USING DETRITAL ZIRCON LASER ABLATION SPLIT-STREAM U-PB-HF DATA TO UNTANGLE LAURENTIAN AND GONDWANAN SOURCES IN SEDIMENTS OF THE CONNECTICUT VALLEY - GASPE BELT

In the Northern Appalachian orogen, the timing of Gondwanan terrane accretion to Laurentia is debated, as well as the real extent of such terranes. Determining the source of detrital sediment that contributed to the infilling of sedimentary basins is one of the key tools able to identify arriving crustal blocks which should generate a distinctive change in detrital sediments.

In order to precisely constrain sediment provenance, we generated new laser-ablation split-stream U-Pb and Lu-Hf isotopic data from detrital zircon grains from Ordovician to Devonian sediments in the Gaspé Péninsula. The split-stream technique was essential in our context, since the zircon grains were small and complex, therefore the U-Pb and Hf data needed to be collected simultaneously. We sampled throughout the stratigraphy of the Connecticut Valley – Gaspé trough (CVGT), a major Upper Ordovician to Devonian post-Taconian sedimentary basin. This basin is associated with late Silurian to Early Devonian crustal extension, which was almost coeval with, or immediately followed, the Salinic orogeny. The CVGT was regionally deformed during the Middle Devonian Acadian orogeny, which has been interpreted as marking the final accretion of the peri-Gondwanan Avalon terrane.

Our detrital zircon U-Pb ages show Laurentian and Appalachian age sources, characterized by abundant Mesoproterozoic (Grenvillian) ages. However, Ordovician samples contain late Paleoproterozoic sources while Devonian samples contain some Ediacaran grains and show an increase in the proportion of Ordovician, Silurian and Devonian ages toward the top of the succession. The εHf data are consistent with Grenvillian and Appalachians sources, however, the Upper Ordovician Garin Formation contains Ordovician zircons with a distinct primitive εHf signature indicating the arrival of a new source. Our results suggest a strong influence of the erosion of Laurentian (cratonic, Grenvillian, and Appalachian) sources during the formation of the CVGT from Ordovician to Silurian time and highlight the proximity of peri-Gondwanan terranes in the Gaspé-Peninsula area during the Late Ordovician. The influence of our identified Gondwanan component would have been missed using only U-Pb data, highlighting the importance of multi-proxy analysis in provenances studies.