Trace element chemistry and U-Pb dating of zircons from oceanic gabbros and their relationship with whole rock composition (Lanzo, Italian Alps)

The U-Pb ages and the trace element content of zircon U-Pb along with major and trace element whole rock data on gabbroic dikes from the Lanzo lherzolitic massif, N-Italy, have been determined to constrain crustal accretion in ocean-continent transition zones. Three Fe-Ti gabbros were dated from the central and the southern part of the massif providing middle Jurassic ages of 161±2, 158±2 and 163±1Ma, which argue for magmatic activity over few millions of years. Zircon crystals are characterized by high but variable Th/U ratios, rare earth element patterns enriched in heavy rare earths, pronounced positive Ce and negative Eu-anomalies consistent with crystallization after substantial plagioclase fractionation. The zircon trace element composition coupled with whole rock chemistry was used to reconstruct the crystallization history of the gabbros. A number of gabbros crystallized in situ, and zircon precipitated from trapped, intercumulus liquid, while other gabbros represent residual liquids that were extracted from a cumulus pile and crystallized along syn-magmatic shear zones. We propose a model in which the emplacement mechanism of gabbroic rocks in ocean-continent transition zones evolves from in situ crystallization to stratified crystallization with efficient extraction of residual liquid along syn-magmatic shear zones. Such an evolution of the crystallization history is probably related to the thermal evolution of the underlying mantle lithospher

Multiple Metamorphic Stages within an Eclogite-facies Terrane (Sesia Zone, Western Alps) Revealed by Th-U-Pb Petrochronology

Convergent plate margins typically experience a transition from subduction to collision dynamics as massive continental blocks enter the subduction channel. Studies of high-pressure rocks indicate that tectonic fragments are rapidly exhumed from eclogite facies to mid-crustal levels, but the details of such dynamics are controversial. To understand the dynamics of a subduction channel we report the results of a petrochronological study from the central Sesia Zone, a key element of the internal Western Alps. This comprises two polymetamorphic basement complexes (Eclogitic Micaschist Complex and Gneiss Minuti Complex) and a thin, dismembered cover sequence (Scalaro Unit) associated with pre-Alpine metagabbros and metasediments (Bonze Unit). Structurally controlled samples from three of these units (Eclogitic Micaschist Complex and Scalaro-Bonze Units) yield unequivocal petrological and geochronological evidence of two distinct high-pressure stages. Ages (U-Th-Pb) of growth zones in accessory allanite and zircon, combined with inclusion and textural relationships, can be tied to the multi-stage evolution of single samples. Two independent tectono-metamorphic ‘slices' showing a coherent metamorphic evolution during a given time interval have been recognized: the Fondo slice (which includes Scalaro and Bonze rocks) and the Druer slice (belonging to the Eclogitic Micaschist Complex). The new data indicate separate stages of deformation at eclogite-facies conditions for each recognized independent kilometer-sized tectono-metamorphic slice, between ∼85 and 60 Ma, with evidence of intermittent decompression (ΔP ∼ 0·5 GPa) within only the Fondo slice. The evolution path of the Druer slice indicates a different P-T-time evolution with prolonged eclogite-facies metamorphism between ∼85 and 75 Ma. Our approach, combining structural, petrological and geochronological techniques, yields field-based constraints on the duration and rates of dynamics within a subduction channe

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U–Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P–T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites (> 850 °C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U–Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure

Tracing the evolution of calc-alkaline magmas: In-situ Sm-Nd isotope studies of accessory minerals in the Bergell Igneous Complex, Italy

The common occurrence of Ca- and Nd-rich accessory minerals titanite, epidote, allanite and apatite in calcalkaline plutonic suites makes them ideal targets for in-situ tracing of the temporal, chemical and isotopic evolution of tonalitic and granodioritic melts. The Sm-Nd isotope composition of accessory phases from the calc-alkaline Bergell Pluton and the peraluminous Novate leucogranite (central Alps) were investigated using laser ablation MC-ICPMS techniques. Grouping of individual LA-MC-ICPMS analyses produced average eNd values with typical uncertainties of <0.3 (2s) epsilon units. SHRIMP dating of magmatic allanite cores of epidote grains from a Bergell gabbro yielded a Th-Pb age of 32.40.4 Ma, which provides a new timing constraint on the emplacement of juvenile members within the Bergell intrusive sequence. The Bergell bulkrock mantle-crust isotopic mixing curve was reproduced, demonstrating that integration of U-Th-rich accessory mineral Nd isotope compositions with crystallisation age is capable of tracing the geochemical evolution of magmatic systems over time.Crucially, the isotopic composition of the mantle end-member was successfully constrained by measurement of magmatic REE-epidote, highlighting the compositional versatility of accessory phases. The removal or addition of feldspar controls the Eu signature of both the bulk-rock and single minerals and therefore presents a unique trace element indicator of magmatic differentiation and assimilation processes in accessory minerals. Therefore the in-situ determination of age, Sm-Nd isotopes and trace elements in accessory minerals permits efficient and accurate reconstruction of complex magmatic processes in calc-alkaline plutonic suites. Sub-grain isotopic heterogeneity in magmatic monazite from the Novate leucogranite was identifiable by LA-MC-ICPMS analysis and emphasizes the additional value of a micro-analytical approach to understanding geological processes

Mineral-scale trace element and U-Th-Pb age constraints on metamorphism and melting during the Petermann Orogeny (Central Australia)

High-pressure amphibolite-facies migmatitic orthogneisses from the Cockburn Shear Zone (CSZ), northern Musgrave Block in central Australia, were formed during the 580-520 Ma intraplate Petermann Orogeny. The shear-zone hosted orthogneisses are of an intermediate bulk composition that promoted the growth of rare earth element (REE)-bearing major phases (garne and hornblende), as well as numerous accessory phases (zircon, titanite, apatite, epidote and allanite), all of which are potential U-Th-Pb geochronometers and are involved in the distribution of REEs. We have integrated petrology and detailed in situ trace element analysis of major and accessory phases in samples collected outside and inside the CSZ to establish the relative timing of metamorphic mineral growth. This paper presents one of the first applications of newly developed in situ dating protocols on metamorphic allanite. Sensitive high-resolution ion microprobe geochronology on metamorphic zircon and allanite indicate that metamorphism and partial melting occurred between 5596 and 5516 Ma. Peak temperatures of 720-750C, determined from rutile included in garnet, necessitate the presence of fluids to flux partial melting in the CSZ quartzofeldspathic rocks. Metamorphic zircon formed during cooling in the presence of melt near the granitic wet solidus at T< 700 C. In contrast, allanite formed at different stages of the CSZ P-T path: (1) as a prograde sub-solidus phase (T&lt;650C) formed in the presence of fluids, and (2) as melt-precipitated Th- and REE-rich overgrowths on pre-existing allanite. The ages of the two growth episodes are not isotopically resolvable by allanite dating. Trace element compositions indicate that in both melted and unmelted rocks, garnet and hornblende growth was primarily controlled by prograde subsolidus hydration reactions that consumed feldspar below the metamorphic peak. REE compositions of the metamorphic zircon and allanite overgrowths that formed in the presence of melt also suggest disequilibrium with garnet. Thus, the major period of garnet and hornblende growth was not coeval with partial melting

Serpentinite dehydration in the subducted lithosphere produces no B isotopic fractionation

The fate of boron (B) and its isotopes during serpentinite dehydration is a matter of debate. To contribute to a better understanding of the B isotopic fractionation upon serpentinite dehydration, we present in situ δ11B analyses of antigorite and olivine from subducted high pressure serpentinites from the Western Alps (Zermatt-Saas and Erro-Tobbio). The different isotopic compositions of antigorite in different parts of the units (δ11B of −5 to +10 ‰ and +13 to +19 ‰ for Zermatt-Saas, and δ11B of up to +26 ‰ for Erro-Tobbio) are inherited from variable serpentinisation conditions on the sea floor. Serpentinite dehydration via the brucite-out reaction during subduction produces metamorphic olivine in oxygen (O) isotopic equilibrium with antigorite. This olivine shows near zero B isotopic fractionation with coexisting antigorite (Δ11BOl-Atg of −0.7 ± 3.4 ‰), which implies little B isotopic fractionation during serpentinite dehydration. In contrast, significant B isotopic disequilibrium (Δ11BOl-Atg of +25 ‰) is found between antigorite and olivine formed in shear bands, shear zones and veins, indicating influx of channelled external fluids, including serpentinite-derived fluids from a protolith with a different isotopic composition

Late Carboniferous Schlingen in the Gotthard nappe (Central Alps) and their relation to the Variscan evolution

AbstractPre‐Mesozoic basements of the Alpine belt commonly contain kilometre‐scale folds with steeply inclined axial planes and fold axes, which are named “Schlingen” folds. The structural evolution of Schlingen folds and their geodynamic significance for the Variscan evolution are unclear. To close this gap, this study investigates a well-preserved Schlingen structure in the Gotthard nappe (Central Swiss Alps). This Schlingen fold evolved by a combination of shearing and folding under amphibolite-facies conditions. Detailed digital field mapping, coupled with petrographical and structural investigations, reveal local synkinematic migmatisation in the fold hinges parallel to axial planes. Zircon crystals from leucosomes in the fold hinges have magmatic cores that yield an age of 449 ± 3 Ma, and rims with a range of dates from ~ 270 to 330 Ma (main cluster at 315 ± 4 Ma). We ascribe this late Carboniferous age to peak metamorphic conditions of the late Variscan Schlingen phase. Moreover, we describe for the first time post-Schlingen, but pre-Alpine transpressional deformation. The investigated Schlingen fold is discussed with respect to comparable structures of the wider Alpine realm, located in the most southern part of the Variscan belt. We propose that Schlingen formation concurred with the crustal-scale transpressional tectonics. This scenario separates, at least in a structural sense, the Southern Variscides from more northern parts (also Gondwana derived), where Schlingen folds are absent.</jats:p

Late Oligocene high-temperature shear zones in the core of the Higher Himalayan Crystallines (Lower Dolpo, western Nepal)

A high\u2010temperature shear zone, Toijem shear zone, with a top\u2010to\u2010the\u2010SW sense of shear affects the core of the Higher Himalayan Crystallines (HHC) in western Nepal. The shear zone developed during the decompression, in the sillimanite stability field, of rocks that previously underwent relatively high\u2010pressure metamorphism deformed under the kyanite stability field. PT conditions indicate that the footwall experienced higher pressure ( 3c9 kbar) than the hanging wall ( 3c7 kbar) and similar temperatures (675\ub0\u2013700\ub0C). Monazite growth constrains the initial activity of the shear zone at 25.8 \ub1 0.3 Ma, before the onset of the Main Central Thrust zone, whereas the late intrusion of a crosscutting granitic dike at 17 \ub1 0.2 Ma limits its final activity. Monazites in kyanite\u2010bearing gneisses from the footwall record prograde metamorphism in the HHC from 3c43 to 33Ma. The new data confirm that exhumation of the HHC started earlier in western Nepal than in other portions of the belt and before the activity of both the South Tibetan Detachment System (STDS) and Main Central Thrust (MCT) zones. As a consequence, western Nepal represents a key area where the channel\u2010flow\u2010driven mechanism of exhumation, supposed to be active from Bhutan to central\u2010eastern Nepal, does terminate. In this area, exhumation of crystalline units occurred by foreland propagation of ductile and, subsequently, brittle deformation

Mechanisms of Crustal Anatexis: a Geochemical Study of Partially Melted Metapelitic Enclaves and Host Dacite, SE Spain

To shed light on the mechanisms of crustal anatexis, a detailed geochemical study has been conducted on minerals and glasses of quenched anatectic metapelitic enclaves and their host peraluminous dacites at El Hoyazo, SE Spain. Anatectic enclaves, composed of plagioclase þ biotite þ sillimanite þ garnet þ glass K-feldspar cordierite þ graphite, formed during the rapid heating and overstepped melting of a greenschist-facies metapelite, and finally equilibrated at 850 508C and 5^7 kbar. Glass appears as melt inclusions within all mineral phases and in the matrix of the enclaves, and has a major element composition similar to that of peraluminous leucogranites. Melt inclusions and matrix glasses have normative quartz^orthoclase^albite compositions that plot in the vicinity of H2O-undersaturated haplogranite eutectics. Melt inclusions show some compositional variability, with high Li, Cs and B, low Y, first row transition elements (FRTE) and rare earth elements (REE), and zircon and monazite saturation temperatures of 665^7508C.They are interpreted as melts produced by muscovitebreakdown melting reactions at the onset of the process of rapid melting and mostly under H2O-undersaturated conditions. Compared with melt inclusions, matrix glasses show less compositional variability, lower large ion lithophile element contents, higher Y, FRTE and REE, and higher zircon and monazite saturation temperatures ( 695^8158C).They are interpreted as former melts recording the onset of biotite dehydration-melting. Matrix glasses in the dacite are compositionally different from glasses in the enclaves, hence the genetic connection between metasedimentary enclaves and dacite is not as straightforward as previous petrographic and bulk major element data suggest; this opens the possibility for some alternative interpretation. This study shows the following: (1) melt inclusions provide a window of information into the prograde evolution of anatexis in the enclaves; (2) melting occurred for the most part under H2O-undersaturated conditions even if, because of the rapid heating, the protolith preserved most of the structurally bound H2O contained at greenschist facies up to the beginning of anatexis, such that the excess H2O maximized the amount of H2O-undersaturated melt generated during anatexis; (3) although a large proportion of accessory minerals are currently shielded within major mineral phases, they have progressively dissolved to a considerable extent into the melt phase along the prograde anatectic path, as indicated by the relative clustering of accessory mineral saturation temperatures and closeness of these temperatures to those of potential melting reactions; (4) the dacite magma was probably produced by coalescence of melt

Dynamics in the Sesia HP terrane: Combined petrochronological and structural analysis

HP terranes dominated by continental crust represent the end result of a sequence of processes that operate at lithosphere scale, i.e. rifting, subduction/accretion, return flow/exhumation. To under\uacstand the dynamics of the subduction channel in complex terranes of this kind, the effects from each stage must be investigated separately, linking the observations and data from kilometers down to micrometer scale. This task recommends an integrative approach. Here we focus on the assembly of the Sesia Zone (SZ), a key element of the internal Western Alps. This terrane comprises two main polymetamorphic base\uacment units and thin trails of a cover sequence that includes post-Permian syn- to post-rift metasediments; the latter show no pre-Alpine metamorphic imprint. The tectonic scenario of Babist et al. (2006) recognizes five main phases in the Alpine structural evolution; their model helped us select areas for detailed structural work and sampling. Our first goal was to relate the early convergent structures (D1, D2) to the P-T evolution and to establish a robust time-frame for the HP-dynamics within and between the tectonic slices. Within the subduction/extrusion channel, problems addressed include the question of tectonic mixing, i.e. temporal and spatial scales of relative and absolute movement of the slices, and the conditions and timing of their final juxtaposition prior to the rapid exhumation of the Sesia Zone as a whole. Mono- and polymetamorphic sediments from different slices display unequivocal evidence of several HP-stages separated in time. Successive stages under eclogite facies conditions occurred between 86 \u2013 65 Ma, as shown by LA-ICP-MS and SHRIMP data on growth zones in accessory allanite, monazite, zircon, and titanite. By using mutual inclusions and overgrowth relationships, the age-data on allanite and monazite can be tied to the multistage evolution of an individual sample. For different rocks, these (over)growth stages can be related to D1- and D2-deformation when micro-, meso- and megastructural observations are combined. Thermobarometry indicates intermittant decompression by ~0.8 GPa between HP phases, hence pressure cycling (aka yo-yo tectonics, Rubatto et al., 2011). This tectonic mobility occurred prior to the final juxtaposition of slices and their exhumation, which involved at least two major deformation phases and lead to widespread retrogression at amphibolite to green-schist facies conditions. Our approach combining structural, petrological, and geochronological techniques yields some field-based constraints on the duration and rates of the dynamics within a subduction channel. It may be useful to compare these to insights from numerical models, provided the latter take into account the specific conditions of the plate convergence, which turns out to have been highly oblique in the present case