BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA

VOL. 60, PP. 603-606, 1 FIG. JUNE 1958

AGE OF THE METAMORPHISM OF THE ROCKS OF THE MANHATTAN PRONG

BY LEON E. LONG AND J. LAURENCE KULP

The igneous-metamorphic complex in the of the latest metamorphic event. Although such vicinity of the city of New York consists of two measurements cannot define the age of the major subprovinces: the New Jersey-New York original sedimentation, they can help clarify Highlands, an elongated belt trending northeast the geologic history of the area. Potassium- from western New Jersey to northwestern argon ages on biotite, phlogopite, and muscovite Connecticut (Fig. 1), and the Manhattan Prong from several different rock units and locations lying south of the Highlands. The rocks of the in the Manhattan Prong are reported here Prong have been classified by some workers (Table 1; Fig. 1). The experimental techniques into the Fordham gneiss, overlain in turn by were those of Carr and Kulp (1957) as modified the Lowerre quartzite, Inwood marble, and by Damon and Kulp (1957). More than a dozen Manhattan schist and gneiss. This is a lithologic replicate analyses of micas in the 300-500 m.y. sequence strikingly analogous to the rocks north age range have established the experimental of the Highlands, where Precambrian gneisses reproducibility as approximately 4-5 per cent. are overlain by a fossiliferous Cambro-Ordo- The radiogenic argon concentration in all sam- vician series of Poughquag quartzite-Wappinger ples reported here was greater than 92 per cent. dolomite-Hudson River pelite. The measured ages found for the metamor- For many years there have been discussion phism of the schist and marble and for the and debate as to whether the highly metamor- pegmatites are in excellent agreement. The phosed rocks constituting the Prong are Paleo- standard deviation for seven out of eight sam- zoic or Precambrian. Berkey (1907) believed ples lies within the uncertainty of the experi- that, since the Poughquag quartzite lies uncon- mental measurements. The agreement among formably upon the gneisses north of the High- these diverse mineral types suggests that the lands, and the Inwood is essentially conformable average apparent age, 366 ± 9 m.y., is close to with the Fordham in the south, there is no com- the true age for the latest major metamorphic pelling reason to correlate these two sequences event. It should be noted that similar ages across the Highlands barrier. He therefore ranging from 320 to 380 m.y. have been re- favored the opinion that all the Prong rocks are ported for pegmatites, granites, and metasedi- Precambrian. On the other hand Balk (1936), ments all along the Atlantic seaboard (Wasser- on the basis of his classic study in Dutchess burg et al., 1957; Eckelmann and Kulp, 1957; County, New York, concluded that the Cambro- Carr and Kulp, 1957; Kulp, Long and Eckel- Ordovician sequence could be traced eastward mann, 1957). This age is tentatively correlated through rapidly increasing grades of meta- with the upper Ordovician, Taconic orogeny. morphic intensity into a sequence identical to The two samples of biotite from the Fordham that of the Prong around New York, and that gneiss appear to be significantly older. This there is no fundamental break between the may be interpreted in two ways: (1) the Ford- Dutchess County rocks and the highly meta- ham represents older basement rock, on which morphosed rocks farther south. He found evi- dence that any unconformity which may have the Lowerre, Inwood, and Manhattan forma- existed between the Fordham and the Inwood tions were deposited, which did not lose all of (the intervening Lowerre quartzite is present its argon during the 365 m.y. recrystallization, in only a few localities) was converted into a or (2) the biotite contained some impurity such pseudo-conformity by the intense deformation as amphibole which may carry excess argon. and shearing. Moie recently Paige (1956) cor- Preliminary results on rubidium-strontium age related undoubted Cambro-Ordovician rocks determinations on Manhattan Prong rocks indi- west of the Hudson River near Peekskill, New cate an age of 350-400 m.y., including the Ford- York, with Inwood marble and Manhattan ham gneiss, but the small ratio of rubidium/ schist east of the river. common strontium in the particular samples The potassium-argon method of age deter- investigated made the error sufficiently large mination makes it possible to measure the time to make it impossible to eliminate one of these 603

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hypotheses. The phenomenon of argon loss ago and that this result is not inconsistent with from older rocks during superimposed meta- a Cambro-Ordovician date for the deposition morphic events has been clearly demonstrated of the Lowerre, Inwood, and Manhattan forma- in the Lamont laboratory for the Spruce Pine, tions.

MANHATTAN PRONG

LONG ISLAND SOUND

MANHATTAN SCHIST PLEISTOCENE "- INWOOD MARBLE • HARRISON DIORITE 0 ! 2 3 4 5 O PEGMATITE MILES X FORDHAM GNEISS FIGURE L- LOCATION OF SAMPLES FOR POTASSIUM-ARGON AGES, MANHATTAN PRONC.

North Carolina, area and is suggested by pre- Further work in the city of New York area liminary work along the contact of the Man- is in progress using potassium-argon, rubidium- hattan Prong with the New Jersey-New York strontium, and uranium-lead methods and will Highlands. be reported later. It is concluded that the age work thus far This research was supported by the Re- (lone suggests that the latest major orogenic search Division of the Atomic Energy Commis- event that produced the metamorphism of the sion. The authors wish to acknowledge the help- rocks of the Manhattan Prong and the intru- ful criticism of P. W. Gast and the assistance sion of the pegmatites occurred about 365 m.y. of F, Bazan in making the argon analyses.

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TABLE 1.—MANHATTAN PRONG POTASSIUM-ARGON AGES* (Probable uncertainty = 4-5% of the apparent age) Radiogenic Apparent age Sample location Mica type K(%) A«» (ppm) A«/K« (m. y.)

Inwood marble, River Ave., Bronx, Phlogopite 8.14 0.206 0.0208 (325 )f N. Y. Manhattan schist, Riverside Church, Biotite 5.00 0.141 0.0232 359 New York, N. Y. Manhattan schist, railroad station, Biotite 3.82 0.114 0.0244 376 Thornwood, N. Y. Inwood marble, quarry, Thornwood, Phlogopite 7.09 0.194 0.0225 349 N. Y. Harrison diorite, Long Ridge Road, Biotite 7.69** 0.228 0.0243 375 Rte. 104, f. 3 miles south of Long Ridge, Conn. B ranch ville pegmatite, quarry, Muscovite 8.49** 0.243 0.0235 364 Branchville, Conn. Discordant pegmatite intruded into Biotite 7.76 0.226 0.0239 368 Fordham gneiss, 2.3 miles east of Tappan Zee Bridge, N. Y. Thru- way Discordant pegmatite intruded into Muscovite 8.47 0.248 0.0240 371 Fordham gneiss, location above Average 366 ± 9 s.d. Fordham gneiss country rock, loca- Biotite 6.69 0.213 0.0262 400 tion above Fordham gneiss country rock, 0.6 mile Biotite 5.78 0.210 0.0299 440 east of Tappan Zee Bridge, N. Y. 0.199 0.0282 Thruway 10 1 1 * Calculated using \f = 4.8 X lO" yr.- and Xe = 0.58 X 10-™ yr.- . f Sample badly weathered; age not included in the average. ** Analyzed by D. Thaemlitz and C. O. Ingamells, Rock Analysis Laboratory, Univ. of Minnesota; others by Ledoux and Co., Teaneck, N. J.

References Cited Kulp, J. L., Long, L. E., and Eckelmann, F. D. 1957, Age of the Southern Appalachian and Balk, R., 1936, Structural and petrologic studies Piedmont metamorphic complex: Geol. Soc. in Dutchess County, New York, Pt. I: Geol. America Bull., v. 68, p. 1758 Soc. America Bull., v. 47, p. 685-774 Paige, S., 1956, Cambro-Ordovician age of the Berkey, C. P., 1907, Structural and stratigraphic "Inwood" limestone and "Manhattan" features of the basal gneisses of the High- schist near Peekskill, New York: Geol. Soc- lands: N. Y. State Museum Bull. 107, p. 361- America Bull., v. 67, p. 391-394 378 Wasserburg, G. J., Pettijohn, F. J., and Lipson, Carr, D. R., and Kulp, J. L., 1957, Potassium- 40 40 argon method of geochronometry: Geol. J., 1957, A /K ages of micas and feldspars Soc. America Bull., v. 68, p. 763-784 from the Glenarm series near Baltimore, Md.: Damon, P. E., and Kulp, J. L., 1957, Determina- Science, v. 126, p. 355-357 tion of radiogenic helium in zircon by stable isotope dilution technique: Am. Geophys. LAMONT GEOLOGICAL OBSERVATORY (COLUMBIA Union Trans., v. 38, p. 945-953 UNIVERSITY), PALISADES, N. Y. Eckelmann, W. R., and Kulp, J. L., 1957, Ura- MANUSCRIPT RECEIVED BY THE SECRETARY OF nium lead method of age determination, Pt. THE SOCIETY, FEBRUARY 3, 1958 II: North American localities: Geol. Soc. LAMONT GEOLOGICAL OBSERVATORY CONTRIBU- America Bull., v. 68, p. 1117-1140 TION No. 283

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