Cenozoic deformation in the Tauern Window (Eastern Alps) constrained by in situ Th-Pb dating of fissure monazite

Ricchi, Emmanuelle; Bergemann, Christian A.; Gnos, Edwin; Berger, Alfons; Rubatto, Daniela; Whitehouse, Martin J.; Walter, Franz

doi:10.5194/se-11-437-2020

Cited by 7 publications

(12 citation statements)

References 61 publications

(135 reference statements)

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“…Epidote U−Pb ages in samples Grimsel-1 and Grimsel-2 yield (early) Miocene ages of 19.2 ± 4.3 and 16.9 ± 3.7 Ma, respectively. These ages are within the uncertainty of each other and can be attributed to the early deformation in the area between 22 and 17 Ma (Handegg phase; Challandes et al, 2008;Rolland et al, 2009). This is corrobo-rated by the presence of green biotite associated with epidote in the epidote-bearing veins (Challandes et al, 2008;Rolland et al, 2009;Wehrens et al, 2017).…”

Section: Geological Constraints On Epidote U−pb Agessupporting

confidence: 56%

“…These ages are within the uncertainty of each other and can be attributed to the early deformation in the area between 22 and 17 Ma (Handegg phase; Challandes et al, 2008;Rolland et al, 2009). This is corrobo-rated by the presence of green biotite associated with epidote in the epidote-bearing veins (Challandes et al, 2008;Rolland et al, 2009;Wehrens et al, 2017). Notably, Grimsel-1 and Grimsel-2 epidote samples yield initial 207 Pb/ 206 Pb ratios that are identical within uncertainty and indicate an inherited radiogenic component.…”

Section: Geological Constraints On Epidote U−pb Agesmentioning

confidence: 53%

“…The Grimsel area is in the Aar Massif, one of the external crystalline massifs of the Alps (Rolland et al, 2009;Wehrens et al, 2017;Herwegh et al, 2020). Here, epidotequartz veins are common in the central Aar granite and in the Grimsel granodiorite, which during the earliest Permian intruded into a polycyclic basement bearing evidence of Ordovician metamorphism and Variscan overprint (Schaltegger and Corfu, 1992;Berger et al, 2017).…”

Section: Geological Context and Field Relationsmentioning

confidence: 99%

“…(1) the Handegg phase (22-17 Ma;Challandes et al, 2008), with stable green biotite in the shear zones (Challandes et al, 2008;Rolland et al, 2009;Wehrens et al, 2017); (2) the Oberaar phase in the southern Aar Massif (14-3.4 Ma; Hofmann et al, 2004;Rolland et al, 2009), with white mica and chlorite stable in the shear zones and metastable biotite Wehrens et al, 2017); and (3) the Pfaffenchopf phase in the northern Aar Massif (< 12 Ma; Herwegh et al, 2020). The epidotequartz veins analyzed in this study were sampled in the Nagra Felslabor tunnel at Grimsel Pass.…”

Section: Geological Context and Field Relationsmentioning

confidence: 99%

“…This may raise questions as to whether the age of 19.2±4.3 Ma dates the formation of Grimsel-1 epidote or the resetting of the U−Pb geochronometer by interaction with a fluid assisting the Alpine deformation. However, epidote is associated with green biotite, hinting that the vein epidote formed during the Handegg phase (22-17 Ma) in the stabil-ity field of green biotite (Challandes et al, 2008;Rolland et al, 2009;Wehrens et al, 2017). Therefore, the age of 19.2±4.3 Ma is interpreted as the crystallization age of epidote.…”

Section: Epidote Ages As Time Of Crystallization In Low-temperature Veinsmentioning

confidence: 99%

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U − Pb geochronology of epidote by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) as a tool for dating hydrothermal-vein formation

et al. 2021

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Abstract. Epidote – here defined as minerals belonging to the epidote–clinozoisite solid solution – is a low-μ (μ=238U/204Pb) mineral occurring in a variety of geological environments and participating in many metamorphic reactions that is stable throughout a wide range of pressure–temperature conditions. Despite containing fair amounts of U, its use as a U−Pb geochronometer has been hindered by the commonly high contents of initial Pb, with isotopic compositions that cannot be assumed a priori. We present a U−Pb geochronology of hydrothermal-vein epidote spanning a wide range of Pb (3.9–190 µg g−1), Th (0.01–38 µg g−1), and U (2.6–530 µg g−1) contents and with μ values between 7 and 510 from the Albula area (eastern Swiss Alps), from the Grimsel area (central Swiss Alps), and from the Heyuan fault (Guangdong Province, China). The investigated epidote samples show appreciable fractions of initial Pb contents (f206=0.7–1.0) – i.e., relative to radiogenic Pb – that vary to different extents. A protocol has been developed for in situ U−Pb dating of epidote by spot-analysis laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) with a magmatic allanite as the primary reference material. The suitability of the protocol and the reliability of the measured isotopic ratios have been ascertained by independent measurements of 238U/206Pb and 207Pb/206Pb ratios, respectively, with quadrupole and multicollector ICP-MS applied to epidote micro-separates digested and diluted in acids. For age calculation, we used the Tera–Wasserburg (207Pb/206Pb versus 238U/206Pb) diagram, which does not require corrections for initial Pb and provides the initial 207Pb/206Pb ratio. Petrographic and microstructural data indicate that the calculated ages date the crystallization of vein epidote from a hydrothermal fluid and that the U−Pb system was not reset to younger ages by later events. Vein epidote from the Albula area formed in the Paleocene (62.7±3.0 Ma) and is related to Alpine greenschist-facies metamorphism. The Miocene (19.2±4.3 and 16.9±3.7 Ma) epidote veins from the Grimsel area formed during the Handegg deformation phase (22–17 Ma) of the Alpine evolution of the Aar Massif. Identical initial 207Pb/206Pb ratios reveal homogeneity in Pb isotopic compositions of the fluid across ca. 100 m. Vein epidote from the Heyuan fault is Cretaceous in age ( 107.2±8.9 Ma) and formed during the early movements of the fault. In situ U−Pb analyses of epidote returned reliable ages of otherwise undatable epidote–quartz veins. The Tera–Wasserburg approach has proven pivotal for in situ U−Pb dating of epidote, and the decisive aspect for low age uncertainties is the variability in intra-sample initial Pb fractions.

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Section: Geological Constraints On Epidote U−pb Agessupporting

confidence: 56%

Section: Geological Constraints On Epidote U−pb Agesmentioning

confidence: 53%

Section: Geological Context and Field Relationsmentioning

confidence: 99%

Section: Geological Context and Field Relationsmentioning

confidence: 99%

Section: Epidote Ages As Time Of Crystallization In Low-temperature Veinsmentioning

confidence: 99%

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U − Pb geochronology of epidote by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) as a tool for dating hydrothermal-vein formation

et al. 2021

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Ion microprobe dating of fissure monazite in the Western Alps: insights from the Argentera Massif and the Piemontais and Briançonnais Zones

et al. 2020

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

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Episodes of fissure formation in the Alps: connecting quartz fluid inclusion, fissure monazite age, and fissure orientation data

et al. 2021

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Fluid assisted Alpine fissure-vein and cleft formation starts at prograde, peak or retrograde metamorphic conditions of 450–550 °C and 0.3–0.6 GPa and below, commonly at conditions of ductile to brittle rock deformation. Early-formed fissures become overprinted by subsequent deformation, locally leading to a reorientation. Deformation that follows fissure formation initiates a cycle of dissolution, dissolution/reprecipitation or new growth of fissure minerals enclosing fluid inclusions. Although fissures in upper greenschist and amphibolite facies rocks predominantly form under retrograde metamorphic conditions, this work confirms that the carbon dioxide fluid zone correlates with regions of highest grade Alpine metamorphism, suggesting carbon dioxide production by prograde devolatilization reactions and rock-buffering of the fissure-filling fluid. For this reason, fluid composition zones systematically change in metamorphosed and exhumed nappe stacks from diagenetic to amphibolite facies metamorphic rocks from saline fluids dominated by higher hydrocarbons, methane, water and carbon dioxide. Open fissures are in most cases oriented roughly perpendicular to the foliation and lineation of the host rock. The type of fluid constrains the habit of the very frequently crystallizing quartz crystals. Open fissures also form in association with more localized strike-slip faults and are oriented perpendicular to the faults. The combination of fissure orientation, fissure quartz fluid inclusion and fissure monazite-(Ce) (hereafter monazite) Th–Pb ages shows that fissure formation occurred episodically (1) during the Cretaceous (eo-Alpine) deformation cycle in association with exhumation of the Austroalpine Koralpe-Saualpe region (~ 90 Ma) and subsequent extensional movements in association with the formation of the Gosau basins (~ 90–70 Ma), (2) during rapid exhumation of high-pressure overprinted Briançonnais and Piemontais units (36–30 Ma), (3) during unroofing of the Tauern and Lepontine metamorphic domes, during emplacement and reverse faulting of the external Massifs (25–12 Ma; except Argentera) and due to local dextral strike-slip faulting in association with the opening of the Ligurian sea, and (4) during the development of a young, widespread network of ductile to brittle strike-slip faults (12–5 Ma).

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Cenozoic deformation in the Tauern Window (Eastern Alps) constrained by in situ Th-Pb dating of fissure monazite

Cited by 7 publications

References 61 publications

U − Pb geochronology of epidote by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) as a tool for dating hydrothermal-vein formation

U − Pb geochronology of epidote by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) as a tool for dating hydrothermal-vein formation

Ion microprobe dating of fissure monazite in the Western Alps: insights from the Argentera Massif and the Piemontais and Briançonnais Zones

Episodes of fissure formation in the Alps: connecting quartz fluid inclusion, fissure monazite age, and fissure orientation data

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