Greenschist Facies Retrogression Research Articles

Preservation of high‐P rocks coupled to rock composition and the absence of metamorphic fluids

AbstractEclogites, blueschists and greenschists are found in close proximity to one another along a 1‐km coastal section where the Cyclades Blueschist Unit (CBU) is exposed on SE Syros, Greece. Here, we show that the eclogites and blueschists experienced the same metamorphic history: prograde lawsonite blueschist facies metamorphism at 1.2–1.9 GPa and 410–530°C followed, at 43–38 Ma, by peak blueschist/eclogite facies metamorphism at 1.5–2.1 GPa and 520–580°C. We explain co‐existence of eclogites and blueschists by compositional variation probably reflecting original compositional layering. It is also shown that the greenschists record retrogression at 0.34 ± 0.21 GPa and T = 456 ± 68°C. This was spatially associated with a shear zone on a scales of 10–100‐m and veins on a scale of 1–10‐cm. Greenschist facies metamorphism ended at (or shortly after) 27 Ma. We thus infer a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression which lasted up to 11–16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time and conclude that greenschist facies retrogression only occurred when and where metamorphic fluids were present. From a tectonic perspective, our findings are consistent with studies showing that the CBU is (a) a high‐P nappe stack consisting of belts in which high‐P metamorphism and exhumation occurred at different times and (b) affected by greenschist facies metamorphism during the Oligocene, prior to the onset of regional tectonic extension.

The nature and significance of the Faroe-Shetland Terrane: Linking Archaean basement blocks across the North Atlantic

Core samples of the continental basement rocks that underlie the eastern Faroe-Shetland Basin (FSB) and its inshore margins west of Shetland reveal a suite of predominantly granodioritic to granitic orthogneisses (including TTG), together with lesser volumes of foliated granitoids and subordinate dioritic to mafic gneisses and amphibolites. A small area of lithologically similar gneisses also crops out onshore at North Roe/Uyea west of the Caledonian front in Shetland. In the core samples, coarse grained gneissose textures and mineralogies consistent with upper amphibolite facies metamorphism are overprinted by a weak, but ubiquitous, static greenschist facies retrogression. Later structures include widespread epidote-quartz veining, and local developments of mylonite, cataclasite, pseudotachylite and phyllonite. Regions associated with the Rona Ridge oil fields (e.g. Clair, Lancaster) also preserve extensive brittle fracturing and associated low temperature hydrothermal mineralization (quartz, adularia, calcite, pyrite/chalcopyrite) with significant fracture-hosted hydrocarbons. New and published U-Pb zircon analyses from the gneisses offshore give a relatively narrow range of Neoarchaean protolith ages (ca 2.7–2.8 Ga) spread over a geographic area of 60,000 km2 west of Shetland. Detrital zircon data from overlying Triassic-Cretaceous sedimentary sequences in the FSB suggest an equivalent limited range of Neoarchaean source materials. Hf isotopic data indicate involvement of Palaeo- to Mesoarchaean crustal sources. Overall, our findings suggest that a major phase of Neoarchaean crustal formation and associated high grade metamorphism dominates basement rocks in the region. They are similar in age and lithology to the protoliths of the nearest onshore Lewisian Complex of NW Scotland (Rhiconich Terrane). However, they lack geochronological or textural evidence for the widespread Proterozoic Laxfordian events (ca 1.7–1.8 Ga) which are widespread in Scotland. This suggests that the Precambrian rocks west of Shetland – the Faroe-Shetland Terrane – can be correlated with the Archaean rocks of the Central Greenland-Rae Craton and that a northern limit of Proterozoic reworking lies just offshore from the north coast of Scotland.

The Origin of A New Pargasite-Schist Hosted Ruby Deposit From Paranesti, Northern Greece

Gem-quality (cabochon) ruby-bearing occurrences (here termed PAR-1 and PAR-5) located near Paranesti, north eastern Greece have been systematically studied for the first time in this paper. Tectonically, the occurrences are located within the Nestos Shear Zone (NSZ). The NSZ separates two distinct geological units. The Rhodope Terrane is a heterogeneous unit of gneisses, mafic, ultramafic, and meta-sedimentary rocks in the hanging wall. The footwall Pangaion-Pirin Complex consists of marbles and acid gneisses of a Mesozoic carbonate platform on pre-Mesozoic continental basement. In this paper, a range of petrographic and geochemical techniques were used to determine (1) any similarities and differences to other mafic-ultramafic hosted ruby deposits worldwide; (2) distinctive geochemical fingerprints for Paranesti; and (3) the likely P-T conditions of formation. Detailed petrographic and whole-rock analyses utilizing ICP-MS, XRF, and XRD have found the Paranesti corundum to be of a mafic/ultramafic protolith with approximately 40 wt.% SiO2, 16 wt.% Mg, 11000 ppm Cr, and 440 ppm Ni. EMPA major element analysis determined the mineral inclusions within the corundum grains to be picotite and hercynite spinels. Pargasite is the dominant amphibole within the corundum-bearing amphibole schist host. The surrounding non-corundum bearing chlorite schist mainly comprises clinochlore. Petrographic examination of the mineral assemblages within the corundum-bearing schists revealed strong fracturing and alignment (parallel to the main regional foliation) of the corundum grains and margarite reaction rims around the corundum. The surrounding non-corundum amphibolites also contain anorthite, along with relict sillimanite, kyanite, and chlorite/muscovite/epidote overprinting. Detailed LA-ICP-MS trace element analysis of the color range of corundum from the two occurrences showed the corundum to be mainly of metamorphic origin, though pale rubies from PAR-5 suggest some metasomatic influence. The corundum displays distinctive geochemical locality signatures, with a combination of high Cr (average 2300 ppm with 15% sample points on core positions >5000 ppm and maximum 8600 ppm); high Si (average 1400 ppm with 40% over 1500 ppm and maximum 2500 ppm), low Mg (average 30 ppm), and very low V, Ti, and Ga. Based on the literature for similar occurrences, and the mineral assemblages observed at Paranesti, the estimated P-T conditions of corundum formation are <7 kbar and <750 °C, similar to the mafic African amphibolite-hosted rubies. This study has found the Paranesti occurrences to be most similar to the Winza, Tanzania ruby deposit, whilst there are some similarities to other high-Cr ruby deposits, primarily the Fiskenaesset, Greenland and metamorphic amphibolitic schist hosted African deposits. The Paranesti corundum most likely formed during regional amphibolite facies metamorphism which created the Nestos Shear Zone, along with a lesser influence (primarily observed in the PAR-5 occurrence) of more localized metasomatism. Subsequent multiple greenschist facies retrogression of the occurrences resulted in the current-day host amphibole-chlorite schist assemblages.

Evidence of Variscan and Alpine tectonics in the structural and thermochronological record of the central Serbo-Macedonian Massif (south-eastern Serbia)

The Serbo-Macedonian Massif (SMM) represents a composite crystalline belt within the Eastern European Alpine orogen, outcropping from the Pannonian basin in the north to the Aegean Sea in the south. The central parts of this massif (south-eastern Serbia) consist of the medium- to high-grade Lower Complex and the low-grade Vlasina Unit. Outcrop- and micro-scale ductile structures in this area document three major stages of ductile deformation. The earliest stage D1 is related to isoclinal folding, commonly preserved as up to decimetre-scale quartz–feldspar rootless fold hinges. D2 is associated with general south-eastward tectonic transport and refolding of earlier structures into recumbent metre- to kilometre-scale tight to isoclinal folds. Stages D1 and D2 could not be temporally separated and probably took place in close sequence. The age of these two ductile deformation stages was constrained to the Variscan orogeny based on indirect geological evidence (i.e. ca. 408-ca. 328). During this period, the SMM was involved in a transpressional amalgamation of the western and eastern parts of the Galatian super-terrane and subsequent collision with Laurussia. Outcrop-scale evidence of the final stage D3 is limited to spaced and crenulation cleavage, which are probably related to formation of large-scale open upright folds as reported previously. 40Ar/39Ar thermochronology was applied on hornblende, muscovite, and biotite samples in order to constrain the age of tectonothermal events and activity along major shear zones. These 40Ar/39Ar data reveal three major cooling episodes affecting the central SMM. Cooling below greenschist facies conditions in the western part of the Vlasina Unit took place in a post-orogenic setting (extensional or transtensional) in the early Permian (284 ± 1 Ma). The age of activity along the top-to-the-west shear zone formed within the orthogneiss in the Božica area of the Vlasina Unit was constrained to Middle Triassic (246 ± 1 Ma). This age coincides with widespread extension related to the opening of the Mesozoic Tethys. The greenschist facies retrogression in the Lower Complex probably occurred in the Early Jurassic (195 ± 1 Ma), and it was related to the thermal processes in the overriding plate above the subducting slab of the Mesozoic Tethys Ocean.

Structural analysis of a subduction-related contact in southern Sesia-Lanzo Zone (Austroalpine Domain, Italian Western Alps)

ABSTRACTA new foliation trajectory map at 1:10000 scale, represented here with an interpretative structural map, is derived from an original field analysis at 1:5000 scale in the southern Sesia-Lanzo Zone (SLZ). It shows the relative chronology of overprinting foliations, characterised by the mineral assemblages that mark superposed fabrics in each rock type. This map and the associated cross-sections, which synthesise the 3D structural outline of the tectonic contact between the Eclogitic Micaschists Complex (EMC), the Rocca Canavese Thrust Sheets and the Lanzo Ultramafic Complex, allow the correlation of the structural and metamorphic imprints that developed in these crustal and mantle complexes during Alpine subduction. Furthermore, the map and cross-sections allow the immediate perception of the metamorphic environments in which the structural imprints developed in each complex successively under eclogite, blueschist and greenschist facies conditions. The represented structural and metamorphic evolution of the southern end of the SLZ (internal Western Alps) has been inferred based on multiscale structural analysis. The dominant fabrics at the regional scale are two superposed mylonitic foliations that developed under blueschist and greenschist facies conditions, respectively. Metamorphic assemblages underlying the successive fabrics in the different metamorphic complexes allow us to identify contrasting metamorphic evolutions indicating that the tectonic contacts between the EMC, the Rocca Canavese Thrust Sheets and the Lanzo Ultramafic Complex developed under blueschist facies conditions and were successively reactivated during the greenschist facies retrogression.

Preservation of Blueschist-facies Minerals along a Shear Zone by Coupled Metasomatism and Fast-flowing CO2-bearing Fluids

AbstractTwo types of blue halo (types I and II) composed of blueschist-facies minerals are centered around a brittle, normal shear zone in greenschist-facies rocks on the island of Syros, Aegean Sea, Greece. The shear zone is steeply dipping and cuts a near-horizontal layer of greenschist-facies rocks (albite + epidote + actinolite + chlorite + quartz). Type I and II blue haloes are 0·3 m and c. 1 m wide respectively, and are seen on both sides of the shear zone. The inner type I haloes are composed of nearly pure glaucophane schist and were formed by metasomatic addition of Na2O and SiO2, and to a lesser extent of K2O and large ion lithophile elements (LILE), coupled with loss of CaO, Al2O3 and MnO. The outer type II haloes consist of a carbonated blueschist-facies assemblage (glaucophane + calcite + phengite + epidote + garnet + quartz). These experienced only slight metasomatic changes (i.e. addition of K2O and LILE), which cannot alone explain halo formation. We present petrological, geochemical and thermodynamic evidence that this assemblage was preserved at greenschist-facies conditions because XCO2 was elevated by flow of a CO2-bearing fluid along the shear zone, which was approximately contemporaneous with greenschist-facies hydration in the surrounding rocks. We further note that the flux of CO2-bearing fluid along the shear zone was rapid with respect to the fluid flux in the surrounding rocks. Mass-balance calculations reveal that the fluid flux within the shear zone was at least 100–2000 times greater than the fluid flux within the surrounding rocks. Mineral textures show greenschist-facies minerals replacing blueschist minerals in the type II haloes, supporting our interpretation that the blueschist-facies minerals were preserved during greenschist-facies retrogression. A simplified P–T vs XCO2 pseudosection confirms that preservation of carbonated blueschist can occur at greenschist-facies conditions in the presence of a CO2-bearing fluid.

In search of transient subduction interfaces in the Dent Blanche–Sesia Tectonic System (W. Alps)

Abstract In this paper we study the Alpine metamorphic history of a major tectonic zone which formed during Alpine orogeny, the Dent Blanche Thrust (DBT). This contact, located in the Northern Western Alps, juxtaposes some ophiolitic metasediment-rich remnants of the Liguro-Piemontese ocean (Tsate Complex) with a composite continental, km-sized complex (Dent Blanche Tectonic System, DBTS) of Adriatic affinity thrusted over the ophiolite. In order to better understand the geodynamic meaning of the DBT region and adjacent units, we have reconstructed the pressure–temperature–time–deformation (P–T–t–d) history of these two units using modern thermobarometric tools, Rb/Sr geochronology, and field relationships. We show that the Tsate Complex is formed by a stack of km-thick calcschists-bearing tectonic slices having experienced variable maximum burial temperatures between 360 °C and 490 °C at depths of ca. 25–40 km. Associated deformation ages span a range between 37 Ma and 41 Ma. The Arolla gneissic mylonites at the base of the DBTS experienced high-pressure (12–14 kbar), top-to-NW deformation at ca. 450 °C between 43 and 48 Ma. A first age of ca. 58 Ma has been obtained for high-pressure ductile deformation in the Valpelline shear zone, atop Arolla gneisses. Some of the primary, peak metamorphic fabrics have been reworked and later backfolded during exhumation and collisional overprint (ca. 20 km depth, 37–40 Ma) leading to the regional greenschist-facies retrogression which is particularly prominent within Tsate metasediments. We interpret the Dent Blanche Thrust, at the base of the Arolla unit, as a fossilized subduction interface active between 43 and 48 Ma. Our geochronological results on the shear zone lining the top of the Arolla unit, together with previous P–T–t estimates on equivalent blueschist-facies shear zones cutting the Sesia unit, indicate an older tectonic activity between 58 and 65 Ma. We demonstrate here that observed younger ages towards lowermost structural levels are witness of the transient, downwards migration of the Alpine early Cenozoic blueschist-facies subduction interface. This down-stepping is interpreted to reflect the progressive underplating acting between 30 and 40 km depth in the Alpine subduction zone between late Cretaceous and late Eocene. Underplating involved first continental material derived from the stretched Adriatic margin followed by underplating of ocean-derived rocks in the Eocene. These results shed light on subduction-zone accretion processes and therefore provide a new perspective for the understanding of geophysical results imaging the plate-interface region in active subduction zones.

ORIGIN OF TANZANITE AND ASSOCIATED GEMSTONE MINERALIZATION AT MERELANI, TANZANIA

Tanzanite is the blue vanadium-bearing variety of zoisite. The only known source of gem-quality tanzanite, to date, is the Merelani area of northeastern Tanzania, a structurally complex region consisting of granulite facies metacarbonate and metasilicate rocks overprinted by amphibolite to greenschist facies retrogression. Gem-quality green grossular garnets, known as tsavorite, are also found in the area, as well as a variety of other rare minerals and gemstones (such as axinite, dravite, fluorapatite, prehnite and tremolite). The δ18O values of tanzanite, tremolite, tsavorite and kyanite from the Merelani area are all high (average of 19.1, 19.5, 18.5 and 18.7‰, respectively) and are consistent with high-T O-isotope equilibrium, and the minimal involvement of externally derived fluids. The whole-rock δ18O values of graphite-bearing gneisses that host the tanzanite range from 16.8 to 21.6‰, whereas the marbles have somewhat higher values (24.0 and 25.0‰). Graphite-free garnet-bearing gneisses from the Telele Hills, ~33 km south of the mining area, have much lower δ18O values (~7 to 10‰). The high loss on ignition (LOI) values, ~8 weight % and ~10 weight %, of the graphite-bearing gneisses from the mining area do not correlate with CaO or MgO content, which is consistent with the presence of up to 10% graphite with minimal carbonate being present. The average δ13C value of the graphite from the garnet-kyanite gneiss in the mining area is −23.8‰, whereas most of the associated carbonate rocks have δ13C values close to zero. This lack of C-isotope equilibration between the adjacent carbonate and silicate rocks, together with the lack of change in bulk-rock δ18O values at the contact between carbonate and silicate units suggests that the high δ18O value of the graphite-bearing gneiss reflects that of its protolith. The average δD value of the tanzanite and tremolite is −77 and −27‰, respectively, which is consistent with their formation at in equilibrium with fluids having δD values of −36 and −11‰, respectively, assuming a temperature of formation of ~630°C. High δ18O values calculated for the fluids in equilibrium with tanzanite are consistent with a strongly rock-buffered system and hence a ‘metamorphic origin’. Calculated fluid δD values are close to zero, and suggest that the fluid is formation water, ultimately of marine or meteoric origin. The negative δ13C of the graphite and the high δ18O values of the rocks and minerals are consistent with previous suggestions that the protolith of the graphite-bearing gneisses was black shale. The major and trace element compositions of the rocks and minerals are consistent with this hypothesis

The geological significance of 40Ar/39Ar and Rb–Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re)crystallization during exhumation?

AbstractThe Attic‐Cycladic crystalline belt in the central Aegean region records a complex structural and metamorphic evolution that documents Cenozoic subduction zone processes and exhumation. A prerequisite to develop an improved tectono‐metamorphic understanding of this area is dating of distinct P–T–D stages. To evaluate the geological significance of phengite ages of variably overprinted rocks, 40Ar/39Ar and Rb–Sr analyses were undertaken on transitional blueschist–greenschist and greenschist facies samples from the islands of Syros and Sifnos. White mica geochronology indicates a large age variability (40Ar/39Ar: 41–27 Ma; Rb–Sr: 34–20 Ma). Petrologically similar samples have either experienced greenschist facies overprinting at different times or variations in ages record variable degrees of greenschist facies retrogression and incomplete resetting of isotopic systematics. The 40Ar/39Ar and Rb–Sr data for metamorphic rocks from both islands record only minor, localized evidence for Miocene ages (c. 21 Ma) that are well documented elsewhere in the Cyclades and interpreted to result from retrogression of high‐pressure mineral assemblages during lower pressure metamorphism. Field and textural evidence suggests that heterogeneous overprinting may be due to a lack of permeability and/or limited availability of fluids in some bulk compositions and that retrogression was more or less parallel to lithological layering and/or foliation as a result of, possibly deformation‐enhanced, channelized fluid ingress. Published and new 40Ar/39Ar and Rb–Sr data for both islands indicate apparent age variations that can be broadly linked to mineral assemblages documenting transitional blueschist‐to‐greenschist‐ and/or greenschist facies metamorphism. The data do not record the timing of peak HP metamorphism, but may accurately record continuous (partial) resetting of isotopic systematics and/or (re)crystallization of white mica during exhumation and greenschist facies retrogression. The form of 40Ar/39Ar phengite age spectra are complex with the lowest temperature steps yielding Middle to Late Miocene ages. The youngest Rb–Sr ages suggest maximum ages of 20.6 ± 0.8 Ma (Syros) and 22.5 ± 0.6 Ma (Sifnos) for the timing of greenschist facies overprinting. The results of this study further accentuate the challenges of interpreting isotopic data for white mica from polymetamorphic terranes, particularly when mixing of populations and/or incomplete resetting of isotopic systematics occurs during exhumation. These data capture the full range of isotopic age variations in retrogressed HP rocks documented in previous isotopic studies, and can be interpreted in terms of the geodynamic evolution of the Aegean.

Fabrics produced mimetically during static metamorphism in retrogressed eclogites from the Zermatt-Saas zone, Western Italian Alps

Lattice preferred orientations (LPOs) are commonly interpreted to form by dislocation creep. Consequently they are used to infer deformation at the metamorphic grade at which the minerals were stable, especially if those minerals show a shape fabric. Here we show that LPOs can occur through mimicry of a pre-existing LPO, so they formed statically, not during deformation. Omphacite and glaucophane LPOs occur in eclogite facies rocks from the Zermatt-Saas Unit of the Northwest Italian Alps. Barroisite grew during greenschist facies retrogression and has an LPO controlled significantly by the eclogite facies omphacite and glaucophane LPOs, rather than directly by deformation. Using spatially resolved lattice orientation data from the three key minerals, collected using electron backscatter diffraction, we deploy a new technique of interphase misorientation distribution analysis to prove this. Barroisite LPO develops by mimicry of omphacite (via a particular lattice orientation relationship) and by direct topotactic and epitactic replacement of glaucophane. LPO in turn influenced anisotropic grain growth, resulting in a barroisite grain shape fabric. Thus regional retrogression during exhumation of the Zermatt-Saas high-pressure rocks was, in large part, static, rather than dynamic as previously interpreted. In general the possibility of mimetic fabrics forming during metamorphic reactions must be borne in mind when interpreting direct structural observations and seismic anisotropy data in terms of deformation, in both crust and mantle.

Monazite stability, composition and geochronology as tracers of Paleoproterozoic events at the eastern margin of the East European Craton (Taratash complex, Middle Urals)

The Precambrian Taratash complex (Middle Urals) is one of the rare windows into the Palaeoproterozoic and earlier history of the eastern margin of the East European Craton. Monazite from intensively deformed rocks within a major amphibolite-facies shear zone in the Taratash complex has been investigated by means of electron-probe microanalysis and laser-ablation SF-ICP-MS. Metamorphic and magmatic cores of monazite from metasedimentary and metagranitoid rocks yield U–Pb ages of 2244 ± 19 and 2230 ± 22 Ma (± 2 σ) and record a previously unknown pre-deformational HT-metamorphic event in the Taratash complex. Subsequent dissolution–reprecipitation of monazite, during shear zone formation under amphibolite-facies conditions, caused patchy zonation and chemical alteration of the recrystallised monazite domains, leading to higher cheralite and huttonite components. This process, which was mediated by a probable (alkali + OH)-bearing metamorphic fluid also caused a total resetting of the U–Pb-system. The patchy domains yield concordant U–Pb-ages between 2052 ± 16 and 2066 ± 22 Ma, interpreted as the age of the shear zone. In line with previously published ages of high grade metamorphism and migmatisation, the data may point to a Palaeoproterozoic orogenic event at the eastern margin of the East European Craton. Post-deformational fluid-induced greenschist-facies retrogression caused partial to complete breakdown of monazite to fluorapatite, REE + Y-rich epidote, allanite and Th-orthosilicate.The retrograde assemblages either form coronas around monazite, or occur as dispersed reaction zones, indicating that the REE, Y, and Th were mobile at least on the thin section scale. The greenschist-facies metamorphic fluid was aqueous and rich in Ca. Monazite affected by advanced breakdown responded to the retrogression by incorporating the cheralite or huttonite components during a fluid-induced dissolution–reprecipitation process. This event did not reset the U–Pb-system but caused partial Pb loss reflected by discordant U–Pb-dates.

Miocene south directed low‐angle normal fault evolution on Kea Island (West Cycladic Detachment System, Greece)

New structural, petrologic, and thermochronologic data from Kea, West Cyclades, define a crustal‐scale ductile shear zone and ductile/brittle low‐angle normal fault (LANF) system. Both the greenschist‐facies shear zone forming the footwall and the overlying LANF zone formed during constrictional strains, with a consistent top‐to‐SW‐S shear sense, with increasing finite strains toward higher structural levels but decreasing temperatures from footwall to hanging wall. The tectonostratigraphy comprises a ∼450 m thick footwall of shallowly dipping schists and calcite marbles representing the Intermediate Unit of the Attic‐Cycladic Crystalline (ACC). Above the footwall is a ∼60 m thick highly strained LANF zone, consisting of phyllonites, cataclastic schists, ultramylonitic calcite marbles, (proto) mylonitic calcite marbles, and cohesive cataclasites that arch over the whole island. These fault rocks exhibit multistage LANFs, evolving from ductile to brittle conditions. An up to ∼50 m thick brecciated limestone and dolostone sequence forms the unmetamorphosed hanging wall which is most probably part of the Upper Unit of the ACC. Multiequilibrium P‐T estimates on chlorite‐white mica pairs in the footwall yield 7–5.5 kbar/360°C–450°C for inclusions in albite and epidote, 5.5–3 kbar/400°C–350°C for the main foliation, and 3–2 kbar/350°C–280°C in localized shear bands (C and C′ foliations). The40Ar/39Ar white mica footwall cooling ages demonstrate that greenschist‐facies retrogression occurred between ∼21 and 17 Ma. Localized, late decimeter thick shear zones were active and dynamically recrystallized before ∼15 to 13 Ma. The LANF on Kea, together with similar structures in South Attica and the West Cyclades define the West Cycladic Detachment System, characterized by ductile to brittle top‐SW‐S shear sense.

Polymetamorphic history of a relict Permian hornfels from the central Gran Paradiso Massif (Western Alps, Italy): a microstructural and thermodynamic modelling study

A petrological and thermobarometric study of the Lago Teleccio hornfelses was undertaken to reconstruct the polymetamorphic evolution and constrain the P–T conditions of Permian contact metamorphism. The Lago Teleccio metasedimentary rocks record a Variscan regional metamorphism characterized by amphibolite facies mineral assemblages including quartz, plagioclase, K-feldspar (Kfs 1), biotite, garnet (Grt 1) and staurolite; this was followed by a late-Variscan mylonitization event. Metamorphism of the Variscan metamorphic rocks at the contact with a Permian granitic intrusion produced static recrystallization and/or new growth of quartz, garnet (Grt 2), plagioclase, K-feldspar (Kfs 2), cordierite, green spinel, biotite and prismatic sillimanite (Contact 1). This thermal event, which occurred at a peak pressure of 0.23–0.35 GPa, temperature of 670–700 °C and aH2O of 0.75–1, was followed either during post-contact metamorphism cooling or, more likely, during the early-Alpine metamorphism by the breakdown of cordierite into an anhydrous kyanite + orthopyroxene + quartz assemblage. The poorly developed early-Alpine eclogite facies metamorphism (Alpine 1) was characterized by relatively anhydrous mineral associations and low strain, which locally produced coronitic and pseudomorphous microstructures in metasedimentary rocks, with scanty formation of jadeite, zoisite and a new high-pressure garnet (Grt 3). Greenschist facies retrogression (Alpine 2) was characterized by the local development of a chlorite- and muscovite-bearing mineral association, suggestive of aqueous fluid incursion. In the hornfelses, the limited extent of metamorphic overprinting is suggested by the fine grain size of the Alpine mineral associations, which formed at the expense of the Permian contact metamorphic associations, and was favoured by the anhydrous mineralogy of the hornfelses.

MODELING THE OCEANIC LITHOSPHERE OBDUCTION: CONSTRAINTS FROM THE METAMORPHIC SOLE OF MIRDITA OPHIOLITES (NORTHERN ALBANIA)

The Hellenic-Dinaric belt of the Eastern Mediterranean sea is characterized by huge and well preserved obducted slices of ophiolites. A well developed and very thick metamorphic sole generally occurs at the base of the obducted ophiolite sequences. The best preserved metamorphic sole of the Hellenic-Dinaric belt crops out in association with the Albanian ophiolites. In the Mirdita area (northern Albania) the metamorphic sole consists of an up to 600 m thick assemblage of metasediments and amphibolites. Four different types of amphibolites, referred as T1 to T4, have been identified on the basis of grain-size and mineralogical assemblages. The protoliths of all the amphibolites are basic rocks with OIB affinity, representative of within-plate magmatism typical of oceanic seamounts; no MORB derived rocks have been identified. The associated metasedimentary rocks are presumably derived from oceanic covers. All the lithologies from the Mirdita metamorphic sole are strongly deformed under lower to upper amphibolite facies conditions. A discontinuous inverted gradient is observed close to harzburgites in the metamorphic sole, that shows high-grade amphibolite facies metamorphism. There are no prograde greenschist facies metamorphic rocks, although all the lithologies from the Mirdita metamorphic sole are affected by greenschist facies retrogression. The different types of amphibolites display different P and T values at metamorphic climax conditions. The temperatures range from 624±9° to 796±50°C in the different units where the pressure is always lower than 0.7 GPa. The boundaries between the amphibolite slices with different peak metamorphism as well as the bodies of the gneisses and micaschists are sheared bands. The structural pattern common to all lithologies identified in the Mirdita metamorphic sole developed through two deformation phases under amphibolite facies conditions, followed by a third phase under greenschist facies. The petrologic and structural characteristics of the metamorphic sole are discussed in order to highlight the kinematics of the obduction process.

Petrology of the eclogites from western Tasmania: Insights into the Cambro-Ordovician evolution of the paleo-Pacific margin of Gondwana

Eclogite facies rocks along the Paleozoic active margin of Gondwana are rare. They are limited to isolated segments of Northern Victoria Land (Antarctica), western Tasmania, and southeastern Australia. New petrological data for mafic rocks and their host garnet-kyanite schists from the Franklin Metamorphic Complex (western Tasmania) permit reconstruction of six main stages of mineral growth for the eclogite. Stages I and II occurred at greenschist/amphibolite-facies conditions (ca. 500–600 °C; 0.55–0.7 GPa for stage II) before attaining high-pressure conditions (at ≈ 600–650 °C; > 1.5 GPa for stage III). The following stages, IV and V, record the decompression from high-pressure conditions to amphibolite-facies (ca. 500–600 °C; 0.4–1.0 GPa). Finally, stage VI represents the late greenschist-facies retrogression. However, the pelitic schist surrounding the eclogite records only the medium-pressure amphibolite-facies stage. The P– T evolution over time outlines a clockwise path that is quite steep in both the prograde and retrograde segments. The latter shows a nearly isothermal decompression between the eclogite and the high-pressure amphibolite-facies stage IV, which was achieved at deep crustal levels (≈ 30 km), and a final decrease in both pressure and temperature from deep/intermediate to shallow crustal levels, with a typical cooling-unloading path. The lack of a complete re-equilibration during the different stages and the high d P/d T for both the prograde and retrograde paths are indicative of a rapid burial and initially rapid exhumation. The similarity of the mafic whole-rock chemical composition, including N, T to E-MORB and of the peak metamorphic age (≈ 500 Ma) between the Tasmanian eclogites and the UHP rocks from Northern Victoria Land, supports the idea that they formed in the context of the same contractional event. However, the different P– T conditions and d P/d T point to different tectono-metamorphic settings for the two sectors of the paleo-Pacific margin of Gondwana during the Ross/Tyennan orogeny.

Structural and 40Ar/39Ar age constraints on the Kulidjik nappe: A record of an early Alpine thrust tectonics in the northeastern Rhodope Massif, Bulgaria

In the northeastern Rhodope Massif, a unique juxtaposition of assumed high-grade basement allochthon onto a low-grade Mesozoic unit is exposed in the Kulidjik nappe. The thrusting is interpreted as post-Jurassic to pre-Late Eocene. The tectonostratigraphy comprises the following units: (i) an upper unit of high-grade basement of the eastern Rhodope consisting of various amphibolite facies rocks with metaophiolite lenses; (ii) an overlaying Mesozoic (including Jurassic radiolarians) low-grade unit; (iii) a structurally overlaying allochthon of the Kulidjik nappe; and (iv) unconformable Late Eocene sedimentary and Oligocene volcanic cover rocks. The basement unit contains a NNE-SSW oriented lineation rarely associated with top-to-the NNE ductile shearing. Greenschist facies retrogression of this unit is diagnostic of local thrusting of the overlying low-grade unit. The greenschists of the low-grade unit represent arc metavolcanic rocks overlain by phyllites. The structural elements in this unit have similar attitudes as the basement unit. Kinematic indicators in the low-grade unit demonstrate internal top-to-the NNE ductile shearing during greenschist facies conditions. The allochthon consists of klippen composed of muscovite-albite gneisses. The flat-lying thrust contact with ductile-brittle deformation contains mylonites and cataclasites. Previously reported local top-to-the SSW ductile shearing in the allochthon is probably a consequence of shear instabilities. Deformation-crystallization relationships revealed fabrics in the allochthon compatible with greenschist up to lower amphibolite facies conditions without retrogression and similar ENE-verging fold patterns in the low-grade unit and the allochthon at the thrust contact. Two muscovite samples from distinct klippen yielded inverse isochron 40Ar/39Ar ages of 156.07±1.30 Ma and 162.62±2.69 Ma, which reflect cooling below 350°C of the allochthon associated with its exhumation during emplacement after greenschist-lower amphibolite facies metamorphism. Structural data indicate NNE-directed tectonic transport of the low-grade footwall unit, which is consistent with emplacement in comparable metamorphic conditions, and the thrust direction of the allochthon in the hanging wall. The subduction-accretionary origin of the low-grade unit and the new age results allow us to link this tectono-metamorphic event to Late Jurassic thrust imbrication of arc units on the Rhodope basement, which is the first time that early Alpine thrust tectonics in the northeastern Rhodope Massif have been recognized.

Albite vein formation during exhumation of high‐pressure terranes: a case study from alpine Corsica

AbstractThe formation of late‐stage veins can yield valuable information about the movement and composition of fluids during uplift and exhumation of high‐pressure terranes. Albite veins are especially suited to this purpose because they are ubiquitously associated with the greenschist facies overprint in high‐pressure rocks. Albite veins in retrogressed metabasic rocks from high‐pressure ophiolitic units of Alpine Corsica (France) are nearly monomineralic, and have distinct alteration haloes composed of actinolite + epidote + chlorite + albite. Estimated P–T conditions of albite vein formation are 478 ± 31 °C and 0.37 ± 0.14 GPa. The P–T estimates and petrographic constraints indicate that the albite veins formed after the regional greenschist facies retrogression, in response to continued decompression and exhumation of the terrane. Stable isotope geochemistry of the albite veins, their associated alteration haloes and unaltered hostrocks indicates that the vein‐forming fluid was derived from the ophiolite units and probably from the metabasalts within each ophiolite slice. That the vein‐forming fluid was locally derived means that a viable source of fluid to form the veins was retained in the rocks during high‐pressure metamorphism, indicating that the rocks did not completely dehydrate. This conclusion is supported by the observation of abundant lawsonite at the highest metamorphic grades. Fluids were liberated during retrogression via decompression dehydration reactions such as those that break down hydrous high‐pressure minerals like lawsonite. Albite precipitation into veins is sensitive to the solubility and speciation of Al, which is more pressure sensitive than other factors which might influence albite vein formation such as silica saturation or Na:K fluid ratios. Hydraulic fracturing in response to fluid generation during decompression was probably the main mechanism of vein formation. The associated pressure decrease with fracturing and fluid decompression may also have been sufficient to change the solubility of Al and drive albite precipitation in fracture systems.

The petrology of two distinct granulite types in the Hengshan Mts, China, and tectonic implications

The Archean to Proterozoic Hengshan Complex (North China Craton), comprises tonalitic and granodioritic gneisses with subordinate mafic lenses, pegmatites and granites. Amphibolite facies assemblages predominate, although granulite-facies relics are widespread, and greenschist-facies retrogression occurs in km-wide shear zones. Mafic lenses, locally abundant, occur as strongly deformed amphibolite (hornblende+plagioclase) boudins or sheets. In contrast to previously published models we find two series of mafic rocks with distinctly different granulite-facies evolutions. In the north of the Complex, relict high-pressure mafic granulites are garnet+clinopyroxene-bearing rocks with a secondary development of orthopyroxene around both garnet (kelyphites) and clinopyroxene (coronas). South of the newly defined central, E–W-trending, Zhujiafang shear zone, numerous mafic boudins and less-deformed dykes exhibit a macroscopically visible magmatic texture with coronitic growth of metamorphic garnet (full of quartz inclusions) between the magmatic plagioclase and pyroxene domains. Additional orthopyroxene (after magmatic augite) and sodic rims to magmatic plagioclase clearly indicate medium-pressure granulite-facies metamorphism. These findings suggest tectonic juxtaposition in this area of three different structural levels of the same Proterozoic-imprinted crust: high-pressure granulite grade in the northern Hengshan, medium-pressure granulite grade in the southern Hengshan and amphibolite- to greenschist-facies grade in the Wutaishan to the SE.

Structural controls on gold mineralisation and the nature of related fluids of the Paiol gold deposit, Almas Greenstone Belt, Brazil

Chemical analyses suggest that the metavolcanic rocks of the Almas Greenstone Belt (AGB), Tocantins State, Brazil have a continental affinity, possibly related to a continental rift environment. They were metamorphosed to amphibolite facies during a regional tectono-metamorphic event (D n ), retrogressed to greenschist facies assemblages and then hydrothermally altered within dextral strike–slip shear zones (D n+1 ). Fracture sets related to D n+2 intersect S n+1 . The Paiol Gold Mine is one of several mineralised zones within metabasic and meta-intermediate rocks of the AGB. It exploits shoots of sulphide–Au–quartz mineralisation that occupy dilational zones approximately perpendicular to an elongation lineation (L n+1 ) within mylonitic foliation S n+1 (S n+1 =S within the S–C fabric). The dilational zones probably formed due to dextral displacement on sinistrally en echelon C surfaces. Minor amounts of gold may have been introduced or remobilised during D n+2 . Coexisting primary and pseudosecondary fluid inclusions in mineralised quartz veins from ore shoots comprise a high-salinity three-phase type (Type II) and a lower salinity two-phase type (Type I). Homogenisation temperatures for Type II inclusions range from 200 to 410 °C and Type I from 90 to 320 °C. The inclusions and their temperature ranges are believed to reflect heat exchange and some mixing between the two fluid types under relatively constant ambient temperatures, but variable (though broadly declining) fluid temperatures. This took place late in D n+1 in conjunction with greenschist facies retrogression and localised hydrothermally induced metasomatism.

Coexisting 3T and 2M1 polytypes of phengite from Cima Pal (Val Savenca, western Alps): Chemical and polytypic zoning and structural characterisation

Randomly oriented phengite flakes, up to 15 cm across and 5 cm thick, occur in a metamorphic dyke at Cima Pal (upper Val Savenca, western Alps). The phengite formed in the Sesia zone at quartz-eclogite-facies conditions ( T ∼ 550°C, P ∼ 16–18 kbar) in the presence of hydrous fluid. A greenschist-facies retrogression (∼ 4–5 kbar and 500°C) locally altered the eclogite-facies assemblage. The mica flakes show yellowish and greenish zoning. This zoning is superposed by a structural zoning of 3 T and 2 M 1 (more abundant) polytypes. Single-crystal X-ray diffraction data were collected and the structures refined from the following samples. \[\mathrm{(K_{0.90}Na_{0.05}) (Al_{1.51}Mg_{0.32}Fe_{0.18}Ti_{0.03}) [Si_{3.40}Al_{0.60}O_{10}](OH)_{2} [yellowish 3\mathit{T}; \mathit{a} 5.220(1), \mathit{c} 29.762(9) {\AA}]}\] , \[\mathrm{(K_{0.95}Na_{0.01}) (Al_{1.40}Mg_{0.34}Fe_{0.27}Ti_{0.03}) [Si_{3.50}Al_{0.50}O_{10}](OH)_{2} [greenish 3\mathit{T}; \mathit{a} 5.228(1), \mathit{c} 29.73(1) {\AA}]}\] , \[\mathrm{(K_{0.98}Na_{0.02}) (Al_{1.55}Mg_{0.24}Fe_{0.21}Ti_{0.02}) [Si_{3.38}Al_{0.62}O_{10}](OH)_{2} [yellowish 2\mathit{M}_{1}; \mathit{a} 5.2132(8), \mathit{b} 9.051(2), \mathit{c} 19.937(5) {\AA}, {\beta} 95.76(2){^\circ}]}\] , and \[\mathrm{(K_{0.99}Na_{0.02}) (Al_{1.42}Mg_{0.33}Fe_{0.24}Ti_{0.04}) [Si_{3.45}Al_{0.55}O_{10}](OH)_{2} [greenish 2\mathit{M}_{1}; \mathit{a} 5.225(1), \mathit{b} 9.057(2), \mathit{c} 19.956(6) {\AA}, {\beta} 95.73(2){^\circ}]}\] . Tetrahedral cation order is absent in both polytypes, although partial octahedral order is present in the 3 T polytype. Colour and limited chemical zoning is attributed to a fluctuation in the fluid-phase composition. The 2 M 1 polytype with disordered pattern of octahedral cations was probably crystallised from the pre-existing 3 T polytype by a solid state reaction. The transformation occurred during the sharp decompression stage leading to the greenschist facies, which triggered nucleation and growth of the 2 M 1 polytype.