DETRITAL ZIRCON LASER ABLATION SPLIT-STREAM U-PB-HF DATA FROM ORDOVICIAN TO DEVONIAN SEDIMENTARY ROCKS IN THE CONNECTICUT VALLEY – GASPÉ BELT: FINGERPRINTING LAURENTIAN AND GONDWANAN SOURCES

The timing and spatial distribution of native and exotic terranes, which were incorporated into Laurentia during Northern Appalachian orogenesis in the Ordovician to Devonian, remains debated. One of the key techniques that can distinguish newly arrived crustal blocks is determining the nature of erosional sources that contributed to the infilling of sedimentary basins. In order to constrain sediment provenance, we report new laser-ablation split-stream U-Pb and Lu-Hf isotopic data obtained from detrital zircon grains from the Ordovician to Devonian sediments in the Gaspé Peninsula. The Connecticut Valley – Gaspé trough (CVGT) is a major Upper Ordovician to Devonian post-Taconian sedimentary basin that extends more than 1000 km from New England to Gaspé peninsula. We sampled throughout the stratigraphy from either side of the basin as well as structurally below it. The formation of the CVGT occurred during a late Silurian to Early Devonian crustal extension event, known as the Salinic disturbance in Gaspé peninsula. The CVGT was regionally deformed and metamorphosed at variable intensities during the Middle Devonian Acadian orogeny, interpreted as marking the final accretion of the peri-Gondwanan Avalon terrane.

Our new detrital zircon ages show mostly Laurentian sources characterized by abundant Mesoproterozoic (Grenvillian) ages. At the base of the basin samples contain abundant late-Paleoproterozoic zircon whereas at the top samples contain Ediacarian grains and show a more important input of Ordovician to Silurian zircons. Epsilon-Hf values are distinctively positive for most of the Grenvillian grains and also show a vertically spread cluster that can be interpreted as a result of mixing of a juvenile mafic magma with older crustal rocks. Our preliminary results suggest a strong influence of the erosion of Laurentian sources during the formation of the CVGT from Ordovician to Silurian time and do not highlight the proximity of peri-Gondwanan terranes in the Gaspé-Peninsula area. We speculate that the peri-Gondwanan terranes now exposed to the southeast of the CVGT were rapidly covered by a combination of underthrusting and sedimentary basin development following accretion to the Laurentian margin, and only exhumed late in the history of the orogen.