Dating Polygenetic Metamorphic Assemblages along a Transect across the Western Alps

Igor M. Villa, Stefan Bucher, Romain Bousquet, Ilka C. Kleinhanns, Stefan M. Schmid
2014 Journal of Petrology  
Multichronometric analyses were performed on samples from a transect in the French^Italian Western Alps crossing nappes derived from the Brianc°onnais terrane and the Piemonte^Liguria Ocean, in an endeavour to date both high-pressure (HP) metamorphism and retrogression history.Twelve samples of white mica were analysed by 39 Ar^4 0 Ar stepwise heating, complemented by two samples from the Monte Rosa nappe 100 km to the NE and also attributed to the Brianc°onnais terrane. One Sm^Nd and three
more » ... Sm^Nd and three Lu^Hf garnet ages from eclogites were also obtained. White mica ages decrease from c. 300 Ma in the westernmost samples (Zone Houille' re), reaching c. 3008C during Alpine metamorphism, to 548 Ma in the internal units to the east, which reached c. 5008C during the Alpine orogeny. The spatial pattern of Eocene K^Ar ages demonstrates that Si-rich HP white mica records the age of crystallization at 47^48 Ma and retains Ar at temperatures of around 5008C. Paleocene^early Eocene Lu^Hf and Sm^Nd ages, recording prograde garnet growth before the HP peak, confirm eclogitization in Eocene times. Petrological and microstructural features reveal important mineralogical differences along the transect. All samples contain mixtures of detrital, syn-D 1 and syn-D 2 mica, and retrogression phases (D 3 ) in greatly varying proportions according to local variations in the evolution of pressure^temperature^fluid activity^deformation (P^T^aD ) conditions. Samples from the Zone Houille' re mostly contain detrital mica. The abundance of white mica with Si46·45 atoms per formula unit increases eastward. Across the whole traverse, phengitic mica grown during HP metamorphism defines the D 1 foliation. Syn-D 2 mica is more Si-poor and associated with nappe stacking, exhumation, and hydrous retrogression under greenschist-facies
doi:10.1093/petrology/egu007 fatcat:ycoibrqdp5glhmqemc3fxoozde