Multiple provenance of detrital zircons from the Permian–Triassic boundary in the Bükk Mts., Hungary

Abstract

Detailed zircon analyses were carried out on samples from the Balvany North section, which contains the Permian–Triassic boundary. A fine-grained sandstone bed within the “boundary shale” contains a significant amount of zircon crystals. Pupin morphological, scanning electron microscopy [secondary electron, back-scattered electron, cathodoluminescence (CL)] and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) investigations were done in selected single grains to combine morphological information with radiometric ages, and infer the provenance of zircons. Three populations are distinguished on the basis of Pupin morphology. Their centres of gravity are S17, S2 and P1. The LA-ICP-MS radiometric ages reveal three main age groups and a smaller one, apart from a few scattered old ages. The largest group, comprising ca. 50% of the measured grains, has an age of 470–440 Ma. The two other main groups, which together represent ca. 25% of the measured grains, are around 280 and 600 Ma. The 280 Ma group contains slightly more grains than the 600 Ma group. A small group has an age of 370–340 Ma. In addition, there are some older grains with ages of 850, 969, 1,050 and 2,150 Ma. Based on the zircon morphology no clear separation of the different age groups could be made. Both the 280 Ma and the 470–440 Ma age groups tend to show zircon crystals derived from three types of sources: crustal granites, calc-alkaline granitoids and alkali granites and/or their volcanic equivalents. Zircons in the 450 Ma group have a magmatic rim around their altered, relict core, as revealed by CL images. Post-magmatic processes also affected the rim. The age of the core is possibly reset during the formation of the magmatic rim. This suggestion is supported by the LA-ICP-MS data, because no difference was seen between the core and the rim. The different roundness of the zircons, the well-sorted crystals and the at least five different source rocks indicate previous concentration of the grains, before their transportation into the “boundary shale”. Austroalpine and Southern Alpine rocks could be the sources of the zircon crystals, which fit well into the paleogeographical model of the area, which suggests that the Bukk Mts. was located in the foreground of the Alpine units.