Palaeoproterozoic U–Pb ages of autochthonous and allochthonous granites from the northern Swedish Caledonides—regional and palaeogeographic implications

Abstract

In the Silurian, Baltica and Laurentia collided to form the Caledonian orogenic belt. During this process, nappes derived from the pre-collisional Baltic margin, from outboard terranes of Baltica, and from exotic terranes of probable Laurentian origin, respectively, were thrust upon the indenting Baltica. The Middle Allochthon is a tectonostratigraphic level inferred to represent the rifted Neoproterozoic margin of Baltica and is heterogeneously composed of telescoped fault-controlled sedimentary basins and subjacent basement plinths. In the Akkajaure area of northern Sweden, the Middle Allochthon consists mainly of granites (Akkajaure Nappe Complex). The Ritsem Granite, a part of the (para-) authochthonous basement below the nappe complexes, yields a U–Pb zircon and titanite age of 1871±11 Ma. Granite and felsic dykes in the lowermost thrust-sheet in the Middle Allochthon yield ages of 1800±2 Ma and 1795±4 Ma, respectively. The granite of the overlying thrust-sheet is 1781±2 Ma old and crosscutting dykes are 1779±7 Ma. These ages constrain the pre-Caledonian position of the nappe complex to the general region west of Lofoten, northwestern Norway, substantiating a link with the Transscandinavian Igneous Belt (TIB) of the Fennoscandian Shield.