FRIEDMAN, GERALD M., Department of Geology, Brooklyn College and Graduate School of the City University of New York, Brooklyn, NY 11210, and Northeastern Science Foundation affiliated with Brooklyn College, Rensselaer Center of Applied Geology, 15 Third Street, P.O. Box 746, Troy, NY 12181-0746

In fluvial meandering facies of the Gilboa Formation (Upper Givetian; uppermost Middle Devonian) within the eastern Catskill Mountains of New York, a well-rounded pebble revealed the texture and composition of metamorphosed diabase. This pebble of greenschist facies, composed for the most part of feldspar and chlorite, derived from mid-oceanic spreading ridges in the Iapetus Ocean. Hydrothermal metamorphism at about 200-400°C two or three kilometers below the ocean floor, transformed diabase into greenschist metamorphic rock. A K-Ar date on feldspar gave an age of 402 ± 11 my. Zircons in the same pebble yielded ca. 400 my. These dates coincide with the early phase of Acadian orogeny. The age of the Gilboa Formation (Upper Givetian) has been dated at 385 to 370 Ma. The ca. 15-30 million-year differential between formation of the diabase body and the final settling of the pebble of metamorphosed diabase in the fluvial deposit, allowed about 15-30 my for converting mid-ocean spreading ridges into the rising Acadian Mountain ranges, and degradation of the mountains by rivers to form the fluvial wedge. Zircon dates in the pebble at around 1200 Ma confirm the Grenville basement as the initial source.

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