GEOCHRONOLOGICAL AND ISOTOPIC EVIDENCE FOR PROGRESSIVE RECYCLING OF OLDER CRUST IN ARC SYSTEMS LINKED TO GANDERIA IN CENTRAL NEWFOUNDLAND

Recent U-Pb geochronological and Sm-Nd isotopic studies in central Newfoundland are aimed primarily at defining the age of important volcanogenic massive sulphide (VMS) deposits within the Victoria Lake Supergroup, a collage of Cambrian and Ordovician rocks representing peri-Gondwanan arc systems within the Iapetus Ocean. However, these data also have interesting implications for the extent and influence of Precambrian crust in the region, and for the progressive accretion of Paleozoic arcs in the region now termed "Ganderia".

An age of ca. 565 Ma from felsic tuffs associated with the Burnt Pond VMS prospect adds to the extent of late Precambrian rocks of both plutonic and volcanic affinity, and indicates that the latter have significant mineral potential. The discrimination of these Precambrian and Paleozoic volcanic sequences is difficult on the basis of their geochemistry, although Sm-Nd data may provide some distinction. Two ages from felsic rocks closely associated with the Duck Pond VMS deposit confirm that it and its host rocks (Tally Pond Group) are of Cambrian age (ca. 514 - 509 Ma). U-Pb SHRIMP data indicate the presence of inherited zircons with ages of ca. 573 Ma and 563 Ma in these rocks. These ages match those obtained directly from late Precambrian rocks in the area, and indicate that the Tally Pond Group likely developed upon this older substrate. Sm-Nd model ages for Tally Pond Group samples are considerably older than ca. 570 Ma, suggesting that the incorporated Precambrian crust had a significantly older heritage. Two ages from the Boomerang VMS deposit indicate that its host rocks form part of a younger (ca. 491 Ma) Ordovician sequence within the Victoria Lake Supergroup. U-Pb SHRIMP data from these samples indicate the presence of inherited zircons with ages of ca. 514 Ma and 510 Ma. These ages closely match those obtained from the Tally Pond Group. Collectively, such patterns support a wider model in which Paleozoic peri-Gondwanan island arcs in central Newfoundland were progressively accreted to a Precambrian crustal fragment, and in which the older arcs provided the foundations for progressively younger sequences.