PLATEAU AGES AND EXCESS ARGON IN PHENGITES - AN AR-40-AR-39 LASER PROBE STUDY OF ALPINE MICAS (SESIA ZONE, WESTERN ALPS, NORTHERN ITALY)

Bulk samples and single grains of high-pressure white micas and a biot ite from two restricted areas (Mucrone and Marine) 10 km apart in the Sesia Zone (Western Alps, Italy) were analysed by the conventional (st ep-heating) and continuous laser probe (step-heating and spot fusion) Ar-40/Ar-39 techniques, respectively, The analysed minerals crystalliz ed during the Eoalpine, eclogitic facies metamorphism. The phengites f rom 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 p hengites from four samples with different bulk-rock compositions displ ay precise plateau ages, which scatter between 69.4 +/- 0.7 and 76.9 /- 0.6 Ma, and homogeneous age maps, whereas two minerals (phengite an d biotite) from the same rock sample provide two very ''discordant'' p lateau ages, 69.4 +/- 0.7 and 140.5 +/- 0.6 Ma, respectively. In the l ight 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 age s 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 tra pped in defects within the structure of the phengite grains at the tim e 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 ma ps. We propose that the age of closure of the phengites to argon loss is less than or equal to 69.4 +/- 0.7 Ma.