MG/FE AND [(MG,FE)SI-AL2] COMPOSITIONAL CONTROL ON ARGON BEHAVIOR IN HIGH-PRESSURE WHITE MICAS - A 40AR/39AR CONTINUOUS LASER-PROBE STUDY FROM THE DORA-MAIRA NAPPE OF THE INTERNAL WESTERN ALPS, ITALY

High-pressure white micas from a restricted area in the Dora-Maira nappe of the western Alps (Italy) were analyzed by the conventional 40Ar/39Ar bulk-sample step-heating method and by the step-heating and the spot-fusion 40Ar/39Ar continuous laser-probe techniques on single grains. High-Si, Ferich phengites from fine-grained gneisses display homogeneous 35-40 Ma plateau and spot-fusion ages, consistent with the upper Eocene 40Ar/39Ar phengite ages documented over the whole internal western Alps. In contrast, phengite separates and single-grains of high-Si and Mg-rich to intermediate composition selected from metapelitic and Mg-rich mica schist samples yield older discordant apparent ages, even when sampled in the same outcrop as the fine-grained gneisses. These local age variations are interpreted to reflect primarily an increasing Ar retentivity with increasing Mg content in phengites. In metapelite-derived white micas, the isotopic discordance is marked by (1) a characteristic bulk-sample upward-convex spectrum shape, (2) single-grain disturbed age spectra, (3) a systematic radially outward decrease in spot ages across individual mica cleavage planes, and (4) strong grain-to-grain spot-age variations in rock sections. In Mg-rich mica schists, the isotopic discordance is observed as (1) a reproducible upward-convex bulk-sample age spectrum with high (140 Ma) intermediate apparent ages, (2) upward-convex single-grain age spectra, and (3) irregular intra- and intergrain age gradients. On geochemical grounds, it is argued that the isotopic discordance occurring in the metapelites is mainly controlled by the compositional changes introduced by the [(Mg,Fe)Si-Al2] celadonitic re-equilibration of the high-pressure phengites during greenschist overprinting. In Mg-rich phengites, this chemical re-equilibration may induce Ar loss below their presumed higher blocking temperature. A model of Ar behaviour in high-pressure white micas is proposed which explains the age variations recorded throughout the different lithologies in terms of a simple interaction between the Mg/Fe compositional control on Ar retentivity and the [(Mg,Fe)Si-Al2] modifications occurring in phengites during progressive overprinting.