ND MODEL AGES AND GEOCHEMISTRY OF DEVONIAN AND ORDOVICIAN SHALES OF WESTERN AND CENTRAL NEW YORK

Representative samples of 26 Devonian and Ordovician shales from the Appalachian Basin of western and central New York were analyzed for major and trace element chemistry. Compared to the Upper Continental Crustal estimates of Rudnick and Gao they are enriched in Al₂O₃, TiO₂, Fe₂O₃, CaO, and especially K₂O, while depleted in SiO₂, MgO, NaO, and Cr₂O₃. In terms of trace elements they are enriched in Rb, U, V, and the REEs and depleted in Sr and As. This is consistent with a highly weathered source and/or clay-rich composition. Enrichment in K and Rb may be from incorporation of material from ash and bentonite beds. REE sums varied from 50-255 ppm (169±55 ppm). Neodymium concentrations ranged from 9.8-50.4 ppm while Sm ranged from 1.78-9.22 ppm. The lowest REE concentrations were measured in carbonaceous shales. A dozen samples were selected for isotopic analysis. ¹⁴⁷ Sm/¹⁴⁴Nd ratios range from 0.1079-0.1275, similar to average shales, and argue against strong diagenetic effects. La/Sm and Dy/Yb ratios were 5.75±0.60 and 1.72±/-0.15 indicating a well-mixed source. Epsilon Nd T_CHUR range from -5.84 to -12.31. Shale samples yield Nd T_DM model ages of 1504 -1988 Ma suggesting sources including: 1) rocks of the Canadian Shield; 2) recycled older sedimentary units; 3) distal, non-North American pre-Grenville rocks; or 4) some combination of the above. The older model ages are somewhat unexpected given the proximity to Taconic, Acadian, and Alleghanian orogenic sources and the Queenston and Catskill deltas, but consistent with previous studies on the provenance of the Utica shale (T_DM ~ 1.9 Ga; Chakrabarti et al., 2007). Ordovician shales (n = 5) yield a slightly older Nd T_DM model age of 1914±54 Ma and Devonian shales (n = 7) yield an average Nd T_DM model age of 1652±96 Ma. This may imply slightly greater Paleozoic juvenile input in Devonian shales or a smaller Archean component. The oldest ages (1913-1988 Ma) where derived from the Utica Shale and the youngest (1504 Ma) was from the Marcellus Shale. The data are consistent with a largely Archean Canadian Shield source and an open seaway carrying detritus longitudinally southward (present coordinates) into the Northern Appalachian Basin, likely intermixing with sediment shed westward from the Taconic and Catskill Mountains during the Ordovician and Devonian periods, respectively.