New geochronological and Sm–Nd constraints across the Pajala shear zone of northern Fennoscandia: Reactivation of a Paleoproterozoic suture

Abstract

Altogether 1130 U–Pb analyses on zircons from 25 samples across the Pajala shear zone in northern Fennoscandia were performed by LA-ICPMS methodology to study the origin of the zone. The samples were also analyzed for their Sm–Nd isotope systematics and geochemically. The new data is tested with existing geological information to elaborate the provenance and maximum depositional ages, the stratigraphic position of studied samples, and finally to assess the tectonic evolution in the Pajala shear zone area.Chromium-enriched metasedimentary rocks are found within and to the east and west of the N-S trending Pajala shear zone. No material originating from Proterozoic felsic igneous (zircons) or mafic (Sm–Nd) rocks was found in these rocks and thus, only Neoarchean or very early Paleoproterozoic ages of deposition are possible. Our new results indicate that the bimodal 1.99–1.97Ga supracrustal rocks of the Rovaniemi Supersuite cover a large area east of the Pajala shear zone. Based on the absence of Archean zircons typical of the metasedimentary rocks covering the Archean basement, it seems that the basement and its sedimentary cover were not exposed at the time of deposition of the Rovaniemi Supersuite. The metasedimentary rocks of the Uusivirka Supersuite, located within the Pajala shear zone, vary from hornblende-bearing metasedimentary rocks, metapelite, and meta-arkose to orthoquartzite. Their characteristic features include a dominant Proterozoic zircon population (65–87%), with a high proportion of analyzed grains yielding ages between 1.96Ga and 1.91Ga, and maximum depositional ages of 1.92–1.91Ga. We record at least three metamorphic events; at ca. 1.92–1.90Ga, ca. 1.85Ga and at 1.82–1.78Ga, of which the youngest can possibly be separated into two events at 1.83–1.82Ga and 1.79–1.77Ga, respectively. We propose a tectonic model in which continental breakup occurred at 2.1–2.05Ga, followed by the development of a magmatic arc (Kittilä arc) at ca. 2.0Ga above a subduction zone. Two Archean continental units (Norrbotten and Karelia) collided at ca. 1.92–1.91Ga, the Kittilä arc was obducted upon the Karelia continent as a foreland fold and thrust belt, and the metasedimentary rocks of the Uusivirka Supersuite were emplaced as foreland deposits. The proposed tectonic model is based on the absence of 2.44Ga layered intrusions, and especially 2.06Ga rocks and 1.99–1.97Ga rocks of the Rovaniemi Supersuite that are found only east of the Pajala shear zone. In our model the Pajala shear zone originated as a divergent plate boundary, and was multiply reactivated after continental collision with both lateral and vertical movements.