A NEW LOOK AT THE TECTONIC PROCESSES INVOLVED IN THE ORDOVICIAN TACONIC OROGENY IN THE NEWFOUNDLAND APPALACHIANS: EVIDENCE FOR MULTIPLE ACCRETION OF (INFANT) ARC TERRANES

Detailed investigations in Newfoundland suggest that the Taconic Orogeny involves a circa 50 Ma long history of Early Palaeozoic, oblique accretion of oceanic and continental arc terranes. The oldest event is the 495-490 Ma obduction of the suprasubduction zone (ssz) 510-500 Ma Lushs Bight oceanic tract onto the peri-Laurentian Dashwoods microcontinent, followed by subduction stepping back into the Humber seaway, which generated the 489-485 Ma ssz Baie Verte oceanic tract (BVOT) and the 1st phase of the Notre Dame arc (488-480 Ma). The BVOT in part occupied a forearc position to the Notre Dame arc built on Dashwoods, but also became the foundation to a coeval (Snooks Arm) oceanic arc along strike. Syn-convergence dextral motion juxtaposed the oceanic and continental arc segments. Arenig loading of the Humber margin coincides with a magmatic gap (479 to 469 Ma) that separates the 1st and 2nd phases of the Notre Dame arc, probably as a result of arc-continent collision. The ~ 462 Ma tonalitic flare-up of the 2nd phase of the Notre Dame arc is related to break-off of the oceanic slab attached to the subducting Humber margin. Arenig to Llanvirn ssz ophiolites and island arc volcanic rocks were generated immediately east of the Dashwoods in the AAT during Notre Dame arc and Humber margin collision. Structural relationships, the presence of boninites and isotope data indicate that the AAT comprises a ~ 480 Ma infant arc generated during west-directed subduction, initiated east of Dashwoods and several thin thrust slices of younger arc volcanic rocks (473 to 460 Ma), separated by incomplete ophiolitic suites that become progressively younger towards the east. Sinistral oblique accretion of the AAT to the Dashwoods started ~ 470 Ma and resulted in strike-slip duplexing of arc segments. All the AAT was accreted to Laurentia by at least 455 Ma when the peri-Gondwanan Popelogan – Victoria Arc started to collide with Laurentia during the final phase of the Taconic Orogeny.