Archean Block in the Core of the Paleoproterozoic Lapland-Kola Orogen: New Data on Composition and Age of Rocks from Poriya Guba Islands

Petrological and geochronological (zircon, U-Th-Pb (LA-ICP-MS)) studies of rocks from the Poriya Guba tectonic mélange exposed on Ozerchanka and Palenyi islands were carried out to decipher composition and tectonic history of the Paleoproterozoic Lapland-Kola orogen (LKO). Tonalite-trondipinite-granodiorite (TTG) (Grr)-Cpx-Opx gneisses dominate on the Ozerchanka island. They contain numerous bodies of mafic granulites and are intruded by syn- and post-tectonic granitoids. The TTG gneisses are Archean in age (>2.6 Ga, T(DM) = 2.9–3.0 Ga). These are strong depleted in heavy REEs, which indicates that their parental melts of their igneous protoliths were formed in equilibrium with a garnet-bearing resitie. Mafic granulite bodies vary widely in geochemical characteristics and likely represent fragments of several Paleoproterozoic mafic intrusions and dikes. Mineral assemblages in the gneisses recorded early granulite-facies (T = 780–820°C and P = 8.6–9.4 kbar) and later amphibolite-facies (T = 640–650°C and P = 6.7–7.3 kbar) metamorphic events that occurred at 1.9 Ga according to zircon ages. On the Palenyi island, Grr-Cpx-Opx gneisses are predominant and vary in composition from andesibasalts to rhyolites. The volcanic protoliths of these rocks have island-arc geochemical signatures, a Paleoproterozoic age of 1958 ± 6 Ma, and juvenile origin (ε(1960) = +1.7 ÷ +3.1; T(DM) = 2.2–2.3 Ga). These rocks were metamorphosed under granulite facies conditions at about 1.9 Ga. Two models can explain the presence of the Archean Ozerchanka block which occurs in the Poriya Guba tectonic mélange composed of the Paleoproterozoic juvenile island arc complexes in the core of the LKO. First, this Archean block could represent a single fragment of Archean lithosphere that was separated during Paleoproterozoic continental rifting and the opening of the Lapland-Kola ocean and subsequently was tectonically juxtaposed with Paleoproterozoic subduction complexes during the Lapland-Kola collisional orogeny. Second, the Archean Ozerchanka block may represent the edge of an adjacent Archean continent exposed in an erosional window within the Paleoproterozoic Poriya Guba tectonic mélange.