The Neoproterozoic Ottfjället dike swarm of the Middle Allochthon, traced geochemically into the Scandian Hinterland, Western Gneiss Region, Norway

Abstract

The Ottfjället swarm of Late Neoproterozoic mafic dikes, cutting Neoproterozoic sandstones, became a key, 30 years ago, to the tectonics of the Scandian Caledonides. The sandstones were deposited in basins related to opening of Iapetus, and intruded by dikes in distal parts of the Baltoscandian margin close to the developing spreading axis. The sandstones and underlying basement rocks were transported, from west of the present Norwegian coast, to as far east as western Sweden during the Silurian-Devonian Scandian Orogeny. The sandstones with dikes make up the Särv Nappe, up to 2 km thick in Sweden, and the quartzites and amphibolites of the Sætra and equivalent nappes in Norway. These form the upper part of the Middle Allochthon. The lower part of the Middle Allochthon includes Middle Proterozoic basement gneisses and rapakivi granites, containing mafic rocks in some places. The dike-bearing quartzite is a key unit due to contrast with similar rocks lacking dikes at lower tectonic levels derived from inboard parts of Baltica.

The Sætra Nappe and equivalents are well constrained on lithotectonic grounds in Norway at Oppdal, Leksdal, and Orkanger. It was also suspected to occur in deep, narrow synclines in the Western Gneiss Region where interlayered feldspathic quartzite and amphibolite are in correct tectonostratigraphic sequence, locally with a total thickness of only 1 to 3 m, and locally with dikes converted to eclogite. To test correlations, 127 samples of mafic rocks were collected from 14 areas west and southwest of Trondheimsfjord into the Western Gneiss region toward Ålesund. Samples include mafic rocks in quartzites and others, some clearly dikes, from the underlying 1190 Ma rapakivi granite/augen gneiss of the Risberget Nappe and adjacent basement gneisses.

Typical Sætra dikes in the Oppdal quarries, and mafic rocks from other quartzites and related rocks, have La_n/Sm_n ratios of 1.0 to 1.8, and Nb/La ratios of 0.8 to 1.4 (Oppdal group). Most REE patterns are moderately LREE-enriched with no or very small, mostly negative, Eu anomalies. All have similar multi-element patterns typically with small positive P anomalies, negative Zr-Hf anomalies and an absence of Nb-Ta anomalies, showing that the dikes are unrelated to arcs and have no notable continental crust component. A subset of Sætra, Risberget, and basement dikes is distinguished by higher La_n/Sm_n ratios of 1.8 to 2.5, but otherwise has very similar characteristics (Ystland Group). These data support correlation of the Sætra Nappe quartzite and dikes into highly deformed parts of the Western Gneiss Region and correlation of nearby dike-rich parts of basement gneiss with the Middle Allochthon.

One sample in quartzite at Ura, at an unusual tectonostratigraphic position, has La_n/Sm_n = 0.7, Nb/La = 0.6, and a multi-element pattern different from Sætra dikes, suggesting it is unrelated to the Ottfjället dikes. Non-Sætra-like amphibolites also occur in the Risberget Nappe, and have La_n/Sm_n ratios of 1.4 to 3, all Nb/La ratios <0.6, and multi-element patterns with sharp negative anomalies for Nb-Ta, P, Zr-Hf and Ti. These are probably Mesoproterozoic magmas and cumulates emplaced into the rapakivi granite protolith of the augen gneiss.

Data from this and earlier studies of correlative nappes in adjacent regions can be geographically partitioned into transitional MORB-like compositions with relatively low LREE enrichment to the north and south, and more LREE-enriched alkaline compositions concentrated at about the latitude of northern Trondheimsfjord (∼64°N). We interpret the zone of alkaline rocks to have been influenced by a nearby source of hot-spot-related enriched mantle during Late Neoproterozoic rifting of Baltica from Rodinia, with striking geochemical and spatial parallels to the modern Mid-Atlantic region.