Constraining deformation phases in the Aar Massif and the Gotthard Nappe (Switzerland) using Th-Pb crystallization ages of fissure monazite-(Ce)

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

doi:10.1016/j.lithos.2019.04.014

Cited by 21 publications

(15 citation statements)

References 44 publications

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“… 2009 ) as well as monazites and adularia from associated clefts (Bergemann 2017 ; Ricchi et al. 2019 ). D3 caused intense massif-internal deformation and led to the development of the highly localised, massif-internal structural relief as observed in Fig.…”

Section: Rscm Results and Interpretationmentioning

confidence: 99%

“… 2017 ; Ricchi et al. 2019 ). The abundance of strike-slip dominated faults decreases from west to east.…”

Section: Field Datamentioning

confidence: 99%

“… 2012 ; Rauchenstein-Martinek 2014 ; Ricchi et al. 2019 ) the age of shearing was estimated at 22–14 Ma. In the southern central Aar Massif, thermodynamic calculations suggest shearing at a temperature of and at a pressure of 6.5 kbar (Challandes et al.…”

Section: Geological Setting and Previous Workmentioning

confidence: 99%

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Structural and thermal evolution of the eastern Aar Massif: insights from structural field work and Raman thermometry

et al. 2021

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The thermo-kinematic evolution of the eastern Aar Massif, Swiss Alps, was investigated using peak temperature data estimated from Raman spectroscopy of carbonaceous material and detailed field analyses. New and compiled temperature-time constraints along the deformed and exhumed basement-cover contact allow us to (i) establish the timing of metamorphism and deformation, (ii) track long-term horizontal and vertical orogenic movements and (iii) assess the influence of temperature and structural inheritance on the kinematic evolution. We present a new shear zone map, structural cross sections and a step-wise retrodeformation. From $$\text{ca.\;26\,Ma}$$ ca.\;26\,Ma onwards, basement-involved deformation started with the formation of relatively discrete NNW-directed thrusts. Peak metamorphic isograds are weakly deformed by these thrusts, suggesting that they initiated before or during the metamorphic peak under ongoing burial in the footwall to the basal Helvetic roof thrust. Subsequent peak- to post-metamorphic deformation was dominated by steep, mostly NNW-vergent reverse faults ($$\text{ca.}$$ ca. 22–14 Ma). Field investigations demonstrate that these shear zones were steeper than $$50^{\circ}$$ 50 ∘ already at inception. This produced the massif-internal structural relief and was associated with large vertical displacements (7 km shortening vs. up to 11 km exhumation). From 14 Ma onwards, the eastern Aar massif exhumed “en bloc” (i.e., without significant differential massif-internal exhumation) in the hanging wall of frontal thrusts, which is consistent with the transition to strike-slip dominated deformation observed within the massif. Our results indicate 13 km shortening and 9 km exhumation between 14 Ma and present. Inherited normal faults were not significantly reactivated. Instead, new thrusts/reverse faults developed in the basement below syn-rift basins, and can be traced into overturned fold limbs in the overlying sediment, producing tight synclines and broad anticlines along the basement-cover contact. The sediments were not detached from their crystalline substratum and formed disharmonic folds. Our results highlight decreasing rheological contrasts between (i) relatively strong basement and (ii) relatively weak cover units and inherited faults at higher temperature conditions. Both the timing of basement-involved deformation and the structural style (shear zone dip) appear to be controlled by evolving temperature conditions.

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Section: Rscm Results and Interpretationmentioning

confidence: 99%

“… 2017 ; Ricchi et al. 2019 ). The abundance of strike-slip dominated faults decreases from west to east.…”

Section: Field Datamentioning

confidence: 99%

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Structural and thermal evolution of the eastern Aar Massif: insights from structural field work and Raman thermometry

et al. 2021

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“…In this context, the role of fluids in the continental crust is particularly relevant because they mediate and influence deformation and meta-124 V. Peverelli et al: U−Pb geochronology of epidote by LA-ICP-MS morphism (e.g., Wyllie, 1977;Etheridge et al, 1983;Johannes, 1984;Pennacchioni and Cesare, 1997;Malaspina et al, 2011;Wehrens et al, 2016). The formation of a hydrothermal vein represents a specific deformation and hydration event in the geological history of the host rock, during which the vein-filling minerals record the geochemical signature of the mineralizing fluid (e.g., Elburg et al, 2002;Barker et al, 2009;Bons et al, 2012;Parrish et al, 2018;Ricchi et al, 2019Ricchi et al, , 2020. By combining different geochemical and geochronological techniques with suitable vein-filling minerals, it is therefore possible to determine when the vein formed and the isotopic signature of the fluids for insight into their origin (e.g., Pettke et al, 2000;Barker et al, 2006;Elburg et al, 2002 hereafter referred to as epidote, produces common rock-forming and vein-filling minerals (e.g., Bird and Spieler, 2004;Franz and Liebscher, 2004;Guo et al, 2014;Zanoni et al, 2016).…”

Section: Introductionmentioning

confidence: 99%

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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“…The formation of a hydrothermal vein represents a specific deformation and hydration event in the geological history of the host rock, during which the vein-filling minerals record the geochemical signature of the mineralizing fluid (e.g. Elburg et al, 2002;Barker et al, 2009;Bons et al, 2012;Parrish et al, 2018;Ricchi et al, 2019;. By combining different geochemical and geochronological techniques to suitable vein-filling minerals it is therefore possible to determine when the vein formed and the isotopic signature of the fluids for insight into their origin (e.g.…”

Section: Introductionmentioning

confidence: 99%

U–Pb geochronology of epidote by LA–ICP–MS as a tool for dating hydrothermal-vein formation

Peverelli

Ewing

Rubatto

et al. 2020

Preprint

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Abstract. Monoclinic epidote is a low-µ (µ = 283U / 204Pb) mineral occurring in a variety of geological environments, participating in many metamorphic reactions and 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 U–Pb geochronology of hydrothermal-vein epidote spanning a wide range of Pb (3.9–190 µg g−1), Th (0.009–38 µg g−1) and U (2.6–530 µg g−1) contents and with µ values between 7–510 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 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 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 by 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–238U / 206Pb) diagram, which does not require corrections for initial Pb and provides the initial 207Pb / 206Pb ratio if all intra-sample analyses are co-genetic. 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 (19.1 ± 4.0 Ma and 16.9 ± 3.7 Ma) epidote veins from the Grimsel area formed during the Handegg 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. 200 m. Vein epidote from the Heyuan fault is Cretaceous in age (108.1 ± 8.4 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.

show abstract

Constraining deformation phases in the Aar Massif and the Gotthard Nappe (Switzerland) using Th-Pb crystallization ages of fissure monazite-(Ce)

Cited by 21 publications

References 44 publications

Structural and thermal evolution of the eastern Aar Massif: insights from structural field work and Raman thermometry

Structural and thermal evolution of the eastern Aar Massif: insights from structural field work and Raman thermometry

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 LA–ICP–MS as a tool for dating hydrothermal-vein formation

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