Basement - cover relationships in the Bodø area : The “basement” rocks in the Heggmovatn and Bodø areas: a Fennoscandian, Laurentian or suspect terrane?

Most maps and publications dealing with the geology of the Bodø-Sulitjelma area have postulated the presence of a major basement culmination; the “Heggmovatn basement dome” between Bodø and Fauske. Granitic gneisses and orthogneisses which dominate the region west of Bodø, have also been considered to be comparable with basement rocks exposed in the Tysfjord and Glomfjord culminations of ~ 1.8 Ga. Whereas orthogneisses are abundant in the Tysfjord and Glomfjord culminations, the Heggmovatn area is dominated by alternating metapsammites and metapelites which are intruded by both pre- and syn/post-tectonic granitoids. U-Pb ages on zircons from the syn/post-tectonic leucogranites indicate a mid-Silurian (~431Ma) emplacement age whiles Neoproterozoic ages (924 – 928 Ma) are recorded in the orthogneisses in both Bodø and Heggmovatn units indicating Grenvillian/Sveconorwegian magmatic activity which has not been previously recorded in this area. The lithologic differences between the Heggmovatn supracrustals and related intrusives, and the nearby basement culminations, as well as the marked differences in ages indicate that neither the Bodø gneisses nor the Heggmovatn units are Baltic basement rocks. In addition to lithologic similarities between the Heggmovatn metasediments and Krummedal Sequence of the East Greenland Caledonides, the obtained data constrains the period of deposition of the Heggmovatn metasediments to be between 1138 – 928 Ma which is remarkably similar to the 1100 – 930Ma depositional age of the Krummedal Sequence. The similarities in clastic zircon populations as well as the ages of their associated intrusives suggests very strongly that the Heggmovatn metasediments represent an exotic thrust sheet closely related to the Krummedal Sequence. The obtained data strengthens the interpretation that the Uppermost Allochthon in the Scandinavian Caledonides represents a fragment of Laurentia crust.