Trace element inventory of detrital garnet: a case study from the Central Alps

Garnet is a popular provenance tracer of orogenic sediments, due to its common occurrence as a detrital phase, and significant major element compositional diversity that reflects variations in the bulk chemistry and equilibration conditions of the source rock. If the source rocks are similar in nature, however, then the major element concentrations of garnet may not differ enough to distinguish detrital contributions from different geological units. Trace elements in garnet, on the other hand, are far more sensitive to small changes in equilibration conditions of both magmatic and metamorphic rocks, and hence offer greater potential for provenance tracing. This contribution develops the use of major and trace elements in detrital garnet as a provenance proxy in the Central Alps and its adjacent northern foreland basin, the Swiss Molasse basin. A compilation of published and own garnet compositions from the Alps (n ≈ 1600) shows that, except for a few exceptions, most alpine garnets are almandine-rich with variable contents of grossular, pyrope and spessartine. Based on this major element data, four partially overlapping clusters of source rocks can be identified: Eclogite-facies garnets with high pyrope and grossular contents, granulite-facies garnets with high pyrope contents, greenschist-facies garnets with high grossular and spessartine contents and finally amphibolite-facies garnets with high almandine contents and a wide spread in grossular and spessartine contents. Within the large cluster of amphibolite-facies garnets, major element concentrations do not seem to show systematical variations amongst different type of source rocks (e.g. between meta-sedimentary or meta-igneous amphibolite-facies rocks), which is inconsistent with the significant petrological diversity of this group. To provide greater resolution on the source rock characteristics, we performed laser ablation ICP-MS analyses on garnets separated from four different amphibolite-facies source rocks from the Central Alps and on detrital garnets from three sedimentary rocks from the Molasse basin. Statistical analysis of the trace element data set focused on the fractionation of high field strength, large ion lithophile, and rare Earth elements. Significantly greater compositional diversity can be identified based on these geochemical discriminators between garnets with otherwise indistinguishable major element characteristics. Our preliminary results show that the combination of major and trace element chemistry can improve the source rock discrimination by up to 40%