Geochronology of Caledonian metamorphic allochthons in the Otta–Heidal region, South Norway; tectonostratigraphic and palaeogeographical implications

Abstract

The Caledonian–Appalachian belt formed in the Early Paleozoic by the assembly and dispersal of continental and oceanic crustal fragments of diverse provenance. The characterization of these terranes and information on their origins are essential in reconstructing the course of the orogeny. The region of northern Gudbrandsdalen in Norway is important for elaborating these questions as it represents a geological transition in architectural style and composition of the Caledonian nappes and the meeting point of tectonic elements with diverse geological histories of both Baltic and exotic origins. In the past century this complexity has generated conflicting interpretations, which we have tested with new mapping and isotope dilution thermal ionization mass spectrometry U–Pb dating. The new data show the following: (1) a metagabbro low in the tectonostratigraphy has an age of 1497\u2009±\u20095\u2005Ma, correlating with the Espedalen Complex of Telemarkian (1550–1480\u2005Ma) affinity; (2) the structurally higher crystalline Rudihøe–Mukampen Suite was metamorphosed at high grade during the Sveconorwegian Orogeny, mainly between 980 and 900\u2005Ma, with older components pointing to a Gothian (1700–1600\u2005Ma) affinity; (3) these ages and mapping indicate that the Rudihøe–Mukampen Suite is a lateral equivalent of the Upper Jotun Nappe; (4) the overlying, dominantly metasedimentary Steinhø Complex was metamorphosed in the Ordovician at about 473\u2009±\u20093\u2005Ma; (5) both the Rudihøe–Mukampen Suite and Steinhø Complex were intruded by trondhjemite dykes at 430–427\u2005Ma. These units are structurally overlain by the lowest elements of the Trondheim Nappe Complex, which includes serpentinite lenses and conglomerates, the products of hyperextension, locally with Ordovician fossils. The new results dispel some of the problematic interpretations proposed earlier and represent new firm elements that will anchor future tectonic reconstructions of the Caledonides in this region.