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

摘要

Using single-zircon stepwise-evaporation Pb-207/Pb-206 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 (Sr-87/Sr-86(init) approximate to 0.7029-0.7035), dated at 333 +/- 3 Ma, and postkinematic granites (Sr-87/Sr-86(init) approximate to 0.7035-0.7038) dated at 327 +/- 2 Ma, whose zircons occasionally have cores with an age of similar to 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 (Sr-87/Sr-86(init) approximate to 0.7042-0.7044), dated at 318 +/- 3 Ma, and undeformed granites (Sr-87/Sr-86(init) approximate to 0.7044) that record a long history of recurrent partial melting and crystallization, from similar to 310 Ma to similar 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 (Sr-87/Sr-86(init) approximate to 0.7049) and peraluminous granitoids (Sr-87/Sr-86(init) approximate to 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 (Sr-87/Sr-86(init) approximate to 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. (C) 2000 Elsevier Science B.V. All rights reserved. [References: 19]