摘要
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.
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.
