Ordovician magmatism, deformation, and exhumation in the Caledonides of central Norway: An orphan of the Taconic orogeny?

Magmatism, contractional deformation, and extension associated with the exhumation of high-pressure rocks in the Scandinavian Caledonides are commonly attributed to the Silurian-Devonian Scandian orogeny, in which eastward thrusting of allochthonous terranes over Baltica was followed by extensional collapse and exhumation. New fieldwork and U-Pb geochronology coupled with recent pressure-temperature estimates within the highest thrust sequence of the Caledonian orogen indicate that an earlier phase of west-directed contractional deformation was punctuated by migmatite-producing events and voluminous magmatism ca. 477-466 Ma and ca. 447 Ma, followed by exhumation in the Late Ordovician. Al-in-hornblende and GASP thermobarometry indicate that emplacement of a suite of 448-445 Ma plutons caused partial migmatization at pressures of 700-800 MPa. Subsequent isothermal exhumation to pressures of 400 MPa occured while the host rocks were still partially molten. Rates of exhumation may have ranged from 2 to 11 mm·yr^-1 or greater. These data provide evidence for a previously unrecognized phase of exhumation in the Caledonides and for aerially extensive west-vergent deformation. Deformation and magmatism associated with these events may be related to Taconic-age orogenesis near Laurentia, where the highest nappe sequences of the Scandinavian Caledonides probably resided during early Paleozoic time.