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Free keywords:
etroeclogite; biotite-orthopyroxene eclogite; Palaeoproterozoic cold
subduction; pseudosection modelling; Zr-in-rutile thermometry; elastic
thermobarometry; Rb/Sr mineral isochron
Abstract:
Pseudosection modelling of a relict garnet-core in Palaeoproterozoic rocks from the Gridino area in the southern Belomorian belt of Karelia reveals peak-pressure eclogite-facies conditions of 610–650°C, 18–20 kbar for two retro-eclogite samples and 610–665°C, 23–26 kbar for a rare Mg-rich biotite-orthopyroxene eclogite, suggesting low initial metamorphic field gradients of 6.6–10°C/km. This confirms an earlier finding in Karelia and, considering other Palaeoproterozoic eclogite occurrences worldwide, that ‘cold’ subduction conditions, characteristic of modern-style subduction, occurred during the Palaeoproterozoic, ∼2 Ga ago, for the first time in Earth history. However, compositions of most other phases in the retro-eclogite were reset by diffusion, deformation and recrystallisation during subsequent pressure release and heating to variable degrees, a reason for earlier overestimations of temperatures. By contrast, peak-pressure conditions for a biotite paragneiss (640–740°C, 15–18 kbar) that occurs close to the biotite-orthopyroxene eclogite locality already show an early resetting of its initial assemblage. High-pressure granulite-facies peak-temperature conditions of the retro-eclogite at 712± 5°C, 9–12 kbar (along a field gradient of 20°C/km) were determined by Zr-in-rutile thermometry and quartz-in-garnet elastic barometry. These conditions were dated by a Rb/Sr mineral isochron for the biotite-orthopyroxene eclogite at 1830±20 Ma for the first time. Using existing ages for the peak-pressure conditions, possible slow overall exhumation rates of <0.9 mm/y between eclogite and the granulite-facies stages could be determined that are compatible with erosion as the main exhumation mechanism. The peak-temperature conditions were possibly established by thermal relaxation during early exhumation. However, a younger Rb/Sr mineral isochron for the biotite paragneiss indicates a characteristic Sr-isotopic disequilibrium distribution caused by diffusion during slow cooling between ∼1800 and 1750 Ma during later exhumation.
