PLATEAU AGES AND EXCESS ARGON IN PHENGITES - AN 4039ARAR LASER PROBE STUDY OF ALPINE MICAS (SESIA ZONE, WESTERN ALPS, NORTHERN ITALY)

Bulk samples and single grains of high-pressure white micas and a biotite from two restricted areas (Mucrone and Marine) 10 km apart in the Sesia Zone (Western Alps, Italy) were analysed by the conventional (step-heating) and continuous laser probe (step-heating and spot fusion) Ar-40/Ar-39 techniques, respectively, The analysed minerals crystallized during the Eoalpine, eclogitic facies metamorphism. The phengites from the Mucrone area display ages which scatter between similar to 104 and similar to 180 Ma. In the Marine area (a 50-m-wide outcrop) the phengites from four samples with different bulk-rock compositions display precise plateau ages, which scatter between 69.4 +/- 0.7 and 76.9 +/- 0.6 Ma, and homogeneous age maps, whereas two minerals (phengite and biotite) from the same rock sample provide two very ''discordant'' plateau ages, 69.4 +/- 0.7 and 140.5 +/- 0.6 Ma, respectively. In the light of the present knowledge of the history of the Alps, ages as old as 180 Ma are unrealistic and are related to excess argon, whereas ages in the range 70-80 Ma, perhaps even 140 Ma, could be significant. We suggest that excess argon, probably conveyed by a fluid phase and trapped in defects within the structure of the phengite grains at the time of their isotopic closure, is the best explanation for the observed spread in apparent ages on a regional to single grain scale, in spite of a nearly universal existence of plateau ages and homogeneous age maps. We propose that the age of closure of the phengites to argon loss is less than or equal to 69.4 +/- 0.7 Ma.