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Relicts of subducted oceanic lithosphere provide key information for the tectonic reconstructions of convergent margins. In the Central Alps, such relicts occur as isolated mafic–ultramafic lenses within the migmatites of the southern Adula nappe and Cima-Lunga unit. Analysis of the major-, minor-, and accessory minerals of these ophiolitic relicts, combined with zircon and rutile U–Pb ages and zircon oxygen isotopes, allows the reconstruction of different stages of their complex evolution. The mafic–ultramafic suite in Valle di Moleno consists of chlorite-harzburgites associated with metarodingites and retrogressed eclogites. Relic omphacite and kyanite in retrogressed eclogites provide evidence for subduction-related metamorphism. Increasing XPrp in the garnet mantle towards the rim documents heating during high-pressure metamorphism up to 800–850 °C. Polyphase inclusions and chemical zoning in garnet suggest fluid-assisted melting during high-pressure metamorphism dated at 31.0 ± 0.9 Ma. In Val Cama, chlorite-harzburgites, metarodingites and calcsilicate-metasediments occur. Detrital zircon ages in the metasediment suggest a Mesozoic deposition. The metarodingite-metaperidotite-metasediment association and the low δ18O signatures of zircon (δ18O 3.0–3.7‰), inherited from seafloor metasomatism of the protoliths, show that the rocks are derived from former altered oceanic crust. Amphibolite facies metamorphism related to the Central Alps Barrovian evolution in Val Cama occurred at 28.8 ± 1.5 Ma. The combined data from Moleno and Cama indicate a rapid transition (~ 2 Ma) from subduction to collisional metamorphism with corresponding exhumation rates of 3–6 cm/year. Fast exhumation tectonics may have been favored by slab break-off or slab extraction. U–Pb dating of rutile from both localities yields ages of ~ 20 Ma, suggesting that these rocks remained at amphibolite-facies conditions for about 10 Ma and underwent a second fast exhumation of 3 cm/year associated with vertical movements along the Insubric line. [ABSTRACT FROM AUTHOR]

The trace-element and isotope geochemistry of rutile are robust tools to determine metamorphic temperatures, age, and host-/source lithologies. The use of rutile as single grain indicator for pressure, temperature, time and composition (P–T–t–X) of the host rock, which is vital in the use of detrital rutile to trace plate-tectonic regimes throughout Earth's history, requires the identification of a pressure dependent trace element in rutile. We investigate the pressure dependence of hydrogen in rutile using polarized in-situ Fourier Transform Infrared (FTIR) spectroscopy. H2O contents in rutile vary between < 10–2500 μg/g H2O with higher contents in samples with higher peak metamorphic pressures, making H2O-in-rutile a viable pressure indicator. The highest H2O contents at ~ 450–2000 μg/g are observed in mafic low temperature eclogite-facies rutile related to modern-style cold subduction conditions. Hydrogen zoning in FTIR maps indicates that H+ is retained at temperatures below 600–700 °C. Ratios of H2O/Zr, using H2O as pressure indicator and Zr as temperature proxy, are a proxy for thermal gradients of metamorphic rutile (i.e. P/T). Low temperature eclogite samples are also characterized by high Fe contents and therefore Fe/Zr-ratios might be used as a first order approximation for H2O/Zr-ratios to identify mafic low temperature eclogite facies rutile. Based on common discrimination diagrams, Nb, W, and Sn can be used to distinguish different host/source rock lithologies of rutile. Combining both H2O/Zr-ratios and Nb, W, and Sn contents can thus identify modern-style cold subduction signatures in rutile. The developed systematics can consequently be used to trace cold-subduction features in the (pre-Proterozoic) detrital record. [ABSTRACT FROM AUTHOR]

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]

Dehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8‰ in δ18O. Thermodynamic and δ18O models show that the large amount of low δ18O H2O required to change the reactive bulk δ18O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 × 105 cm3/cm2, which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 × 10–20 m2) is comparable to the permeability determined experimentally for blueschist and serpentinites. [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]

Microscale dating of distinct domains in minerals that contain relics of multiple metamorphic events is a key tool to characterize the polyphase evolution of complex metamorphic terranes. Zircon and allanite from five metasediments and five metaintrusive high-pressure (HP) rocks from the Eclogite Micaschist Complex of the Sesia Zone were dated by SIMS and LA-ICP-MS. In the metasediments, zircon systematically preserves detrital cores and one or two metamorphic overgrowths. An early Permian age is obtained for the first zircon rim in metasediments from the localities of Malone Valley, Chiusella Valley and Monte Mucrone (292 ± 11, 278.8 ± 3.6 and 285.9 ± 2.9 Ma, respectively). In the Malone Valley and Monte Mucrone samples, the early Permian ages are attributed to high-temperature metamorphism and coincide with the crystallization ages of associated mafic and felsic intrusions. This implies that magmatism and metamorphism were coeval and associated to the same tectono-metamorphic extensional event. In the Malone Valley, allanite from a metasediment is dated at 241.1 ± 6.1 Ma and this age is tentatively attributed to a metasomatic/metamorphic event during Permo-Triassic extension. Outer zircon rims with a late Cretaceous age (67.4 ± 1.9 Ma) are found only in the micaschist from Monte Mucrone. In metagabbro of the Ivozio Complex, zircon cores yield an intrusive age for the protolith of 340.7 ± 6.8 Ma, whereas Alpine allanite are dated at 62.9 ± 4.2 and 55.3 ± 7.3 Ma. The Cretaceous ages constrain the timing of the HP metamorphic stage. The presence of zircon overgrowth only in the central area of the Eclogite Micaschist Complex is attributed to local factors such as (1) multiple fluid pulses at HP that locally enhanced zircon dissolution and recrystallization, and (2) slightly higher temperatures reached in this area during HP metamorphism. [ABSTRACT FROM AUTHOR]

An amendment to this paper has been published and can be accessed via the original article. [ABSTRACT FROM AUTHOR]

Ion probe 208Pb/232Th fissure monazite ages from the Argentera External Massif and from the high-pressure units of the Western Alps provide new insights on its Cenozoic tectonic evolution. Hydrothermal monazite crystallizes during cooling/exhumation in Alpine fissures, an environment where monazite is highly susceptible to fluid-mediated dissolution-(re)crystallization. Monazite growth domains visualized by BSE imaging all show a negative Eu anomaly, positive correlation of Sr and Ca and increasing cheralite component (Ca + Th replacing 2REE) with decreasing xenotime (Y) component. The huttonite component (Th + Si replacing REE and P) is very low. Growth domains record crystallization following chemical disequilibrium in a fissure environment, and growing evidence indicates that they register tectonic activity. Fissure monazite ages obtained in this study corroborate previous ages, recording crystallization at ~ 36 Ma, ~ 32–30 Ma, and ~ 25–23 Ma in the high-pressure regions of the Western Alps, interpreted to be respectively related to top-NNW, top-WNW and top-SW thrusting in association with strike-slip faulting. During this latter transpressive phase, younger fissure monazite crystallization is recorded between ~ 20.6 and 14 Ma in the Argentera Massif, interpreted to have occurred in association with dextral strike-slip faulting related to anticlockwise rotation of the Corsica-Sardinia Block. This strike-slip activity is predating orogen-parallel dextral strike-slip movements along and through the internal part of all other External Crystalline Massifs (ECM), starting only at ~ 12 Ma. Our combined compositional and age data for hydrothermal monazite track crystallization related to tectonic activity during unroofing of the Western Alps for over more than 20 million years, offering chronologic insights into how different tectonic blocks were exhumed. The data show that fissures in the high-pressure units formed during greenschist to amphibolite facies retrograde deformation, and later in association with strike-slip faulting. [ABSTRACT FROM AUTHOR]

In the Dent Blanche Tectonic System, the Mont Morion biotite-bearing granite is a km-scale intrusion preserved in a low-strain volume. Zircon saturation thermometry suggests that it crystallised from a melt that reached about 800 °C. U-Pb zircon and allanite geochronology indicates crystallization of the magma in the Permian (290 ± 3 Ma; 280 ± 8 Ma, respectively). Migmatitic biotite-gneiss and amphibolite are found as xenoliths within the Mont Morion granite and constitute its country-rocks. In two samples of migmatitic biotite-gneiss zircon has metamorphic overgrowths that yield U-Pb ages of 285 ± 3 Ma and 281 ± 4 Ma, and are thus contemporaneous with the intrusion of the granite. The Mont Morion granite with its country-rocks of migmatitic biotite-bearing gneiss and amphibolite was thus emplaced at middle crustal levels while amphibolite facies metamorphism affected its country rocks. The magmatic and metamorphic record in the Mont Morion area reflects the high-temperature regime and lithospheric thinning of the Adriatic continental margin during Permian. [ABSTRACT FROM AUTHOR]

Monazite is a key accessory mineral for metamorphic geochronology, but interpretation of its complex chemical and age zoning acquired during high-temperature metamorphism and anatexis remains a challenge. We investigate the petrology, pressure-temperature and timing of metamorphism in pelitic and psammitic granulites that contain monazite from the Greater Himalayan Crystalline Complex (GHC) in Dinggye, southern Tibet. These rocks underwent isothermal decompression from pressure of >10 kbar to ~5 kbar at temperatures of 750-830 °C, and recorded three metamorphic stages at kyanite (M), sillimanite (M) and cordierite-spinel grade (M). Monazite and zircon crystals were dated by microbeam techniques either as grain separates or in thin sections. U-Th-Pb ages are linked to specific conditions of mineral growth on the basis of zoning patterns, trace element signatures, index mineral inclusions (melt inclusions, sillimanite and K-feldspar) in dated domains and textural relationships with co-existing minerals. The results show that inherited domains (500-400 Ma) are preserved in monazite even at granulite-facies conditions. Few monazites or zircon yield ages related to the M-stage (~30-29 Ma), possibly corresponding to prograde melting by muscovite dehydration. During the early stage of isothermal decompression, inherited or prograde monazites in most samples were dissolved in the melt produced by biotite dehydration-melting. Most monazite grains crystallized from melt toward the end of decompression (M-stage, 21-19 Ma) and are chemically related to garnet breakdown reactions. Another peak of monazite growth occurred at final melt crystallization (~15 Ma), and these monazite grains are unzoned and are homogeneous in composition. In a regional context, our pressure-temperature-time data constrains peak high-pressure metamorphism within the GHC to ~30-29 Ma in Dinggye Himalaya. Our results are in line with a melt-assisted exhumation of the GHC rocks. [ABSTRACT FROM AUTHOR]

The Dora-Maira whiteschists derive from metasomatically altered granites that experienced ultrahigh-pressure metamorphism at ~750 °C and 40 kbar during the Alpine orogeny. In order to investigate the P–T–time–fluid evolution of the whiteschists, we obtained U–Pb ages from zircon and monazite and combined those with trace element composition and oxygen isotopes of the accessory minerals and coexisting garnet. Zircon cores are the only remnants of the granitic protolith and still preserve a Permian age, magmatic trace element compositions and δ18O of ~10 ‰. Thermodynamic modelling of Si-rich and Si-poor whiteschist compositions shows that there are two main fluid pulses during prograde subduction between 20 and 40 kbar. In Si-poor samples, the breakdown of chlorite to garnet + fluid occurs at ~22 kbar. A first zircon rim directly overgrowing the cores has inclusions of prograde phlogopite and HREE-enriched patterns indicating zircon growth at the onset of garnet formation. A second main fluid pulse is documented close to peak metamorphic conditions in both Si-rich and Si-poor whiteschist when talc + kyanite react to garnet + coesite + fluid. A second metamorphic overgrowth on zircon with HREE depletion was observed in the Si-poor whiteschists, whereas a single metamorphic overgrowth capturing phengite and talc inclusions was observed in the Si-rich whiteschists. Garnet rims, zircon rims and monazite are in chemical and isotopic equilibrium for oxygen, demonstrating that they all formed at peak metamorphism at 35 Ma as constrained by the age of monazite (34.7 ± 0.4 Ma) and zircon rims (35.1 ± 0.8 Ma). The prograde zircon rim in Si-poor whiteschists has an age that is within error indistinguishable from the age of peak metamorphic conditions, consistent with a minimum rate of subduction of 2 cm/year for the Dora-Maira unit. Oxygen isotope values for zircon rims, monazite and garnet are equal within error at 6.4 ± 0.4 ‰, which is in line with closed-system equilibrium fractionation during prograde to peak temperatures. The resulting equilibrium ∆18Ozircon-monazite at 700 ± 20 °C is 0.1 ± 0.7 ‰. The in situ oxygen isotope data argue against an externally derived input of fluids into the whiteschists. Instead, fluid-assisted zircon and monazite recrystallisation can be linked to internal dehydration reactions during prograde subduction. We propose that the major metasomatic event affecting the granite protolith was related to hydrothermal seafloor alteration post-dating Jurassic rifting, well before the onset of Alpine subduction. [ABSTRACT FROM AUTHOR]

Fluids are considered a fundamental agent for chemical exchanges between different rock types in the subduction system. Constraints on the sources and pathways of subduction fluids thus provide crucial information to reconstruct subduction processes. The Monviso ophiolitic sequence is composed of mafic, ultramafic and minor sediments that have been subducted to ~80 km depth. In this sequence, both localized fluid flow and channelized fluids along major shear zones have been documented. We investigate the timing and source of the fluids that affected the dominant mafic rocks using microscale U-Pb dating of zircon and oxygen isotope analysis of mineral zones (garnet, zircon and antigorite) in high-pressure rocks with variable degree of metasomatic modification. In mafic eclogites, Jurassic zircon cores are the only mineralogical relicts of the protolith gabbros and retain δO values of 4.5-6 ‰, typical of mantle melts. Garnet and metamorphic zircon that grew during prograde to peak metamorphism display low δO values between 0.2 and 3.8 ‰, which are likely inherited from high-temperature alteration of the protolith on the sea floor. This is corroborated by δO values of 3.0 and 3.6 ‰ in antigorite from surrounding serpentinites. In metasomatized eclogites within the lower shear zone, garnet rim formed at the metamorphic peak shows a shift to higher δO up to 6 ‰. The age of zircons in high-pressure veins and metasomatized eclogites constrains the timing of fluid flow at high pressure at around 45-46 Ma. Although the oxygen data do not contradict previous reports of interaction with serpentinite-derived fluids, the shift to isotopically heavier oxygen compositions requires contribution from sediment-derived fluids. The scarcity of metasediments in the Monviso sequence suggests that such fluids were concentrated and fluxed along the lower shear zone in a sufficient amount to modify the oxygen composition of the eclogitic minerals. [ABSTRACT FROM AUTHOR]

The Kokchetav complex in Kazakhstan contains garnet-bearing gneisses that formed by partial melting of metasedimentary rocks at ultrahigh-pressure (UHP) conditions. Partial melting and melt extraction from these rocks is documented by a decrease in KO and an increase in FeO + MgO in the restites. The most characteristic trace element feature of the Kokchetav UHP restites is a strong depletion in light rare earth elements (LREE), Th and U. This is attributed to complete dissolution of monazite/allanite in the melt and variable degree of melt extraction. In contrast, Zr concentrations remain approximately constant in all gneisses. Using experimentally determined solubilities of LREE and Zr in high-pressure melts, these data constrain the temperature of melting to ~1,000 °C. Large ion lithophile elements (LILE) are only moderately depleted in the samples that have the lowest U, Th and LREE contents, indicating that phengite retains some LILE in the residue. Some restites display an increase in Nb/Ta with respect to the protolith. This further suggests the presence of phengite, which, in contrast to rutile, preferentially incorporates Nb over Ta. The trace element fractionation observed during UHP anatexis in the Kokchetav gneisses is significantly different from depletions reported in low-pressure restites, where generally no LREE and Th depletion occurs. Melting at UHP conditions resulted in an increase in the Sm/Nd ratio and a decoupling of the Sm-Nd and Lu-Hf systems in the restite. Further subduction of such restites and mixing with mantle rocks might thus lead to a distinct isotopic reservoir different from the bulk continental crust. [ABSTRACT FROM AUTHOR]

We report here the discovery of UHP eclogites in the Ulfas Valley within the Texel Complex (Meran, NE Italy), which has important implications on the evolution of the Austroalpine nappe stack in the Eastern Alps. Exsolved K-feldspar and phengite lamellae within omphacite relics, together with an unusually high K-content of the host clinopyroxene, point to equilibrium pressures at least close to the quartz/coesite transition. The age of the high-pressure metamorphism is constrained by U-Pb dating of metamorphic zircon at 85 ± 4 Ma. These new data suggest pressure conditions for the Cretaceous eclogitic metamorphism of the Texel Complex higher than hitherto reported data, possibly indicating that several sectors of the Africa-Adria derived Austroalpine margin were subducted to mantle depths during the early stages of the Alpine orogeny. [ABSTRACT FROM AUTHOR]

This study investigates the behaviour of the Zr-in-rutile and Ti-in-zircon thermometers in granulite facies metapelites from the Ivrea-Verbano Zone lower crustal section. U-Pb ages of zircon constrain the timing of regional amphibolite-granulite facies metamorphism to 316 ± 3 Ma and record zircon recrystallisation and resetting of U-Pb ages at 276 ± 4 Ma and 258 ± 3 Ma. Zr-in-rutile thermometry records peak contact metamorphic temperatures related to intrusion of mafic magmatic rocks and gives peak temperatures between 900-930 °C and 1,000-1,020 °C that are consistent with the geological settings of the samples. Ti-in-zircon temperatures of 700-800 °C and 810-870 °C record growth or re-equilibration of zircon after cooling from peak temperatures. Ti-in-quartz thermometry for one sample records both peak and retrograde temperatures. Some rutiles in all samples record resetting of Zr-in-rutile temperatures at ~750-800 °C. Electron microprobe profiles across individual rutiles demonstrate that Zr expulsion occurred by recrystallisation rather than by diffusive exchange. Exsolution of small needles of baddelyite or zircon from rutile is an important method of Zr redistribution, but results in no net Zr loss from the grain. The demonstration that Zr-in-rutile thermometry can robustly record peak temperatures that are not recorded by any other thermometer emphasises the relevance of this technique to investigating the evolution of high-grade metamorphic terranes, such as those that characterise the lower crust. [ABSTRACT FROM AUTHOR]

The petrology and timing of crustal melting has been investigated in the migmatites of the Higher Himalayan Crystalline (HHC) exposed in Sikkim, India. The metapelites underwent pervasive partial melting through hydrous as well as dehydration melting reactions involving muscovite and biotite to produce a main assemblage of quartz, K-feldspar, plagioclase, biotite, garnet ± sillimanite. Peak metamorphic conditions were 8-9 kbar and ~800 °C. Monazite and zircon crystals in several migmatites collected along a N-S transect show multiple growth domains. The domains were analyzed by microbeam techniques for age (SHRIMP) and trace element composition (LA-ICP-MS) to relate ages to conditions of formation. Monazite preserves the best record of metamorphism with domains that have different zoning pattern, composition and age. Zircon was generally less reactive than monazite, with metamorphic growth zones preserved in only a few samples. The growth of accessory minerals in the presence of melt was episodic in the interval between 31 and 17 Ma, but widespread and diachronous across samples. Systematic variations in the chemical composition of the dated mineral zones (HREE content and negative Eu anomaly) are related to the variation in garnet and K-feldspar abundances, respectively, and thus to metamorphic reactions and P-T stages. In turn, this allows prograde versus decompressional and retrograde melt production to be timed. A hierarchy of timescales characterizes melting which occurred over a period of ~15 Ma (31-17 Ma): a given block within this region traversed the field of melting in 5-7 Ma, whereas individual melting reactions lasted for time durations below, or approaching, the resolution of microbeam dating techniques (~0.6 Ma). An older ~36 Ma high-grade event is recorded in an allocthonous relict related to mafic lenses. We identify two sections of the HHC in Sikkim that traversed similar P-T conditions at different times, separated by a tectonic discontinuity. The higher structural levels reached melting and peak conditions later (~26-23 Ma) than the lower structural levels (~31-27 Ma). Diachronicity across the HHC cannot be reconciled with channel flow models in their simplest form, as it requires two similar high-grade sections to move independently during collision. [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]

The high-K calc-alkaline volcanic rocks along the Neogene Volcanic Province of SE Spain represent crustal anatectic melts mixed with mantle components during the opening of the Alborán Sea. Partially melted metapelitic enclaves, along with the geochemical signature, provide evidence of their crustal source. U–Pb SHRIMP geochronology on monazite and zircon from enclaves and their hosting lavas in the localities of El Hoyazo, Mazarrón and Mar Menor reveals variable delays between the melting at depth and the eruption of the volcanics. These data indicate that: (1) the most important event of anatexis in the Neogene spanned at least the 3 m.y. interval between 12 and 9 Ma; (2) there is no trend in age of crustal melting; and (3) the delay between magma generation and extrusion varies from more than 3 m.y. at El Hoyazo to ~0.5 m.y. and possibly 2.5 m.y. at Mar Menor, with no significant delay measurable at Mazarrón. The variable time delay between anatexis and lava extrusion indicates that radiometric ages of volcanics may provide misleading information on the timing of magma genesis occurring at depth. This highlights the pitfall of basing detailed geodynamic models on volcanic extrusion ages alone. [ABSTRACT FROM AUTHOR]

U-Pb geochronology with ion microprobe (SHRIMP) analysis has been carried out on eclogite-facies rocks of the Beigua Unit, an ophiolitic slice of the Voltri Massif, Western Alps. The investigated samples are eclogites and high-pressure metasomatic rocks (metarodingites and centimetre-sized Ti-clinohumite-bearing dykes). Zircon contained in an eclogitic metagabbro and a metarodingite preserves magmatic zoning patterns and trace element compositions. The zircon ages of 160±1 and 161±3 Ma are interpreted to date the crystallization of the gabbroic protoliths. Ti-clinohumite dykes in the same unit contain baddeleyite crystals in textural equilibrium with Ti-clinohumite, diopside, chlorite and magnetite, which form the eclogite-facies assemblage in these rocks. Baddeleyite also contains inclusions of such minerals, indicating its formation at high pressure. The baddeleyite has cathodoluminescence intensity and chaotic patterns similar to metamorphic zircon. It contains a significant amount of Hf (1.3–1.7 wt%), traces of Ti, Y, Nb, Ta, REE, U and Th. Its chondrite-normalised trace element pattern has strong enrichment in middle REE, positive Ce-anomaly and small negative Eu-anomaly. This represents the first report of baddeleyite formed during regional metamorphism, and suggests that this mineral could (re)crystallize easier than zircon under low-temperature, high-pressure conditions. The age of the baddeleyite is interpreted as likely dating the eclogite-facies metamorphism in the Beigua Unit at 33.6±1.0 Ma. This age is very close to the Early Oligocene age of the overlying Tertiary continental breccias and conglomerates, which contains clasts of high-pressure rocks. This sedimentary record, which is unique for Alpine high-pressure units, is direct evidence of fast exhumation of the Beigua eclogites. The young age for the HP metamorphism of the Beigua ophiolite makes a revision of either the palaeogeography prior to collision, or of the subduction setting in the entire region, necessary. [ABSTRACT FROM AUTHOR]

Zircon and monazite from three restitic enclaves and one host dacite have been dated by ion microprobe (SHRIMP), with the aim of characterising their Miocene history and defining the timing relationships between crustal melting and eruption in the high-K calc-alkaline volcanics of the Neogene Volcanic Province of SE Spain. The studied samples are from the volcanic edifices of El Joyazo (Cerro del Hoyazo) and Mazarrón. Zircons in the enclaves are characterized by a thin euhedral rim overgrowing a detrital core. The core-rim boundary is marked by tiny glass inclusions of S-type granitic composition, which attest to the growth of zircon rims during a crustal melting event. At El Joyazo, where lavas erupted at 6.3 Ma (Zeck and Williams 2002), U-Pb ages of zircon overgrowths define an age of anatexis of 9.63±0.26 Ma (95% c.l.), in agreement with monazite ages of 9.74±0.21 Ma (95% c.l.). At Mazarrón, the age of anatexis provided by monazite at 9.13±0.18 Ma (95% c.l.) overlaps with that of melt-precipitated zircon in the host dacite, dated at 9.06±0.53 Ma (95% c.l.). These results indicate that after partial melting, the enclaves and the syngenetic S-type melts resided at depth for >3 m.y. at El Joyazo. Compared with the results from Mazarrón, the long residence time obtained at El Joyazo is probably due to the greater depth of melting (c. 25 km vs. c. 15 km). At such depth, corresponding to the Miocene palaeo-Moho, the more ductile regime of the crust is likely to have favoured magma ponding. The thermal anomaly beneath the Neogene Volcanic Province, which generated the S-type crustal melts, is today visible from geophysical data and can be traced back to the Lower Miocene. As a consequence, residence times longer than determined in this work may be expected. [ABSTRACT FROM AUTHOR]

Examines the stability of aluminum-rich and fluoride-rich titanite in metacarbonate. Association of eclogites with high pressure; Chemical and isotopic composition of titanite; Use of high-resolution ion microprobe.

Presents information on a combined isotopic and petrological study of metamorphic zircon from the diamondiferous rocks in the Kokchetav Massif, Kazakhstan. Relation of zircon growth to metamorphic conditions; Properties of zircon; Behavior of zircon during high temperature-high pressure metamorphism.

Focuses on a study that reported an extensive field-based study of zircon and monazite in the metamorphic sequence of the Reynolds Range in the Northern Territory. Geological setting; Sample description; Methodology; Zircon and monazite response to prograde metamorphism; Time and duration of regional metamorphism.

Presents a study which was conducted to better understand the Jurassic opening and the Alpine closure of the Tethys ocean, emphasizing the fundamental importance of the geology of the Zermatt-Saas-Fee ophioligies (ZSFO). Formation of the Alphine chain; What led to the subduction of oceanic and continental crust under the Adriatic margin; Methods which were utilized to conduct the study; Geochronological results of the study.