Timanian blueschist-facies metamorphism in the Kvarkush metamorphic basement, Northern Urals, Russia

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Abstract
The core of the Kvarkush Anticline, on the western side of the Northern Urals, contains pre-Ordovician metasediments and minor metabasites that have undergone multistage deformation and transitional greenschist- to blueschist-facies metamorphism. Metamorphic conditions for the blueschist-facies assemblages are estimated at 350–400 °C and 7–8 kbar. The metamorphic rocks are cut by mafic dykes, one of which yields a Sm-Nd mineral age of 398 ± 37 Ma (2σ). The age of the high-pressure/low-temperature ( HP/LT ) metamorphism has been controversial. New Rb-Sr mineral data yield ages for the waning of blueschist-facies metamorphism at c. 536 Ma (535 ± 6 Ma, 535.8 ± 6.5 Ma, 536 ± 19 Ma, all 2σ), demonstrating that the specific conditions required for blueschist-facies metamorphism were attained in latest Neoproterozoic to earliest Cambrian times. The new data are the first well-constrained documentation of HP/LT metamorphism within the Timanian Orogen. We suggest that easterly-directed subduction of the East European continental margin (present-day coordinates) during the Timanian Orogeny, involving compressional thickening of the margin sediments, is the most likely scenario for the formation of the Kvarkush blueschists. The Kvarkush HP/LT metasediments, together with lower greenschist-facies and subgreenschist-facies metasediments further west, form parts of a Timanian orogenic wedge on an east-dipping thrust ramp. Reworking during the Uralian orogenesis was of minor importance.