The Heggmovatn supracrustals, North Norway—A late Mesoproterozoic to early Neoproterozoic (1050–930 Ma) terrane of Laurentian origin in the Scandinavian Caledonides

Rocks of the “Heggmovatn dome”, North Central Scandinavian Caledonides have traditionally been interpreted as a fold nappe involving Baltican basement rocks. They are also considered to be comparable in age to the orthogneisses in the Glomfjord dome to the south, the Rishaugfjell dome to the east and the Tysfjord culmination to the north, dated to ca. 1.7–1.8 Ga. However, while these basement culminations are generally dominated by orthogneisses, the Heggmo area is composed of alternating metapsammites and metapelites intruded by both pre-and post-tectonic granitoids. U–Pb ID-TIMS ages on magmatic zircons from the syn/post-tectonic granitoids indicate a mid-Silurian emplacement age (ca. 430 Ma) whereas zircons from a migmatitic biotite-rich pre-tectonic intrusive indicate a Grenvillian/Sveconorwegian crystallization age (ca. 925 Ma). U–Pb LA-ICPMS ages on clastic zircons from the Heggmo metapsammites, coupled with ages from cross-cutting granitoid intrusives constrain the period of deposition of the sedimentary sequence to be between ca. 1050 Ma and ca. 930 Ma.There are strong similarities in lithologies, depositional age, age of clastic zircon populations and magmatic activity, to Laurentian derived supracrustal sequences, particularly, the Krummedal Sequence in Northeast Greenland. Based on these similarities and on its high tectonic position in the Caledonian nappe pile, the Heggmo rocks are reinterpreted to represent an exotic terrane of Laurentian affinity, probably derived from present day NE Greenland, and thrust onto Baltica as part of the Uppermost Allochthon during the Scandian phase (E. Devonian) of the Caledonian orogeny.

Figure optionsDownload as PowerPoint slideHighlights
► The Heggmo rocks previously thought to represent Baltic basement of ca. 1.8 Ga.
► We report U–Pb zircon data on clastic and magmatic zircons from the Heggmo terrane.
► Depositional age of metasediments is 1052–950 Ma and intrusions aged 925 and 431 Ma.
► Lithologies, clastic zircons and magmatism similar to Laurentian Krummedal Sequence.
► We interpret the Heggmo terrane to be an exotic thrust sheet derived from Laurentia.