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Unpublished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
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(*) QE 40 Z899 1997 D36
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