Zircon U-Pb ages and Lu-Hf isotopes from basement rocks associated with Neoproterozoic sedimentary successions in the Sparagmite Region and adjacent areas, South Norway: the crustal architecture of western Baltica

摘要

The Neoproterozoic Hedmark Group (Lower Allochthon), Valdres Group (Middle Allochthon) and Engerdalen Group (Middle Allochthon) contain allochthonous basement slices with tectonic and depositional contacts to sedimentary rocks. The basement slices were transported from the western margin of the continent Baltica during the Caledonian Orogeny. Zircon U-Pb and Lu-Hf isotopes reveal that allochthonous basement rocks in the Osen-Rola Nappe Complex, forming the depositional substrate of the Hedmark Group, are related to the Transscandinavian Igneous Belt (TIB) in terms of age and petrogenetic history. The Kvitvola Nappe Complex of the Middle Allochthon, carrying the sedimentary Engerdalen Group, contains c.1620 Ma and 1180 Ma augen gneisses. The oldest augen gneiss has TIB-like Hf characteristics, while the younger gneiss is more juvenile. These basement rocks may originally have been separated by a major lithotectonic boundary. A sample from the basement slice, located stratigraphically beneath the Valdres Group in the Valdres Nappe Complex, is dated at c.1480 Ma and also has juvenile Hf composition. The apparent younging of basement ages from Lower to Middle Allochthon suggests age zonation at the SW Baltican margin. The juvenile 1180 Ma and 1480 Ma samples are probably derived from the same crust as what is today known as SW Norway or “Telemarkia”. Crustal rocks of presentday SW Norway must have continued further to the northwest (according to present geography) and were probably bordered by TIB rocks in the NE. We also present new U-Pb and Lu-Hf data from the autochthonous basement in front of and tectonostratigraphically beneath the Osen-Roa Nappe Complex, sampled along a transect from the Romerike Complex across the Mylonite Zone to Lake Storsjoen. Hf isotopes show the juvenile character of the gneisses on the western side of the Mylonite Zone and typical TIB 3 compositions on the eastern side. Hafnium isotopes from the Hustad Igneous Complex in the Western Gneiss Region (WGR) show strong similarity with the TIB, which supports a genetic connection between the TIB and WGR. Hafnium isotopes from the late Sveconorwegian Fla granite show crustal contamination, which supports models where the TIB continues underneath the Gothian domain.