Dating polygenetic metamorphic assemblages along a transect across the Western Alps

Villa, Igor Maria; Bucher, Stefan; Bousquet, Romain; Kleinhanns, Ilka C; Schmid, Stefan M (2014). Dating polygenetic metamorphic assemblages along a transect across the Western Alps. Journal of petrology, 55(4), pp. 803-830. Oxford University Press 10.1093/petrology/egu007

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Multichronometric analyses were performed on samples from a transect in the French-Italian Western Alps crossing nappes derived from the Briançonnais terrane and the Piemonte-Liguria Ocean, in an endeavour to constrain the high-pressure (HP) metamorphism and the retrogression history. 12 samples of white mica were analysed by 39Ar-40Ar stepwise heating, complemented by 2 samples from the Monte Rosa 100 km to the NE and also attributed to the Briançonnais terrane. One Sm-Nd and three Lu-Hf garnet ages from eclogites were also obtained. White mica ages decrease from ca. 300 Ma in the westernmost samples (Zone Houillère), reaching ca. 300 °C during Alpine metamorphism, to < 48 Ma in the internal units to the East, which reached ca. 500 °C during Alpine orogeny. The conventional “thermochronological” interpretation postulates Cretaceous Eo-Alpine HP metamorphism and younger “cooling ages” in the higher-temperature samples. However, Eocene Lu-Hf and Sm-Nd ages from the same samples cannot be interpreted as post-metamorphic cooling ages, which makes a Cretaceous eclogitization untenable. The age date from this transect require instead to replace conventional “thermochronology” by an approach combining age dating with detailed geochemical, petrological and microstructural investigations.
Petrology reveals important mineralogical differences along the transect. Samples from the Zone Houillère mostly contain detrital mica. White mica with Si > 6.45 atoms per formula unit becomes more abundant eastward. Across the whole traverse, HP phengitic mica forms the D1 foliation. Syn-D2 mica is Si-poorer and associated with nappe stacking, exhumation, and hydrous retrogression under greenschist facies conditions. D1 phengite is very often corroded, overgrown or intergrown by syn-D2 muscovite. Most importantly, syn-D2 recrystallization is not limited to S2 schistosity domains; microchemical fingerprinting shows that it also can form pseudomorphs after crystals that could be mistaken to have formed during D1 based on microstructural arguments alone. Thereby the Cl concentration in white mica is a useful discriminator, since D2 retrogression was associated with a less saline fluid than eclogitization.
Once the petrological stage is set, geochronology is straightforward. All samples contain mixtures of detrital, syn-D1 and syn-D2 mica, and retrogression phases (D3) in greatly varying proportions according to local pressure-temperature-fluid activity-deformation conditions. The correlation of age vs. Cl/K clearly identifies 47 ± 1 Ma as the age of formation of syn-D1 mica along the entire transect, including the Monte Rosa nappe samples. The inferred age of the greenschist-facies low-Si syn-D2 mica generation ranges within 39-43 Ma, with local variations. Coexistence of D1 and D2 ages, and the constancy of non-reset D1 ages along the entire transect, are strong evidence that the D1 white mica ages are very close to formation ages. Volume diffusion of Ar in white mica (activation energy E = 250 kJ/mol; pressure-adjusted diffusion coefficient D’0 < 0.03 cm2 s-1) has a subordinate effect on mineral ages compared to both prograde and retrograde recrystallization in most samples. Eocene Lu-Hf and Sm-Nd garnet ages are prograde and predate the HP peak.

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