Mimmi Nilsson

PLATE TECTONICS BEFORE 2.0 Ga: EVIDENCE FROM PALEOMAGNETISM OF CRATONS WITHIN SUPERCONTINENT NUNA

Laurentia, the core of Paleo- to Mesoproterozoic supercontinent Nuna, has remained largely intact since assembly 2.0 to 1.8 billion years ago [Ga]. For earlier times, previous paleomagnetic data on poorly dated Paleoproterozoic mafic intrusions yielded ambiguous estimates of the amount of separation between key cratons within Nuna such as the Slave and Superior. Recent developments in paleomagnetism and U-Pb baddeleyite geochronology, including new results reported herein, yield sufficiently precise data to generate partial apparent polar wander paths for both the Slave and Superior craton from 2.2 to 2.0 Ga. Our new apparent polar wander comparison confirms earlier speculations that processes similar to plate tectonics, with relative motion between the Slave and Superior cratons, were operative leading up to the final assembly of supercontinent Nuna.

New U–Pb baddeleyite age, and AMS and paleomagnetic data for dolerites in the Lake Onega region belonging to the 1.98–1.95 Ga regional Pechenga–Onega Large Igneous Province

Abstract A new U–Pb baddeleyite age of 1970 ± 3 Ma for the Unoi dolerite sill of the Onega structure of Karelia craton matches other 1.98–1.95 Ga units across the Kola craton (Pechenga) and widely separated parts of the Karelian craton, including the Lake Onega sill area and a extensive NW-trending dolerite dyke swarm. Herein these coeval units are combined into the Pechenga–Onega Large Igneous Province. The sills in the Lake Onega area exhibit similar geochemical patterns, although the Unoi dolerite sills appear less contaminated and less differentiated than the Pudozhgora intrusion, Gabnev sill and Koikary-Svatnavolok and Palieyeozero sills but are similar to other doleritic sills in the northern part of the Onega structure. New AMS data from sills are consistent with emplacement along the same NW–SE trend as the dykes, consistent with the dykes acting as their feeder system. Paleomagnetic data obtained on 1.98–1.95 Ga magmatic rocks result in a new robust paleopole for the Karelian craton and accentuate its variable paleolatitude and paleoorientation during the Paleoproterozoic.

Mesoproterozoic dykes in the Timmiarmiit area, Southeast Greenland: evidence for a continuous Gardar dyke swarm across Greenland’s North Atlantic Craton

Abstract During the Proterozoic, several mafic dykes with variable trends and mineralogies intruded the Archaean basement of Southeast Greenland. Some of the younger ENE-trending dykes are interpreted to represent a prolongation of the Mesoproterozoic Gardar Province, and have been termed Timmiarmiit dykes. Extrapolations of their trends across the inland ice sheet coincide with the northernmost so-called brown dykes (BD’s) which are part of the Gardar Province. Baddeleyite U–Pb ID-TIMS analyses for three ENE-trending Timmiarmiit dykes give ages of 1277 ± 4, 1275 ± 3 and 1268 ± 4 Ma, which are slightly younger than the oldest (BD0 = 1284–1279 ± 3 Ma, Upton 2013) and only dated generation of dykes in the Gardar Province, and thereby indirectly provide a possible age for the two younger dyke generations (BD1 and BD2). The Timmiarmiit–Gardar correlation is strengthened by a rigorous multivariate statistical analysis, on the basis of all major and trace elements. Thus, a coherent ENE-trending trans-Greenlandic dyke swarm is constituted. The major and trace element data of the Timmiarmiit dykes show that they crystallised from comparably evolved mantle-derived magma with minor crustal contamination and indicate a strong contribution of a metasomatised subcontinental lithospheric mantle component in the evolution of melts. This component was probably influenced by supra-subduction zone metasomatism during the Palaeoproterozoic Ketilidian orogeny. The data presented here, in addition to recent plate reconstruction models, give new evidence for a petrogenetic link between rift-related Mesoproterozoic magmatism in North America, South Greenland and Central Scandinavia which possibly formed in response to back-arc basin formation.

Quadrupole LA-ICP-MS U/Pb geochronology of baddeleyite single crystals

We present a straightforward method for accurate and precise U/Pb dating of baddeleyite using quadrupole LA-ICP-MS. To establish the validity of our ICP-MS data we used FC-4b baddeleyite from the Duluth complex as external standard, whereas baddeleyite from the Phalaborwa (South Africa) and the Sorkka (SW Finland) intrusions were treated as unknowns. The analytical protocol includes the use of a matrix matched standard, which allows for an accurate downhole-fractionation and laser-induced interelement fractionation correction. The quadrupole ICP-MS results for the Phalaborwa and Sorkka samples analyzed are accurate within error of the published ID-TIMS U/Pb ages and with a precision of 0.3–0.7% (2σ) from concordia ages of ten to twenty analyses per sample. No orientation effects were observed.

New U-Pb Ages for Mafic Dykes of SE Greenland

approximately three dozen preserved Archean cratonic fragments now dispersed globally, but its affinity to formerly neighboring supercontinent cratonic blocks remains poorly known. Bounding orogenic belts (e.g. Nagssugtoquidian, Torngat, Makkovik-Ketilidian) involve either extensive structural reworking of once rifted margins, or represent a hybrid of accreted juvenile material. Mafic dykes preserved internal to these blocks provide excellent time markers and allow for highresolution age comparisions of magmatic and extensional histories between cratons, particularly when coupled with paleomagnetic and geochemical studies. Southern Greenland comprises the core of NAC, while Nain Province in Labrador, Canada and the Lewisian Complex in Scotland constitute fragments rifted during the opening of the Labrador Sea and North Atlantic, respectively. The preserved Archean bedrock of Greenland has experienced multiple events of Paleoproterozoic dyke intrusion: at ca. 2.50, 2.42-2.37, 2.22-2.10 and 2.05-2.02 Ga. While the bedrock geology of southern West Greenland is well studied, the Archean of remote south East Greenland has been understood at a reconnaissance level until only recently. During a 2012 expedition led by the Geological Survey of Denmark and Greenland (GEUS), over 70 mafic dykes from the area were sampled in order to better understand possible Proterozoic and younger additions and modifications to the Archean block. Here, we present ages for five mafic dykes from SE Greenland and one from SW Greenland. Four dykes from the Skjoldungen area yield ages of 2166 ± 14 Ma, 2158.2 ± 7.5 Ma (both E-trending), 2137 ± 11 Ma (ENE-trending) and 2124 ± 11 Ma (NE-trending). The latter is indistinguishable from an age of 2124.9 ± 9.0 Ma for an E-trending SW Greenland dyke from the Nuuk area. This younger generation of dykes was previously unknown from the Greenland portion of North Atlantic Craton, but has temporal equivalents in northern Nain Province: 2121.0 ± 1.5 Ma ENE-trending Tikkigatsiagak dykes and likely correlative, regionally extensive ENE-ESE-trending swarms of dykes (Napaktok, Domes), extending to 2142 ± 2 Ma Avayalik dykes in northernmost Labrador (Hamilton et al., 1998; Connelly, 2001). In north-central Labrador the gabbro dykes are associated with a major anorthositediorite-monzonite plutonic complex (Arnanunat Suite) emplaced between 2135-2109 Ma (Hamilton, 1998; Ryan et al., 1999). 2165-2140 Ma dykes in SE Greenland may be related to the Iggavik suite of dykes in SW Greenland, which have been estimated at 2180 ± 100 Ma for the NEtrending Naujat dyke (Rb-Sr; Kalsbeek and Taylor, 1985). A N-S-trending, 70m-thick dolerite dyke from the Tingmiarmit area in SE Greenland yields a preliminary UPb baddeleyite age of ca. 1630 Ma. This is in precise agreement with the 1635-1622 Ma Melville Bugt dyke swarm (MBDS) in northwest Greenland (Halls, Hamilton & Denyszyn, 2011), which they proposed to extend under the inland ice to SE Greenland. The new results confirm this, and suggest that the MBDS likely extends over a strike length of at least 2000 km. Both the Tingmiarmit dyke and the Melville Bugt swarm sensu stricto have a characteristic porphyritic texture with plagioclase megacrysts. This generation of dykes could be related to the PD (‘Porphyritic dykes’) suite described by Chadwick (1969), which trend 120° in the Paamiut area of SW Greenland. These were proposed to have an emplacement age of ~1600 Ma, and if these dykes represent a subset of the MBDS, the volume and areal extent of the swarm would be immense. Current efforts are aimed at testing this hypothesis.