MINERALOGICAL STUDY OF PEGMATITE DIKES WITHIN THE ADIRONDACK PIEDMONT, ST. LAWRENCE COUNTY, NEW YORK

The purpose of our study was to mineralogically characterize felsic pegmatite dikes within part of the Adirondack piedmont, St. Lawrence County, New York and determine if the dikes were emplaced prior or subsequent to the Grenville metamorphic event.

The Adirondack piedmont is underlain by Precambrian metasedimentary (Grenville series, ~1.1Ga) and meta-igneous rocks (mostly granite gneiss). Flat-lying Paleozoic strata, including the basal Cambrian Potsdam sandstone, have mostly been eroded off of the underlying metamorphic rocks. The metasedimentary and meta-igneous rocks of the area host relatively small (≤5 m wide), compositionally variable, and spatially distinct felsic pegmatite dikes that generally cross-cut host rock metamorphic fabric and macroscopically appear to be unmetamorphosed.

Five representative pegmatite samples (VK08-1 to VK08-5) were collected in situ from five different localities within St. Lawrence County. Transmitted and reflected light microscopy and the electron microprobe were used to determine the mineralogy of each pegmatite sample and if each sample exhibits evidence of metamorphism. Microscopic analysis of twenty regular and polished thin sections revealed the most abundant mineral phases within each pegmatite to be quartz, plagioclase feldspar (An₂₀-An₄₈), and microcline, with subordinate orthoclase, tourmaline, calcic pyroxene, biotite, muscovite, phlogopite, ilmenite, and rutile.

All pegmatite dikes in this study appear to have been metamorphosed. Microscopic evidence includes undulatory extinction of quartz, bent lamellae within some plagioclase feldspars, and recrystallization of primary minerals to coarser-grained equivalents. This suggests the pulse(s) of magmatic activity responsible for the emplacement of the pegmatite dikes was either prior to or coeval with the Grenville metamorphic event.