ALBERT

Description: A detailed tectonic analysis of the magmatic evolution of the Early to Middle Ordovician west-facing Popelogan arc in New Brunswick and adjacent Maine is presented based on combining new U-Pb zircon radiometric age dates (thermal ionization mass spectrometry and sensitive high-resolution ion microprobe) with existing age constraints on the various magmatic events recognized in this arc system. The Popelogan arc had a life span of nearly 20 m.y. (ca. 476–457 Ma) and becomes progressively younger toward the northwest. Existing lithogeochemical and isotope data combined with field relationships and new zircon inheritance data indicate that the Popelogan arc was built on the leading edge of Ganderia and represents a predominantly continental calc-alkaline arc. Incursions of arc to within-plate–like tholeiitic magmatism correspond to rifting events temporally and spatially linked to trenchward migration of the arc magmatic axis, isolation of arc ribbons, and formation of mafic oceanic and transitional crust in the associated Tetagouche backarc basin. Arc-trench migration and backarc basin opening were caused by a retreating subduction zone. Slab retreat prior to 467 Ma was at least partially accommodated by arc migration, but thereafter, it was mainly accommodated by extension and spreading in the associated Tetagouche backarc basin. New U-Pb zircon radiometric ages and lithogeochemistry of calc-alkaline felsic tuff beds interlayered with the oceanic backarc rocks of the Fournier Supergroup provide a direct link between the Tetagouche backarc rocks and coeval calc-alkaline arc activity in the Popelogan arc. The oldest incursion of arc tholeiite (470–467 Ma) corresponds to cessation of the ca. 476–470 Ma calc-alkaline Meductic phase of the Popelogan arc, the onset of the calc-alkaline Balmoral phase (467–457 Ma) farther to the west, and formation of oceanic and mafic-dominated transitional crust in the Canoe Landing Lake Formation of the Tetagouche backarc basin. A less-well-constrained second incursion of tholeiitic magmatism between 465 and 459 Ma corresponds to rifting of the Tetagouche arc ribbon and formation of the ophiolitic Devereaux complex in the Fournier Supergroup. Cessation of calc-alkaline arc magmatism and eruption of transitional and alkalic mafic volcanic rocks between 459 and 455 Ma may represent slab breakoff following accretion of the Popelogan arc to composite Laurentia, and/or ridge subduction immediately prior to arc-continent collision.