Paleomagnetic studies of rapakivi complexes in the Fennoscandian shield – Implications to the origin of Proterozoic massif-type anorthosite magmatism

Johanna Salminen,Sten-Åke Elming,Satu Mertanen,Chong Wang,Bjarne Almqvist,Mohsen Oveisy Moakhar

doi:10.1016/j.precamres.2021.106406

Abstract

Paleomagnetic studies have been performed on five rapakivi related complexes in Sweden and Finland. Poles of varying quality have been defined and the majority of the ∼1640–1497 Ma poles are clustering on low latitudinal positions. By combining data from ∼1500 Ma intrusions a new high-quality pole (Plat: 13°N; Plon: 190°E; A95: 11°, K: 14) for Baltica has been defined. Tectonic reconstructions, on the basis of the new data and previously published high-quality data, indicate that Baltica experienced stable low latitude to equatorial positions during 1640–1470 Ma, temporally coinciding with globally pronounced rapakivi-anorthosite magmatism. Our study argues against single hotspot source for ∼1640–1620 Ma, ∼1590–1520 Ma, and 1470–1410 Ma rapakivi-anorthosites, but supports a model of large-scale superswell under a stationary low-latitude position of supercontinent Nuna for the origin of rapakivi-anorthosite magmatism. However, a possibility for convergent tectonism as the origin cannot be ruled out.

Highlights

The type locality of the Proterozoic rapakivi granites, the Wiborg batholith, extends from the southeastern Finland to Russia comprising a varied series of granitic rocks (e.g. Haapala et al, 1990; Ramo, 1991; Ramoet al., 2014)
We describe briefly the geology of the sampled Kopparnas dyke swarm related to Obbnas rapakivi granite in southeastern Finland and the Ragunda, Stromsbro, Rodoand Noran rapakivi complexes and associated dyke swarms in central Sweden
We suggest that used modern demagnetizing techniques here are capable to isolate the characteristic remanent magnetization (ChRM), where as in the earlier study of Piper (1980) the ChRM was masked by secondary remanence

Summary

Introduction

The type locality of the Proterozoic rapakivi granites, the Wiborg batholith, extends from the southeastern Finland to Russia comprising a varied series of granitic rocks (e.g. Haapala et al, 1990; Ramo, 1991; Ramoet al., 2014). Though, according to Ashwal and Bybee (2017) consensus on the tectonic environment for Proterozoic massif-type anorthosites has gradually shifted towards convergent margin settings (Corrigan and Hanmer, 1997; Scoates and Chamberlain, 1997; Ashwal, 2010) They suggest a tectonic model with an Andean-type continental arc setting, where magmatism is initiated during active convergent margin processes, to be most consistent with the overall observations made so far for massif-type anorthosites and related rocks. Kukkonen and Lauri (2009) have proposed a link between ~1850 and 1780 Ma Svecofennian late orogenic thermal evo lution and the rapakivi granite magmatism in Finland caused by thick ened crust In their collisional model the middle-lower crust and the upper mantle continued warming 200 m.y. after the collision, until Mesoproterozoic times

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Discussion

Conclusion