Deformation model of the Cheka pluton of alkaline granitoids: petromagnetic and geochemical data (Southern Urals)

Introduction. This study investigates the Cheka block (pluton) of alkaline granitoids (Southern Urals, Chelyabinsk Oblast). The objective of this study was to further investigate its existing deformation model after previous studies though the methods of fracture analysis, petromagnetic studies and geochemical analysis.

The Cheka pluton is composed of the Cheka Mountain and has a meridional strike and dimensions of 6.5 km long and 1-2 km wide. The pluton is composed of alkaline rocks of three intrusion phases: first – monzodiorites, second – alkaline syenites, third – alkaline granites and granosyenites. The pluton is Triassic and intrudes Carboniferous volcanics. The western contact of the Cheka pluton is limited by a dextral fault. The pluton is situated in the Magnitogorsk zone.

During the formation of the pluton, extension changed to compression. This led to formation of a right-lateral transpression setting with a system of meridional strike-slip and near-slip extension zones. These changes were followed by low-grade metamorphism.

This study can be split into two sections: structural analysis and geochemical/isotopes description. The first part was partially conducted previously and presented in 2022.

Materials and methods. To reconstruct structural evolution of the pluton a combination of petromagnetic studies, magnetic mineralogy and fracture analysis were used as well as supporting aerial and satellite imagery. 62 core samples and over 180 fracture measurements from 7 locations were used for each method respectively. Petromagnetic data was collected by drilling procedures, processed using MFK-1 kappabridge at room temperature and after heating to 470 °C and analyzed in Anisoft 5.1.08 software. Magnetic mineralogy lab analyses were performed with interpretation using Max UnMix software. Fracture analysis was conducted in Stereonet v.11.3.0.

As the second part of the study geochemical analyses were conducted – silicate geochemistry and ICP-MS at 6 locations.

Results and discussion. Petromagetic studies showed the magma flow to have an orientation of 036°. Analysis of tectonic fractures points to the Riedel fracture model with main fracture zone orientation (compression) of 039°. Since the magma flow and compression orientation match a deformation model can be constructed. Also based on the magma flow orientation, types of protectonic fractures were identified (S, Q, L).

Geochemical analyses showed that the elemental signature of the pluton matches the upper crust the best and shows signs of subduction. Silicate geochemistry shows a clear trend in Na₂O concentration, while K₂O concentrations do not. This pattern is interpreted as a sign of low-grade metamorphism (prehnite-pumpellyite facies).

A full deformation model was created based on two methods with additional supporting data providing strong evidence for the Riedel based deformation model, which corresponds to previous structural and geochemical findings. The model suggests that the Cheka pluton was formed in a general right-lateral transpression setting with following tectonic developments and related low-grade metamorphism.

Financial support. The reported study was funded by RFBR and Czech Science Foundation according to the research project № 19-55-26009. Centre of collective usage ‘Geoportal’, Lomonosov Moscow State University (MSU), provided access to remote sensing data.