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Early Paleozoic Cenerian (Sardic) geodynamic relationships of peripheral eastern north Gondwana affinities : revisiting the Ordovician of the Getic/Kučaj nappe (eastern Serbia)

Spahić Darko, Tančić Pavle, Barjaktarović Dejan

Geological Quarterly

2023

Vol. 67, No. 1

art. no. 5

Regional tectonic synthesis suggests that a segment of the bipartite eastern Gondwana-type Carpathian-Balkan nappe-stacked Getic/Kučaj/Supragetic basement (eastern Serbia) experienced Cambro-Ordovician Cenerian (Sardic) accretionary-type deformation. The Ordovician basement of the Alpine Getic/Kučaj nappe exposes an earlier-mapped shallow-marine transgressive-type Fe-silicate-rich ironstone sequence. The Ordovician ironstone is used as second-order evidence of a hitherto untraceable tectonically-driven unconformity. Early Paleozoic compression is consistent with the controversial latest Cambrian to intra-Ordovician Cenerian (Sardic) interval, documented by (i) a 488 Ma metamorphic event and available detrital zircon data (Serbo-Macedonian gneissic unit), (ii) a deformed Lower Ordovician Getic/Kučaj brachiopod assembly, and (iii) an intra-Ordovician unconformity dividing the Supragetic basement/”Vlasina complex”. The data further imply that mafic gabbro-dominating sills, cropping out in the northern Getic/Kučaj unit, are consistent with Ordovician back-arc activity. The Getic/Kučaj gabbro is Ordovician in age, piercing a Neoproterozoic–Cambrian (Lower Ordovician) Supragetic/”Vlasina complex”, overlain by a transgressive Silurian–Devonian sedimentary sequence. The emergence of Ordovician mafic intrusions reflects submarine volcanism, while deep-water redox conditions were capable of a sustained supply of Fe (similar to Sardinia). In terms of tectono-palaeogeographic reconstructions, the origin of Ordovician shortening and mafic volcanism is often challenged. The latter is broadly analogous with the embryonic eastern Rheic Ocean, corresponding additionally to the Armorican spur and related intra-continental magmatism.

Rietveld crystal structure refinement of the natural rhombohedral grossular-andradite garnet from Serbia

Tančić Pavle, Kremenović Aleksandar

Geological Quarterly

2022

Vol. 66, No. 1

art. no. 7

Four macroscopically visible B–E (rim) zones of the natural grossular-andradite garnet with Grs58-64subAdr36-42>Sps2composition were crystallographically studied. The applied procedure includes the Rietveld refinements of the crystal structures in a series of six space groups, which were followed by the comparative analysis of the R-values, site occupancy factors, and the bond lengths and angles. It was established that it crystallized in the rhombohedral R-3c space group. Various polyhedral distortions and structural order-disorder variations between the studied zones were also observed, determined and discussed. Therefore, rhombohedral symmetry of the studied grandite could be treated as primary and residual strain as the second cause for its slight optical anisotropy.