METAMORPHIC EVOLUTION OF THE SESIA-LANZO ZONE, WESTERN ALPS - TIME CONSTRAINTS FROM MULTISYSTEM GEOCHRONOLOGY

The Sesia-Lanzo Zone is a polymetamorphic unit containing Hercynian gr anulite relics overprinted by eclogite and greenschist facies metamorp hism and deformation during the Alpine orogeny. Different parts of the unit record different stages on the P-T-deformation evolution, allowi ng multi-system isotopic studies to unravel the precise timing of the metamorphic history. New Rb-Sr white mica and U-Pb sphene data constra in the age of eclogite facies metamorphism and deformation to 60-70 Ma . This substantially alters the common view of early- to mid-Cretaceou s eclogite facies metamorphism in this unit. The new results are more consistent with the established geotectonic framework for the Alpine o rogeny, since they do not require a prolonged period of depressed geot hermal gradient at a time when the region was in extension. It is also more concordant with recent studies of other units that demonstrate p ost-Cretaceous high-pressure metamorphism. Step-heated Ar-40-Ar-39 ana lysis of phengites yields good plateaux giving ages older than the cor responding Rb-Sr age. Such anomalously high ages indicate the presence of radiogenic argon-rich fluids in the grain boundary network under t he fluid/pressure conditions acting during this high-pressure metamorp hic event. The U-Pb sphene ages are variable in poly metamorphic rocks , and show inheritance of older Pb or sphene crystals into the high-pr essure event. Two monometamorphic assemblages yield concordant ages at 66 +/- 1 Ma, reflecting crystallisation of the eclogite facies assemb lage. The Gneiss Minuti Complex (GMC) lies structurally below the Eclo gitic Micaschists, and its pervasive greenschist facies fabric yields tightly clustered Rb-Sr white mica ages at 38-39 Ma. This greenschist event did not affect the majority of the EMC. The Ar-40-Ar-39 ages of micas formed at this time were very disturbed, whereas micas surviving from an earlier higher pressure assemblage had their Ar-40-Ar-39 syst em reset. The greenschist event did not strongly affect U-Pb systemati cs in Hercynian age sphenes, suggesting that the GMC did not uniformly suffer an eclogite facies metamorphism during the Alpine cycle, but w as juxtaposed against the EMC later in the orogeny. This model still r equires that the locus of deformation and metamorphism (and possibly f luid flux) moved outboard with time, leaving the Sesia-Lanzo basement as a shear-bounded unreactive block within the orogenic wedge.