Abstract

Abstract: The last magmatic stage before the Sveconorwegian orogeny in the Baltic Shield is represented by bimodal, c.1.16 Ga volcanics from the Bandak Group in southern Norway and largely coeval basalts from the Dal Group in SW Sweden. In both areas, the volcanic rocks are intercalated with sedimentary units and the onset of basin development is marked by deposition of clastic sediments.

In the lower part of the Bandak Group, the Morgedal basalts are relatively evolved and geochemical signatures suggest assimilation of lower crustal components. In contrast, the subsequent Gjuve basalts are more primitive and record assimilation of upper crustal rocks similar to exposed basement rocks.

In the Bandak Group, the chemistry of the basalts changes within the stratigraphy, such that the older Morgedal basalts (MgO = 5.2–8.8 wt%, initial ϵNd 2.81–3.96, Zr/Y = 4.0–5.1, Zr/Nb = 34–50, La/Nb = 2.4–3.7) are more fractionated and have more enriched Nd isotope ratios compared to the Gjuve basalts (MgO = 6.4–12.3 wt%, initial ϵNd 4.05–4.97, Zr/Y = 3.4–4.7, Zr/Nb = 20–30, La/Nb = 1.5–2.4). This suggests that there earliest basalts are more contaminated, and it appears that the composition of the crustal component changed with time.

A thin felsic volcanic unit, the Dalen Formation, separates the two basalt sequences. It represents melting of upper crustal rocks, triggered by injection of mafic magmas into crustal magma chambers. In the Dal Group, basalts form a relatively thin unit (< 500 m) where flows with high MgO values (> 9%) were least affected by assimilation of crustal components.

Whole-rock geochemistry and Nd isotope ratios for the Bandak and Dal basalts indicate shallow melting of sub-continental lithospheric mantle in response to extension in a continental back-arc setting, related to subduction along the western margin of Baltica. These constraints envision a similar tectonic evolution with that of eastern Laurentia, which support models of a pre-Grenvillian supercontinent with a long-lived, active margin that reached western Baltica.

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