PETROGENESIS OF LATE EDIACARAN RHYOLITE OF THE SILVER HILL FORMATION, COLDBROOK GROUP, IN AVALONIA OF SOUTHERN NEW BRUNSWICK, CANADA

The Coldbrook Group is a suite of volcanic and epiclastic sedimentary rocks emplaced in a rift setting between 560-550 Ma (U-Pb zircon ages). The Silver Hill Formation represents the last eruptive event of the Coldbrook Group at about 550 Ma and includes large volumes of rhyolitic flows and tuffs that can be traced for more then 50 km through the Avalonian Caledonia terrane of southern New Brunswick. These silicic volcanic rocks have been interpreted to be cogenetic with the spatially associated granitic Bonnell Brook and other plutons of similar age. Textural observations, mineral and whole-rock geochemistry, Molar Element ratio (MER) analysis, Sm-Nd isotopic data, and zircon chemistry and U-Pb dating are being utilized to investigate the origin of the Silver Hill rhyolite and its possible link with the associated felsic plutons, and to investigate the hypothesis that these rocks represent a super-volcano/eruption environment. Preliminary geochemical results show that the rhyolitic and granitic rocks have similar concentrations of major and trace elements, all displaying, for example, high SiO₂ (70-79 weight %), low TiO₂ and wide variation in Zr and Y concentrations compared to somewhat older volcanic components of the Coldbrook Group. MER analysis indicates quartz and mainly alkali-feldspar sorting, for both rhyolite and granite. However, the rhyolite has slightly higher silica concentrations and displays more scatter in the MER data compared to the granitic rocks. MER analysis also suggests that the rhyolite has experienced loss of K, Na and Ca, processes not observed in the data for the granitic rocks. The chemical similarities suggest that the rhyolitic and granitic rocks are genetically related and that the rhyolite may represent a pre-eruptive melt extracted from a crystal mush, represented by the Bonnell Brook and associated plutons.