GEOCHRONOLOGICAL, PETROLOGIC AND KINEMATIC CONSTRAINTS ON THE EVOLUTION OF THE ERR-PLATTA BOUNDARY, PART OF A FOSSIL CONTINENT-OCEAN SUTUREIN THE ALPS (EASTERN SWITZERLAND)

The boundary between the Austroalpine Err nappe and the Penninic Platt a unit represents part of a Late Cretaceous continent-ocean suture pre served in the highest stockwerk of the Tertiary Alpine orogen. Subduct ion and nappe stacking (D1) and subsequent extensional uplift (D2) alo ng Tart of this suture are dated with the K-Ar method on two generatio ns of syn-kinematic white mica in the Err nappe and with Ar-Ar step-he ating of syn- to post-kinematic riebeckitic amphiboles in the Platta n appe. The first generation of white micas associated with isoclinal fo lds and W-directed mylonitic thrusting along nappe contacts yields a 7 6 to 89 Ma age range, whereas the second generation of white micas tha t grew during tight folding and top-to-the-E extensional D2 shearing y ields 67 to 80 Ma ages. The K-Ar white mica ages are interpreted as ag es of formation at the approximately 300 degrees C temperature of lowe r green-schist facies regional metamorphism in the southern part of th e Err nappe. The two amphibole samples from the Platta nappe yield 69 and 73 Ma Ar-Ar ages, interpreted as syn-D2 formational ages. The Si-c ontent of the white micas in chlorite-K-feldspar-quartz-bearing meta-r adiolarites in the southern part of the Err nappe indicate a pressure decrease from D1 to D2 deformation, probably accompanied by an increas e in temperature. This can be related to D2 extensional uplift accommo dated along reactivated D1 nappe contacts in the footwall of the Julie r-Bernina nappe. When combined with stratigraphic evidence, the radiom etric ages and deformation-crystallization relations indicate clockwis e P-T paths for the Err, Platta, and Margna nappes: Suturing of the Au stroalpine continental and Liguro-Piemontese oceanic crust began with near surface thrusting and flysch sedimentation between 88 and 100 Ma and culminated with D1 folding and nappe stacking under high P/T green schist facies conditions sometime during the period 80 to 88 Ma. The u nroofing of this suture was a multi-stage process involving Late Creta ceous D2 extension under greenschist facies conditions, Early Tertiary D3 thrusting under lower anchizonal conditions, and mid-Tertiary D4 n ormal faulting above the uplifting Lepontine dome.