Robert Handler^* 1, Franz Neubauer ¹

Dating of detrital minerals, e.g. white mica, preserved within non-metamorphic to low-grade metamorphic clastic sequences, has proved to be a powerful tool for correlation of sedimentary sequences with possible metamorphic/plutonic source areas. ⁴⁰Ar/³⁹Ar ages reported from detrital white mica have classically been interpreted to reflect cooling through, or formation at or below respective closure temperatures (e.g. 350-400°C) of white mica within their previous host-rocks prior to uplift/exhumation, erosion, transport, and subsequent deposition. However, conventional incremental heating techniques require bulk-grain samples of about 10 to 100 µm sample material, and therefore an uniform source area for the detrital mineral assemblage is assumed. Our analyses on detrital white mica from recent fluviatile systems focused on the question, if such an assumption is justified, or not, respectively, if it is possible to differentiate contrasting source areas within one sedimentary unit. We tried to decipher contrasting sources by (1) careful mineral separation, and (2) by incremental heating of bulk-grain samples, because we expected a different degassing behaviour of different mica populations, which should be detectable in the Ar-release spectra.

For the present investigation we have chosen the Salzach river system including its confluents in the province Salzburg, Austria, because different source areas with contrasting tectonometamorphic evolution and various degree of Alpine metamorphic overprint contribute their sedimentary load into the Salzach river via various confluents. White micas were prepared from sands and concentrated by standard separation techniques including sieving and magnetic separation. Two types of analyses were performed, (1) total fusion dating of single-grains, and (2) step-wise analyses of bulk-grain samples, each containing c. 10-20 grains. For both analyses samples were heated using an IR laser ablation system, isotopic analyses were performed on a VG MicroMass 3600 noble-gas mass-spectrometer in the Institute for Geology, University Salzburg.

Total fusion ⁴⁰Ar/³⁹Ar ages of single-grain analyses yielded ages of cooling of respective source areas. Within different fluviatile systems 5 age groups can be distinguished: (1) Cadomian (c. 800-640 Ma), (2) early Variscan (c. 360-340 Ma), and (3) late Variscan (c. 330-280 Ma) ages are reported in sediments, which were derived from non-metamorphic to low-grade metamorphic clastic sequences of the Graywacke Zone and basal (Permian-Skythian) sequences of the Northern Calcareous Alps; (4) early Alpine (c. 95-80 Ma) ages are reported for detrital white mica from creeks which drain areas affected by Cretaceous Alpine low- to medium-grade metamorphism, (5) late Alpine (c. 35-20 Ma) ages are reported from creeks draining Alpine Tertiary metamorphic areas. Samples taken from distal parts of the Salzach river indicate mixing of detrital white mica populations of all age groups.

Compared to the single-grain experiments, incremental heating analyses reflect two types of Ar release spectra: (1) plateau-type spectra indicate ages, which can be interpreted as cooling/formation ages of respective source areas. Surprisingly, no indication for "mixed plateau ages" - e.g. mixing of Ar isotopes from different white mica populations during heating - can be observed; (2) completely disturbed age spectra mostly reflect distinct ages of one or more of the age groups listed above. Only few "mixed ages" are reported in the release spectra, most of the ages can be attributed to one of the 5 age groups. However, both single-grain total fusion and incremental heating experiments indicate no systematic age variation depending on grain size and / or magnetic susceptibility.

Results from single-grain heating experiments indicate, that (1) different white mica populations even exist in small fluviatile systems which drain areas of presumably the same tectonometamorphic history. This implies that in ancient clastic sequences we mostly deal with mixed mica populations. (2) It is not possible to physically separate different detrital white mica populations by different grain size or magnetic susceptibility. Stepwise heating experiments of mixed mica populations indicate that these can be identified in the Ar release spectra, as rare "mixed ages", but more common as distinct, geologically meaningful ages, which are reported during various steps of Ar-release spectrum.