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Kemp, K.M., 1994

Tephrostratigraphy of the Late Cretaceous Kanguk Formation, Banks Island, Arctic, Canada

Bibliographic Reference

Kemp, K.M., 1994, Tephrostratigraphy of the Late Cretaceous Kanguk Formation, Banks Island, Arctic, Canada: Toronto, Ontario, Canada, University of Toronto, M.S. thesis.


Vitric submarine fallout ash beds found in the Late Cretaceous Kanguk Formation on Banks Island in Arctic Canada are high K, peraluminous virtually aphyric rhyolites to high-silica rhyolites. The typical phenocryst assemblage is quartz + plagioclase +/- albite + ilmenite + zircon + biotite. Other minerals present include primary apatite and rutile, and accidental staurolite, almandine, and andradite. Within individual units, feldspar, ilmenite, zircon, and biotite tend to have a wide range of major element compositions, suggesting that they are not all in equilibrium with the glass in which they are found. The glasses may have suffered slight alkali exchange during hydration; however, other elements, notably the rare-earth-elements (REE), appear to have been unaffected. Trace-element composition of glass, particularly REE, Sc, Cs, Th, and Hf, allows each tephra unit (with one exception) to be characterized. Collectively, heavy rare-earth-element (HREE) depletion with increasing SiO2, rock/glass REE patterns that do not reproduce fractionation trends, and high Th relative to U contents suggest that most of the compositional differences between tephra units are due to original differences in the magma produced by partial melting rather than differentiation through fractional crystallization. The magma was generated in a continental volcanic arc probably situated in eastern Alaska. Montmorillonite, produced by devitrification of volcanic glass, may show up to 200% enrichment in REE over the glass from which it was derived. This enrichment is probably due to mass loss during the formation of montmorillonite. However, clays subjected to acidic solutions are depleted in REE and other trace elements relative to the glass from which they were derived. Whole-rock REE patterns from highly altered tephra may be distinctly unlike those typically found in volcanic glasses. Chemical correlation using tephra layers altered to montmorillonite should only be attempted when acidic solutions have not percolated through the bed. The cores of thick tephra beds are less likely to be altered by acid-leaching.

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