Excess 40Ar-free phengite in ultrahigh-pressure metamorphic rocks from the Lago di Cignana area, Western Alps

Ar/Ar analyses of phengites and paragonites from the ultrahigh-pressure inetamorphic rocks (zoisite-clinozoisite schist, garnet-phengite schist and piemontite schist) in the Lago di Cignana area, Western Alps were carried out with a laser probe step-heating method using single crystals and a spot dating method on thin sections. Eight phengite and two paragonite crystals give the plateau ages of 37-42 Ma with 96-100% of Ar-39 released. Each rock type also contains mica crystals showing discordant age spectra with age fractions (20-35 Ma) significantly younger than the plateau ages. Phengite inclusions in garnet give ages of 43.2 +/- 1.1 Ma and 44.4 +/- 1.5 Ma, which are significantly older than the spot age (36.4 +/- 1.4 Ma) from the matrix phengites, and the plateau ages from the step-heating analyses. Inclusion ages (43 and 44 Ma) are consistent with a zircon SHRIMP age (44 +/- 1 Ma) in this area. These results suggest that the oceanic materials that underwent a simple subduction related UHPM, form excess Ar-40-free phengite and that the peak metamorphism is ca. (44)Ma or little older. We suggest that matrix phengites experienced a retrogression reaction changing their chemistry contemporaneously with deformation related to the exhumation of rocks releasing significant radiogenic Ar-40 from the crystals. This has lead to the apparent ages of the matrix phengites that are significantly younger than the inclusion age. (c) 2006 Elsevier B.V. All rights reserved.