Muscovite recrystallization and saddle-shaped 40 Ar/ 39 Ar age spectra: example from the Blond granite (Massif Central, France)

Alexandrov, Pavel; Ruffet, Gilles; Cheilletz, Alain

doi:10.1016/s0016-7037(01)00895-x

Cited by 46 publications

(28 citation statements)

References 24 publications

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“…at temperature lower than the isotopic closure of the K-Ar system in the biotite. Our results in the area of the Joyeuse fault are very similar to those obtained by Alexandrov et al (2002) (1) excess or inherited 40 Ar is not likely because the ages are in the same order than those obtained in the southern Maures Massif (Morillon et al 2000) and in the Tanneron (Demoux et al 2008), (2) the 40 Ar/ 39 Ar plateau ages on muscovites from the surrounding rocks are younger in samples collected at decreasing distances of the Rouet granite (which displays the youngest age in the area), (3) even they allow plateau age calculation, spectra shapes and inconsistent ages in the same sample suggest post-isotopic closure disturbances in muscovite grains which could be the result of recrystallization processes. Thus, we propose that the scattering of the ages between 303 and 318 Ma on both side of the Joyeuse fault (e.g.…”

Section: Age Distribution Around the Joyeuse Faultsupporting

confidence: 91%

“…The age scattering is significantly higher than the error margins of the calculated plateau ages, even in the same sample, and thus should have a geological significance. Even though they allow plateau age calculation, muscovite 40 Ar/ 39 Ar ages around the Joyeuse fault give saddle and staircase shaped spectra, which have been interpreted by many authors as the result of mixing of inherited and neocrystallized white micas due to partial recrystallization (Dunlap et al 1991;Dunlap 1997;Cheilletz et al 1999;Castonguay et al 2001;Alexandrov et al 2002) rather than excess argon component (McDougall and Harrison 1999). Dunlap et al (1991) and Cheilletz et al (1999) argued that the saddle-shaped age spectra could result from the mixing of mineral phases, e.g.…”

Section: Age Distribution Around the Joyeuse Faultmentioning

confidence: 99%

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Exhumation processes during post-collisional stage in the Variscan belt revealed by detailed 40Ar/39Ar study (Tanneron Massif, SE France)

Corsini

Bosse

Féraud

et al. 2008

Int J Earth Sci (Geol Rundsch)

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International audienceDetailed 40Ar/39Ar geochronology on single grains of muscovite was performed in the Variscan Tanneron Massif (SE France) to determine the precise timing of the post-collisional exhumation processes. Thirty-two plateau ages, obtained on metamorphic and magmatic rocks sampled along an east–west transect through the massif, vary from 302 ± 2 to 321 ± 2 Ma, and reveal a heterogeneous exhumation of the lower crust that lasted about 20 Ma during late Carboniferous. In the eastern part of the massif, the closure of the K–Ar isotopic system is at 311–315 Ma, whereas in the middle part of the massif it closes earlier at 317–321 Ma. These cooling paths are likely to be the result of differential exhumation processes of distinct crustal blocks controlled by a major ductile fault, the La Moure fault that separates both domains. In the western part of the massif, the ages decrease from 318 to 303 Ma approaching the Rouet granite, which provides the youngest age at 303.6 ± 1.2 Ma. This age distribution can be explained by the occurrence of a thermal structure spatially associated to the magmatic complex. These ages argue in favour of a cooling of the magmatic body at around 15 Ma after the country rocks in the western Tanneron. The emplacement of the Rouet granite in the core of an antiform is responsible for recrystallization and post-isotopic closure disturbances of the K–Ar chronometer in the muscovite from the host rocks. These new 40Ar/39Ar ages clearly outline that at least two different processes may contribute to the exhumation of the lower crust in the later stage of collision. During the first stage between 320 and 310 Ma, the differential motion of tectonic blocks limited by ductile shear zones controls the post-collisional exhumation. This event could be related to orogen parallel shearing associated with crustal-scale strike-slip faults and regional folding. The final exhumation stages at around 300 Ma take place within the tectonic doming associated to magmatic intrusions in the core of antiformal structures. Local ductile to brittle normal faulting is coeval to Upper Carboniferous intracontinental basins opening

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Section: Age Distribution Around the Joyeuse Faultsupporting

confidence: 91%

Section: Age Distribution Around the Joyeuse Faultmentioning

confidence: 99%

Exhumation processes during post-collisional stage in the Variscan belt revealed by detailed 40Ar/39Ar study (Tanneron Massif, SE France)

Corsini

Bosse

Féraud

et al. 2008

Int J Earth Sci (Geol Rundsch)

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“…Consequently, these rocks too do not seem to have been affected by ductile deformation and dynamic recrystallisation related to the strike-slip fault, the movement of which is dated by synkinematic 277 Ma-old granites (Wang et al 2008a, this issue). Cheilletz et al (1999) and Alexandrov et al (2002) have pointed out that saddle-shaped age spectra, similar to the one obtained for muscovite XJ679, may result from the presence of different reservoirs in partially recrystallised mica grains with distinct argon compositions that degas over a different energy interval: a primary, unrecrystallised or inherited domain (low and high temperature steps) and a newly formed or recrystallised one (saddle minimum in the intermediate to high temperature steps). As suggested by Castonguay et al (2007), the younger subdomains formed by growth or recrystallisation could characterise the last isotopic record during a protracted (re)crystallisation history.…”

Section: Meaning Of Permian 40 Ar/ 39 Ar Agesmentioning

confidence: 59%

“…Since the early laboratory experiments at low temperature (B200°C) and pressure by Gerling and Ovchinnikova (1962) and Kulp and Engels (1963) it is known that interaction of circulating solutions with the lattice of minerals can produce cation exchange and that this process can affect their age. Studies in a number of metamorphic terrains worldwide have revealed that fluid-assisted recrystallisation, which affects ionic bonds in minerals, plays a prominent role during exchange or loss of radiogenic daughter isotopes and hence in isotope resetting (Andriessen 1991;Miller et al 1991;Kerrich and Ludden 2000;Jenkin et al 2001;de Jong et al 2001;Alexandrov et al 2002). Such processes partly occur by low-temperature fluid reaction below the normally used closure temperature, at least for the less robust radiogenic isotope systems (Miller et al 1991;Kerrich and Ludden 2000;Jenkin et al 2001).…”

Section: Permian Ages: Structurally Controlled Fluid-mediated Recrystmentioning

confidence: 99%

New 40Ar/39Ar age constraints on the Late Palaeozoic tectonic evolution of the western Tianshan (Xinjiang, northwestern China), with emphasis on Permian fluid ingress

Jong

Wang

Faure

et al. 2008

Int J Earth Sci (Geol Rundsch)

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154

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International audienceLaser-probe dating of mylonite whole-rock samples from the North Tianshan—Main Tianshan fault zone that cross-cuts the North Tianshan domain's southern margin yielded 40Ar/39Ar spectra with 255–285 Ma ages. Biotite from an undeformed, Early Carboniferous granite, which cuts the steep mylonitic foliation in the Proterozoic basement of the Yili arcs's southern margin, gave a 263.4 ± 0.6 Ma plateau age (1σ). Pre-Carboniferous metasediments overlying this basement yielded plateau ages (1σ) of 253.3 ± 0.3 (muscovite) and 252.3 ± 0.3 (biotite) Ma. The Permian ages of mylonites date movement on these ductile, dextral strike-slip shear zones, whereas the mica ages are interpreted by recrystallisation as a result of fluid flow around such transcurrent faults. We propose that the Tianshan's Permian syn-tectonic bimodal magmatism was created in a non-plume-related Yellowstone-like extensional–transtensional tectonic regime. Gold mineralisation, tracing aqueous flow in the crust, peaked in Permian time and continued locally into the Triassic. The picture is emerging that a convective fluid system partly driven by magmatic heat, existed in a strongly fractured and weakened crust with an elevated heat flow, leading to regional-scale isotope resetting. We suggest that surprisingly young isotopic ages in the literature for early orogenic (ultra)high-pressure metamorphism are similarly due to fluid-mediated recrystallisation

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“…radiogenic argon (Dalrymple et Lamphere, 1969). A mixture of mineral phases can also results in older or younger ages for the first and final steps (Figure V.2.3, B and C) (Alexandrov et al, 2002 ;Wijbrans et McDougall, 1986). Another style of age spectrum is that resulting from a biotite chloritization.…”

Section: V2a U/pb Isotopic System On Monazitementioning

confidence: 99%

Evolução estrutural e térmica de um batólito sin-cinemático no orógenos Neoproterozóico Araçuaí (leste do Brasil)

Mondou¹

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AcknowledgementsMy first acknowledgments are for my two co-directors, Marcos Egydio-Silva, and Alain Vauchez, which permitted me to realize this work. I thank them to took me in a country that I enjoyed discover with them, especially during the field missions. And a special thank for their patience and their disponibility during the preparation of this manuscript and for their comments and reflexions in general.A great thank to Maria Irene Raposo who initiate me to the rock magnetism and help me to complete most of the experiments. Thank you for the discussions about sciences and other.I thank the correctors Ricardo Trindade and Jean-Luc Bouchez for accepting to criticize this work, and the others board members Jérôme Bascou, Maria-Helena Hollanda and Andréa Tommasi for their participation to the reflexion.I also thank all those who participated to this work, people from Geoscience Montpellier and from the Instituto de Geociências.Of course, I thank my family who supported me in my choices. My friends, from Brazil and France, for the precious moments of relaxation. And finally, thank you Lara, your support during this experience has been the greatest help I received. AbstractThe allochtonous domain of the Neoproterozoic Araçuaí belt involves large amounts of magma, widespread partial melting, granulitic facies and high geotherm, characterising this belt as a hot orogen. The Galiléia tonalitic suite, emplaced within host metasediments and deformed at magmatic state, represents a huge batholith that strongly influenced the mechanical behaviour of this middle crust. The anisotropy of magnetic susceptibility (AMS) measured through this batholith and used as a petrofabric proxy, combined to a detailed magnetic mineralogy investigation, permitted to characterize the paramagnetic behaviour of the Galiléia suite and therefore to highlight a complex 3D strain deformation. The observed structures developed within the viscous magma resulted from a combination of tangential tectonics induced by the compression, and gravitational forces arising from the load of the overlying crust. The kinematics of the batholith is compatible with that already described for ductile rocks of hot orogens. U/Pb dating on zircons and monazites together with 40 Ar/ 39 Ar dating on amphiboles, muscovites and biotites permitted to define the thermal evolution of the Galiléia batholith and its host metasediments and constrain the timing of the deformation. The Galiléia batholith emplaced during an important magmatic, tectonic and thermal event at ~580 Ma. Temperature remained high during the first ~50 Ma of the thermal evolution, promoting a seemingly constant deformation of the batholith at magmatic state during several tens of millions years. Such high temperature conditions and stable deformation kinematics during protracted periods of time are supposed to be characteristic of hot orogen. The slow cooling rate of ~10°C/Ma evidenced after ~500 Ma probably indicate a very slow exhumation probably only conducted by erosion. ResumoA faixa Araçuaí, ...

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Muscovite recrystallization and saddle-shaped 40 Ar/ 39 Ar age spectra: example from the Blond granite (Massif Central, France)

Cited by 46 publications

References 24 publications

Exhumation processes during post-collisional stage in the Variscan belt revealed by detailed 40Ar/39Ar study (Tanneron Massif, SE France)

Exhumation processes during post-collisional stage in the Variscan belt revealed by detailed 40Ar/39Ar study (Tanneron Massif, SE France)

New 40Ar/39Ar age constraints on the Late Palaeozoic tectonic evolution of the western Tianshan (Xinjiang, northwestern China), with emphasis on Permian fluid ingress

Evolução estrutural e térmica de um batólito sin-cinemático no orógenos Neoproterozóico Araçuaí (leste do Brasil)

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