Early Carboniferous rifting in the Southern Urals: New isotopic dating of plutonic and volcanic complexes

Early Carboniferous igneous rocks are widespread in the Southern Urals. We have obtained new stratigraphic and isotopic data on plutonic and volcanic complexes, allowing us to determine correlation of their age and to construct a new geodynamic model.

The prevailing tectonic setting in the Southern Urals during the Early Carboniferous was sinistral transtension. Volcanic and plutonic complexes in transtensional zones were synchronously formed along large submeridional orogen-parallel strike-slip faults, but are particularly abundant within two N–S-trending zones: Magnitogorsk and East Ural.

The upper Tournaisian–lower Visean sequence in the Magnitogorsk zone consists mainly of moderately alkaline volcanic rocks, basalt and rhyolite are predominant, but pyroclastic, volcano-sedimentary, terrigenous, and carbonate rocks are also widespread. The middle Visean sequence consists of moderately alkaline basalt, andesite, dacite including lavas, tuffs and tuffites. The thickness of the Lower Carboniferous volcanic group varies from 1200 to 5500 m. The age of the volcanic rocks has been proved by findings of foraminifera in limestone interbeds. The oldest volcanic rocks appear in upper Tournaisian, while the youngest are found in the middle upper Visean. New U–Pb zircon dating using SHRIMP is now in progress.

Volcanic rocks in the East Ural zone occur within a few tectonic sheets. The sequence consists of lavas and tuffs of basalt, basaltic andesite, andesite and rhyolite. The total thickness of the sequence varies from 800 to 1500 m. The age of the sequence is determined by findings of fossil plants as middle Visean.

We studied eight plutons in the Magnitogorsk and six in the East Ural zones. Most of them record several intrusive phases. The composition of the rocks varies from gabbro to granodiorites and granites from normal to moderately alkaline series. We combined our new isotopic data on zircons (SHRIMP) with published ages and came to the following conclusions.

• Two main stages of Early Carboniferous plutonism can be distinguished in the Southern Ural. The first began simultaneously in both zones at the Devonian/Carboniferous boundary (ca. 356–357 Ma) and then changed to volcanic activity at around 346 Ma in the Magnitogosk zone and at around 340 Ma in the East Ural zone, respectively. The second stage began after the termination of volcanic activity and corresponds to 334–327 Ma interval in both zones. So, stages of active volcanism and plutonism alternate in time.
• Early Carboniferous rifting began with intrusion of plutons, usually associated with transtensional zones under oblique collision. The subsequent volcanic stage corresponds to local extension. The next stage of plutonism began just after volcanism termination and marked a cessation of tectonic activity.

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.