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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