EVIDENCE FOR EXCESS ARGON DURING HIGH-PRESSURE METAMORPHISM IN THE DORA-MAIRA MASSIF (WESTERN ALPS, ITALY), USING AN ULTRA-VIOLET LASER-ABLATION MICROPROBE AR-40-AR-39 TECHNIQUE

Ultra-high pressure eclogite/amphibolite grade metamorphism of the Dor a Maira Massif in the western Alps is a well established and intensive ly studied event. However, the age of peak metamorphism and early cool ing remains controversial. The Ar-40-Ar-39 step-heating and laser spot ages from high pressure phengites yield plateau ages as old as 110 Ma which have been interpreted as the time of early cooling after the hi gh pressure event. Recent U/Pb and Sm/Nd results challenge this assert ion, indicating a much younger age for the event, around 45 Ma, and he nce a radically different timing for the tectonic evolution of the wes tern Alps. In a new approach to the problem, samples from the undeform ed Hercynian metagranite, Brossasco, were studied using an ultra-viole t laser ablation microprobe technique for Ar-40-Ar-39 dating. The new technique allowed selective in situ analysis, at a spatial resolution of 50 mu m, of quartz, phengite, biotite and K-feldspar. The results d emonstrate the frequent occurrence of excess argon with high Ar-40-Ar- 36 ratios (1000-10000) and a strong relationship between apparent ages and metamorphic textures. The highest excess argon ratios are always associated with high closure temperature minerals or large diffusion d omains within single mineral phases. The best interpretation of this r elationship seems to be that excess argon was incorporated in all phas es during the high pressure event, then mixed with an atmospheric comp onent during rapid cooling and retrogression, producing a wide range o f argon concentrations and Ar-40-Ar-36 ratios. Step-heating analysis o f minerals with this mixture would produce linear arrays on a Ar-36/Ar -40 versus Ar-39/Ar-40 correlation diagram, leading to geologically me aningless plateau ages, older than the true closure age. In the presen t case, some ages in the range 60-110 Ma could be explained by the pre sence of excess argon incorporated around 40-50 Ma ago. Similar result s found in other high pressure terrains in the Alps may reconcile the argon geochronometer with other systems such as Rb/Sr, U/Pb or Sm/Nd. This study therefore calls for an increasing use of high resolution in situ sampling techniques to clarify the meaning of (40)A/(39)A ages i n many high pressure terrains.