Abstract

A belt of alkalic to alkalic-calcic pipes, lamprophyres, and intrusive complexes of probable Late Ordovician age extends from the Peach Lake–Croton Falls area of Westchester County, New York, about 100 km S80°W to Beemerville, New Jersey. The rocks are intruded across a Taconic dynamothermal belt ranging from sillimanite grade on the east to chlorite grade on the west. Foliated Middle Ordovician and older rocks containing slaty cleavage or coarser grained metamorphic fabrics are crosscut by dikes that are posttectonic to late syntectonic relative to the Taconian dynamothermal events. Analysis of thrust faults and fold structures within this belt suggests that this cross-grain belt of alkalic to weakly alkalic rocks intruded steeply dipping brittle fractures trending N40° to 50°W, N35° to 60°E, and N70° to 80°E and along more ductile zones trending N20° to N70°E. This plutonic activity was restricted to a discrete east-west belt. The clear posttectonic to syntectonic nature of the dikes and the spatial and temporal association with Ordovician tectonism suggest that magmatic activity may be related to fracturing of mantle rock at the junction of two salients of Taconic age that intersect in the New York recess. The tectonic setting of the intrusive belt is unusual because it is associated in space and time with compressional tectonics of the Taconic orogeny, rather than with extensional rifting.

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