A AR-40 AR-39 LASER PROBE STUDY OF MICAS FROM THE SESIA ZONE, ITALIANALPS - IMPLICATIONS FOR METAMORPHIC AND DEFORMATION HISTORIES/

Two samples from the Eclogite Micaschist Complex (EMC) and the Seconda Zona Diorito-Kinzigitica (IIDK) of the Sesia Zone have been studied u sing a high-spatial resolution laser probe Ar-40/Ar-39 technique with the aim of investigating the complexities of argon behaviour in metamo rphic rocks and comparing their thermal histories. Data from a single large phengite grain from the EMC show a range of ages from mid-Jurass ic to Upper Cretaceous. These 'apparent age' variations are spatially related to both location within the grain and to intragrain microstruc ture. Modelling of the data shows that the profile formed by the diffu sion of an excess argon component into the grain, parallel to the mica cleavage. Profile asymmetry is explained by temporal variations in mi crostructural development enabling excess argon to enter the grain at different times in different places. The temperatures of the initiatio n of deformation and the possible time-scales for the deformation can be calculated as a function of cooling rate. All estimates suggest def ormation at greenschist facies, in accord with the observed retrograde mineral assemblage. Absolute temperature estimates for deformation va ry by less than 22 degrees C for different cooling rates of 10 and 30 degrees C Ma(-1) but vary by 80 degrees C with different estimates of diffusion parameters. The duration of deformation was for at least 2 M a at 10 degrees C Ma(-1) or 0.7 Ma at 30 degrees C Ma(-1). Biotites fr om the IIDK sample record a Permian to Upper Cretaceous age range that correlates with grain size, the smallest grain sizes yielding the you ngest ages. This relationship is best explained by a partial resetting of biotites during an Alpine thermal event initiated not more than 70 Ma ago. Modelling of these data suggest that the sample never exceede d 300 degrees C during the Alpine. The profoundly different thermal hi stories of the two units - the EMC recrystallized at 550 degrees C whi lst the IIDK remained below 300 degrees C - suggests that they may not have been juxtaposed until much later than the eclogite facies metamo rphism.