MULTI-SAMPLE COMPARISON OF DETRITAL ZIRCON AGE SPECTRA OF LOWER PALEOZOIC UNITS FROM THE VARISCAN-APPALACHIAN OROGENIC BELT

The possibility to date detrital heavy minerals and, therefore, to constrain the age of igneous and/or high-grade metamorphic protoliths of sedimentary rocks gains growing significance. The increasing datasets demand statistical and quantitative evaluation tools that allow a fast comparison of data. However, the objective extraction of provenance information requires thoughtful selection of those parts of the dataset that actually carry provenance information and omission of parts that are shared by all rocks. Here, we present a simple yet powerful approach of semi-automatic provenance end-member identification.

The age distributions of sedimentary rocks should not be compared with the one of magmatic rocks because of different preservation. Furthermore, comparison also will fail when age spectra contain grains that do not provide any source information, e.g. discordant grains and ages that reflect the youngest and shared orogenic event exclusively producing juvenile zircon. The similarity of the rocks can be determined by using Multidimensional Scaling (MDS) that transfers the Kolmogorov-Smirnov distances between the age distributions into Euclidean distances. The shape of the resultant MDS configuration depends on the number of sources and sedimentary mixing and recycling process during the source-to-sink transport. A simple two-source scenario results in an elongate MDS configuration with the provenance end-members at the ends of the configuration. Because the distance between the end-members is proportional to the mixing between the sources, MDS provides a semi-quantitative measure for the fraction of sedimentary source mixing. In a multi-source scenario, provenance end-members can be objectively identified by unsupervised density-based clustering algorithms.

The approach is exemplified for the Variscan-Appalachian belt, which provides a wide range of tectonic fragments of potentially contrasting provenance that have been juxtaposed during the Late Paleozoic orogeny. We compiled 78 015 detrital zircon U–Pb ages from 987 Precambrian to Lower Paleozoic sedimentary rocks of the orogenic belt from 196 publications. Our results reveal four provenance end-members contributing to the sedimentary rocks in the Gondwana-Laurussia plate boundary zone providing crucial paleogeographic constraints.