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
Return to MS Theses completed in the Geological
Sciences Program, University at Albany