Petrology and geochemistry of Ordovician K-bentonites in New York State: Constraints on the nature of a volcanic arc
- State Univ. of New York, Albany (USA)
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 {mu}m in diameter that generally are neither detrital contamination from the surrounding black shale nor igneous phenocrysts. The dominant phases are garnet (two groups; Gr{sub 15-20}Alm{sub 45-75}Py{sub 35-0}Sp{sub 2-5}; Gr{sub 2-5}Alm{sub 55-85}Py{sub 10-40}Sp{sub 1}), plagioclase feldspar (An{sub 80-10}), alkali feldspar (Or{sub 99-0}), clinopyroxene (Wo{sub 50-40}En{sub 50-30}Fs{sub 0-30}), and orthopyroxene (Wo{sub 1-2}En{sub 77-32}Fs{sub 32-66}), 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 {sup 40}Ar/{sub 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).
- OSTI ID:
- 6250461
- Journal Information:
- Journal of Geology; (USA), Journal Name: Journal of Geology; (USA) Vol. 98:2; ISSN JGEOA; ISSN 0022-1376
- Country of Publication:
- United States
- Language:
- English
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FEDERAL REGION II
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ORIGIN
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OXYGEN COMPOUNDS
PALEOZOIC ERA
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PLUTONIC ROCKS
POTASSIUM COMPOUNDS
PYROXENES
QUARTZ
RADIOISOTOPES
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SEDIMENTARY ROCKS
SHALES
SILICATE MINERALS
SILICATES
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