MAGMATIC EVOLUTION OF THE AVALONIAN ARC SYSTEM REVEALED BY EDIACARAN STRATA IN EASTERN NEWFOUNDLAND: NEW RESULTS FROM LASER ABLATION SPLIT STREAM (LASS) DETRITAL ZIRCON U-PB GEOCHRONOLOGY AND HF ISOTOPE GEOCHEMISTRY

West Avalonia in the Canadian Appalachians is underlain by Tonian to Ediacaran rocks that record ~200 m.y. of convergent margin activity. In the Avalon Zone of eastern Newfoundland, conventional TIMS zircon U-Pb studies have reported that West Avalonia was largely built by arc-related events at ca. 760, 730, 685-670, 640-590, and 590-565 Ma. These magmatic events document the secular evolution of Avalonia during its early arc (760-670 Ma), accretionary (665-650 Ma), continental arc (640-590 Ma), and late arc (590-565 Ma) phases. Deep-marine to alluvial plain successions that were deposited during the late Ediacaran erosion of the Avalonian arc are >10 km-thick and offer excellent opportunities to investigate the coupled magmatic-sedimentary evolution of a Neoproterozoic convergent margin. To test and develop new working hypotheses for Avalonian arc evolution, we used the novel LASS (laser ablation split stream) technique to constrain the U-Pb and Hf isotope compositions of Neoproterozoic detrital zircons in upper Ediacaran strata of the St. John’s area, eastern Avalon Peninsula.

A pilot study with three fluvial sandstone samples and one arc-related mixtite sample confirmed that Avalonian crust in eastern Newfoundland grew during a series of magmatic events from 760-565 Ma. Most samples show a unimodal age peak ~620 Ma, with shoulders and minor age peaks ~760, 680, 660, 640, 600, 595, 580, and 565 Ma. The majority the 640-590 Ma zircons, likely from the continental arc phase of Avalonian development, yield radiogenic Hf isotope values (initial εHf = +2 to +9). Ongoing studies of four additional sandstone samples will refine these constraints and determine if observed excursions to lower values (initial εHf = -2 to 0) were coeval with 640-590 Ma high-volume events, similar to published models for magmatic flare-ups in some compressive arc systems (e.g., DeCelles et al., 2009, Nature Geoscience). Hf model ages for the continental arc phase zircons typically range from 800-1300 Ma, which is consistent with tectonic scenarios for Avalonia being partially built on early Neoproterozoic to Mesoproterozoic crust. Tonian detrital zircons, likely from the early arc phase, yield early Mesoproterozoic to Paleoproterozoic Hf model ages and may point to older basement domains beneath West Avalonia.