PEGMATITE EMPLACEMENT AND GRANITOID MAGMATISM IN THE APPALACHIAN MOUNTAIN BELT

The emplacement of granitic pegmatites in the Appalachian mountains represent parts of repeated episodes of felsic magmatism that occurred during prolonged multiple continental-arc collisions and crustal extension. Granitic pegmatites are widely distributed throughout the Appalachian belt and consist of a diversified spectrum of pegmatite types characterized by: 1) variable mineral paragenesis, textures and internal structure; 2) bulk compositions that may include enrichment in rare alkalis (e.g., Li, Rb, Cs), high-field strength elements (e.g., Nb, Ta, Sn), rare-earth elements (including Y, U and Zr), and volatile components (e.g., B, F); 3) relationships to syn-tectonic, post-tectonic and anorogenic granitic plutons intruded at shallow to deep crustal levels; and by 4) emplacement into greenschist to amphibolite facies metamorphic rocks. These criteria allow for the division of the Appalachian Pegmatite Province into the New England, Mid-Atlantic and Southern subprovinces. The New England subprovince (Maine, New Hampshire, Massachusetts, Connecticut and Rhode Island) is best characterized by relatively shallow emplaced pegmatites with patterns of rare-element enrichment (e.g., Li, Rb, Cs, Be, Ga, Sn, Ta>Nb, B, P, and F) that can lead to extreme fractionation and highly diversified mineralization. Parental granites generally show S-type chemistry. The Mid-Atlantic (southern New York, New Jersey, Pennsylvania, Maryland and Virginia) and Southern (North Carolina, South Carolina, Georgia and Alabama) subprovinces host pegmatites that were emplaced at deeper levels with primitive to moderately evolved geochemical signatures. Parent granitoids may be absent to poorly differentiated with S- to I-type signatures. Pegmatites enriched in Nb>Ta, Ti, Y, REE, Zr, Be, U, Th, and F are typically related to granitoids with A-type chemistry and are more prominent in the New England subprovince.

Although the magmatic and tectonic histories of pegmatite generation in the Appalachians has never been fully defined, field relations and radioactive age determinations suggest that pegmatite emplacement spans a period in excess of 400 Ma and largely postdates major orogenic events (e.g., Taconic, Acadian and Alleghenian).