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

Abstract

The Ottfja ¨llet 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 Sa ¨rv Nappe, up to 2 km thick in Sweden, and the quartzites and amphibolites of the Saetra 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 Saetra 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 Saetra 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 Saetra, 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 Saetra 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 Saetra dikes, suggesting it is unrelated to the Ottfja ¨llet dikes. Non-Saetra-like amphibolites also occur in the Risberget Nappe, and have La n /Sm n ratios of 1.4 to 3, all Nb/La ratios <...

U–Pb zircon geochronology of eclogites from the Scandian Orogen, northern Western Gneiss Region, Norway: 14–20 million years between eclogite crystallization and return to amphibolite-facies conditionsThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

“…The southern ''basal'' contact of Blåhø 2 is similarly marked with, in addition at Oksvollneset, very rare impure quartzite and amphibolite no more than 1-2 m thick, interpreted as part of the Saetra Nappe. Basement B, exposed through Dryna, south Midøya, and part of Otrøya, is special, consisting of highly deformed layered pink basement gneiss cut by less deformed late Neoproterozic mafic dikes, geochemically equivalent to those in the Saetra Nappe (Hollocher et al 2007a). Basement B could be the substrate upon which quartzites of the Saetra Nappe were deposited, therefore, it is not part of the proximal Baltican basement, but probably a far-travelled thrust slice of the Middle Allochthon.…”

“…The central segment also includes a small amount of typical Saetra Nappe, never more than 1-2 m thick (Hollocher et al 2007a). In the northern segment, the Blåhø Nappe reached UHP eclogite-facies conditions.…”

“…These include the Risberget Nappe, mainly containing Mesoproterozoic rapakivi granite (Handke et al 1995), the Saetra Nappe comprising Neoproterozoic quartzites intruded by late Neoproterozoic diabase dikes (Hollocher et al 2007a), the Blåhø Nappe comprising high-grade mica schists and amphibolites, and the Støren Nappe comprising Ordovician intrusive, volcanic, and sedimentary rocks, including ophiolite fragments in greenschist or low-amphibolite facies. Locally, near Trondheim, the Støren Nappe contains distinctive Early to Middle Ordovician fossils long known to be of Laurentian affinities (Cocks 2000).…”

“…A challenging problem for subduction models is that Baltican continental basement of the Western Gneiss Region is commonly accompanied by thin to very thin veneers consisting of one to as many as four thrust nappes, also recognized in the frontal part of the orogen (Robinson 1995(Robinson , 1997Tucker et al 2004;Hollocher et al 2007a). In the Moldefjord belt (Fig.…”

Orogenic to postorogenic (1.20-1.15 Ga) magmatism in the Adirondack Lowlands and Frontenac terrane, southern Grenville Province, USA and Canada

“…Trace elements from fi ve samples from the Edwardsville pluton were determined by ICP-MS at ACME Analytical Laboratories, and by XRF on pressed-powder pellets at Colgate University (Chiarenzelli et al, 2010b). Trace elements for 20 samples from Rockport and Hyde School granite gneisses were determined using methods described in Hollocher et al (2007). Trace elements for 16 samples from the Frontenac suite were determined with an Agilent HP4500 ICP-MS (Harpp et al, 2005).…”