Single-zircon evaporation ages and Rb–Sr dating of four major Variscan batholiths of the Urals: A perspective on the timing of deformation and granite generation

Abstract

Using single-zircon stepwise-evaporation 207Pb/ 206Pb and whole-rock Rb–Sr methods, we have dated the main facies of four Variscan batholiths of the Urals — Syrostan, Verkhisetsk, Murzinka and Dzhabyk — from different geodynamic environments. Syrostan and Verkhisetsk are subduction-related batholiths of the accreted terrains in the west, closely related to the Main Uralian Fault. Syrostan is composed of metaluminous epidote-bearing synkinematic gabbros, diorites and granodiorites ( 87Sr/ 86Sr init≈0.7029–0.7035), dated at 333±3 Ma, and postkinematic granites ( 87Sr/ 86Sr init≈0.7035–0.7038) dated at 327±2 Ma, whose zircons occasionally have cores with an age of ∼1800 Ma. The presence of these relict zircon cores probably indicates the contribution of Russian Platform materials to subduction-related magmas. Verkhisetsk, 250 km north of Syrostan, comprises deformed amphibole- and epidote-bearing granodiorites ( 87Sr/ 86Sr init≈0.7042–0.7044), dated at 318±3 Ma, and undeformed granites ( 87Sr/ 86Sr init≈0.7044) that record a long history of recurrent partial melting and crystallization, from ∼310 Ma to ∼275 Ma. Murzinka and Dzhabyk are continental-type batholiths in the east Uralian zone. Dzhabyk, in the southern Urals, postdates the Kartali seismic reflector and is composed of several coeval intrusions of metaluminous monzonites ( 87Sr/ 86Sr init≈0.7049) and peraluminous granitoids ( 87Sr/ 86Sr init≈0.7043) dated at 291±4 Ma. Murzinka, 600 km north of Dzhabyk, also comprises two coeval intrusions of peraluminous granites with different chemical and isotopic compositions ( 87Sr/ 86Sr init≈0.7045 and 0.7093) dated at 254±5 Ma. These data indicate that the age of cessation of movements along the Main Uralian Fault and the transition from subduction to continent–continent collision migrated northwards. The age of granite magmatism also migrated gradually northwards and eastwards, suggesting an oblique convergence with a SE movement of the converging continent, so that the closure of the Uralian palaeo-ocean started in the south and then migrated progressively northwards.