Avalonian Igneous Activity in the Manhattan Prong, Southeastern New York

DOUGLAS G. MOSE Department of Geology, Brooklyn College, Brooklyn, New York 11210, and Department of Geology, Florida State University, Tallahassee, Florida 32306 JOHN HAYES Department of Geology, Brooklyn College, Brooklyn, New York 11210

ABSTRACT The lowermost of the metasedimentary units is the Fordham Gneiss. It is unconformably overlain by the Lowerre Quartzite, a The Manhattan Prong in southeastern New York consists probable correlative of the Cambrian Poughquag Quartzite that primarily of a Cambrian-Ordovician sequence (Lowerre Quartzite, overlies Precambrian crystalline rocks in the Hudson Highlands. Inwood Marble, Manhattan Schist) that unconformably overlies Many workers have therefore considered the Fordham Gneiss to be the Fordham Gneiss of uncertain age. Rb-Sr whole-rock analyses of the same age as the Hudson Highlands (Merrill, 1898; Berkey, the Pound Ridge Granite Gneiss, a rock unit within the Fordham 1907; Berkey and Rice, 1919; Balk, 1936). The Hudson Highlands Gneiss, indicate that the granite gneiss formed 596 ± 19 m.y. ago, are typical of Appalachian Grenville exposures and are thought to with an initial 87Sr/86Sr ratio of 0.7287 ± 0.0130. Field relations be about 1,000 to 1,150 m.y. old (Tilton and others, 1960; Rat- suggest that the Pound Ridge Granite Gneiss was an Avalonian cliffe and others, 1972; Dallmeyer, 1972). On the other hand, volcanic rock or accumulation of migmatitic fluids. As such, this Prucha and others (1968) were able to distinguish between the Rb-Sr age determination is a minimum age for the Fordham Gneiss. Fordham Gneiss in the Manhattan Prong and the Precambrian Key words: geochronology, absolute age, intrusions, metamorphic rocks, Precambrian, faults.

INTRODUCTION

The Manhattan Prong, which trends northeastward across southeastern New York, is an extension of similar rocks from Connecticut into New York. It consists primarily of marble, schist, and medium- to high-grade gneiss (Fig. 1). Several interpretations of the stratigraphy and chronological age of these rocks have been published in the past 80 years, and refinements in our understand- ing of the enigmatic belt continue to be made. Recent isotopic work by Long (1969) suggests that the Yonkers Granite Gneiss in the Manhattan Prong is post-Grenville (less than 1 b.y. old) but pre-Taconic (more than 450 m.y. old) in age. This intermediate age is similar to that of other igneous rocks in the Appalachians, produced during what is known as the Avalonian "orogeny" (Lilly, 1966) or the Avalonian "deformation" involving late Precambrian rifting and formation of block-faulted basins filled with arkose, graywacke, and lava flows (Bird and Dewey, 1970; Hughes, 1970; Hughes and Bruckner, 1971; Rodgers, 1972). The Rb-Sr whole-rock analyses of the Pound Ridge Granite Gneiss presented here demonstrate the presence of additional Av- alonian igneous rocks in the northern part of the Manhattan Prong.

REGIONAL GEOLOGY

The Manhattan Prong is bounded on the west by the Triassic Newark Group (Sanders, 1963), on the north by the Precambrian Hudson Highlands (Tilton and others, 1960; Ratcliffe and others, 1972), on the east by distinctly different rocks of the Hartland Formation (Rodgers and others, 1959), and on the south by Cenozoic and younger sedimentary rocks and sediments. Metasedimentary rocks in the Manhattan Prong consist of the Fordham Gneiss, the Lowerre Quartzite, the Inwood Marble, and the Manhattan Schist. Rocks of metaigneous or metasedimentary nature include the Yonkers Granite Gneiss and the Pound Ridge Granite Gneiss. Igneous rocks include the Cortlandt Complex, the Figure 1. Generalized geologic map of Manhattan Prong. Contacts from Peekskill Granite, and smaller intrusive bodies. Fisher and others (1970).

Geological Society of America Bulletin, v. 86, p. 929-932, 3 figs., July 1975, Doc. no. 50707.

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gneisses in the Hudson Highlands. In short, stratigraphic correla- the Fordham Gneiss and appear to be either metavolcanic or tions have not yet resolved the age of the Fordham Gneiss beyond metasedimentary units within the Fordham Gneiss (Isachsen, 1964; showing that it is probably no older than 1 b.y. and no younger Lessing, 1967; Hall, 1968a). Zircons from the Yonkers Granite than Early Cambrian. Gneiss yield a nearly concordant U-Pb age of 515 m.y. (Grauert The uppermost of the metasedimentary units is the Manhattan and Hall, 1973). This is very similar to the Rb-Sr whole-rock iso- Schist. It consists of three members, the lowermost (unit A) of chron age of 575 ± 30 m.y. obtained for this granite gneiss by Long which is thought to be Middle Ordovician on the basis of regional (1969). correlation of an unconformity at its base (Hall, 1966, 1968a). The Pound Ridge Granite Gneiss has been mapped by Scotford This has been confirmed by the discovery of Middle Ordovician (1956), Lessing (1967), and Prucha and others (1968). It is a pelmatozoan fragments in unit A (Ratcliffe and Knowles, 1968; quartz-microcline-microperthite rock with small amounts of biotite Ratcliffe, 1968). Units B and C of the Manhattan Schist appear to and muscovite. It is foliated and structurally concordant with the have been thrust onto unit A (Hall, 1968b). The Lowerre Quartz- surrounding Fordham Gneiss. Prior to the work reported here, the ite, the Inwood Marble, and the Manhattan Schist (unit A) se- Pound Ridge Granite Gneiss appeared to be another Avalonian quence in the Manhattan Prong appear to be equivalent to igneous rock similar to the Yonkers Granite Gneiss, or if not, pos- Cambrian-Ordovician rocks in eastern New York, north of the sibly an older rock, which could more clearly resolve the age of the Hudson Highlands (Balk, 1936; Hall, 1968b). Fordham Gneiss.

PREVIOUS RADIOMETRIC WORK IN THE ANALYTICAL TECHNIQUE MANHATTAN PRONG Samples weighing approximately 10 kg each were collected for U-Pb age determinations on zircons from metasedimentary rocks isotopic analyses. Each sample was crushed and split to approxi- in the Manhattan Prong show a large spread on a concordia dia- mately 0.5 g. The samples were spiked with MSr and 87Rb before gram with 207Pb/206Pb ages as old as 1,300 m.y. (Grauert and Hall, being dissolved in about 25 ml of HF and 0.5 ml of doubly distilled 1973). Of these, the analyses of zircons from the Fordham Gneiss HC104. Sample digestions were done in 50-ml Teflon beakers. Rb are particularly interesting. This formation has been divided into and Sr fractions were obtained using cation exchange columns. five members by Hall (1968a, 1968b). Zircons from member D Mass spectrometric analyses were done on a 12-in. radius, 60° (rusty sillimanite-garnet schist and gneiss) fall on a chord between sector field, single-focusing mass spectrometer equipped with a 1,000 m.y. and 430 m.y., as do zircons from neosomes in a migma- triple-filament thermionic source, a Faraday cup collector, a vibrat- tite in member C (gray biotite-hornblende gneiss with quartz- ing reed electrometer, and an expanded scale strip-chart recorder. feldspar layers and amphibolites). This indicated to Grauert and Hall that the Fordham Gneiss underwent an orogenic event 1,000 m.y. ago. An alternative interpretation is that the zircons in these members of the Fordham Gneiss are detrital zircons formed during Grenville time in the Hudson Highlands to the north and west. If this is the case, the U-Pb ages are useful in determining source areas for the sedimentary precursors of the rocks in the Manhattan Prong but provide little help in determining the time of sedimentation. A similar conclusion has been reached about the Rb-Sr whole-rock studies of these metasedimentary units (Senechal, 1973; Spanglet and others, 1974). Age determinations on mica from rocks of the Manhattan Prong, made with the K-Ar and Rb-Sr single-mineral technique, have also not been useful for studies of depositional chronology. These age determinations, however, have been useful in revealing the Paleozoic metamorphic chronology. At first, these mica ages sug- gested that the Manhattan Prong was most recently metamorph- osed about 360 m.y. ago (Long and Kulp, 1962). However, "•"Ar/^Ar incremental heating studies on similar rocks in Dutchess County, New York, now indicate that the metamorphism actually took place about 450 m.y. ago (Bence and Rajamani, 1972). The • p-20 360-m.y. age represents the time at which these micas were uplifted and cooled enough to retain radiogenic argon. The Rb-Sr data presented in this paper are from the Pound Ridge Granite Gneiss. This rock is located at the northern end of the same • P-21 folded structure in which the Yonkers Gneiss is found. The Pound Ridge and Yonkers Granite Gneisses are generally concordant with

TABLE 1. ANALYTICAL RESULTS FOR THE POUND RIDGE GRANITE GNEISS

Sample no. "Rb "Sr "Rb/*6Sr "Sr/,6Sr* (ppm) (ppm) atomic ratio atomic ratio

P-l 68.18 2.563 25.56 0.9550 ± 0.0018 P-2 112.0 0.241 460.3 4.6771 ± 0.0178 P-3 73.47 1.961 37.00 1.0403 ± 0.0044 P-l 9 93.30 1.661 55.53 1.1677 ± 0.0010 P-20 48.89 0.235 205.6 2.4981 ± 0.0040 P-21 58.45 0.547 105.6 1.5523 ± 0.0003 Figure 2. Approximate locations of Pound Ridge Granite Gneiss sam- 86 8B Corrected for fractionation, assuming Sr/ Sr = 0.1194. ples collected for isotopic analyses.

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Twelve analyses of the Eimer and Amend standard SrC03 (Lot metasedimentary or metavolcanic unit within the Fordham Gneiss 492327) gave an average 87Sr/86Sr ratio of 0.7080 ± 0.0003 (l

ACKNOWLEDGMENTS

The analytical work at Florida State University was supported by National Science Foundation Grant GA-33559. Sample prepara- tions were made with facilities provided by the Institute of Geochemistry and Chronology at Brooklyn College of the City University of New York. Co-workers who helped us to improve the manuscript include R. D. Dallmeyer, L. M. Hall, P. Lessing, L. E. Long, and N. M. Ratcliffe.

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