HR: 0830h
AN: V51G-0359    [PDF]
TI: Paleotectonic Significance of the Upper Cretaceous Frontier Bentonites, Wyoming.
AU: * Khandaker, N I
EM: kdaker@york.cuny.edu
AF: Department of Natural Sciences, York College (CUNY), Jamaica, New York, NY 11451
AB: The major problems concerning the Upper Cretaceous Frontier bentonites in north-central Wyoming are (1) their source areas, (2) paleotectonic setting, and (3) their original ash composition. Detailed trace-element geochemistry, detrital mineralogy, and field relationships associated with the bentonite beds were used to assess the composition and to characterize the provenance of the volcanism. Geochemical plots using Ti-Zr-Y and Nb-Zr-Y contents indicate a lower group (Peay bentonite) of thin tephra of dominantly alkali basaltic composition and an upper group (Torchlight bentonites) of thick and variable tephras of andesitic to basaltic andesite composition. Geochemical signatures within the bentonites indicate that the volcanism in the northerly source region changed composition with time. The progression from homogeneous but generally silica-undersaturated (Peay bentonites) to consistently silica-saturated magmas (Torchlight bentonites) with time suggests an increase in the degree of partial melting and a decrease of crustal involvement with time. Furthermore, the overall composition and immobile trace element geochemistry of the bentonites indicate that the source volcanoes had a similar plate-continental affinity and were spatially related.
DE: 8404 Ash deposits
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting