ABSTRACT
More than 20 bentonites occur in the Middle Ordovician black shales of the Utica Formation that outcrop along tributaries to the Mohawk River in New York State. Pristine melt inclusions within quartz and apatite phenocrysts are commonly found in at least 40% of the bentonites. Constraints on magmatic processes can be obtained by examining the major-, minor- and trace-element variations among different melt inclusions in quartz and apatite phenocrysts from individual bentonites. The melt inclusions in apatite range in composition from rhyodacitic to rhyolitic, while those in quartz are high-silica rhyolites. Most bentonites within the Middle Ordovician sequence in New York contain melt inclusions with 5-6 wt.% K20 (high-K). However, the bentonites within a restricted stratigraphic interval (approximately 30 m thick) that occurs in the C. americanus graptolite zone contain melt inclusions with 2-3 wt.% K20 (low-K). These inclusions typically have higher MgO, CaO, MnO, TiO2 and FeO abundances relative to the high-K glasses. Commonly in this 30 m restricted interval, bentonite layers display bimodal distributions of elemental abundances. In some cases, one of the groups of inclusions within the bimodal bentonites appears to be chemically identical to melt inclusions within other bentonites in this interval. The bimodal bentonites could be composed of the air-fall components of two different eruptions. Another possibility is that the bimodality reflects zoned magma chambers. The two different models may hold important implications for the interpretation of bentonite correlations.

Dannenmann, S., 1997. Analysis of glasses present in quartz and apatite phenocrysts from Ordovician K-bentonites in the Mohawk Valley, NY. Unpublished MSc. thesis, State University of New York at Albany. 92 pp., +x
University at Albany Science Library call number:  SCIENCE Oversize (*) QE 40 Z899 1997 D36

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