Preservation of Hercynian 40Ar/39Ar ages through high-pressure low-temperature Alpine metamorphism in the Western Alps
European Journal of Mineralogy Volume 2 Number 3 (1990), p. 343 - 361
published: Jun 21, 1990
manuscript accepted: Feb 14, 1990
manuscript received: May 31, 1989
ArtNo. ESP147050203002, Price: 29.00 €
Abstract Pre-Triassic crystalline rocks of the Briançonnais Zone (Western Alps) have been overprinted by an Eocene high-pressure, low-temperature tectono-metamorphic event at about 350-400°C and 7-10 kbar. 40Arl39Ar step-heating studies were performed on micas and glaucophanes from rocks containing both pre-Alpine and Alpine assemblages. Pre-Alpine muscovites and biotites from overprinted high-temperature metamorphic rocks were not substantially affected by argon resetting and indicate ages of 340-360 Ma, which are interpreted as dating their initial closure to argon. White micas from overprinted igneous assemblages exhibit complex age spectra resulting from binary mixing between pre-Alpine muscovite and Alpine phengite. Glaucophane yields a saddle-shaped release pattern with a minimum age of 54 ± 1 Ma, indicating a maximum age for Alpine overprinting and trapping of excess argon. Phengite from the Triassic cover records a plateau-age of 37 ± 1 Ma related to the thermal peak of Alpine metamorphism. Hercynian ages are preserved in micas during the Alpine overprinting, even though temperatures close to or higher than the assumed argon closure temperature for micas were reached. This is a combined effect of the high P/T gradient during the overprinting event, of the short duration of the tectonic and metamorphic processes, and, probably, of low fluid mobility in the basement.