E. O. Walsh

Protolith ages and exhumation histories of (ultra)high-pressure rocks across the Western Gneiss Region, Norway

The timing of protolith formation, ultrahigh-pressure (UHP) subduction, and subsequent exhumation for the ultrahigh-pressure to high-pressure units across the eastern part of the Western Gneiss Region, Norway, were assessed using U/Pb zircon, Th/Pb monazite, and 40 Ar/ 39 Ar white mica ages. U/Pb zircon ages from eclogites demonstrate that oceanic and continental allochthons were emplaced onto the Baltica basement before the entire mass was subducted to (ultra)high pressure. Eclogites within the allochthons across the entire Western Gneiss Region are Caledonian and show a degree of zircon (re)crystallization that increases with peak pressure, permitting the interpretation that the entire region underwent synchronous subduction. 40 Ar/ 39 Ar white mica ages of 399 Ma indicate that the eastern part of the Western Gneiss Region had been exhumed to shallow crustal levels while UHP metamorphism was ongoing farther west, indicating a westward dip to the slab. The 40 Ar/ 39 Ar white mica ages also show a clear east-to-west gradient across the entire Western Gneiss Region, indicating that the Western Gneiss Region rose diachronously to crustal levels from east to west between 399 and 390 Ma.

Strain within the ultrahigh-pressure Western Gneiss region of Norway recorded by quartz CPOs

Abstract Electron back-scatter diffraction (EBSD) was used to measure the crystal preferred orientations (CPOs) from 101 samples across the ultrahigh-pressure Western Gneiss region of Norway to assess slip systems, sense of shear, CPO strength, and strain geometry. The CPOs suggest a dominance of prism ⟨a⟩ slip, with lesser amounts of prism [c] slip and basal ⟨a⟩ slip; there are few Type I and Type II girdles. The major structural feature in the study area – the high-strain, top-W, normal-sense Nordfjord–Sogn Detachment Zone – is characterized by asymmetric and strong CPOs; an eastern domain with strong asymmetric CPOs shows top-E shear. Strain throughout the study area was characterized by a mix of plane strain and constriction with no evidence of flattening. Adjacent gneiss and quartzite/vein samples have similar CPOs.

Geochemistry of amphibolite-facies volcanics and gabbros of the støren nappe in extensions west and southwest of Trondheim, Western Gneiss Region, Norway: a key to correlations and paleotectonic settings

The Upper Allochthon of the Scandinavian Caledonides is composed of thrust slices of a variety of metamorphosed volcanic, plutonic, and sedimentary rocks. The rocks are largely interpreted to have been formed in Cambrian to Ordovician oceanic to peri-continental arc environments that were thrust onto Baltica during the Silurian-Devonian Scandian Orogeny. We review the literature on Upper Allochthon igneous rock geochemistry broadly. We also present 87 new analyses of metamorphosed igneous rocks taken from the Surnadal and Moldefjord synforms and the Rissa area in the northern part of the Western Gneiss Region, where highly deformed Upper Allochthon rocks are preserved. Here the Upper Allochthon is dominated by basaltic volcanics and gabbros with sparse intermediate and felsic rocks. Most of the mafic rocks are transitional between MORB- and arc-type compositions in tectonic discriminant diagrams and for REE and multi-element patterns. We present two new discriminant diagrams to illustrate this subtle transitional characteristic. We interpret these volcanics as having been erupted in a mature back-arc basin environment where mantle sources for the magmas were only slightly influenced by the subduction zone component from an adjacent arc. Another set of mafic volcanic rocks, coming only from the Storås and Rissa areas, were extruded in an oceanic arc-type environment. These two geochemical types, back-arc and oceanic-arc, are closely matched by Støren Group ophiolites in the Trondheim Region which are dominated by basaltic volcanics and gabbros and generally lack felsic igneous and sedimentary rocks. Felsic igneous rocks, sedimentary rocks, and calc-alkaline and alkaline volcanics are more abundant in stratigraphically younger units of the Upper Allochthon, including the Lower and Upper Hovin Groups. We correlate the Upper Allochthon igneous rocks in the Moldefjord and Surnadal synforms and the Rissa area with ophiolites of the Støren Group. Early oceanic-arc and back-arc volcanic rocks, followed by more calc-alkaline to alkaline volcanics, are the dominant pattern for ophiolite sequences in the rest of the Upper Allochthon of the Scandinavian Caledonides. We envisage a paleotectonic scenario in which a Late Cambrian to Tremadocian, oceanic-arc system developed above a subduction zone dipping oceanward from a microcontinent that had earlier rifted away from Baltica or possibly Ganderia. The ophiolitic and primitive arc rocks are inferred to have been obducted in Late Tremadocian–Early Floian time upon epicontinental rocks flanking the microcontinent, which was then drifting rapidly across the Iapetus Ocean approaching Laurentia. A new arc and marginal basin developed, following a subduction polarity reversal, with the Dapingian-Darriwilian sedimentary infill being replete with Laurentian faunas, and locally punctuated by calc-alkaline volcanics and dikes, with fringing reefal limestones. The entire volcano-sedimentary assemblage was later affected by Scandian (Silurian–Early Devonian) orogenesis during emplacement of the major nappes onto the Baltoscandian margin.

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

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 Lan/Smn 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 Lan/Smn 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 Lan/Smn = 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 Lan/Smn 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.

Application of major- and trace-element geochemistry to refine U-Pb zircon, and Sm/Nd or Lu/Hf sampling targets for geochronology of HP and UHP eclogites, Western Gneiss Region, Norway

Abstract The geochemistry of kyanite and orthopyroxene eclogites (7 samples) indicate that they are gabbroic cumulates. Incompatible trace elements in these rocks occur at low concentrations compared to regionally associated eclogites that are compositionally similar to basaltic magmas (11 samples). Eclogites with cumulate protoliths commonly contain <10 ppm Zr, <1.2 ppm Sm, and <0.2 ppm Lu, compared to generally >100 ppm Zr, >4 ppm Sm, and >0.4 ppm Lu for basaltic eclogites. Because of low Zr concentrations, igneous and metamorphic zircons are rare or absent in these eclogites. Samarium and Lu concentrations are also low in the kyanite and orthopyroxene eclogites, but they have parent/daughter Sm/Nd ratios of 0.23-0.51 and Lu/Hf ratios of 0.22-0.60, higher than most associated basaltic eclogites at 0.22-0.38 and 0.11-0.18, respectively. These results suggest that kyanite and orthopyroxene eclogites are poor targets for zircon geochronologic work, but are good targets for Sm/Nd and Lu/Hf mineral/whole rock geochronology because of their high parent/daughter ratios.

Ordovician–early Silurian intrusive rocks in the northwest part of the Upper Allochthon, mid-Norway: Plutons of an Iapetan volcanic arc complex

The Scandinavian Caledonides are a late Silurian–early Devonian orogen that is an amalgam of thrust sheets derived from the Baltican margin, Iapetan arcs, and the Laurentian margin. Here we discuss intrusive rocks in two Iapetan arc complexes of Ordovician–early Silurian age that are now part of the Upper Allochthon in the central-western part of Norway. Samples from the older of the two arcs were collected near Lensvik and Rissa, on both sides of Trondheimsfjord. These include mafic to felsic, mostly tholeiitic metamorphosed intrusive rocks, with one previously reported age of 482 Ma. These plutons intrude volcanic rocks of the Støren Group, a suite of 495 to 482 Ma oceanic arc rocks that include ophiolites. We assign these plutonic rocks to the Støren Group, based on their similar ages and strong geochemical similarities with Støren Group plutonic and volcanic rocks in the inner Trondheimsfjord region. Part of the Støren Group is unconformably overlain by earliest Middle Ordovician sedimentary sequences that contain warm-water Laurentian faunal assemblages. Plutonic rocks of the younger arc have previously reported ages of 477 to 440 Ma, are calc-alkaline, span a composition range from ultramafic to granitic, and include a large fraction of adakites. Rocks in the Upper Allochthon with similar ages and lithologic and geochemical characteristics also occur farther north, across the Grong-Olden culmination, and in southwestern Norway, indicating an extensive arc complex. Throughout that extent, the plutonic rocks are spatially associated with older ophiolite-bearing oceanic arc assemblages, thought to be equivalent to the Støren Group. The Helgeland Nappe of the Uppermost Allochthon contains an extensive suite of igneous rocks that have almost identical characteristics to the younger Upper Allochthon arc we describe. They are also spatially associated with 500 to 480 MA ophiolites, and in addition cut metamorphosed sedimentary rocks that contain evidence for development near or on the Laurentian margin. This evidence suggests that the calc-alkaline rocks of the Upper and Uppermost Allochthons may belong to the same arc complex. The fact that similar-age calc-alkaline arc rocks occur in the Taconian arc system of western New England, the Notré Dame arc of Newfoundland, and in the Midland Valley Terrane and related units in Scotland and Ireland, suggest extensive calc-alkaline Iapetan arc complexes that developed before and during late Ordovician closing of an arm of Iapetus.