Geochronological constraints on the exhumation of the austroalpine seckau nappe (eastern alps)

Abstract

New Rb-Sr biotite age data from meta-granitoids of the Seckau Nappe (Eastern Alps) are discussed to constrain its tectonometamorphic evolution. Within the basement of the Seckau Nappe a clear distinction between S- and I- Type granitoids can be established. The S- type granites are mainly part of the structurally uppermost sections and are covered by Permian to Late Triassic metasedimentary sequences of the Rannach-Formation. Within the AFM diagram all granitoids are characterized by a calcalkaline trend. This suggests formation of the melts during a subduction process. The granitoids are related to both pre-plate collision, syn-collision and post-collision uplift settings. The Rb-Sr biotite age data (biotite - whole rock isochrons) range from 76 to 86 Ma, with eight samples showing ages around 84 to 86 Ma. The Rb-Sr biotite-ages are interpreted to date cooling below 300±50°C. Therefore the investigated part of the Seckau Nappe cooled down below ~300°C at about 85 Ma in the Santonian. Referring to microstructural observations, the contact between the basement of the Seckau Complex and the Rannach-Formation was strongly overprinted by the formation of distinct shear zones that are characterised by extensional fabrics. Kinematics is characterized by conjugate sets of top-to-the-WNW and top-to-the-ESE. As feldspar is deformed by cataclastic deformation mechanisms, whereas quartz shows deformation mechanisms of dislocation creep and bulging dynamic recrystallization we assume that the conditions of deformation along the Seckau Complex with the Rannach-Formation are in the range of 300°C to 400°C. A comprehensive interpretation of the Rb-Sr biotite age data combined with deformation structures suggests that extension-reated exhumation of the Seckau complex occurred around 85 to 80 Ma.