Abstract

The Helgeland Nappe Complex forms part of the Uppermost Allochthon in the central Scandinavian Caledonides. It consists of metasupracrustal rocks of assumed Precambrian age which are juxtaposed with metasupracrustal rocks present as non-conformable cover sequences to subjacent fragments of Early Ordovician ophiolitic rocks. All of these rock units have been intruded by the Bindal Batholith, which ranges in composition from mafic gabbro to leucogranite. In this paper, U-Pb zircon ages from five plutons in the southern part of the batholith are presented. These are a porphyritic granite of the Andalshatten pluton (447 ± 7 Ma), a tonalite from the Kråkfjellet pluton (443 ± 7 Ma), a granodiorite west of Gåsvassfjellet (437 ± 4 Ma), the Kalvvatnet monzonite (435 ± 10 Ma), and a monzodiorite near Tosbotn (430 ± 7 Ma). The dates are essentially concordant and are considered to reflect the crystallization ages of the intrusions.

Based on regional correlations and the age of the oldest of the dated plutons, it is concluded that internal accretion and polyphase tectonometamorphism of the Helgeland Nappe Complex took place in Ordovician times. The final Silurian (Scandian) thrusting of the nappe stack across the continent of Baltica appears to post-date emplacement of the Bindal Batholith.

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