Abstract

U-Pb and 40Ar/39Ar isotopic data combine with structural and petrological information to allow insights into the timing of Caledonian tectonic burial and exhumation of lower crustal rocks now exposed in the Lofoten Islands of North Norway (latitude 68° N). Severely retrogressed eclogites occur in rare lenses or, even more rarely, hydrated shear zones within Archaean and Proterozoic granulite-facies Baltic basement gneisses. The timing of high-pressure metamorphism in Lofoten has been difficult to determine because retrogression has disturbed mineral isotopic systems and zircon apparently was not generated, leaving its tectonic significance uncertain. Recently discovered pre- and post-kinematic felsic injections provide the opportunity to bracket the age of eclogitization. U/Pb analyses of zircon and xenotime from a prekinematic syenogranite dyke that cuts the mafic host to one of the retro-eclogite lenses intruded at 1800 ± 5 Ma, and we interpret the strong disturbance of its U-Pb system at 478 ± 41 Ma to approximate the age of eclogitization. Omphacite breakdown textures imply rapid isothermal decompression during initial uplift, followed by slow uplift. Syn-upper-amphibolite-facies thrust emplacement of the overlying Leknes Group at ca. 464 Ma implies that the Lofoten basement resided in the lower crust for ca. 14 m.y.40Ar/39Ar cooling dates from the retro-eclogites trace their exhumation path into the middle crust (hornblende ca. 433 Ma) where they resided before being slowly elevated to levels at the base of the ductile-brittle transition in the early Carboniferous (muscovite ca. 343 Ma). The Middle Ordovician Lofoten eclogites likely formed at a time similar to the “early” group of Eocaledonian eclogites (ca. 505–450 Ma) found in the Tromsø, Seve, and Bergen Arcs allochthons that were later thrust onto Baltica during the main Scandian (Siluro–Devonian) collision. Lofoten eclogites appear to be ca. 50 m.y. older than the “late” group of autochthonous, Scandian (ca. 425–400 Ma) high-pressure and ultrahigh-pressure (UHP) eclogites of the Western Gneiss Region (WGR), and the former preserve a much longer (ca. 100 m.y.) exhumation history. To date, there is no evidence to indicate either UHP or Scandian eclogite-facies metamorphism in Baltic basement along coastal Norway north of latitude 64° N. Middle Ordovician eclogites of Scandinavia are contemporaneous with Taconic eclogites found in thrust nappes of the Appalachian Orogen. If the Lofoten eclogites correlate to those in the Tromsø Nappe Complex (Uppermost Allochthon), then both terranes might represent Laurentian relics emplaced during the Scandian and left orphaned on the conjugate side of the orogen when the North Atlantic began to open in the Eocene. Lithologic, petrologic, kinematic, provenance, and palinspastic information favor, however, correlation with eclogites in transitional Baltic-Iapetan crust of the Seve Nappe Complex (Upper Allochthon), which provides a piercing point linking eclogites in autochthonous Baltic crust now exposed in the most internal parts of the orogen with those transported and preserved in the Swedish foreland. We suggest a broad twofold subdivision for the Eocaledonian eclogite provinces into those that are (1) Baltic derived, and (2) exotic with respect to Baltica. Middle Ordovician eclogites falling under category (1) can further be subdivided into those occurring in autochthonous Baltic basement (i.e., Lofoten), and those in thrust translated terranes. Apparently, continental crust in a collisional setting can be subducted to mantle depths and show only very sparse evidence of this tectonic history.

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