Exhumation rates in the Gran Paradiso Massif (Western Alps) constrained by in situ U–Th–Pb dating of accessory phases (monazite, allanite and xenotime)

Manzotti, Paola; Bosse, Valérie; Pitra, Pavel; Robyr, Martin; Schiavi, Federica; Ballèvre, Michel

doi:10.1007/s00410-018-1452-7

Cited by 51 publications

(40 citation statements)

References 97 publications

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“…Rates and duration of metamorphism -in particular those of HP metamorphism that bring invaluable insight into deeply buried terranes -are key parameters to understand the geodynamic evolution of orogens. Numerous studies have attempted to constrain the age of HP metamorphism and the subsequent exhumation history by combining several geochronometers, such as U-Pb in zircon, monazite, rutile or titanite, and Lu-Hf and/or Sm-Nd in garnet (e.g., Anczkiewicz et al, 2007Kylander-Clark et al, 2007, 2008Cheng et al, 2008Cheng et al, , 2009Cheng et al, , 2012Warren et al, 2012;Colett et al, 2018;Lotout et al, 2018;Manzotti et al, 2018;Butler et al, 2018, Cheng et al, 2018. The link between these ages and the metamorphic history of the rock, i.e.…”

Section: Introductionmentioning

confidence: 99%

From Burial to Exhumation: Emplacement and Metamorphism of Mafic Eclogitic Terranes Constrained Through Multimethod Petrochronology, Case Study from the Lévézou Massif (French Massif Central, Variscan Belt)

Lotout

Poujol

Pitra

et al. 2020

Journal of Petrology

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Linking mineral growth and time is required to unravel the evolution of metamorphic rocks. However, dating early metamorphic stages is a challenge due to subsequent retrograde overprinting. A fresh eclogite and a former eclogite retrogressed under amphibolite facies from the southern French Massif Central (Lévézou massif, Variscan belt) were investigated with a large panel of geochronometers (U–Pb in zircon, rutile and apatite, Lu–Hf and Sm–Nd in garnet) in a petrological context tightly constrained by petrographic observations, trace element analyses and phase equilibrium modelling. Both samples recorded similar HP conditions at 18–23 kbar and 680–800°C, whereas the retrogressed eclogite later equilibrated at 8–9·5 kbar and c.600°C. In the retrogressed sample, most of the zircon grains are characterized by negative Eu anomalies and HREE enrichment, and yield an Ordovician U–Pb date of 472·3 ± 1·7 Ma, interpreted as the emplacement age of the mafic protolith. In agreement with other data available for the Variscan belt, and based on zircon trace element record and whole-rock geochemistry, this age is considered to represent the magmatism associated with the extreme thinning of the continental margins during the Ordovician. In the same sample, a few zircon rims show a weaker HREE enrichment and yield a date of 378 ± 5·7 Ma, interpreted as a prograde pre-eclogitic age. Lu–Hf garnet dating from both samples yields identical dates of 357 ± 13 Ma and 358·0 ± 1·5 Ma inferred to approximate the age of the high-pressure metamorphic peak. Fresh and retrogressed samples yield respectively 350·4 ± 7·7 Ma and 352 ± 20 Ma dates for Sm–Nd garnet dating, and 367·8 ± 9·1 Ma and 354·9 ± 9·5 Ma for U–Pb rutile dating. Apatite grains from the retrogressed sample give a mean age of 351·8 ± 2·8 Ma. The similarity between all recorded ages from distinct chronometers and radiometric methods (U–Pb, rutile, apatite; Lu–Hf, garnet; Sm–Nd, garnet) combined with P–T estimations from high-pressure metamorphic rocks equilibrated under different conditions testifies to very fast processes that occurred during the Variscan orogeny, highlighting a major decompression of 15–8·5 kbar in less than 7 Myr, and suggesting mean exhumation rates in excess of 6·3 mm/yr.

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Section: Introductionmentioning

confidence: 99%

From Burial to Exhumation: Emplacement and Metamorphism of Mafic Eclogitic Terranes Constrained Through Multimethod Petrochronology, Case Study from the Lévézou Massif (French Massif Central, Variscan Belt)

Lotout

Poujol

Pitra

et al. 2020

Journal of Petrology

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“…It records high‐pressure (HP) metamorphic peaks ranging between 440–550 °C and 1.2–1.5 GPa in continental crust slices (Austroalpine klippen) and in metaophiolite units (Dal Piaz et al, and references therein). Ages of the high‐pressure metamorphic peaks in the subduction complex are comparable among different units, ranging between 49 and 40 Ma in the Austroalpine klippen and between 45 and 42 Ma in metaophiolites (Dal Piaz et al, ) and around 42 Ma in the Penninic Gran Paradiso units (e.g., Manzotti et al, ). These eclogitic peak ages differ from those revealed by metaophiolites from the northern Aosta Valley (e.g., 65 Ma in the Valtournenche area) (Rebay et al, ) or in the Sesia Lanzo Zone (e.g., 79–65 Ma) (Rubatto et al, ), suggesting that the subducted tectonic units in this region of the Western Alps attained their maximum P‐T imprint diachronously.…”

Section: Introductionmentioning

confidence: 99%

Structural Evolution and Metasomatism of Subducted Metaophiolites in the Northwestern Alps

et al. 2019

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A subduction complex of the northwestern Alps consists of serpentinites, eclogitic metagabbros, flysch-like metasediments, meta-ophicarbonates, and gneissic slices. Unlike other subduction complexes, it contains unusual hybridized rocks described here for the first time in the northwestern Alps. They are preserved as patches interstitial in the metagabbro and as layers within metagabbros and serpentinites. The hybridized rocks are made of high modal zoisite/clinozoisite + white mica pseudomorphs of lawsonite, garnet, and amphibole associated with an Alpine eclogite-facies fabric. While these eclogitic metagabbros are chemically comparable to oceanic oxide gabbros from the ultraslow Southwest Indian Ridge, the layers are extremely enriched in Al 2 O 3 and CaO and depleted in TiO 2 , MgO, and SiO 2 relative to metagabbros. Patches have a geochemical signature that is intermediate between that of layers and metagabbros. Trace element compositions of hybridized rocks suggest a contribution from a fluid derived from a mixed source made of sediments and serpentinites. Except for Ba, Rb, and K, layers are comparable to the global subducting sediments, indicating a sedimentary contribution, whereas the enrichment in Cr indicates a serpentinite contribution. Metasediment dehydration and chemical exchange of Ca and Sr have resulted in significant lawsonite crystallization in the subduction zone, as reflected by the ubiquitous presence of lawsonite pseudomorphs. In light of the unique textures and geochemical signature of the lawsonite pseudomorph-bearing hybridized rocks, an origin by fluid-rock interaction and Ca-metasomatism in the subduction environment is inferred and considered in the Western Alps context.

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“…When chemical variations are related to isotopic variations, it is then possible to distinguish different stages in the petrological evolution of the host rock and then to its geodynamic history (e.g. Manzotti et al 2018). Each different, coexisting monazite generations records a point in the P-T-A-X-D-t path of a rock, disentangling its polymetamorphic evolution.…”

Section: (Th+u)-pb Petrochronometrymentioning

confidence: 99%

“…As a recent example, Manzotti et al (2018) propose to evaluate the exhumation rate of the Gran Paradiso and Money Units (Western Alps) by studying the relative timing of the growth and dissolution of the accessory phases. Combining thermodynamic modelling with HP inclusion, textural and chemical data from both major and accessory phases, these authors were able to date the metamorphic peak and retrograde evolution by fixing P-T conditions of the crystallization episodes of allanite, monazite and xenotime (Fig.…”

Section: Example 4 Polyclism and Polymetamorphismmentioning

confidence: 99%

“…Note the presence of HP inclusions, which confirm the crystallization of the monazite during HP metamorphism conditions. Money Units (a) and Gran Paradiso (b), modified after Manzotti et al (2018). BSE images and Y X-ray maps of matrix monazites in the Rhodope samples (Didier et al 2014) Circles show the location of the LA-ICPMS pits (11 µm) and their corresponding 208Pb/232Th ages (2σ level).…”

Section: Figure Captionsmentioning

confidence: 99%

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Petrochronology and hygrochronology of tectono-metamorphic events

Bosse

Villa

2019

Gondwana Research

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U-Th-Pb petrochronology is based on the incontrovertible fact that the diffusion of radiogenic Pb is negligibly small relative to retrograde reaction rates. Multi-element maps demonstrate that patchy textures tightly correspond to (U+Th)-Pb age variations, requiring that fluid-induced dissolution/ reprecipitation is the principal cause of Pb mobility. Attempts to model intracrystalline core-rim Pb zonations as diffusive transport are not legitimate unless genuine bell-shaped diffusion profiles in minerals can be documented, which happens only exceptionally. Monazite and zircon intra-grain age maps confirm that coupled dissolution-reprecipitation and retrogression reactions assisted by fluids control (Th+U)-Pb ages, not temperature. The chemical zonations observed in many (Th+U)-bearing mineral chronometers (e.g. monazite, allanite, xenotime, zircon) provide petrological constraints. Linking petrology with textures and the isotope record allows reconstructing entire segments of the P-T-AX -D-t history of a rock and its geodynamic environment. The dearth of mathematically sound diffusion profiles equally applies to the isotope record of micas and feldspars. The tight link between petrology, microtextures, chemical composition and geochronology also pertains to Rb-Sr and K-Ar. Overdetermined multi-mineral Rb-Sr isochrons with excess scatter, and spatially resolved/stepwise release 39 Ar-40 Ar results, demonstrate ubiquitous correspondence between relict phases and isotopic inheritance. Many rock-forming minerals are highly retentive of Sr and Ar, unless they are obliterated by retrograde reactions. The rates of dissolution in fluid-controlled reactions are several orders of magnitude faster at upper and mid-crustal levels than diffusive reequilibration rates. Thus, as a rule Rb-Sr and K-Ar chronometers date their own formation. Accurately establishing P-T paths of monometamorphic rocks requires assessing petrologic equilibrium using multivariate thermodynamic software. Dating complex parageneses of polymetamorphic, unequilibrated rocks requires labor-intensive disentangling by: (i) qualitative identification of relicts, retrogression reactions, and chemically open systems by imaging techniques (e.g. cathodoluminescence, element maps, etc.); (ii) microchemical analyses at the µm-scale quantifying heterochemical disequilibrium phases and assigning them to a P-T-AX segment; (iii) spatially resolved/stepwise release, relating the chemical signature of the analyzed mineral to its age. K-Ar and Rb-Sr usually provide a different perspective on the P-T evolution of a rock than does (Th+U)-Pb, as K+Rb-rich minerals (phyllosilicates and especially feldspars) mostly form later and react/dissolve faster in the retrograde path than U-rich accessory phases (e.g. Mukai et al., 2014). The present paper reviews these general principles by means of well-understood examples, both successful and insuccessful in matching the independently known external constraints.

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Exhumation rates in the Gran Paradiso Massif (Western Alps) constrained by in situ U–Th–Pb dating of accessory phases (monazite, allanite and xenotime)

Cited by 51 publications

References 97 publications

From Burial to Exhumation: Emplacement and Metamorphism of Mafic Eclogitic Terranes Constrained Through Multimethod Petrochronology, Case Study from the Lévézou Massif (French Massif Central, Variscan Belt)

From Burial to Exhumation: Emplacement and Metamorphism of Mafic Eclogitic Terranes Constrained Through Multimethod Petrochronology, Case Study from the Lévézou Massif (French Massif Central, Variscan Belt)

Structural Evolution and Metasomatism of Subducted Metaophiolites in the Northwestern Alps

Petrochronology and hygrochronology of tectono-metamorphic events

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