Browsing by Subject "U-PB GEOCHRONOLOGY"

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  • Piispa, Elisa J.; Smirnov, Aleksey V.; Pesonen, Lauri J.; Mitchell, Roger H. (2018)
    We present new paleomagnetic and geochemical data from a suite of the similar to 1144-Ma ultramafic lamprophyre dikes that outcrop in the Canadian Shield northeast of Lake Superior (Ontario, Canada). Nineteen of 22 sampled dikes yielded consistent characteristic remanent magnetization directions of normal (n = 5) and reversed (n = 14) polarity. The primary origin of characteristic remanent magnetization is bolstered by positive baked contact tests and a reversal test. The group mean direction (D = 306.4 degrees, I = 72.1 degrees, alpha(95) = 5.5 degrees, N = 19) obtained from the lamprophyre dikes is statistically indistinguishable from the group mean direction (D = 297.4 degrees, I = 65.5 degrees, alpha(95) = 8.3 degrees, N = 8) previously reported for the nearly coeval similar to 1142-Ma Abitibi dikes. The geochemistry of the lamprophyre dikes suggests strong affinity with magmas derived from ocean island basalt-type mantle sources, consistent with the mantle plume hypothesis for the formation of the similar to 1.1-Ga North American Midcontinent Rift. The similarity in age, trend, paleomagnetism, and geochemistry indicates that the lamprophyre and Abitibi dike suites represent the earliest magmatic event associated with the commencement of rifting. The combined mean direction (D = 303.1 degrees, I = 70.2 degrees, alpha(95) = 4.5 degrees, N = 27) corresponds to a paleomagnetic pole at P-lat = 55.8 degrees N, P-long = 220.0 degrees E (A(95) = 7.3 degrees). The new pole merits the highest classification on the Q-scale of paleomagnetic reliability and represents a key pole defining the North American apparent polar wander path during the late Mesoproterozoic. Combined with high-quality data from the similar to 1108-Ma Coldwell Complex, our data indicate an equatorward motion of Laurentia at 3.8 +/- 1.4 cm/year, comparable with the present-day velocities of continental plates, before switching to extremely rapid motion between similar to 1108 and similar to 1099 Ma. Plain Language Summary Similar to a magnetic tape, rocks can retain the direction of ancient Earth's magnetic field. Scientists use this record (known as paleomagnetism) to reconstruct past positions of continents and to decipher the geological history of our planet. We investigated paleomagnetism and chemical composition of the similar to 1.14 Ga-old intrusive rocks called lamprophyres exposed in Northwestern Ontario (Canada). We found that the paleomagnetic field directions recorded in lamprophyres are indistinguishable from those recorded by another similar age suite of basaltic intrusions called the Abitibi dikes, from the same area. The combined data from these rocks allowed us to constrain the position of an ancient supercontinent called Laurentia at similar to 1.14 billions of years ago more accurately than it was possible before. Our results convincingly show that, during that time, Laurentia moved with a velocity comparable to present-day plate velocities, before switching to an extremely rapid motion approximately 35 millions of years later. The lamprophyre and Abitibi rocks also share similar chemical signatures, close to those observed for ocean island basalts (e.g., Hawaii). These observations support the hypothesis that a failed ocean opening attempt called the North American Midcontinent Rift was instigated by the arrival of a hot mantle material upwelling to the Earth surface.
  • Luttinen, Arto; Kurhila, Matti; Puttonen, Riina; Whitehouse, Martin; Andersen, Tom (2022)
    New U-Pb ages for zircons constrain the duration of silicic magmatism and timing of coeval mafic magmatism across the main rift zone of the Karoo large igneous province in Mozambique. Our 190 +/- 2 Ma, 188.4 +/- 0.9 Ma, 181.7 +/- 1.0 Ma, 180 +/- 3 Ma, 178 +/- 2 Ma and 172 +/- 2 Ma ages support periodicity of Karoo magmatism previously inferred from 40Ar/39Ar age data. The - 190-188 Ma ages confirm early onset of magmatism and the - 182-178 Ma ages correlate the bimodal volcanic successions of the Lower Zambezi and the Movene Formation with widespread silicic magmatism across the rift zone. The - 172 Ma age corresponds to waning magmatic activity. The age range and Hf isotopic compositions of zircons indicate up to -9 Ma lifespan for the Jurassic silicic magma chambers and suggest that the -2700-400 Ma xenocrysts represent crustal sources of the host rocks. The available chronological data indicate that the -183 Ma main phase magmatism was largely confined within the main Karoo and Kalahari basins and that the preceding and subsequent phases were mainly associated with the Karoo rift zone. Judging from geochemical literature, different kinds of magmas were erupted during the successive magmatic phases. We calculate from published geochemical data that the mafic main phase magmas were relatively poor in CO2 and SO2 and the lava stacking patterns point to low eruption rates, which suggests that degassing of sedimentary wall-rocks of intrusions probably triggered the coeval Pliensbachian-Toarcian extinction. In contrast, the mafic late phase magmas were rich in CO2 and SO2 and at least some of the lavas indicate high eruption rates. We propose that efficient degassing from widespread mafic magmatism and explosive eruption of over 30,000 km3 of silicic magmas in the Karoo rift zone linked the -182-178 Ma late phase magmatism with contemporaneous global biosphere crises. (c) 2022 The Authors. Published by Elsevier B.V. on behalf of International Association for Gondwana Research. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/ 4.0/).
  • Luoto, Toni; Salminen, Johanna; Obst, Karsten (2021)
    Baltica and Laurentia form the core of the hypothesized Mesoproterozoic supercontinent Nuna in most paleogeographical reconstructions. Long gaps still exist in the Mesoproterozoic paleomagnetic record of Baltica, and different relative configurations for Baltica and Laurentia have been presented. This study presents new paleomagnetic data obtained for mafic dykes on Bornholm (Denmark, southwest Baltica). We provide a new 1.326 +/- 0.010 Ga Bornholm Group I paleomagnetic key pole (Plat: 06 degrees N, Plon: 165 degrees E, K: 21, A95: 6 degrees) for Baltica. This pole supports the low-latitude equatorial core of Nuna at 1.33 Ga, where Kola Peninsula and Northern Norway of Baltica were facing northeastern Greenland of Laurentia. Based on statistically different magnetization directions with Group I and differences in Nb-Zr-Y systematics, we propose a separate Bornholm Group II paleomagnetic pole. This undated, poor-quality pole indicates a paleolatitude of ca. 50 degrees, possibly reflecting an age difference compared to Group I, accompanied with the continental drift. On Bornholm, the wide Listed and Kas dykes of uncertain age yield significantly different paleomagnetic results compared to the other studied dykes there. In addition, the virtual geomagnetic poles (VGPs) of these dykes are 45 degrees apart from each other. On the basis of similar Nb-Zr-Y systematics with the dykes of the 0.98-0.94 Ga Blekinge Dalarna Dolerite Group (Sweden) and overlapping paleomagnetic data with the high-quality 0.95-0.94 Ga paleomagnetic poles of Baltica, an early Neoproterozoic age for the magnetization is proposed. The relatively big discrepancy between Listed and Kas VGPs could stem from an unaveraged paleosecular variation or from a small but significant age difference during rapid plate movements.
  • Jarvinen, Ville; Halkoaho, Tapio; Konnunaho, Jukka; Heinonen, Jussi S.; Ramo, O. Tapani (2021)
    Several mafic-ultramafic layered intrusions were emplaced in the Fennoscandian Shield during wide-spread mantle-sourced magmatism at 2.5-2.4 Ga. The Narankavaara intrusion (surface area 5 x 30 km(2)), northeastern Finland, contains a 1.5-2 km thick basal dunite (not dated), and a 1.5 km thick layered series (2436 +/- 5 Ma). A newly discovered marginal series between the basal dunite and the layered series indicates that the basal dunite is older, and highlights the need for further study on their relationship. Along its southern basement contact, the basal dunite contains a 200-300 m thick zone of olivine ortho- and mesocumulates, but the bulk of it is composed of olivine adcumulates and lesser olivine-orthopyroxene heteradcumulates. Based on whole-rock geochemistry, the basal dunite is divided into a low-Fe zone (average FeOt 10.2 wt% and Ni 2250 ppm) and a high-Fe zone (average FeOt 12.5 wt% and Ni 1700 ppm). Both zones have high MgO (32-47 wt%) and varying Cr (830-5160 ppm) and Al2O3/TiO2 (16-26). Textural and geochemical layering is similar along the 30 km strike of the basal dunite. A LREE-enriched high-MgO basaltic parental magma composition (13-18 wt% MgO) is inferred for the basal dunite from olivine-melt mixing trends in orthocumulates. The dunite exhibits at least two geochemical reversals as well as abundant low-porosity adcumulates, poikilitic chromite, and bimodal olivine, suggesting formation in a high-volume open magmatic system. Significant similarity in major and trace element compositions with the Narankavaara layered series and the Burakovsky intrusion and Vetreny belt extrusives in Russian Karelia suggests that the basal dunite belongs to the Fennoscandian 2.5-2.4 Ga mafic layered intrusions. An Archean komatiitic origin for the dunite body cannot be completely ruled out, however. Distinct Ni-depletion in olivine is found in the basal dunite from the low-Fe zone to the high-Fe zone (3200 versus 2200 ppm). This depletion does not correlate with Fo contents, which suggests that it is not related to olivine fractionation. The basal dunite may thus have potential for Ni-(Cu-Co-PGE) sulfide mineralization.
  • Bispo-Santos, Franklin; D’Agrella-Filho, Manoel S.; Pesonen, Lauri J.; Salminen, Johanna M.; Reis, Nelson J.; Silva, Julia Massucato (2020)
    In recent years, there has been a significant increase in the paleomagnetic data of the Amazonian Craton, with important geodynamic and paleogeographic implications for the Paleo-Mesoproterozoic Columbia supercontinent (a.k.a., Nuna, Hudsoland). Despite recent increase of paleomagnetic data for several other cratons in Columbia, its longevity and the geodynamic processes that resulted in its formation are not well known. A paleomagnetic study was performed on rocks from the similar to 1535 Ma AMG (Anorthosite-Mangerite-Rapakivi Granite) Mucajai Complex located in the Roraima State (Brazil), in the northern portion of the Amazonian Craton, the Guiana Shield. Thermal and AF treatments revealed northwestern/southeastern directions with upward/downward inclinations for samples from twelve sites. This characteristic remanent magnetization is mainly carried by Ti-poor magnetite and in a lesser amount by hematite. Site mean directions were combined with previous results obtained for three other sites from the Mucajai Complex, producing the dual polarity mean direction: Dm = 132.2 degrees; Im = 35.4 degrees (N = 15; alpha(95) = 12.7 degrees; k = 10.0) and a paleomagnetic pole located at 0.1 degrees E, 38.2 degrees S (A(95) = 12.6 degrees; K = 10.2). The Mucajai pole favours the SAMBA (South AMerica-BAltica) link in a configuration formed by Amazonia and Baltica in Columbia. Also, there is geological and paleomagnetic evidence that the juxtaposition of Baltica and Laurentia at 1.76-1.26 Ga forms the core of Columbia. The present paleomagnetic data predict a long life 1.78-1.43 Ga SAMBA connection forming part of the core of the supercontinent. (c) 2019 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.