GEOCHEMISTRY OF NEOPROTEROZOIC MUDROCKS FROM THE WINDERMERE SUPERGROUP

The Windermere Supergroup consists of rift to post-rift clastics shed from the western margin of Laurentia from 736-569 Ma. This study investigated the geochemistry of deep-marine mudrocks interlayered with sandstone deposited in basin-floor lobe complexes in the Middle and Upper Kaza Group (~2000 m) overlain by base-of-slope leveed channels of the Isaac Formation (~1500 m), Cariboo Mountains, British Columbia. In the two study areas, Castle Creek and Eagle Valley, vertically-dipping, glacially polished, km-thick by km-wide exposures of greenschist facies turbidites crop out. Sixteen samples of the Isaac Formation and 31 from the underlying Middle and Upper Kaza Group were analyzed for major and trace elements. Despite differences in physical appearance, the chemistry of all strata is nearly identical, with the exception of slightly higher concentrations (~2x) of redox sensitive elements in the deeper water, and stratigraphically lower, Kaza Group. Few, if any, significant vertical geochemical trends were observed, however, on the average the Isaac Formation has less CaO and REEs. Compared to crustal averages both the Isaac and Kaza Formations are enriched in Al2O3, Fe2O3, TiO2, K2O, and REEs, and depleted in SiO2, CaO, and NaO. These trends are similar to other greenschist facies mudrocks and are likely related to metamorphic changes including the recrystallization of clay minerals and release of mobile elements. Total carbon and sulfur concentrations are low in all samples (C – 0.11 and 0.14%; S – 0.18 and 0.16), however a phosphatic unit (4.29% - P2O5) was identified near the Kaza and Isaac contact. Redox sensitive ratios for both formations consistently indicate oxic conditions. While enriched in iron, Fe/Al ratios range are .44 and .47 for the Isaac and Kaza, respectively. A felsic source is indicated for both units, however, the overlying Isaac Formation is less so, perhaps suggesting progressive unroofing of high level plutons.