GEOCHEMICAL CHARACTERIZATION OF GRENVILLE-AGE TOURMALINE (SCHORL- DRAVITE- UVITE) IN THE ADIRONDACK LOWLANDS, ST. LAWRENCE COUNTY, NEW YORK
Tourmaline crystals range from 0.05 mm to 2.00 cm in length, and are typically subhedral to euhedral. Color varies from black to green to golden-brown in hand sample. Microprobe analyses define symmetrical zoning within crystals and chemically distinct populations from different sample localities. Composition ranges from near end-member uvite to dravite to dravite-schorl. The uvite crystals have the greatest chemical variability from rim to core (Mg/Fe=6.0 to 27.0). Compositional variations correlate with the optical properties; pleochroism, birefringence, and refractive index.
Ternary diagrams comparing Mg-Ca-Fe, and Fe-Mg-Al were constructed (after the method of Henry and Guidotti, 1985) and indicate distinct chemical populations of tourmaline. The plots define source-rock compositions of Li-poor granitoids, metacarbonates, and calc-silicate rocks (agreeing with field observations).
Chemical zoning profiles in metamorphic tourmalines suggest relatively constant metamorphic conditions, with late stage sharp changes of pressure/temperature possibly associated with the development of the Carthage-Colton mylonite zone. Chemical distinctions between tourmaline populations are best defined by the Mg/ Fe ratio, while Na/Ca and Al ratios lead to inconclusive results. Microprobe traverses parallel to the tourmaline C-axis and across the C-axis define potential chemical exchange relationships during growth. The strongest correlation is between the Mg/ Fe and Na/Ca ratios for individual crystals.