PRE-ALPINE EVOLUTION OF THE AUSTROALPINE BASEMENT (EASTERN ALPS): CONSTRAINTS FROM IN-SITU LA-ICP-MS U-PB ZIRCON GEOCHRONOLOGY WITHIN THE SECKAU NAPPE SYSTEM

The Seckau Complex (SC) of the Eastern Alps, as part of the Austroalpine Silvretta-Seckau Nappe System, was given little attention due to its presumably monotonous composition, structure, and tectonometamorphic evolution during the Alpine orogeny. The SC basically assembles locally migmatized paragneisses, (garnet-) micaschists and amphibolites that were intruded by different types of granitoids, (quartz monzodiorites, granodiorite and various granites). These units represent parts of pre-Variscan/Variscan lithosphere that was reworked during Alpine orogeny. Despite the lack of precise geochronological ages, the granitoids were generally attributed to be part of the widely distributed intrusions related to the Variscan orogeny during the collision of Gondwana and Laurussia, that were variably overprinted during the Alpine orogeny.

New U-Pb zircon ages and geochemical data from the SC allow to subdivide the SC into three lithostratigraphic sub-units (1) Ingering Metamorphic Suite (IMS), (2) Hochreichart Plutonic Suite(HPS) and (3) Hintertal Plutonic Suite(HtPS). The IMS comprises fine-grained biotite-plagioclase-garnet gneisses yielding U-Pb zircon ages in the range between 2.7 Ga and 2.0 Ga, and zircon grains clustering from 572 ± 7 Ma to 559 ± 11 Ma. These gneisses form the host rock for the large volumes of metagranitoids that are divided into the HPS and the HtPS. The HPS, predominantly situated in the central part of the SC, comprises highly fractionated melts with mainly S-type characteristics and U-Pb zircon ages between 508 ± 9 Ma and 486 ± 9 Ma. The HtPS, dominating the north-eastern and south-western part of the SC, is composed of early Variscan metagranitoids with ages between 365 ± 11 Ma and 343 ± 12 Ma indicating a magmatic fractionation trend, seen in variable Rb/Sr ratios. The HtPS can be further subdivided into (a) the Griessstein Pluton characterized by S-type metagranitoids and (b) the Pletzen Pluton with intermediate to acidic metagranitoids of I-type affinity. This study thus expands on previous hypotheses that assigned metagranitoids of the SC to the Variscan orogeny and also demonstrates that the IMS gneisses, related to the Cadomian orogeny, were affected by a massive pre-Variscan magmatic event at around 500 Ma prior to early Variscan-related magmatism.