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

Abstract

Petrological and geochronological (zircon, U-Th-Pb (LA-ICP-MS)) studies of rocks from the Poriya Guba tectonic mélange exposed on the Ozerchanka and Palenyi islands were carried out to decipher the composition and tectonic history of the Paleoproterozoic Lapland-Kola orogen (LKO). Tonalite–trondhjemite–granodiorite (TTG) (Grt)–Cpx–Opx gneisses dominate on 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, Т_Nd(DM) = 2.9–3.0 Ga). The HREE depletion in the gneisses indicates that the parental melts of their igneous protoliths were formed in equilibrium with a garnet-bearing residue. Mafic granulite bodies vary widely in geochemistry and likely represent fragments of several Paleoproterozoic mafic intrusions and dikes. Mineral assemblages in the gneisses recorded the early granulite-facies (Т = 780–820°С and Р = 8.6–9.4 kbar) and later amphibolite-facies (Т = 640–650°С and Р = 6.7–7.3 kbar) metamorphic events that occurred at 1.9 Ga according to zircon ages. On Palenyi Island, the banded Grt–Cpx–Opx gneisses are predominant and vary in composition from basaltic andesites to rhyolites. The volcanic protoliths of these rocks have island-arc geochemical signatures, a Paleoproterozoic age of 1958 ± 6 Ma, and juvenile origin (ε_Nd(1960) = +1.7 ÷ +3.1; Т_Nd(DM) = 2.2–2.3 Ga). These rocks were metamorphosed under the granulite-facies conditions at about 1.9 Ga. Two models can explain the presence of the Archean block in the Poriya Guba tectonic mélange, which is 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 opening of the Lapland–Kola ocean and then was tectonically juxtaposed with Paleoproterozoic subduction complexes during the Lapland-Kola collisional orogeny. Second, the Archean block may represent the margin of an adjacent Archean continent exposed in an erosional window within the Paleoproterozoic Poriya Guba tectonic nappe.