Granitic pegmatites in the southeastern Adirondacks: their use as indicators of temperature, pressure, and fluid conditions during a late stage of the Grenville Orogeny

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