Comparative analysis of U-Pb dating of zircons from Early Carboniferous volcanites and Middle Triassic alkaline granitoids of the Magnitogorsk zone (Southern Urals)

Introduction. Determination of the age of igneous roc Comparative analysis of U-Pb dating of zircons from Early Carboniferous volcanites and Middle Triassic alkaline granitoids of the Magnitogorsk zone (Southern Urals)

ks by the U-Pb isotope method using zircons is currently one of the main dating methods. Here we present new isotopic data of zircons from alkaline granitoids of the Cheka massif and zircons from acidic volcanites of the lower Carboniferous of the Magnitogorsk zone (Southern Urals).

Materials and methods. The Middle Triassic isotopic age of the Cheka massif was determined by the Rb-Sr isochron method. Currently, we obtained new seven U-Pb dates based on zircons isolated from various phases of the massif. Early Carboniferous volcanites are represented by a contrast moderately alkaline series. Volcanites have been sampled at two points. The U-Pb dating was performed at the All-Russian Geological Research Institute using SHRIMP-II.

Results. At least two zircon populations of early Carboniferous isotopic age have been identified in acid volcanites. The first population is represented by full   crystals and their fragments 100-200 microns in size. They have a short-prismatic habit and a clear oscillatory zonation. This population is predominant in all samples. Zircons have a moderate content of U and Th. The population is homogeneous with average concordant age is 348.5 ± 3.1 Ma.

Zircons of the second population were found in all samples. They are small (about 50 microns), perfectly faceted crystals with an increased content of U and Th. Their isotopic ages (344 and 351 Ma) are entirely fit the age range of the first population. Thus, completely different in morphology and composition, zircons have the same isotopic age.

Two most representative samples of alkaline granitoids, provide zircons 150-250 microns in size. They are light in the cathodoluminescent image, with a clear fine oscillatory zonation and weakly expressed sectorial. The range of isotopic ages of these zircons in is 342.6–376.6 Ma, and their average concordant age is almost the same: 353.9±4.0 and 352.7±3.9 Ma.

Discussion. U-Pb dating of zircons from acidic volcanites confirmed their Tournaisian age. The morphology and composition of zircons turned out to be an important key to understanding the age of volcanites intruded by the alkaline granitoids.

Inherent zircons in alkaline granitoids may not be crystallized at all, since all zirconium should be concentrated in alkaline dark-colored minerals. In this case, only the inherited zircon will remain in the rock. In addition, the dissolution of inherited zircons can also occur in alkaline melts.

Early Carboniferous zircon grains in all samples of alkaline granitoids are similar to those from volcanites. They have a typically magmatic appearance and zonation and the concentration and ratio of uranium and thorium are also typical. At the same time, alkali-rich fluid-saturated magmatites are usually characterized by a Th/U ratio close to or significantly higher than 1. Uranium and thorium concentrations are usually very high. The described features most likely indicate the xenogenic nature of Early Carboniferous zircons in relation to granitoids.

Financial support. The research has been funded by RFBR (research project № 19-55-26009).