Timanide (Ediacaran-Early Cambrian) Metamorphism at the Transition from Eclogite to Amphibolite Facies in the Beloretsk Complex, SW-Urals, Russia

Abstract

The Beloretsk Metamorphic Complex in the SW Urals formed at a convergent eastern margin of Baltica during the Neoproterozoic-Early Cambrian Timanide orogeny. It comprises three major units with lenses of facies-critical metabasites within metasedimentary rocks: A lowermost eclogite unit, an intermediate garnet amphibolite unit and an upper amphibolite-greenschist unit. Pressure (P)-temperature (T)-paths of four rocks from the two lowermost units were determined mainly by PT pseudosection techniques showing similar clockwise loops at different peak metamorphic, water-saturated conditions: A phengite-bearing eclogite shows peak PT conditions of 16.5–18.5 kbar/525–550 °C (stage I) followed by stage II at 11.5–13.0 kbar/585–615 °C. A garnet amphibolite from the intermediate unit yields lower peak conditions of 11.7–14.5 kbar/480–510 °C (stage I) followed by stage II at 9.5–11.0 kbar/535–560 °C. However, a granite gneiss in the eclogite unit shows similar maximum pressures as the eclogite, but higher temperatures at 15.6–16.2 kbar/660–675 °C, whereas a garnet micaschist contains comparable high pressure relicts, but underwent an advanced midcrustal reequilibration at 7.5–9.0 kbar/555–610 °C. We dated the eclogite by a 7-point Rb/Sr mineral isochron (phengite, omphacite, apatite) at 532.2±9.1 Ma interpreted as age of crystallisation of the eclogitic peak PT assemblage. This age is the youngest compared to the known Timanide metamorphic and magmatic ages.

Keywords

Urals / eclogite / high pressure metamorphism / Timanide orogeny / Rb-Sr dating / PT pseudosections / PT-path

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