Abstract

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 occurred 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.

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