A hyperextension assemblage, imbricated in Archean–Paleoproterozoic crust, at the base of the Kalak Nappe Complex in the northern Scandinavian Caledonides

Abstract

The break-up of continents generates dyke swarms, basins and hyperextension assemblages. Once incorporated into new orogens, the latter can provide crucial information about origins and tectonic processes. The Nålfjell Complex in the Caledonian Skillefjord Nappe of northern Norway has many of the characteristics of a hyperextension assemblage, notably the presence of solitary serpentinite bodies exposed by the exhumation of serpentinized mantle and now embedded in schists, amphibolites, mylonites, marbles and felsic gneisses. The Skillefjord Nappe is a lithologically diverse, imbricated and discontinuous allochthon. It consists of felsic gneisses and dykes dated by zircon U–Pb to c. 3100, 2940, 2830, 2510 and 1800–1750 Ma. These rocks yield titanite ages of 2810–2700, 1750, 1660–1590 and 430–420 Ma. Metagabbro intruded at 1995 Ma. The ages and evolution of the Skillefjord Nappe are distinct from those of the structurally higher Svaertholt Terrane (>1030 Ma sediments deformed and intruded by granite at 980–950 Ma) and the Sørøy Terrane (characterized by a multi-stage 900–500 Ma evolution). The assembly of these disparate elements was completed at 420 Ma. The timing of hyperextension is uncertain. The serpentinites have a primitive Pb isotopic composition indicating old depleted mantle lithosphere. Extensional processes from the Paleoproterozoic to Ediacaran are considered. Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/cc/caledonian-wilson-cycle