A 2.44 GA SYN-TECTONIC MAFIC DYKE SWARM IN THE KOLVITSA BELT, KOLA PENINSULA, RUSSIA: IMPLICATIONS FOR EARLY PALAEOPROTEROZOIC TECTONICS IN THE NORTH-EASTERN FENNOSCANDIAN SHIELD

Abstract

The Kolvitsa Belt in the south-western Kola Peninsula formed coeval with the earliest Palaeoproterozoic rift-belts in the Fennoscandian Shield. The Palaeoproterozoic history of this belt comprises the deposition of the 2.47 Ga Kandalaksha amphibolite (metabasalt) sequence onto 2.7 Ga granitoid gneisses, the intrusion of the 2.45–2.46 Ga Kolvitsa Massif of gabbro-anorthosite and the subsequent multiple injection of mafic dykes and magmatic brecciation, followed by the intrusion of 2.44 Ga dioritic dykes, and extensive shearing at 2.43–2.42 Ga. The gabbro-anorthosite and dykes contain high-pressure garnet-bearing assemblages that have previously been considered as evidence for metamorphism in a compressional setting of the Kolvitsa Belt at 2.45–2.42 Ga, i.e. coeval with the formation of the Imandra–Varzuga rift-belt and layered mafic intrusions in an extensional setting. The Kochinny Cape study area on the White Sea coast presents an unique remnant of a 2.44 Ga mafic dyke swarm that endured ca. 1.9 Ga collision but preserved its primary structural pattern well. All these dykes were intruded along numerous NW-trending shear zones within the Kolvitsa Massif and contain angular xenoliths of sheared gabbro-anorthosite. Every new batch of mafic melt underwent shearing during or immediately after solidification, and later dykes intruded into already sheared dykes. Thus, rocks of the Kolvitsa Massif and its dyke complex were successively injected into a large-scale shear zone which was active from ca. 2.46 to 2.42 Ga. Multiple injection of mafic melts, the presence of mutually intruding, composite, sheared mafic dykes, of magmatic breccias with gabbroic groundmass, and of host rocks fragments (showing no evidence of tectonic stacking at the time of brecciation), all indicate an extensional setting. Shearing was also extensional as it occurred simultaneously with the multistage magmatism. The asymmetric morphology of deformed dykes, and asymmetric flexures within weakly deformed lenses show that all these extensional shear zones, apart from a few exceptions, are dextral, were formed in a transtensional setting and are attributed to general W–E to WSW–ENE extension. Structural data available for 2.4–2.5 Ga magmatic rocks elsewhere in the Kola region suggest that the same kinematics operated on a regional scale. The presence of the garnet-bearing assemblages in gabbro-anorthosite and dykes may be explained by crystallisation and shearing of the magmatic rocks at deep crustal levels. Alternatively, corona development might have occurred much later as a result of tectonic loading due to the juxtaposition and overthrusting of the Umba Granulite Terrane onto the Kolvitsa Belt at ca. 1.9 Ga. In view of the field evidence and published ages, an overall extensional setting rather than a combination of compressional and extensional zones is preferable for Palaeoproterozoic tectonics in the north-eastern Fennoscandian Shield at 2.5–2.4 Ga.