A multivariate discrimination scheme of detrital garnet chemistry for use in sedimentary provenance analysis

Abstract

Garnet chemistry provides a well-established tool in the discrimination and interpretation of sediment provenance. Current discrimination approaches, however, (i) suffer from using less variables than available, (ii) subjective determination of discrimination fields with strict boundaries suggesting clear separations where in fact probabilities are converging, and (iii) significant overlap of compositional fields of garnet from different host-rock groups. The new multivariate discrimination scheme is based on a large database, a hierarchical discrimination approach involving three steps, linear discriminant analysis at each step, and the five major host-rock groups to be discriminated: eclogite-(A), amphibolite-(B) and granulite-(C) facies metamorphic rocks as well as ultramafic (D) and igneous rocks (E). The successful application of statistical discrimination approaches requires consideration of the a priori knowledge of the respective geologic setting. This is accounted for by the use of prior probabilities. Three sets of prior probabilities (priors) are introduced and their advantages and disadvantages are discussed. The user is free to choose among these priors, which can be further modified according to the specific geologic problem and the level of a priori knowledge. The discrimination results are provided as integrated probabilities of belonging to the five major host-rock groups. For performing calculations and results a supplementary Excel® spreadsheet is provided. The discrimination scheme has been tested for a large variety of examples of crystalline rocks covering all of the five major groups and several subgroups from various geologic settings. In most cases, garnets are assigned correctly to the respective group. Exceptions typically reflect the peculiarities of the regional geologic situation. Evaluation of detrital garnets from modern and ancient sedimentary settings of the Western Gneiss Region (Norway), Eastern Alps (Austria) and Albertine Rift (Uganda) demonstrates the power to reflect the respective geologic situations and corroborates previous results. As most garnet is derived from metamorphic rocks and many provenance studies aim at reconstructing the tectonic and geodynamic evolution in the source area, the approach and the examples emphasize discrimination of metamorphic facies (i.e., temperature-pressure conditions) rather than protolith composition.

Miocene basement exhumation in the Central Alps recorded by detrital garnet geochemistry in foreland basin deposits

“…The exact time of their surficial exposure is unknown, but it is believed to post-date the youngest preserved Molasse deposits. This interpretation is based on the lack of granitic pebbles attributable to the external massifs in the Molasse (Trümpy, 1980) and on structural reconstructions (e.g. Pfiffner, 1986) in combination with thermochronological data (e.g.…”

“…This is mostly due to the large compositional overlap of garnet sourced by diverse amphibolite facies metamorphic rocks (e.g. metasedimentary versus meta-igneous; Krippner et al, 2014;Tolosana-Delgado et al, 2018).…”

“…However, many gneisses in the Central Alps preserve a prealpine amphibolite facies metamorphic signature as well Table 4. Results from classification following Mange and Morton (2007) and Tolosana-Delgado et al (2018). Using the linear discriminant method of Tolosana-Delgado et al (2018), garnet was attributed to one single class if the probability for that class was ≥ 50 %.…”

“…Results from classification following Mange and Morton (2007) and Tolosana-Delgado et al (2018). Using the linear discriminant method of Tolosana-Delgado et al (2018), garnet was attributed to one single class if the probability for that class was ≥ 50 %. Several grains were assigned mixed probabilities with <50 % per class; these are listed separately below (italic numbers).…”

“…Several grains were assigned mixed probabilities with <50 % per class; these are listed separately below (italic numbers). Mange and Morton (2007) Tolosana- Delgado et al (2018) Types after Mange and Morton (2007) Figure 5. Relative frequency of the four most common endmembers almandine, grossular, spessartine, and pyrope in the three detrital samples from the Molasse basin.…”