Zircon age depth-profiling sheds light on the early Caledonian evolution of the Seve Nappe Complex in west-central Jämtland

• The Scandinavian Caledonides comprise nappe stacks of far-travelled allochthons that record closure of the Iapetus Ocean and subsequent continental collision of Baltica and Laurentia. The Seve Nappe Complex (SNC) of the Scandinavian Caledonides includes relics of the outermost Baltoscandian passive margin that were subducted to mantle depths. The earliest of the deep subduction events has been dated to ca. 500-480 Ma. Evidence of this event has been reported from the northern exposures of the SNC. Farther south in the central and southern segments of the SNC, (ultra)high-pressure rocks have yielded younger ages in the range of ca. 470-440 Ma. This study provides the first record of the early Caledonian event in the southern SNC. The evidence has been obtained by depth profiling of zircon grains that were extracted from the Tvaraklumparna microdiamond-bearing gneiss. These zircon grains preserve eclogite facies overgrowths that crystallized at 482.6 +/- 3.8 Ma. A second, chemically-distinct zircon overgrowth records granulite facies metamorphism at 439.3 +/- 3.6 Ma, which corroborates previous geochronological evidence for granulite facies metamorphism at this time. Based on these results, we propose that the entire outer margin of Baltica was subducted in the late Cambrian to early Ordovician, but the record of this event may be almost entirely eradicated in the vast majority of lithologies by pervasive late Ordovician to early Silurian metamorphism.

Subject headings

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geologi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geology (hsv//eng)

Keyword

Caledonian Orogeny

Seve Nappe Complex

Zircon geochronology

HP

UHP metamorphism

Double dunking

Depth profiling

Publication and Content Type

ref (subject category)

art (subject category)