Provenance and exhumation of an exotic eclogite-bearing nappe in the Caledonides: a U–Pb and Rb–Sr study of the Jæren nappe, SW Norway

Abstract

The provenance and exhumation history of paragneisses that enclose the 471-458 Ma eclogites in the Caledonian Jaeren nappe, SW Norway, were constrained by performing U-Pb analysis on detrital zircon and Rb-Sr dating of mica. Zircon grains from four samples were analysed by laser ablation inductively coupled plasma mass spectrometry, providing detrital U-Pb ages between 3130 and 620 Ma. These data suggest Laurentian provenance, implying the existence of an early Caledonian (Taconian) HP fragment of Laurentian crust in the southern Scandinavian Caledonides. Several rims and recrystallized zircon patches yielded U-Pb ages averaging at 469 AE 6 Ma (2ó). These are identical to the Lu-Hf ages of the eclogites and underscore the importance of Taconian metamorphism in this part of the Caledonides. The main phengite Rb-Sr age cluster at 424.4 AE 4.6 Ma provides time constraints on the amphibolite-facies recrystallization of former high-Si phengite. Five out of seven biotite Rb-Sr ages were identical, yielding a weighted mean age of 413.6 AE 2.6 Ma. Low-Si phengite and biotite in three samples gave c. 400 Ma ages. The Rb-Sr data record the mid-crustal emplacement of the exotic Jaeren nappe and its subsequent unroofing along the western edge of Baltica during the backsliding and collapse of the Scandian orogenic front.Supplementary material: The sample locations, U-Pb isotope data and ages for zircon, and the phengite and biotite major element data are available at http://www.geolsoc.org.uk/SUP18448.

A plate tectonic scenario for the Iapetus and Rheic oceans

“…Early to Middle Ordovician fossils of Laurentian affinity in the Støren nappe (Reed, 1932;Pedersen et al, 1992) and Archean to Paleoproterozoic inherited zircons in Early to Middle Ordovician granitoids intruding the Karmøy ophiolite (Pedersen and Dunning, 1997) suggest that this intraoceanic arc was close to Laurentia, at least by the Early Ordovician. The apparent termination of intraoceanic subduction in the late Early to early Middle Ordovician was coincident with HP metamorphism in the Jaeren nappe (Smit et al, 2011) and widespread mid-to-Late Ordovician deformation in the Köli and Lyngen nappes (Slagstad et al, 2013;Corfu et al, 2014), possibly signifying collision of the intraoceanic arc with Laurentia at that time. Such a scenario could also explain the Early Ordovician HP complexes presently preserved in western Svalbard, as well as the timing of subduction initiation beneath northeast Greenland-Svalbard in the Middle Ordovician.…”

Root zone of a continental rift: the Neoproterozoic Kebnekaise Intrusive Complex, northern Swedish Caledonides

“…from syn-rift sedimentary rocks occurring as xenoliths in the KIC. Detrital zircons from other late Neoproterozoic sedimentary rocks deposited in Iapetus-related rift basins along the margin of Baltica have yielded ages as young as 600 Ma (Bingen et al, 2005;Smit et al, 2011;Lamminen et al, 2015). Thus, even zircons with ages around 600 Ma may be inherited, and represent an earlier phase of rift-related magmatic activity.…”

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

“…Some of the structurally higher tectonic units of the Køli Nappe Complex host a Laurentian fauna (Bergström 1979;Bruton & Bockelie 1979Spjeldnaes 1985;Bruton & Harper 1988), testifying to the far-travelled origin of these units with respect to Baltica. Recent studies have indicated that terranes within the Seve Nappe Complex are of exotic or Laurentian origin (Root & Corfu 2011;Smit et al 2011). Similarly, it has been suggested that the Kalak Nappe Complex in northernmost Norway, previously thought to be part of the Baltica-derived Middle Allochthon, is exotic with respect to Baltica and should be incorporated in the Upper Allochthon (e.g.…”