The pre-Alpine basement of the Apuseni Mountains (Romanian Carpathians) comprises three terranes: Someş, Baia de Arieş and Biharia. Metaigneous rocks within these terranes record Late Cambrian–Ordovician U–Pb zircon crystallization ages. Two samples from an orthogneiss layer in the Someş terrane yielded an age of 459.8 ± 2.7 Ma. In the Baia de Arieş terrane, an augen gneiss yielded an age of 470.8 ± 3.8 Ma, a porphyroid yielded an age of 467.8 ± 3.8 Ma and two metagranites yielded ages of 467.8 ± 4.7 Ma and 467.1 ± 3.9 Ma, respectively. Three samples from a metagranitoid outcrop located in the lower part of the Biharia terrane yielded an age of 495.0 ± 2.1 Ma. Three other samples from a metabasite layer situated lithostratigraphically several hundred metres above the metagranitoid yielded an age of 477.8 ± 3.2 Ma. Detrital U/Pb zircon ages from a paragneiss interlayered within the Baia de Arieş terrane scatter between 548 and 3518 Ma. Detrital zircon ages together with the inherited ages recorded by magmatic zircon populations suggest a northeastern Gondwanan provenance for the Someş, Baia de Arieş and Biharia terranes, adjacent to the Arabian–Nubian Shield. Lead loss in some zircons is interpreted to have taken place during the Variscan orogeny.
Supplementary material:
Coordinates for samples from the Apuseni Mountains, Palaeozoic palaeogeographical models of Gondwana, Laurentia and Baltica to the formation of Pangaea, sample preparation and analytical procedures, sample description, and age measurements of zircon grains are available at
http://www.geolsoc.org.uk/SUP18425
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Like the Alps and Western Carpathians, the Apuseni Mountains represent a fragment of the Variscan orogen involved in the Alpine crustal shortenings. Thus the more extensive Alpine tectonic unit in the Apuseni Mountains, the Bihor Autochthonous Unit is overlain by several nappe systems. During the Variscan orogeny, the Bihor Unit was a part of the Some terrane involved as the upper plate in subduction, continental collision and finally in the orogen collapse and exhumation. The Variscan thermotectonic events were marked in the future Bihor Unit by the large Muntele Mare granitoid intrusion, an S-type anatectic body. Zircon U-Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) dating yielded a weighted mean age of 290.9 ± 3.0 Ma and a concordia age of 291.1 ± 1.1 Ma. U-Pb isotope dilution zircon analyses yielded a lower intercept crystallization age of 296.6 + 5.7/-6.2 Ma. These two ages coincide in the error limits. Thus, the Muntele Mare granitoid pluton is a sign of the last stage in the Variscan history of the Apuseni Mountains. Many zircon grains show inheritance and/or Pb loss, typical for anatectic granitoid, overprinted by later thermotectonic events.
The Padeş suite from the Poiana Ruscă Mountains is a component of the Padeş Paleozoic terrane. The Padeş terrane evolved as an island arc between the Cadomian Sebeş-Lotru and Făgăraş terranes, the main parts of the Getic crystalline in the Southern Carpathians. Evaporated single zircon grains offered 394±20 Ma, 546±20 Ma, 655±19 Ma, 1305±17 Ma, and 1538±17 Ma ages. The 394 Ma age has been interpreted as an early Variscan collision age, 546 Ma as the protolith age and the other ages as signifying Cadomian and Saharan detrital zircons. Similar to the terranes in the Apuseni Mountains, the Padeş terrane has a North African-Gondwanan provenance.
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