Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 66 (1996)No.
1. (January), Pages 139-146
Authigenic Feldspar as an Indicator of Paleo-Rock/Water Interactions in
Permian Carbonates of the Northern Calcareous Alps, Austria
Christoph Spotl (1), Martin Kralik (2), Michael J. Kunk
(3)
ABSTRACT
Dolostones interbedded with Upper Permian evaporites at the base of the
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 crystals. Both feldspar types have
essentially identical K-feldspar end-member compositions with >= 99.5 mole
% Or component. Low oxygen
isotope
ratios (+16.1
to +18.1
SMOW) suggest precipitation from 18O-enriched, saline fluids
at temperatures in excess of
140°C. 40 r/39Ar plateau-age spectra of five
samples range from 145 ± 1 to 144 ±1 Ma (Early Berriasian)
and suggest that both types of feldspar were formed within an interval
that did not exceed
2 m.y. Rb/Sr model 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 (detachment)
and by an increase in the subsurface temperature in response to thrust
loading.