| dc.description.abstracteng | This PhD thesis investigates the application of heavy minerals in sedimentary provenance analysis, with the main focus on garnet geochemistry. The work is mainly based on an intensive literature study, fieldwork, conventional heavy mineral analysis, electron microprobe analysis of garnet, and statistical data evaluation.
The thesis is subdivided into six chapters. After an introduction into the subject (Chapter 1), we make an attempt to evaluate six different garnet discrimination diagrams (one binary diagram and five ternary diagrams) commonly used by many researchers (Chapter 2). A large dataset was compiled (N=3532) encompassing major element compositions of garnets derived from various host lithologies, including metamorphic, igneous, and mantle-derived rocks, in order to test the applicability of the various discrimination schemes. The dataset contains mineral chemical data collected from the literature complemented with some new data (N=530) from garnet-bearing metamorphic and ultramafic rocks in Austria and Norway. Discrimination of the tested diagrams only works for a small group of garnets derived from mantle rocks, granulite-facies metasedimentary rocks, and felsic igneous rocks. For other garnet types, the assignment to a certain type of host rock remains ambiguous. We further apply compositional biplot analysis to derive some hints towards future perspectives in detrital garnet discrimination. In Chapter 3, we present results from fieldwork in the southern Tauern window in Austria. Here, we test the application of heavy minerals and garnet geochemistry for sedimentary provenance analysis 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. Additionally, the influence of grain-size is studied in detail by considering grain-size fractions ranging from coarse silt to coarse sand (32 to 1000 μm). 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 data underline strong grain-size control on sediment composition including single grain compositional variations. In Chapter 4, we present heavy mineral data and garnet geochemistry of stream sediments and bedrocks from the catchment area draining the Almklovdalen peridotite massif in the Western Gneiss Region in SW Norway. The data reveal that a wider grain-size spectrum than the commonly used grain-size range of 63−125 μm needs to be considered to avoid misleading interpretations. Remarkably is the almost total lack of pyrope-rich garnets in the sediments, which point to the few garnet peridotites exposed in the area. Garnets with pyrope content exceeding 50 % are only found in the 500−1000 μm grain-size fraction. In Chapter 5, we present heavy mineral and garnet geochemical data of recent stream sediments and bedrocks from three different catchment areas (the Flatraket and Ulvesund body and the island of Runde) in the Western Gneiss Region in SW Norway. We want to test to what extent the heavy minerals and the garnet geochemistry from a single stream-sediment sample per catchment reflect the geological situation in the source area. In overall, the heavy mineral and garnet geochemical data very well reflect the geological situation in the source area, which confirms the application and the importance of heavy minerals in sedimentary provenance analysis. Geochemical data of heavy minerals usually show a wider distribution in the sediments than the data of heavy minerals measured in the bedrocks. However, our results demonstrate that this is not always the case. Some garnets measured in the bedrocks, especially lower grade and ultrahigh-grade metamorphic garnets are only of secondary importance or they are lacking in the sediments. In Chapter 6, we have a look which elements are most useful to discriminate between the several garnet bearing groups. We present first ideas about the compilation of our garnet data and for a step-wise classification of garnets. For this we use the data from our compiled database. At the final end we want to establish a new scheme in which detrital garnets are assigned to a specific rock type with the highest probability. However, this is not yet included in this thesis since we need to work more on that subject. | de |