Delano, J. W., Schirnick, C., Bock, B., Kidd, W. S. F., Heizler, M. T., Putman, G. W., De Long, S. E., and Ohr, M.
Altered volcanic ashes (K-bentonites) in the late Ordovician (Caradocian) Utica shale of New York State are the product of explosive arc volcanism. Most of the 30 K-bentonites examined in this investigation contain fragmental crystals and rock fragments (microliths) up to 600 (xm in diameter that generally are neither detrital contamination from the surrounding black shale nor igneous phenocrysts. The dominant phases are garnet ($$two groups; Gr_{15_20} Alm_{45_75} Py_{35_0} Sp_{2-5}; Gr_{2_5} Alm_{55_85} Py_{10-40} Spi$$), plagioclase feldspar ($$An{80-10}$$), alkali feldspar ($$Or_{99_0}$$), clinopyroxene $$(Wo_{50-40} En_{50-30} Fs_{0-30}$$), and orthopyroxene ($$Wo_{1-2} En_{77_32} Fs{32_66}4$$), accompanied by lesser quantities of hornblende, aluminosilicate, quartz, sphene, Fe-Ti oxides, apatite, and zircon. Most of the fragmental crystals appear to be derived from the same source as the metamorphic microliths, which possess minerals with similar compositions. Both crystals and microliths are interpreted as xenocrysts and xenoliths from the ancient continental crust on which the Ordovician arc was constructed. They became entrained in the volcanic plume during explosive eruptions. A Precambrian age acquired on K-feldspars from one K-bentonite using the $$^{40}Ar/^{39}Ar$$method shows that these xenocrysts were derived from depths of less than 5-10 km in the microcontinent at the time of late Ordovician volcanism. The occurrence of xenocrysts and xenoliths in these K-bentonites underscores the importance of performing detailed petrology on ash layers prior to the onset of more sophisticated tasks (e.g., isotopic age determinations; regional stratigraphic correlations of K-bentonites based upon chemical compositions).