2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 14
Presentation Time: 1:30 PM-5:30 PM


JOHNS, Shannon M., HELMSTAEDT, Herwart H. and KYSER, T. Kurt, Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, ON K7L 3N6, johns@geoladm.geol.queensu.ca

The ~1.9 Ga Bravo Lake Formation (BLF) is a complex inter-layered package of metamorphosed volcanic and sedimentary rocks at the northern margin of the Trans-Hudson Orogen on central Baffin Island. The formation is a member of an extensive Paleoproterozoic passive margin and basin succession. The initiation of intrabasinal mafic rift volcanism, forming the BLF, resulted in shelf collapse and a rapidly subsiding turbidite-filled basin. Geological mapping indicates that much of the BLF represents lava flows and high-level intrusions within an evolving submarine volcanic rift setting. Primary volcanic features are relatively undeformed and include amygdaloidal pillow lava, radial columnar and tortoise-shell jointed pillows, hydroclastic breccia, extensional sheeted dyke swarms, laminated mafic sediments, massive and fragmental lava flows, and layered and megacrystic intrusions. Major, trace, and REE geochemistry suggest that these rocks are within-plate alkali and tholeiitic basalts, and show distinctive fractionation trends and crustal contamination. During the emplacement of this sequence, mafic lavas and magmas interacted with Proterozoic seawater and underwent hydrothermal alteration. Himalayan-scale mountain building followed during the Trans-Hudson orogeny, which deformed, fragmented, and metamorphosed much of the BLF. Petrographic studies indicate that the rocks have undergone extensive mineralogical replacement during hydrothermal alteration, dry closed-system upper amphibolite facies regional metamorphism, and later retrograde metamorphism. Stable oxygen and hydrogen isotopic compositions measured from relict hydrothermal minerals are used to see through the metamorphism, and suggest that Paleoproterozoic seawater was isotopically similar to modern seawater. Stable isotope measurements of whole rocks, and metamorphic minerals from each of the phases reflect a complex history of fluid evolution that affected this submarine volcanic system.