GONDWANAN - SOUTH AMERICAN CONNECTIONS IN TACONIAN, ACADIAN-CALEDONIAN, AND YOUNGER OROGENIC RECORDS IN THE NORTHERN APPALACHIANS AND NORWEGIAN CALEDONIDES (II)

The Scandinavian Caledonides have long been understood as a Silurian - Early Devonian (Scandian) orogen, with emplacement of far-traveled thrust sheets onto the northwest margin of Baltica. Various evidence suggests this involved NW subduction of Baltica beneath Laurentia, including recent U-Pb ages on HP and UHP eclogites from 420 (latest Silurian) to 400 Ma (Early Devonian), including some in proximal parts of Baltican basement. The upper part of the Upper Allochthon involves 500-480 Ma ophiolites like those emplaced on the Laurentian margin in the Taconian, voluminous magmatic-arc rocks remarkably similar in character and age (480 to 441 Ma) to the Shelburne Falls - Bronson Hill arc, and, in the highest thrust sheets, fossils of Laurentian affinity. The Uppermost Allochthon, long thought as possibly Laurentian, has recently revealed three important features. Medium- to high-grade marbles, are dated as Late Neoproterozoic through Cambrian by C-Sr isotope stratigraphy. Some Cambrian marbles show clear sedimentary features indicating primary deposition on the SE-facing margin of an extensive carbonate bank identical to that known on the Cambrian margin of Laurentia. Further, the same rocks contain strong evidence of NW-directed, probable Taconian thrusting, older than the pervading Scandian structures and regional metamorphism. Elsewhere in the Uppermost Allochthon, high-grade metamorphic rocks cut by highly deformed plutons in the range 480-460 Ma were thrust NW over much lower-grade magmatic arc rocks and ophiolites, then stitched by plutons in the range 448-441 Ma. This high-grade thrust sheet may represent rocks exotic to Ordovician Laurentia, thrust emplaced onto the assembled Ordovician arcs and Laurentian margin in the Taconian, then emplaced onto Baltica and metamorphosed again in the Scandian.

These aspects of Appalachian-Caledonian geology, with potential connections to South America, illustrate the importance of thinking on a very large scale, including examination of modern parallels, while paying attention to the smallest details, adapting to the wide range of new petrologic and geochronologic techniques, and being fearless in attempting to carry stratigraphic correlation into medium- and high-grade metamorphic rocks, which may be the only place where some key records are preserved.