Lars Eivind Augland

Late Ordovician to Silurian ensialic magmatism in Liverpool Land, East Greenland: new evidence extending the northeastern branch of the continental Laurentian magmatic arc

New U–Pb ID-TIMS geochronological and whole-rock geochemical data from the Hurry Inlet Plutonic Terrane in Liverpool Land provide evidence of a Late Ordovician to Silurian magmatic arc in the East Greenland Caledonides. These voluminous granitoid rocks range from meladiorite to monzonite and granite, they are alkali-calcic to calc-alkaline and magnesian, and have characteristic arc granitoid trace element signatures. Zircon data give ages of 446 ± 2 and 438 ± 4 Ma for two phases of the Hurry Inlet Composite Pluton, 426 ± 1 Ma for a meladioritic xenolith in the anatectic Triaselv granite, and 424 ± 1 Ma for the Hodal-Storefjord Pluton. The Late Ordovician plutons can be correlated with similar plutons in the uppermost nappes of the Scandinavian Caledonides, likely representing the northern branch of magmatic arcs formed on the Laurentian margin. Magmatism appears to have continued sporadically until about 425 Ma when a major, short-lived, magmatic event formed the bulk of the batholith on Liverpool Land. This activity was likely mantle-driven and can be correlated with the Newer Granites in Scotland, for which a slab break-off mechanism has been proposed. The increased heat flow from this process can also explain the generation of the crustally derived, syntectonic, two-mica granites, which are the areally most important Caledonian suite in East Greenland.

Age, structural setting, and exhumation of the Liverpool Land eclogite terrane, East Greenland Caledonides

The close spatial relationship between Devonian high-pressure rocks (eclogites) and Ordovician–Silurian calc-alkaline plutonic rocks, as observed in Liverpool Land, NE Greenland, is not easily explained by existing tectonic models for the Caledonide orogen. New field studies and isotope dilution–thermal ionization mass spectrometry U-Pb geochronology demonstrate, however, that the association is just coincidental, because the two rock groups are located within distinct terranes separated by a composite structure. The major element is the Gubbedalen shear zone, a N-dipping shear zone dominated by a penetrative top-up-to-the-S ductile fabric. Superimposed brittle-ductile top-down-to-the-N shear zones are typical of the structurally uppermost part of the shear zone. The contact against the hanging wall is the N-dipping, brittle Gubbedalen extensional detachment fault. A zircon age of 399.5 ± 0.9 Ma for an eclogite body is interpreted to represent the time of high-pressure metamorphism of the footwall. The host gneiss was migmatized between ca. 388 Ma and ca. 385 Ma, as constrained by the ages of a pegmatite predating migmatization and crosscutting granites. Coeval synkinematic granites intrude along amphibolite-grade, top-to-the-S high-strain zones in the Gubbedalen shear zone. Juxtaposition of the Ordovician–Silurian plutonic terrane (hanging wall) against the Early to mid-Devonian eclogite terrane (footwall) is best explained by a tectonic model involving early mid-Devonian buoyancy-driven exhumation followed by late mid-Devonian syncontractional extension related to thrusting on the Gubbedalen shear zone in a dextral strike-slip zone. Subsequent exhumation through the brittle-ductile transition occurred by extension on early semiductile structures and the overprinting Gubbedalen extensional detachment fault, and erosion.

The Bratten–Landegode gneiss complex: a fragment of Laurentian continental crust in the Uppermost Allochthon of the Scandinavian Caledonides

Abstract This report presents new field observations and geochronology (isotope-dilution thermal ionization mass spectrometry/ID-TIMS and secondary-ion mass spectrometry/SIMS) on an igneous complex and its country rocks in the Bodø area, northern Norway, traditionally interpreted to represent the (par)autochthonous crust of Baltica (c. 1.8 Ga). Field observations however indicate that the rocks are allochthonous and comprise the uppermost tectonostratigraphic level in the area. The presence of a migmatitic megacrystic granite with an emplacement age of 946 Ma strongly supports such an interpretation and indicates that the Bratten–Landegode gneiss complex is exotic with respect to Baltica. The 946 Ma granite intrudes metasedimentary rocks. The rocks were metamorphosed in the Late Ordovician and intruded by granitic pegmatites and diorites at 430 and 427 Ma, respectively. The Bratten–Landegode gneiss complex shows a close correlation with Mesoproterozoic–Neoproterozoic rock complexes in the East Greenland Caledonides and we interpret it to be a Laurentian pre-Caledonian continental fragment. The discovery of Laurentian pre-Caledonian continental crust in the Uppermost Allochthon calls for a revision of the tectonostratigraphy of this part of the Caledonides and provides important constraints on the sequence of events on the Laurentian margin prior to continent–continent collision between Baltica and Laurentia as well as on intercontinental interactions during the Caledonian orogeny.

Terrane transfer during the Caledonian orogeny: Baltican affinities of the Liverpool Land Eclogite Terrane in East Greenland

Abstract: The Liverpool Land basement horst, East Greenland, comprises two allochthonous Caledonian terranes with very different tectonomagmatic and metamorphic histories, separated by the composite Gubbedalen Shear Zone. The Liverpool Land Eclogite Terrane in the footwall consists of orthogneiss formed at 1645–1640 Ma, affected by eclogite-facies metamorphism at 400 Ma and subsequently migmatized and intruded by granites at 388–385 Ma. The Liverpool Land Eclogite Terrane contrasts with the upper plate, which consists of arc-related Late Ordovician to Early Silurian magmatic rocks and was not affected by the Devonian metamorphism seen in the lower plate. Conversely, Ordovician–Silurian rocks are absent from the lower plate. Mesoproterozoic protolith ages, Early Devonian high-pressure metamorphism, and the occurrence of garnet peridotite distinguish the Liverpool Land Eclogite Terrane from all other terranes in East Greenland, including the large North East Greenland Eclogite Province, whereas these features are characteristic for the Western Gneiss Region in Norway. These observations indicate that the Liverpool Land Eclogite Terrane is exotic with respect to Laurentia and a Baltican origin seems likely. The amalgamation of the Baltican crustal slice with Laurentia probably occurred by imbrication of subducted Baltican crust in a bulk transpressional setting.

Early Ordovician to Silurian evolution of exotic terranes in the Scandinavian Caledonides of the Ofoten–Troms area – terrane characterization and correlation based on new U–Pb zircon ages and Lu–Hf isotopic data

Abstract Isotope dilution thermal ionization mass spectrometry U–Pb dating and coupled Lu–Hf solution inductively coupled plasma mass spectrometry analyses of zircon were acquired from magmatic rocks along two transects across the Scandinavian Caledonides in the Troms–Ofoten region of Norway to explore possible correlations and gain insight into the evolution of far-travelled nappes within the Upper and Uppermost Allochthons. One pulse of magmatic activity was recorded at c. 489 Ma in the Tromsø Nappe. In the underlying Nakkedal Nappe, a magmatic pulse was recorded at c. 450 Ma, being contemporaneous with eclogite facies metamorphism in the area. Tonalites in the structurally underlying Lyngen and Gratangseidet ophiolitic complexes, both forming the substratum to carbonate–schist–quartzite sequences (Balsfjord and Evenes groups, respectively), yielded ages of 481 and 474 Ma. Obtained ϵHf(t) values are, however, distinctly different and indicate a juvenile origin for the Gratangseidet tonalite (ϵHf(474)=+9.57) and the presence of Palaeoproterozoic source material for the Lyngen tonalite (ϵHf(481)=−16.8 to −2.3). The 474 Ma age of the Snaufjellet granite intruding the Bogen Group structurally above the Evenes Group requires a thrust between the two units. An age of 435 Ma on the Heia gabbro of the Nordmannvik Nappe is comparable to that of the Råna norite (437 Ma) in the Narvik area, supporting a correlation with the Narvik Nappe Complex. The youngest intrusion dated to 425 Ma is a leucogranite that intruded the Balsfjord Group. The Lu–Hf data indicate a common source for several of the magmatic rocks in the Uppermost Allochthon, as well as Laurentian arc granitoids from East Greenland and possibly from Newfoundland and related areas.

Constraining shifts in North Atlantic plate motions during the Palaeocene by U-Pb dating of Svalbard tephra layers

Radioisotopic dating of volcanic minerals is a powerful method for establishing absolute time constraints in sedimentary basins, which improves our understanding of the chronostratigraphy and evolution of basin processes. The relative plate motions of Greenland, North America, and Eurasia changed several times during the Palaeogene. However, the timing of a key part of this sequence, namely the initiation of compression between Greenland and Svalbard, is currently poorly constrained. The formation of the Central Basin in Spitsbergen is inherently linked to changes in regional plate motions, so an improved chronostratigraphy of the sedimentary sequence is warranted. Here we present U-Pb zircon dates from tephra layers close to the basal unconformity, which yield a weighted-mean 206Pb/238U age of 61.596 ± 0.028 Ma (2σ). We calculate that sustained sedimentation began at ~61.8 Ma in the eastern Central Basin based on a sediment accumulation rate of 71.6 ± 7.6 m/Myr. The timing of basin formation is broadly coeval with depositional changes at the Danian-Selandian boundary around the other margins of Greenland, including the North Sea, implying a common tectonic driving force. Furthermore, these stratigraphic tie points place age constraints on regional plate reorganization events, such as the onset of seafloor spreading in the Labrador Sea.