M.S. (Master of Science)
Department of Geology
Lava--Ontario--Alona Bay; Geochemistry--Ontario--Alona Bay; Petrogenesis--Ontario--Alona Bay; Geology; Stratigraphic
An 1100 m section of Keweenawan volcanics consisting of at least 107 separate basalt flows is exposed at Alona Bay, Ontario, several kilometers north of Mamainse Point. The flows represent fluid, subaerial flood basalts. Initial melts are high Mg and Ni, low Al olivine tholeiites which alternate between flows low in incompatible elements (low Ti basalts) and flows high in incompatible elements (high Ti basalts). These flows comprise the lower 700 m of the section. While Zr/Y ratios are similar for these two groups, high Ti basalts have higher Ti/Zr (101) and lower Zr/Nb (12) ratios than low Ti basalts (84 and 17 respectively). The upper 300 m of lavas have higher Al and lower Mg and Ni concentrations. These upper flows are low in all incompatible elements except Nb and have incompatible element ratios which distinguish them from stratigraphically lower flows (lower Zr/Nb and Zr/Y). Critical compositional distinctions within the section are believed to result from variation in depth and degree of partial melting, and source heterogeneity. Lavas low in the section were apparently generated at the greatest depth (65-100 km) within the subcontinental lithosphere. Changes in incompatible element ratios between high and low Ti basalts can be explained by melting a garnet lherzolite source, with Zr behaving more compatibly than Nb and Ti
Gmitro, Todd T., "Chemostratigraphy and petrogenesis of Keweenawan lavas at Alona Bay, Ontario" (1990). Graduate Research Theses & Dissertations. 1772.
xi, 243 pages, maps
Northern Illinois University
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