40AR/39AR METAMORPHIC AGES FROM THE MANHATTAN PRONG AND ROWE-HAWLEY ZONE OF NEW YORK AND CONNECTICUT

Hornblende and mica 40Ar/39Ar age spectra have been determined along two subparallel traverses of the high-grade and polymetamorphosed terrane of southwesternmost New England. One traverse begins at the Hudson River near the Cortlandt complex in an area of little or no Acadian overprinting and extends to sillimanite grade rocks in Manhattan. The other starts from near Goldens Bridge, NY in the sillimanite + K-feldspar zone and extends 40 km to Long Island Sound into sillimanite zone rocks.

At the Hudson River near the Cortlandt complex, Fordham Gneiss hornblende below the sillimanite isograd is variously reset, but two samples give slightly discordant spectra and yield minimum ages of 450 and 434 Ma, suggesting a lack of Acadian degassing of Taconic hornblende. To the south, hornblende from the type locality of the Yonkers Gneiss has a flat, slightly discordant spectrum with a preferred age of 387 Ma. Amphibolite from Manhattan B at Inwood Park in Manhattan has a plateau age of 378 Ma. At Goldens Bridge, Fordham Gneiss hornblende gives a plateau age of 414 Ma which we interpret as a cooling age from Taconic metamorphism. In sillimanite zone rocks south and east from Goldens Bridge, hornblende ages are younger. Three kilometers south of Goldens Bridge, allochthonous Manhattan C cover rocks, composed of interlayered migmatitic grt-bt gneiss and amphibolite, give a hornblende plateau age of 383 Ma. Muscovite from a leucocratic granitic dike that intrudes this same Manhattan C cover gives a plateau age of 352 Ma. Samples collected east of Cameron's Line are in the sil-ms zone. Leucocratic bt-hnb gneiss of the Harrison Gneiss collected north of Rye gives a discordant hornblende spectrum with a minimum age of 373 Ma. Amphibolite from the Hartland Formation in the Mamaroneck quadrangle in Rye gives a flat, nearly concordant spectrum with a preferred age of 364 Ma. We interpret these hornblende ages in the range 387-364 Ma as Acadian cooling ages.

Our data provide the basis for recognizing a boundary between Taconic and Acadian metamorphism in the Manhattan Prong. The similarity in hornblende cooling ages from Goldens Bridge to Rye and across adjacent Connecticut based on other studies, suggests that high-grade rocks in this region sustained rather uniform uplift and cooling following high-grade Acadian metamorphism.