FORMATION OF THE KHEDOZEROBOLSHOZERSKAYA STRUCTURE OF THE BALTIC SHIELD IN THE LIGHT OF NEW GEOCHEMICAL AND GEOCHRONOLOGICAL DATA

Abstract

Based on existing and newly obtained geological materials, complemeted by new petrogeochemical, geochronological and isotope-geochemical data, a new interpretation of the history of the geological development of the Khedozero-Bolshozerskaya structure in Central Karelia has been proposed. It is generally believed that the three strata identified within this structure consistently and successively overlap each other. This study established that sedimentation and volcanic activity were disparate both spatially and temporally, and occurred in different tectonic settings. The lower terrigenic and middle volcanogenic strata have tectonic contacts, while the upper volcanogenic stratum is a complex of thin transverse dikes. The accumulation of more ancient terrigenic sediments in the lower strata (of metagraywacke nature) occurred in the south of the structure, began no earlier than 2753 ± 6 Ma and was completed before the start of volcanic activity. The sedimentation conditions were similar to those of primitive oceanic island arcs. Volcanic activity (2712 ± 6 – 2703 ± 7 Ma), which is associated with the formation of the two upper strata, took place in the north of the structure, started with lava outflows of andesibasalts, andesites and dacites from the middle strata, and ended with the intrusion of upper-strata rhyolite porphyry dikes. The volcanism occurred in the settings similar to modern mature volcanic arcs. In the course of further geological development involving complex tectonic deformations and areal granitization, the southern terrigenic and the northern volcanogenic strata spatially converged.