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.