The first results of U-Pb dating of the granitoids of the Neplyuevka pluton(The Southern Urals)

Introduction. The Neplyuevka pluton is situated in the Chelyabinsk region of the Southern Urals, in the western part of the Eastern Uralian megazone. The area of pluton is 20×14 km. The Neplyuevka intrusion is comprised of 4 phases: 1) gabbro and diorites, 2) quartz diorites and granodiorites, 3) adamellites, 4) leukogranites. The granitoids intrude the terrigenous rocks of the Lower Ordovician. The overlying terrigenous deposits of the Upper Visean contain granitic lithoclasts. The isotope Rb-Sr ages of the rocks comprising the Neplyuevka pluton lie in the range of 346-340 Ma. The variations in the isotopic ages are consistent with the order of individual phases: 346 Ma for the 2nd phase, 342 for the 3rd phase and 340 for the 4th phase. So, according to the isotope ages, the evolution of the pluton took at least 6 Ma. The pluton also contains Cisuralian leucogranites (278 Ma).

Materials and methods. We have studied the zircon grains extracted from 4 specimens: 1 – granodiorites of the 2nd phase, 2 and 3 – adamellites from the 3rd phase, 4th – leucogranites of the 4th phase. The specimens themselves have been collected from the exact locations and the same rock types as previous sampling for Rb-Sr isotope dating. We have studied the morphology of the grains and their internal structure using cathodoluminescence imaging. The U-Pb dating was performed at the Russian Geological Research Institute (VSEGEI) using SHRIMP-II.

Results and discussion. The zircon grains from the granodiorites and adamellites are represented by transparent and light-yellow, idiomorphic, bipyramidal-prismatic crystals, 200-600 μm in length. The crystals are characterized by medium intensity of luminosity with apparent medium-contrast coarse and fine oscillatory zonation. By their obtained isotopic ages the zircons can be divided into 2 populations 1) 334-342 Ma and 2) 354-356 Ma. The isotopic ages of the 1st population are close to those obtained by Rb-Sr dating.

The formation of the Neplyuevka complex in the beginning of the Carboniferous signifies an important event in the geodynamic evolution of the Southern Urals – the fast transition from island arc magmatism, which continued during the whole Devonian, to the rift magmatism, which ceased only in the middle of the Visean. The termination of island arc magmatism is usually explained by the slab delamination at the Devonian-Carboniferous boundary, while the initiation of the rifting is attributed to the oblique collision between the Paleo-Ural island arc with the Laurussia in the middle of the Tournaisian. The obtained data allows us to reevaluate the age of the rifting initiation, assigning it to the Devonian-Carboniferous boundary.

Financial support. The research has been funded by RFBR and CNF as a part of the research project № 19-55-26009. Interpretation of U-Pb data was carried out within the framework of the government assignment of IGEM RAS.