New data on the age (SHRIMP II) of protolith and paleoproterozoic transformations of the Archean Keivy terrain (Kola Peninsula)

The Keivy terrain is a unique structure principally different from adjacent territories. It comprises four complexes of magmatic and supracrustal rocks: the supposed basement composed of the tonalite- trondhjemite association (TTG), the Lebyazhinskaya Series of acid and intermediate metavolcanic rocks, the Keivy Series of highaluminum schists (1), and the complex of alkaline granites, sometimes associating with ultrabasic and alkaline rocks (2). The tectonic boundaries of the terrain control the location of basic and utrabasic intrusions including the layered plutons of the Panskaya and Fedorovskaya tundra. Of special importance for the solution of the problems of peri� odization of endogenous problems are the studies in which the U-Pb study of zircons provided the Neoarchean age of metavolcanic rocks of the Leb� yazhinskaya Series (2750 Ma) (3), alkaline granites and granosyenites of the western part of the Keivy ter� rain (2630-2674 Ma) (4, 5), and plutons of the Pan� skaya and Fedorovskaya tundra (2500 Ma) (6). This demonstrates that the considered structure is not only entirely composed of Archean formations, but its sep� aration took place at the end of the Neoarchean. This study was aimed at local U-Pb (SHRIMP) dating of the processes of Paleoproterozoic tectonic and meta� morphic transformation of Archean structures within the considered terrain. The structural geological investigations of the Keivy terrain demonstrating that later it underwent radical transformation allowed us to make a conclu� sion on the presence of nappe structures in the studied area of the Keivy terrain and on the formation of this terrain with the Bolshie Keivy nappe as one of its com� ponents (7-9) in the course of the thrust formation (10) during the Neoarchean-Paleoproterozoic colli� sional processes in the southwestern boundary of the Kola fragment of the Archean craton. The Bolshie Keivy nappe is the upper part of the tectonostratigraphic section of the Keivy terrain and composed of plates and lenses of blastomylonitic, mainly highaluminum metamorphic and metaso�