GEOLOGY AND ISOTOPE DATING OF THE SUMIAN-SARIOLIAN LOWER PROTEROZOIC VOLCANOPLUTONIC COMPLEX IN NORTHERN KARELIYA, THE PAANAJARVI-TSIPRINGASTRUCTURE

The rocks of the Paanajarvi-Tsipringa structure, northern Kareliya, we re studied by the U-Pb method using zircons to obtain geochronological datings for the following: plagiomicrocline granites (2710 +/- 110 Ma ), which intrude the granite-gneisses of the Archean basement and are covered by a pre-Sumian weathering crust; the acid metavolcanics lying at the base of the Lower Proterozoic section (2432 +/- 22 Ma); and th e comagmatic acid volcanics of the Nuorunen granites (2450 +/- 72 Ma). The petrochemical characteristics of the Sumian-Sariolian basic metav olcanics of the Paanajarvi-Tsipringa structure show an affinity with v olcanics of continental rifts. A persistent geochemically bimodal (and resite-basalt-dacite-rhyolite) association of volcanics can be traced, in our opinion, at different levels in the pre-Jatulian part of the s ection of the structure. Therefore, this association, in combination w ith clastogenic metasedimentary and sedimentary-volcanogenic units and tilloids, is regarded as a separate Sumian-Sariolian tectonic-facies complex with group rank. The geological, petrochemical, and geochronol ogical data described in the paper testify that the Sumian-Sariolian s edimentary-volcanogenic rocks and the stratified basic and acid intrus ives were formed in an active tectonic setting during an early stage i n the development of a continental rife between 2.45 and 2.40 Ga as pa rt of a single volcanoplutonic complex. The K-Ar method was used to da te the last endogenic processes of the region as late- and post-Svecof ennian.