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

Ultra-high pressure eclogite/amphibolite grade metamorphism of the Dora Maira Massif in the western Alps is a well established and intensively studied event. However, the age of peak metamorphism and early cooling remains controversial. The 40Ar-39Ar 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 high pressure event. Recent U/Pb and Sm/Nd results challenge this assertion, indicating a much younger age for the event, around 45 Ma, and hence a radically different timing for the tectonic evolution of the western Alps. In a new approach to the problem, samples from the undeformed Hercynian metagranite, Brossasco, were studied using an ultra-violet laser ablation microprobe technique for 40Ar-39Ar 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 demonstrate the frequent occurrence of excess argon with high 40Ar-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 domains within single mineral phases. The best interpretation of this relationship seems to be that excess argon was incorporated in all phases during the high pressure event, then mixed with an atmospheric component during rapid cooling and retrogression, producing a wide range of argon concentrations and 40Ar-Ar-36 ratios. Step-heating analysis of minerals with this mixture would produce linear arrays on a Ar-36/40Ar versus 39Ar/40Ar correlation diagram, leading to geologically meaningless plateau ages, older than the true closure age. In the present case, some ages in the range 60-110 Ma could be explained by the presence of excess argon incorporated around 40-50 Ma ago. Similar results 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 in many high pressure terrains.