Tectonometamorphic evolution of the Åreskutan Nappe – Caledonian history revealed by SIMS U–Pb zircon geochronology

Abstract

Secondary ionization mass spectrometry (SIMS) U-Pb dating of zircons from the Å reskutan Nappe in the central part of the Seve Nappe Complex of western central Jämtland provides new constraints on the timing of granulite-amphibolite-facies metamorphism and tectonic stacking of the nappe during the Caledonian orogeny. Peak-temperature metamorphism in garnet migmatites is constrained to c. 442 + 4 Ma, very similar to the ages of leucogranites at 442 + 3 and 441 + 4 Ma. Within a migmatitic amphibolite, felsic segregations crystallized at 436 + 2 Ma. Pegmatites, cross-cutting the dominant Caledonian foliation in the Nappe, yield 428 + 4 and 430 + 3 Ma ages. The detrital zircon cores in the migmatites and leucogranites provide evidence of Late Palaeoproterozoic, Mesoproterozoic to Early Neoproterozoic source terranes for the metasedimentary rocks. The formation of the ductile and hot Seve migmatites, with their inverted metamorphism and thinning towards the hinterland, can be explained by an extrusion model in which the allochthon stayed ductile for a period of at least 10 million years during cooling from peaktemperature metamorphism early in the Silurian.In our model, Baltica-Laurentia collision occurred in the Late Ordovician-earliest Silurian, with emplacement of the nappes far on to the Baltoscandian platform during the Silurian and early Devonian, Scandian Orogeny lasting until c. 390 Ma.

COSC-1 – drilling of a subduction-related allochthon in the Palaeozoic Caledonide orogen of Scandinavia

“…Åreskutan Nappe) of migmatites and paragneisses (Arnbom, 1980) with a previous ultra-high-pressure metamorphic history (Klonowska et al, 2015); and an upper, amphibolite-dominated unit with micaschists and psammites. The high-grade metamorphism and leucogranite intrusions of the Seve Nappes have yielded Early Silurian ages (Gromet et al, 1996;Williams and Claesson, 1987;Ladenberger et al, 2014). The entire Middle Allochthon was derived from west of the Norwegian coast and the upper units transported at least 400 km eastwards during Scandian orogeny (Gee, 1978).…”

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

“…At present 'Scandian' is generally intended to indicate the period between c. 430 and c. 380 Ma that encompasses the collisional history starting with development of eclogite-facies assemblages in the Lindås Nappe and ending during uplift of the Western Gneiss Region and deposition of Devonian clastic sedimentary rocks. Ladenberger et al (2013) and Gee et al (2013a) propose, however, to extend the term to c. 440 Ma to cover the late Ordovician metamorphic evolution of the Seve nappe, based on the interpretation that this activity represents the initial stages of the collision with Baltica. There is, however, as yet no consensus on the validity of that assumption, the alternative being that the Seve nappe is just another exotic terrane with a nonBaltic pre-history prior to its thrust incorporation into the Scandinavian Calednides (Corfu et al 2011).…”

Seismic imaging in the eastern Scandinavian Caledonides: siting the 2.5 km deep COSC-2 borehole, central Sweden

“…Subductionrelated metamorphism along the Baltica margin already took place in the early to middle Ordovician Majka et al, 2012) and the initial stages of continent-continent collision are believed to have occurred around 445 Ma (e.g., Ladenberger et al, 2012Ladenberger et al, , 2014. Thrust tectonics, which dominated throughout the collision, resulted in the emplacement of allochthonous units both westwards onto the Laurentian platform of Greenland (Higgins and Leslie, 2000), with displacements of the higher allochthons at least 200 km, and eastwards onto the Baltoscandian platform, with displacements of more than 500 km (Gee, 1978).…”