ABSTRACT.
Granitic pegmatites associated with the Grenville orogeny intrude
a
mid-Proterozoic, upper amphibolite to near granulite facies
terrane in
the southeastern Adirondack Mountains of New York State. The
inferred anatectic,
fluid-saturated, and minimum melt characteristics of the
pegmatites suggest
that they can be used as analogs of the "granite" or
"haplogranite" system,
and as indicators of crustal temperature, pressure, and fluid
conditions
during a late stage of the Grenville orogeny. Bulk chemical
compositions
of seven simple, undeformed pegmatites in the southeastern
Adirondacks
indicate that they are generally depleted in silica and sodium,
and enriched
in potassium, relative to experimental data on phase equilibria of
the
granite-pure H2O system, when pH2O=7 kb. Bulk chemical
compositions of
coexisting feldspars from four of the seven pegmatites indicate
equilibrium
conditions of crystallization. Zirconium and P205 contents of five
of the
pegmatites indicate minimum melt temperatures of 644-720º
Centigrade,
while zirconium concentrations of two of the pegmatites indicate
temperatures
of roughly 850º Centigrade. Appreciable amounts of fluorine
and chlorine
in biotites separated from three of the seven pegmatites suggest
that additional
volatile components present in the fluid phase during initial
melting may
have generated a silica- and sodium-depleted, and
potassium-enriched minimum
melt. Pegmatites in the southeastern Adirondacks which are
enriched in
sodium and silica, and depleted in potassium, intrude
metasedimentary gneisses
and amphibolitic gneisses associated with carbonates.
These pegmatites may have been produced under higher CO2 activity
fluids,
relative to the granite-pure H2O system. Sharp pegmatite-host
gneiss contacts
and disrupted foliations in the wall rock are indicative of an
injection
mechanism of emplacement. Similarities in chemistry and mineralogy
between
pegmatites and host gneisses suggest hydrothermal interaction
between the
intruding magma and the host gneiss. The mineralogy and chemistry
of the
host gneisses indicates an approximately 5 meter wide,
sodium-enriched
restite within the host gneisses on both sides of the pegmatites,
and a
less than one meter thick wide zone affected by an infiltrating,
volatile-rich
vapor phase, produced as the melt crystallized. The local nature
of the
pegmatites and association with high strain zones suggest that
they represent
the final "pulse" of the Grenville orogeny in the southeastern
Adirondacks,
which brought an upper level felsic slice to a lower, hotter
region along
ductile high strain zones, and initiated localized partial
melting. Alternatively,
the evidence for hydrothermal interaction between the pegmatites
and their
host gneiss is suggestive of low pressure intrusion, possibly
related to
uplift following the Grenville orogeny. The association of the
some of
the pegmatites with extensive migmatization and mylonites, and the
lack
of any chilled margins, however, indicate significant, albeit not
necessarily
peak, metamorphic pressure.
Mihalich, J.P., 1987. Granitic pegmatites in the southeastern
Adirondacks:
their use as indicators of temperature, pressure, and fluid
conditions
during a late stage of the Grenville Orogeny. Unpublished
MSc. thesis,
State University of New York at Albany. 123 pp., +ix
University at Albany Science Library call number: SCIENCE
Oversize
(*) QE 40 Z899 1987 M55
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