Detrital rutile U-Pb geochronology as a tracer of convergence in the External Western Carpathians

The collision of Europe (Laurusia) and Alcapa (part of Adria) lead to the formation and later erosion of high-pressure rocks in the Carpathian arc. Since metamorphic rutile requires relatively high pressure to crystallize, its formation during orogeny is indicative for a subduction setting. To better understand the closure of the Alpine Tethys Ocean in the Western Carpathians, U-Pb geochronology was applied to detrital rutile from medium grained sandstones of the Magura and Silesian Nappes. Twelve samples were collected along a transect through the Magura Nappe and three samples from the Silesian Nappe were added as a reference. Approximately 200 rutile grains were separated from each sandstone and around half of them were selected for further analysis. The dated rutile shows significant differences in age, as well as in appearance (shape, inclusions, zoning etc.) suggesting derivation from various sources.

The most prominent age peaks represent the Variscan (c. 400-280 Ma) and Alpine (c. 160-90 Ma) tectonic events, which are well-represented in all but the oldest dated sample. It is noteworthy that four distinct Alpine maxima were detected in the rutile dataset. The two most prominent peaks of 137-126 Ma and 115-105 Ma are found in the majority of the samples. In two sandstone samples, deposited in the Eocene – Oligocene and the Late Cretaceous – Paleocene, the youngest peak of 94-90 Ma appears. Another peak of 193-184 Ma is also present in these two samples, as well as in another sandstone deposited between the Paleocene and Eocene. In addition, most of the dated sandstones show some Proterozoic ages (approx. 1770 Ma, 1200 Ma, 680 Ma and 600 Ma).

Tentatively, we propose that recognizable events include the Jurassic subduction of the Meliata Ocean (~180-155 Ma), and the Cretaceous thrust stacking and exhumation of the Veporic and Gemeric domains (140-90 Ma). The abundance of Alpine rutile in all but the oldest dated sandstone suggests no physical barrier for supply of detrital material derived from the southern and central Alcapa (part of Adria) to a sedimentary basin developed north of the alleged Oravic (Czorsztyn) continental sliver within the Alpine Tethys Ocean. The lack of young Alpine ages in the oldest sandstone could be a result of either a natural boundary between the basin and the orogen or a lack of rutile-bearing rocks at the surface at that time.

In a broader sense, we propose that synorogenic deposits of the Outer Western Carpathians contain detritus from the formerly subducted, exhumed and imbricated oceanic and continental crustal domains at the southern margin of the ALCAPA microcontinent.

This research is funded by the National Science Centre, Poland, project no. 2021/43/B/ST10/02312.