Fluid inclusions in quartz related to subsequent stages of foliation development during a single metamorphic cycle (Schneeberg Fault Zone, Eastern Alps, Austria)

摘要

Macro- and micro-scale structures, fluid inclusions, quartz rheology and host mineral chemistry were used to establish the P-T-d evolutionary path of monometamorphic metapelites of the Schneeberg Fault Zone. Four selected quartz generations indicate formation conditions during stages of prograde and retrograde metamorphism. Earliest foliation development S1 is outlined by quartz foam microstructures that are armored within foliation-parallel garnet layers which reflect individual growth characteristics during the prograde and early retrograde stage. The prograde path is linked with garnet growth at low differential stress up to peak pressure conditions of ca. 10 kbar. Final garnet growth during increasing temperatures and WNW-directed shearing up to <600 °C predates exhumation along an isothermal decompressional path and increasing differential stress. Exhumation was accompanied by the formation of younger quartz generations and the formation of a second foliation S2 that was incorporated into intense folding. Minimum pressure condition for S2 is about 4.5 kbar resulting from the gradient of fluid inclusion density isochores estimated from S2-related pressure shadows in combination with the temperature range for dislocation creep in quartz aggregates between 500 and 600 °C. The dominance of carbonic fluid inclusions indicates decarbonation associated with H2O leakage as a main process during Alpine monometamorphism in the metasedimentary host rocks. Aqueous fluid inclusions are arranged along late fluid planes and reflect a wide range in densities with variable salinities. This study corroborates the successful approach of combining mineral growth, fluid inclusions and macro- to micro-scale deformation stages to constrain a prograde to retrograde growth history of metamorphic minerals like garnet and quartz even though these underwent polyphase deformation and upper amphibolite facies metamorphism. It supports previous arguments about low differential stress at high-pressure conditions and allows speculations about the location of the metasediments of the Schneeberg Fault Zone nearby the low strength plate interface during late stage of Cretaceous subduction and subsequent early exhumation.