A MINERALOGICAL AND GEOCHEMICAL INVESTIGATION OF A GRANITIC PEGMATITE NEAR DeKalb JUNCTION, ST. LAWRENCE COUNTY, NEW YORK

The McLear pegmatite, located 3.3 km southwest of DeKalb Junction, St. Lawrence County, New York, was a source of feldspar for use in the ceramics industry between 1907 and 1933. The pegmatite consists of large (≤1m) phenocrysts of alkali feldspar and plagioclase feldspar with smaller phenocrysts of quartz and titanite; accessory phases include calciobetafite, uraninite, and thorite (Munschauer, 2009). Tremolite, diopside, and chlorite occur at the contact between the pegmatite and the siliceous marble host rock.

The goals of this study were to 1) determine the age of the pegmatite, 2) identify the source(s) of the magma, and 3) determine the temporal relationship between the pegmatite and regional Mesoproterozoic metamorphic events.

Preliminary Rb-Sr isotope data yield a calculated age of 1110 Ma for igneous titanite, comparable to previously-determined ages of 1217 Ma (zircon; Lupulescu, 2010), 1170 Ma (zircon; Chiarenzelli, 2008), and 1094 Ma (uraninite; Shaub 1940). Forthcoming Rb-Sr, Sm-Nd, U-Pb, and Pb-Pb isotope data will help elucidate magma source and further refine pegmatite age.

Although the pegmatite exhibits no macroscopic metamorphic textures, undulose extinction within quartz phenocrysts suggests it was emplaced prior to or during regional metamorphism.