AUTHIGENIC FELDSPAR AS AN INDICATOR OF PALEO-ROCK WATER INTERACTIONS IN PERMIAN CARBONATES OF THE NORTHERN CALCAREOUS ALPS, AUSTRIA

Dolostones interbedded with Upper Permian evaporites at the base of th e Northern Calcareous Alps contain abundant authigenic K-feldspar. Two petrographically, structurally, and isotopically distinct generations of K-feldspar can be distinguished: crystals composed of an inclusion -rich core and a clear rim, and optically unzoned, transparent crystal s. Both feldspar types have essentially identical K-feldspar end-membe r compositions with greater than or equal to 99.5 mole % Or component. Low oxygen isotope ratios (+16.1 parts per thousand to +18.1 parts pe r thousand SMOW) suggest precipitation from O-18-enriched, saline flui ds at temperatures in excess of similar to 140 degrees C. Ar-40/Ar-39 plateau-age spectra of five samples range from 145 +/- 1 to 144 +/- 1 Ma (Early Berriasian) and suggest that both types of feldspar were for med within an interval that did not exceed similar to 2 m.y. Rb/Sr mod el ages range from 152 to 140 Ma, assuming that the burial diagenetic regime was buffered with respect to strontium by the associated marine Permian evaporites. Authigenic K-feldspar records two distinct events of hot brine flow, most likely triggered by tectonic movements (detac hment) and by an increase in the subsurface temperature in response to thrust loading.