, 944 the basement and the sedimentary cover. (e) Enlarged view of mylonitic granite (see panel a for 945 location). (f) Enlarged view of weakly deformed (but altered) granite (see panel a for location
, Unconformity between the basement 948 and the Triassic sediments. (b) Chlorite-quartz bearing veins at the contact between the 949 basement and the basal breccia in the Triassic sediments. (c) Two generations of chlorite-quartz 950 bearing veins within Triassic sandstones and pelites, Figure 3 Field structures in the Triassic sediments. (a)
, backscattered electron images (BSE) (panels b, 953 d, f and h) of granitic basement with increasing deformation, Figure 4 Photomicrographs
, except Wm: white mica; (a) Undeformed but altered granite; (b) BSE image 955 of a Chl + Wm + Ttn pseudomorph after Bt; (c) weakly deformed and altered granite with dark, 2010.
, Chl-Wm pseudomorphs after Bt folded to form kink bands; (d) BSE images of a kinked Chl 957 flake; (e) weakly deformed sample with opening fractures. Chlorite flakes are cross cut by 958 secondary mm-size fractures. (f) BSE image of a Chl + Ttn flake after Bt (Bt relicts still visible), 959 cross cut by a fracture filled with secondary chlorite; (g) mylonite; (h) folded Chl + Wm + Ant 960 flake cross cut by a secondary fracture in a highly deformed sample
, Figure 5 Backscattered images of accessory minerals in the granitic basement. (a) and (b)
, magmatic Ilm breaking down to Ant + Cal, then to Ttn. (c) Anatase 1 folded within a Chl flake 965 and replaced by Ttn. (d) Magmatic zircon (core) with metamict rims (low CL intensity
, Ant, preserved in the core of titanite). (g) Chlorite flake (ChlA) in sample ZA16-79 969 with both types of anatase. (h) Relationships between titanite and anatase in sample, pp.16-128
, Figure 6 Macro-and micro-scale petrography of the Triassic sediments
, Backscattered electron image of panel a, showing the occurrence of anatase, Fe-oxides and Mnz 976 within neocrystallized Chl and Wm. The inset show the patchy texture of monazite grains
, Chlorite-bearing veins and shear planes forming 'en-echelon' microstructures in sample ZA16-978 12. (d) Two generations of chlorite (in veins and shear planes) in samples ZA16-12 (zoom of 979 panel c)
, (b) Quantified SEM map of FeO content in chlorite of 984 sample ZA16-82 (fractured sample). (c) Elementary Al vs Mg (%) content of all analysed 985 chlorites; (d) Trends and compositional groups identified in chlorite in samples collected east 986 of Lake Urdiceto
, Figure 8 Chemical composition of white mica. (a) Compositional triangular plot and trends for 990 white mica in the basement. (b) Compositional ternary diagram of white mica in the 991 sedimentary samples, Mus: muscovite, Cel: celadonite, Pyr: pyrophillite
, Maxime Waldner, vol.3
, ISTeP UMR, vol.8, issue.7193
, , p.38000
, , p.11
, *corresponding author:laura.airaghi@univ-orleans.fr, now at: Université d'Orléans, CNRS-13 INSU, Institut des Sciences de la Terre
, Pre-kinematic greenschist-facies metamorphism is often observed in granites and basement 29 units of mountains belts, but rarely dated and accounted for in orogenic cycle reconstructions
, In the 305 Ma-old Variscan basement of the Bielsa massif 34 (located in the Axial Zone of the Pyrenees), successive fluid-rock interaction events are 35 recorded in granites at temperatures below 350°C. Combined microstructural and petrographic 36 analysis, low-temperature thermobarometry and in situ U-Th/Pb dating of anatase, titanite and 37 monazite show extensive pre-orogenic (pre-Alpine) and pre-kinematic alteration related to 38 feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270-350°C 39 at 230-300 Ma. This event is followed by a second fluid-rock interaction stage marked by new 40 crystallisation of phyllosilicates at temperatures of 200-280°C and associated with the 41 formation of mylonitic shear zones and fractures parallel to the shear planes. U-Pb anatase and 42 monazite ages as well as the microtextural relationships of accessory minerals suggest an age 43 for this event at 40-70 Ma, consistent with independent regional geology constraints. The 44 Variscan basement was therefore softened at late-to post Variscan time, at least 150-200 Ma net of mylonites. The associated deformation, both distributed at the scale of the Bielsa 47 massif and localized at decametric scale in mylonitic corridors, precedes the strain localization 48 along the major thrusts of the Axial zone, Studying pre-kinematic alteration is challenging because of its usual obliteration by subsequent 31 syn-kinematic metamorphism often occurring at conditions typical of the brittle-ductile 32 transition
, Key words: U-Th/Pb anatase-titanite-monazite dating, mylonites, chlorite-white mica 53 thermobarometry
, Granites deformed at the brittle-ductile transition show mineral assemblages characteristic of 58 low-grade metamorphism. Greenschist-facies metamorphic reactions often result in the growth 59 of phyllosilicates (e.g. white mica, chlorite) whose presence strongly affect the rheological 60 behaviour of the upper crust through softening, strain localization and fluid migration
, , 1996.
, , 2004.
, The breakdown of 80 % of feldspar into white 64 mica, 2012.
, Reactions forming low-grade mineral assemblages responsible 66 for crustal softening are generally observed to be syn-kinematic, occurring during the major 67 deformation phases of an orogenic cycle, 1998.
, Static or quasi-static 69 hydrothermal alteration is also observed in granites and has been widely documented, especially 70 in large igneous provinces, 1985.
, At a microscopic scale, pre-kinematic and syn-kinematic transformations are difficult 77 to distinguish, particularly so if the rocks have been deformed through several orogenic cycles
, The timing and conditions of pre-kinematic metamorphic reactions and their effects on (i) the
, syn-kinematic transformation and (ii) the rheological response of the upper crust to successive 80 compressive phases remain therefore poorly constrained. This study addresses these issues by 81 focussing on the Bielsa massif
, This massif is an excellent natural laboratory to study how granitic rocks 83 record distinct low-grade metamorphic events. The massif is part of the central Pyrenean belt, 84 which experienced at least two well-known deformation phases
, The Bielsa massif is located in the most external basement domain of 87 the Pyrenees (implying it experienced the lowest peak metamorphic temperatures with respect 88 to Alpine tectonics), south of the zones of large-scale shear bands that have affected other 89 granitic basement units of Pyrenees, vol.86, 2017.
, raising questions about 92 the link between low-grade metamorphism and strain accommodation mechanisms. Early 93 feldspar destabilisation has been proposed to have a major control on subsequent mechanical 94 softening in Bielsa, 90 addition, structural and microstructural studies show significant distributed shortening at the 91 scale of the massif, 1988.
, In this contribution we estimate the conditions of crystallization of both pre-kinematic and syn-97 kinematic greenschist-facies mineral assemblages in the Bielsa granite and its Triassic 98 sedimentary cover. Microstructural and petrographic analyses were combined with low-99 temperature thermobarometry and in situ U-Th/Pb dating of zircon
, Attempt to date anatase (polymorph of rutile) by the U/Pb method were also undertaken. The 101 implications for the timing of crustal softening relative to the major shortening phases are
, kinematic (pre-Alpine) alteration controls the deformation pattern and strain localisation in the
, The Axial zone (central Pyrenees) is part of a S-verging thrust system made of
, The plutons of the Axial zone (i.e. Bielsa, Neuville, 110 Maladetta massifs etc., Figure 1a) were emplaced into Paleozoic sediments during Late
, , pp.320-300, 1997.
, These plutons and their Paleozoic sedimentary cover constitute the basement to the Permian 113 and Mesozoic sedimentary units (Fig. 1a). Several tectono-metamorphic stages are documented 114 in the metasedimentary units Axial zone but are difficult to distinguish within the basement
, 2015) and related to extensive 118 tectonics centered on the SE Bay of Biscay. Basement thrusts in the central-western Pyrenees 119, pp.20-30, 2014.
, In 121 the hanging wall of the Gavarnie thrust, an older deformation event (c. 48 Ma) is recognised in 122 the massifs of Néouvielle and Eaux Chaudes, pp.32-36, 1998.
, Close to Lake Urdiceto (Figure 1c-d), granitic to granodioritic 127 rocks, the S-verging thrust system structurally located between the Gavarnie 126 and Guarga thrusts (Figure 1b), 2004.
, the Alpine compressional phase (Figure 1d, 2003.
, 2019) with a wavelength of ~1 km. Several sub-vertical S-or N-dipping normal 131 or reverse steep faults bound the antiforms and synforms (Figure 1d)
, They exhibit a sub-vertical to N-dipping cleavage (e.g. Figure 2b 133 and 3c), with nord side up sense of shear kinematic indicators above the sediment-cover
, At the 135 decametric scale, undeformed and weakly deformed granitic rocks alternate with zones of 136 distributed ductile deformation, zones of fracturing and faulting and mylonites (Figures 1c-d 137 and 2a-f). A pervasive N-dipping schistosity is widespread in the basement, 2019.
, In some shear 143 zone, the dips of mylonitic planes varies from S-dipping to N-dipping, with an apparent sense 144 of shear from top-to-north to top-to-south (Figure 2b-d). Some of the shear bands at the 145 boundary between the Triassic sediments and the late Variscan granite (regionally trending 146 N110, Figure 1c) contain deformed fragments of the Triassic red sandstone (Román-Berdiel et 147 al., 2004; Figure 2b), indicating that they are post-Triassic. The high density of mylonitic 148 corridors and the pervasive schistosity development result in a distributed deformation pattern 149 (as observed in other crystalline units as the Aar massif, Mylonitic corridors, vol.168, p.203, 2017.
, At the boundary between the Triassic sediments and the granite/granodiorite, sub-156 horizontal quartz and chlorite-bearing veins which are 1-5 cm-wide are observed (yellow square 157 in Fig. 3b). Fifty meters from the contact, Triassic sediments show two generations of chlorite-158 and quartz-bearing veins: (1) sub-horizontal veins parallel to the ones observed at the contact 159 between the basement and the sedimentary rocks produced by vertical extension
, 162 3. SAMPLE DESCRIPTION AND CHRONOLOGY OF MINERAL ASSEMBLAGES
, Twenty-seven samples were collected along two transects east and west of Lake Urdiceto, from 164 the undeformed granite (e.g. ZA16-84 and B11-16, Figure 4a, b) up to the zone of intense 165 mylonitisation (e.g. ZA16-96, ZA16-98; Figures 1c, 4g)
, In the basement unit, we distinguished the following rock fabrics: Undeformed samples
, In some of those samples the schistosity is associated to mm-scale 175 fractures (e.g. ZA16-82, ZA16-78, ZA16-128, Figure 4e-f and h) that are parallel to the 176 subvertical shear planes observed at an outcrop scale in mylonites (C in Figure 2a), Figure 4a), where the magmatic, euhedral shape of grains is still preserved. Weakly 173 deformed rocks, pp.16-84
, In the samples defined as mylonite, both brittle and ductile deformation features are identified
, All samples, regardless of the degree of the strain, exhibit extensive alteration of the 181 magmatic mineral assemblage originally defined by quartz, K-feldspar and plagioclase, biotite 182 ± amphibole, apatite, zircon (all in textural equilibrium, Figures 4a-b) and ilmenite preserved 183 as inclusions in the less altered amphibole and biotite grains (Figure 5a-b). The alteration is 184 comprised by the breakdown of plagioclase and K-feldspar
, Biotite and amphibole break down to chlorite 186 flakes (ChlA, abbreviations are from Whitney & Evans, 2010) ± white mica (20-100 µm in 187 size) ± prehnite (Figure 4a) and a Ti-bearing phase (anatase or titanite or both, Figure 4b, d, f 188 and g), + albite + calcite ± rare epidote (Figure 4a
, In the undeformed samples, relicts of feldspar, biotite and amphibole are still visible and their 191 euhedral magmatic grain shapes are preserved (Figure 4a)
, Quartz grains contain sparse, 195 micrometric fluid inclusions and are cross-cut by microcracks. Allanite is locally observed in 196 textural equilibrium with the magmatic minerals, Euhedral magmatic zircon grains are found 197 within relicts of feldspar or at the boundaries between feldspar and amphibole
, Quartz grains partly preserve their magmatic texture, but are 206 fractured, with fractures subparallel to the kink axes. Grains are cross-cut by fluid inclusion 207 trails. Originally euhedral grains of ilmenite are first altered to aggregates of anatase + calcite 208 then to titanite (Figure 5a-b), following the reaction proposed by Hansen, Reimink, & Harlov 209 (2010) for low temperature conditions. Anatase is itself replaced by titanite, in textural 210 equilibrium with chlorite flakes (Figures 5a-b). Anatase and titanite are systematically aligned 211 with the chlorite flakes, within the S-planes. When they are folded to form kinks, anatase relicts 212 are mainly preserved in the microfold hinges (Figure 5c), Cathodoluminescence imaging of zircon in samples ZA16-84 and ZA16-119 typically reveals 199 primary oscillatory growth zoning without internal complexity. However, some zircon grains, 200 especially in sample ZA16-119, show metamictization with porous domains and lobate rims 5-201
, These fractures are filled with secondary chlorite (ChlB) ± white 219 mica (Figure 4f). Sheets of ChlB are oriented parallel to the fracture walls in contrast to ChlA 220 grains which are mainly perpendicular (Figure 4f). Furthermore, ChlB is rarely associated with 221 biotite breakdown products as prehnite and Ti-bearing phases (Figure 4f), weakly deformed samples showing millimetric fractures, biotite deformation and 216 stretching along the main schistosity S is more pronounced
, Anatase 225 is observed: (1) as ~70 µm sub-euhedral relicts in titanite cores at the rims of the ChlA flakes 226 (as in the undeformed samples, type-1 Ant, Figure 4h, 5b-c, f-h and Figure S1) and (2) as grains 227 20-30 µm in size not associated with titanite within ChlA flakes along the ancient biotite Page, Titanite is observed in the same microstructural position as in undeformed samples (folded 224 within ChlA flakes), p.10
, Figure 4h, 5f-h and Figure S1). Locally, type-2 anatase is observed 229 within ChlB-bearing fractures (e.g. ZA16-73, Figure 5i). Type-2 anatase of type 2 is often 230 observed close to type-1 anatase. Magmatic accessory minerals such as apatite and zircon are 231
, In mylonitic samples, the schistosity is defined by folded chlorite + white mica flakes 233 and interconnected layers of white mica (Figure 4g). Flakes are folded up to kinks
, and most of them are fractured, with cracks subparallel to the C planes (Figure 4g)
, Within the grains, evidences of ductile deformation are observed such as deformation lamellae. samples. In mylonites (ZA16-96, ZA16-98), only one type of anatase is 240 observed (small anatase grains of type 2 predominate)
, The largest grains of monazite (20-40 µm) (Mnz1) are lobate, and stretched along the C planes 244 with fractures perpendicular to them and exhibit irregular grain boundaries, micrometric large 245 fractures, patchy textures with bright and dark spots in BSE images and 'inclusion-like 246 dissolution figures' (as described by Grand, Homme et al, 2018.
,
, Small monazite grains (~10 µm) with rounded shape 248 share features with that of ZA16-88, aligned along the main foliation or within alteration halos 249 marking the reaction front along secondary fractures subparallel to the C (Figure 5j). In sample 250 ZA16-98 monazite is locally Th and Si-enriched, 2018.
, summary, the granitic rocks experienced a first transformation of the original 253 magmatic mineral assemblage of Qz + Fs + Pl + Bt ± Amph (+ Ilm + Ap + Zr ± Aln) to Qz +
, They therefore 256 predate any deformation stage. ChlB is rarely associated with secondary reaction products and 257 helps to define the C-parallel fractures. It is considered to precipitate from a fluid during a 258 second syn-kinematic (syn C planes) alteration event. The varying characteristics of anatase, 259 only found in deformed samples, suggest that in these samples two generations of anatase may 260 be preserved. That type-2 anatase is located within ChlA flakes along former biotite cleavage 261 suggests that type-2 Ant grew after type-1 Ant on ChlA rims. The chloritization reaction front 262 and replacement is indeed expected to progress from the grain boundaries to the core, along the 263 biotite cleavages. The proximity of type-2 anatase to type-1 anatase grains indicates however 264 transport scale of the anatase-forming elements limited to some dozen of µm, Ttn or Ant then Ttn) ± Pr ± KFs. Zircon and titanite grains with anatase core 255 (type-1 Ant) are stretched in the main schistosity and folded during shearing
, Ant may have crystallized during the shearing. Mnz1 is affected by deformation and mainly 267 observed within the main schistosity, while Mnz2 (and type-2 Ant, see below) are associated
, At the brecciated contact between the Triassic sediments and the chlorite-quartz bearing 273 vein in contact with the basement (Figure 6a), the Triassic sediments are composed of quartz, 274 chlorite, white mica, Fe-oxides and clasts in a calcite matrix (sample ZAL18-11). The vein is 275 composed of mm-size quartz grains recrystallized by bulging and hosting a dense net of fluid 276 inclusions, interlayered with elongate chlorite and white mica
, 20-40 µm large anatase grains, monazite, REE-278 bearing carbonates and tourmaline are occasionally observed (Figure 6b). The textural aspect 279 of monazite in sediments is similar to that of monazite in the basement, chlorite grain boundaries, abundant Fe-oxides
, Chlorite developed along the vein-host rock interface with long axes parallel to the 285 vein opening direction (Figure 6c). Veins are stretched and displaced by 30-50 µm secondary 286 chlorite and quartz-bearing layers cross-cutting the foliation, forming 'en échelon' fractures 287 (Figure 6c-d). Monazite in pelites forms euhedral grains 5-10 µm in diameter, The red-coloured, foliated pelitic rocks are comprised of white mica, chlorite and quartz 281 grains on the scale of tents of microns (± detrital monazite, apatite and anatase, samples ZAL18-282 12 and ZAL18-13)
, MnTiO 3 (Mn, Ti), orthoclase (K, Al), hematite (Fe), albite (Na) and Cr 2 O 3 (Cr) 296 as standards for measuring the elements written in parentheses. The compositional variability 297 of chlorite and white mica in different microstructural sites was characterised by element 298 mapping using both the EPMA and a Zeiss SUPRA-55VP scanning electron microscope (SEM) in energy-dispersive spectroscopy (EDS) mode at 15 keV acceleration voltage, 30 nA Page 174 of 203 XMapTools, CAMECA SX-FIVE electron probe micro-analyser (EPMA) at ISTeP (Sorbonne Université). size, in wavelength-dispersive spectroscopy (WDS) mode, using diopside 295, 2014.
, The analytical protocol and data processing are detailed in Appendix 1
, Titanite, anatase and zircon were dated in-situ by the U-Pb LA-ICP-MS technique at
, Dublin (Ireland), on the same thin sections as used for the microstructural and 311 petrological studies. U-Pb ages were determined using a Photon Machines Analyte Exite 193
, 300 shots were ablated per analysis with 314 a laser spot size of 24 µm (Figure S1). The laser repetition rate and fluency were set at 15 Hz, 315 1.60 J/cm 2 and 11 Hz and 2.25 J/cm 2 for anatase/titanite and zircon respectively, and typically 316 50 analyses were obtained on each sample (usually 1-2 spots per grain) and plotted on a Tera-317 Wasserburg diagram (Tera and Wasserburg, 1972) with 2? error ellipses, Agilent 7900 ICPMS. For titanite, zircon and anatase
, The 321 "VizualAge" DRS was employed for zircon data reduction. Common Pb in the titanite and 322 rutile standards was corrected using the 207 Pb-based correction method. For unknowns, the 323 initial Pb in titanite and anatase was determined from the upper intercept, UcomPbine" data reduction scheme (DRS) of Chew, Petrus, and Kamber, 2014.
, Pb/ 206 Pb axis) of the discordia line defined by the analyses on Tera-Wasserbutg Concordia
, The age for each population of grains was calculated from the lower intercept of the discordia 326 line on Tera-Wasserburg Concordia. line with the Pb evolution model
, The 328 primary standards employed, the secondary standard ages and further details on the analytical 329 procedure are reported in Supplementary Material S1. Titanite and anatase included in ilmenite 330 were not dated due to their small size. When anatase core relicts were preserved in titanite grains 331 (type-1 Ant), it was rarely possible to ablate separately relicts of anatase and their titanite host 332 due to the small grain size (e.g. Figure S1). In anatase-dominated samples, where the bulk of 333 grain was anatase, these analyses were classified and processed as anatase, in titanite dominated 334 samples with only minor relict anatase the analyses were classifies as titanite. The contribution 335 of relicts of anatase to the calculated age was estimated by comparison with results from 336 samples where the only anatase or titanite only were sampled
, Monazite ages were calculated from the EPMA U, Pb and Th contents. The associated 341 detection limits were 90 ppm for Pb, 120 ppm for U and 115 ppm for Th. For each monazite 342 analysis, a chemical U-Th-Pb age was determined (see Montel, 1996.
, The crystallization temperature and the minimum content of XFe 3+ (Fe 3+ /(Fe 2+ + Fe 3+ ))
, from element abundances at a reference pressure 348 of 3 kbar and H 2 O activity at fixed unit value. The absolute uncertainty on the crystallization 349 temperature estimates is around 50°C, 23 selected samples were estimated using the Chl-Qz-H 2 O 347 thermodynamic model of Vidal et, vol.176, p.203, 2006.
, formulated for of pre-kinematic and syn-kinematic chlorite and white mica, 2013.
, Chlorite generally exhibits compositional variation at the µm-scale with high variability in Fe
, Mg and Al contents (especially in discretely deformed granites), as shown in Figure, vol.7
, In samples collected east of Lake Urdiceto, two groups of chlorite are recognized
, The first, named Chl1, is ubiquitous from undeformed granites (pre-kinematic) to mylonites, 361 and mainly observed in ChlA flakes (Figure 7a). correspond, 2006.
, The second group of chlorites, Chl2, is mainly observed in 365 syn-kinematic ChlB (Figure 7a, c-d) and exhibits higher Al content (Al= 2.5-2.86 apfu) and 366 lower Mg content, The Fe content increases slightly from Chl1, pp.15-17
, In undeformed samples, only homogeneous Chl1 in chlorite flakes 368 is observed (e.g. sample ZA16-84). In the most deformed samples, chlorite flakes (ChlA) 369 exhibit µm-scale Mg and Al zoning and portions of Chl1 are pseudomorphically replaced by
, Chl1 is also found as relicts in ChlB in fractures, wrapped by Chl2 (Figure 7a), Chl1 in flakes is Fe-rich (1.97-2.13 apfu) and Mg-poor
, A micron-scale 374 zoning within Chl1 flakes is observed miming cleavage planes of the original magmatic crystal 375 (biotite or amphibole) in discretely deformed samples, while Chl2 within ChlB within fractures homogeneous (Figure 7b), Figure 7b, c, e) while the Al content is typically constant (Table S1)
, In the Triassic sediments, chlorite is Al-enriched compared to chlorite 379 in the basement, but exhibits similar compositional trends (Figure 7c-e), Two groups are 380 recognized in samples ZAL18-13 and ZAL18-12: one is Mg-depleted (1.7-1.97 apfu)
, 68-2.78 apfu) and is observed within the matrix (Figure 7c-e), pp.18-29
, White mica in 386 fractures, veins and in the deformed matrix (syn-kinematic) exhibits lower muscovite (55-70 %) 387 and higher (or similar) celadonite content (20-30 %) than white mica in chlorite flakes (pre-388 kinematic, 70-80 % muscovite, 10-20 % celadonite, Figure 8a). The composition of white mica 389 in sedimentary samples is similar to that of white mica from feldspar in basement samples 390 (Figure 8b), but with lower variability. While in samples ZAL18-12 and ZAL18-11 one single 391 group of white mica is observed, in sample ZAL18-13 two groups are identified: white mica in 392 the veins have higher celadonite contents (18-28%) than in the matrix (10-18%). with the chlorite thermometers of, White mica in the granitic basement shows 55-80 % muscovite (Ms), pyrophyllite (Prl) 385 content in the 0-20% range and up to ~30 % celadonite content (Cel, Figure 8a), vol.178, p.203, 2006.
, The entire iron content was considered 402 divalent for the thermometer of Bourdelle et al. (2013) as recommended in that study. differences between Chl1 and Chl2, samples recording two compositional groups 405 of chlorites, p.220, 2019.
, In samples where dominant chemical variations involve Al and
, Crystallisation 413 temperatures calculated for chlorite in mylonites (e.g. ZA16-96, ZA16-98, ZA16-128, ZA16-414 97) are intermediate between the Chl1 and Chl2 crystallisation temperatures. Two ranges of 415 temperatures were also obtained for chlorite in sedimentary samples (ZAL18-12). The highest 416 temperatures (240-350°C) are generally associated with chlorite in veins, while lowest 417 temperatures (180-280°C) are mostly associated with chlorites in C planes (with variability, 418 especially for samples collected Est and West of the Lake of Urdiceto, Fig. 1c), Chl1 records lower temperatures (170-280°C) than Chl2, vol.409, pp.11-16
, To test for the statistical validity of the two temperature intervals (240-350°C and 180-280°C)
, In the basement, two 423 main peaks at 260 and 330°C persist, 2006.
, The moderate celadonite contents of white mica (accounting for the Tschermark substitution 427 only) suggest crystallization pressures in the range 1-4.5 kbar following the calibration of
, Following this calibration, the higher celadonite content (and Si apfu content) of white 431 mica in fractures, veins and shear bands indicates higher pressure conditions than in white mica 432 growing from feldspar or from biotite, 1988.
, In situ U-Th/Pb dating of magmatic zircon and hydrothermal anatase, titanite and 435 monazite
, Results for titanite, anatase and monazite dating are shown in Figure 10a-c and summarized in 437 Table 1; the raw isotopic ratios and age data for zircon
, The entire geochronological dataset is combined with previous literature age 439 constraints of metamorphic events, derived from other localities of the Axial zone in Figure 440 10d. The composition of monazite together with all concordias for zircon, titanite and anatase 441 are provided in Figure S2. Zircon analyses yield weighted mean Concordia ages at 300-310 Ma 442 (Figure 10d and f)
, Titanite grains contain on average 16 ppm of U, 1.7 ppm of Th and 50 ppm of common
, Titanite in ZA16-84 provides the age constraint with the lowest uncertainty (313 ± 11, 447 MSWD of 1.1, Figure 10a) and exhibits the lowest common Pb content
, The age of anatase is difficult to constrain because of Page 180 of 203 common Pb and low radiogenic Pb contents. When possible, two Tera-Wasserburg 452 concordia lower intercepts for anatase U-Pb age data were defined for each sample, delimiting 453 the possible youngest and oldest grain populations (e.g. Figure 10b). In this case, youngest ages 454 cluster at c. 30-80 Ma while oldest ages cluster at c. 240-280 Ma (Figure 10b, d). The mylonitic 455 granite ZA16-96 exhibits the young age population only (Tera-Wasserburg concordia lower 456 intercept of 39 ± 8 Ma, Anatase contains on average lower amounts of U (8 ppm) and common Pb 449 (24 ppm), and higher Th contents
, The U-Th/Pb ages (with Pb > 461 EPMA detection limits of ~100 ppm) range between 180 and 325 Ma (Figure 10c), Patchy monazite in samples ZA16-98 and ZAL18-11 shows low and variable U and Th 459 contents (Figure 10e and Table S2), 2007.
, Table S2) are observed within Th-depleted monazite grains, indicating late partial replacement 464 of monazite grains by recrystallization and dissolution-reprecipitation (Figure 10e). The oldest 465 ages generally correspond to monazite with relatively high Th content (ThO 2 > 2.4 wt %) and of monazite has a lower Th content
, both samples ZA16-98 and ZAL18-11
, Assuming a Pb 470 content equal to the EPMA detection limit (~90 ppm), a maximum age for this late stage 471 crystallisation has been calculated at c. 100 Ma in sample ZA16-98 and at c. 20 Ma in sample 472 ZAL18-11 for the domains with the highest Th (ThO 2 =7.6 wt.%) and U contents (UO 2 =0.9750 473 wt.%) (Figure 10c)
, from thorite-monazite domains. No ages could be obtained for monazite from the sedimentary
, Estimated crystallisation temperatures are far below the closure temperature of Pb diffusion in 481 rutile (580-630°C), titanite (575-700°C), monazite (~800 °C) and zircon (~900 °C, Rubatto, vol.482, 2017.
, and references therein). Therefore, U-Th/Pb dates are interpreted as 483 crystallisation ages. Zircon is dominantly of magmatic origin as attested by its microtextures, 2017.
, Figure 10d and Table 1), are in agreement with previously published ages, 2017.
, Since metamict domains show 490 typical textures of dissolution-reprecipitation under low-T conditions (Geisler et al., 2003) and 491 are observed in undeformed samples, the c. 290 Ma age may indicate re-opening of the system 492 during the early pre-kinematic fluid-rock interaction
, Titanite ages are less accurate but cluster within uncertainty of the zircon ages, p.300
, Late Variscan) (Figure 10d and Table 1)
, Wasserbourg Concordia diagram is well constrained due to the large variation in U:Pb ratio of 498 the titanite grains (Figure 10a and Figure S2). Therefore, the titanite ages marginally older than 499 their corresponding zircon ages may originate from local incorporation of radiogenic PB at the 500 time of titanite crystallization (see also Essex and Gromet, pp.182-203, 2000.
, This signs the incorporation of radiogenic Pb from the breakdown of a U-504 rich mineral phase such as rutile or anatase, 502 is often significantly lower (i.e. more radiogenic) than the crustal evolution model of Stacey 503 and Kramers, 1975.
, Anatase does not record the 509 magmatic stage since it is observed to replace magmatic ilmenite. If type-1 Ant or titanite 510 recrystallize into type-2 Ant, inter and intra-grain variations in the radiogenic Pb and U content 511 could be inherited from the parent mineral given the small transport scales for anatase-forming 512 elements deduced from microtextural evidence (see section 3.1). Such variations and possible 513 incorporation of type-1 Ant in spot ablations of titanite (see section 4) contribute to the large 514 spread of the anatase ages (e.g. samples ZA16-79, ZA16-73). However, a recurrently young 515 signal of c. 40-80 Ma is exclusively observed only in samples with both types of anatase and in 516 mylonites. As solid-state Pb diffusion can be excluded due to the low metamorphic grade, the 517 youngest U-Pb anatase ages likely indicate partial (re-)crystallization of the smallest anatase 518 grains (type-2 Ant), in agreement with microtextural observations, The U/Pb ages obtained for anatase are less well constrained than the titanite due to 507 higher common Pb, lower radiogenic Pb contents and the smaller spread in U-Pb ratio of the 508 anatase grains
, The two age end-members are shown on Figure 10d for all anatase-bearing samples
, Monazite is unlikely to record the magmatic stage since (1) magmatic allanite and apatite 523 incorporate the rare earth elements and phosphorous necessary for monazite crystallization
, monazite is observed to replace magmatic allanite or apatite and (3) the oldest monazite ages 525 are younger than the zircon ages, Monazite ages in both samples ZA16-98 and ZAL18-11
, The mineral 529 assemblage corresponding to the monazite ages of c. 200 Ma remains difficult to identify as 530 textural and chemical evidences is lacking. The ages of 190-250 Ma obtained in monazite of 531 chlorite-bearing veins in sediments indicate that monazite grains were likely inherited from the 532 underlying basement and partially re-crystallized during vein opening and fluid circulation soon
, Within this temperature range, a first fluid-538 rock interaction event is related to the breakdown of the original magmatic assemblage and the 539 growth of Ttn, Chl1, anatase (type-1 Ant) at 270-350°C. This alteration event predates all 540 deformation stages since the earliest deformation phase deforms this mineral assemblage and 541 the alteration event is also observed in undeformed samples (e.g. Figure 4a-b). During the first 542 alteration event, the source of Al and Ca for chlorite and titanite (± prehnite) growth is the 543 contemporaneous breakdown of plagioclase and the albitization of alkali-feldspar (always 544 largely consumed), irrespective of the strain intensity. Calcium may also derive from external 545 fluids as attested by calcite growth within weakely deformed granite matrix, Detailed microstructural and petrological observations show a poly-phase alteration history in 537 the Bielsa basement at temperatures below 350°C, p.230
, This age range is in agreement with 40 Ar/ 39 Ar ages of biotite of 548 c. 280 and 250 Ma obtained in the Bielsa and Maladeta massifs respectively, 2007.
, The second fluid-rock interaction event is related to the syn-kinematic growth of Chl2 550 during the formation of C-S structures (see section 3) at ~200-280°C. The pervasive schistosity, 551 kinking, shearing and fracturing are likely to have occurred at the same time since they are, vol.184, p.203
, The S-forming stage is therefore post-late Triassic in age at least. 554 In such deformed samples, the youngest ages are yielded by anatase (in samples bearing 555 both types of anatase) and by monazite; this suggests that deformation triggered the partial re-556 crystallization of anatase and monazite. The youngest anatase ages only (c.40 Ma) and the 557 youngest monazite ages (c. 50 Ma) are obtained in mylonites (ZA16-96 and ZA16-98), 552 kinematically compatible. The S planes are observed in both the basement and in the Triassic 553 sediments (see section 2)
, In sample ZA16-79 an age population at c. 120 Ma is 562 observed, 1998.
, Chlorite temperatures calculated with the method of Vidal et al. (2006) and Bourdelle 565 et al. (2013) are consistent despite differences in how they employ the oxidation state of Fe
, 2015) and with temperatures calculated for the overlying sediments, suggesting that fluids 570 were freely circulating between the basement and the overlying sediments, The calculated range of temperatures (200-350°C) is also consistent with hydrothermal 567 conditions typically associated with the appearance of prehnite and titanite in the cleavages of 568 biotite, pp.200-280, 1985.
, Figure 9b), with spatial variations (east and west of Lake Urdiceto, Figure 9a) due to local 574 variations in fluid composition. The systematic differences between temperatures estimated for
, Chl1 and Chl2 support however the existence of two different fluid-rock interaction events, vol.185, p.203
, The moderate celadonite content of white mica, especially in fractures, veins and shear 577 bands (Figure 8a and c) suggests crystallization at a pressure of 1-5 kbar (1 -4.5 kbar, following 578 the formulation of Massone and Schreyer, 1988.
, No paragonite nor margarite 580 components are observed within our white mica dataset. Pyrophyllite substitution, which 581 increases the Si content of muscovite, is not predominant compared to the Tschermak 582 substitution (Figure 8a), 1986.
, , 2007.
, This low-grade petro-chronological dataset allows the timing of low-grade
, metamorphism and deformation to be discussed in the light of the Variscan and Alpine orogenic 590 cycles, with major implications for the crustal rheology and the evolution of the deformation
, Petro-chronological results and microstructural analyses show that Late to post-Variscan lower composition) and amphibole into weaker, hydrated minerals such as white mica and 598 chlorite, without associated deformation (as suggested by, vol.186, p.203, 2019.
, In the Bielsa massif, 602 pre-orogenic sub to lower greenschist-facies metamorphic reactions have two effects: they 603 enhance grain size reduction (e.g. from magmatic feldspars to minute white micas) and also 604 replace strong minerals such as feldspar and amphibole with mechanically weaker 605 phyllosilicates, Both processes are known to result in enhanced 606 softening of the granitic rocks, 1993.
, Early pre-kinematic alteration therefore considerably softened the 608 granite at least 150 Ma before the major Alpine shortening phases. Furthermore, the massive 609 growth of pre-kinematic and pre-Alpine white mica and chlorite indicates high fluid-rock ratios 610 during Late-to post-Variscan times and may strongly influence the water availability in the 611 rocks at the grain boundaries during deformation. This has been shown to have major effects 612 on the mechanisms of strain accommodation in other major phases as quartz, 2010.
, This suggests that granite alteration continued during syn-kinematic Alpine 616 shortening. In mylonites, syn-kinematic chlorite and white mica partly replace the pre-617 kinematic chlorite and mica grains, this time without needing huge supplementary amounts of 618 external fluid. Consequently, in the latter fluid-rock interaction event, there is no evidence for 619 additional large-scale reaction-driven softening outside mylonitic zones, consistent with 620 observations reported for the Néouvielle massif (~ 15 km to the northwest, Wayne & McCaig, 621 1998). The crystallisation of new phyllosilicates and the larger amount of fluid inclusions in 622 quartz in fractured and mylonitic samples as well as the partial age resetting up to, Sericitisation of feldspar and chloritisation of biotite is observed to increase with 615 deformation
, The 627 crystallization of phyllosilicates during a pre-orogenic alteration event may weaken fault zones 628 by at least 30% as phyllosilicates have much lower coefficients of friction than undeformed and 629 unaltered crystalline rocks, Zones of fractures and faults indicate that granitic rocks were also deformed by frictional 626 behaviour, as commonly observed at temperatures of 200-350°C, 2017.
, The pervasive schistosity (S) observed within the Bielsa massif along the studied transects 636 indicates that the deformation is distributed at the scale of the massif (Figure 1b). The existence 637 of mylonitic corridors shows a phase of strain localization with variations in the sense of shear 638 at the kilometric to decametric scale, but their high density also results in a distributed 639 deformation at the massif scale. At Bielsa, both S and C forming stages post-date the Late to 640 post-Variscan pervasive alteration that softened the basement, the Maladeta massif, pp.50-60
, less pervasive pre-kinematic alteration is observed as well as localized 642 deformation and lower density of mylonitic corridors
, This indicates that pre-orogenic crustal softening controlled the deformation 644 patterns observed on a outcrop scale, in particular the frequency of mylonitic bands the location 645 of faults and of fractures, 1994.
, In the Bielsa massif, strain localization in mylonitic bands may have occurred during 647 the intense folding of the basement and the sedimentary cover in the Eocene as indicated by the 648 young ages from the mylonites. During this stage, the re-activation of steep faults may have formed different phase strength contrasts. Consequently, ductile strain and reactions 651 interacted positively to produce narrow ductile shear zones (as described by Holyoke & Tullis, Furthermore, deformation at this stage may strongly influence the breakdown on residual 653 feldspar to muscovite, vol.652, 1985.
, The youngest ages obtained in this study (c. 40 Ma) are older than the activation of the
, This may 659 indicate that the distributed shortening in Bielsa and at a larger scale predates the localization 660 of the strain along some of the major thrust of the Axial zone. These findings are consistent 661 with the distributed / localized modes of shortening documented in the external Western Alps 662 (Bellahsen et al., 2014), in the central Alps (Wehrens, Baumberger, Berger & Herweg, 2017) 663 and in the Longmen Shan where pre-kinematic and a syn-kinematic greenschist-facies 664 metamorphic events were also recognized, Gavarnie and Guarga thrusts (Figure 1b), in the hanging and footwall of the Bielsa unit, dated 657 at 32-36 Ma et 20-30 Ma respectively, 1998.
, , 1986.
, The early alteration stages may be linked to protracted magmatic activity 672 recorded in the Pyrenean domain throughout the entire Permian, 2016.
, Late Variscan magmatic activity has also been invoked as the cause of 674 hydrothermal alteration in the French Massif Central (Cathelineau, 1986.
, Permian to Triassic ages similar to those obtained for fluid-rock interaction in the Bielsa massif have also been documented in the Variscan basement of the Eastern, 1990.
, Iberian Central System and in NE Spain by fluid inclusion and isotopic studies
, The associated hydrothermal 679 alteration in these cases is related to hydrothermal cells formed during extensional tectonic 680 events rather than to plutonism, 681 the Bielsa massif, high fluid-rock ratios have been proposed to result from ingress of seawater 682 and meteoric fluids during the extensional phase following the Variscan orogeny, 2000.
, Although Cretaceous ages 689 were obtained for samples which contain both types of anatase, the large U/Pb age uncertainties 690 for this time period in our petro-chronological dataset mean that strong evidence for extensive 691 fluid circulation, mineral replacement and deformation during Cretaceous times is lacking. This 692 is in contrast to the eastern Pyrenees where a hydrothermal event characterized by newly formed 693 rutile and titanite associated with Cretaceous rifting overprints the 'post, the Pyrenees, the main post-Variscan metamorphic phase 688 related to fluid circulation is dated at 110 Ma, 2000.
, This may indicate that, within the Pyrenean belt, metamorphism related to rifting has only
, The granitic basement rocks in the southern Axial zone of the Pyrenees (Bielsa massif) acted 700 as reactive rocks at low temperatures (T < 350 °C) over a long (c. 300 Ma) polyphase history, vol.190, p.203
, At an outcrop scale, the Bielsa pluton and the Triassic sedimentary cover exhibit a pervasive
, Within the granitic basement, partial mineral replacement and 704 compositional variations at the micron-scale record an extensive fluid-rock interaction event at 705~270-350°C in Late to post-Variscan times (230-300 Ma), as indicated by the titanite, anatase 706 and zircon U/Pb ages dataset. This event is ubiquitous and particularly well preserved in the 707 less deformed areas of the massif. It is responsible for pre-kinematic (pre-Alpine) plagioclase 708 and amphibole replacement with growth of phyllosilicates, some tens of million years after 709 pluton emplacement. A second syn-kinematic fluid-rock interaction event is marked by the 710 (re)crystallisation of phyllosilicates at ~200-280°C and is related to the formation of mylonitic 711 corridors and fractures parallel to the C shear planes. In the most deformed samples, despite the 712 low radiogenic Pb content
, As the strength of phyllosilicate-rich 715 material is lower by up to an order-of-magnitude than that of the original granite, the granitic 716 massif was significantly softened (and hydrated) at least 150-200 Ma before the major Alpine 717 shortening phase. The dense network of mylonites in the Bielsa pluton resulted in distributed 718 deformation at the massif scale before the localization of the strain along the major thrusting 719 systems of the Axial zone. Pre-orogenic crustal softening controlled the deformation patterns effort can provide valuable information allowing
, This study was founded by the CNRS-BRGM-TOTAL 'OROGEN' project
, Duchene and the anonymous review for their constructive comments and remarks
, We thank Michel Fialin, Nicolas Rividi and Omar Boudouma for their help with the electron 731 microscopy at ISTeP. DC acknowledges support from Science Foundation Ireland grants
, Research Centre). iCRAG is funded under 733 the SFI Research Centres Programme and is co-funded under the European Regional
, We also thank Gary O'Sullivan (Trinity College) for his support with the 735 LA-ICP-MS analysis
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, Additional Supporting Information may be found online in the supporting information tab for
, Tera-Wasserburg diagrams for all analysed anatase, titanite and zircon
, Original istopic data for zircon, titanite and anatase. Graphs represents weighted maps and cross section. (a) Schematic structural map of the Pyrenees, Red square: Bielsa, 933 N: Néouvielle, M: Maladeta, vol.932, 2004.
, Geological map of the studied area. (d) Cross sections along transects AA' and BB
, yellow: shear planes (C). (b) Second example of mylonitic 940 corridor a few hundred meters eastward of transect BB', at the contact with the Triassic 941 sediments. Schistosity (S, filled white lines) is subparallel to S-dipping shear planes (C, marked 942 in yellow) due to high strain. (c) Mylonitic bands within the basement with C planes varying 943 from S-dipping to N-dipping. (d) N-dipping shear planes in a mylonite at the contact between 944 the basement and the sedimentary cover. (e) Enlarged view of mylonitic granite (see panel a for 945 location). (f) Enlarged view of weakly deformed, Figure 2 Field structures around Lake Urdiceto. (a) Mylonitic corridor along the transect BB' 939 of Figure 1c. White: schistosity (S)
, Unconformity between the basement 948 and the Triassic sediments. (b) Chlorite-quartz bearing veins at the contact between the 949 basement and the basal breccia in the Triassic sediments. (c) Two generations of chlorite-quartz 950 bearing veins within Triassic sandstones and pelites, Figure 3 Field structures in the Triassic sediments. (a)
, backscattered electron images (BSE) (panels b, 953 d, f and h) of granitic basement with increasing deformation. Abbreviations are from Whitney 954 & Evans (2010), except Wm: white mica; (a) Undeformed but altered granite; (b) BSE image 955 of a Chl + Wm + Ttn pseudomorph after Bt, Figure 4 Photomicrographs
, Chl-Wm pseudomorphs after Bt folded to form kink bands; (d) BSE images of a kinked Chl 957 flake; (e) weakly deformed sample with opening fractures. Chlorite flakes are cross cut by 958 secondary mm-size fractures. (f) BSE image of a Chl + Ttn flake after Bt (Bt relicts still visible), 959 cross cut by a fracture filled with secondary chlorite; (g) mylonite; (h) folded Chl + Wm + Ant 960 flake cross cut by a secondary fracture in a highly deformed sample
, Figure 5 Backscattered images of accessory minerals in the granitic basement. (a) and (b)
, Anatase 1 folded within a Chl flake 965 and replaced by Ttn. (d) Magmatic zircon (core) with metamict rims (low CL intensity) in 966 sample ZA16-119. (e) Chlorite flakes (ChlA) with type-1 Ant cross cut by ChlB-bearing 967 fractures with type-2 Ant (sample ZA16-73). (f) Type-2 Ant rimming titanite after anatase 968 (type-1 Ant, preserved in the core of titanite). (g) Chlorite flake (ChlA) in sample ZA16-79 969 with both types of anatase, magmatic Ilm breaking down to Ant + Cal, then to Ttn. (c)
, Figure 6 Macro-and micro-scale petrography of the Triassic sediments
, Backscattered electron image of panel a, showing the occurrence of anatase, Fe-oxides and Mnz 976 within neocrystallized Chl and Wm. The inset show the patchy texture of monazite grains
, Chlorite-bearing veins and shear planes forming 'en-echelon' microstructures in sample ZA16-978 12. (d) Two generations of chlorite (in veins and shear planes) in samples ZA16-12 (zoom of 979 panel c)
, (b) Quantified SEM map of FeO content in chlorite of 984 sample ZA16-82 (fractured sample). (c) Elementary Al vs Mg (%) content of all analysed 985 chlorites; (d) Trends and compositional groups identified in chlorite in samples collected east 986 of Lake Urdiceto
, Figure 8 Chemical composition of white mica. (a) Compositional triangular plot and trends for 990 white mica in the basement. (b) Compositional ternary diagram of white mica in the 991 sedimentary samples, Mus: muscovite, Cel: celadonite, Pyr: pyrophillite
, Figure 9 Thermobarometric results. (a) Chlorite temperatures calculated with the method of
, ) (rectangles). (b) Monte Carlo simulation Page, vol.202, p.203, 2006.
, 995 for the temperature dataset calculated for basement chlorite (3000 iterations, see the text for 996 details). (c) Pressure range estimated from the celadonite content in white mica, following the 997 calibration of Massonne and Schreyer, 1988.
, Figure 10 Geochronology results. (a) Representative Tera-Wasserburg concordia lower 1002 intercept age defined by titanite analyses in an undeformed sample (ZA16-84). (b) Tera
, Wasserburg Concordia lower intercept ages defined by various anatase populations (red: old, 1004 blue: young) in a deformed sample (ZA16-128). (c) Frequency distribution of monazite ages
, Note the 1007 ages calculated for domains where Pb is below the EPMA detection limit are maximum ages. 1008 (d) Summary of geochronological results (this study) integrated with previously published data 1009 for metamorphic events in the Axial zone. U-Pb Ant-Ttn indicate the ages obtained for analysis 1010 where a titanite locally
, Th content of monazite in sample ZAL18-11. (f) Probability density plot of all zircon ages for 1012 samples ZA16-84 and ZA16-119
, Table 1 Summary of sample descriptions and ages