New data on low-grade metamorphism in the Briançonnais domain of the Prealps, Western Switzerland
Jaboyedoff, Michel; Thelin, Philippe
European Journal of Mineralogy Volume 8 Number 3 (1996), p. 577 - 592
published: Jun 17, 1996
manuscript accepted: Jan 16, 1996
manuscript received: Dec 6, 1994
ArtNo. ESP147050803010, Price: 29.00 €
Abstract The nappes of the Median Prealps (Switzerland) originated from the Briançonnais and Subbriançonnais domains and are currently located on the northern margin of the Helvetic nappes. This unit is classically divided into the plastic and rigid zones of the Median Prealps. Numerous measurements by XRD of the Scherrer width at half-peak height (001 reflection of illite) and of the illite/smectite ratio in mixed-layer minerals, show evidence for an increase in the degree of metamorphism from diagenesis in the plastic zone of the Median Prealps to very lowΛow-grade conditions in the trailing part of the rigid zone, along a NW-SE cross section. Detailed work on illite/smectite (I/S) mixed-layer minerals was performed on samples from plastic Median Prealps; their XRD patterns (air-dried and glycolated) were modelled using the NEWMOD program in order to determine the proportions of illite and smectite components in mixed-layer minerals. Both the illite content of the I/S clays and the size of coherent diffracting domains increase during deep diagenesis and very low-grade metamorphism. The characterization of the illite/smectite mixed-layer minerals and of illites using the Scherrer width allows the identification of regional trends varying from diagenesis to very low-grade metamorphism. These trends suggest that the main metamorphic event probably affected the Median Prealpine nappes during transport from their Penninic origin; the process responsible for this Alpine metamorphism was progressive burial by thrust stacking, probably during the Late Eocene.