40Ar/39Ar dating in coesite-bearing and associated units of the Dora Maira massif, Western Alps
Monié, Patrick; Chopin, Christian
European Journal of Mineralogy Volume 3 Number 2 (1991), p. 239 - 262
published: Apr 18, 1991
manuscript accepted: Nov 2, 1990
manuscript received: Mar 29, 1990
ArtNo. ESP147050302004, Price: 29.00 €
Abstract In the Dora Maira massif, internal Western Alps, a coherent coesite-bearing terrain is overlain by coesite-free eclogite- and blueschist-facies units. 40Ar/39Ar step-heating and laser probe studies of Si3.50 to Si3.75 phengites in the least retrogressed high-pressure assemblages from the different units yield similar closure ages of 90-115 Ma (mostly in the range 95-105 Ma), interpreted as reflecting cooling after the peak of metamorphism. Low-Si phengites from greenschist-facies retrograde assemblages give ages of 35-41 Ma which date the main episode of ductile deformation observable in the massif as well as the imbrication of the different units. Younger biotite ages of 28-34 Ma may record the fading away of the low-pressure evolution or a late thermal pulse related to the post-metamorphic magmatism further north in the Sesia and Canavese Zones. Overprinted high-pressure assemblages give complex age spectra resulting from variable amounts of argon loss controlled by deformation, recrystallisation and the chemical evolution of the mica (several generations of high- to low-Si phengites). The same coesite-bearing sample yields an 40Ar/39Ar phengite age of 105 ± 2 Ma, intermediate between its U-Pb zircon age of 121-29+12 Ma and phengite/whole-rock Rb-Sr age of 96 ± 4 Ma. The reversal in the Ar-Ar and Rb-Sr closure ages of phengite is not thought to reflect the presence of trapped excess argon ; it is probably a real feature, suggesting that the closure temperature for phengite Ar-Ar may be occasionally higher than previous estimates of 350-400°C.