Abstract
The present study of Paleozoic K-bentonites demonstrates that the
geochemistry
of melt inclusions and the morphology of zircons can be studied by
inexpensive
and simple-to-use methods, which rely on phenocrysts. Constraints
are obtained
that lead to (a) the origin of these altered volcanic ashes, (b)
the geochemistry
of ash-to-K-bentonite-alteration, and (c) the reliable correlation
of extensively
altered volcanic ashes (i.e. K-bentonites).
Silicic melt inclusions (i.e. non-devitrified) have been found in
quartz
and zircon phenocrysts contained within Ordovician and Devonian
K-bentonites
from New York State, the Upper Mississippi Valley, and
Pennsylvania. Origin,
source, and tectonic setting of the volcanism that produced these
Paleozoic
volcanic ashes (i.e. K-bentonites) are constrained by the
geochemistry
of these inclusions. The major element compositions of the
inclusions,
which are small samples of the non-degassed pre-eruptive melt
trapped during
growth of the phenocrysts, indicate that the K-bentonites were
generated
by explosive eruptions of rhyolitic, high-K type magmas in a
continental
volcanic arc. The geochemistry of melt inclusions may furthermore
be used
for correlation of these volcanic ashes since stratigraphically
distinct
K-bentonites contain inclusions with different major element
composition.
Diagenetic alteration of the rhyolitic ashes to K-bentonites has
strongly
affected their mineralogy and bulk geochemistry. The major element
composition
of altered K-bentonites, which apparently depends on the
composition of
the dominating clay minerals and other authigenic phases (e.g.,
pyrite,
calcite), has been
compared with unaltered melt inclusions and shows the direction
and
magnitude of the geochemical changes that occur during diagenesis.
Relative
to aluminum, substantial amounts of Si, Na, K and Mn have been
lost, whereas
Ti, Fe and Mg have been gained in the K-bentonites. The
surrounding sediments,
which are enriched in SiO2 compared with sediments further away,
apparently
acted as a sink for the silica released from the volcanic ash. The
observed
enrichment of TiO2 in the K-bentonites relative to aluminum seems
best
explained as a result of contamination by pelagic, TiO2-rich clay
particles
that have settled into the voids within the unaltered volcanic
ashes.
The morphology of zircon populations from several K-bentonites has
been studied using the classification scheme of Pupin and
Turco(1972b).
Applied as a petrogenetic indicator, the morphologies suggest
crystallization
of the zircons in I-type magmas at temperatures common for silicic
volcanic
rocks (i.e. >750ºC). It can be demonstrated that
stratigraphically
different K-bentonites contain zircon populations that are
morphologically
distinct and can be used for correlation. At least two different
K-bentonites
seem to be correlated between New York State and the upper
Mississippi
Valley based on the morphology of zircons.
The trace element abundances of Hf, Ti, P, Y, Yb, Ce, U and Th in
individual
zircons from several K-bentonites have been analyzed by electron
microprobe.
Single grains have been selected from layers which can be
correlated by
stratigraphy and by zircon morphology. It was found, however, that
the
geochemistry of zircons from stratigraphically different layers is
indistinguishable
and can not be used for correlation of the Paleozoic K-bentonites.
Schirnick, C., 1990. Origin, sedimentary geochemistry, and
correlation
of Middle and Late Ordovician K-bentonites: constraints from melt
inclusions
and zircon morphology. Unpublished MSc. thesis, State
University
of New York at Albany. 209 pp., +xii
University at Albany Science Library call number: SCIENCE
Oversize
(*) QE 40 Z899 1990 S35
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