Early Ordovician to Silurian evolution of exotic terranes in the Scandinavian Caledonides of the Ofoten–Troms area – terrane characterization and correlation based on new U–Pb zircon ages and Lu–Hf isotopic data

Abstract

Isotope dilution thermal ionization mass spectrometry U-Pb dating and coupled Lu-Hf solution inductively coupled plasma mass spectrometry analyses of zircon were acquired from magmatic rocks along two transects across the Scandinavian Caledonides in the TromsOfoten region of Norway to explore possible correlations and gain insight into the evolution of far-travelled nappes within the Upper and Uppermost Allochthons. One pulse of magmatic activity was recorded at c. 489 Ma in the Tromsø Nappe. In the underlying Nakkedal Nappe, a magmatic pulse was recorded at c. 450 Ma, being contemporaneous with eclogite facies metamorphism in the area. Tonalites in the structurally underlying Lyngen and Gratangseidet ophiolitic complexes, both forming the substratum to carbonate -schist-quartzite sequences (Balsfjord and Evenes groups, respectively), yielded ages of 481 and 474 Ma. Obtained 1 Hf(t) values are, however, distinctly different and indicate a juvenile origin for the Gratangseidet tonalite (1 Hf(474) ¼ + 9.57) and the presence of Palaeoproterozoic source material for the Lyngen tonalite (

New perspectives on the Caledonides of Scandinavia and related areas: introduction

“…Augland et al (2013a) investigated the Bratten -Landegode gneiss complex, previously interpreted to represent Baltican crust, and show that it is instead an allochthonous piece of Mesoproterozoic crust, probably derived from Laurentia. A verification of the composition and provenance of nappes in northern Norway is also the theme of the second paper by Augland et al (2013b) where they document a complex and long-lived magmatic evolution from the Cambrian to the Silurian in various different tectonic units.…”

Structural and Metamorphic Implications of the Final Emplacement of the Lyngen Nappe

“…The major lithotectonic units of this region are (from structurally lowest to highest): the autochthonous Dividal Group, and the Kalak, Vaddas, Kåfjord, Nordmannvik, Lyngen, Nakkedal, and Tromsø nappes (Figure 9 & Figure 10). Augland et al (2014). Figure 9 showing relative nappe relationships.…”

“…The Uppermost Allochthon currently consists of the Lyngen, Nakkedal, and Tromsø nappes, and contains Caledonian intrusive rocks interpreted to have originated near or within the continental margin of Laurentia Roberts et al, 2002;Barnes et al, 2007;Roberts et al, 2007;Augland et al, 2012). The Nakkedal Nappe is composed of quartzofeldspathic paragneisses and the Skattøra Migmatite Complex (Augland et al, 2014). Capping this nappe stack is the Tromsø NC, which is a complex suite of kyanite-mica-schists, amphibolites, anorthosites, calc-silicates, marbles, gneisses, and eclogites .…”

“…These two suites are usually separated by the oceanic, high-temperature, ductile Rypdalen Shear Zone (Slagstad, 1995;Augland et al, 2014). At the base of the LMC lies the Kjosen Unit, a mylonitic greenschist-amphibolite.…”

“…At the base of the LMC lies the Kjosen Unit, a mylonitic greenschist-amphibolite. This unit then transitions into the underlying Koppangen FM (Formation), composed predominantly of a crenulated phyllite, which then transitions through a greenschist-facies shear zone (Augland et al, 2014) into the schists and gneisses of the Nordmannvik Nappe below. Unconformably overlying the LMC is the Balsfjord Group, represented by metamorphosed Late Ordovician/Early Silurian sedimentary rocks (Bjørlykke & Olaussen, 1981) with an increasing metamorphic grade towards the overlying Nakkedal Nappe .…”