Paper No. 26-5
Presentation Time: 9:25 AM
AGE-CONSTRAINED LITHOGEOCHEMICAL INVESTIGATIONS OF VOLCANIC ROCKS IN NORTHWESTERN AVALON TERRANE, NEWFOUNDLAND
Northwestern Avalon Terrane in Newfoundland is underlain by: calc alkaline to tholeiitic magmatic arc rocks of the Love Cove Group (LCG); adjacent, mainly deep-marine basin fill of the Connecting Point Group (CPG) and; the unconformably overlying terrestrial-to-marine Musgravetown Group (MG). New lithogeochemical data and U-Pb zircon ages better constrain the tectono-magmatic setting and evolution of these sequences. Previous geochronological constraints on these largely (Acadian?) fault-bound belts include: 620 Ma for the LCG; 610 Ma near the stratigraphic middle of the CPG; 605-600 Ma for the CPG-MG boundary (Sweet Bay area) and; 569 Ma for alkaline rhyolite flows previously considered the basal volcanic unit of the MG (Bull Arm Formation: BAF). However, banded rhyolite near Bull Arm (Isthmus of Avalon; 120 km south of this study area) and a schistose crystal-lithic tuff from the LCG type locality yielded ages of 605 Ma and 589 Ma, respectively. Although the 620 Ma and 589 Ma rocks are lithologically similar, the former (Group F1) is characterized geochemically by deeper Nb troughs, lower HREE abundances and higher Th/Nb ratios relative to the latter (Group F2). The 605 Ma BAF rhyolite from the Isthmus of Avalon is similar to Group F2, but more enriched in most incompatible trace elements. Group F2 and Isthmus of Avalon rhyolite overlap the volcanic arc and within-plate granite fields on most geochemical discrimination diagrams. The 569 Ma BAF rhyolites, Group F3, are distinct from older BAF and LCG rocks: they are alkaline (A-type), have higher HFSEs, lower Th abundances and likely formed in an extensional setting. Strongly deformed MG rocks, including most Group F2 samples, were previously mapped as LCG. Accurate lithostratigraphy of volcanic groups of Avalonia is further hampered by assumptions that NNE-trending faults are coincident with lithological contacts. Results of this study highlight the importance of multidisciplinary (lithostratigraphic, lithogeochemical and geochronological) investigations in regional correlations. We propose that the BAF be elevated to Group status, the MG to Supergroup status, and that the use of LCG be restricted to ca. 620 Ma magmatic rocks, including intermediate compositions, having demonstrable, mature volcanic arc signatures.