ARCHEAN ECLOGITES IN THE BELOMORIAN MOBILE BELT, BALTIC SHIELD

Abstract

Eclogites of Archean (2720± 8 Ma) and Proterozoic (2416.1± 1.3 Ma) age were found in the Belomorian Mobile Belt of the Baltic Shield near the village of Gridino in Karelia. The unique Archean eclogites described in this paper and the symplectitic apoeclogitic rocks that developed after them in the course of their retrogression are components of a complicated polygenetic chaotic complex, which is thought to be composed of extensively migmatized rocks of a tectonic melange zone. The eclogite-bearing complex is cut by gabbronorite dikes (2.43-2.44 Ga) and small plagiogranite intrusions and veins (which were dated by the U-Pb zircon method at 2701.3 ± 8.1 Ma in Stolbikha Island). Zircons from the symplectitic apoeclogites were dated by the U-Pb method on a NORDSIM ion microprobe at the Archean (2720 ± 8 Ga). The petrochemical characteristics of the eclogites are comparable with those of metabasalts (amphibolites) of the ophiolite-like complex in the Central Belomorian Mafic Zone. This paper reports the results of the detailed geological, petrological, and mineralogical examination of the complex and the P-T metamorphic parameters of the eclogites during their prograde and retrograde metamorphic evolution. The prograde metamorphism (P = 14.0-17.5 kbar, T = 740-865°C) of the eclogites occurred in a geodynamic environment in which the processes of "warm" subduction could take place. The trajectory of the multistage subisothermal decompression during the retrograde stage with the pressure decreasing from 14.0 to 6.5 kbar at T= 770-650°C was controlled by the exhumation of the eclogites.