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

The Kolvitsa Belt in the south-western Kola Peninsula formed coeval with th e earliest Palaeoproterozoic rift-belts in the Fennoscandian Shield. The Pa laeoproterozoic history of this belt comprises the deposition of the 2.47 G a Kandalaksha amphibolite (metabasalt) sequence onto 2.7 Ga granitoid gneis ses, the intrusion of the 2.45-2.46 Ga Kolvitsa Massif of gabbro-anorthosit e and the subsequent multiple injection of mafic dykes and magmatic breccia tion, followed by the intrusion of 2.44 Ga dioritic dykes, and extensive sh earing at 2.43-2.42 Ga. The gabbroanorthosite and dykes contain high-pressu re garnet-bearing assemblages that have previously been considered as evide nce for metamorphism in a compressional setting of the Kolvitsa Belt at 2.4 5-2.42 Ga, i.e. coeval with the formation of the Imandra-Varzuga rift-belt and layered mafic intrusions in an extensional setting. The Kochinny Cape s tudy area on the White Sea coast presents an unique remnant of a 2.44 Ga ma fic dyke swarm that endured ca. 1.9 Ga collision but preserved its primary structural pattern well. All these dykes were intruded along numerous NW-tr ending shear zones within the Kolvitsa Massif and contain angular xenoliths of sheared gabbro-anorthosite. Every new batch of mafic melt underwent she aring 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 pr esence of mutually intruding, composite, sheared mafic dykes, of magmatic b reccias 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 simultane ously 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, we re 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-a northosite and dykes may be explained by crystallisation and shearing of th e magmatic rocks at deep crustal levels. Alternatively, corona development might have occurred much later as a result of tectonic loading due to the j uxtaposition and overthrusting of the Umba Granulite Terrane onto the Kolvi tsa Belt at ca. 1.9 Ga. In view of the field evidence and published ages, a n overall extensional setting rather than a combination of compressional an d extensional zones is preferable for Palaeoproterozoic tectonics in the no rth-eastern Fennoscandian Shield at 2.5-2.4 Ga. (C) 2001 Elsevier Science B .V. All rights reserved.