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

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,watersaturated conditions: A phengite-bearing eclogite shows peak PT conditions of 16.5–18.5 kbar/525–550 ℃(stage I) followed by stage II at 11.5–13.0 kbar/585–615 ℃. A garnet amphibolite from the intermediate unit yields lower peak conditions of 11.7–14.5 kbar/480–510 ℃(stage I) followed by stage II at 9.5–11.0 kbar/535–560 ℃. 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 ℃,whereas a garnet micaschist contains comparable high pressure relicts,but underwent an advanced midcrustal reequilibration at 7.5–9.0 kbar/555–610 ℃. 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.