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Fluid–rock interaction within the altered oceanic crust and across the slab–mantle boundary during subduction facilitates element transfer, but the dynamics of fluid transport and fluid–rock exchange during upward fluid migration are still unclear. A study of metamorphic fluid–rock interaction within a section of subducted oceanic crust was carried out on eclogites and metasediments of the ultra-high-pressure Lago di Cignana Unit (NW Italian Alps). The P–T modeling of a quartzschist shows that garnet grew during the prograde and sporadically during the retrograde path and that phengite mainly records the peak to retrograde conditions. Microscale geochemical analysis of garnets has revealed a systematic evolution of oxygen isotopic composition with garnet major element zonation, with extreme within-sample core–rim variations in δ18O between 18 and 4‰ providing evidence for external fluid influx. Garnet in eclogites and calcschists, as well as garnet cores in quartz-rich lithologies, shows normal compositional zoning, as expected for prograde garnet growth, and a relatively constant oxygen isotopic composition. The outer garnet growth zones within a few metasediments show reverse or discontinuous zoning and progressively lower δ18O. Despite major element zoning, the isotopic composition of mica is homogeneous across chemical zoning in one eclogite and one quartzschist, but shows 6‰ variability in another quartzschist. In the underlying Zermatt–Saas serpentinites, antigorite from nine serpentinite samples shows some variation in δ18O, with average δ18O values for individual samples ranging from 1 to 6‰. These results provide evidence for two main stages of external fluid infiltration: (i) fluids from the dehydration of mafic lithologies entered the sequence at peak conditions around 3 GPa, as indicated by the oxygen composition of intermediate zones of mica and garnet, and (ii) low δ18O fluids from serpentinites infiltrated parts of the sedimentary package during exhumation prior to 1.5 GPa, as recorded by the 4‰ garnet outer rims. Samples recording external fluid infiltration are concentrated in the lower part of the sequence, indicating channelized fluid flow, suggesting focused fluid infiltration due to permeability contrasts between metasedimentary and eclogitic lithologies. Channelized fluid flow in the ultra-high-pressure metasediments of Lago di Cignana has not resulted in systematic decarbonation of the metasediments. [ABSTRACT FROM AUTHOR]

The age and P-T conditions of Variscan high-P(HP) metamorphism in the Palaeozoic basement of the western Alps remain poorly constrained, but is nevertheless crucial to build a consistent tectonic scenario for the southeastern domain of the Variscan Belt. We report here the results of a structural, petrological, thermobarometric, and geochronological investigation of an eclogite-bearing unit exposed in the northeastern part of the Belledonne Massif (France). This unit is mostly composed of metasediments that are locally migmatized and contain decametre-to hectometre-scale lenses of orthogneiss and amphibolites. SIMS U-Pb dating of magmatic zircon cores in two retrogressed ecogites yields ages at 456 ± 4 Ma and 448 ± 6 Ma, which are interpreted to date the emplacement of the magmatic protoliths. The peak pressure stage in the retrogressed eclogites is estimated to be >1.4 GPa at 690-740° C, and was followed by decompression from 1.4 to ca. 1.0 GPa at 700-800° C. By contrast, the investigated migmatitic metasediment does not present any trace of HP metamorphism, but instead preserves prograde evolution from sub-solidus conditions (ca. 0.8-1.1 GPa and 600-700° C) to supra-solidus conditions (1.1-1.4 GPa and 700-780° C). A later stage of retrogression below ca. 0.5-0.8 GPa and 570-610° C is recorded in both lithologies, and is taken to indicate cooling and exhumation to upper crustal levels. Metamorphism was roughly coeval in the retrogressed eclogites and in the migmatitic metasediment. Metamorphic zircon rims yield U-Pb dates scattering between 340 and 310 Ma in both lithologies. In the migmatitic metasediment, a distinct younger age at 306 ± 3 Ma is interpreted to represent late stages of melt crystallization. In the retrogressed eclogites, zircon zoning and chemical composition (Th/U and REE) indicate initial crystallization of the rims during the HP stage followed by protracted growth during decompression to granulitic/amphibolitic conditions. Rutile U-Pb dating in one eclogite sample yields an age of 340 ± 11 Ma similar to the oldest zircon ages and is interpreted to approximate the age of the peak pressure metamorphism. Retrogression in the amphibolite facies is correlated with the development of a penetrative, N30° subvertical mylonitic S2 foliation. Regionally, this deformation occurs in a dextral transpressive corridor interpreted to represent a crustal-scale shear zone active during the mid-late Carboniferous. We therefore suggest that this structure has driven the exhumation of eclogites from the lower crust and their mixing with mid-crustal felsic lithologies devoid of HP assemblages. [ABSTRACT FROM AUTHOR]

Garnet is one of the most robust and ubiquitous minerals that record element zoning during crustal metamorphism. In addition to major elements, zoning in trace elements can provide a wealth of information to document the changing conditions of garnet growth and modification. However, mapping trace elements at low concentrations, over large areas and with high resolution has remained a major challenge. We present a comprehensive investigation of the TE distribution in garnet from three Alpine samples that underwent a complex evolution at different metamorphic conditions. The TE distribution in garnet grains is mapped in 2D in thin section with a novel approach using laser ablation inductively coupled plasma time of flight mass spectrometry (LA-ICP-TOFMS) to achieve a lateral resolution of 5 µm and limits of detection for the heavy rare earth elements (REE) down to 0.2 µg/g. Comparison with major element zoning measured by electron probe microanalysis and trace elements measured by conventional LA-ICPMS spot analysis testifies to the accuracy of the measurements. Garnet in an amphibolite-facies metapelite from Campolungo, Central Alps, that recorded metamorphism to 600 °C preserves Y + REE trace element zoning that closely matches that of Ca. In this sample, there is no notable diffusive modification for trace elements. Y + REE zoning is dominated by Rayleigh fractionation in the core and by the sporadic breakdown of accessory phases producing annuli in the rim of the garnet. A granulite-facies garnet from Malenco, Eastern Central Alps, formed during subsolidus heating, followed by peritectic melting reactions up to temperatures of 800–850 °C. Major and trace element zoning are decoupled indicating diffusional resetting of major elements, whereas trace elements still largely document the growth history. Enrichment of trace elements in the garnet mantle may be related to the consumption of biotite (V, Cr) and the dissolution of zircon (Zr) and monazite (Y + REE) in the melt. Diffusion of Y + HREE at the core–mantle boundary occurred over a length scale of ~ 200 µm. Garnet in an eclogite from the Sesia Zone, Western Alps (P ~ 2 GPa, T ~ 600 °C), displays pronounced fluid-related veinlets, visible in FeO, MgO and MnO, which cross-cut the primary growth zoning. Surprisingly, complex Y + REE and Cr zoning is not affected by the veinlets, indicating that they did not form by a crack-seal mechanism but are rather related to a selective replacement process. The trace element maps provide a detailed insight into the growth and modification of garnet and thus allow assessment of equilibrium versus disequilibrium processes, and assist in determination of P–T conditions, garnet dating, diffusion modelling as well as documenting fluid-induced modifications. [ABSTRACT FROM AUTHOR]

Burial of supracrustal sedimentary material is one of the processes that contribute to the formation of the lower continental crust. We investigate the rate of this process in Permian lower continental crust from the Malenco Unit (eastern Central Alps) by constraining the age of protolith deposition and subsequent high-grade metamorphism of felsic, garnet-rich granulites. Detrital zircon cores, many of which are small (<50 μm), were dated by SIMS and yield concordant dates ranging from 387 ± 21 Ma (2σ) to 2380 ± 16 Ma, with the vast majority of the dates falling in the range 450–1000 Ma. For the seven cores in each sample that gave the youngest concordant dates, replicate analyses were carried out. None of these cores had all measured U-Pb dates agree within error, indicating significant disturbance by the Permian granulite facies metamorphism. A 206Pb/238U age of 424 ± 6 Ma for a core with two overlapping replicate analyses is the best available constraint on the maximum depositional age of the sedimentary protolith. This maximum depositional age also coincides with the first significant population in the probability density plot of detrital core dates, which is a double peak at ∼425 Ma and ∼450 Ma. Zircon detrital cores are separated from metamorphic overgrowths by a small seam of zircon that is riddled with tiny inclusions of quartz, biotite and muscovite. The first generation of metamorphic zircon rims has a 206Pb/238U age of 272.9 ± 2.8 Ma, displays steep heavy rare earth element (HREE) patterns and a moderate negative Eu-anomaly, and returns Ti-in-zircon temperatures of 740–780 °C. The second generation of metamorphic rims has a 206Pb/238U age of 263.8 ± 2.6 Ma, a flat HREE pattern and a pronounced negative Eu-anomaly, and gave Ti-in-zircon temperatures of 780–810 °C. Collectively these observations indicate that the first zircon rim formed on the prograde path at the onset of partial melting where muscovite was present but the rocks contained little garnet and no K-feldspar. Therefore, the metapelites resided at lower amphibolite facies, subsolidus conditions up until the intrusion of a Permian gabbro, which was emplaced shortly after the formation of the first generation of metamorphic zircon and caused heating to granulite facies conditions and widespread partial melting. There is no record of high-pressure metamorphism preceding granulite facies conditions, indicating that felsic lower crust in the Malenco unit was not formed by relamination. The Malenco unit was situated at the north-eastern active margin of Gondwana 420 Ma ago. Subduction-related accretion of sediments at lower crustal levels shortly after their deposition could explain the formation of this felsic lower crust that resided at subsolidus conditions for up to 100 My prior to Permian extension, gabbro intrusion and granulite facies metamorphism. • Detrital and metamorphic zircon ages constrain timescales of lower crust formation. • >100 My between sedimentation and granulite facies metamorphism of metapelites. • Lower crust remained at subsolidus conditions for ∼100 My before granulitisation. • No record of high-pressure metamorphism preceding granulite facies conditions. • Single dates of detrital zircon cores can give erroneous maximum depositional ages. [ABSTRACT FROM AUTHOR]

The timing and dynamics of fluid-induced melting in the typical Barrovian sequence of the Central Alps has been investigated using zircon chronology and trace element composition. Multiple zircon domains in leucosomes and country rocks yield U–Pb ages spanning from ~32 to 22 Ma. The zircon formed during Alpine melting can be distinguished from the inherited and detrital cores on the basis of their age, Th/U (<0.1) and trace element composition. Ti-in-zircon thermometry indicates crystallization temperatures around 620–700°C. Their composition allows discriminating between (1) zircon formation in the presence of early garnet, (2) zircon in equilibrium with abundant L-MREE-rich accessory phases (allanite, titanite and apatite) typical of metatonalites, and (3) zircon formed during melting of metasediments in feldspar-dominated assemblages. The distribution of zircon overgrowths and ages indicate that repeated melting events occurred within a single Barrovian metamorphic cycle at roughly constant temperature; that in the country rocks zircon formation was limited to the initial stages of melting, whereas further melting concentrated in the segregated leucosomes; that melting occurred at different times in samples a few meters apart because of the local rock composition and localized influx of the fluids; and that leucosomes were repeatedly melted when fluids became available. The geochronological data force a revision of the temperature–time path of the migmatite belt in the Central Alps. Protracted melting over 10 My followed the fast exhumation of Alpine eclogites contained within the same region and preceded fast cooling in the order of 100°C/Ma to upper crustal levels. [ABSTRACT FROM AUTHOR]

Exhumed portions of subducted crust provide the only direct observational evidence for investigating the fluid flow within and out of the descending slab. The Sesia Zone consists of accreted continental fragments of polymetamorphic and mono-metamorphic lithologies such as metagranitoids, metasediments and metabasites that experienced pre-Alpine amphibolite to granulite facies metamorphism and blueschist to eclogite facies metamorphism during the Alpine orogenic cycle. Because the granulitic protoliths were inherently anhydrous, the formation of eclogitic assemblages necessarily required addition of water either during the pre- or early-Alpine evolution. Reconstructing the conditions of the hydration stages can shed light on the dynamics and potential drivers of crustal reworking. An approach that combines petrology and geochronology with the study of oxygen isotope and trace element geochemistry has been applied to different lithologies from the Eclogitic Micaschist Complex (EMC). Garnet, which mainly forms by dehydration reaction, shows a variety of textures with major and trace element compositional zoning not always reconcilable with simple growth zoning, but also indicating resorption and replacement processes driven by fluid influx (Giuntoli et al., 2018). However, the origin of the fluid and the degree of fluid-rock interaction remain largely unknown.Garnets in metasediments show a notable decrease in δ18O values of up to 5.5 ‰ from the pre-Alpine cores to the Alpine rims. Chronology and trace element patterns of zircon and allanite as well as trace elements in garnet corroborate Permian high-grade metamorphism overprinted by Alpine high-pressure metamorphism, and hint to Triassic reworking at low-grade metamorphic conditions. Texturally, subsequent pulses of externally derived fluids are related to fracturing and resorption of pre-Alpine garnet, new growth of high-pressure garnet and late atoll garnet formation with pervasive growth of phengite, with a distinct δ18O signature. On the other hand, garnets in metabasites have a typical growth zoning, locally affected by late replacement textures that show no significant isotopic shift in δ18O composition and are therefore ascribed to internally buffered fluids. Petrological and isotopic modelling is used to investigate two possible scenarios: (1) the interaction with sea-water during Triassic rifting or (2) the pervasive input of fluids derived from dehydration of mafic and ultramafic rocks during Alpine subduction. Giuntoli, F., Lanari, P., & Engi, M. (2018). Fracturing, dissolution-precipitation, and diffusion processes recorded by garnet textures of the central Sesia Zone (western Italian Alps). Solid Earth, 9(1), 167-167. [ABSTRACT FROM AUTHOR]

Aqueous fluids play a critical role during metamorphic processes in the Earth's crust because they have first-order influence on element transport, reaction kinetics and heat transfer. Understanding their fluxes, sources and interaction with minerals is fundamental for the comprehension of these processes. How, when and where does fluid-mineral interaction take place in metamorphic rocks are basic issues that remain poorly constrained, partly because the fluids eventually escape the rock system leaving a concealed, often invisible path behind.As fluid interaction can occur during events separated in time and under different physicochemical conditions, micro sampling of distinct minerals and mineral zones is best suited to resolve stages of fluid-rock interaction. I will present an approach that combines in situ oxygen isotope measurements in key minerals, with textural analysis, chemical zoning and chronology to address key questions concerning sources, pathways, and composition of fluids in the crust. Are fluids moving in channels or can they penetrate pervasively at depth? How do fluids released from sediments, basalts and serpentinites interact with other lithologies? In subducted crust, can we distinguish metasomatism inherited from the oceanic stage from interactions with subduction zone fluids? Using case studies from the Alps and Turkey, I will show that the nature of the protolith and ocean floor alteration is preserved in relict accessory phases within eclogites that have been fully modified at high-pressure conditions. High-pressure shear zones and lithological boundaries show evidence of intense fluid metasomatism at depth, but significant fluid exchanges are not limited to such discontinuities. Pervasive fluid exchange between mafic, ultramafic and metasedimentary rocks can be recognised during prograde subduction metamorphism. In most cases analyses of distinct mineral zones enable identification of multiple pulses of fluids during the rock evolution. [ABSTRACT FROM AUTHOR]

The Oisans–Pelvoux massif belongs to the Paleozoic basement of the External western Alps and records high temperature (HT) metamorphism associated with intense migmatization during the syn to post-collisional stages of the Variscan orogeny. Metamorphic assemblages related to the early collision stages have been obliterated making it difficult to constrain the earlier evolution of the unit. In this study, we report the finding of new bodies of high pressure (HP) mafic granulites, which preserve relics of the prograde evolution. Relics of Mn-rich garnet cores containing inclusions of epidote, titanite, chlorite and rare white micas constrain an early prograde stage at 460–550 °C and 0.4–1.0 GPa. The HP assemblage consists in garnet + clinopyroxene+ quartz + rutile ± plagioclase ± amphibole ± biotite and yield peak-P conditions at 650–730 °C and 1.5–1.7 GPa. Decompression was associated with heating to HT granulite-facies conditions of 800–870 °C and 0.6–0.9 GPa, which led to the development of granoblastic polycrystalline mosaics over the resorbing HP assemblage. The U-Pb dating of magmatic zircon cores constrains the emplacement age of the mafic protoliths at 479 ± 5 Ma. Timing of the prograde to HP evolution is estimated around 345–330 Ma based on rutile U-Pb dating, which is contemporaneous with HP recorded in the other crystalline massifs of the western Alps. Zircon metamorphic rims yields U-Pb dates scattering between 337 and 294 Ma, which are interpreted to record crystallization during decompression and heating to granulite facies. Metamorphic conditions during the early prograde stage precludes scenarios involving subduction of a cold passive margin and are better reconciled with thickening of an orogenic wedge during collision. In contrast with other Variscan HP relics of the external Alps, the HP rocks of the Oisans–Pelvoux massif record heating to HT granulitic conditions during decompression, which may result from longer residence time in the lower crust before exhumation. • Prograde evolution is recorded by Variscan high-P granulites in the western Alps. • High-P conditions of ca. 1.5–1.7 GPa and 650–730 °C were reached around 345–330 Ma. • High-T overprint at ca. 800–870 °C and 0.6–0.9 GPa occurred during decompression. • The prograde apparent geotherm is inconsistent with a cold subduction setting. [ABSTRACT FROM AUTHOR]