Paper No. 3
Presentation Time: 1:45 PM
ORDOVICIAN K-BENTONITES IN WESTERN VERMONT: MINERALOGIC, STRATIGRAPHIC AND GEOCHEMICAL EVIDENCE FOR THEIR OCCURRENCE AND TECTONIC SIGNIFICANCE
Evidence from the field, geochemical and mineralogical analyses suggest the presence of three K-bentonite beds in uppermost Beekmantown (middle Ordovician) carbonates on the shores of Lake Champlain. These beds range in thickness from 5 to 50 cm and are distinguished in the field by their very fine-grained, shaly appearance, sharp basal and upper contacts, lateral continuity over hundreds of meters in outcrop, and recessive weathering. Thin section analysis reveals an illitic clay-rich matrix with tear-shaped clasts interpreted as devitrified volcaniclastic spherules and lack of sedimentary layering common to shales. Whole-rock geochemistry documents high concentrations of K2O (7.4 8.4 wt %) and low SiO2/Al2O3 ratios (2.2 2.7), both characteristics of K-bentonites. XRD analysis of the clay fraction indicates the presence of R3 interstratified illite/smectite with 95% illite layers as well as accessory chlorite and potassium feldspar, and low concentrations of quartz (12 14%). By comparison, Ordovician shales in the study area contain 4% K2O, SiO2:Al2O3 ratios of 3.2 4.2, abundant chlorite and quartz, and accessory plagioclase with no potassium feldspar. Apatite was detected in thin section we are currently carrying out heavy mineral separations designed to detect zircon. The K-bentonite beds plot mostly in the volcanogenic field on a SiO2/Al2O3 vs TiO2/Al2O3 diagram whereas the Ordovician shales plot in the detrital field. Rare earth elements are enriched in LREE (~100x chondrite) relative to HREE (~10-20x chondrite) with (La/Sm)n ratios of ~ 4.0 and (Gd/Yb)n ratios of ~ 2.5. Relative to average upper continental crust, the possible K-bentonites are enriched in immobile elements Sc, Cr and Ni and depleted in Nb, Ta, P, Zr and Hf. The age of the proposed K-bentonite beds, as indicated by the age of the enclosing carbonates, is approximately the same as the 485 to 470 Ma Shelburne Falls volcanic arc, a potential source of the volcanogenic material. Preliminary classification using immobile trace elements suggests the K-bentonites were derived from basalt or basaltic andesite ash; in that case, the source volcano was probably close to the site of deposition of the ash.