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European journal of mineralogy : EJM 35(2), 171-187 (2023). doi:10.5194/ejm-35-171-2023, Published by Copernicus Publications, Göttingen

International audience; In orogens, pinpointing the timing of mineralization is often complex due to superimposition of magmatic, metamorphic and tectonic events. Unravelling the tectono-metamorphic evolution of deformed terranes is essential for understanding the formation of orogenic mineralization. In the Bossòst dome of the Pyrenean Axial Zone, undated stratabound Pb-Zn mineralization previously considered to be SEDEX-like and devoid of evidence for significant remobilization/deformation, was recently interpreted as orogenic mineralization and structurallycontrolled by Variscan tectonics. However, relations with the poly-magmatic and metamorphic events reported in the Pyrenean Axial Zone are still poorly constrained. In the Bossòst dome, laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Th-Pb dating was performed on zircon to constrain the formation ages of two undeformed granitic dykes, and on monazite to identify both the metamorphic imprint recorded by two metapelites and the formation of stratabound Pb-Zn mineralization in the Bentaillou deposit. Late-Carboniferous early-Permian magmatic zircon ages (315-280 Ma) are commonly found in these undeformed dykes (307.4 ± 4.7 Ma and 283 ± 15 Ma), consistent with the main magmatic event, extensively recorded in the Pyrenean Axial Zone. Monazite crystals with co-genetic textural relationships with stratabound sphalerite from Bentaillou were dated at 309 ± 11 Ma. We propose that the stratabound mineralization is formed during one main Variscan remobilization event associated with the first Variscan deformation event (D1) and probable metamorphic fluid circulation in pre-existing metal-rich Ordovician sedimentary levels. Moreover, monazite ages in metapelite rocks (289.1 ± 5.9 Ma and 290.0 ± 3.8 Ma) probably date both the end of high T-low P metamorphism and late fluid circulation in the Variscan tectonothermal event. Visean-Serpukhovian ages (340-325 Ma) are found in one inherited zircon and in two monazites included in garnet and staurolite, which probably record the end of the first Variscan magmatic-metamorphic event, mainly recorded in the core of the Pyrenean Axial Zone. Based on our ages and a regional synthesis, we propose a new tectono-metamorphic model for the Bossòst dome and the related formation of Pb-Zn mineralization.

International audience; The inherited localization model for shear zone development suggests that ductile deformation in the middle and lower continental crust is localized on mechanical anisotropies, like fractures, referred to as shear zone brittle precursors. In the Neves area (Western Tauern Window, Eastern Alps), although the structural control of these brittle precursors on ductile strain localization is well established, the relative timing of the brittle deformation and associated localized fluid flow with respect to ductile deformation remains in most cases a matter of debate. The present petrological study, carried out on a brittle precursor of a shear zone affecting the Neves metagranodiorite, aims to determine whether brittle and ductile deformations are concomitant and therefore relate to the same tectonic event. The brittle precursor consists of a 100–500 µm wide recrystallized zone with a host mineral‐controlled stable mineral assemblage composed of plagioclase–garnet–quartz–biotite–zoisite±white mica±pyrite. Plagioclase and garnet preserve an internal compositional zoning interpreted as the fingerprint of Alpine metamorphism and fluid–rock interactions concomitant with the brittle deformation. Phase equilibrium modelling of this garnet‐bearing brittle precursor shows that metamorphic garnet and plagioclase both nucleated at 0.6 ± 0.05 GPa, 500 ± 20°C and then grew along a prograde path to 0.75 ± 0.05 GPa, 530 ± 20°C. These amphibolite facies conditions are similar to those inferred from ductile shear zones from the same area, suggesting that both brittle and ductile deformation were active in the ductile realm above 500°C for a depth range between 17 and 21 km. We speculate that the Neves area fulfils most of the required conditions to have hosted slow earthquakes during Alpine continental collision, that is, coupled frictional and viscous deformation under high‐fluid pressure conditions ~450°C. Further investigation of this potential geological record is required to demonstrate that slow earthquakes may not be restricted to subduction zones but are also very likely to occur in modern continental collision settings.

The crustal architecture of slow-spread ocean crust results from complex interactions between magmatism, hydrothermalism, and tectonics. IODP Hole U1473A (809 m depth) was drilled during IODP Expeditions 360 and 362T at the summit of the Atlantis Bank, a gabbroic massif exhumed at the Southwest Indian Ridge (SWIR). In this study, we identify and quantify plastic deformation processes in oceanic gabbros and active slip-systems in plagioclase from 112 thin sections sampled throughout Hole U1473A.We describe deformed zones using petrographic observations and modern Electron Backscattered Diffraction (EBSD) analyses made all along the core. Ductile deformation is widespread and is sometimes strongly localized. It initiated during accretion under magmatic conditions and continued until late brittle conditions. Porphyroclastic microstructures testify to post-magmatic, solid-state, high-temperature (HT) deformation. Plagioclase represents ~60% of rock’s volume and is the dominant phase accommodating deformation in the gabbro. It shows strong dynamic recrystallization accommodated by dislocation creep, forming a fine-grained matrix. Strain localizes in mylonitic and ultramylonitic zones, and these shear zones are often overprinted by lower temperature deformation.EBSD analyses reveal weak to moderate crystallographic preferred orientations (CPO) of plagioclase first developed during early magmatic flow, that has produced a primary fabric with a (010) foliation plane and a [100] lineation axis. This CPO is persistent during subsequent plastic deformation and strain localization and is observed in almost all samples. However, a detailed investigation of internal misorientations measured at subgrains reveals the activity of at least 4 to 5 slip systems in plagioclase grains: , and maybe . The strength of CPO is first increasing from slightly foliated gabbros to mylonites before decreasing significantly in ultramylonites, which could be explained by orientation scattering after subgrain rotation recrystallization and grain boundary processes (e.g., nucleation, grain boundary sliding).

International audience; Crustal architecture at slow-spreading oceanic ridges results from complex interactions between magmatism, hydrothermalism, and tectonics. IODP Hole U1473A was drilled during Expeditions 360 and 362T at the summit of the Atlantis Bank, a gabbroic massif exhumed at the Southwest Indian Ridge. In this study, we identify and quantify plastic deformation processes in gabbroic lithologies and active slip systems in plagioclase from 115 microstructural domains throughout Hole U1473A. We describe deformed zones using petrographic observations and electron backscattered diffraction analyses made all along the core. Ductile deformation is widespread, and in places strongly localized in mylonitic and ultramylonitic zones. Plagioclase represents ∼60% of rock's volume and is the dominant phase accommodating deformation in samples. It shows strong dynamic recrystallization accommodated by subgrain rotation in the dislocation creep regime, forming a fine-grained matrix. Electron backscattered diffraction analyses reveal weak to moderate crystallographic preferred orientations of plagioclase as a result of plastic deformation and strain localization, producing a fabric characterized by (010) parallel to the foliation plane and [100] parallel to the lineation. The fabric strength is first increasing from slightly deformed lithologies to mylonites before decreasing significantly in ultramylonites. This could be explained by orientation scattering after recrystallization, and a change of active slip systems. Subsequent granular flow has likely occurred in some samples. A detailed investigation of intracrystalline misorientations measured at plagioclase subgrain boundaries reveals the activity of four dominant slip systems: [001](010), [100](001), ½[110](001), and ½[111](001). These slip systems reflect decreasing temperatures during CPO development and subgrain wall formation

International audience; The contribution presents microstructural and petrological results obtained on garnet-bearing felsic migmatites from the Khondalite Belt (North China Craton). The study focused on low-melt fraction felsic migmatites in which peritectic garnet are strongly elongated and show plastic deformation. These peritectic garnet grains show different typology depending on their petrographic location within the layered migmatite. An elongated shaped garnet with sillimanite and biotite inclusion lies within Al-rich layer whereas poecilithic garnets are located within S-rich layers, forming a garnet-quartz aggregate. The study presents petrological, microstructural observation and EBSD analyses of the garnet grains and garnet-quartz aggregates. Phase diagram sections modeling allows to constrain the P–T conditions of the deformation at 850–1000 °C and 0.6–1.1 GPa. At such high-to ultra-high temperature conditions, a low melt fraction was produced, between 1 and 12% (generally under 7%). Forescattered diffraction and misorientation maps with Orientation Pole Figure argue for dislocation creep with development of subgrains in garnets. Our results suggest that at high-to ultra-high temperature, the quartz-feldspar-garnet solid framework of felsic migmatites strongly accommodate the deformation and contribute to weakening of migmatite even at low melt fraction.

International audience; Rare metals (Ge, Ga, In, Cd) are key resources for the development of green technologies and are commonly found as trace elements in base-metal mineral deposits. Many of these deposits are in orogenic belts and the impact of recrystallization on rare metal content and distribution in sphalerite needs to be evaluated. Based on laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses, and micro-imaging techniques such as laser-induced breakdown spectroscopy (LIBS) and electron backscattered diffraction (EBSD), we investigate the minor and trace element composition related to sphalerite texture for three types of mineralization from the Pyrenean Axial Zone (PAZ). Vein mineralization (type 2b) appears significantly enriched in Ge and Ga compared to disseminated and stratabound mineralizations (type 1 and type 2a, respectively). In vein mineralization, the partial recrystallization induced by deformation led to the remobilization of Ge, Ga, and Cu from the sphalerite crystal lattice into accessory minerals. We propose that the association of intragranular diffusion and fluid-rock reaction were likely responsible for the formation of patchy-oscillatory zoning in sphalerite, and the crystallization of Ge-rich accessory minerals. Chemical and textural heterogeneity is common in sphalerite from various world-class deposits and a full understanding of these heterogeneities is now crucial to assess the rare metal potential, and associated extraction processes of deformed base-metal ores.

Rare metals are essential to the development of the “green” technologies that are at the core of low-carbon societies. In nature, these metals are frequently present in trace amounts scattered in base metal ore deposits, but the physico-chemical processes that are responsible for their concentration into strategic minerals are still poorly understood. Based on laser-induced breakdown spectroscopy (LIBS), coupled with electron backscattered diffraction (EBSD) analysis, this study shows that plastic deformation and subsequent syntectonic recrystallization of sphalerite (zinc sulfide, ZnS) led to the spatial redistribution of germanium (Ge): from a background level of a few hundreds of parts per million in undeformed primary sphalerite to tens of weight-percent in neocrystallized Ge minerals. During dynamic recrystallization, Ge is likely released from the crystal lattice of parent sphalerite and subsequently concentrated in Ge minerals, leaving behind a Ge-depleted, recrystallized sphalerite matrix. Identifying how rare metals concentrate through deformation and syntectonic recrystallization at the mineral scale is essential to understand the spatial redistribution and localization at the deposit scale. This study highlights the importance of coupling in situ chemical mapping analysis with macro- and microstructural characterization when targeting rare metals in deformed ore.

International audience; The late Carboniferous accretionary system of the South Tianshan orogen (North-Western China) underwent complex structural and polymetamorphic evolution. Combined petrological, geochronological and microstructural analysis of (ultra)high-pressure (UHP) metabasites (eclogites and blueschists) enclosed in metapelites show a relict coarse-grained eclogitic fabric S2 surrounded by a dominant fine-grained eclogite and blueschist facies retrograde fabric S2. The S2 fabric is reworked by upright folds F3 that are responsible for a major shortening of the whole accretionary system. For both the eclogite and blueschist, peak and retrograde P–T conditions have been thermodynamically constrained at 25–26 kbar and 425–500 °C and 10–13 kbar and 500−550 °C respectively, suggesting a shared exhumation history. The garnet-whole rock-amphibole isochron in the blueschist yielded Lu–Hf and Sm–Nd ages of 326.0 ± 2.9 Ma and 318.4 ± 3.9 Ma respectively, interpreted to date the prograde to peak metamorphic assemblage. The retrograde path of the eclogite is characterized by heterogeneous omphacite recrystallization into a mylonitic fine-grained matrix and crystallization of blue amphibole. Microstructures in both pristine porphyroclastic and recrystallized fine-grained domains in the eclogite indicate a gradual evolution from constriction-dominated (L>S-type) to flattening-dominated (S>L-type) type of deformation, increase of fabric intensity reflected by gradually growing M-indexes and the development of lattice preferred orientation (LPO) typical for dislocation creep under slightly hydrated conditions. Recrystallization of the matrix in the blueschist is homogeneous, which indicates a matrix dominated channel flow during exhumation. These LPOs evolutions suggest a significant mechanical coupling with the upper plate concomitant with oroclinal bending of the Kazakh orocline. Lock up of Kazakh orocline is responsible for further stress increase resulting in horizontal shortening of South Tianshan accretionary wedge and development of D3 upright folding and steepening of the whole sequence.

International audience; Germanium (Ge) is often found as trace element in undeformed sphalerite (ZnS). However, the presence of Ge-minerals (oxides, chloritoids and/or sulphides with up to 70 wt% Ge) is remarkable in Pb-Zn deposits from the Variscan Pyrenean Axial Zone. Their abundance is controlled by the chemical and/or the mechanical processes that affect rare element concentration from sulphides which have undergone deformation and metamorphism. In this study, we document the microstructures and chemical heterogeneities in sphalerite, based on EBSD (electron backscatter diffraction) coupled to LA-ICPMS in situ analyses. Deformation induces the dynamic recrystallization of sphalerite. Recrystallized domains have low Ge contents (1-50 ppm Ge) whereas porphyroclastic sphalerite grains commonly show higher Ge concentrations (up to 650 ppm Ge). Ge-minerals (up to 70 wt% Ge) are exclusively hosted by the Ge-poor recrystallized domains. We propose that Ge was removed from the sphalerite crystal lattice during sulphide recrystallization, and was subsequently concentrated in Ge-minerals, leaving behind a Ge-depleted fine-grained recrystallized sphalerite matrix. Numerous sulphide ore types enriched in rare elements like Pyrenean deposits may present recrystallization features and we suggest evaluating the potential of such deposits by integrating chemical and structural informations at the micrometer scale using state-of-the-art analytical techniques in exploration methods.

International audience; Mass balance calculations have been performed through a comparison of published graphical and statistical approaches applied to two contrasted geological settings: (i) the development of a greenschist-facies ductile shear-zone that recorded a weak volume change but significant mass transfers, and (ii) the formation of exoskarns in metacarbonate xenoliths that recorded a large volume decrease related to huge mass transfers. The comparison of the four mass-balance approaches shows that, if uncertainties are ignored, (1) they yield similar results concerning the mobile vs immobile behaviour of many components; (2) they yield similar mass-change values on bulk rock and on individual chemical elements (bulk-rock mass-change values differ by a maximum of ca. 15 % between graphical and statistical treatments of the metacarbonate xenolith evolution). The main difference concerns the uncertainties on mass changes (for bulk rocks and individual elements), which are much larger with the graphical than with the statistical approaches when uncertainties on chemical elements are taken into account, as they should be.The main advantage of the graphical methods is their rapid implementation and the clarity of the diagrams. Their main disadvantages are that uncertainties on each chemical element and bulk compositions are not taken into account and the difficulty in choosing an accurate immobility field to precisely define errors. Graphical methods need to be completed by a statistical treatment that gives absolute mass transfer results. The statistical approaches have the advantage of taking into account the chemical heterogeneities of the compared populations, in conjunction to a precise data treatment. The statistical treatment is an important and necessary step to decipher and to be pertinent in interpreting mobility/immobility of chemical elements, and, thus, in the absolute quantification of mass and volume changes.

International audience; The Wadi Dima area (Oman Ophiolite) exposes partially altered to highly serpentinized harzburgites that are cross-cut by intense (>20 Vol%) carbonate veining. We identified a sequence of 3 types of carbonate veins with compositions ranging from calcite to dolomite (Mg/Ca = 0-0.85). Type 1 carbonates occur as a fine diffuse vein network, locally replacing olivine cores, penetrative into the serpentinized harzburgites. They have depleted trace elements abundances (e.g., Yb

Observations from deep boreholes at several locations worldwide, laboratory measurements of frictional strength on quartzo-feldspathic materials, and earthquake focal mechanisms indicate that crustal faults are strong (apparent friction μ ≥ 0.6). However, friction experiments on phyllosilicate-rich rocks and some geophysical data have demonstrated that some major faults are considerably weaker. This weakness is commonly considered to be characteristic of mature faults in which rocks are altered by prolonged deformation and fluid-rock interaction (i.e., San Andreas, Zuccale, and Nankai Faults). In contrast, in this study we document fault weakening occurring along a marly shear zone in its infancy (

Potassium–argon method on muscovite and biotite and chemical U–Th–Pb method on monazite have been used to date various tectonic and thermal processes affecting the Silesian orogenic wedge at the eastern Variscan front (NE Bohemian Massif). This wedge is composed of three structural units showing an increasing Barrovian metamorphic gradient from the east to the west and LP–HT reworking related to voluminous granite intrusion of the Žulova Pluton in the central part. Three groups of ages are reported: 1) Mississippian (340–320 Ma) K–Ar muscovite ages from the western kyanite zone and easternmost biotite–chlorite zone, ~320 Ma U–Th–Pb ages of monazite inclusions in syn-burial S1fabric preserved in garnets of the kyanite zone, 2) Pennsylvanian–Early Permian (~310–290 Ma) K–Ar ages on muscovite and biotite and matrix monazite from the sillimanite and staurolite zones of the central part of the wedge, 3) Early to Mid-Permian K–Ar muscovite and biotite and U–Th–Pb matrix monazite ages (~280–260 Ma) from the southern part of the area, adjacent to the Sudetic fault system. The sequence of obtained ages is interpreted as reflecting the Mississippian formation of the Silesian orogenic wedge that was followed by crustal-scale detachment related to Pennsylvanian–Early Permian intrusion of a voluminous Žulova Pluton accompanied by important fluid flow. Finally, the southern part of the studied domain was probably reworked by Permian fault system associated with renewed fluid activity.

This paper presents quantitative microstructural and crystallographic preferred orientation (CPO) analyses of an Alpine amphibolite facies shear zone developed in the Fibbia metagranite (Gotthard massif, Central Alps). The weakly deformed metagranite and orthogneiss at the margins of the shear zone are characterized by a bulk strain partitioning between harder coarse-grained monomineralic aggregates, derived from quartz and K-feldspar porphyroclasts, and softer fine-grained plagioclase-bearing shear bands. A characteristic feature is a dilatant fracturing of strong quartz and feldspar aggregates. CPOs and microtextures suggest that quartz and K-feldspar aggregates are dynamically recrystallized via dislocation creep while plagioclases show evidences of fluid-assisted diffusive mass transfer and grain boundary sliding. In the mylonite and ultramylonite shear zone core, the porphyroclasts-derived quartz and K-feldspar layers are broken-down to produce a polyphased matrix that is characterized by a homogeneous micron-scale grain size and regular/random distribution. Here, the deformation of the whole aggregate occurs via a fluid-assisted dissolution–precipitation creep and grain boundary sliding, referred as a fluid-assisted granular flow. We propose a model of shear zone formation associated with the nucleation of shear zone followed by lateral widening of the sheared domain. The lateral broadening of the shear zone is driven by (1) the increase in fluid pressure in permeable albite–oligoclase shear bands that results in expulsion of fluids to the shear zone margins and hydraulic fracturing of strong aggregates, and (2) the thermodynamic re-equilibration via metasomatic reactions of the shear zone walls.

In order to address the question of the processes involved during shear zone nucleation, we present a petro-structural analysis of millimetre-scale shear zones within the Roffna rhyolite (Suretta nappe, Eastern central Alps). Field and microscopic evidences show that ductile deformation is localized along discrete fractures that represent the initial stage of shear zone nucleation. During incipient brittle deformation, a syn-kinematic metamorphic assemblage of white mica + biotite + epidote + quartz precipitated at ca. 8.5 ± 1 kbar and 480 ± 50 °C that represent the metamorphic peak conditions of the nappe stacking in the continental accretionary wedge during Tertiary Alpine subduction. The brittle to ductile transition is characterized by the formation of two types of small quartz grains. The Qtz-IIa type is produced by sub-grain rotation. The Qtz-IIb type has a distinct CPO such that the orientation of c-axis is perpendicular to the shear fracture and basal and rhombhoedric slip systems are activated. These Qtz-IIb grains can either be formed by recrystallization of Qtz-IIa or by precipitation from a fluid phase. The shear zone widening stage is characterized by a switch to diffusion creep and grain boundary sliding deformation mechanisms. During the progressive evolution from brittle nucleation to ductile widening of the shear zone, fluid–rock interactions play a critical role, through chemical mass-transfer, metasomatic reactions and switch in deformation mechanisms.

Alpine deformation in the Grimsel granodiorite (Aar massif, Central Alps) at greenschist facies conditions (6.5 ± 1 kbar for 450� C±2 5� C) is characterized by the development of a network of centimetre to decametre localized shear zones that surround lenses of undeformed granodiorite. Localization of deformation is assumed to be the result of a first stage of extreme localization on brittle precursors (nucleation stage) followed by a transition to ductile deformation and lateral propagation into the weakly deformed granodiorite (widening stage). A paradox of this model is that the development of the ductile shear zone is accompanied by the crystallization of large amounts of phyllosilicates (white mica and chlorite) that maintains a weak rheology in the localized shear zone relative to the host rock so that deformation is localized and prevents shear zone widening. We suggest that chemical processes, and more particularly, the metamorphic reactions and metasomatism occurring during re-equilibration of the metastable magmatic assemblage induced shear zone widening at these P-T-X conditions. These processes (reactions and mass transfer) were driven by the chemical potential gradients that developed between the thermodynamically metastable magmatic assemblage at the edge of the shear zone and the stable white mica and chlorite rich ultramylonite formed during the first stage of shear zone due to localized fluid infiltration metasomatism. P-T and chemical potential projections and sections show that the process of equilibration of the wall rocks (l-l path) occurs via the reactions: kf + cz + ab + bio + MgO + H2O = mu + q + CaO + Na2O and cz + ab + bio + MgO + H2O = chl + mu + q + CaO + Na2O. Computed phase diagram and mass balance calculations predict that these reactions induce relative losses of CaO and Na2 Oo f� 100% and � 40% respectively, coupled with hydration and a gain of � 140% for MgO. Intermediate rocks within the strain gradient (ultramylonite, mylonite and orthogneiss) reflect various degrees of re-equilibration and metasomatism. The softening reaction involved may have reduced the strength at the edge of the shear zone and therefore promoted shear zone widening. Chemical potential phase diagram sections also indicate that the re-equilibration process has a strong influence on equilibrium mineral compositions. For instance, the decrease in Si-content of phengite from 3.29 to 3.14 p.f.u, when white mica is in equilibrium with the chlorite-bearing assemblage, may be misinterpreted as the result of decompression during shear zone development while it is due only to syn-deformation metasomatism at the peak metamorphic condition. The results of this study suggest that it is critical to consider chemical processes in the formation of shear zones particularly when deformation affects metastable assemblages and mass transfer are involved.

A lower amphibolite Alpine shear zone from the Fibbia metagranite (Gotthard Massif, Central Alps) has been studied to better understand the parameters controlling strain localization in granitic rocks. The strain gradient on the metre-scale shows an evolution from a weakly deformed metagranite (QtzI- KfsI-AbI-BtI ±P l II-ZoI-PhgI-Grt) to a fine banded ultramylonite (QtzII-KfsII-AbII-PlII-BtII- PhgII ± Grt-ZoII). Strain localization is coeval with dynamic recrystallization of the quartzofeldspathic matrix and a modal increase in mica, at the expense of K-feldspar. The continuous recrystallization of plagioclase during deformation into a very fine-grained assemblage forming anastomosed ribbons is interpreted as the dominant process in the shear zone initiation and development. The shear zone initiated under closed-system conditions with the destabilization of metastable AbI-ZoI porphyroclasts into fine-grained (20-50 lm sized) AbII-PlII aggregates, and with minor crystallization of phengite at the expense of K-feldspar. The development of the shear zone requires a change in state of the system, which becomes open to externally derived fluids and mass transfer. Indeed, mass balance calculations and thermodynamic modelling show that the ultramylonite is characterized by gains in CaO, FeO and H2O. The progressive input of externally derived CaO drives the continuous metamorphic recrystallization of the fine-grained AbII-PlII aggregate into a more PlII-rich and finer aggregate. Input of water favours the crystallization of phengite at the expense of K-feldspar to form an interconnected network of weak phases. Thus, recrystallization of 50% of the bulk rock volume would induce a decrease of the strength of the rock that might contribute to the development of the shear zone. This study emphasizes the major role of metamorphic reactions and more particularly plagioclase on strain localization process. Plagioclase represents at least one-third of the bulk rock volume in granitic systems and forms a stress- supporting framework that controls the rock rheology. Therefore, recrystallization of plagioclase due to changes in P-T conditions and ⁄ or bulk composition must be taken into account, together with quartz and K-feldspar, in order to understand strain localization processes in granites.

International audience; Several studies have proposed a model for shear zone nucleation in granitoids, in which an initial anisotropy, frequently a brittle structure or a compositional heterogeneity, is required to localize and initiate the ductile shearing in granitoids. This model is referred as an inherited-localization model. This brittle precursor is also the loci of intense fluid-rock interactions that induce mineral and chemical transformations that control the behavior of shear zone. However, Oliot et al. (2010, 2014) suggest that brittle precursors are not necessary required to initiate the ductile shearing in granites. Indeed, the early recrystallization of metastable phases (like plagioclase) into a fine-grained metamorphic assemblage, stable at the P-T-fluid conditions of the deformation, might induce local weakening required for strain localization and shear zone nucleation. We will define this model as the metamorphic-localization model.In both models, shear zone nucleation appears to be associated with metamorphic reactions, fluid flow and mass transfer. The goal of this contribution is to document and quantify the mineralogical and chemical changes involved during the process of shear zone nucleation in granitoids, and to discuss their role. Answering these questions requires to study shear zones in which petrological and microstructural evidences of the deformation have not been obliterated by subsequent ductile deformation. One of the best example is located in the Neves area (Tauern Window, Eastern Alps, Italy) where a Variscan granodiorite is affected by an Alpine amphibolite facies deformation strongly localized on precursor fractures. The studied samples are mm to cm-wide shear zones with distinct mineralogical evolution. In the undeformed granodiorite, the mineralogical assemblage consists of quartz, K-feldspar, biotite and a fine-grained assemblage of albite, epidote and white mica developed at the expense of the metastable magmatic plagioclase. In addition to the breakdown of biotite and K-feldspar into chlorite and phengite, the most noticeable metamorphic reaction is the crystallization of garnet on former plagioclase site. Shape, grain size and chemical zoning of garnet evolve continuously along the strain gradient. The highest strained zone is characterized by the crystallization of Fe-and Ti-oxide while albite and epidote are no more present. The interior of the ultramylonitic shear zone is later reactivated under brittle conditions with the formation of a dense micro-scale network of fractures, which are filled with K-feldspar and epidote. To quantify the chemical mass transfer associated with these reactions, quantified high resolution X-ray mapping has been performed along the strain gradient. In addition, the evolution of chemical zoning of garnet as a function of the strain is used to estimate the P-T conditions of the deformation and model the amount of chemical mass transfer.Although macroscopic field observations clearly suggest that shear zones nucleate on pre-existing brittle fractures, there is no microscopic evidence of cataclasis or shear fracture. Micro-scale observations suggest that the earliest stages of shear zone formation are characterized by large metasomatic recrystallization. These chemical and microstructural changes induce local variations in time and space of the viscosity that will control the subsequent development of the shear zone. For instance the late brittle reactivation in the ultramylonitic shear zone core might be due to a relative hardening induced by garnet crystallization.

Determining the P-T conditions at which metasomatism occurs provides insight into the physical conditions at which fluid-rock interaction occurs in the crust. However, application of thermodynamic modeling to metasomatized rocks is not without pitfalls. As with “normal” metamorphic rocks, the main difficulty is to select mineral compositions that were in equilibrium during their crystallization. This essential task is particularly difficult in metasomatized rocks because it is often difficult to distinguish textures produced by changes in P-T conditions from those caused by fluid-rock interactions and associated changes in bulk composition. Furthermore, the selection of minerals in equilibrium in metasomatized rocks is made difficult by the great variability of scale of mass transfer (see Chaps. 4 and 5), and therefore equilibrium, which varies from micrometer- to hand-sample or larger scale, depending on the amount of fluid involved and the fluid transport mechanisms (e.g. pervasive or focused). Finally, another major limitation that is discussed in detail in Chap. 5, is that fluid composition coming in or out of the rock is unknown. Since fluid is a major phase component of the system, neglecting its impact on the phase relations might be problematic for thermobarometry. Despite these pitfalls, we describe in this contribution examples where thermobarometry has been apparently successfully applied. We emphasize that pseudosection thermobarometry is particularly suitable for metasomatized rocks because the effects of mass transfer can be explored through P-T-X phase diagrams. Application of thermodynamic modeling to metasomatized rocks requires (1) detailed mineralogical and textural investigation to select appropriate mineral compositions, (2) essential geochemical analyses to define the relative and absolute mass changes involved during the metasomatic event(s), and (3) forward modeling of the effects of mass transfer on phase relations.

International audience; This study addresses the mechanical and chemical behavior of allanite during shear zone formation under high-pressure metamorphism. Understanding physico-chemical processes related to the retention or resetting of Pb isotopes in allanite during geological processes is essential for robust petrochronology. Dating of allanite in meta-granodiorite showing variable amounts of strain (from an undeformed protolith to mylonite) at Monte Mucrone (Sesia Zone, NW Italy) gave surprising results. Based on structural and petrographic observations the shear zones at Mt Mucrone are Alpine, yet allanite located within an eclogite facies mylonite yielded Permian ages (208Pb/232Th average age: 287 ± 7 Ma). These mm-sized allanite grains are rimmed by an aggregate of coarse-grained garnet + phengite, thought to derive from former epidote. These aggregates were immersed in a weak matrix that experienced granular flow, and they were thus chemically and mechanically shielded during Alpine mylonitization. In undeformed samples (8a and 8b), two populations of epidote group minerals were found. Allanite forms either coronas around Permian monazite or individual grains with patchy zoning. Both types yield Permian ages (208Pb/232Th age: 291 ± 5 Ma). On the other hand, grains of REE-rich clinozoisite of Cretaceous age are found in undeformed rocks. These grains appear as small fragments with embayed surface outlines and minute satellites or rims around Permian allanite. These (re)crystallized grains are Sr-rich and show mosaic zoning. These results indicate that allanite crystals retained their chemical and isotopic characteristics, and thus their Permian age, as a result of strong strain partitioning between the epidote group porphyroclasts and the eclogite facies matrix in HP-mylonites. The observed partial mobilization of Pb isotopes, which lead to the Cretaceous-aged rims or grains in undeformed samples was facilitated by (re)crystallization of allanite and not by mere Pb diffusion alone under the HP conditions.

International audience; In order to date low-temperature deformation, intensely trained muscovite porphyroclasts and neocrystallized shear band phengite from greenschist-facies shear zones have been dated by 40Ar/39Ar method in the Argentera-Mercantour massif. Shear zones are featured by gradual mylonitization of a Variscan granite, gneiss and Permian pelite protolith (300-315Ma) during the Alpine orogenic event. Mineralogical and textural observations indicate that phengites and chlorites developed from biotite and plagioclase in fluid system during deformation following dissolution-transport-precipitation reactions of the type biotite + plagioclase + aqueous fluid = chlorite + albite + phengite + quartz + titanite + K-bearing fluid in the granite-gneiss mylonite. Contrariwise, phengite developed at the expense of clays following substitution reaction in pelite mylonite. Based on conventional thermobarometry on phengite and chlorite and Pressure-Temperature-aqueous fluid (P-T-MH2O) pseudosections calculated with shear zone bulk compositions, the conditions during shear deformation were estimated at 375±30 °C and 4.8-7±1 kbar in an H2O-satured system. In this low temperature environment, 40Ar/39Ar analysis of the Variscan muscovite for various grades of ductile strain intensity shows a limited 40Ar/39Ar isotopic resetting, all ages scattering between 296 and 315 Ma. Under conditions of intense ductile deformation and large scale fluid circulation, muscovite grains formed during the Variscan retain their much older ages. 40Ar/39Ar dating of very fine grained synkinematic phengite grains, neoformed during the Alpine history, give consistent plateau ages (34-20 Ma) for each shear zone. In detail, 40Ar excess can be detected in the pelite mylonitic sample where phengites crystallized by substitution process while the other mylonitic samples where phengites grow from fluid-induced reactions do not evidence any 40Ar excess. These results demonstrate that the 40Ar/39Ar dating of neocrystallized synkinematic white mica allows the determination of precise ages of deformation and fluid activity. Together with precise thermobarometry undertaken on the basis of mineral chemistry and whole-rock composition, 40Ar/39Ar dating of white mica leads to the reconstitution of precise depth-deformation history of low-grade (b400 °C) metamorphic units. At the Argentera-Mercantour massif scale, several stages of shear zone development at 15-21 km depth are dated between 33 and 20Ma. In the SE part of the massif shear zone ages are well constrained to be either (1) 33.6±0.6 Maor in the range (2) 26.8±0.7Ma-26.3±0.7 Ma. In the West of the massif, younger shear zone ages range between (3) 22.2±0.3 Ma and (4) 20.5±0.3 Ma.

International audience