ORDOVICIAN ARC MAGMATISM: COMPARISON OF THE NEW ENGLAND APPALACHIANS AND CALEDONIDES OF CENTRAL NORWAY

Extensive metamorphosed calc-alkaline intrusive rocks and volcanics appear in gneiss domes and in the east limb of the Green Mountain - Berkshire anticlinorium, western New England. These lie east of a belt of early Ordovician ophiolite fragments of probable fore-arc origin and a sequence of Cambrian - Mid-Ordovician continental slope/rise sediments of Laurentian affinity in a "Taconian accretionary prism". A common interpretation is that the volcanic and intrusive rocks belong to a "Bronson Hill magmatic arc" above an east-dipping subduction zone, and that abortive subduction of Laurentia produced features of the Taconian orogeny. Radiometric ages of the intrusives range from 488 to 442Ma (Late Tremadoc to Earliest Silurian) with most abundant activity 465-444Ma. These compare with emplacement of the Giddings Brook allochthon near Albany at 452Ma and 443 Ma for the end of high-grade Taconian metamorphism. Exposures closer to the New England coast, with volcanics and intrusives 471-460 Ma, may be an eastern exposure of the same arc terrane. Støren and Helgeland Nappes of central Norway, contain calc-alkaline Ordovician to earliest Silurian volcanics and intrusives thrust southeastward onto Baltica during the Devonian Scandian orogeny. At high tectonic levels above extensional detachments associated with Devonian basins, the intrusives are only weakly metamorphosed/deformed and yield ages 448-430Ma At low levels similar rocks suffered strong ductile deformation during and after extensional emplacement against higher grade gneisses of deeply subducted Baltica basement. These have the lineated/foliated appearance of gneiss dome rocks in New England and yield ages 482-444Ma. The oldest magmatic rocks are early Ordovician ophiolites, locally emplaced upside down and then covered by arc volcanics. Ophiolites are now exposed on both sides of the intrusive terrane, confusing paleotectonic interpretation. Possibly the Norwegian rocks represent ophiolite and arc rocks originally emplaced on Laurentia in the Taconian collision, then re-emplaced on Baltica during early Devonian continental collision.