Potassium-argon and rubidium-strontium age measurements have been made on mica separated from many rock types of the Manhattan Prong and New Jersey-New York Highlands (Reading Prong). Most of the rocks from the Manhattan Prong were dated within experimental error at 360 m.y. (million years). Since this age is identical with those obtained by the U-Pb method by other workers on intrusive pegmatites, it probably represents the last major metamorphic event in the area. The isotopic age of mica in the Highlands at least 10 miles from the zone where complete re-crystallization took place 360 m.y. ago is about 840 m.y. Apparent ages of mica in a transition zone range between these two. The ages in the transition zone are determined by the extent of re-crystallization which in turn is strongly dependent on the local structure. If the 1150-m.y. zircon age obtained by Tilton and others (1960) in the Highlands is the primary age of the basement in this region, major metamorphic events took place at 1150, 840, and 360 m.y. Relict ages as old as 480 m.y. in the Manhattan Prong suggest a fourth event if the rocks of the Prong were deposited in the Cambrian or later. This study illustrates both the difficulties and the effectiveness of isotopic geochronometry in areas of complex metamorphic history.

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