Grain-size dependence of garnet composition revealed by provenance signatures of modern stream sediments from the western Hohe Tauern (Austria)

Heavy minerals are valuable indicators about the geological framework in the source area. The heavy mineral garnet is one of the most widespread heavy minerals in orogenic sediments and its geochemistry provides important information about metamorphic conditions. The application of heavy minerals and garnet geochemistry for sedimentary provenance analysis is tested for modern stream sediments collected along three rivers draining the Eclogite Zone and adjacent geological source units of the western Hohe Tauern area in the central Eastern European Alps. For comparison with the stream sediments, rock outcrops exposed in this area were also sampled. The chosen area is very well investigated and provides an excellent place to constrain the relations between source rocks and sediment in first-order drainages. The influence of grain size is studied in detail by considering grain-size fractions ranging from coarse silt to coarse sand (32 to 1000μm). In all grain-size fractions the heavy mineral assemblages are characterised to a variable extent by epidote, zoisite, garnet, and green calcic amphibole. In the smaller grain-size fraction apatite is more frequent, whereas in the coarser grain-size fractions an increase of green calcic amphibole and garnet can be observed. Electron microprobe analysis of detrital garnet shows the dominance of almandine-rich garnet. Stream sediments within and downstream of the Eclogite Zone show an increase of pyrope-rich garnets. Interestingly, in all samples, grossular-rich garnets are more frequent in the smaller grain sizes and pyrope-rich garnets are more frequent in the coarser grain sizes. This is controlled by the original finer size distribution of grossular in the source rocks rather than being a hydraulic effect. The heavy mineral assemblages and garnet geochemical data reflect the geological setting of the study area, hence confirming the general strength of these methods in sedimentary provenance analysis. However, the data underline strong grain-size control on sediment composition including single-grain compositional variations.