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Hypogene dissolution-precipitation processes strongly affect the petrophysical properties of carbonate rocks and fluid migration pathways in sedimentary basins. In many deep carbonate reservoirs, hypogene cavernous voids are often associated with silicified horizons. The diagenesis of silica in carbonate sequences is still a poorly-investigated research topic. Studies exploring the complexity of silica dissolution-precipitation patterns in hypogene cave analogues are therefore fundamental to unravel the diagenetic and speleogenetic processes that may affect this kind of reservoir. In this work, we investigated an exhumed and silicified Neoproterozoic carbonate sequence in Brazil hosting a 1.4 km-long cave. Quartz mineralization and silicified textures were analyzed with a multidisciplinary approach combining petrography, fluid inclusion microthermometry, silicon-oxygen stable isotope analyses and U-Th-Pb dating of monazite crystals. We found that an early silicification event caused the replacement of the dolostone layers with micro-crystalline quartz forming chert nodules. This event was likely associated with mixing fluids (ancient Neoproterozoic seawater and hydrothermal solutions sourced from the underlying Mesoproterozoic basement) at relatively low temperatures (ca. 50-100 degrees C) and shallow depth. After the tectonic deformation produced by the Brasiliano orogeny, silica dissolution was promoted by high temperature and alkaline hydrothermal solutions rising from the quartzite basement along deep-rooted structures. Hypogene hydrothermal alteration promoted the dissolution of the cherty layers and the precipitation of chalcedony and megaquartz. Homogenization temperatures from primary fluid inclusions in megaquartz cement indicate minimum formation temperatures of 165-210 degrees C. Similar temperature estimates (110-200 degrees C) were obtained from the delta Si-30 and delta O-18 isotope systematics of quartz precipitated from hydrothermal solutions. The dissolved salts in the fluid inclusions were evaluated as NaCl + CaCl2 from microthermometric data combined with cryogenic Raman spectroscopy, corresponding to salinity ranging between 17 and 25 wt.%. No reliable age constraints for hydrothermal silica dissolution-precipitation phases were obtained from monazite U-Th-Pb dating. However, our results, interpreted in the regional context of the Sao Francisco Craton, suggest that the Cambrian tectono-thermal events could have been amongst the possible drivers for this hypogene process in the basin.

Late Mesozoic volcanic rocks comprising mainly basalt and basaltic-andesite to dacite occur in south-central Vietnam (Dalat zone) and to a lesser extent in southwestern Vietnam (Bay Nui area). Mineral and whole-rock chemistry indicate a calc-alkaline affinity for samples in the Dalat zone and a high-K calc-alkaline to shoshonitic affinity for rocks in the Bay Nui area. Mineral characteristics and variation diagrams of selected elements suggest that fractional crystallization dominated during magma differentiation. The Bay Nui volcanic rocks generally are more enriched in potassium and LILEs (large-ion lithophile elements) than volcanic rocks from the Dalat zone, which may indicate a more evolved nature or crustal assimilation. The similar chemical characteristics and eruption/emplacement age range of volcanic and plutonic rocks (ca. 90–110 Ma) of equal silica concentration indicate that the magma feeding the volcanic eruptions had the same source as that of the plutonic rocks. The observed mineral and whole-rock compositions with enrichment in LILEs, depletion in HFSEs (high field strength elements), and noticeably negative Nb, Ta, and Ti anomalies are characteristic for arc signatures. Zircon U-Pb geochronological data for the volcanic rocks indicate an age range of 95–105 Ma for the eruption. These geochemical and geochronological data link this Late Mesozoic volcanism with continental arc magmatism driven by the subduction of the Paleo-Pacific beneath eastern Indochina. Zircon xenocrysts with a likely magmatic origin cluster around 350 Ma and 250 Ma, indicating two earlier magmatic events most likely related to the subduction of the Paleo-Tethys beneath western Indochina and the subsequent Indosinian orogeny.

Allanite-group minerals are known to incorporate not only U and Th but also initial, non-radiogenic Pb. Allanite can therefore be analyzed in order to assess its crystallization age as well as the ambient Pb composition at the time of crystallization. Whereas allanite age dating has been the focus of many studies, constraining its initial Pb composition has received much less attention. We collected a series of Phanerozoic, allanite-bearing magmatic rock samples (volcanic, plutonic, pegmatite) and measured both the age and initial Pb composition of allanite by laser ablation-multi collector-inductively coupled plasma-mass spectrometry. We show that allanite data can be corrected for mass bias and fractionation using zircon (for U/Pb and Th/Pb ratios) and glass (for Pb/Pb ratios) as reference material as long as allanite is not metamict. A lower intercept age and y-axis intercept Pb composition can be determined by linearly regressing U-Pb data in a Tera-Wasserburg diagram, and a 230Th disequilibrium correction is highly recommended. We find a good agreement between our allanite U-Pb dates and published U-Pb zircon ages for the same localities. Our initial Pb compositions are validated by a fair agreement with Pb isotopic data measured on co-genetic feldspars from the same samples. The initial Pb composition of samples ranging from ca. 530 to 18 Ma reveals fluctuations in initial 207Pb/206Pb ratio, which points to different degrees of crustal (elevated μ=238U/204Pb) contribution. These variations could be due to post-magmatic deformation, weathering or metamorphism, but we believe that they rather reflect differences in initial magma composition. We thus emphasize the usefulness of allanite initial Pb compositions to discuss the source of igneous rocks.

The Inthanon Zone is regarded as the main suture between the Indochina and Sibumasu blocks and comprises ultramafic rocks, marine sediments and crystalline basement rocks. The gneissic basement is exposed in two different structural domains: (1) the Inthanon core complex located to the west of the Chiang Mai basin, and (2) the Mae Ping shear zone located to the south of the core complex. Here, we present new petrological and geochronological results from gneisses and schists of the Inthanon zone. Four different mineral assemblages can be recognised in gneisses and schists: (1) garnet–muscovite–biotite±sillimanite±chlorite schist, (2) garnet–muscovite–biotite–plagioclase–K-feldspar gneiss, (3) tourmaline-bearing muscovite–biotite– orthogneiss, and (4) migmatitic biotite gneiss. These rocks typically contain accessory ilmenite, pyrite, apatite, tourmaline, monazite, xenotime, and zircon. In-situ Th-U-total Pb dating of monazite reveals at least two metamorphic events, one in the Early Jurassic and another one in the Early Paleocene. A garnet–muscovite–biotite–sillimanite schist sample shows matrix micas and fibrolithic sillimanite wrapped around garnet porphyroblasts. Multi-equilibrium thermobarometry using Tweequ (Berman, 1996) yields metamorphic peak conditions of 0.5 GPa and 570 °C. Monazite dating yields two age populations at 189±5 and 61±7 Ma. A second sample belonging to this group contains chlorite instead of sillimanite and has a main schistosity with tightly folded relicts of a former fabric. Garnet porphyloblasts exhibit pressure shadows with quartz and mica. Monazite dating gives a single age population of 65±6 Ma. Garnet–muscovite–biotite–plagioclase–K-feldspar gneiss samples show corona textures with plagioclase, quartz, biotite, and muscovite around garnet porphyroblasts, indicative of pressure decrease. P–T conditions of 0.6–0.7 GPa and 680–700 °C were calculated using the garnet-biotite-plagioclase-quartz and garnet-biotite geothermobarometers. The formation of coronae around garnet occurred during exhumation at slightly lower conditions of 0.4–0.5 GPa and 640–660 °C. Monazite dating yields a main population at 189±5 Ma with few 50-70 Ma dates. Tourmaline-bearing muscovite–biotite–plagioclase–K- feldspar gneiss samples are characterized by an ultramylonitic texture. Large K-feldspar augen and tourmaline porphyroclasts are surrounded by a fine-grained, foliated matrix of quartz, and feldspar. The mineral assemblage indicates middle amphibolite grade. Monazite dating of this sample yields two populations at 192±3 and 58±4Ma. Migmatitic biotite–gneiss samples preserve a biotite–plagioclase–K-feldspar–quartz assemblage in both the melanosome and leucosome. Monazite dating provides a single population of 61±2 Ma. Two tectono-metamorphic events are revealed by our data: a widespread medium P-T regional metamorphic phase, and a younger overprint of unclear grade (low to high T assemblages are found) but significant spatial extent. While the first event was coeval with abundant plutonism during Sukhothai-Sibumasu collision (~185 Ma), the second one (~60 Ma) does not appear to be connected with regional plutonic activity and might be related to large scale shearing as seen in the Mae Ping and Three Pagoda shear zones.

The Eastern Greywacke Zone is composed of three Alpine nappes. From bottom to top these are (1) the Veitsch nappe (Early Carboniferous to Permian molasse), (2) the Silbersberg nappe with intercalated slivers of the Kaintaleck Metamorphic Complex and Permian phyllites and conglomerates as cover, and (3) the Noric nappe (mainly Ordovician to Devonian shelf sediments and Permian cover). All units experienced Eo-Alpine lower greenschist facies metamorphism. Due to the development of ductile shear zones during Alpine nappe stacking, the Kaintaleck Complex was dismembered and emplaced as lens-shaped bodies of 10-100m thickness that stretch from West (Kalwang, Upper Styria) to East (Gloggnitz, Lower Austria) below the Noric nappe of the Eastern Greywacke Zone. Lithologically, the Kaintaleck Complex is represented by a mafic suite, comprising amphibolite, garnet-amphibolite, greenschist and serpentinite, and a felsic suite that consists mostly of gneiss and mica-schist (some of them garnet-bearing). The felsic suite corresponds to metamorphosed clastic sediments and granitoids, whereas the mafic suite represents most likely a former oceanic crust. This work tries to constrain the P-T-t path of the Kaintaleck Metamorphic Complex by applying U-Th/Pb monazite and zircon dating and geothermobarometry. Based on whole rock geochemistry, amphibolites from the locality of Frauenberg represent tholeiitic basalts with an E-MORB affinity, whereas garnet-amphibolites, amphibolites and greenschists from the localities of Prieselbauer, Oberdorf, Unteraich, Kalwang, Arzbach and Schlöglmühl show a T-MORB signature. Samples from the localities of Stübminggraben and Utschgraben have a N-MORB affinity. Garnet-amphibolite samples show distinct plagioclase-epidote-rich symplectitic coronae, which are indicative of decompression from former eclogite-facies conditions. P-T estimations based on Zr-in-rutile thermometry and phengite barometry yield up to 720°C and 19 kbar for the felsic suite, and 700°C and 21 kbar for the mafic suite, both interpreted as peak metamorphic conditions. Monazite dating by EPMA in garnet-mica-schist from the localities of Prieselbauer, Arzbach, Schlöglmühl and Oberdorf, revealed weighted average U-Th-total Pb dates of 351 ± 4 Ma, 358 ± 16 Ma, 349 ± 3 Ma and 362 ± 6 Ma, which are interpreted as reflecting peak Variscan metamorphism. Monazite in these samples is partly replaced by an apatite-allanite-corona, related to monazite-breakdown due to Alpine lower grade metamorphic overprint. Preliminary LA-MC-ICP-MS U/Pb age dating results of zircon grains from a garnet-amphibolite from the Prieselbauer locality yield a Devonian mean date of 400 ± 4 Ma ascribed to the protolith formation.

Two volcanic phases took place within the Styrian basin: an older one during the Miocene and a younger one during the Pliocene. While the alkaline basalts of the Pliocene phase have been studied in detail in the recent years (e.g. Ali et al. 2013), the Miocene volcanism around Bad Gleichenberg has not received much attention. The hills of Gleichenberg and Bschaidkogel are composed of trachyandesites and rhyolites with a calc-alkaline to shoshonitic affinity. A crystallization age of 13 ± 1 Ma was determined by K/Ar dating (Balogh et al., 1990), which clearly differentiates the Gleichenberg formation from the younger Pliocene alkaline basaltic magmatic formations that characterize the Styrian Basin. To further characterize and determine the age of the Gleichenberg volcanic formation, samples from different locations around Bad Gleichenberg were analyzed by petrological and geochemical methods. The majority of the samples are trachyandesites that follow typical calc-alkaline differentiation trends with SiO2 varying from 57 to 62 wt%; CaO, FeO and MgO decrease with increasing SiO2, while Al2O3, K2O, Na2O and P2O5 increase with increasing SiO2. The second main lithology – although much less voluminous - are rhyolites with SiO2 contents >71 wt%. Biotite and apatite are commonly found in both rock types. The fluorine content is high in both biotite and apatite with values up to 2 and 3 wt%, respectively. TiO2 contents in biotite can be as high as 8 wt%. U-Pb zircon dating by LA-MC-ICP-MS was carried out on zircons from two samples. Idiomorphic zircons, partly with abundant apatite inclusions or visible zircon cores and zonation, could be found in almost all samples. No age difference between zircon cores and rims or between zoned and homogeneous grains was observed. The analyzed samples yielded homogeneous crystallization ages of ~14 Ma for both volcanic rock types. Therefore, the Gleichenberg formation is likely related to the Balatonmária and Bükkalja volcanic fields in the Western Pannonian Basin System (Harangi et al. 1995). Based on similar geochemical characteristics, both occurrences can be related to the opening of the Styrian/Pannonian Basin, which resulted from slab retreat after collision between the Adriatic plate and Europe. ALI, S., NTAFLOS, T., UPTON, B. (2013). Petrogenesis and mantle source characteristics of Quaternary alkaline mafic lavas in the western Carpathian–Pannonian Region, Styria, Austria. Chemical Geology. 337. 10.1016/j.chemgeo.2012.12.001. BALOGH, K., HARALD, L., PÉCSKAY, Z., RAVASZ, CS., SOLTI, G. (1990). K/Ar radiometric dating of the Tertiary volcanic rocks of East-Styria and Burgenland. MÁFÍ Évi Jel. 1988-ról, 451-468. HARANGI, S., VASELLI, O., TONARINI, S., SZABÓ, C., HARANGI, R., CORADOSSI, N. (1995). Petrogenesis of Neogene extension-related alkaline volcanic rocks of the Little Hungarian Plain volcanic field (Western Hungary). Acta Vulcanologica, 7, 173-188.

Zircon megacrysts are unusually large crystals (> 5 mm) that are commonly associated with mantle-derived kimberlites, carbonatites, alkali basalts and syenitic pegmatites (e.g., Hoskin and Schaltegger 2003). Such zircons form during relatively short timespans and therefore, are often used as reference material for U-Pb geochronology. Here, we determine the geochemical and isotopic (U-Pb, Hf, O) characteristics of a little-known occurrence of zircon megacrysts at Kawisigamuwa, Sri Lanka.The dark brown megacrysts are euhedral, commonly elongate crystals with double pyramidal terminations and have faintly corroded crystal surfaces. The zircons consist of oscillatory zoned and nearly featureless cathodoluminescence-bright patches, some of which appear to follow sealed cracks. All zircon domains show a low to moderate FWHM of the ν3 (SiO4) Raman band (2.5 to 7.3 cm-1), have a low to moderate radiation damage (total α-dose mainly

The affiliation of the Ennstal Phyllite Zone (EPZ) to either the micaschist units of the Koralpe-Wölz nappe system (KW-NS) to its south or to nappes of the “Greywacke Zone” to its north and east is still debated. Due to similarities with phyllites of the “Greywacke Zone” in the north and phyllonitic micaschists in the south, no clear lithological boundary between these units is observable. Petrographic observations suggest a continuous eoalpine metamorphic gradient with no metamorphic gap between the KW-NS and the EPZ. In order to clear this debate and further constrain the tectonic and temporal evolution of these units, we present new LA-MC-ICP-MS U/Pb age dating results for metapelite samples from the EPZ as well as for the adjacent units of the KW-NS.Two samples (EA09 and SP02) from the central EPZ and one sample (SP62) from the northernmost part of the Wölz-Complex of the KW-NS were selected for detrital zircon age dating. The distribution of approximately 150 dates per sample reveals major peaks at the Ediacaran-Cryogenian boundary (624 – 646 Ma), a smaller peak at the Neoproterozoic-Mesoproterozoic boundary (~1000 Ma) followed by a hiatus and a smaller peak in the mid-Paleoproterozoic (~2000 Ma). All samples show similar mid-Paleoproterozoic and Neoproterozoic-Mesoproterozoic peaks. Sample SP62 contains one grain of Cambrian age (523 Ma) and one grain of mid-Ordovician age (460 Ma) whereas the youngest zircons from the EPZ samples yield Ediacaran ages of 629 Ma and 625 Ma. The lack of zircons of Ordovician age in samples EA09 and SP02 indicate an affiliation of the EPZ with the basal units of the “Greywacke Zone”.We also dated metamorphic allanite and REE-bearing epidote rims which are interpreted to form at low pressure and temperature conditions in metapelites. Allanites from the EPZ yield metamorphic ages of 105 ± 3.5 Ma in the northern part of the unit and 279 ± 6 Ma in the southern part. Allanite cores from two micaschist samples from the northern and central Wölz-Complex yield ages of 316 ± 21 Ma and 286 ± 11 Ma. Their respective epidote rims yield eoalpine ages of 98 ± 2 Ma and 96 ± 2 Ma. One micaschist sample from the Rappold-Complex yields ages of 326 ± 9 Ma for the allanite cores and 101 ± 1 Ma for the epidote rims. These ages are interpreted as prograde crystallization of allanite and epidote and give us petrochronological information about three distinct metamorphic events: Variscan, Permian and Eoalpine. By gathering three distinct eoalpine ages within the EPZ and the KW-NS, we can further constrain the metamorphic evolution of the eoalpine lower plate.

Large plutons are common within the Inthanon Zone in Northwestern Thailand. These igneous rocks are also known as Central Granitoids Belt in mainland Southeast Asia. They are interpreted to be part of the suture zone between Sibumasu and Indochina and were emplaced mainly in the Upper Triassic.Here, we present new petrological and geochemical data for the Central Granitoids Belt. A geochronological study on selected samples will follow. The sampled granitoids can be separated into three groups: (1) biotite granite, (2) hornblende granite, (3) syenite/monzonite. The samples consist of various light colored to dark grey granitoids due to the type and amount of mafic minerals (biotite or hornblende) present. The general mineral assemblage of all the intrusive igneous rocks is quartz + plagioclase + K-feldspar + biotite + apatite + zircon ± allanite ± titanite ± ilmenite. The biotite granites are mostly composed of biotite aggregates associated with accessory minerals: zircon, ilmenite, and apatite. The syenite/monzonite group usually contains additional clinopyroxene and hornblende. Plagioclase and hornblende of the syenite/monzonite group commonly exhibit a sieve texture.The biotite granite group is typically peraluminous and belongs to the high-K calk-alkaline to shoshonitic series. The hornblende granite group is mostly peraluminous and of predominantly shoshonitic affinity. The syenite/monzonites are typically metaluminous but also belong to the shoshonitic series. The chondrite normalized rare earth element (REE) patterns are quite similar for all igneous rocks with elevated LREE, pronounced negative Eu anomaly and a flat HREE segment. The granite tectonic discrimination plots after Pearce et al. (1984) classify most samples as syn-collision granites (syn-COLG) and when using the Batchelor and Bowden (1985) discrimination diagram as syn-, late, and post-collisional.The intrusive igneous rocks from Northwestern Thailand were presumably emplaced in a syn- to post-collisional setting when the Sibumasu block collided with the Sukhothai terrane and was eventually amalgamated to the Indochina block. This led to the closure of the Palaeotethys along the eastern area of the Sibumasu block.Batchelor, R.A. and Bowden, P. (1985) Petrogenetic Interpretation of Granitoid Rock Series Using Multicationic Parameters. Chemical Geology, 48, 43-55.Julian A Pearce, Nigel BW Harris, Andrew G Tindle (1984). Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology, 25, 956-983.

The alteration of magmatic monazite and its consequences for monazite geochronology are explored in granitoids from the western part of the Ryoke belt (Iwakuni-Yanai area, SW Japan). Biotite-granite samples were collected in two plutons emplaced slightly before the main tectono-metamorphic event: the first one, a massive granite (Shimokuhara) adjoins schistose rocks affected by greenschist facies metamorphism; and the second, a gneissose granite (Namera) adjoins migmatitic gneiss that experienced upper-amphibolite facies conditions. Despite contrasting textures, the granite samples have similar mineral modes and compositions. Monazite in the massive granite is dominated by primary domains with limited secondary recrystallization along cracks and veinlets. It is variably replaced by allanite+apatite±xenotime±Th-U-rich phases. The outermost rims of primary domains yield a weighted average 206Pb/238U date of 102 ± 2 Ma while the Th-U phases show Th-U-Pb dates of 58 ± 5 and 15 to 14 ± 2–3 Ma. Monazite in the gneissose granite preserves sector- or oscillatory-zoned primary domains cross-cut by secondary domains enriched in Ca, Y, U, P, and containing numerous inclusions. The secondary domains preserve concordant 206Pb/238U dates spreading from 102 ± 3 to 91 ± 2 Ma while primary domain analyses are commonly discordant and range from 116 to 101 Ma.Monazite alteration textures in the two granites chiefly reflect differences in their post-magmatic histories. In the massive granite, monazite replacement occurred via a nearly stoichiometrically balanced reaction reflecting interaction with an aqueous fluid enriched in Ca+Al+Si±F during hydrothermal alteration of the granitic assemblage, likely below 500 °C. In the gneissose granite, a small amount of anatectic melt, probably derived from the neighboring metasedimentary rocks, was responsible for pseudomorphic recrystallization of monazite by dissolution-reprecipitation above 600 °C. Regardless of whether monazite underwent replacement or recrystallization, primary monazite domains preserve the age of magmatic crystallization for both plutons (102 ± 2 and 106 ± 5 Ma). Conversely, the age of monazite alteration is not easily resolved. Monazite replacement in the massive granite might be constrained using the Th-U-rich alteration products; with due caution and despite probable radiogenic Pb loss, the oldest date of 58 ± 5 Ma could be ascribed to chloritization during final exhumation of the granite. The spread in apparently concordant 206Pb/238U dates for secondary domains in the gneissose granite is attributed to incomplete isotopic resetting during dissolution-reprecipitation, and the youngest date of 91 ± 2 Ma is considered as the age of monazite recrystallization during a suprasolidus metamorphic event. These results reveal a diachronous, ca. 10 Ma-long high-temperature (HT) history and an overall duration of about 15 Ma for the metamorphic evolution of the western part of the Ryoke belt.

The Ryoke plutono-metamorphic belt exposed in SW Japan is the type locality for low-Pressure/high-Temperature (LP/HT) metamorphism. The Ryoke metamorphic field gradient is, however, a complex object shaped by several deformation phases, multiple magmatic pulses and protracted metamorphism. In the western part of the Ryoke belt (Iwakuni-Yanai area), a petrological and geochronological study of two plutons emplaced before metamorphism is used to explore the behaviour of magmatic monazite along the LP/HT gradient and constrain the thermal history of the belt. We compare a massive granite adjoining schistose rocks affected by greenschist facies metamorphism with a gneissose granite adjoining migmatitic gneiss affected by upper-amphibolite facies conditions. Despite contrasting textures, the granite samples have similar mineral modes and compositions. Monazite in the massive granite is dominated by primary domains with limited secondary recrystallization, and is variably replaced by allanite+apatite±xenotime±Th−U-rich phases. Primary domains yield an average 206Pb/238U date of 102 ± 2 Ma while Th−U phases show Th−U−Pb dates of ca. 58 and 15−14 Ma. Monazite in the gneissose granite preserves sector- or oscillatory-zoned primary domains cross-cut by inclusion-rich secondary domains enriched in Ca, Y, U, P. Primary domain analyses are commonly discordant (116−101 Ma) while secondary domains preserve concordant 206Pb/238U dates spreading from 102 ± 3 to 91 ± 2 Ma.Despite alteration, primary monazite domains preserve the age of magmatic crystallization for both plutons (102 ± 2 Ma and 106 ± 5 Ma). In the massive granite, monazite replacement is ascribed to the influx of aqueous fluid enriched in Ca+Al+Si±F during hydrothermal alteration below 500 °C. The oldest date (58 ± 5 Ma) obtained from the Th−U-rich alteration products is regarded as a minimum age for chloritization during final exhumation of the granite. In the gneissose granite a small amount of anatectic melt was responsible for a pseudomorphic recrystallization of monazite by dissolution-reprecipitation above 600 °C. The spread in 206Pb/238U dates for the secondary domains is attributed to incomplete isotopic resetting during dissolution-reprecipitation, and the youngest date of 91 ± 2 Ma is considered as the age of monazite recrystallization during a suprasolidus metamorphic event. These results reveal a diachronous, ca. 10 Ma-long HT history and an overall duration of about 15 Ma for the metamorphic evolution of the western part of the Ryoke belt.

The spatiotemporal relationship between granitoid intrusions and low-pressure/temperature type regional metamorphism in the Ryoke belt (Mikawa area) is investigated to understand the tectono-thermal evolution of the upper- to middle-crust during a Cretaceous flare-up event at the Eurasian active continental margin. Three plutono-metamorphic stages are recognized; (1) 99–84 Ma: intrusion of granitoids (99–95 Ma pulse) into the upper crust and high-T regional metamorphism reaching sillimanite-grade (97.0 ± 4.4 Ma to 88.5 ± 2.5 Ma) in the middle crust, (2) 81–75 Ma: intrusion of gneissose granitoids (81–75 Ma Ma pulse) into the middle crust at ~19–24 km depth, and (3) 75–69 Ma: voluminous intrusions of massive to weakly-foliated granitoids (75–69 Ma pulse) at ~9–13 km depth and formation of contact metamorphic aureoles. Cooling of the highest-grade metamorphic zone below the wet solidus of granitic rocks is estimated at 88.5 ± 2.5 Ma. At ca. 75 Ma, the upper-middle crustal section underwent northward tilting, resulting in the exhumation of regional metamorphic zones to ~9–13 km depth. Although the highest-grade metamorphic rocks and the 99–95 Ma pulse granitoids preserve similar U–Pb zircon ages, the absence of spatial association suggests that the regional metamorphic zones were mainly produced by a transient thermal anomaly in the mantle and thermal conduction through the crust, supplemented by localized advection due to granitoid intrusions. The successive emplacement of granitoids into shallow, deep and shallow levels of the crust was probably controlled by the combination of change in thermal structure of the crust and tectonics during granitoid intrusions.

International audience; Magma transfer between lower and upper continental crust is a fundamental process linking the dominantly mafic composition of the lower crust with the more felsic composition of the upper crust. We explore the mechanisms of mafic magma ascent and emplacement in the middle crust by studying a mid-crustal gabbroic to dioritic magmatic system (Sondalo complex, Eastern Central Alps, N Italy). We characterize the structure and Anisotropy of Magnetic Susceptibility (AMS) fabric of concentric gabbroic to dioritic intrusions. The significance of AMS fabrics is discussed using Anisotropy of Anhysteretic Remanent Magnetization (AARM) and Crystallographic Preferred Orientation (CPO) data acquired on different test sites. The magmatic and magnetic fabrics of the pluton were acquired during its emplacement in the Permian and were not subsequently tilted: the fabrics are essentially vertical, indicating vertical magma transfer through the crust with a two-phase intrusion history. (1) The concordant orientation between the magmatic foliation and the host-rock xenoliths in the center of the pluton suggest that the first magma ascent phase occurred along pathways subparallel to the vertical fabric of the host metasedimentary rocks. (2) The second magma ascent phase was controlled by a change in the rheology of the host-rock and the mafic magma. Heat dissipation to the contact aureole induced partial melting, thereby lowering the mechanical strength of the host-rocks, whereas the viscosity of the mafic magma increased due to cooling and associated fractional crystallization. This caused an en-masse rise of the pluton resulting in the formation of a structural aureole, i.e. a vertical foliation in the contact aureole and a weaker but concordant magmatic foliation at the rim of the pluton. This ascent phase accounts for the P−T evolution recorded by metasedimentary rocks in the contact aureole of the pluton.

Utilizing microstructures of Cl-bearing biotite in pelitic and felsic metamorphic rocks, the timing of Cl-rich fluid infiltration is correlated with the pressure-temperature-time (P-T-t) path of upper amphibolite- to granulite-facies metamorphic rocks from Perlebandet, Sor Rondane Mountains (SRM), East Antarctica. Microstructural observation indicates that the stable Al2SiO5 polymorph changed from sillimanite to kyanite + andalusite + sillimanite, and P-T estimates from geothermobarometry point to a counterclockwise P-T path characteristic of the SW terrane of the SRM. In situ laser ablation inductively coupled plasma mass spectrometry for U–Pb dating of zircon inclusions in garnet yielded ca. 580 Ma, likely representing the age of garnet-forming metamorphism at Perlebandet. Inclusion-host relationships among garnet, sillimanite, and Cl-rich biotite (Cl > 0.4 wt%) reveal that formation of Cl-rich biotite took place during prograde metamorphism in the sillimanite stability field. This process probably predated partial melting consuming biotite (Cl = 0.1–0.3 wt%). This was followed by retrograde, moderately Cl-bearing biotite (Cl = 0.1–0.3 wt%) replacing garnet. Similar timings of Cl-rich biotite formation in different samples, and similar f(H2O)/f(HCl) values of coexisting fluid estimated for each stage can be best explained by prograde Cl-rich fluid infiltration. Fluid-present partial melting at the onset of prograde metamorphism probably contributed to elevate the Cl concentration (and possibly salinity) of the fluid, and consumption of the fluid resulted in the progress of dehydration melting. The retrograde fluid was released from crystallizing Cl-bearing partial melts or derived externally. The prograde Cl-rich fluid infiltration in Perlebandet presumably took place at the uppermost part of the footwall of the collision boundary. Localized distribution of Cl-rich biotite and hornblende along large-scale shear zones and detachments in the SRM supports external input of Cl-rich fluids through tectonic boundaries during continental collision.

We revisit the origin of the metamorphic field gradient exposed in the Ryoke Belt (SW Japan) — the type locality for low pressure/high temperature (L P /H T ) conditions — based on combined structural, petrological and geochronological data from the Iwakuni–Yanai area (W Ryoke). In this area metasedimentary rocks pass from schist in the north to partly migmatitic paragneiss in the south, and are surrounded by numerous granitoid bodies. Three deformation phases are emphasized; an accretionary-stage or burial deformation of weak intensity (D0), a vertical shortening (D1) which generated a subhorizontal, locally extensional foliation in the southern gneiss zone, and E–W upright folding (D2) which reworked both the northern schist and southern gneiss zones. Crystallization–deformation relationships indicate that, in the schist zone, most andalusite, cordierite and mica porphyroblasts formed after D0 and before D1. Syn-D1 mineral assemblages define E–W trending metamorphic zones with increasing grade from north to south, and rare syn-D2 parageneses reveal a similar pattern. Continuous magmatic activity lasted from ca. 105 to 94 Ma. The oldest granitoids (Shimokuhara, Soo, Namera), found in the western part of the belt, were emplaced below schistose rocks at 105–100 Ma. Subsequently, syn- to post-D1 granodiorite (Gamano) intruded concordantly with the foliation of southern, high-grade gneissic rocks from 100 to 94 Ma. A large, syn-D1 pluton (Kibe) intruded the center of the belt at 98 Ma, whereas a younger granite (Iwakuni) and its satellite dykes were emplaced to the northeast at 96–94 Ma. Constraints on the timing of D1 (103–99 Ma) and the likely diachronous D2 phase (100–94 Ma) suggest a deformation continuum during magma intrusion and metamorphism. We conclude that the exposed metamorphic field gradient is an apparent, but fortuitous, alignment of P – T data resulting from spatially and temporally distinct thermal events. Peak temperature conditions in the relatively shallow schist zone were attained during a first, pre-tectonic, contact metamorphic event. A second, regional metamorphic overprint arose from the syn-tectonic intrusion of granitoids in the deeper gneiss zone. A younger contact metamorphic event locally affected the eastern part of the schist zone. The revised tectono-thermal history highlights the composite and diachronous character of the Ryoke metamorphic field gradient.

International audience; The end of an orogenic cycle is commonly associated with a general extensional regime, widespread magmatism and complex metamorphic overprints. The Austroalpine domain in SE Switzerland and N Italy preserves a polyphase tectonic history spanning the Carboniferous Variscan orogeny, the Late Carboniferous-Early Permian post-Variscan extension and the Jurassic rifting. In this study, the late-and post-Variscan evolution of the Austroalpine domain is explored by constraining the timing of intrusion of the Sondalo gabbroic complex in the Campo unit. U-Pb zircon dating on magmatic and metamorphic rocks and trace element geochemistry in zircon are performed using laser ablation coupled with mass spectrometry. The ages of both magmatic and metamorphic rocks range between 289 ± 4 and 285 ± 6 Ma. Trace elements in zircon from magmatic rocks indicate that zircon grew in two different chemical systems: an initial one devoid of garnet and with already crystallized plagioclase, and a second one with an increasing modal amount of garnet and lacking (initial) plagioclase. The second system probably reflects mixing of the initial magma Manuscript submitted to International Journal of Earth Sciences 2 with melt derived from the surrounding partially molten metapelites. New results, existing age data and P-T estimates allow to describe the late-to post-Variscan evolution of the Austroalpine domain. These data are compared and integrated in the framework of the Permian geodynamic setting in Western Europe. We present new arguments to consider the "Permian event" in Europe as a process which was temporally distinct from the collapse of the Variscan orogen. Late-Variscan re-equilibration of the crustal thickness occurred between 310 and 290 Ma (i.e. during the collapse phase). Permian mafic magmas emplaced from 290 to 270 Ma formed either during or even after the latest stages of the Variscan collapse.

We use textural and geochronological arguments to constrain monazite petrogenesis and the age of metamorphic events in medium-grade metasedimentary rocks from the European Variscan Belt (Orlica-Śnieznik Dome, Czech Republic/Poland). The relationships between monazite, allanite and other major and accessory phases are characterized for staurolite-, sillimanite- and kyanite-grade, garnet-bearing mica schist samples. Garnet porphyroblasts with prograde compositional zoning include allanite in the core and monazite at the rim. Allanite is virtually absent from the matrix; it shows replacement textures involving a cluster of Th ± U ± Y-rich silicates or phosphates. Monazite, with frequent micro-cracks or trimmed grain boundaries, abounds in the mica-rich matrix; it occurs as lone grains close to partially resorbed garnet, apatite or staurolite, and as subhedral and compositionally zoned aggregates associated with allanite and xenotime. Electron microprobe analyses of monazite define Pb vs. Th* isochrons (ages = 363–343 Ma) with positive intercepts (Pb 0 = 40–120 ppm) which suggest the presence of initial Pb and age overestimation. Isotopic analyses obtained by laser ablation coupled with mass spectrometry confirm the minor presence of initial Pb which leads to variable discordance in the 206 Pb/ 238 U– 207 Pb/ 235 U system. Conversely, the 206 Pb/ 238 U– 208 Pb/ 232 Th system is not visibly affected; analyses are concordant and 208 Pb/ 232 Th ages range from 358 to 299 Ma (± 7–8 Ma) regardless of monazite texture or composition. They define a main group at 330–310 Ma, with few older outliers. All observations suggest that a first monazite generation (MnzI) formed via allanite breakdown at prograde to peak P – T conditions (5–7 kbar/575–640 °C). A comparison with previous geochronological results reveals that the age of this prograde reaction is, however, preserved by only few domains yielding 360–340 Ma. Instead, our results emphasize the importance of retrograde processes which built the dominant age population of 330–310 Ma. The latter ages are ascribed to a combination of (1) transient monazite growth after allanite and controlled by P availability, (2) variable recrystallization/replacement of older monazite grains, and (3) minor monazite neoformation due to the resorption of garnet or apatite. The young monazite grains or domains (MnzII) mostly formed during fluid influx and chloritization; they constrain the end of the orogenic history when mid-crustal rocks were exhumed and re-hydrated at temperatures below ~ 300 °C.

A section of the orogenic middle crust (Orlica-nienik Dome, Polish/Czech Central Sudetes) was examined to constrain the duration and significance of deformation (D) and intertectonic (I) phases. In the studied metasedimentary synform, three deformation events produced an initial subhorizontal foliation S1 (D-1), a subsequent subvertical foliation S2 (D-2) and a late subhorizontal axial planar cleavage S3 (D-3). The synform was intruded by pre-, syn- and post-D-2 granitoid sheets. Crystallization-deformation relationships in mica schist samples document I1-2 garnet-staurolite growth, syn-D-2 staurolite breakdown to garnet-biotite-sillimanite/andalusite, I2-3 cordierite blastesis and late-D-3 chlorite growth. Garnet porphyroblasts show a linear Mn-Ca decrease from the core to the inner rim, a zone of alternating Ca-Y- and P-rich annuli in the inner rim, and a Ca-poor outer rim. The Ca-Y-rich annuli probably reflect the occurrence of the allanite-to-monazite transition at conditions of the staurolite isograd, whereas the Ca-poor outer rim is ascribed to staurolite demise. The reconstructed P-T path, obtained by modelling the stability of parageneses and garnet zoning, documents near-isobaric heating from similar to 4kbar/485 degrees C to similar to 4.75kbar/575 degrees C during I1-2. This was followed by a progression to 4-5kbar/580-625 degrees C and a subsequent pressure decrease to 3-4 kbar during D-2. Pressure decrease below 3kbar is ascribed to I2-3, whereas cooling below similar to 500 degrees C occurred during D-3. In the dated mica schist sample, garnet rims show strong Lu enrichment, oscillatory Lu zoning and a slight Ca increase. These features are also related to allanite breakdown coeval with staurolite appearance. As Lu-rich garnet rims dominate the Lu-Hf budget, the 344 +/- 3Ma isochron age is ascribed to garnet crystallization at staurolite grade, near the end of I1-2. For the dated sample of amphibole-biotite granitoid sheet, a Pb-Pb single zircon evaporation age of 353 +/- 1Ma is related to the onset of plutonic activity. The results suggest a possible Devonian age for D-1, and a Carboniferous burial-exhumation cycle in mid-crustal rocks that is broadly coeval with the exhumation of neighbouring HP rocks during D-2. In the light of published ages, a succession of telescoping stages with time spans decreasing from c. 10 to 2-3Ma is proposed. The initially long period of tectonic quiescence (I1-2 phase, c. 10Ma) inferred in the middle crust contrasts with contemporaneous deformation at deeper levels and points to decoupled P-T-D histories within the orogenic wedge. An elevated gradient of similar to 30 degrees Ckm(-1) and assumed high heating rates of c. 20 degrees CMa(-1) are explained by the protracted intrusion of granitoid sheets, with or without deformation, whereas fast vertical movements (2-3Ma, D-2 phase) in the crust require the activity of deformation phases.

第7回極域科学シンポジウム:[OG] 地圏11月29日（火）国立極地研究所 1階 交流アトリウム

A geological synthesis of the Palaeozoic Vosges Mountains (NE France) is presented using existing observations and new data. The geodynamic evolution involves: (1) Early Palaeozoic sedimentation and magmatism; (2) Late Devonian subduction triggering back-arc spreading; (3) early Lower Carboniferous continental subduction, continent–continent collision and polyphase deformation and metamorphism of the orogenic root; and (4) late Lower Carboniferous orogenic collapse driven by thermal weakening of the middle crust. The evolution is integrated within the framework of the European Variscan Belt. The Northern Vosges comprise sediments of Rhenohercynian affinity separated from Tepla-Barrandian metasediments by a Lower Carboniferous magmatic arc. The latter is correlated with the Mid-German Crystalline Rise, and is ascribed to the south-directed subduction of the Rhenohercynian Basin. The Saxothuringian–Moldanubian suture is thought to be obliterated by the magmatic arc, while the Lalaye–Lubine Fault is interpreted as the Tepla-Barrandian–Moldanubian boundary. The Central Vosges are paralleled with the Moldanubian domain of the Bohemian Massif where identical lithologies record the Devonian–Carboniferous SE-directed subduction of the Saxothuringian passive margin below the Moldanubian upper plate. The Southern Vosges represent the upper Moldanubian crust and are linked to the southern Black Forest. The presence of an oceanic domain to the south of the Vosges–Black Forest remains unclear.

Eclogite, felsic orthogneiss and garnet–staurolite metapelite occur in a 5 km long profile in the area of Miedzygorze in the Orlica–Śnieznik dome (Bohemian Massif). Petrographic observations and mineral equilibria modelling, in the context of detailed structural work, are used to document the close juxtaposition of high-pressure and medium-pressure rocks. The structural succession in all lithologies shows an early shallow-dipping fabric, S1, that is folded by upright folds and overprinted by a heterogeneously developed subvertical foliation, S2. Late recumbent folds associated with a weak shallow-dipping axial-plane cleavage, S3, occur locally. The S1 fabric in the eclogite is defined by alternation of garnet-rich (grs = 22–29 mol.%) and omphacite-rich (jd = 33–36 mol.%) layers with oriented muscovite (Si = 3.26–3.31 p.f.u.) and accessory kyanite, zoisite, rutile and quartz, indicating conditions of ∼19–22 kbar and ∼700–750 °C. The assemblage in the retrograde S2 fabric is formed by amphibole, plagioclase, biotite and relict rutile surrounded by ilmenite and sphene that is compatible with decompression and cooling from ∼9 kbar and ∼730 °C to 5–6 kbar and 600–650 °C. The S3 fabric contains in addition domains with albite, chlorite, K-feldspar and magnetite indicating cooling to greenschist facies conditions. The metapelites are composed of garnet, staurolite, muscovite, biotite, quartz, ilmenite and chlorite. Chemical zoning of garnet cores that contain straight ilmenite and staurolite inclusion trails oriented perpendicular to the external S2 fabric indicates prograde growth, from ∼5 kbar and ∼520 °C to ∼7 kbar and ∼610 °C, during the formation of the S1 fabric. Inclusion trails parallel with the S2 fabric at garnet and staurolite rims are interpreted to be a continuation of the prograde path to ∼7.5 and ∼630 °C in the S2 fabric. Matrix chlorite parallel to the S2 foliation indicates that the subvertical fabric was still active below 550 °C. The axial planar S2 fabrics developed during upright folding are associated with retrogression of the eclogite under amphibolite facies conditions, and with prograde evolution in the metapelites, associated with their juxtaposition. The shared part of the eclogite and metapelite P–T paths during the development of the subvertical fabric reflects their exhumation together.

In a Barrovian metamorphic sequence, garnetiferous mica schists document a heterogeneously developed superposition of sub-orthogonal fabrics and multiple garnet growth episodes. In the variably deformed domains, four types of garnet porphyroblasts have been defined based on inclusion trail patterns. Modelled garnet zoning in the MnNCKFMASHTO system indicates a prograde evolution from 4-4.5 kbar and 490-510 � C to 5-6 kbar and 520-550 � C in the earliest subhorizontal fabric progressing towards 6.5-7.5 kbar and 560-590 � C in the subsequent subvertical foliation. This fabric is heterogeneously deformed into a shallow-dipping retrograde foliation associated with garnet resorption. In situ electron backscatter diffraction measurements of ilmenite inclusions in individual garnet grains yield precise data on included planar and linear elements. Consistent orientations of internal foliations, lineations and foliation intersection axis sets indicate a superposition of three sub-orthogonal foliation systems. Weak variations of internal records with increasing intensity of deformation suggest that a moderate buckling stage occurred, but apparent lack of porphyroblast rotation is interpreted as a result of dominant passive flow. Coupling the orientation of internal fabric sets with P-T estimates is used to complement the tectono-metamorphic evolution of the thickened crust. We demonstrate that garnet porphyroblasts preserve features which reflect large-scale tectonic processes in orogens.

In the Orlica-Snieznik Dome (NE Bohemian massif), alternating belts of orthogneiss with high-pressure rocks and belts of mid-crustal metasedimentary-metavolcanic rocks commonly display a dominant subvertical fabric deformed into a subhorizontal foliation. The first macroscopic foliation is subvertical, strikes NE-SW and is heterogeneously folded by open to isoclinal folds with subhorizontal axial planes parallel to the heterogeneously developed flat-lying foliation. The metamorphic evolution of the mid-crustal metasedimentary rocks involved successive crystallization of chlorite-muscovite-ilmenite-plagioclase-garnet, followed by staurolite-bearing and then kyanite-bearing assemblages in the subvertical fabric. This was followed by garnet retrogression, with syntectonic crystallization of sillimanite and andalusite parallel to the shallow-dipping foliation. Elsewhere, andalusite and cordierite statically overgrew the flat-lying fabric. With reference to a P-T pseudosection for a representative sample, the prograde succession of mineral assemblages and the garnet zoning pattern with decreasing grossular, spessartine and X-Fe are compatible with a P-T path from 3.5-5 kbar/490-520 degrees C to peak conditions of 6-7 kbar/similar to 630 degrees C suggesting burial from 12 to 25 km with increasing temperature. Using the same pseudosection, the retrograde succession of minerals shows decompression to sillimanite stability at similar to 4 kbar/similar to 630 degrees C and to andalusite-cordierite stability at 2-3 kbar indicating exhumation from 25 km to around 9-12 km. Subsequent exhumation to similar to 6 km occurred without apparent formation of a deformation fabric. The structure and petrology together with the spatial distribution of the metasedimentary-metavolcanic rocks, and gneissic and high-pressure belts are compatible with a model of burial of limited parts of the upper and middle crust in narrow cusp-like synclines, synchronous with the exhumation of orogenic lower crust represented by the gneissic and high-pressure rocks in lobe-shaped and volumetrically more important anticlines. Converging P-T-D paths for the metasedimentary rocks and the adjacent high-pressure rocks are due to vertical exchanges between cold and hot vertically moving masses. Finally, the retrograde shallow-dipping fabric affects both the metasedimentary-metavolcanic rocks and the gneissic and high-pressure rocks, and indicates that the similar to 15-km exhumation was mostly accommodated by heterogeneous ductile thinning associated with unroofing of a buoyant crustal root.

Plutonic bodies of the Central and Southern Vosges Mts can be assigned to two major early Carboniferous magmatic events: a Visean Mg–K event ( c. 345 and 340–336 Ma) and a younger S-type event (329–322 Ma). New petrological, geochemical and Sr–Nd isotopic data highlight the existence of two groups of Mg–K intrusions that might be related to the nature of their primary magma sources; that is, CHUR-like and enriched mantle, which interacted with juvenile and mature crustal material, respectively. The differences between these two groups are explained by a geodynamic scenario involving deep subduction and relamination of the Saxothuringian continental crust under the Moldanubian continent. The relaminated radiogenic Saxothuringian material is thought to have been responsible for dehydration melting of both subducted crust and underlying metasomatized mantle, thereby generating the Mg–K magma subsequently emplaced at middle crustal depth. During their ascent, the mafic magmas interacted with crustally derived felsic melts. Significantly later ( c . 10–15 myr) a widespread mid-crustal anatexis occurred, generating voluminous granite intrusions from mixed crustal sources (paragneisses and/or immature felsic–intermediate metaigneous rocks mixed with Mg–K plutons). The principal heat source for such a major melting event is related to the presence of Mg–K plutons rich in heat-producing elements, which were responsible, after the time lag specified, for a temperature increase at mid-crustal levels by in situ radiogenic heat production. The current study underlines the importance of deep continental crust subduction and relamination for the magmatism and development of collisional orogens. Supplementary material: Analytical methods and data, and supplementary figures, are available at http://www.geolsoc.org.uk/SUP18795.

The relationships between the intrusion of gneissose granitoids and the attainment of regional high-T conditions recorded in metamorphic rocks from the Ryoke belt of the Mikawa area, central Japan, are explored. Seven gneissose granitoid samples (tonalite, granodiorite, granite) were collected from three distinct plutonic bodies that are mapped as the so-called “Older Ryoke granitoids.” Based on bulk-rock compositions and U–Pb zircon ages obtained by laser ablation inductively coupled plasma mass spectrometry, the analyzed granitoids can be separated into two groups. Gneissose granitoids from the northern part of the area give weighted mean 206Pb/238U ages of 99 ±1 Ma (two samples) and 95 ±1 Ma (one sample), whereas those from the southern part yield 81 ±1 Ma (two samples) and 78–77 ±1 Ma (two samples). Regional comparisons allow correlation of the northern granitoids (99–95 Ma) with the Kiyosaki granodiorite, and mostly with the Kamihara tonalite found to the east. The southern granitoids are tentatively renamed as “78–75 Ma (Hbl)−Bt granite” and “81–75 Ma Hbl−Bt tonalite” (Hbl, hornblende; Bt, biotite). and seem to be broadly coeval members of the same magmatic suite. With respect to available age data, no gneissose granitoid from the Mikawa area shows a U–Pb zircon age which matches that of high-T metamorphism (ca 87 Ma). The southern gneissose granitoids (81–75 Ma), although they occur in the highest-grade metamorphic zone, do not seem to represent the heat source which produced the metamorphic field gradient with a low dP/dT slope.

International audience; We combine structural observations, petrological data and 40Ar-39Ar ages for a stack of amphibolite-facies metasedimentary units that rims high-pressure (HP) granulite-facies felsic bodies exposed in the southern Bohemian Massif. The partly migmatitic Varied and Monotonous units, and the underlying Kaplice unit show a continuity of structures that are also observed in the adjacent Blanský les HP granulite body. They all exhibit an earlier NE−SW striking and steeply NW-dipping foliation (S3) which is transposed into a moderately NW-dipping foliation (S4). In both the Varied and Monotonous units, the S3 and S4 foliations are characterized by a Sil-Bt-Pl-Kfs-Qtz-Ilm±Grt assemblage, with occurrences of post-D4 andalusite, cordierite and muscovite. In the Monotonous unit, minute inclusions of garnet, kyanite, sillimanite and biotite are additionally found in plagioclase from a probable leucosome parallel to S3. The Kaplice unit shows rare staurolite and kyanite relicts, a Sil-Ms-Bt-Pl-Qtz±Grt assemblage associated with S3, retrogressed garnet−staurolite aggregates during the development of S4, and post-D4 andalusite, cordierite and secondary muscovite. Mineral equilibria modelling for representative samples indicates that the Varied unit records conditions higher than ~7 kbar at 725 °C during the transition from S3 to S4, followed by a P−T decrease from ~5.5 kbar/750 °C to ~4.5 kbar/700 °C. The Monotonous unit shows evidence of partial melting in the S3 fabric at P−T above ~8 kbar at 740-830 °C and a subsequent P−T decrease to 4.5-5 kbar/700°C. The Kaplice unit preserves an initial medium-pressure prograde path associated with the development of S3 reaching peak P−T of ~6.5 kbar/640 °C. The subsequent retrograde path records 4.5 kbar/660 °C during the development of S4. 40Ar-39Ar geochronology shows that amphibole and biotite ages cluster at c. 340 Ma close to the HP granulite, whereas adjacent metasedimentary rocks preserve c. 340 Ma amphibole ages, but biotite and muscovite ages range between c. 318 and c. 300 Ma. The P−T conditions associated with S3 imply an overturned section of the orogenic middle crust. The shared structural evolution indicates that all mid-crustal units are involved in the large-scale folding cored by HP granulites. The retrograde P-T paths associated with S4 are interpreted as a result of a ductile thinning of the orogenic crust at a mid-crustal level. The 40Ar-39Ar ages overlap with U-Pb zircon ages in and around the HP granulite bodies, suggesting a short duration for the ductile thinning event. The post-ductile thinning late-orogenic emplacement of the South Bohemian plutonic complex is responsible for a re-heating of the stacked units, reopening of argon system in micas and a tilting of the S4 foliation to its present-day orientation.

International audience; In order to portray the main differences and similarities between the Northeastern Variscansegments (French Massif Central (FMC), Vosges, Black Forest and Bohemian Massif (BM)),we review their crustal-scale architectures, the specific rock associations and lithotectonicsequences, as well as the ages of the main magmatic and metamorphic events. This review demonstratessignificant differences between the ‘Moldanubian’ domains in theBMand the FMC. On thisbasis we propose distinguishing between the Eastern and Western Moldanubian zones, while theVosges/Black Forest Mountains are an intermediate section between the BM and the FMC. Theobserved differences are the result of, first, the presence in the French segment of an earlylarge-scale accretionary system prior to the main Variscan collision and, second, the duration ofSaxothuringian/Armorican subduction, which generated long-lived magmatic arc and back-arcsystems in the Bohemian segment, while the magmatic activity in the FMC was comparablyshort-lived.

International audience; The present work examines the lithological, structural, geochemical and geochronological records from the Klippen Belt located in the southern Vosges Mountains (NE France). The Klippen Belt is represented by discontinuous exposures of serpentinized harzburgite, ophicalcite, gabbro, gneiss and polymictic conglomerate overlain by deep marine pelitic sediments. Structural data and Bouguer anomalies reveal that the Klippen Belt coincides with a significant discontinuity now occupied by a granitic ridge. Gabbro geochemistry indicates a MOR-type affinity similar to recent slow-spreading ridges, but positive Ba, Sr, Th or U anomalies do not exclude the influence of fluids expelled from a subduction zone. A Sm-Nd isochron age of 372 +/- A 18 Ma is thought to reflect gabbro emplacement from a highly depleted mantle source (epsilon(Nd) = +11.3), and U-Pb zircon ages from a gneiss sample indicate that the basement found in the Klippen has a Neoproterozoic origin. Combined data indicate the formation of a deep basin during Late Devonian rifting. The Klippen lithologies could testify for the presence of an ocean-continent transition environment subsequently inverted during the Early Carboniferous. Basin inversion during the Middle Visean was probably controlled by rift-related structures, and resulted in folding of the sedimentary successions as well as exhumation along thrust zones of deep parts of the basin represented by the Klippen Belt. Based on correlations with the neighbouring Variscan massifs, it is proposed that the southern Vosges sequences represent a back-arc basin related to the North-directed subduction of the southern Palaeotethys Ocean. This geodynamic reconstruction is tentatively correlated with similar ophiolitic remnants in the northern part of the French Massif Central (Br,venne) and with the evolution of the southern Black Forest. The Late Devonian ophiolites are interpreted as relicts of small back-arc marginal basins developed during general closure of the Palaeozoic subduction systems.

International audience; U-Pb zircon dating is combined with petrology, Zr-in-rutile thermometry and mineral equilibria modelling to discuss zircon petrogenesis and the age of metamorphism in three units of the Variscan Vosges Mountains (NE France). The monotonous gneiss unit shows results at 700-500 Ma, but no Variscan ages. The varied gneiss unit preserves ages between 600 and 460 Ma and a Variscan group at 340-335 Ma. Zircon analyses from the felsic granulite unit define a continuous array of ages between 500 and 340 Ma. In varied gneiss samples, zoned garnet includes kyanite and rutile and is surrounded by matrix sillimanite and cordierite. In a pseudosection, it points to peak conditions of ~16 kbar/850 °C followed by isothermal decompression to 8-10 kbar/820-860 °C. In felsic granulite samples, the assemblage K-feldspar-garnet-kyanite-Zr-rich rutile is replaced by sillimanite and Zr-poor rutile. Modelling these assemblages supports minimum conditions of ~13 kbar/925 °C, and a subsequent P-T decrease to 6. 5-8. 5 kbar/800-820 °C. The internal structure and chemistry of zircons, and modelling of zircon dissolution/growth along the inferred P-T paths are used to discuss the significance of the U-Pb ages. In the monotonous unit, inherited zircon ages of 700-500 Ma point to sedimentation during the Late Cambrian, while medium-grade metamorphism did not allow the formation of Variscan zircon domains. In both the varied gneiss and felsic granulite units, zircons with a blurred oscillatory-zoned pattern could reflect solid-state recrystallization of older grains during HT metamorphism, whereas zircons with a dark cathodoluminescence pattern are thought to derive from crystallization of an anatectic melt during cooling at middle pressure conditions. The present work proposes that U-Pb zircon ages of ca. 340 Ma probably reflect the end of a widespread HT metamorphic event at middle crustal level.

[1] The contribution of lateral forces, vertical load, gravity redistribution and erosion to the origin of mantled gneiss domes in internal zones of orogens remains debated. In the Orlica-Snieznik dome (Moldanubian zone, European Variscan belt), the polyphase tectono-metamorphic history is initially characterized by the development of subhorizontal fabrics associated with medium- to high-grade metamorphic conditions in different levels of the crust. It reflects the eastward influx of a Saxothuringian-type passive margin sequence below a Tepla-Barrandian upper plate. The ongoing influx of continental crust creates a thick felsic orogenic root with HP rocks and migmatitic orthogneiss. The orogenic wedge is subsequently indented by the eastern Brunia microcontinent producing a multiscale folding of the orogenic infrastructure. The resulting kilometre-scale folding is associated with the variable burial of the middle crust in synforms and the exhumation of the lower crust in antiforms. These localized vertical exchanges of material and heat are coeval with a larger crustal-scale folding of the whole infrastructure generating a general uplift of the dome. It is exemplified by increasing metamorphic conditions and younging of 40Ar/39Ar cooling ages toward the extruded migmatitic subdomes cored by HP rocks. The vertical growth of the dome induces exhumation by pure shear-dominated ductile thinning laterally evolving to non-coaxial detachment faulting, while erosion feeds the surrounding sedimentary basins. Modeling of the Bouguer anomaly grid is compatible with crustal-scale mass transfers between a dense superstructure and a lighter infrastructure. The model implies that the Moldanubian Orlica-Snieznik mantled gneiss dome derives from polyphase recycling of Saxothuringian material.