Neoarchean A-type acid metavolcanics in the Keivy Terrane, northeastern Fennoscandian Shield: Geochemistry, age, and origin

Abstract

The Lebyazhka acid metavolcanics occupying 40% of the Keivy Terrane in the Arctic Region of the Fennoscandian Shield belong to mainly alkali-calcic and calc-alkaline series and are classified as peraluminous to metaluminous metadacites, metarhyodacites, metarhyolites and metatrachyrhyolites. Geochemical characteristics of most of these metavolcanics are very close or identical to those discriminative of A-type granites. Compositionally these rocks are similar and often identical to the Keivy A-type peralkaline granites. On variation plots and geochemical spider and discriminant diagrams the Lebyazhka acid metavolcanics and Keivy peralkaline granites share the same trends and patterns and fall into fields of post-orogenic, within-plate and A-type granites. A U–Pb (SHRIMP II) zircon age of the Lebyazhka acid metavolcanics is 2678 ± 7 Ma. The first thermal event recorded by the U–Pb isotope system of magmatic zircon occurred at ca. 1.90 Ga, an age corresponding to the termination of the Paleoproterozoic Lapland-Kola collisional orogeny. The youngest zircon generation is dated at 1805 ± 23 Ma ago and is assumed to have grown during metasomatic or hydrothermal alteration.The finding of relics of baddeleyite, a mineral crystallizing only in mafic and ultramafic igneous rocks, in zircon from the Lebyazhka acid metavolcanics along with the chemical similarity of these metavolcanics and the Keivy peralkaline granites and their emplacement simultaneously with the Keivy gabbro-anorthosites 2.68–2.66 Ga ago, obviously suggests that these three types of rocks should have a common origin. Based on new data and existing tectonic models, the origin of the Lebyazhka acid metavolcanics as well as the Keivy peralkaline granites, gabbro-anorthosites and nepheline syenites is concluded to be linked with underplating by basaltic magma of the lower crust of the eastern Kola Province 2.68 Ga ago. This underplating led to anorogenic bimodal magmatism including the 2678 Ma Lebyazhka acid metavolcanics, the 2674–2663 Ma Keivy peralkaline granites, the 2674–2659 Ma Keivy gabbro-anorthosites, and the 2645 Ma Sakharjok nepheline syenites and subalkaline gabbro. The underplating also caused partial melting of the Mesoarchean continental crust and involvement of this older crustal component into felsic melts parental for the Lebyazhka acid metavolcanics and the Keivy peralkaline granites.