Abstract
Within outcrops of the black Utica shales south of the Mohawk
Valley
narrow re-entrants parallel with stratification are visible, often
accompanied
-by a marked orange-staining of the underlying strata. These
rentrant features
indicate the presence of bentonites weathered back faster then the
enclosing
shales. These bentonites are presumably magmatic ashes related to
extensive
volcanism when an island arc collided with the North American
continent
in the late Ordovician about 450 million years ago (Taconic
orogeny).
Bentonites are good stratigraphic markers because of the short
time-interval
they represent and the large area they cover. Therefore they are
commonly
used as correlation tools. In this study an attempt was made to
use the
minerals (i.e. garnets) as a correlation tool in contrast to
earlier attempts
to correlate bentonites by chemical fingerprinting using bulk
chemistry
(Huff, 1986; Kolata et al., 1986, 1987).
The mineral content of these bentonites was separated and
analyzed.
In this study special attention was paid to the garnets and
garnet-bearing
rock fragments. The garnets were analyzed using an electron
microprobe
for major elements and trace elements, i.e. Ti, Sc, Zn and Y. In
the single
crystals as well as in the rock fragments two groups of garnets
are observed:
(1) pink grains containing 0.4-5 mol.% grossular, 56-79 mol.%
almandine,
12-40 mol.% pyrope, 1-5 mol.% spessartine, and <2.2 mol.%
andradite;
and (2) dark grains containing 14-24 mol.% grossular, 48-76 mol.%
almandine,
2-35 mol.% pyrope, 1-9 mol.% spessartine, and <7 mol.%
andradite.
For the trace elements some general trends are observed,
especially
a positive correlation of TiO2 with CaO, with up to 1150 ppm TiO2
in the
CaO-rich garnets. A negative correlation of Sc and CaO is
observed, leading
to higher amounts of Sc (100-200 ppm) in the CaO-poor garnets
compared
to the CaO-rich garnets which contain generally less than 100 ppm
Sc. Yttrium
does not show a distinct behavior with changing CaO content, but
it appears
to favor slightly garnets with higher CaO content.
Geothermobarometry calculations
(Berman and Perkins, 1987; Lindsley and Anderson, 1983; Ellis and
Green,
1979; Fuhrman and Lindsley, 1988; Ghent, 1976; Kohn and Spears,
1990) were
attempted on rock fragments with suitable mineral assemblages.
Calculated
temperatures and pressures included: (1) 600ºC on pyroxene
pairs;
(2) 605-625ºC and 6.5-7.5 kbar on a rock fragment containing
garnet-hornblende-quartz-plagioclase;
(3) 450-550ºC on feldspars; (4) 600-650ºC on
clinopyroxene-garnet
pairs; and (5) pressures of 4.5 kbar at 600ºC for the
assemblage garnet-
plagioclase-aluminosilicate and quartz, which support the view
that the
garnets and rock fragments are derived from a metamorphic source
of about
granulite-facies grade.
The incorporation of xenocrysts and xenoliths into the bentonites
can
be explained by the disruption and integration of Grenville-like
crust
into the ash during eruption. This explanation is consistent with
the ages
derived from a K-feldspar aliquot of one of the bentonites using
the Ar/Ar
method of dating, which shows a thermal history similar to
Grenville crust
K-feldspars and an age of 550-750 million years. This is too old
to be
derived from metamorphism during the Taconic event, nor can these
feldspars
be explained as authigenic phases; therefore, the best explanation
is a
different source than the volcanism at 450 million years, and a
minimum
age for this source of 750 million years, and probably older.
The subject of this investigation is split into two parts: (1)
Garnets
as means of correlation and (2) garnets as indicators for the
source of
the magmas creating the bentonites. Both lines of investigation
were unsuccessful.
The xenocrystic garnets in the bentonites cannot be used for
correlation
purposes and are also of very limited use to define a source area
because
of their metamorphic character.
Bock, B., 1990. Geochemistry of garnet xenocrysts and
garnet-bearing
xenoliths in Ordovician bentonites. Unpublished MSc. thesis, State
University
of New York at Albany. 113 pp., +x
University at Albany Science Library call number: SCIENCE
Oversize
(*) QE 40 Z899 1990 B63
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