Scientific publications

New data are reported on the geology and petrology of zoisitites, whose geological position in the Gridino eclogite-bearing complex of the Belomorian Province (as part of a clastic component in the foliated granitoid matrix) is similar to that of eclogites. The relict association Pl≥82–83 ± Di suggests an anorthositic origin for the protolith of the zoisitites. The prograde trend of metamorphism comprises: 1) a stage at which white mica of the muscovite – seladonite series with Si concentrations of 3.15–3.55 p. f. u. was formed after calcium-rich plagioclase with phengites and paragonites occurring among them, and 2) a stage at which early association minerals were completely replaced by zoisites with XPs concentrations rising from 0.10 to 0.17. The Р-Т conditions of the prograde trend – 9 kbar / 590 °С to 23.5 kbar / 820 °С – are consistent with high-pressure amphibolite – eclogite facies. The Р-Т conditions of retrograde processes were 1) Zo0.10 – 13 kbar / 600 °С; 2) Zo0.08 – 8 kbar / 570 °С. Anorthosite as the protolith of zoisitites is assumed to be fragments of a layered gabbro-anorthosite massif similar in relict mineralogo-petrological characteristics and age to amphibolitized striated apo-eclogites from Stolbikha Island: the eclogites and zoisitites are dated at ~ 2.72 Ga and the age of their magmatic protolith is ~ 2.75 Ga. The predominance of plutonic rocks in the composition of the clastic component suggests an eclogite-facies metamorphic event in a collision regime. Eclogitized rocks could have occurred as lower-crustal material exhumed at the initial stage of Archean collision in a decompression channel during the formation of a deep continental transtension zone.