ISOTOPIC AGES ON SOME IGNEOUS AND METAMORPHIC ROCKS IN THE VICINITY OF NEW YORK CITY

Abstract

The ages of igneous and metamorphic rocks in the New York City area were determined by isotopic techniques. All analyses were made by isotope dilution and mass spectrometric methods. From the data it was concluded that the metamorphism of the New Jersey-New York Highlands began about ll00 to 1200 million years ago. Another regional event occurred about 850 million years ago which recrystallized the mica in the metamorphic rocks of the entire area, but was not of sufficient magnitude to cause serious chemical alteration of the zircon. The latest recrystallization of Manhattan Prong rocks occurred about 365 million years ago. Scattered evidence points to a methmorphic history of these rocks prior to that time. Northeast of the Hudson River where the Highlands were strongly remetamorphosed in Lower Paleozoic time, apparent ages between 1200 and 365 million years were found. (M.C.G.)