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Unique provenances of Lower Triassic and Cretaceous sandstones of Tisia: U-Pb zircon and 40Ar/39Ar ages

We undertook a combined single-grain laser-ICP-MS U-Pb zircon and single-grain 40Ar/39Ar white mica age dating study of provenance of Permian rift and Lower Cretaceous synorogenic greywackes of the westernmost part of the Tisia block (Medvenica and Samogorska Mts.). The principal aim is to reveal the paleogeographic relationships of this part of Tisia. Sample CRO-40C is a red sandstone of Samogorska Mts. with a supposed Permian age. For zircons, we found the following U-Pb age groups from young to old: The youngest age 246.1 ± 1.3 Ma revealing a potentially maximum Early Triassic age of these redbeds. Seven zircon grains range between 319.4 ± 1.8 Ma and 367.7 ± 2.3 Ma showing the importance of Variscan magmatism in the source region. A similar large age population (eight grains) ranges between 417.3 ± 5.0 Ma and 489.0 ± 2.5 Ma and argues for Ordovician to Silurian magmatism similar as four grains between 583.1 ± 5.2 Ma and 676.7 ± 4.4 Ma indicating Panafrican events. Interestingly, single grains are at 829.9 ± 4.6 Ma (Late Grenvillian) and 2072 ± 8 Ma (Early Proterozoic). Detrital white mica grains are currently measured with the 40Ar/39Ar method. As the next younger time-step we investigated sample CRO-39A, which is a Lower Cretaceous greywacke of Samogorska Mts. With the U-Pb method, we found the following age groups respectively single grains: a single grain at 216.0 ± 3.3 Ma indicates Late Triassic magmatism in the source region. Four grains range from 249.5 ± 3.8 to 254.2 ± 3.9 Ma showing magmatism at the Permian/Triassic boundary. Six grains range between 308.4 ± 4.7 to 363.1 ± 5.5 Ma (Variscan tectonothermal events). Further single grains are at 455.1 ± 6.9 Ma, 545.0 ± 8.2 Ma. 629.8 ± 9.4 Ma, 907.0 ± 13.2 Ma. 987 ± 14 Ma, 1671 ± 23 Ma, 1852 ± 25 Ma and 1940 ± 26 Ma and indicate Ordovician, Panafrican, Grenvillian and Early Proterozoic sources. The Ar-Ar ages of white mica of the same sample are as follows: A single grain is at 253.2 ± 1.6 Ma (Permian/Triassic boundary), thirteen grains between 304.7 ± 5.7 Ma and 369.3 ± 4.9 Ma constrain dominant Variscan tectonic events, five grains are between 430.3 ± 6.0 Ma and 460.2 ± 4.7 Ma and reveal Mid Ordovician to Silurian events, and a single grain is at 510.7 ± 21.6 Ma (late Panafrican). Sample CRO-27 is again a Lower Cretaceous greywacke from Medvenica Mts. Three grains are between 247.9 ± 5.7 Ma and 263.9 ± 1.6 Ma, fifteen grains between 286.9 ± 1.8 Ma and 369.1 ± 2.3 Ma, and nine grains range from 386.8 ± 2.4 Ma to 430.0 ± 2.7 Ma. The first two populations are similar to Samogorska Mts., the later population reveals a significant difference to Samogorska. In summary, the new data argue that Variscan tectonothermal events are dominant in the source of this part of Tisia. However, some other features are virtually unique. These include the missing Mesozoic Ar-Ar white ages in the Lower Cretaceous greywackes compared to previous reports from Dinarides (Ilic et al., 2005), tectonothermal events at the Permian/Triassic boundary monitored by both U-Pb zircon and Ar-Ar white mica ages, and the presence of a small population of Grenvillian U-Pb zircon ages arguing for likely Amazonian provenance within Gondwana. Furthermore, compared with Alps (Neubauer et al., 2007), the new data reveals a much higher importance of Ordovician to Silurian tectonothermal events, whereas Panafrican detritus is present but much less important than in Alps.

Lower Triassic; Cretaceous; sandstones; Tisia; U-Pb; zircon; 40Ar/39Ar ages

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