Rb-Sr isotopic dating of pseudomorphs after lawsonite in metabasalts from the Glockner nappe, Tauern Window, Eastern Alps

Abstract

Pseudomorphs after lawsonite from the Glockner nappe (south-central Tauern Window, Austria), a metamorphic sequence of metasediments intercalated with metavolcanics of mid-ocean ridge basalt origin, have been dated using Rb-Sr mineral systematics. The basalts were metamorphosed in blueschist facies, as evident from pseudomorphs after lawsonite and relics of glaucophane, and then overprinted at greenschist-facies conditions. The foliated fabric suggests late syndeformative greenschist-facies formation of the pseudomorphs. The pseudomorphs show the mineral assemblage Ab +Ep +Phe +Chl +Cal +Ttn +Act ± Qtz ± Ap. Minerals from the pseudomorphs and the matrix yield an Oligocene internal mineral isochron age of 29.82 ± 0.54 Ma. This age corresponds to the late prograde-early retrograde decomposition of lawsonite and the syntectonic crystallization of the greenschist facies matrix assemblage. The new data imply that the last penetrative deformation and thermal overprint of the Glockner nappe in the central Tauern Window only slightly postdates the exhumation of the nearby Eclogite zone by about 2 Ma. The history of Alpine nappe stacking, exhumation and passage of the rocks through their thermal peak conditions is thus constrained to a surprisingly narrow time frame in the Oligocene.