U-PB ZIRCON AGES FROM THE BINDAL BATHOLITH, AND THE TECTONIC HISTORY OF THE HELGELAND NAPPE COMLEX, SCANDINAVIAN CALEDONIDES

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 metasupracr ustal rocks present as non-conformable cover sequences to subjacent fr agments of Early Ordovician ophiolitic rocks. All of these rock units have been intruded by the Bindal Batholith, which ranges in compositio n from mafic gabbro to leucogranite. In this paper, U-Pb zircon ages f rom 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 Krakfjellet pluton (443 +/- 7 Ma), a granodio rite west of Gasvassfjellet (437 +/- 4 Ma), the Kalvvatnet monzonite ( 435 +/-10 Ma), and a monzodiorite near Tosbotn (430 +/- 7 Ma). The dat es are essentially concordant and are considered to reflect the crysta llization ages of the intrusions. Based on regional correlations and t he age of the oldest of the dated plutons, it is concluded that intern al 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 t o post-date emplacement of the Bindal Batholith.