviso and Queyras units) onto the distal Briançonnais units (D1 and D2 late Eocene deformation phases) under blueschist-facies conditions. Subsequent deformation during the Early Oligocene (D3 deformation phase) took place under greenschist-facies conditions and was associated with back-folding and -thrusting in the units overlying the Dora-Maira massif and with exhumation related to normal reactivation of former thrusts within the latter massif. Two large cover units, detached from their former distal Briançonnais basement, are redefined as the Maira-Sampeyre and Val Grana Allochthons (shortly: Maira-Grana Allochthons = MGA) including, (i) the Val Maira-Sampeyre unit involving Lower and Middle Triassic formations, seemingly detached from the Dora-Maira units during the subduction process, and (ii) the Val Grana unit with Middle-Upper Triassic and Early-Middle Jurassic formations, which was probably detached from the Maira-Sampeyre unit and correlates with the “Prepiemonte units” known from the Ligurian Alps to the Swiss Prealps. Three major shear zones involving tectonic mélanges of oceanic and continental rocks at the base of the Val Grana, Maira-Sampeyre and Dronero units testify to an early phase of exhumation within the subduction channel in front of the Adria plate. We present a new metamorphic map based on published and new petrological data, including new thermometric data obtained by Raman spectroscopy of carbonaceous material (RSCM). The TRSCM values range from ~ 400 °C to > 500 °C, going from the most external Val Grana unit and overlying Queyras schists to the uppermost Dora-Maira unit. During the Late Triassic, the width of the Briançonnais s.l. domain can be restored at ~ 100 km, whereas it reached ~ 150 km after the Jurassic rifting. A significant, second rifting event affected the Briançonnais domain during the Late Cretaceous-Paleocene, forming the Longet-Alpet chaotic breccias, which deserve further investigations.

Blueschist mylonitic zones accommodating syn-subduction exhumation of deeply buried continental crust: the example of the Rocca Canavese Thrust Sheets Unit (Sesia\u2013Lanzo Zone, Italian Western Alps)

The Rocca Canavese Thrust Sheets Unit (RCTU) is a subduction-related mélange that represents the eastern-most complex of the Sesia–Lanzo Zone (SLZ), bounded by the Periadriatic (Canavese) Lineament that separates the Alpine subduction complex from the Southalpine domain. The RCTU is limited to the south by the Lanzo Massif (LM) and to the east by the Eclogitic Micaschists Complex (EMC). Particularly the tectonic contact area of the RCTU, adjacent to the neighbouring SLZ and the LM is characterised by a 100–200-m-thick mylonitic to ultra-mylonitic zone (MZ) that was active under blueschist-to greenschist-facies conditions. Despite the dominant mylonitic structure, some rocks (garnet-bearing gneiss, garnet-free gneiss and orthogneiss) still preserve pre-mylonitic parageneses in meter-sized domains. The scarcity of superposed structures and the small size of relicts impose a detailed microstructural analysis supported by chemical investigation to reconstruct the tectono-metamorphic history of the MZ. Therefore, we integrated the classical meso- and microstructural analysis approach with a novel quantitative technique based on the Quantitative X-Ray Map Analyzer (Q-XRMA), used to classify rock-forming minerals starting from an array of X-ray elemental maps, both at whole thin section and micro-domain scale, as well as to calibrate the maps for pixel-based chemical analysis and end-member component maps, relevant for a more robust conventional geothermobarometer application as well for calculating reliable PT pseudosections. Pre-Alpine relicts are garnet and white mica porphyroclasts in the garnet-bearing gneiss and biotite and K-feldspar porphyroclasts in garnet-free gneiss and orthogneiss, respectively, providing no PT constraints. The Alpine evolution of the MZ rocks, has been subdivided in three deformation and metamorphic stages. The first Alpine structural and metamorphic equilibration stage (D1 event) occurred at a pressure of ca. 1.25–1.4 GPa and at a temperature of ca. 420–510 °C, i.e. under blueschist-facies conditions. The D2 event, characterised by a mylonitic foliation that is pervasive in the MZ, occurred at ca. 0.95–1.1 GPa and ca. 380–500 °C, i.e. under epidote-blueschist-facies conditions. The D2 PT conditions in the MZ rocks are similar to those predicted for the blocks that constitute the RCTU mélange, and they overlap with the exhumation paths of the EMC and LM units. Therefore, the RCTU, EMC and LM rocks became coupled together during the D2 event. This coupling occurred during the exhumation of the different tectono-metamorphic units belonging to both continental and oceanic lithosphere and under a relatively cold thermal regime, typical for an active oceanic subduction zone, pre-dating Alpine continental collision.

Temporal and spatial mineralogical changes in clasts from Mariana serpentinite mud volcanoes: Cooling of the hot forearc-mantle at subduction initiation

Mafic and ultramafic clasts (mostly ~1–5 cm in size) were recovered from three different serpentinite mud volcanos in the Mariana forearc during Integrated Ocean Drilling Program (IODP) Expedition 366. Mafic clasts from drill sites distant from the trench bear lawsonite, Al-rich riebeckite, jadeitic pyroxene (~80 mol% jadeite), and aragonite as metamorphic minerals. In contrast, mafic clasts from drill sites closer to the trench are characterized by prehnite–pumpellyite-facies mineral associations and/or the presence of analcime and natrolite. An occurrence of antigorite-bearing ultramafic clasts becomes progressively more frequent with distance from the trench. One amphibolite clast from a mud volcano near the trench also has prehnite filling veins, and it also occurs as pseudomorphs after plagioclase. Amphibolite clasts at other mud volcanoes distant from the trench are partially overprinted by blueschist-facies minerals. The apparent metamorphic grades increase with distance from the trench; these metamorphic conditions represent the increasing depth from zeolite- to lawsonite–blueschist-facies conditions in a subduction zone. Considering the consistency of the low-temperature metamorphic grade of mafic and ultramafic clast mineralogy in each mud volcano, they likely reflect the thermal structure of the slab-mantle interface before the ascent. As a result, these clasts were brought up to the seafloor en masse by the serpentinite mudflow. The polymetamorphosed amphibolite clasts suggest cooling of the hot forearc-mantle at the initiation of Mariana subduction in the Eocene. The ultramafic clasts in the mud volcanoes distant from the trench frequently contain Ca amphibole and talc, which indicates hot mantle hydration by metasomatic fluids released from the slab at subduction initiation.

Metamorphic Conditions of Neotethyan Meliatic Accretionary Wedge Estimated by Thermodynamic Modelling and Geothermobarometry (Inner Western Carpathians)

Metamorphic evolution of an accretionary wedge can be constrained by a reconstructed P–T conditions of the oceanic and continental margin fragments. This paper deals with the metamorphic overprinting of the Inner Western Carpathians (IWC) Meliatic Triassic–Jurassic paleotectonic units after the closure of the Neotethyan Meliata Basin. Medium to high-pressure and lower temperature conditions were estimated by Perple_X pseudosection modelling, combined with garnet–phengite, calcite–dolomite and chlorite thermometers and chlorite–phengite and phengite barometers. The Late Jurassic subductional burial to a maximum 50 km depth was estimated from the Bôrka Unit continental margin fragments at 520 °C and 1.55 GPa. This is compatible with the metamorphic peak garnet–glaucophane–phengite assemblage of blueschist facies in metabasites. The Jaklovce Unit oceanic fragments were subducted to maximum 35–40 km at 390–420 °C and 1.1–1.3 GPa. Metabasalts and metadolerites contain winchite, riebeckite, actinolite, chlorite, albite, epidote and phengite. A glaucophane-bearing metabasalt recorded an intra-oceanic subduction in blueschist-facies conditions. Rare amphibolite-facies metabasalts of this unit indicate the base of an inferred oceanic crust sliver obducted onto the continental margin wedge. The Meliata Unit oceanic/continental margin flysch calciclastic and siliciclastic metasediments suggest the burial to approximately 15–20 km at 250–350 °C and 0.4–0.6 GPa. This is indicated by a newly formed albite, K-feldspar, illite–phengite and chlorite associated with quartz and/or calcite and dolomite in these rocks. Magnesio-hastingsite to magnesio-hornblende bearing metagabbro with newly formed metamorphic magnesio-riebeckite and actinolite is an inferred detached Meliatic block tectonically emplaced in a Permian salinar mélange in the Silica Nappe hanging wall. Reconstructed P–T paths indicate variable metamorphic conditions from the medium-pressure to high-pressure subduction of the Bôrka and Jaklovce units to the Meliata Unit shallow burial in an accretionary wedge during Late Jurassic to Early Cretaceous Meliaticum evolution. Mélange blocks of Meliaticum incorporate different juxtaposed Meliatic paleotectonic units exposed in nappe outliers overlying the IWC Gemeric and Veporic superunits.

The Growth of Sodic Amphibole at the Greenschist- to Blueschist-facies Transition (Dent Blanche, Western Alps): Bulk-rock Chemical Control and Thermodynamic Modelling

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.

Tectonic evolution of the Tianshan Akeyazi metamorphic complex (NW China)

The South Tianshan orogen (northwest China) comprises a thick volcanoclastic HP-UHP metamorphic belt, the Akeyazi (high-pressure) metamorphic complex (AMC), exhibiting well-preserved eclogite- and blueschist-facies rocks and pervasive deformation throughout all units. In order to clarify the tectonic evolution of this fossil subduction zone, which remains controversial due to the lack of systematic field observations combining structural and petrological data, we herein present the results of extensive mapping of lithologies, deformation patterns and petrological evolutions.The AMC is composed of a high-pressure (HP) to ultra-high-pressure (UHP) central domain – the HP-UHP unit – separated in the North by a major detachment from a greenschist-facies domain. A gradational transition in lithologies (i.e., interlayered micaschists, volcanoclastic, and mafic rocks) is observed within the HP-UHP unit, from more pelitic in the North to more mafic in the South. Tectonic patterns and contrasting lithologies allow to recognize five distinct sub-units stacked at HP to UHP conditions. Pervasive ductile deformation revealed by penetrative schistosities, folding, shear planes and boudinage structures is characterized by successive deformation stages which are attributed to burial (Dn-1), peak deformation accompanying detachment from the slab and/or deep nappe stacking (Dn, Dn+1) and exhumation (Dn+2, Dn+3). During exhumation under blueschist-facies conditions, the HP-UHP nappe stack underwent pervasive shearing, in particular across km-scale extensional shear zones with consistent top to the north shear senses reworking inherited thrust contacts. The entire HP-UHP unit reached peak-metamorphic conditions at ∼2.5 GPa and ∼540 °C. Peak pressures of 2.52 ± 0.25 GPa, consistent with the widespread occurrence of coesite in the HP-UHP unit, suggest a maximum burial depth of ∼80 km which is thought to reflect a transition between mechanical coupling and decoupling at the plate interface. This tectono-metamorphic evolution is set back in a tentative geodynamic model.

Paleo-Tethyan tectonic evolution of Lancangjiang metamorphic complex: Evidence from SHRIMP U-Pb zircon dating and 40Ar/39Ar isotope geochronology of blueschists in Xiaoheijiang-Xiayun area, Southeastern Tibetan Plateau

Blueschists are sporadically exposed as lenses within the Lancangjiang metamorphic complex, and represent unique components of the Paleo-Tethys. In this paper, we present geochemical and geochronological results of blueschists to decipher their origin and tectonic significance. The whole-rock geochemical analyses revealed strong similarities with ocean island basalt (OIB), and further discrimination diagrams confirm an affinity to a within-plate setting. Combined studies on blueschists using cathodoluminescence (CL) imaging, SHRIMP U-Pb dating of zircon domains and 40Ar/39Ar dating of phengite and glaucophane provide evidence of their magmatic origin and metamorphic evolution. Slightly oscillatory zoned or compositionally homogeneous zircon grains/domains, as well as structureless zircon rims, yield ages from 231.6±3.7 to 225.3±4.8Ma, recording the blueschist facies metamorphic event. In contrast, the captured zircon grains and cores with a major age peak at ~241Ma as well as several minor older age peaks indicate the multiple provenance of the zircons. 40Ar/39Ar step heating analyses on single grains of phengite and glaucophane separated from blueschists yield plateau ages ranging from 242.5±1.4 to 228.7±1.5Ma which are interpreted to reflect high-pressure metamorphism.This study provides geochemical and geochronological constraints on the tectonic evolution of the Paleo-Tethyan ocean, which was closed and subsequently subducted as a result of the collision of the Simao and Baoshan Blocks. During subduction in Trassic (243 to 225Ma), the protoliths of blueschists underwent blueschist facies conditions.

Insights on Variscan geodynamics from the structural and geochemical characterization of a Devonian-Carboniferous gabbro from the Austroalpine Domain (Western Alps)

Ivozio kyanite-bearing eclogites represent one of the main pre-Alpine mafic complexes of the Sesia Lanzo Zone (Austroalpine Domain) whose protoliths have been dated at Late Devonian to Early Carboniferous. The complex consists of various HP mafic-ultramafic rocks such as amphibole-bearing eclogites, zoisite-bearing eclogites, amphibole-epidote-bearing eclogites, quartz-bearing eclogites, chlorite-bearing amphibolites and ultramafites surrounded by eclogitic micaschists and jadeite-bearing metagranitoids. Multiscale structural analyses, integrated with geochemical investigation, allowed to distinguish heterogeneities of eclogite types consequent to strain partitioning and different degrees of metamorphic re-equilibration during Alpine polyphase deformation history or to pristine magmatic differences. Five groups of Alpine structures (D 1 to D 5 ) have been detected: they consist of folds, foliations and shear zones developed during the Late Cretaceous to Eocene subduction-exhumation path. The metamorphic mineral assemblages related to the deformation stages indicate a polyphase deformation (pre-D 1 to D 3 ) under eclogite-facies conditions followed by a blueschist re-equilibration (during D 4 ), localized shearing and successive folding and discrete shearing (D 5 ) under greenschist-facies conditions. Four veining stages, developed under the eclogite- and blueschist-facies conditions have been documented. Whole rock major and trace element composition of main igneous lithotypes suggest a subduction-related context for the emplaced of the Ivozio mafic Complex. It also suggests that all rocks are not co-genetic and that they likely emplaced in the continental crust. Igneous products with arc and back-arc geochemical affinity (from 380 to 345 Ma) are described in the European Variscan Belt from French Central Massif to Bohemian Massif, supporting the interpretation that the Ivozio mafic Complex protoliths are the witness of such Devonian-Carboniferous arc magmatism in the Alpine chain. TO DOWNLOAD THE GEOLOGICAL MAP SEE THE IN THE MENu SUPPLEMENTARY FILE (ON THE RIGHT COLUMN)

Analysis of fabric evolution and metamorphic reaction progress at Lago della Vecchia-Valle d’Irogna, Sesia-Lanzo Zone, Western Alps

ABSTRACTThe Lago della Vecchia-Valle d’Irogna rocks are part of the Eclogitic Micaschists Complex (EMC) of the Sesia-Lanzo Zone, western Austroalpine domain. The 1:10,000 scale map includes metaintrusive, minor micaschist, banded gneiss, and metabasic boudins. The multiscale structural analysis reveals successive magmatic and tectono-metamorphic stages: during M0 the metaintrusive protoliths emplaced; D1 took place under eclogite-facies conditions; during D2 stage, a pervasive foliation developed under retrograde blueschist-facies conditions; D3–D4 and D5 structures developed under greenschist-facies conditions; during M6 andesitic dykes intruded. The mapped degree of fabric evolution (FE) and metamorphic transformation (MT) related to D2-foliation shows that the MT was not only controlled by bulk rock and mineral compositions, but also by FE. The development of a pervasive blueschist-facies D2-foliation is in contrast with the eclogitic dominant fabric generally recorded in the EMC. This difference suggests that FE and MT are potentially responsible for km-scale heterogeneities in the tectono-metamorphic record.

Deformation behavior of continental crust during subduction and exhumation: Strain distribution over the Tenda massif (Alpine Corsica, France)

In order to address the question of strain localization within continental units during subduction and exhumation, a large-scale portion of an exhumed continental crust was structurally revisited. The Tenda massif (Alpine Corsica) has recorded burial (D1; top-to-the-SW kinematics) down to blueschist-facies conditions followed by exhumation (D2; top-to-the-NE kinematics). It was so far regarded as a quite rigid unit with strain localization at the upper contact with the overlying oceanic material, the East Tenda Shear Zone (ETSZ), where previous studies were focused. A structural analysis carried out from the core to the boundaries of this continental unit shows instead that deformation is pervasive in the whole section. A reappraisal of the finite structure shows that this unit corresponds to a N140°E elongated dome of D2 fabrics severely overprinting earlier D1 structures, locally preserved in the core of the dome. Field observations show that deformation was distributed at maximum burial, pervasively affecting both the Tenda massif and the overlying tectonically coupled oceanic material. Strain remained distributed during the first steps of exhumation that probably started during the last stages of subduction. After a regional switch from compression to extension, deformation localized toward the ETSZ and other internal shear zones such as the newly described Saleccia Shear Zone, as shown by strain intensity mapping in the eastern granitic protoliths. Doming occurred during this localization when crossing the brittle-ductile transition. This large-scale structural study shows that lithological boundaries controlled the place where strain localized, while the general style of deformation (distributed vs localized) was controlled by depth (i.e. pressure-temperature conditions), interacting with other parameters such as fluid circulation and protolith composition.

Fluid-controlled deformation in blueschist-facies conditions: plastic vs brittle behaviour in a brecciated mylonite (Voltri Massif, Western Alps, Italy)

AbstractA blueschist-facies mylonite crops out between two high-pressure tectono-metamorphic oceanic units of the Ligurian Western Alps (NW Italy). This mylonitic metabasite is made up of alternating layers with different grain size and proportions of blueschist-facies minerals.The mylonitic foliation formed at metamorphic conditions of T = 220–310 °C and P = 6.5–10 kbar. The mylonite shows various superposed structures: (i) intrafoliar and similar folds; (ii) chocolate-tablet foliation boudinage; (iii) veins; (iv) breccia.The occurrence of comparable mineral assemblages along the foliation, in boudin necks, in veins and in breccia cement suggests that the transition from ductile deformation (folds) to brittle deformation (veining and breccia), passing through a brittle–ductile regime (foliation boudinage), occurred gradually, without a substantial change in mineral assemblage and therefore in the overall P–T metamorphic conditions (blueschist-facies).A strong fluid–rock interaction was associated with all the deformative events affecting the rock: the mylonite shows an enrichment in incompatible elements (i.e. As and Sb), suggesting an input of fluids, released by adjacent high-pressure metasedimentary rocks, during ductile deformation. The following fracturing was probably enhanced by brittle instabilities arising from strain and pore-fluid pressure partitioning between adjacent domains, without further external fluid input.Fluids were therefore fixed inside the rock during mylonitization and later released into a dense fracture mesh that allowed them to migrate through the mylonitic horizon close to the plate interface.We finally propose that the fracture mesh might represent the field evidence of past episodic tremors or ‘slow earthquakes’ triggered by high pore-fluid pressure.

Geochronological challenges posed by continuously developing tectonometamorphic systems: insights from Rb–Sr mica ages from the Cycladic Blueschist Belt, Syros (Greece)

AbstractIs metamorphism and its causative tectonics best viewed as a series of punctuated events or as a continuum? This question is addressed through examination of the timing of exhumation of the Cycladic Blueschist Belt (CBB). The cause of scatter beyond analytical error in Rb–Sr geochronology was investigated using a suite of 39 phengite samples. Rb–Sr ages have been measured on phengite microsamples drilled from specific microstructures in thin sections of calcschists and metabasites from the CBB on Syros. The majority are from samples that have well‐preserved blueschist facies mineral assemblages with limited greenschist facies overprint. The peak metamorphic temperatures involved are below the closure temperature for white mica so that crystallization ages are expected to be preserved. This is supported by the coexistence of different ages in microstructures of different relative age; in one sample phengite from the dominant extensional blueschist facies fabric preserves an age of 35 Ma while post‐tectonic mica, millimetres away, has an age of 26 Ma. The results suggest that micro‐sampling techniques linked to detailed microstructural analysis are critical to understanding the timing and duration of deformation in tectonometamorphic systems. North of the Serpentinite Belt in northern Syros, phengite Rb–Sr ages are generally between 53 and 46 Ma, comparable to previous dates from this area. South of the Serpentinite Belt phengite in blueschist facies assemblages associated with extensional fabrics linked to exhumation have ages that range from 42 Ma down to c. 30 Ma indicating that extensional deformation while still under blueschist facies conditions continued until 30 Ma. No age measurements on samples with unambiguous evidence of deformation under greenschist facies conditions were made; two rocks with greenschist facies assemblages gave phengite ages that overlap with the younger blueschist samples, suggesting blueschist facies phengite is preserved in these rocks. Two samples yielded ages below 27 Ma; one is from a post‐tectonic microstructure, the other from a greenschist in which the fabric developed during earlier blueschist facies conditions. These ages are consistent with previous evidence of greenschist facies conditions from c. 25 Ma onwards. The data are consistent with a model of deformation that is continuous on a regional scale.

Metamorphic history of riebeckite- and aegirine-augite-bearing high-pressure–low-temperature blocks within the Siuna Serpentinite Mélange, northeastern Nicaragua

The Siuna Serpentinite Mélange (SSM) is a subduction-zone-related complex that contains diverse blocks of igneous and sedimentary origin, overprinted by various metamorphic conditions. The SSM is located at the southern border of the Chortís block and marks the boundary between continental and oceanic crusts in the western margin of the Caribbean Plate. The serpentinite matrix mainly consists of lizardite/chrysotile, Cr-rich spinel, and relict orthopyroxene that suggest a harzburgitic protolith and an upper mantle supra-subduction zone origin. Blocks within the southern and central regions range from Jurassic pelagic sediments to mafic/intermediate igneous rocks that are metamorphosed to various degrees, ranging from prehnite-pumpellyite/greenschist to likely blueschist facies (e.g. riebeckite-bearing metashale) conditions. In contrast, the northern section encloses almost exclusively epidote-amphibolite facies metabasite blocks, and minor mica- and chlorite-rich rocks of metasomatic origin, respectively. Some of the epidote-amphibolite blocks contain relic garnet-rich zones embedded in an amphibole-rich matrix. The garnets appear to record two generations of growth and contain mineral inclusions such as amphibole, apatite, titanite, aegirine-augite, and quartz. Thermobarometric estimates for the garnet-rich zones and epidote-amphibolite-rich matrix suggest a prograde blueschist facies at ~1.2 GPa and 400–450°C, an eclogite facies metamorphic peak at 1.5–1.7 GPa and 565–614°C, and a post-peak epidote-amphibolite facies metamorphism. These pressure and temperature estimates indicate a classical clockwise PT path that has been observed in many palaeo-subduction zone environments worldwide. Phengite Ar–Ar dating of mica-rich rock yields 140 Ma and suggests an Early Cretaceous exhumation along the southern edge of the continental Chortís block.

Accretion, underplating and exhumation along a subduction interface: From subduction initiation to continental subduction (Tavşanlı zone, W. Turkey)

We herein reappraise the pressure–temperature (PT) evolution of the high-pressure and low-temperature (HP–LT) Tavşanlı zone (western Turkey) in order to (i) better characterize rock units exhumed along a cooling subduction interface, from birth to steady state and (ii) constrain exhumation and detachment dynamics, as well as mechanical coupling between plates. Based on PT estimates and field observations three oceanic complexes are recognized between the HP–LT continental margin and the obducted ophiolite, with PT estimates ranging from incipient metamorphism to blueschist-facies conditions. PT conditions for the continental unit are reappraised to 24kbar and ~500°C on the basis of pseudosection modelling and Raman spectroscopy on carbonaceous material. A tentative reconstruction of the subduction zone evolution is proposed using available radiometric and palaeogeographic data and recent thermomechanical modelling. Both PT conditions and field observations point out to the slicing of km-sized units at different preferred depths along the subduction interface, thus providing constraints on the dynamics of accretion and underplating. In particular, the comparison of PT estimates for the Tavşanlı zone and for other broadly similar fossil subduction settings (i.e., Oman, Corsica, New Caledonia, Franciscan, Schistes Lustrés) suggests that units are detached preferentially from the slab at specific depths of 30-40km (i.e., downdip of the seismogenic zone) and ~80km. We propose that these depths are controlled by major changes in mechanical coupling along the plate interface, whereas exhumation through time would rather be controlled by large-scale geodynamic boundary conditions.

P–T–t evolution of eclogite/blueschist facies metamorphism in Alanya Massif: time and space relations with HP event in Bitlis Massif, Turkey

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.

Geodynamics of the Tavşanlı zone, western Turkey: Insights into subduction/obduction processes

Abstract The tectono-metamorphic evolution of the high-pressure low-temperature (HP–LT) Tavsanli zone (Western Anatolia, Izmir–Ankara suture zone) is herein reappraised to highlight processes occurring along a fossil subduction interface, from initial obduction stages to continental subduction. Structural and petrological data allow in particular to constrain the nature, internal structure and PT conditions of the oceanic accretionary complex sandwiched between the subducted continental margin of the Anatolide–Tauride Block (Orhaneli unit) and the non-metamorphic obducted ophiolite on top. Two distinct oceanic units (termed complexes 1 and 2) are recognised on top of one another, with metamorphic conditions ranging from incipient HP-LT imprint (complex 1) to blueschist facies conditions (complex 2). Based on the first occurrence of Fe–Mg carpholite and on pseudosection calculations, PT estimates of 250–350 °C and 11–13 kbar are inferred for complex 2. The internal structure of the accretionary complex points to the underplating of kilometre-scale units at different depths along the plate interface and to contrasting dynamics with respect to both the underlying continental unit and the ophiolite. Inter-plate mechanical coupling within the Tavsanli zone is compared to the Oman case-study. The variable HP–LT overprint of the metamorphic sole places further constraints on regional scale tectonics, the accretionary dynamics and on the rapid thermal reequilibration of the subduction interface.

Petrogenetic relationships between jadeitite and associated high-pressure and low-temperature metamorphic rocks in worldwide jadeitite localities: a review

Jadeitite-bearing serpentinite-matrix melange is distributed in the Caribbean (Guatemala, Cuba, and Dominican Republic), circum-Pacific (Japan, Western USA, and Papua New Guinea), Alpine-Himalayan (Italy, Iran, Greece, and Myanmar), and Caledonian (Russia and Kazakhstan) orogenic belts, and always contains high-pressure, low-temperature (H P -L T ) metamorphic rocks. There are also jadeitite xenoliths in kimberlitic pipes in the Colorado Plateau (USA). The oldest occurrences of jadeitite are Early Paleozoic in Japan, Russia, and Kazakhstan, suggesting subduction-zone thermal structures evolved the necessary high pressure/temperature conditions for jadeitite formation since Early Paleozoic; the youngest occurrence is a xenolith from the Colorado Plateau. Major occurrences consist principally of fluid precipitates (P-type) that infiltrated the mantle wedge; fewer occurrences document metasomatic replacement (R-type) of plagiogranite, metagabbro and eclogite, and both types may be possible in the same occurrence or system. The P-T conditions for jadeitite formation can be extended beyond the previously argued limits of blueschist-facies conditions. Some jadeitite formed at epidote amphibolite and others at eclogite facies conditions. Available geochronological data of both jadeitite and associated H P -L T rock show temporal discrepancies between jadeitite formation and H P -L T metamorphism at some localities. The close association between older jadeitite and younger H P -L T rock in a single melange complex implies different histories for the subduction channel and jadeitite-bearing melange. Jadeitite-bearing serpentinite melange can stay at the mantle wedge for a considerable time and, as a result, experience multiple fluid-infiltration events. The subduction channel can occasionally incorporate overlying serpentinized mantle wedge material due to tectonic erosion. With time, the disrupted mantle wedge containing jadeitite veins is mixed with younger blueschists, exhumed eclogites and various fragments of suprasubduction-zone lithologies. Consequently, recrystallization and re-precipitation of jadeitite are reactivated along a slab–mantle wedge interface. All these possible scenarios and their combinations yield a complicated petrological record in jadeitite. With further investigation, the rock association of jadeitite–H P -L T metamorphic rocks–serpentinite has the potential to yield a greater understanding of subduction channels and overlying mantle wedge.

Strain development and kinematic significance of the Alpine folding on Andros (western Cyclades, Greece)

The general trend of both fold axes and stretching lineation in the Cycladic Blueschist Unit is NE–SW to NNE–SSW. This orientation forms a large angle (almost perpendicular) with respect to the Hellenic trend that is inferred from the main thrusts on mainland Greece. Thus, the kinematic significance of the stretching parallel folding in the Cycladic Blueschist Unit is non-trivial. Since within the western Cyclades, the NE-trending folds are best exposed on the island of Andros, it is a key locality for understanding the timing, style and kinematic significance of folding. Here we show that the NE-trending folds on Andros formed within the stability field of glaucophane, after the peak high-pressure metamorphism and simultaneously with the early stage of retrogression. The axes-parallel stretching was non-rotational; it started during the NE folding at blueschist-facies conditions, and continued long afterward and well into the retrograde greenschist overprint. Furthermore, we present the result of a finite strain calculation which shows that the large NE folds could not have been reoriented at ~ 90° as previously thought. Instead it is suggested that these folds formed under constrictional strain regime during regional NE–SW extension, and represent coeval transverse NW–SE shortening and vertical thinning. This implies that NE extension and southwest directed rollback of the active margin prevailed in the western Aegean between the Eocene and early Miocene.