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U-Pb zircon data of Middle-Upper Triassic magmatism in Southern Alps and NW Dinarides: Implications for the Southeast Mediterranean tectonics

Middle to upper Triassic magmatism is known to be widespread in central Southern Alps and in Dinarides since a long time and was increasingly reported from Western and Eastern Carpathians as well as from Hellenides. To constrain the whole age range and potential trends of magmatism, we present here six new laser-ablation ICP-MS U-Pb zircon ages mainly from thick tuff and subordinate stocks from the eastern part of Eastern Alps and northwestern Dinarides. Our new ages range from 245.8 ± 1.8 Ma (Bärental, South Karawanken unit) to 223.7 ± 1.5 Ma (Lojše, Sava fold region). The porphyritic andesite stock of Fužine (NW Dinarides) gave 233.7 ± 1.5 Ma. Foliated tuffaceous metasandstone previously believed to represent a Carboniferous deposit of the Tolmin nappe turned out to have an age of 233.7 ± 1.5 Ma, similar as tuffaceous sericite schists of the same tectonic unit with age of 223.7 ± 1.5 Ma. Green lapilli tuffs of Riofreddo have an age of 236.5 ± 5.5 Ma. All tuffs also bear zircon xenocrysts. In those of the Southalpine unit, two age groups were found, ca. 444 ± 6 to 456 ± 8 Ma and 603 ± 7. At Fužine, a xenocrystic grain yields 397 ± 5 Ma. We argue, therefore, that magmatism evolved on a Panafrican to Ordovician resp. Lower Devonian continental basement although magmatic rocks of these age groups are largely unknown in Sourthern Alps and NW Dinarides. Interestingly, many high-precision ages of tuffs from the Alpine-Carpathian region are available in the literature with the main purpose to correlate with biostratigraphy. These ages center with ca. 241 to 238 Ma mostly around the older age group reported by our new ages, whereas younger ages are scarce. A regional survey is showing a wide range of petrographic and geochemical compositions with mainly calc-alkaline acidic compositions and rare mafic magmas to highly alkaline compositions including syenite plutons (e.g., Eisenkappel in Alps, Ditrau in Eastern Carpathians) and A-granites (Hrončok, Vepor unit of Western Carpathians), whereas those of Dinarides and Hellenides are largely of calc-alkaline composition. Consequently, the significance of these magmatic suites includes elements of quite different significance. In terms of the geodynamic composition, these magmas are late within the rift process and rather at the stage of rift to drift transition. We speculate that some magmas were generated by impingement of a mantle plume into the evolved rift.

U-Pb zircon; data; Middle-Upper Triassic; magmatism; Southern Alps; NW Dinarides

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